Manufacturing Processes Chapter 3 - Mechanical Properties Of Materials PDF

Manufacturing Processes Chapter 3: Mechanical properties of materials Dr. Queen Tannous Spring 2024  Lecture Content Stress-Strain Relationships Hardness Effect of Temperature on Properties Fluid Properties...

Manufacturing Processes Chapter 3: Mechanical properties of materials Dr. Queen Tannous Spring 2024  Lecture Content Stress-Strain Relationships Hardness Effect of Temperature on Properties Fluid Properties Viscoelastic Behavior of Polymers Chapter 3: Mechanical Properties of Materials Slide 2 of 56 Mechanical Properties in Manufacturing Mechanical properties determine a material’s behavior when subjected to mechanical stresses: Properties include elastic modulus, ductility, hardness, and various measures of strength Dilemma: mechanical properties that are desirable to the designer, such as high strength, usually make manufacturing more difficult Chapter 3: Mechanical Properties of Materials Slide 3 of 56 Stress-Strain Relationships Three types of static stresses to which materials can be subjected: Tensile: stretching the material Compressive: squeezing the material Shear: causing adjacent portions of the material to deform Stress-strain curve: basic relationship that describes mechanical properties for all three types Chapter 3: Mechanical Properties of Materials Slide 4 of 56  Tensile Test Most common test for studying stress-strain relationship, especially for metals In the test, a force pulls the material, elongating it and reducing its diameter Chapter 3: Mechanical Properties of Materials Slide 5 of 56  Tensile Test Specimen ASTM (American Society for Testing and Materials) specifies preparation of test specimen Chapter 3: Mechanical Properties of Materials Slide 6 of 56  Tensile Test Setup Tensile testing machine Chapter 3: Mechanical Properties of Materials Slide 7 of 56  Tensile Test Sequence (1) No load; (2) uniform elongation and area reduction; (3) maximum load; (4) necking; (5) fracture; (6) final length Chapter 3: Mechanical Properties of Materials Slide 8 of 56  Engineering Stress Defined as force divided by original area: 𝑭 𝝈= 𝑨𝟎 Where σ = engineering stress (Pa) F = Applied force (N) A0 = Original area of the test specimen (m2) Chapter 3: Mechanical Properties of Materials Slide 9 of 56  Engineering Strain Defined at any point in a tensile test as the deformation in elongation: ∆𝑳 𝑳 − 𝑳𝟎 𝜺= = 𝑳𝟎 𝑳𝟎 Where ε = engineering strain (mm/mm or dimensionless) L = length at any point during elongation (mm) Lo = original gage length (mm) Chapter 3: Mechanical Properties of Materials Slide 10 of 56  Typical Engineering Stress-Strain Plot Typical engineering stress-strain plot in a tensile test of a metal. Two regions: o Elastic region o Plastic region Chapter 3: Mechanical Properties of Materials Slide 11 of 56 Elastic Region in Stress-Strain Curve Relationship between stress and strain is linear o Hooke's Law: 𝝈𝒆 = 𝑬𝜺 o where E = modulus of elasticity Material returns to its original length when stress is removed E is a measure of the inherent stiffness of a material o It’s a property of the material and its value differs for different materials Chapter 3: Mechanical Properties of Materials Slide 12 of 56 Yield Point in Stress-Strain Curve As stress increases, a point in the linear relationship is finally reached when the material begins to yield. Yield point Y can be identified by the change in slope at the upper end of the linear (elastic) region Y is a strength property Other names for yield point: o Yield strength o Yield stress o Elastic limit Chapter 3: Mechanical Properties of Materials Slide 13 of 56 Yield Point in Stress-Strain Curve The start of yielding is usually difficult to see in a plot of test data (it does not usually occur as an abrupt change in slope),Y is typically defined as the stress at which a strain offset of 0.2% from the straight line has occurred. More specifically, it is the point where the stress–strain curve for the material intersects a line that is parallel to the straight portion of the curve but offset from it by a strain of 0.2%. Chapter 3: Mechanical Properties of Materials Slide 14 of 56 Plastic Region in Stress-Strain Curve Yield point marks the beginning of plastic deformation The stress-strain relationship is no longer guided by Hooke's Law. It is guided by the flow stress model. As load is increased beyond Y, elongation proceeds at a much faster rate than before, causing the slope of the curve to change dramatically Chapter 3: Mechanical Properties of Materials Slide 15 of 56  Tensile Strength in Stress-Strain Curve Elongation is accompanied by a uniform reduction in cross-sectional area, consistent with maintaining constant volume. Finally, the applied load F reaches a maximum value, and engineering stress at this point is called the tensile strength TS (a.k.a. ultimate tensile strength). 