LECTURE PRESENTATION
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
Outline
Fundamental Mechanical Properties
Strength
- 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
Strength
- Many types (discussed later)
Stiffness
- Resistance to deformation under load in an elastic state
Hardness
- Resistance to indentation or abrasion
Elasticity
- Ability to deform without taking a permanent set when the load is released
Plasticity
- 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
Generally:
- 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
- Kind of stress
- 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
Load
- Typically measured in units of force
- Pounds-weight (lbf) or newtons (N)
- Torsion tests loads are moments measured in foot-lbs or newton-meters
Stress
- 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
Deformation
- 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.
Strain
- 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
Stiffness
- 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
http://www.youtube.com/watch?v=OaiynlvxFbk
- 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
Elasticity
- 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
Ductility/Malleability
Brittleness
- 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.
Slippage
- 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
Buckling
- 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
Hysteresis
- Loss of energy when transferring load
- Elastic hysteresis is hysteresis within elastic region
Toughness
- 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
