Introduction to Materials Technology PDF

Sergio Bonilla López Material’s Technology Sergio Bonilla López CHAPTER 1: INTRODUCTION TO MATERIALS TECHNOLOGY...

Sergio Bonilla López Material’s Technology Sergio Bonilla López CHAPTER 1: INTRODUCTION TO MATERIALS TECHNOLOGY Materials Technology International bachelor’s degree in Industrial Engineering (UPNA) 1 Sergio Bonilla López Material’s Technology 1.1. The importance of materials Introduction: Every engineering work ends up with the manufacturing of something: - Structures - Machinery - Components Usually, design is made to fulfil certain requirements, specifications and making a product capable of achieving certain functionalities. Requirements: - Costs - Qualities - Whole life of the product: - Ambient effect - Consumption - Generating process - Recycle/reuse. Technology evolution, that is humankind evolution, is entirely based on the materials evolution. Advances are achieved in two different ways: 1. Materials science. Relationship microstructure-properties. - Try something in a lab and see what happens. 2. Materials technology. Relationship properties-microstructure - Trivial objective. What I do to obtain something. Property: Definition: Response of a material against an external input. Utility: by properties we can compare materials. Note: Property definition must be unique, without any specific material, shape, and size relationship → Otherwise, comparison is not possible. - If the response against the external input is because of something related to the concrete analysed piece (size,shape…) → we are not talking about a property. Examples: - Optical properties: Light reflection depending on the polishing of a material (rugosidad superficial/surface roughness). Since it will produce a roughness surface where the light would or not reflect in a straight way. - Espejos → polished aluminium (los de great telescopes). - Stress – strain example. 2 Sergio Bonilla López Material’s Technology Now, let’s distinguish if Stress-strain is a property or not: Technical properties usually are grouped as follows: - Mechanical - Magnetic - Electrical - Optical - Thermal - Chemical Selecting a material: Whenever a new design is made, there are many potential choices of materials for its implementation. Things to consider when selecting a material (for a given design): 1. Working conditions for the material with respect to the required ones. 2. Safety: Internal conditions (very important for us, engineers) and external aggressive conditions causing degradation (temperature, corrosion…). 3. Costs: Economic considerations + Transportation costs. 4. Amount of material needed. On many occasions, opposite considerations may arise. - Worst case: Strength vs. Ductility → In some cases, a material can be strong but not very ductile, which means it can withstand high loads but deforms or breaks easily when subjected to significant stresses. On the other hand, a material can be ductile but not very strong, meaning it can deform easily but cannot withstand large loads before breaking. So, a medium point selection must be done. However, with materials technology, it is possible modifying some behaviours. 3  Sergio Bonilla López Material’s Technology How can be - Manufacturing processes. modified these - Treatments of different classes (heat, surface ….). behaviours? - Local modifications of materials: (E.g., In a ship we improve the material to get anti-corrosion state, making the boat more expensive, but it may live longer). At the end, in most of the occasions, materials selection in engineering is mainly an economical decision. Not only material price is important but also the amount of material used. And if it could be recycled and how → iron (steel) → which can be melted and combined over and over again. The selection of the material has strong influence even in the safety coefficient: - Brittle (frágil) → High safety coefficient. - Ductile → Low safety coefficient. Materials usually employed in engineering developments are the following: - Metals (Ferrous and Non-ferrous): 75-80% - Plastics (Raw ones as elastomers or reinforced): 15-20% - Ceramics (for high Temperatures): 2-5% Material selection is made based upon two different kinds of properties: 1. Qualitative properties. These are non-numerically quantifiable ones. Nevertheless, they impose important differences in behaviour. - Homogeneity. - Plasticity. - Isotropy. - Ductility-fragility (brittleness). - Anisotropy. - Malleability - Elasticity. - Weldability - Linearity. 2. Quantitative properties. These are numerically quantifiable ones that allow for direct comparison of materials for their selection. - Tensile strength. Strength. - Toughness. - Proportional limit. - Hardness. - Elastic limit. - Notch (fracture) toughness. - Yield stress. Upper and lower. - Creep. - Elasticity modulus or Young - Viscoelasticity. Modulus. - Specific heat: thermal - Shear strength. conductivity, coefficient of - Torsion strength. thermal expansion (CTE). - Resilience. 