Testing of Materials PDF

Testing of Materials You have a future envisioned for your product. You see an outstanding performance—high sales, low returns and happy customers. But before you can consider your product successful, you need to know if it will do what you’re asking it to do. Will it perform as expected? How can yo...

Testing of Materials You have a future envisioned for your product. You see an outstanding performance—high sales, low returns and happy customers. But before you can consider your product successful, you need to know if it will do what you’re asking it to do. Will it perform as expected? How can you be sure? 1 Understanding how materials behave in their natural state and under certain conditions will help them to understand why objects are made of specific materials. For this reason we have to know the materials properties. A materials property is an intensive often quantitative, property of a solid. Quantitative properties may be used as a metric by which the benefits of one material versus another can be assessed, thereby aiding in materials selection. Mechanical properties Electrical properties Magnetic properties Thermal properties Optical properties Acoustical properties Atomic properties Chemical properties Environmental properties Some properties are easily observable features, such as transparency which is an optical property, others are less obvious and need to 2 have tests carried out on them.  MECHANICAL PROPERTIES  The mechanical properties of a material are those properties that involve a reaction to an applied load. The mechanical properties of metals determine the range of usefulness of a material and establish the service life that can be expected.  Mechanical properties are also used to help classify and identify material. The most common properties considered are strength, ductility, hardness, impact resistance and fracture toughness.  The application of a force to an object is known as loading. Materials can be subjected to many different loading scenarios and a material’s performance is dependant on the loading conditions. 3 There are five fundamental loading conditions; Tension, Compression, Bending, Shear, Torsion. Tension is the type of loading in which the two sections of material on either side of a plane tend to be pulled apart or elongated. Compression is the reverse of tensile loading and involves pressing the material together. 4 Loading by bending involves applying a load in a manner that causes a material to curve and results in compressing the material on one side and stretching it on the other. Shear involves applying a load parallel to a plane which caused the material on one side of the plane to want to slide across the material on the other side of the plane. Torsion is the application of a force that causes twisting in a material. 5  Tensile Test Experiment One material property that is widely used and recognized is the strength of a material. But what does the word “strength” mean? “Strength” can have many meanings, so let us take a closer look at what is meant by the strength of a material. We will look at a very easy experiment that provides lots of information about the strength or the mechanical behavior of a material, called the tensile test. 6 Tensile Test Experiment (stress-strain test) Standard tensile specimen with circular cross section We apply weight to the material gripped at one end while the other end is fixed. We keep increasing the weight (often called the load or force) while at the same time measuring the change in length of the sample. 7 machine  Stress-Strain Diagram ultimate tensile strength 3 necking  UTS yield Fracture strength y 5 2 Elastic region Plastic slope =Young’s (elastic) modulus Region yield strength Plastic region ultimate tensile strength Elastic fracture σ  Eε Region 4 σ 1 E ε Strain (  ) (DL/Lo) 8  Tensile Strength, TS After yielding, the stress necessary to Stres-strain curve continue plastic deformation in metals increases to a maximum point (M) and then decreases to the eventual fracture point (F). All deformation up to the maximum stress is uniform throughout the tensile sample. However, at max stress, a small constriction or neck begins to form. Subsequent deformation will be confined to this neck area. Fracture strength corresponds to the stress at fracture. 9 Tensile Strength: Comparison Room T values Based on data in Table B4, Callister 6e. a = annealed hr = hot rolled ag = aged cd = cold drawn cw = cold worked qt = quenched & tempered AFRE, GFRE, & CFRE = aramid, glass, & carbon fiber-reinforced epoxy composites, with 60 vol% fibers. 10  Yield Strength Typical stress-strain behavior for a metal showing elastic and plastic deformations, the proportional limit P and the yield strength σy, as determined using the 0.002 strain offset method (where there is noticeable plastic deformation). P is the gradual elastic to plastic transition. 11 Yield Strength: Comparison Room T values a = annealed hr = hot rolled ag = aged cd = cold drawn cw = cold worked qt = quenched & tempered 12  Elastic Deformation 1. Initial 2. Small load 3. Unload bonds stretch return to initial  Elastic means reversible. F 13  Linear Elastic Properties Hooke's Law:  = E  Units: E: [GPa] or [psi] Modulus of Elasticity, E: (Young's modulus) 14  Young’s Moduli: Comparison Graphite Metals Composites Ceramics Polymers Alloys /fibers Semicond 1200 1000 Diamond 800 600 Si carbide 400 Tungsten Al oxide Carbon fibe rs only Molybdenum Si nitride E(GPa) 200 Steel, Ni Tantalum Si crystal CFRE(|| fibers)* Platinum Cu alloys Ara mid fibers only 100 Zinc, Ti 80 Silver, Gold Glass-soda AFRE(|| fibers)* 60 Aluminum Glass fibe rs only Magnesium, GFRE(|| fibers)* 40 Tin Concrete 109 Pa 20 GFRE* CFRE* Composite data based on Graphite GFRE( fibers)* reinforced epoxy with 60 vol% 10 8 of aligned carbon (CFRE), CFRE( fibers)* 6 AFRE( fibers)* aramid (AFRE), or glass (GFRE) Polyester 4 PET fibers. PS PC Epoxy only 2 PP 1 HDPE 0.8 0.6 Wood( grain) PTFE 0.4 LDPE 15 0.2  Plastic Deformation (Metals) 1. Initial 2. Small load 3. Unload F linear linear elastic elastic Plastic means permanent.  