Engineering Materials II (ME2301) Notes - SEM2 (2) PDF

School of Mechanical and Aeronautical Engineering ENGINEERING MATERIALS II ME2301  Module Overview __________________________________________________________________________ MODULE OVERVIEW 1 Introduction Engine...

School of Mechanical and Aeronautical Engineering ENGINEERING MATERIALS II ME2301  Module Overview __________________________________________________________________________ MODULE OVERVIEW 1 Introduction Engineering Materials II is a second year core module for the Diploma in Bioengineering, Diploma in Mechanical Engineering and Diploma in Aeronautical Engineering Full-time Course. It is also a third year module in the Diploma in Mechanical Engineering Part-time Course. 2 Module Aims This module will provide students with a foundation of the principles governing the properties of engineering materials and their behaviour during processing and service. It also provides students with sufficient knowledge of both metallurgical processes practised widely in industries as well as methods of assessing methods of materials defects. 3 Module Contents The topics within the module and their allocated lecture hours are listed below Topic Title Hours 1 Materials Selection Methodology 2 2 Failure of Metals 6 3 Ceramics and Composites 5 4 Non-Destructive Testing 6 5 Corrosion 5 6 Heat Treatment of carbon and alloy steels 6 EMII/Overview School of Mechanical & Aeronautical Engineering Ver 6.0 1  Module Overview __________________________________________________________________________ 4 Teaching Plan The timetable for lectures, tutorial and laboratories are outlined below: Week Lecture Schedule Lab / Tut Schedule 1 Materials Selection Briefing 2 Failure of Metals Materials selection Case Study / 3 Failure of Metals Experiment 1 4 Failure of Metals Tut 1 Failure of Metals / 5 Ceramics & Composites Experiment 2 6 Ceramics & Composites Tut 2 Ceramics & Composites / 7 Non-Destructive Testing Experiment 3 8 MST 9-11 BREAK BREAK 12 Non-Destructive Testing Tut 3 Non-Destructive Testing / 13 Non-Destructive Testing Experiment 4 14 Corrosion Tut 4 Corrosion / 15 Corrosion Experiment 5 16 Corrosion Tut 5 Heat Treatment / 17 Heat Treatment Makeup 18 Heat Treatment /Revision Makeup Lab / Revision Note: Due to unforeseen circumstances, the actual schedule of lectures, tutorials and laboratory experiments may be adjusted to accommodate resources constraints. In such events, students affected will be informed and by teaching staff concerned and informed of respective make-up classes (where applicable). EMII/Overview School of Mechanical & Aeronautical Engineering Ver 6.0 2  Module Overview __________________________________________________________________________ 5. Assessment The assessment consists of a Mid-Semester Test, a Semester Examination, Laboratory Reports, Tutorials and a Case Study. The weightage and format are as follows: Assessment Weightage Format Mid-Semester Test 20% 20 Multiple-Choice Questions: 20 marks (MST) 2 Structured Questions: 30 marks 50 marks Semester Examination 40% 20 Multiple-Choice Questions: 20 marks (EXAM) 5 Structured Questions: 80 marks 100 marks Laboratory Reports 20% 5 reports = 5x4% = 20 marks (LAB) Tutorials 10% A total of five tutorials are assessed during tutorials: Punctuality: 20% Attitude: 20% Participation: 30% (CA) Preparation effort: 30% Case Study 10% Attendance on demonstration session and tutorials and report on ‘Materials Selection’ Total 100% Note: The weightage assigned above may be subjected to changes. Students will be informed of any such change. EMII/Overview School of Mechanical & Aeronautical Engineering Ver 6.0 3  Module Overview __________________________________________________________________________ 6 Resource Material 6.1 Text There is no single book that adequately covers all the topics in the module. Students will be given lecture handouts, tutorials and laboratory material by the department. 6.2 References The following references are available in the library. 1. Budinski, K. G. and Budinski, M. K., Engineering Materials: Properties and selection, 9th Edition, Pearson. 2. Higgins R. A. (1993), Engineering Metallurgy Pt 1: Applied Physical Metallurgy, 6th Edition, London: Edward Arrnold 3. William D. Callister Jr. Materials Science and Engineering, 10th Edition, New York, John Wiley & Sons 4. Flinn & Trojan, Engineering Materials and Their Applications, 4th edition, John Wiley & Sons 5. Chandler H. (1998), Metallurgy for Non-metallurgists, ASM International. 6. Ashby M. F., Materials Selection for Mechanical Design (4th Edition), Amsterdam, Elseiver Butterworth-Heinemann. EMII/Overview School of Mechanical & Aeronautical Engineering Ver 6.0 4 MODULE: Engineering Materials II 1-1 Syll No: ME 2301 TOPIC No: 1 MATERIALS SELECTION METHODOLOGY Keywords: Selection factor, functionality, processibility, reliability, go/no go criteria, weighting factor, scaled property, material performance index Objectives: Students should be able: 1.1 To explain the principles of materials selection. 1.2 To recognise the relationship between materials selection and manufacturing processes. 1.3 To know the materials performance requirements. 1.4 To carry out the selection procedure. Software: CMS (Cambridge University); Cambridge Materials Selector developed by Cambridge University Engineering department. It implements the selection procedures developed in the text Ashby, M. F. (1992) “Materials Selection in Mechanical Design”, Butterworth/Heinemann. EMII\Materials Selection School of Mechanical & Aeronautical Engineering Ver 6.0 MODULE: Engineering Materials II 1-2 Syll No: ME 2301 TOPIC No: 1 Introduction: Materials selection is concerned with selecting the “best” material for an application. An integrated approach considering the entire process of design is adopted. This will involve the consideration of material properties, processing methods, economics, recycling and aesthetics. Information about materials is needed at each stage. The nature of the information needed in the early stages differs greatly in its level of precision and breadth from that needed later. As there is an enormous range of materials from which to choose, the approach is to eliminate as many materials as quickly as possible; then concentrate on the promising candidates. Principles of Materials Selection: The four steps to consider in arriving at the optimum material choice are: The design specification Precise definition of functions which the product must perform, service conditions in which the product may operate and number required and rate of production. Detailed design considerations will not normally be made. It may be required to return to this step to offer help to the designer with any problem area. It is important to ensure all relevant service conditions and product requirements are known. The manufacturing processes Consideration is given to the problems of how to make the product or shape. Constraints imposed by the manufacturing process include shape and size of the product, commonly available form of the supplied material, numbers of product required, and rate of production. The numbers of products is important e.g. > 104 products per annum allows the possibility of higher tooling costs in some processes (e.g. die casting, injection moulding). Translating specification into relevant materials properties Prepare a list of properties, which must be considered, for the product. Not all properties will be of equal importance, e.g. some may be mandatory, for others, compromise will be possible. Use a selection factor of 1 to 5 where 1 is considered imperative and 5 unimportant. Thus, 4 is desirable but of low level of importance. The selection factors are applied to the appended checklist. EMII\Materials Selection School of Mechanical & Aeronautical Engineering Ver 6.0 MODULE: Engineering Materials II 1-3 Syll No: ME 2301 TOPIC No: 1 CHECKLIST Establish selection factors from the list  Factor (1: high