4 Sergio Bonilla López Material’s Technology Qualitative properties: Homogeneity: A material is said to be homogeneous when the properties are the same in every point inside it. - Manufacturing processes may affect homogeneity of a material. Strictly talking → there are not pure homogeneous materials. - As it depends on the scale we study. At an engineering level→most of the materials can be considered homogeneous. Examples: Isotropy vs. Anisotropy: Isotropy: when it has the same properties along every direction of it. - Vast majority of engineering materials are isotropic. - Manufacturing processes can affect the isotropy of the material. Anisotropy: opposite behaviour to isotropy. Different behaviour in each direction. - Material characterization is extremely complicated. - Use of pure anisotropic materials is very strange in engineering. Difficult to see in nature (almost impossible). - But appears when materials are mixed → Composite Materials (Chapter 4). 5 Sergio Bonilla López Material’s Technology Elasticity: Is the property that allows the material to recover its initial geometry after elimination of the external loads. - We always work with materials that follow Hooke’s Law: - Linearity: ▪ Relationship between stress and strain is a straight line. ▪ It is very common simplify the real behaviour of a material to a pure linear one to simplify the calculations. Plasticity: It is the property by which a material increases its strain without applying additional external loads (without increasing its stress). - All deformations that suffer under plastic state are not recovered after retiring the load → this happens after overpassing a specific part in deformation process (limit point). - This property can be a critical behaviour of the material → change its shape → make it useless. It is uncommon the use of pure plastic materials in engineering (one use materials). - Pure elastic material → Gels → Used for protection (hits absorption). 6 Sergio Bonilla López Material’s Technology It is quite common working with a combination elastic-plastic. Example: Combination of elastic-plastic work. Cars: When a car is involved in an accident, we want it to absorb the most amount of energy possible, even if it breaks, for the occupants to suffer the least damage. Therefore: Head of the car: low elastic limit and huge plastic ability (to absorb the energy). Structure: high strength metals (more elastic material / alta resistencia). Ductility-fragility (brittleness): Represents the total strain that supports a material until it reaches its breaking stress. A material can be ductile or fragile, depending on: - How much strain it is needed to reach the breaking point, therefore will be determined by the ultimate strain ( ) - If the strain is large, material is ductile if it is low, material is brittle. Also, for the same material, there can be some variations of this property. Ductility: Refers to the ability of a material to deform under stress before fracturing. Ductile materials can undergo significant plastic deformation without breaking. They have great capability to elongate at plastic behaviour. - Great property → since it will warn before breaking (by shape or noise) → useful for safety reasons. Fragility: The opposite of ductility. It describes how easily a material breaks or shatters when subjected to stresses or impacts. Fragility is useful in some cases: - Gas storage: Is better to have small cracks than elongation till explosions when pressure increases. - We use small valves to liberate the gas if pressure increases. - Some parts of an aircraft are designed to be broken (hangings pants), under certain impacts to maintain the wheel structure. 7 Sergio Bonilla López Material’s Technology Some conditions under which ductile materials can have brittle behaviours: 1. Impacts: Ductile materials are generally known for their ability to undergo significant plastic deformation before failure. However, under high-velocity impact loading, the deformation process may be too rapid for the material to exhibit its typical ductile behaviour. Instead, the material may fracture in a more brittle manner, showing less plastic deformation. If you apply load suddenly and you don’t give the material time to relocate molecules, it breaks as a fragile material, although it’s ductile. (F vs. t) Special Ductile material: Manganese steel alloy. When subjected to impact or pressure, the steel undergoes plastic deformation and strain hardening. This means that, with repeated impacts or pressure, the material becomes progressively harder and more resistant to deformation. 2. Low temperatures: At low temperatures we can lose ductility, so ductile materials may become more susceptible to brittle fracture. This phenomenon is often referred to as ``ductile-to-brittle transition´´. Examples: - Steel, which is ductile at room temperature, can become brittle at extremely low temperatures, leading to unexpected fractures. - Therefore, some steels by adding some modifications can resist better to low temperatures (till -20 ºC). - Some materials improve behaviour with lower temperatures: Al, Carbon fibers. 