Mechanical Properties of Materials


Reading Assignment

  • 10th ed. Chapter 2
    • 2.1 Introduction
    • 2.2 Static Properties
    • 2.3 Dynamic Properties
    • 2.4 Temperature Effects
    • 2.5 Machinability
    • 2.8 Testing Standards and Concerns


Fundamental Mechanical Properties


  • Ability to resist the application of loads without failure

Mechanical Properties

  • Deal with behavior of materials under applied loads
  • Compare with chemical, thermal, electrical/magnetic, acoustic and optical properties
  • Examples: Strength in: tension, compression, shear, torsion and flexure; Static, impact and endurance stiffness, toughness, elasticity, plasticity, ductility, hardness


  • Many types (discussed later)


  • Resistance to deformation under load in an elastic state


  • Resistance to indentation or abrasion


  • Ability to deform without taking a permanent set when the load is released


  • Ability to deform outside the elastic range and not rupture (think PlayDoh)

Energy Capacity

A function of strength and stiffness.  The ability to absorb energy or have work done on it.
  • Resilience – is energy capacity in the elastic region
  • Toughness is energy required to rupture a material

Materials testing

Methods and procedures used to measure and determine mechanical properties of materials
  • Measuring an applied force or load and
  • Measuring the specimen’s dimensional change

Mechanical Test Considerations

No single test can determine all properties.
Factors related to testing conditions
  •  The manner in which the load is applied
    • Kind of stress
      • Tension, compression, torsion, shear, flexure
    • Rate the stress is developed
      • Static, long-time, dynamic (inertial effects present)
    • Number of cycles of load application
  •  The condition of the material specimen
  • The surrounding conditions (environment)

Basic Types of Loading

  • Tension
  • Compression
  • Torsion
  • Shear
  • Flexure

Testing Conditions

Conditions refer to conditions of the specimen and the environment

Stress and Strain


  • Typically measured in units of force
  • Pounds-weight (lbf) or newtons (N)
  • Torsion tests loads are moments measured in foot-lbs or newton-meters
  • Intensity of internally distributed forces
  • Typically, force per unit area
    • e.g., PSI, or MPa
  • Typically calculated based on the original cross-sectional area of the specimen.

  • s = stress developed in specimen
  • F = applied load
  • A = original cross-sectional area of specimen
  • Denotes the change in form of a body due to stress, thermal change, moisture absorption , or any other cause.
  • Usually a linear change in dimensions and is measured in units of length.
  • Change in length per unit length
  • e = strain
  • lf = final gage length
  • lo = initial gage length

(Considered dimensionless, sometimes in/in or mm/mm)

Three Types of Strain
  • Tensile Strain
  • Compressive Strain
  • Shear Strain
  • Nominal stresses and strains (Engineering Strain)
  • True stresses and strains
  • Axial and lateral strain
    • Poisson’s ratio
    •  PR = (lateral strain)/(axial strain)
Stress-Strain Diagram
  • Ability to resist deformation under load
    • Determined by the rate of stress to given strain
  • While below the proportional limit, the modulus of elasticity is the ratio of stress to corresponding strain (higher E, stiffer material
  • Young’s modulus is E in tension,
  • Modulus of rigidity (G instead of E)  is under shear stress
  • Remember Hooke’s Law:

Elastic & Plastic Deformation

  • The property of a material to recover from a deformation.
  • Elasticity may be altered by changing temperature, or the by prolonged or rapid loading
  • Three measures of elastic strength:
    • Elastic limit (not commonly used)
    • Proportional limit
    • Yield strength (sy)
  • Upper and lower Yield points

Proportional Limit

Plastic Deformation
  • Slip
  • Dislocation, not breaking of bonds between atoms
  • Occurs during plastic deformation
  • Requires a certain level (applied load) of energy to occur
  • Move in particular paths through the lattice (slip planes)
Plasticity – The ability of a material to endure a permanent deformation without rupture.
  • The ability to be stretched before rupture while maintaining a load
  • Measured in % elongation and percent reduction



  • Brittle material fractures with little or no elongation

Categories of Strength

Ultimate Strength
  • Maximum stress that can develop (highest point on the stress strain curve)
Tensile Strength
  • Maximum tensile stress
Compressive Strength
  • Maximum compressive stress
Fatigue Strength
  • Maximum stress that can be applied over a given number of cycles without causing failure.
Fatigue Limit
  • Maximum stress below which a material will not fail regardless of the number of cycles.

Types of Failure

Failure Definition
  • The change in any characteristic that renders a material unsatisfactory for use.
  • Movement of parallel planes within a material in parallel directions
Creep (plastic flow)
  • Constant slippage at a constant volume without material disintegration
Separation, cleavage, fracture
  • Occurs when applied stress is greater than the internal binding forces of a material under tensile loading
  • Occurs when a material is unable to resist a compressive stress

Energy Capacity

  • Ability to absorb and store energy
Elastic Resilience
  • Energy or work required to stress a material up to its elastic limit
Modulus of Resilience
  • Energy stored per unit volume for a material at its elastic limit.  Max. energy that can be reclaimed.
  • Area under elastic portion of the stress strain curve
  • Loss of energy when transferring load
  • Elastic hysteresis is hysteresis within elastic region
  • Measure of energy required to cause a material to rupture.  It is represented graphically by the total area under the stress strain curve.
Modulus of toughness
  • The amount of energy required to rupture a material under static load.
  • Toughness Units are J/m3
  • ASTM D-256, plastics and ASTM E-23 for metals

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