Manufacturing Processes: Chapter 3 Quiz

Manufacturing Processes: Chapter 3 - Mechanical Properties of Materials

• This chapter focuses on the mechanical properties of materials in manufacturing.
• Mechanical properties determine how a material responds to mechanical stresses.
• Properties include elastic modulus, ductility, hardness, and various strength measures.
• High strength materials are desirable for designers, but often make manufacturing more difficult.
• Stress-Strain Relationships: Materials experience three types of static stresses: tensile (stretching), compressive (squeezing), and shear (causing adjacent parts to deform). The stress-strain curve describes the relationship between stress and strain for these three types.
• Tensile Test: A common test that measures deformation when an external force elongates and decreases the diameter of a material (especially metals).
• Tensile Test Specimen: ASTM (American Society for Testing and Materials) sets standards on specimen preparation.
• Tensile Test Setup: A machine (tensile testing machine) with two crossheads, one fixed, one moving, applies a force to a test specimen in the machine to measure strain and stress.
• Tensile Test Sequence: The stages of the tensile test are shown graphically to illustrate the various stages: (1) no load, (2) uniform elongation and area reduction, (3) maximum load, (4) necking, (5) fracture, (6) final length.
• Engineering Stress: Defined as the applied force divided by the original cross-sectional area of the specimen. Stated as a formula.
• Engineering Strain: Measures deformation, the change in length divided by the original length, at any point during a tensile test. Stated as a formula.
• Typical Engineering Stress-Strain Plot: A graph displays two key regions: elastic (linear) and plastic where the slope changes.
• Elastic Region in Stress-Strain Curve: The graph shows a linear relationship between stress and strain. Hooke's Law states that stress is directly proportional to strain (σ = Eε), with E being the modulus of elasticity (a measurement of the material's stiffness). The material recovers its original shape when the stress is removed.
• Yield Point in Stress-Strain Curve: The yield point is when a material begins to deform permanently; identified by a change in slope. It is a crucial strength property, also referred to as yield strength, yield stress, and elastic limit.
• Yield Point in Stress-Strain Curve (more detail): Yield strength, Y, isn't necessarily an abrupt change in slope but rather where a 0.2% strain is drawn from the initial elastic region line.
• Plastic Region in Stress-Strain Curve: The relationship between stress and strain isn't linear and is characterized by the flow stress model as stress increases. Elongation occurs at a faster rate than before as stress increases.
• Tensile Strength in Stress-Strain Curve: The maximum load, Fmax and engineering stress at the point is called the tensile strength, TS, or ultimate tensile strength (UTS). Expressed as a formula.
• Ductility in Tensile Test: The material's ability to strain plastically before rupturing. Measured with the elongation (EL), the difference between the final and original lengths, after elongation. Stated as a formula.
• True Stress: The instantaneous stress at any point on the specimen in the test is calculated by dividing the applied force by the actual or instantaneous area resisting the load. Expressed as a formula.
• True Strain: Evaluates instantaneous elongation per unit length. Expressed as a formula.
• True Stress-Strain Curve: A graph to present true stress versus true strain to illustrate a more realistic assessment of the material's response to deformation; it accounts for the decreasing cross-sectional area during the test. Also presents the Elastic region where the relationship is still σ = Εε.
• Strain Hardening in Stress-Strain Curve: True stress increases continuously in the plastic region until necking, which demonstrates that the metal becomes stronger with an increase in strain.
• True Stress-Strain in Log-Log Plot: The graph plots strain and stress on a logarithmic scale. A linear relationship is expressed as the formula σ = Kec, where K is the strength coefficient, and n is the strain hardening exponent.
• Flow Curve: A straight line on a log-log plot illustrating the mathematical relationship between true stress (σ) and true strain (ε). Expressed as a formula.
• Categories of Stress-Strain Relationship: Perfectly Elastic. This describes materials that have a linear stress-strain relationship defined by E. They fail by fracturing before yielding to plastic flow.
• Stress-Strain Relationships: Elastic and Perfectly Plastic describes materials that behave linearly until yielding where the stress remains constant but the strain continues to increase. Flow curve shows K = Y, n = 0. Metals behave in this way at high temperatures (above recrystallization).
• Stress-Strain Relationships: Elastic and Strain Hardening materials behave linearly until yielding, and the stress required increases as the strain increases.
• Compression Test: Applies a compressive load to a cylindrical specimen between two plates to measure the resulting height change and cross-sectional decrease.