3. Triaxiality: Triaxiality refers to the state of stress in a material. Ductile materials usually deform more easily under tensile stresses. However, under certain triaxial stress conditions, such as compression combined with shear, ductile materials may exhibit a more brittle response. This is particularly relevant in situations where the stress state deviates from simple tensile loading ( can be seen thanks to size of Mohr circles). 8 Sergio Bonilla López Material’s Technology Example: In some structural components subjected to complex stress conditions, like those found in certain engineering applications, ductile materials might display a brittle response due to high triaxiality. Malleability: This property represents the deformation capability of a material under compressive loading. - It’s a quite important property specially for manufacturing parts, especially if very thin plates and shims are required. Weldability: Determines how a material can be manufactured depending on its structure. - It allows how to cut and decompose. Weldability refers to the ease with which a material can be welded or joined together by the process of welding. It is an important consideration in various industries, especially in manufacturing and construction. 9 Sergio Bonilla López Material’s Technology Quantitative properties: Tensile strength: Resistencia a la tracción, this is the most important mechanical property of a material. Maximum stress can a material resist without breaking. - External loads produce stresses on the materials. The problem comes when the external loads produce a stress higher than the ultimate stress →it breaks. - By correcting the cross section diameter, we can increase tensile strength. We study tensile strength instead of compressive strength because: Most times tensile strength < compressive strength (except in composites). This way the security coefficient is higher →better to ensure. Stress -strain graph analysis: It is usual to work with this figure (for ductile steels) and not the compression strength. Where: Proportional limit → till point B, till them the relationship stress-strain is linear. Elastic limit → till. It is usual working with the conventional EL 0.2%. Point at which it stops behaving elastically. In the case of steel: proportional limit = elastic limit. (rest of materials not usually) Yield stress → till point C, point at which it starts behaving plastically. R → the cross section remains being the same. Necking → ESTRICCIÓN, when the material starts to lose section. → Ultimate stress/strength. 10  Sergio Bonilla López Material’s Technology Elasticity (Young) modulus: the key parameter in the stiffness (Rígidez) The modulus of elasticity is a mechanical property of a material that describes its ability to deform elastically under the application of a load and recover its original shape when the load is removed. (Relationship between stress and strain under loading). Its attribute to each family of materials (it can barely change), most of steels have the same (E = 270 GPa). - Very useful steel E for engineering applications. - To improve stiffness → look for E of materials → best steel (but expensive). - Hooke’s Law. - Limiting designs due to deformation is very usual. - Modifying this property is very difficult once selected a material. ➔ If you want to change the stiffness, just change the geometry since E is cte. - Sometimes modulus tangents and secants are used. Hago una paralela a la línea recta de la función (psrte elastic) For nonlinear calculations → require intermediate level y donde me corta con la curva obtengo el elastic limit. and calculations at intermediate points (no problem with technology) Shear strength: Amount of pure shear a material can withstand. Shear → Esfuerzo cortante. - Measures strength of materials against shear. - It is quite important in certain joints → remaches stresses for a ductile material can - Also used in cutting processes. be approximated as the following - But not usually required for methods nowadays. when no having R data: Ultimate shear: Yield point shear: 11 Sergio Bonilla López Material’s Technology Belts are preloaded. Why? If you don’t expose the joint to preloaded joints and a force F is applied → The material joint will be under shear stresses (and we don’t want this) Torsion strength: If forces that tend to twist a body are applied to it, the body undergoes torsion. This is the type of stress that a key experiences when turning in a lock. - Equivalent to that diagram of but with. - Yield does not appear. - Doesn’t present a clear elastic limit (due to differences in distance of the stresses). - When the external surface of the material reaches the elastic limit: Torsion strength and stiffness → G - When all the surface reaches same elastic limit → it breaks. If we don’t have torsion strength information about a material, we should apply tensile formulas. 