1. Chapter 11 Mechanical Properties of Materials
Strengths
Chapter 11 Mechanical Properties of Materials

2. Tension Test Most common used test for metals and ductile materials
Universal testing machine used
Can also test compression
Shear
Bending
Round specimen made to ASTM spec clamp into machine
The tensile force P acting on the specimen at any time during the test is indicated
The corresponding change in length between two marks is recorded
Corresponding values of stress versus strain can be calculated.

3. Stress – Strain diagram
Graphical representation of the tension test
Elastic Stage – straight line to the proportional limit (stress proportional to strain)
Deformation is consider elastic in this range – means specimen will return to its initial size and shape
Beyond elastic limit only part of the deformation can be recovered – deformation after becomes permanent – called plastic deformation.
Yield Stage – curve becomes horizontal – specimen continues to elongate without any significant increase in load
Strain hardening stage – ability of the material to resist deformation is regained after the yield stage has passed
Modulus of elasticity – figure 11-5 page 389
Example 11-1 page 390

4. Mechanical Properties of Materials
Strength – greatest stress that it can withstand without failure
Stiffness – ability of material to resist deformation
Elasticity – property of material that enables it to regain its original unreformed length once the load is removed
Ductility – ability of material to undergo a lot of plastic deformation before rupture
Brittleness – undergoes very little plastic deformation before rupture is said to be brittle
Hardness – the resistance of a material to penetration
Machinability – the ease with which a material can be machined
Resilience – capacity of a material to absorb energy within the elastic range
Toughness – capacity of a material to absorb energy without fracture

5. Allowable stresses and factor of safety
To provide margin of safety in design , machine and structural members are designed for a limited stress level – called the allowable stress – which is the maximum stress considered to be safe when a member made of a given material is subjected to a known loading condition
Formula = 11-5, 11-6
Elastic design with ductile materials significant deformation may occur and render the structural element unusable when the yield strength is reached
A factor of safety of 2 means that the member can withstand a maximum load equal to twice the load for which the member is designed before failure
Choosing appropriate value for the factor of safety depends on
Variation in material properties
Uncertainty in the method of analysis
Problems in manufacturing
Environmental conditions
Uncertainty in loading conditions
Risk and liability

6. Allowable stresses and factor of safety
Example 11-2
Example 11-3

7. Stress Concentrations
When an axially loaded subjected to central axial tension or compression the normal stress is unifromly distributed in the cross section
Stress raisers in flat axially loaded members and the resulting stress distributions can change this equation
Stress is concentration adjacent to the stress raiser
Stress may be several times greater then the average stress over the net cross sectional area
The max stress is called the stress concentration
The max stress vs the average stress is called the stress concentration factor
Examples –figure
Examples 11-4
Examples 11-5

8. Elastic Design Versus Plastic Design
Up to this point design of axially loaded members based on allowable stress – allowable stress design
Elastic design – assumes that once a material starts to yield it can no longer carry any additional load
Elastoplastic materials – highly ductile metals – hooke law is valid up to the yield point – once yield point is reached the stress and the stress strain diagram can be modeled – a material is considered elastoplastic
Maximum plastic load is reached when every point in the section has yield
Another design approach assumes that failure will not occur until every point in a critical section has yielded – called ultimate strength design
Example 11-6
Example 11-7