Material Properties and Stress-Strain Curve Quiz

Manufacturing Processes - Chapter 3: Mechanical Properties of Materials

• Mechanical properties define a material's behavior under mechanical stress. These include elastic modulus, ductility, hardness, and various strength measures.
• Desirable material properties (like high strength) often make manufacturing more challenging.
• Stress-Strain Relationships involve three static stress types: tensile (stretching), compressive (squashing), and shear (deforming adjacent portions).
• A stress-strain curve shows the relationship between stress and strain for these three types of stress.
• Tensile tests are used to study the stress-strain relationship, especially for metals.
• In a tensile test, a force elongates the material and reduces its diameter.
• ASTM (American Society for Testing and Materials) standards specify the preparation of tensile test specimens.
• Tensile tests involve a specific sequence: no load, uniform elongation/area reduction, maximum load, necking, fracture, and final length.
• Engineering stress is calculated as force divided by the original area of the specimen.
• Engineering strain is the deformation in elongation, calculated as the change in length divided by the original length.
• A typical engineering stress-strain plot for a metal shows an elastic and a plastic region.
• Hooke's Law describes the linear relationship between stress and strain in the elastic region. Stress is equal to the elastic modulus times strain.
• The elastic modulus (E) represents a material's stiffness.
• The yield point (Y) marks the transition from elastic to plastic deformation. It's identified by a change in slope on the stress-strain curve.
• Other strength properties include yield strength, yield stress, and elastic limit.
• Yield points are more often defined as the calculated stress at which a 0.2% offset from the initial elastic region line is reached, rather than an abrupt change in slope.
• The plastic region shows a non-linear and often faster rate of elongation as stress increases beyond the yield point. Models like the flow stress model guide this relationship.
• Tensile strength (TS) is the maximum stress point on the stress-strain curve.
• Ductility measures a material's ability to deform plastically without fracturing; elongation (EL) is a measure of ductility calculated as the change in length at fracture divided by the original length.
• True stress accounts for changes in the specimen's cross-sectional area during deformation. True stress is calculated by dividing force by the instantaneous area.
• True strain is a more realistic representation of instantaneous elongation. Calculated as the natural log of the ratio of current length to the original length.
• A true stress-strain curve can be plotted from the engineering stress-strain curve. The slope of the true stress-strain curve in the plastic region is an indication of strain hardening.
• Strain hardening shows the material's increasing strength as strain increases.
• The flow curve, presented as a log-log plot in the plastic region, displays a linear relationship between true stress and true strain. The strength coefficient (K) and strain hardening exponent (n) help describe this relationship.
• Perfectly elastic materials only deform elastically and then fracture before yielding into the plastic region. Ceramics, many cast irons, and thermosetting polymers show this behavior.
• The stress-strain relationship for elastic and perfectly plastic materials shows a horizontal line in the plastic region.
• Metals show elastic and strain hardening behavior when cold-worked, with a non-linear increase in stress with strain in this region.
• The compression test applies a force that squeezes a cylindrical test specimen between plates, resulting in changes in height and diameter. Engineering stress and strain calculations are also used in compression tests.
• For compression tests, true stress-strain curves are nearly identical to true stress-strain curves in a tensile test.
• Brittle materials (like ceramics) are usually tested using bending tests (or flexure tests) because tensile tests are not suitable. Bending tests result in tensile and compressive stresses on these materials.
• Brittle failure occurs when the tensile strength of the outer fibers of the specimen is exceeded. Cleavage is a common failure type in brittle materials at low temperatures.
• Shear properties describe the application of opposing stresses, resulting in shearing along the specimen. Shear stress is calculated as the applied force divided by the area over which the deflection occurs. Shear strain is calculated as the deflection of the element divided by the initial distance. A torsion test produces a typical shear stress/strain curve.
• Shear elastic and shear plastic relationships describe the relationship between shear stress and shear strain in the elastic and plastic regions of a shear test respectively. Shear strength can be calculated indirectly from tensile strength.
• Hardness is a material's resistance to permanent indentation. Materials exhibit better scratching and wear resistance the harder they are. Hardness can be assessed non-destructively using tests like Brinell and Rockwell tests.
• Brinell hardness is often used to test metals and non-metals with low to medium hardness and is closely correlated to ultimate tensile strength.
• Rockwell tests use a cone-shaped indenter under a minor and major load to assess hardness.
• Temperature affects various material properties including strength and ductility. Properties decrease as temperature increases, with some exceptions.
• Some materials (like ceramics or low-carbon high-temperature steels) maintain hardness at elevated temperatures (or retain hot hardness) while others show reduced hardness when heated.
• Recrystallization in metals describes the formation of new grains in deformed metals when heated above a certain recrystallization temperature, which is generally around half the melting point. This allows the metal to revert to plastic behavior after significant deformation. Hot working involves forming a metal at temperatures above its recrystallization temperature.
• Fluids flow and take the shape of the container holding them, with viscosity representing a fluid's resistance to flow. Fluid properties can be considered in many manufacturing processes involving liquefied materials like molten metals or polymers.
• Polymers and thermoplastic polymer melts' viscosity is influenced by flow rate, making their behavior non-Newtonian. Their behavior in terms of viscosity is considered as pseudoplastic meaning their viscosity decreases with increasing shear rate.
• Viscoelastic materials show both elastic and viscous behavior, exhibiting strain that depends on time and temperature.