priority ; 5: low) Property Factor Requirement Mechanical Stiffness-static and dynamic f (Temp, time) Yield strength Or appropriate proof stress Impact strength f (T, ε) Fatigue strength < 0.5 Tm K Creep strength > 0.5 Tm K Hardness Wear resistance etc. Fracture toughness* KIC and critical crack length Chemical Atmospheric corrosion Pollution effects Aqueous corrosion Pollution effects Elevated temperature oxidation and corrosion Stress corrosion cracking Weathering and ageing of coatings e.g. paint Physical Thermal conduction Metals are good conductors Thermal expansion Phase-change volume changes Electrical properties Metals are good electrical conductors Magnetic properties Some metals and alloys are ferromagnetic Density Safety Toxic During processing or operation Flammability Aesthetics Colour Effects of coatings on chemical and mechanical properties Surface finish Ability to polish Economics Raw material cost Commodity markets, energy costs, political-economic considerations Fabrication cost Relate to manufacturing processes Cost of ownership Lifetime of product in relation to the design specification Recovery of material From manufacturing and also recycling or refurbishing * Measure resistance to crack propagation EMII\Materials Selection School of Mechanical & Aeronautical Engineering Ver 6.0 MODULE: Engineering Materials II 1-4 Syll No: ME 2301 TOPIC No: 1 Examine the data and select material After completing the above three stages without prejudgement, the selection process can now begin. The classes of materials, which will satisfy the specification at least in terms of modulus, will already be known and from this point the selection is made by satisfying firstly all of the factors 1 then the factors 2 and 3. A factor of 4 would be used to make a final decision. List important selection factors 1. ________________________________ 2. ________________________________ 3. ________________________________ 4. ________________________________ Select a material system Metals Plastics Ceramics Others Select a specific material In general there is no single solution to the problem but there may be several solutions of equal value only distinguishable by parameters of factor 3 or 4. These solutions are equally valid. Some interaction might be allowed in that, for example once a choice is made, one may go back, in the light of the selection, and make modifications to the manufacturing process. The method suggested is not intended to be totally inflexible in application and inevitably for many products the system is controlled by economic factors which might not be assessed but ought to be recognised by a materials specialist. EMII\Materials Selection School of Mechanical & Aeronautical Engineering Ver 6.0 MODULE: Engineering Materials II 1-5 Syll No: ME 2301 TOPIC No: 1 Relationship between Materials Selection and Processing: A material has to be selected to translate the design-on-paper (or computer) into reality. There is no benefit in selecting the most ideal material when it cannot be processed economically into the required shape. It is important to match materials to processes carefully as each process will give better result with some materials than others. Materials selection and manufacturing processes go hand-in-hand. Processing is to achieve shape and dimensions, properties, and finish. Shape and dimensions The three basic methods to obtain the shape are flow processes, fabrication by joining and machining. Flow processing includes the liquid casting of metals, injection moulding of plastics, slip casting of ceramics, mechanical working of metals and the densification of powder compacts. Fabrication is accomplished by mechanical, metallurgical or chemical methods of joining. Mechanical methods include riveting and bolting and other diverse methods of clipping and fixing. Metallurgical methods include welding, brazing and soldering. Chemical methods involve the use of adhesives, glues or cements. Machining is expensive in terms of energy and labour, wasteful of basic resources and requiring a good deal of costly capital equipment. However, machining has the ability to combine high quality with large throughput. Almost any shape can be produced from a solid block of material provided the price can be paid. Properties The properties of an engineering component derive mainly from the material of which it is made. Properties in metals can be modified during successive stages of processing which may not be possible for other materials. For example, the separate processes in making a sheet metal will include solidification of an ingot, re-heating, hot-rolling, cold-rolling and annealing in a complex sequence of operations of which its properties will have been manipulated to suit its final use. On the other hand, once the melt for making an injection moulding plastics component has been prepared, there is less in subsequent procedure that can modify its properties. The ability to control properties of a component during processing thus allows these to be better matched to applications. EMII\Materials Selection School of Mechanical & Aeronautical Engineering Ver 6.0 MODULE: Engineering Materials II 1-6 Syll No: ME 2301 TOPIC No: 1 Finish It includes engineering tolerance, surface quality, surface protection and appearance. It is a property that can be precisely specified in terms of standard limits and fits and parameters of surface topography. Performance Requirements: The performance requirements are divided into functional requirements, processibility requirements, cost, reliability and resistance to service conditions. Functionality It is related to the required characteristics of the component. The evaluation can be simple. For example if the component carries a uniaxial load, the yield strength of a material is used to relate the load-carrying of the component. Evaluation can also be complex and predictions are based on simulated service tests. Processibility It is a measure of its ability to be worked or shaped into a finished component. Processibility can be defined as castability, weldability, machinability, etc. It is important to note that processing will always affect the material properties, so that processibility is closely related to functional requirements. FUNCTION: To transmit loads, heat, pressure, store energy, etc. at optimum efficiency. MATERIAL SHAPE PROCESS There is interaction between function, material, process and shape. Function dictates the choice of materials. The shape is chosen to perform the intended function using the material. Process is influenced by material properties. EMII\Materials Selection School of Mechanical & Aeronautical Engineering Ver 6.0 MODULE: Engineering Materials II 1-7 Syll No: ME 2301 TOPIC No: 1 Cost In many applications, there is a cost limit for a material intended to meet the component’s requirements. The cost of stock materials is often less than cost of processing. Total cost to consumer Purchase cost Cost of ownership Variable cost Fixed cost Manufacturer’s Maintenance profit Repairs Insurance Cost of basic Factory overheads H. P. interest materials Administration Cost of Sales and marketing manufacture R&D Reliability Reliability is defined as the probability that a component will perform its function adequately for the intended time under specified conditions without failure. A component’s reliability is difficult to measure because it depends not only on materials properties but also affected by its production and processing history. Failure analysis is used to predict different ways a component fails. The causes of failure can be traced back to defects in materials and processing. Service conditions The operating environment plays an important role in determining the material performance requirements. Corrosive environments can adversely affect most materials in service. Thermal stresses should be avoided in elevated temperature applications. Wear resistance is a requirement where relative movement between