Mechanical Properties of Materials
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Questions and Answers

What does a P-B ratio of less than 1 indicate regarding the oxide film?

  • The oxide film can transition to protective.
  • The oxide film is protective.
  • The oxide film is ideal for protection.
  • The oxide film is nonprotective. (correct)

    • Which statement best describes a distinguishing feature of semiconductors?

  • They have a small band gap and create electron holes when excited. (correct)
  • They operate solely at absolute zero temperature.
  • They have a large band gap that permits minimal conduction.
  • They have no band gap and easily conduct electricity.

    • What type of magnetism is characterized by materials that exhibit a magnetic effect only in an external field?

  • Antiferromagnetic
  • Ferromagnetic
  • Diamagnetic (correct)
  • Paramagnetic

    • What color do materials typically appear, based on their interaction with light?

    <p>The color of light they reflect.</p> Signup and view all the answers

    Which type of magnetism is permanent and associated with metallic materials like iron, cobalt, and nickel?

    <p>Ferromagnetic</p> Signup and view all the answers

    What happens when the value of K is greater than the material property Kc?

    <p>The specimen will fail.</p> Signup and view all the answers

    How does the radius of curvature (p) affect the stress concentration factor (Kt)?

    <p>A larger radius decreases Kt values.</p> Signup and view all the answers

    Which of the following correctly describes fatigue in materials?

    <p>Fatigue is responsible for approximately 90% of mechanical engineering failures.</p> Signup and view all the answers

    What are the stages of fatigue failure in materials?

    <p>Crack initiation, crack growth, rapid propagation.</p> Signup and view all the answers

    Under what condition does creep occur in metals?

    <p>At or above 0.4 times the melting temperature.</p> Signup and view all the answers

    What characterizes elastomeric polymers?

    <p>They show large recoverable strains at low stresses.</p> Signup and view all the answers

    How is tensile strength defined in the context of polymers?

    <p>The stress at fracture.</p> Signup and view all the answers

    What does the Schmid factor indicate?

    <p>It determines the slip direction in crystallographic structures.</p> Signup and view all the answers

    What happens during viscoelastic deformation of a polymer?

    <p>The material exhibits both elastic and viscous behaviors at the glass transition temperature.</p> Signup and view all the answers

    Which of the following tests measures hardness in materials?

    <p>Brinell test</p> Signup and view all the answers

    Which of these statements about dislocations is true?

    <p>Movement of dislocations is termed slip.</p> Signup and view all the answers

    What is the primary factor affecting the ductility of a material?

    <p>Presence of dislocations and their movement.</p> Signup and view all the answers

    What do plastic polymers exhibit after initial elastic deformation?

    <p>Yielding and plastic deformation.</p> Signup and view all the answers

    What is stress defined as in mechanical properties?

    <p>Force per area</p> Signup and view all the answers

    When tension is applied to a material, how does strain behave?

    <p>It is positive as the material elongates</p> Signup and view all the answers

    What does Poisson's ratio express?

    <p>The lateral strain compared to axial strain</p> Signup and view all the answers

    What is a primary method to mitigate creep in materials?

    <p>Raising the melting temperature</p> Signup and view all the answers

    Which of the following definitions pertains to plastic behavior in materials?

    <p>Permanent deformations that do not recover shape after stress</p> Signup and view all the answers

    Which statement accurately describes oxidation?

    <p>Loss of electrons at the anode</p> Signup and view all the answers

    What type of corrosion is specifically caused when two different metals are in contact in the presence of an electrolyte?

    <p>Galvanic corrosion</p> Signup and view all the answers

    How is the shear stress calculated?

    <p>The total force divided by the area of the section</p> Signup and view all the answers

    Stress corrosion cracking occurs under which conditions?

    <p>Tensile stress in corrosive environments</p> Signup and view all the answers

    What phenomenon does Hooke's Law describe in materials?

    <p>Relationship between stress and strain in elastic materials</p> Signup and view all the answers

    Which type of corrosion occurs in stagnant areas depleted of oxygen?

    <p>Crevice corrosion</p> Signup and view all the answers

    During a tensile test, what kind of data is primarily collected?

    <p>Force-displacement data</p> Signup and view all the answers

    What is the characteristic of intergranular corrosion?

    <p>It occurs along grain boundaries</p> Signup and view all the answers

    What happens to a material when an elastic modulus is applied?

    <p>The material's shape returns to original upon load removal</p> Signup and view all the answers

    Erosion-corrosion is a result of which two factors combined?

    <p>Chemical attack and mechanical abrasion</p> Signup and view all the answers

    Which of the following best describes reduction in an electrochemical context?