plastic 16  Ductility, %EL Ductility is a measure of the plastic l f  lo deformation that has been sustained at % EL  x100 fracture: lo A material that suffers very little plastic deformation is brittle. Ao  Af Another ductility measure: % AR  x100 Ao Ductility may be expressed as either percent elongation (% plastic strain at fracture) or percent reduction in area. %AR > %EL is possible if internal voids form in neck. 17  Toughness Toughness is the ability to absorb Lower toughness: ceramics energy up to fracture (energy per unit volume of material). Higher toughness: metals It is approximated by the area under the stress-strain curve. A “tough” material has strength and ductility. 18 Stress-Strain Results for Steel Sample 19  Terminology Load : The force applied to a material during testing. Strain gage or Extensometer: A device used for measuring change in length (strain). Engineering stress: The applied load, or force, divided by the original cross- sectional area of the material. Engineering strain: The amount that a material deforms per unit length in a tensile test. Young's Modulus: This is the slope of the linear portion of the stress-strain curve, it is usually specific to each material; a constant, known value. Yield Strength: This is the value of stress at the yield point, calculated by plotting young's modulus at a specified percent of offset (usually offset = 0.2%). Yield strength is a measure of resistance to plastic deformation. Ultimate Tensile Strength: This is the highest value of stress on the stress- strain curve. Percent Elongation: This is the change in gauge length divided by the original gauge length. 20  Terminology Stress and strain: These are size-independent measures of load and displacement, respectively. Elastic behavior: This reversible behavior often shows a linear relation between stress and strain. To minimize deformation, select a material with a large elastic modulus (E or G). Plastic behavior: This permanent deformation behavior occurs when the tensile (or compressive) uniaxial stress reaches sy. Toughness: The energy needed to break a unit volume of material. Ductility: The plastic strain at failure. 21  Hardness Hardness is a measure of a material’s resistance to localized plastic deformation (a small dent or scratch). Quantitative hardness techniques have been developed where a small indenter is forced into the surface of a material. The depth or size of the indentation is measured, and corresponds to a hardness number. The softer the material, the larger and deeper the indentation (and lower hardness number).  Macrohardness - Overall bulk hardness of materials measured using loads >2 N.  Microhardness Hardness of materials typically measured using loads less than 2 N using such test as Knoop (HK). Hardness Testers  Nano-hardness - Hardness of materials measured at 1– 10 nm length scale using extremely small (~100 µN) 22 forces.  Hardness Resistance to permanently indenting the surface. Large hardness means: -resistance to plastic deformation or cracking in compression. -better wear properties. 23 Adapted from Fig. 6.18, Callister 6e. (Fig. 6.18 is adapted from G.F. Kinney, Engineering Properties and Applications of Plastics, p. 202, John Wiley and Sons, 1957.) Hardness Testing Techniques 24 Conversion of Hardness Scales Also see: ASTM E140 - 07 Volume 03.01 Standard Hardness Conversion Tables for Metals Relationship Among Brinell Hardness, Vickers Hardness, Rockwell Hardness, Superficial Hardness, Knoop Hardness and Scleroscope Hardness 25 Correlation between Hardness and Tensile Strength Both hardness and tensile strength are indicators of a metal’s resistance to plastic deformation. For cast iron, steel and brass, the two are roughly proportional. Tensile strength (psi) = 500*BHR 26 27  Impact Fracture Testing Impact testing is used to ascertain the fracture characteristics of materials at a high strain rate and a triaxial stress state. In an impact test, a notched specimen is fractured by an impact blow, and the energy absorbed during the fracture is measured. There are two types of tests – Charpy impact test and Izod impact test. The ability of a material to withstand an impact blow is referred to as notch toughness. The energy absorbed is the difference in height between initial and final position of the Charpy test hammer. The material fractures at the notch and the structure of the cracked surface will help indicate whether it was a brittle or ductile 28 fracture.  Impact Test (Charpy) Data for some of the Alloys In effect, the Charpy test takes the tensile test to completion very rapidly. The impact energy from the Charpy test correlates with the area under the total stress-strain curve (toughness) 29  Impact Test: The Izod Test Generally used for polymers. Izod test is different from the Charpy test in terms of the configuration of the notched test specimen 30 Material Variability… … or “how do we know what we have?” Types of Variance  Material  Sampling Cumulative  Testing Errors vs. Blunders  Precision and Accuracy Precision – “variability of repeat measurements under carefully controlled conditions” Accuracy – “conformity of results to the true value” Bias – “tendency of an estimate to deviate in one direction”  Accuracy vs. Precision Bias Precision Accuracy Precision without without and Accuracy Precision Accuracy  Basic Statistics 1  n 2  2 n   xi  x   x i  i 1 s  x i 1 n 1    n     Arithmetic Mean Standard Deviation “average” “spread”  Basic Statistics Need both average and mean to properly quantify material variability For example: mean = 40,000 psi and st dev = 300 vs. mean = 1,200 psi and st. dev. = 300 psi  Coefficient of Variation A way to combine ‘mean’ and ‘standard deviation’ to give a more Standard useful description of the deviation material variability n%   100 s x mean