different parts exists. EMII\Materials Selection School of Mechanical & Aeronautical Engineering Ver 6.0 MODULE: Engineering Materials II 1-8 Syll No: ME 2301 TOPIC No: 1 Selection The large number of materials and processing methods make selection Procedure: process a difficult task. The selection process should not be carried out haphazardly to avoid the risk of overlooking a possible attractive alternative solution. A systematic selection procedure must be adopted to minimise the risk. Materials and process selection should not be viewed only in terms of new product development but rather for reviewing the type of materials and processes used in making an existing product. The many reasons in reviewing include: Taking advantage of new materials or processes. Improving service performance, including longer life and higher reliability. Meeting new legal requirements. Accounting for changed operating conditions. Reducing cost and making the product more competitive. Using a table of go/no go criteria allows early elimination of unsuitable materials from the initial list of selected materials. Considerable experience and knowledge are required to reject a material at this stage. Requirements Material Primary (F1) Secondary (F2) Cost Decision PR1 PR2 PR3 SR1 SR2 M1 A O A A A E Reject M2 A A A O A A M3 U A A A A A Reject M4 A O A A O A M5 A A A A A E Reject M6 A A A U A A U : Under provision O : Over provision E : Excessive A : Acceptable In most applications it is necessary that a selected material satisfy more than one performance requirement. The weighted properties method can be used when several properties are taken into consideration. In this method, each material property (those given selection factor 1) is further refined by comparison and assigned a certain weight by varying degrees of merit. EMII\Materials Selection School of Mechanical & Aeronautical Engineering Ver 6.0 MODULE: Engineering Materials II 1-9 Syll No: ME 2301 TOPIC No: 1 Each property is listed and compared with one another, two at a time. The important of the two is given a 1 (one) and the less important given a 0 (zero). In this way, the total number of positive responses N is n(n-1)/2 where n is the number of properties. Properties P1 P2 P3 P4 P5 Row Weights total ω P1 - 1 0 0 1 2 0.2 P2 0 - 1 1 1 3 0.3 P3 1 0 - 0 0 1 0.1 P4 1 0 1 - 1 3 0.3 P5 0 0 1 0 - 1 0.1 N=5 Number of positive responses 10 1.0 N = n(n-1)/2 = Since different properties have different numerical values, a scaling factor is used to standardise these differences. numerical value of property β = scaled property = X 100 largest value under consideration Where it is more important to have low values, e.g. density, the scale factor is formulated as follows: lowest value under consideration β = scaled property = X 100 numerical value of property For properties that are not readily quantified, e.g. weldability, vague terms such as poor, fair, excellent are best abandoned in favour of numerical ratings. Property Matl 1 Matl 2 Matl 3 Matl 4 Matl 5 Weldability Excellent Good Fair Poor Bad Relative rating 5 4 3 2 1 Scaled property 100 80 60 40 20 The material performance index γ is summation of all the properties. γ = ∑ βω As cost is usually with a high weighting factor, it may be useful to emphasise cost by applying it as a moderator to the material performance index. γ γ' = where c = total material cost per unit weight; ρ = density cρ EMII\Materials Selection School of Mechanical & Aeronautical Engineering Ver 6.0 MODULE: Engineering Materials II 1-10 Syll No: ME 2301 TOPIC No: 1 Example 1. The materials selection for a cryogenic storage vessel for liquefied gas is being evaluated on the basis of low temperature fracture toughness, fatigue strength, stiffness, thermal expansion and cost. Determine the weighting factors for these properties. Solution 1. Property 1 2 3 4 5 Positive ω response 1. Fracture toughness - 1 1 1 1 4 0.4 2. Fatigue strength 0 - 0 1 0 1 0.1 3. Stiffness 0 1 - 0 0 1 0.1 4. Thermal expansion 0 0 1 - 0 1 0.1 5. Cost 0 1 1 1 - 3 0.3 n=5 N = n(n-1)/2 = 10 1.0 Matl Go / No go Toug Fatig Stiff Expan Cost γ Cor Wel Ava (0.4) (0.1) (0.1) (0.1) (0.3) 304 S S S 100 70 93 80 50 79 9% Ni S S S 85 100 97 100 83 89 Al alloy S S U 3% Ni S S S 40 70 100 94 100 72 304 : 304 stainless steel 9%, 3% Ni : 9%, 3% Nickel steel S: Satisfactory U : Unsatisfactory Cor : Corrosion resistance Wel : Weldability Avai : Available in thick plate Toug : Fracture toughness Fatig : Fatigue strength Stiff : Stiffness Expan : Thermal expansion Several go/no go criteria are included. The materials performance index indicates that 9% nickel steel is the best choice. EMII\Materials Selection School of Mechanical & Aeronautical Engineering Ver 6.0 MODULE: Engineering Materials II 1-11 Syll No: ME 2301 TOPIC No: 1 Example 2. Consider the use for an aircraft wing from the listed materials. Material σYS KIC ρ E Cost (MPa) (Mpa√m) (tonnes/m ) (GPa) (S$/tonne) 3 Al alloy 1 350 45 2.7 70 1650 Al alloy 2 550 25 2.7 70 1960 Ti alloy 880 60 4.5 110 15400 Stainless steel 900 100 7.8 200 1400 Solution 2. The raw data need to be standardised. Since the wing can be simply represented as a wide plate cantilever, standardisation is carried through: BENDIN G BUCKLING / TENSION  1  E 3 Stiffness (bending) is maximised by  ... Eq 1  ρ    σ  Strength (buckling/tension) is maximised by  ... Eq 2  ρ 2  K IC  Crack resistance is maximised by  ... Eq 3  σ YS  Material Eq 1 β Eq2 β Eq 3 β Cost β γ= ∑βω Al alloy 1.53 100 130 64 16.5 100 16500 85 87 1 Al alloy 1.53 100 204 100 2.1 13 1960 71 71 2 Ti alloy 1.06 69 196 96 4.6 28 15400 9 51 SS 0.75 49 115 56 12.3 75 1400 100 70 Taking equal weightage ω = 1/4 = 0.25 for all properties The high cost of titanium alloy and high density of steels leaves aluminium alloys as the main contenders. For supersonic military fighter jets, it is necessary to include resistance to elevated temperature since high frictional heat is generated at high speed. EMII\Materials Selection School of Mechanical & Aeronautical Engineering Ver 6.0 MODULE: Engineering Materials II 1-12 Syll No: ME 2301 TOPIC No: 1 The Role of Computer: The use of computer allows an integration of design, materials selection and processing. Furthermore, the huge availability of materials, especially plastics and composites, has made the task of materials selection more difficult. Much of the data on materials now exist in computerised form are producer-based, relating to individual classes of materials. There remains the problem of properly non-biased assessment, completeness, uniformity of testing methods, comparability of data, etc. The use of graphical relationship approach from the data is particularly effective in the initial sorting stages of a selection procedure. Such graphical approach is implemented in Cambridge Materials Selector (CMS) software. In this approach, the data for the properties of all materials are presented as a set of Materials Selection Charts. Ashby, M. F. (1992) “Materials Selection in Mechanical Design”, Butterworth/Heinemann EMII\Materials Selection School of Mechanical & Aeronautical Engineering Ver 6.0 MODULE: Engineering Materials II 1-13 Syll No: ME 2301 TOPIC No: 1 Ashby, M. F. (1992) “Materials Selection in Mechanical Design”, Butterworth/Heinemann EMII\Materials Selection School of Mechanical & Aeronautical Engineering Ver 6.0 MODULE: Engineering Materials II 1-14 Syll No: ME 2301 TOPIC No: 1 Example 3. Use the appropriate materials selection chart(s) to select suitable materials for the construction of a minimum weight thin-walled spherical pressure vessel. The strength of the vessel must exceed 400 MPa. Pressure: p Radius of sphere: r Thickness of sphere: t pr Hoop stress: σ = 2t Solution 3. pr σ= … Eq 1 2t Weight of the spherical vessel, W = 4πr2tρ … Eq 2 where ρ is the density of the material Minimising weight is achieved by reducing t; i.e. pr W t= (from Eq 1) and t = (from Eq 2) 2σ 4 πr 2 ρ W pr ρ Combining, 2 = or W = (2πr 3 p )  4πr ρ 2σ σ σ Thus, to minimise weight, we need to maximise  . ρ σ   is called the performance index, P. ρ Using the strength-density materials selection chart and set P = 30; two lines are drawn. Line 1: A slope line (σ/ρ = 1) at P = 30 Line 2: A horizontal line with σ = 400 MPa EMII\Materials Selection School of Mechanical & Aeronautical Engineering Ver 6.0 MODULE: Engineering Materials II 1-15 Syll No: ME 2301 TOPIC No: 1 Materials whose positions are above the 2-lines are deemed acceptable. These include engineering ceramics; engineering composites; and engineering alloys such as aluminium alloys, titanium alloys and steels. Ceramics can be eliminated due to their brittleness. Let us consider the following candidate materials: Materials ρ σ cost, c tonne/m3 MPa S$/kg Aluminium alloys 2.8 595 4.8 Steel, medium carbon 7.9 1990 1.1 Steel, low alloy 7.9 2450 1.6 Stainless steel, austenitic 8.1 2240 14.4 Titanium alloys 4.8 1630 96 GFRP laminate 2.0 750 7.2 Materials P = σ/ρ Consider cost, σ/cρ MPa.m3/tonne MPa.kg.m3/S$.tonne Aluminium alloys 213 (6) 44 (4) Steel, medium carbon 252 (5) 229 (1) Steel, low alloy 310 (3) 194 (2) Stainless steel, austenitic 277 (4) 19 (5) Titanium alloys 340 (2) 3.5 (6) GFRP laminate 375 (1) 52 (3) ( ) Ranking with (1) being the best. On the basis of P alone, GFRP is the best followed by titanium alloy and low alloy steel. In real engineering applications, economics is often an over-riding consideration. Taking cost into consideration, the ranking shows that medium carbon steel is the best choice; followed by low alloy steel. EMII\Materials Selection School of Mechanical & Aeronautical Engineering Ver 6.0 MODULE: Engineering Materials II 1-16 Syll No: ME 2301 TOPIC No: 1 Strength-density materials selection chart. (From Ashby M. F., 1992 “Materials Selection in Mechanical Design”, Butterworth/Heinemann). Those materials lying above P=30 MPa.m3/tonne and =400 MPa lines are acceptable. EMII\Materials Selection School of Mechanical & Aeronautical Engineering Ver 6.0 MODULE: Engineering Materials II 2-1 Syll No: ME 2301 TOPIC No: 2 FAILURE OF METALS Keywords: Chevron marks, transition temperature, notch, S-N curves, beach markings, stress concentration, creep curve Objectives: Students should be able: 2.1 To understand the causes of metal failure 2.2 To understand ductile and brittle failure 2.3 To understand fatigue and creep failure in metals EMII/Failure of Metals School of Mechanical & Manufacturing Engineering Ver 6.0 MODULE: Engineering Materials II 2-2 Syll No: ME 2301 TOPIC No: 2 METAL FAILURE Introduction : In an engineering plant, the failure of one unit or even a small component may result in complete shut down or at least a serious loss of production. It could lead to damage to equipment or even the loss of lives. The understanding of modes of failure would in one way or other help to determine the possible cause(s) of failure and prevent any recurrences. Many failures, whether due to cracking, corrosion, poor design or fabrication, environmental conditions or other causes, fortunately occur gradually. Usually 3 stages develop: - (a) initiation (b) growth and; (c) propagation. The first stage could be detected during manufacture though it may be developed during operation. The propagation stage is reached when the growth becomes unstable and comparatively rapid. With brittle materials however the process may occur with great rapidity especially if unfavourable environmental conditions are suddenly imposed and catastrophic failure may result. Sources of : In a failure investigation or analysis, therefore, a systematic and Failure comprehensive approach has to be adopted before corrective action can be recommended or applied. Poor designing This can arise from lack of understanding of the effect of stress raisers or stress distribution e.g. use of too sharp a fillet radius at a change in section of a shaft or a similar part that is subjected to bending or torsional loading. Keyways and drilled holes are also sources of stress concentration which will lead to early failure of metal parts. Material Selection Always take into consideration the type of stress and environment the material will be subjected to, i.e. whether the stress system is static or a cyclic type or a combination of cyclic loading with corrosion. When time factor is involved, then consideration should be given to the rate of wear or erosion of the part and perhaps the effect of temperature on its properties. EMII/Failure of Metals School of Mechanical & Manufacturing Engineering Ver 6.0 MODULE: Engineering Materials II 2-3 Syll No: ME 2301 TOPIC No: 2 Imperfections In Materials Surface defects and internal flaws can reduce the overall strength of a material. They can initiate crack or induce crack propagation leading to complete failure. Material defects introduced during processing such as gas porosity may be present in cast or welded products. These defects could serve as origins for metal failures. Deficiencies In Processing Cold forming and related operations such as deep drawing and bending, produce high residual stresses. These operations sometimes produce localised stress areas, cracks or microcracks and cause loss of ductility. Surface defects and metallurgical changes caused by processing have an influence on fatigue strength, resistance to brittle fracture and corrosion resistance. Machining and grinding often leave residual stresses. Severe grinding is a source of overheating and consequent softening; it has been known to produce cracking in hardened steels. Misalignment Misalignment of shafts, gears, bearings, seals and couplings is frequently a factor contributing to service failures. Improper Service Conditons The operation of equipment under abnormally severe operating conditions of speed, loading, temperature and chemical environment, or without regularly scheduled maintenance, inspection and monitoring is often a major contribution to the occurrence of service failures. Inspection, with the aid of non destructive testing equipment for instance, and monitoring procedures should be employed to check for defects and the rate of deterioration at regular scheduled intervals. Inadequate Maintenance This deficiency is frequently a contributing factor in service failures. Maintenance procedures should be thoroughly re-evaluated when failures reoccur despite regularly scheduled maintenance. EMII/Failure of Metals School of Mechanical & Manufacturing Engineering Ver 6.0 MODULE: Engineering Materials II 2-4 Syll No: ME 2301 TOPIC No: 2 Types of failure of metals Metal failures can be broadly grouped into four categories: (a) Ductile Fracture (b) Brittle Fracture (c) Fatigue (d) Creep Ductile Fracture Nature: Ductile fracture results from the application of an excessive tensile force to a metal that has the ability to deform permanently prior to fracture. The classic example of a ductile failure is a tensile specimen that has necked down before its fracture. It is sometimes referred to as an overload failure. Identification : Metal subjected to ductile failure shows considerable permanent deformation. The fractured surfaces of the specimen show a 'cup and cone' appearance with the 'cup' showing many irregular surfaces at about 45o to the tensile axis which accounts for the fibrous look. In addition, ductile fractures usually show transgranular cracking when examined microscopically. Causes : Ductile fractures occur at stresses above the yield strength. This implies that one of the following could have occurred:- (a) The material was not strong enough (incorrect heat treatment of steel or wrong material used). (b) The service conditions (loads) differed from those anticipated by the designer. (c) Abnormal loading conditions were applied. The latter two reflect design problems. However, they are different in that (b) concerns normal loading conditions and a drastic mistake by the designer, whereas (c) concerns loading from an outside source, for example, an explosion in a