𝑭𝒎𝒂𝒙 𝝈𝑻𝑺 = 𝑨𝟎 Chapter 3: Mechanical Properties of Materials Slide 16 of 56  Ductility in Tensile Test Ability of a material to plastically strain without fracture Ductility measure = elongation EL 𝑳𝒇 − 𝑳𝟎 𝑬𝑳 = 𝑳𝟎 Where EL = elongation, Lf = specimen length at fracture, L0 = original specimen length. Lf is measured as the distance between gage marks after two pieces of specimen are put back together Chapter 3: Mechanical Properties of Materials Slide 17 of 56 True Stress Stress value obtained by dividing the instantaneous area into applied load: 𝑭 𝝈𝑻 = 𝑨 where 𝜎𝑇 = true stress; F = force; and A = actual (instantaneous) area resisting the load Chapter 3: Mechanical Properties of Materials Slide 18 of 56  True Strain Provides a more realistic assessment of "instantaneous" elongation per unit length 𝑳 𝒅𝑳 𝑳 𝜺=න = 𝒍𝒏 𝑳𝟎 𝑳 𝑳𝟎 Chapter 3: Mechanical Properties of Materials Slide 19 of 56 True Stress-Strain Curve True stress-strain curve for previous engineering stress- strain plot Chapter 3: Mechanical Properties of Materials Slide 20 of 56  Strain Hardening in Stress-Strain Curve Note that true stress increases continuously in the plastic region until necking In the engineering stress-strain curve, the significance of this was lost because stress was based on the original area value It means that the metal is becoming stronger as strain increases This is the property called strain hardening Chapter 3: Mechanical Properties of Materials Slide 21 of 56 True Stress-Strain in Log-Log Plot True stress-strain curve (plastic region portion) plotted on log-log scale Chapter 3: Mechanical Properties of Materials Slide 22 of 56 Flow Curve Because it is a straight line in a log-log plot, the relationship between true stress and true strain in the plastic region is 𝝈 = 𝑲𝜺𝒏 where K = strength coefficient; and n = strain hardening exponent Chapter 3: Mechanical Properties of Materials Slide 23 of 56 Categories of Stress-Strain Relationship: Perfectly Elastic Behavior is defined completely by modulus of elasticity E Fractures rather than yielding to plastic flow Brittle materials: ceramics, many cast irons, and thermosetting polymers Chapter 3: Mechanical Properties of Materials Slide 24 of 56  Stress-Strain Relationships: Elastic and Perfectly Plastic Stiffness defined by E Once Y reached, deforms plastically at same stress level Flow curve: K = Y, n = 0 Metals behave like this when heated to sufficiently high temperatures (above recrystallization) Chapter 3: Mechanical Properties of Materials Slide 25 of 56  Stress-Strain Relationships: Elastic and Strain Hardening Hooke's Law in elastic region, yields at Y Flow curve: K > Y, n > 0 Most ductile metals behave this way when cold worked Chapter 3: Mechanical Properties of Materials Slide 26 of 56 Compression Test Applies a load that squeezes the ends of a cylindrical specimen between two platens Compression force applied to test piece and resulting change in height and diameter Chapter 3: Mechanical Properties of Materials Slide 27 of 56 Compression Test Setup Chapter 3: Mechanical Properties of Materials Slide 28 of 56 Engineering Stress in Compression As the specimen is compressed, its height is reduced and cross-sectional area is increased 𝐅 𝛔=− 𝐀𝟎 where Ao = original area of the specimen Chapter 3: Mechanical Properties of Materials Slide 29 of 56  Engineering Strain in Compression Engineering strain is defined as: ∆𝒉 𝒉 − 𝒉𝟎 𝜺= = 𝒉𝟎 𝒉𝟎 Since height is reduced during compression, value of ε is negative (the negative sign is usually ignored when expressing compression strain) Chapter 3: Mechanical Properties of Materials Slide 30 of 56 Stress-Strain Curve in Compression Shape of plastic region is different from tensile test because cross section