LECTURE #10: MECHANICAL PROPERTIES

OF MATERIALS

ENGR 151: Materials of Engineering

DUCTILITY

Measure of the

degree of plastic

deformation that has

been sustained at

fracture

Material with little or

no plastic deformation

at fracture is brittle

High degree of plastic

deformation at

fracture is ductile

DUCTILITY

Percent Elongation:

Percent Reduction:

RESILIENCE

Capacity of a material to absorb

energy when it is deformed

elastically and then, to have energy

recovered

Modulus of resilience (Ur): the strain

per unit volume required to stress a

material from unloaded state to

yielding

Area under the engineering stress-

strain curve

RESILIENCE

Assuming linear stress-strain relationship (linear elastic region)

Resilient materials have high yield strengths and low moduli of

elasticity (used for spring applications)

Stress increases relatively slowly with strain

Can tolerate relatively high stress levels

TOUGHNESS

Measure of ability of a material to absorb

energy up to fracture

Area under stress-strain curve up to fracture

TYPICAL MECHANICAL PROPERTIES

TYPICAL MECHANICAL PROPERTIES

RESILIENCE

Concept Check (Table 6.3, pg. 170)

a) Which will experience the greatest percent

reduction in area? Why?

b) Which is the strongest? Why?

c) Which is the stiffest? Why?

RESILIENCE

Concept Check (Table 6.3, pg. 170)

a) B, because it shows the largest strain value

b) D, on account of its largest yield and tensile strengths

c) E, since it has the largest elastic modulus

TRUE STRESS

Must take into account the thinning of cross-sectional

area in plastic deformation

True stress: defined as the load F divided by the

instantaneous cross-sectional area Ai over which

deformation is occurring:

TRUE STRAIN

If no volume change occurs during deformation

TRUE STRAIN

Relationship between true and engineering stress and strain

TRUE STRESS AND STRAIN

Only valid to the onset of necking

You can approximate the true stress-strain

curve from onset of plastic deformation to the

point at which necking begins:

K and n are constants that are material-

dependent.

TRUE STRESS AND STRAIN

EXAMPLE PROBLEMS

EXAMPLE PROBLEMS

EXAMPLE PROBLEMS

HARDNESS

A measure of a material’s resistance to localized plastic deformation (small dent or scratch)

Measured by hardness tests:

Simple and inexpensive

Nondestructive test

Other properties may be estimated from hardness data (Figure 6.19)

HARDNESS TESTS

Brinell Hardness Test Indenter: 10mm sphere of steel or tungsten carbide

Varying loads

Vickers Microhardness Indenter: Diamond Pyramid

Varying loads

Knoop Microhardness Indenter: Diamond Pyramid

Varying loads

Rockwell Indenter: diamond cone, steel spheres

Specified loads

HARDNESS TESTS (TABLE 6.5)

HARDNESS TESTS

Rockwell Hardness vs. Superficial Rockwell

Hardness Scales (Table 6.6, pg. 176)

Notice different range of load

HARDNESS TESTS

Difference in scales

DESIGN/SAFETY FACTORS

Safe stress or working stress

N = factor of safety

σy = yield strength

DESIGN/SAFETY FACTORS - EXAMPLE

DESIGN/SAFETY FACTORS - EXAMPLE

HW (DUE MONDAY, MARCH 27)

Chapter 6

6.37, 6.42, 6.45, 6.51, 7.12, 7.17