pressurized vessel. EMII/Failure of Metals School of Mechanical & Manufacturing Engineering Ver 6.0 MODULE: Engineering Materials II 2-5 Syll No: ME 2301 TOPIC No: 2 Brittle Fracture Nature Brittle fracture is characterised by the very small amount of work absorbed, very little deformation by a crystalline appearance of the surfaces of fracture, often with chevron (river like) patterns pointing to the origin of fracture. It can occur at a low stress with great suddenness. Figure 1: Brittle fracture surface with chevron marks Conditions The marking on the fracture surfaces of brittle failures usually indicate For Fracture that the failure originated from a severe stress concentration, often a crack- like defect. Three conditions are required to produce a brittle failure. (a) Ambient temperature below the transition temperature (b) Presence of a notch (severe stress concentration) (c) Existence of a tensile stress. Transition It is the range of temperature over which the mode of fracture of the Temperature material, when notched, rapidly changes from ductile to brittle. Only the BCC metals have a ductile/brittle transition temperature. The transition temperature of BCC metals e.g. mild steels, are usually determined by Charpy V notch test. In this test, a series of notched specimen is tested over a range of temperatures. By plotting the result of the energy absorbed against temperature as well as by observing the change in the appearance, the change of transition in fracture mode occurring over a range of temperatures (from ductile to brittle fracture) can be found (Figure 2). EMII/Failure of Metals School of Mechanical & Manufacturing Engineering Ver 6.0 MODULE: Engineering Materials II 2-6 Syll No: ME 2301 TOPIC No: 2 Figure 2: Brittle-ductile transition behaviour of steels When the service temperature is above the brittle-ductile transition temperature, the stress required to make the crack grow will be large. Below the transition temperature the metal is brittle and the stress necessary to cause crack growth, therefore, will be reduced, i.e. small. Different tests may arrive at a different transition temperature range for the same material. The same test may also show different transition temperatures for the same material thickness. In other words, although the effect of temperature on notched ductility is important, it is not the only factor which affects the transitional behaviour of the material. These factors include such things as the size and thickness of the material, the rate of loadings and the type of microstructure. The Notch It is important to realise that a brittle fracture starts from a pre-existing crack or sharp defect. To understand the process of brittle fracture, it is necessary to appreciate the effects of such a notch. In the presence of stress concentration, an applied stress may be amplified or concentrated at the tip of the notch. The magnitude of the localised stress depends on the geometry and orientation of the crack or notch. EMII/Failure of Metals School of Mechanical & Manufacturing Engineering Ver 6.0 MODULE: Engineering Materials II 2-7 Syll No: ME 2301 TOPIC No: 2 Tensile Stress To produce any fracture, a tensile stress is required in the material. The tensile stress may be produced by an externally applied loading or the residual stress due to welding or more usually a combination of both. If the magnitude of the stress is high enough and the other necessary conditions for fracture are present then a brittle failure will occur. It is more difficult to initiate than to propagate a brittle crack. Therefore notches and cracks should be avoided in materials or structures that may fail by brittle fracture. Weld defects especially cracks and also heat treatment cracks form severe stress concentration and are all potential fracture initiation points. Remedy The risk of brittle fracture may be eliminated by removing any of the three conditions necessary to cause it. That is to say, at a service temperature above the transition temperature, even fairly large crack-like defects can be tolerated and the problem becomes one of static strength only. Alternatively even if the material is brittle at the service temperature there is no risk of fracture provided there are no defects present. The transition temperature in steels can be lowered by: (a) decrease in C content to below 0.15% C (b) decrease in rate of loading (impact loading) (c) a decrease of depth notch or increase in radius notch (d) increase the nickel content to about 2 to 5% (e) reduce the grain size by adding grain refining elements; e.g. A1, Nb. Identification Brittle fractures can usually be distinguished from ductile fractures by an almost complete absence of plastic deformation. Fractures occur at stresses below the yield stress and are often flat and shiny. It is often possible to trace a fracture path to its origin by interpreting chevron markings on the fracture surfaces. With a brittle fracture, the fracture origin is often a notch or a small crack. When examined microscopically, brittle fractures may be transgranular or intergranular. EMII/Failure of Metals School of Mechanical & Manufacturing Engineering Ver 6.0 MODULE: Engineering Materials II 2-8 Syll No: ME 2301 TOPIC No: 2 Fatigue Failure Introduction The term "fatigue" is used to describe the failure of a material under a repeatedly applied stress. The stress required to cause failure, if it is applied a large number of times, is much less than that necessary to break the material with a single pull. Under normal conditions steel structures may be designed to a permissible static stress of approx. 2/3 the yield stress for the material. This gives an adequate margin against the onset of yield and a bigger margin against ultimate failure. In other words, there is a factor of safety. This is perfectly true provided it is understood that one is considering statically loaded structure i.e. the stress remains constant with time. Unfortunately, very few structures in service are subjected to purely static loading. Variations in live load applied during normal operations, changes in temperature or pressure vibrations caused by wind loads, or dynamic loads in machinery will all give rise to fluctuations of the working stresses in a structure. These changes in stress may range from a simple cyclic fluctuation to a completely random variation. Although these may be within the limit of the permissible static stress, the structure can no longer be regarded as safe without first considering the possibility of fatigue failure. The only realistic approach to the design of a structure subjected to fatigue loading is to relate the working stresses to fatigue strength data for the particular service condition. Fatigue test The fatigue properties of different materials can be compared using the "Wohler" machine. In this test, the specimen in the form of a cantilever, forms the extension of a shaft, driven by an electric motor. Dead loading is applied to the specimen through a ball bearing. As the specimen rotates there is a sinusoidal variation of stress which is greatest at the surface and zero at the centre. The dimensions of the specimen vary according to the make of the machine. A series of specimens are subjected to reversals of stresses, each at a different value, until failure occurs or until 10 million cycles have been endured. From the results a stress-cycle (S/N or S/log N) curve is plotted from which we can establish the relationship between the stress and the number of cycles to failure for any particular type of loading. EMII/Failure of Metals School of Mechanical & Manufacturing Engineering Ver 6.0 MODULE: Engineering Materials II 2-9 Syll No: ME 2301 TOPIC No: 2 Figure 3: S (Stress) – N (Cycles to failure ) curves. A: Ferrous metals; SL: Endurance Limit B: Non-ferrous metals For the ferritic steels below 2000C and A1-Mg alloys at room temperature, the S/N curve tends to become horizontal after about 10 million cycles at a stress