increases Calculated value of engineering stress is higher Chapter 3: Mechanical Properties of Materials Slide 31 of 56 Tensile Test vs. Compression Test Although differences exist between engineering stress- strain curves in tension and compression, the true stress-strain relationships are nearly identical Since tensile test results are more common, flow curve values (K and n) from tensile test data can be applied to compression operations When using tensile K and n data for compression, ignore necking, which is a phenomenon peculiar to strain induced by tensile stresses Chapter 3: Mechanical Properties of Materials Slide 32 of 56 Testing of Brittle Materials Hard brittle materials (e.g., ceramics) possess elasticity but little or no plasticity o Conventional tensile test cannot be easily applied Often tested by a bending test (also called flexure test) o Specimen of rectangular cross-section is positioned between two supports, and a load is applied at its center Chapter 3: Mechanical Properties of Materials Slide 33 of 56 Bending Test Bending of a rectangular cross section results in both tensile and compressive stresses in the material: (1) initial loading; (2) highly stressed and strained specimen Chapter 3: Mechanical Properties of Materials Slide 34 of 56 Testing of Brittle Materials Brittle materials do not flex- They deform elastically until fracture Failure occurs because tensile strength of outer fibers of specimen are exceeded Failure type: cleavage- common with ceramics and metals at low temperatures, in which separation rather than slip occurs along certain crystallographic planes Chapter 3: Mechanical Properties of Materials Slide 35 of 56 Shear Properties Application of stresses in opposite directions on either side of a thin element: (a) shear stress and (b) shear strain Chapter 3: Mechanical Properties of Materials Slide 36 of 56 Shear Stress and Strain Shear stress defined as: 𝑭 𝝉= 𝑨 where F = applied force; and A = area over which deflection occurs Shear strain defined as: 𝜹 𝜸= 𝒃 where  = deflection element; and b = distance over which deflection occurs Chapter 3: Mechanical Properties of Materials Slide 37 of 56  Torsion Stress-Strain Curve Typical shear stress-strain curve from a torsion test Chapter 3: Mechanical Properties of Materials Slide 38 of 56 Shear Elastic Stress-Strain Relationship In the elastic region, the relationship is defined as: 𝝉 = 𝑮𝜸 where G = shear modulus (shear modulus of elasticity) For most materials, G ≈ 0.4E, where E = elastic modulus Chapter 3: Mechanical Properties of Materials Slide 39 of 56 Shear Plastic Stress-Strain Relationship Relationship similar to flow curve for a tensile test Shear stress at fracture = shear strength S o Shear strength can be estimated from tensile strength: S  0.7(TS) Since cross-sectional area of test specimen in torsion test does not change as in tensile and compression, engineering stress-strain curve for shear  true stress- strain curve Chapter 3: Mechanical Properties of Materials Slide 40 of 56 Hardness Resistance to permanent indentation Good hardness generally means material is resistant to scratching and wear Most tooling used in manufacturing must be hard for scratch and wear resistance Chapter 3: Mechanical Properties of Materials Slide 41 of 56 Hardness Tests Commonly used for assessing material properties because they are quick and convenient (non-destructive) Variety of testing methods are appropriate due to differences in hardness among different materials Most well-known hardness tests are Brinell and Rockwell Other test methods are also available, such as Vickers, Knoop, Scleroscope, and durometer Chapter 3: Mechanical Properties of Materials Slide 42 of 56 Brinell Hardness Test Widely used for testing metals and nonmetals of low to medium hardness A hard ball is pressed into specimen surface with a load of 500, 1500, or 3000 kg Chapter 3: Mechanical Properties of Materials Slide 43 of 56 Brinell Hardness Test Brinell hardness (HB) exhibits a close correlation with the ultimate tensile strength TS of steels, leading to the relationship : TS = Kh (HB) Where Kh is a constant of proportionality: If TS is expressed in MPa, Kh = 3.45; If TS is in lb/in2, then Kh = 500. Chapter 3: Mechanical Properties of Materials Slide 44 of 56  Rockwell Hardness Test Another