    <p>Gain of electrons at the cathode</p> Signup and view all the answers

    Study Notes

    Mechanical Properties

    • Stress is the force perpendicular to a given area. The units are Pascals (Pa) or pounds per square inch (psi).
    • Strain is the change in length over the original length when a force is applied. It tells you how much deformation has occured. A positive value means the object has elongated due to tension, a negative value means the object has shortened due to compression.
    • Shear Stress occurs when a force is parallel to the area. Shear stress is calculated the same way as normal stress. Shear strain is calculated using the tangent of the angle of deformation.
    • Elastic Behavior occurs when a material returns to its original size and shape after a load is removed. Elastic behavior is linear and proportional to the stress applied. It follows Hooke's Law (Stress = Modulus * Strain). Within this region, the deformation is non-permanent.
      • The Elastic Modulus or Young's Modulus is a measure of material stiffness. It is the slope of the stress-strain curve in the elastic region. It is also proportional to the strength of the interatomic bonding forces.
    • Plastic Behavior occurs when a material permanently deforms, i.e. it does not return to its original size and shape after a load is removed.
    • Stress-Strain Curve
      • A Tensile Test is used to gather stress-strain data. A sample is loaded in a universal testing machine and moved at a constant speed to test the sample's response to increasing load. A Force-Displacement relationship is obtained, and then transformed into a stress-strain curve.
      • Yield Strength is the maximum point of the stress-strain curve just beyond the linear-elastic region.
      • Tensile Strength is the stress at fracture.
    • Polymers are sensitive to strain rate, temperature, and environment.
      • Brittle Polymers experience failure in the elastic region and fracture, for example bakelite.
      • Plastic Polymers exhibit elastic deformation initially, followed by yielding and plastic deformation, for example PMMA, HDPE.
      • Elastomeric Polymers deform elastically, with large recoverable strains at low stresses, for example rubber bands.
      • Viscoelastic Polymers exhibit both viscous and elastic behavior. The behavior depends on temperature. A viscous fluid at high temperatures, rubbery at intermediate temperatures, and like glass at low temperatures. The glass transition temperature is the intermediate temperature where the polymer exhibits both elastic and viscous behavior.

    Plastic Deformation

    • Dislocations are one-dimensional defects within a material. There are three types: edge, screw, and mixed.
      • The movement of dislocations is called slip.
      • Slip occurs on close-packed planes and along close-packed directions.
      • Materials with dislocations that can move will be ductile, while materials where dislocations cannot move will be brittle.
      • The Schmid Factor defines the shear stress on the dislocations. Once this factor is greater than the material's critical resolved shear stress, the dislocations will move.
      • Frank-Reed Sources are dislocation factories where dislocations frequently originate.
    • Strengthening Mechanisms use various methods to impede the motion of dislocations.
      • Grain Size Reduction: Reduces the distance that dislocations can travel.
      • Solid Solution Strengthening: Adding impurities can create internal stress that hinders dislocation motion.
      • Strain Hardening: Deformation of a material strengthens it by increasing the density of dislocations.
      • Precipitation Strengthening: Precipitating a second phase in the material that hinders dislocation movement.
    • Stress Concentration is a measure of how stress increases at local defects such as corners and crack tips.
      • The Stress Concentration Factor (Kt) is a factor used to determine the increase in stress in a component with a defect. The larger the crack length, the larger the stress concentration factor, and the more likely the crack will propagate. A sharp corner will yield a higher stress concentration than a rounded corner.
    • Fatigue is failure caused by cyclic loading. The stress may not even be high enough to lead to failure in an otherwise static condition. The most common type of mechanical engineering failure.
      • Fatigue Limit is a stress value below which an infinite number of cycles are possible without failure.
      • Fatigue Stages:
        • Crack Initiation
        • Crack Propagation
        • Rapid Propagation
        • Failure
    • Creep is the permanent deformation of a material over long periods of time. It does not occur in metals until heated to or above 0.4 times their melting point (use Kelvin degrees!).
      • Creep Resistance can be increased by:
        • Increasing Grain Size
        • Increasing Melting Temperature
        • Increasing Modulus
        • Precipitation Strengthening

    Environmental Damage

    • Oxidation is the loss of electrons. It occurs at the anode. Anodic materials have a negative electrode potential.
    • Reduction is the gain of electrons. It occurs at the cathode. Cathodic materials have a positive electrode potential.
    • Corrosion is the degradation or deterioration of a material due to chemical reactions. Types of corrosion include:
      • Galvanic Corrosion: Two different metals are in contact with each other, and there is the presence of water or an electrolye.
      • Crevice Corrosion: Occurs at crevices or under deposits of debris.
      • Pitting Corrosion: Small pits and holes form, nearly perfectly vertical.
      • Intergranular Corrosion: Occurs at grain boundaries, especially common is stainless steels.
      • Stress Corrosion Cracking: Occurs when a tensile stress is applied to a sample in a corrosive environment.
      • Erosion Corrosion: Occurs from fluid motion and abrasion.
    • Pilling-Bedworth Ratio (P-B Ratio) is a value that helps predict the protectiveness of an oxide layer.
      • A P-B Ratio less than 1 indicates an oxide film is not protective.
      • A P-B Ratio close to 1 is ideal for a protective oxide film.
      • A P-B Ratio between 1-2 is usually protective, and above 2 is typically not protective.

    Other Properties

    • Electrical Properties of materials are defined by their band gap, which is the energy difference between the valence band and the conduction band.
      • Conductors have a small band gap, so electrons require only a small amount of energy to move to the conduction band, making them good conductors of electricity.
      • Semiconductors have a small band gap, and require a moderate amount of energy to move electrons to the conduction band.
      • Insulators have a very large band gap, making it very difficult to move electrons to the conduction band.
    • Optical Properties of materials involve their interaction with light.
      • Intensity of Light: The summation of transmitted, absorbed, and reflected light. The color of a material is defined by the wavelengths of light not absorbed or transmitted.
    • Magnetic Properties are materials' responses to an external magnetic field. Some of the types of magnetism are:
      • Diamagnetism: A weak, temporary form of magnetism that only occurs when an external magnetic field is present.
      • Paramagnetism: Weak permanent magnetism caused by dipoles within each atom.
      • Ferromagnetism: Very strong permanent magnetism. Occurs in metals like iron.
      • Antiferromagnetism: The magnetic poles cancel each other out, resulting in no magnetic effect.
      • Ferrimagnetism: Permanent magnetization occuring in ceramics with aligned spins.

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    Exam 2 Study Guide - Filled PDF

    Description

    This quiz covers the fundamental mechanical properties of materials, including stress, strain, shear stress, and elastic behavior. Understand key concepts such as Young's Modulus and the implications of deformation under different forces. Test your knowledge on how materials respond to various stresses and the significance of elasticity in engineering.

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