which is called the fatigue limit (the fatigue limit is the highest stress which regardless of the number of times it is repeated, will not cause fracture). Endurance Limit Endurance limit is the limiting stress below which a metal will withstand an indefinitely large number of cycles of stress without failure by fatigue fracture. Above this limit failure by fatigue occur. Steels have true endurance limits but most nonferrous metals do not show true endurance limits (see Figure 3). In the latter cases, the term endurance strength is used. It is defined as the repeated stress at which failure will not occur before a stated number of stress cycles. EMII/Failure of Metals School of Mechanical & Manufacturing Engineering Ver 6.0 MODULE: Engineering Materials II 2-10 Syll No: ME 2301 TOPIC No: 2 Identification Fatigue fractures exhibit no visible sign of deformation of the material and for this reason are very difficult to see particularly in the initial stages. A fatigue crack starts as a very narrow opening which grows in size as the repeated stressing is maintained until it extends through a considerable part of the material. (Figure 4). The fracture surface has a characteristic appearance. It consists of two areas, one portion is smooth and shows ripple markings of striations spreading out from some discrete points indicating where the fractures were initiated. These striations are often a feature of service failures associated with non-uniform conditions and intervals of rest. While the remainder of the surface has either a crystalline or a fibrous appearance which indicates the final tearing, which occurs when the area can no longer sustain the load. The final fracture can be brittle or ductile or a combination of both. Figure 4: Typical fatigue fracture surface on steel (Fractured Shaft) EMII/Failure of Metals School of Mechanical & Manufacturing Engineering Ver 6.0 MODULE: Engineering Materials II 2-11 Syll No: ME 2301 TOPIC No: 2 Factors Several factors affect the behaviour of metals under conditions of fatigue loading. They are related to stress concentration effects, design, processing methods, surface conditions and tensile strength. Stress Concentration Figure 5 shows the S-N curves for three series of mild steel plates specimens tested under conditions giving an axial stress variation from zero to a maximum tension. Figure 5: Effect of stress concentration on fatigue curves Curve "a" is from plane plate specimens, curve "b" is from identical specimens with a small hole drilled through the centre and curve "c" from specimens with small V notches cut in both edges. There is a considerable variation in fatigue strength of the three types of specimens. The reason for this is that both the hole and V notches produce stress concentration i.e. the level of stress in the material is raised above the average. EMII/Failure of Metals School of Mechanical & Manufacturing Engineering Ver 6.0 MODULE: Engineering Materials II 2-12 Syll No: ME 2301 TOPIC No: 2 Local stress concentration produced by notches, keyways, oil holes, screw thread, machining marks and other irregularities e.g. undercut, lack of root penetration, etc. in welded joints, on the surface are starting points for the promotion of fatigue failures. Other stress raisers are scratches, tool marks, rough surfaces, quenching and grinding cracks, sharp changes in section, poor fillets as well as inclusions in the metal, corrosion pits etc. It is important to avoid creating such areas of stress concentration if possible. Design Improvement of design sometimes involves a simple reduction in the severity of section changes; this can be achieved by using fillets and more rounded contours. Rounded bottom keyways have proved very successful in reducing fatigue failure. Liberal fillets should replace sharp right-angle cuts in machine parts. Processing Fatigue usually initiates at a surface because stresses are normally higher there, particularly since most parts experience bending loads resulting in substantially higher stresses in the outermost fibres. Stress raisers are likely to be present as a result of surface irregularities which can result from processing. Improvement and modification of processing methods offer possibilities for increasing resistance to fatigue failure. It may be necessary to change to an entirely different method of manufacture e.g. from a weldment to a forged and machined part. In other cases, improvements in a given method may be sufficient, such as modification of a mould design to eliminate shrinkage voids. When a machinery operation is involved, specifications calling for a better surface finish may yield the required improvement in performance. Tensile strength Although not directly proportional, the fatigue strength and the tensile strength of a metal are directly related. The increase in tensile strength can be effected by alloying the metal for alloys which can be solid solution strengthened. In unstable alloys, however, e.g. age-hardened material, alloying has been found to produce disappointingly low increase in fatigue strength. EMII/Failure of Metals School of Mechanical & Manufacturing Engineering Ver 6.0 MODULE: Engineering Materials II 2-13 Syll No: ME 2301 TOPIC No: 2 Surface Conditions Fatigue resistance can be either weakened by adverse conditions or strengthened by hardening the surface by nitriding, carburising, etc. Uniformly hardening the surface of the material is an useful method of improving fatigue behaviour. The surface can be work hardened or case hardened to introduce compressive stress on it by means of: (a) Shot-peening - a stream of steel shots is made to impinge on the surface (b) Cold rolling (c) Case hardening - for steel components by means of nitriding or carburising. Here the N or C is allowed to diffuse into the surface of the steel at elevated temperatures to produce a hard layer containing nitride or carbide phases in the material. EMII/Failure of Metals School of Mechanical & Manufacturing Engineering Ver 6.0 MODULE: Engineering Materials II 2-14 Syll No: ME 2301 TOPIC No: 2 Creep of Metals Introduction The problem of producing alloys capable of supporting stress at high temperature is one of the most exciting and important tasks facing the metallurgist at the present time. Alloys for aircraft engines are already being used at a bright red heat (800 0C) and have a satisfactory service life of several thousand hours. In the field of rocketry, alloys capable of withstanding even higher stresses and temperatures are necessary. The skin of the rocket would become white hot during its re-entry into the earth's atmosphere though for a short time. Similar creep problems arise in the nuclear reactor field, wherein the fuel elements and other components get distorted due to creep at high temperatures. On a more mundane level, there is a constant demand for alloys, usually steels, required for the manufacture of boilers and turbine equipment for the power industry. Many other industries use high temperature alloys. Two in particular are the chemical and petroleum industries, since many of the products they make must be processed at high temperatures. These are but a few examples which could be quoted to indicate the widespread use of metals and alloys at high temperatures. An understanding of the natures of the creep process in metals and what is meant by creep resistance is thus of vital importance. Creep is the slow plastic deformation of metals under a constant stress. Creep can take place and lead to fracture at static stresses much smaller than those which will break the specimen when loaded quickly. Creep becomes important when: (a) Metals and alloys are stressed at a temperature in excess of about 1/3 Tm where Tm is the melting point in Kelvin. Lead and tin creep at room temperature whilst molybdenum, tungsten and nickel base alloys creep at about 10000C. For example, the creep for lead (Tm: 327.5°C) can take place from a temperature as low as {[1/3 x (273 + 