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