widely used test A cone shaped indenter is pressed into specimen using a minor load of 10 kg, thus seating indenter in material Then, a major load of 150 kg is applied, causing indenter to penetrate beyond its initial position Additional penetration distance d is converted into a Rockwell hardness reading by the testing machine Chapter 3: Mechanical Properties of Materials Slide 45 of 56 Rockwell Hardness Test (1) Initial minor load and (2) major load Chapter 3: Mechanical Properties of Materials Slide 46 of 56 Effect of Temperature on Properties General effect of temperature on strength and ductility Chapter 3: Mechanical Properties of Materials Slide 47 of 56 Hot Hardness Ability of a material to retain hardness at elevated temperatures Typical hardness as a function of temperature for several materials Chapter 3: Mechanical Properties of Materials Slide 48 of 56 Recrystallization in Metals Most metals strain harden at room temperature according to the flow curve (n > 0) But if heated to sufficiently high temperature and deformed, strain hardening does not occur Instead, new grains form that are free of strain. The metal has recrystallized The metal behaves then as a perfectly plastic material; that is, n = 0 Chapter 3: Mechanical Properties of Materials Slide 49 of 56 Recrystallization Temperature Recrystallization temperature of a given metal = about one-half its melting point (0.5 Tm) as measured on an absolute temperature scale. Recrystallization takes time The recrystallization temperature is specified as the temperature at which new grains are formed in about one hour Chapter 3: Mechanical Properties of Materials Slide 50 of 56 Recrystallization and Manufacturing Recrystallization can be exploited in manufacturing Heating a metal to its recrystallization temperature prior to deformation allows a greater amount of straining Lower forces and power are required to perform the process Forming a metal at temperatures above its recrystallization temperature is called hot working Chapter 3: Mechanical Properties of Materials Slide 51 of 56 Fluid Properties and Manufacturing Fluids flow : they take the shape of the container that holds them Many manufacturing processes are accomplished on materials converted from solid to liquid by heating (solidification processes) Examples: o Metals are cast in molten state o Glass is formed in a heated and fluid state o Polymers are almost always shaped as fluids Chapter 3: Mechanical Properties of Materials Slide 52 of 56 Viscosity in Fluids Viscosity is the resistance to flow Flow is a defining characteristic of fluids, but the tendency to flow varies for different fluids Viscosity is a measure of the internal friction when velocity gradients are present in the fluid The more viscous the fluid, the higher the internal friction and the greater the resistance to flow Reciprocal of viscosity is fluidity Chapter 3: Mechanical Properties of Materials Slide 53 of 56 Viscosity Viscosity can be defined using two parallel plates separated by a distance d and a fluid fills the space between the plates Chapter 3: Mechanical Properties of Materials Slide 54 of 56 Flow Rate and Viscosity of Polymers Viscosity of a thermoplastic polymer melt is not constant o It is affected by flow rate o Its behavior is non-Newtonian A fluid that exhibits this decreasing viscosity with increasing shear rate is called pseudoplastic This behavior complicates analysis of polymer shaping processes such as injection molding Chapter 3: Mechanical Properties of Materials Slide 55 of 56 Newtonian versus Pseudoplastic Fluids Viscous behaviors of Newtonian and pseudoplastic fluids Polymer melts exhibit pseudoplastic behavior For comparison, the behavior of a plastic solid material is shown Chapter 3: Mechanical Properties of Materials Slide 56 of 56 Viscoelastic Behavior Material property that determines the strain that the material experiences when subjected to combinations of stress and temperature over time Combination of viscosity and elasticity Chapter 3: Mechanical Properties of Materials Slide 57 of 56  Elastic Behavior vs. Viscoelastic Behavior (b) Response of a (a) Response of viscoelastic elastic material material Material in (b) takes a strain that depends on time and temperature Chapter 3: Mechanical Properties of Materials Slide 58 of 56