327.5)]} – 273 = -73.8°C. (b) Steam and chemical plants operating at 450° to 550°C. (c) Gas turbines which are working at high temperatures (800° - 900°C). (d) Furnace Parts. EMII/Failure of Metals School of Mechanical & Manufacturing Engineering Ver 6.0 MODULE: Engineering Materials II 2-15 Syll No: ME 2301 TOPIC No: 2 High Performance of metals in service at an elevated temperature is governed Temperatures by more than the strength and ductility as these properties are measured at room temperature or lower. Time becomes a factor, because at high temperatures metal will creep, that is the section under stress will continue to deform although the load is maintained constant. When metal at room temperature is stressed at some level below the yield point, the load which is applied at one end of the specimen is transmitted to the other end in a fraction of a second and the elastic deformation or strain then is complete. At elevated temperatures, however, stresses below the yield point cause not only the normal elastic strain, but, progressive plastic strain (creep deformation) which continues with time. The rate at which metals creep under load increases rapidly with increasing temperature. Consequently, the time over which a metal or article under load will deform too much to be suitable for its application can vary from many years at a slightly elevated temperature to a few minutes at a temperature near the melting point. As would be expected, metals and alloys differ considerably in their creep rate. At room temperature only the low melting point metals e.g. Pb, Sn and Zn are subjected to creep under load. At elevated temperatures all metals will creep. In fact, if the temperature and stress are sufficiently high, the metal will creep until rupture occurs. Creep Curve The behaviour of a metal under a high temperature creep condition can be evaluated by subjecting a specimen of a metal to a constant load at a specified temperature and drawing a curve of strain vs time. A typical creep rupture curve obtained is shown in the Figure 6. After a certain amount of instantaneous elastic and plastic deformation shown by OE, the curve of the elongation vs time shows three distinct stages namely: (a) The Primary Stage EP (creep), when relatively rapid extension takes place but at a decreasing rate. This is of interest to a designer since it forms part of the total extension reached in a given time and may affect clearances. This stage could be interpreted metallurgically as a result of strain or work hardening i.e. dislocation movement is becoming more difficult. (b) The Secondary Stage PS (Creep), or period during which creep occurs at a more or less constant rate, i.e. steady rate. This is the important part of the curve for most applications. During this period the properties of the material remain constant; i.e. work hardening is balanced out by thermal softening processes (Note: creep strain also involves sliding of grains boundaries i.e. fine grained material creeps more rapidly than coarse grains). EMII/Failure of Metals School of Mechanical & Manufacturing Engineering Ver 6.0 MODULE: Engineering Materials II 2-16 Syll No: ME 2301 TOPIC No: 2 (c) The tertiary Creep Stage SX, when the rate of extension accelerates and finally leads to rupture. The use of alloys in this stage should be avoided but the change from secondary to tertiary stage is not always easy to determine from creep curves for some materials. This stage could be explained as the formation of necking, crack formation and structural changes. Figure 6: A Typical Creep Curve Unlike elastic deformation, creep deformation is permanent and the elongation remains if the load is removed. At low temperatures and low stresses, only primary creep occurs and extension may eventually cease. At high temperatures and stresses (Figure 7), tertiary creep predominates i.e. acceleration is generally due to the growth of cracks in the material leading to stress concentration and ultimate catastrophic failure. Figure 7: Variations of creep rate with stress and temperature EMII/Failure of Metals School of Mechanical & Manufacturing Engineering Ver 6.0 MODULE: Engineering Materials II 2-17 Syll No: ME 2301 TOPIC No: 2 Creep To reduce creep the following strengthening methods are used: Resistant Alloys (a) High melting point metal (b) Solid solution strengthening (c) Precipitation or dispersion hardening (so long as the second phase is not easily dissolved in matrix). Finely dispersed precipitates are necessary for high creep resistance. Many nickel-based superalloys, e.g. Nimonics and Inconel, contain small amounts of Al and/or Ti which will form fine precipitates of intermetallic compounds. Fine inert oxide particles may be introduced to the metal matrix to enhance creep resistance in components manufactured by the powder metallurgy technique. This technique is used for certain SAP (sintered aluminium powder) alloys. EMII/Failure of Metals School of Mechanical & Manufacturing Engineering Ver 6.0 MODULE: Engineering Materials II 3-1 Syll No: ME 2301 TOPIC No: 3 CERAMICS AND COMPOSITES Keywords Ionic bonding, covalent bonding, electrostatic, dislocation, sintering, isostatic, macroscale, particulate, laminate, fibre, matrix, rule of mixture, pultrusion, fracture toughness Objectives Students should be able: 7.1 To understand the applications of ceramics 7.2 To understand the fundamentals of composites 7.3 To understand the applications of composites EMII/Ceramics & Composites School of Mechanical & Aeronautical Engineering Ver 6.0 MODULE: Engineering Materials II 3-2 Syll No: ME 2301 TOPIC No: 3 CERAMICS Introduction Ceramics are complex compounds joined by ionic or covalent bonds. They are hard, brittle, high melting point materials with low electrical and thermal conductivity, good chemical and thermal stability, and high compressive strengths. Ceramics are divided into two groups: traditional ceramics and engineering ceramics. Traditional ceramics include bricks and tiles used in the construction industry and electrical porcelain in the electrical industry. Engineering ceramics consist of pure or nearly pure compounds such as aluminium oxide (Al2O3), silicon carbide (SiC), and silicon nitride (Si3N4). Exercise 1 (a) Briefly describe ionic bonding. (b) Briefly describe covalent bonding. Solution 1 (a) In ionic bonds, the atoms are held together as charged ions (particles). The electrostatic attraction between the unlike (+ve and -ve) charges gives most of the bonding. So the ions pack densely to give as many plus and minus charges close together. (b) In covalent bonds, an atom bonds by sharing electrons with its neighbours to give a fixed number of directional bonds. They are like a child's lego set units that snap together so that the number and position of neighbours (units) are rigidly fixed. The resulting structures are different from ionic. Packing is thus by forming three-dimensional networks of chains or sheets. Exercise 2 Explain why ceramics are hard and brittle. Solution 2 The electrostatic forces in ionic bonds make dislocation movement easy on some planes, but difficult on others. In covalent bonds, the localised bonds present an enormous resistance to dislocation movement. These bonds make ceramic hard. Brittleness in ceramics is associated with the number of extremely small defects such as cracks, voids and inclusions present in the material. The amount of energy required to fracture the material in the presence of these defects is thus reduced. The manufacturing processes must be carefully controlled since these defects can reduce the strength of a ceramic to only a few percent of its ideal strength; thereby rendering the ceramic part useless. EMII/Ceramics & Composites School of Mechanical & Aeronautical Engineering Ver 6.0 MODULE: Engineering Materials II 3-3 Syll No: ME 2301 TOPIC No: 3 Processing Most ceramic products are produced by compacting powders or particles into shapes which are then heated to a high enough