12 Sergio Bonilla López Material’s Technology Resilience: Measures the capacity of a material to store elastic energy. - With the Hooke Law, applicable in most of the engineering materials, it is quite important in certain joints. - Area below elastic line on diagrams of or. This relationship is obtained by developing Hooke’s Law and area of the triangle formed below the curve. - This quality is required in some car structure pieces → in order to not have deformation of the piece for small impacts → piece can stop elastic energy of the impact, so no deformation is shown in the car. Toughness: (Tenacidad) Measures the capability of a material to store energy. - Total energy → elastic + plastic - Useful to deformation of car pieces to absorb most of energy during big impacts (high – load). - High toughness value → ductile material (and opposite for brittleness). In a first course approximation, toughness can be computed as: (elastic limit * ultimate strain) As we can see in the graph, ductile fracture comes after than in a brittle material, since ductile materials are better and have more toughness. Depend on temperature: Temperature → → → Ductile Temperature → → → Brittle 13 Sergio Bonilla López Material’s Technology Previous figure is obtained in a test performed with Charpy V-notch pendulum hammer, used to obtain toughness data. 1. Compute energy of free pendulum motion. 2. Hit pendulum against material wall in order to compare and see energy difference → the energy store at the material. Test mode at temperatures prepound for zones. Hardness: Dureza Resistance of a material to be scratched (rayado) → to localized plastic deformations. - It has great effect on the surface wear and in contact components. Components whose surface will interact with other materials. Hardness conceptually measures with a machine: a prefetching material hits slowly and produce plastic deformation into the testing material. - Relationships between hardness and ultimate strength. There are different test procedures to measure hardness. - Hardness Brinnell, Vickers, Shore, Rockwell … 14 Sergio Bonilla López Material’s Technology Fracture toughness: Measure of a material’s resistance to brittle fracture when a crack is present. - In plastic materials it is especially important. - Starting data for the study of fracture. Important when working with long fatigue samples and huge stresses. 1º Mode (KIC) 2º Mode (KIIC) 3º Mode (KIIIC) Opening mode: produced Produced by pure shear Tearing mode: produced by normal stresses. stresses. by out of plane shear stresses. In the edges of the crack → increase the reaction force → catastrophe Important property depending on the conditions in which is going to work → steel for ships structures → consider a certain length of possible crack. - Aluminium structure not high fracture toughness → test aircraft fracture toughness → visual cracks → not allowed to fly. - Some plastics can have good toughness, but not good fracture toughness → adhesive type → with minor cacks → easy fracture appearance. Yield → fluencia Creep: Fluencia en caliente This is a phenomenon that produces additional strain in a material at high temperatures with static loading (termofluencia). - Temperatures are lower to those that can produce metallographic changes in the material. - As temperature increases → some propertiesare modified (lossing qualities) o For example: aluminum have extremely bad qualities at high T. - Creeptox → derivative of crrep with respect to time. - Design for creep → must have creep specifications for steels. o Evaluate creep behaivour ith Larson-Muller parameter: (18.3 + 𝑙𝑜𝑔𝑡) 𝐿𝑀𝑃 = 𝑇 100 15  Sergio Bonilla López Material’s Technology There are 3 creep zones: 1. Primary creep: the speed at which creep starts to appear (is fast and the curve loose slope). 2. Secondary creep: Constant creep rate. Linear relationship: line-extension. Balance between the hardening by precipitation and annealing. 3. Tertiary creep → extension starts to grow so fast and reach total failure of material. Behaviour depends both on temperature as on the stress level of the material: Creep does not increase stress but produces some necking, decreasing cross section. Supposing constant stress and variation in temperature: Supposing constant temperature and variation in stresses: In the analysis seen in the graph: T1 > T2 > T3 In the analysis seen in the graph: 1> 2> 3 We can see that the secondary creep period: 2 is double the 1, but in 3 takes longer. Same conclusion as before: Conclusion: →2nd period last less → Breaks before Temperature →2nd period last less → Breaks before Creep at plastic materials: Viscoelasticity and viscoplasticity: Like creep but produced at lower temperatures (room temperature). - Viscoelasticity: Constant increase of strain overtime but once you take out the external load, we recover the initial geometry. (No critical) - Viscoplasticity: Constant increase of strain overtime till it breaks (We do not distinguish 3 creep periods). There’s no initial extension. 16 Sergio Bonilla López Material’s Technology Note: Parameters to measure creep in metals → Larson Miller parameters. The Larson-Miller parameter describes the equivalence of time at temperature for a steel under the thermally activated creep process of stress rupture. It permits the calculation of the equivalent times necessary for stress rupture to occur at different temperatures. (How much creep a material will survive). In Chapter 2 will be presented: Fatigue and Dynamics. 17