• Engineering Stress in Compression: Similar to tensile tests with the formula to calculate stress where the height of the specimen decreases.
• Engineering Strain in Compression: Strain is calculated by comparing the change in original height and the change in height during compression. Negative values are common and usually discarded from the calculations.
• Stress-Strain Curve in Compression: Describes the relationship between stress and strain where the shape of the plastic region is different from a tensile test due to an ever-increasing cross-section during this type of test.
• Tensile Test vs. Compression Test: True stress-strain curves in tension and compression are very similar. Testing from tensile tests can be used to predict the results of a compression test because the cross-sectional area doesn't change in a compression test.
• Testing of Brittle Materials: A bending test (also called a flexural test) is tested instead of a tensile test. A rectangular specimen is placed between supports, with a load applied at the center.
• Bending Test: Bending a specimen of rectangular cross-section results in both tensile and compressive stresses within the material. Graph or image shows the stresses.
• Testing of Brittle Materials (Failure): Brittle materials fail by cleavage- a common failure type with ceramics and metals at low temperatures with separation occurring rather than slip along the crystallographic planes.
• Shear Properties: Describes the application of stresses in opposite directions on either side of a thin element. Provides a concept of shear strength in relation to stress and strain.
• Shear Stress and Strain: Shear stress is illustrated with a formula. Shear strain is illustrated with a formula and shows how shear is related to deflection over a distance.
• Torsion Stress-Strain Curve: A graph illustrated from a torsion test to give the Shear stress versus Shear strain for a material.
• Shear Elastic Stress-Strain Relationship in the elastic region, the shear stress is illustrated with the following formula.
• Shear Plastic Stress-Strain Relationship: Illustrates the relationship of shear stress at failure = shear strength. The shear strength can be estimated from tensile strength and the cross-sectional area of the sample doesn't change so engineering stress and true stress-strain curves are similar
• Hardness: Resistance of a material to permanent indentation, directly related to scratching and wear resistance.
• Hardness Tests: Specific tests for characterizing the hardness of materials, which are quick and convenient like the Brinell and Rockwell tests which provide non-destructive measurement.
• Brinell Hardness Test: A widely used method that presses a hard sphere into a specimen surface using a standard load. The formula displays how the diameter of the indentation is used in the calculation for the Brinell test.
• Brinell Hardness Test (further detail): Brinell hardness provides a close correlation with ultimate tensile strength TS in steel, providing a close correlation in stress.
• Rockwell Hardness Test: A technique that uses an indenter (cone, ball, or diamond) under two types of loads. A initial load followed by a second load to calculate the difference and arrive at the Rockwell hardness reading.
• Effect of Temperature on Properties: A graph shows that tensile and yield strengths, and ductility all decrease as temperature increases. The graph is a visualization based on temperature.
• Hot Hardness: The graph displays the capability of a material to maintain hardness at high temperatures, such as in ceramics and high-alloy steel.
• Recrystallization in Metals: Most metals gain strength at room temperatures but if heated they can lose strength. The new grains formed during heating are free of strain; this temperature is typically one-half the melting point in absolute temperature or about one hour for the new grains to form.
• Recrystallization Temperature: Specific to each material, typically around one-half of its melting point. Specifies the temperature where new grains form in about an hour.
• Recrystallization and Manufacturing: Heating metals above their recrystallization temperature allows for greater deformation with less force compared to at room temperature; this process is called hot working.
• Fluid Properties and Manufacturing: Fluids take the shape of their container. Many processes (e.g., casting, glass forming) make use of the fluid properties of materials at elevated temperatures.
• Viscosity in Fluids: Resistance to flow. The internal friction when there is a difference in velocity. Viscous fluids have higher internal friction.
• Viscosity: Defined by using parallel plates separated by a distance, with the fluid filling the space between the two plates.
• Flow Rate and Viscosity of Polymers: Viscosity of thermoplastic polymer melt is affected by flow rate.
• Newtonian versus Pseudoplastic Fluids: A graph shows pseudoplastic fluids' decrease in viscosity with an increase in shear rate, which highlights viscosity is not constant. The graph also includes characteristics of plastic solids.
• Viscoelastic Behavior: Material property that illustrates how a material responds to a combination of stress and temperature over time. The property is a combination of viscosity and elasticity.
• Elastic Behavior vs. Viscoelastic Behavior: A graph illustrates the difference between elastic (fixed) vs. viscoelastic (time-dependent) behavior. The viscoelastic response depends on time.