temperature to bond the particles together. Exercise 3 Describe the 3 basic steps in processing ceramic components. Solution 3 The 3 basic steps are: Powder production. For traditional ceramics, the powders are usually prepared by grinding. For advanced engineering ceramics, a more complex method such as vapour-phase deposition is used. Compacting or pressing. The ceramic particles are pressed in the dry or wet condition into a die to form "green" shaped products. Firing or sintering. The rigidity and strength of the ceramic product will increase. Sintering causes additional shrinkage of the ceramic body as the pore size between the particles is reduced. Figure 1: Pressing of powder (a) & (b) Filling (c) Pressing (d) Ejection EMII/Ceramics & Composites School of Mechanical & Aeronautical Engineering Ver 6.0 MODULE: Engineering Materials II 3-4 Syll No: ME 2301 TOPIC No: 3 Figure 2: Slip casting (a) Drain casting in porous plaster of Paris mould (b) Solid casting Applications Application Requirements Examples Wear parts High hardness, Silicon carbide, Seals low friction Alumina Bearing Valves Nozzles Cutting tools Hot hardness, Silicon nitride Lathe tools high strength Milling cutters Heat engines Thermal insulation, Silicon carbide, Diesel components high temperature strength, Silicon nitride, Gas turbines fuel economy Zirconia Medical implants Surface bonds to tissue, Bioglass, alumina, Teeth corrosion resistance, Zirconia Joints biocompatibility Construction Improved durability, Advanced cermets Highways lower overall cost and concrete Bridges & Buildings EMII/Ceramics & Composites School of Mechanical & Aeronautical Engineering Ver 6.0 MODULE: Engineering Materials II 3-5 Syll No: ME 2301 TOPIC No: 3 High Temperature Applications of Ceramics The high strength and creep resistance of ceramics permits the design of engines and turbines that can operate at higher temperature and more efficiently. The oxidation resistance is not only more superior but its greater hardness ate elevated temperatures gives seals and bearings better resistance to wear. All these properties depend on the strength of the covalent bond compared with metallic bond. However, the same bonding and structures make ceramics having little plasticity leading to brittle fracture. Extensive efforts are being made to develop ceramics with sufficient ductility and to realise a greater potential of ceramics. In using ceramics, the below three factors must be evaluated. 1) Resistance to thermal shock 2) Creep 3) Effect of different atmospheres at high temperatures EMII/Ceramics & Composites School of Mechanical & Aeronautical Engineering Ver 6.0 MODULE: Engineering Materials II 3-6 Syll No: ME 2301 TOPIC No: 3 COMPOSITES Introduction Composites are produced through a mixture of two or more materials on a macroscale. Two or more distinctly different materials are combined together to form a composite which possesses properties that are superior to the properties of the individual components. Composites are thus selected to give unusual combinations of properties. The three categories of composites are: particulate, laminate and fibre. Exercise 4 (a) State 5 factors that controlled the behaviour of composites. (b) What is meant by “rule of mixture” in determining the properties of a composite? Solution 4 (a) The overall behaviour of composites depends on: Properties of the components, Size and distribution of the components, Volume fraction of the components, Shape of the components, Nature and strength of the bond between the components. (b) Since composites consist of identifiable components, each possessing its own individual properties, the strength and moduli can thus be expressed by the rule of mixture, i.e. Composite strength = (strength of component 1 x fraction component 1) + (strength of component 2 x fraction component 2) +... Or in symbol form, σc = σ1V1 + σ2V2 and Ec = E1V1 + E2V2 (for two components) Where σ = strength and V = volume fraction EMII/Ceramics & Composites School of Mechanical & Aeronautical Engineering Ver 6.0 MODULE: Engineering Materials II 3-7 Syll No: ME 2301 TOPIC No: 3 Particulate Composites Particulate composites aim to improve the mechanical properties of a base material by adding small particles of another material that have improved mechanical properties. The particles can be stronger and harder than the matrix, as is the case with steels and heat treatable aluminium alloys. They can be soft and tougher than the matrix, as is the case of rubberised polymers. Exercise 5 (a) Explain how you would increase the strength and hardness of a particulate composite. (b) How does the addition of rubber particles toughened particulate composites? Solution 5 (a) In increasing strength and hardness, the composite consists of fine, hard particles imbedded in a softer, tough matrix. The tensile strength and modulus are increased to far above that of the parent matrix due to restriction in the movement of dislocations and cracks, but often the toughness drops. Ideally the particles are very fine, hard and evenly dispersed throughout the matrix. (b) Rubber toughened polymers (e.g. ABS) derive their toughness from the small rubber particles they contain. A crack intersects and stretches the rubber particles. These particles act as little springs, clamping the crack shut, and thereby increase the load needed to make it propagate. Figure 3: Toughening by rubber particles Laminate composites Within this group there are the simple forms of laminated composites such as plywood and laminated windscreens, where the reasons behind the laminating differ. For wood its limitation is directional properties, so to remove this directionality, the plies are laminated at 90o to each other. So now the properties are the same in two directions instead of one (in the original wood). EMII/Ceramics & Composites School of Mechanical & Aeronautical Engineering Ver 6.0 MODULE: Engineering Materials II 3-8 Syll No: ME 2301 TOPIC No: 3 Exercise 6 Describe how the limitation of “shattering” of glass windscreens is prevented by laminating with a polymer. Solution 6 For glass its limitation is not so much its strength, but its ability to keep its shape on fracture. Normally when glass fractures, it will shatter throwing the small pieces of glass everywhere. If two glass plates are "stuck" together by a polymer adhesive (such as polyvinyl butyral), the strength will increase over one plate. It also will prevent the glass from shattering, thus improving the safety factor. In the advanced materials area, a fibre-reinforced polymer such as boron reinforced polyester is laminated to an aluminium or titanium honeycomb to produce a high stiffness, high strength laminate. Figure 4: An example of laminate composite Fibre composites The fibre composites improve strength, stiffness, fatigue properties and strength-to-weight ratio by incorporating strong, stiff, brittle fibres into a softer, more ductile matrix. The matrix material transmits the load to the fibres while the fibres carry most of the applied load. Figure 5: Load transmits from matrix to fibre EMII/Ceramics & Composites School of Mechanical & Aeronautical Engineering Ver 6.0 MODULE: Engineering Materials II 3-9 Syll No: ME 2301 TOPIC No: 3 Mechanics of fibre reinforcement Exercise 7 (a) Explain the functions of the matrix in a fibre composite. (b) List 5 factors that influenced the strength of fibre composites. Solution 7 (a) The matrix is used to bind together the fibres and to protect their surfaces from damage or chemical attack. The matrix separates the individual fibres and prevents brittle cracks from spreading across the composite. The matrix can also be considered as a medium to transfer and distribute the load to the fibres. The bond between the fibre and the matrix must be strong enough to prevent interfacial separation or fibre pull-out under axial loads. (b) The strength of fibre composites is determined by: Strength of the fibres, Orientation of the fibres w