CHE3166: HANDOUT 4

Mechanical Properties and Testing

(Hardness, Impact, Creep and Fatigue)

LEARNING OBJECTIVES

Material’s response to:

• Excessive Loading: Tensile Test

• Localized Loading: Hardness Test

• Sudden Intense Loading: Impact Test

• Loading at High Temperatures: Creep Test

• Cyclic Loading: Fatigue Test

Localized Loading: Hardness / Hardness Test

Hardness: Resistance to

• localized plastic deformation

• penetration of the surface

Hardness Test Data are used for:

• rough but quick comparison of strength

of materials

• characterization of wear resistance

• quick examination of heat treatment

• quick testing for materials quality control

Hardness Tests are quick and simple

Hardness / Hardness Test

Hardness Test

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

Vickers Micro-hardness Test: Diamond pyramid

P is the applied load

Correlation between

Hardness and Tensile StrengthTS (MPa) = 3.45 x HB TS (psi) = 500 x HB

Response to Sudden Loading: Impact

Increase in Loading

Rate results in:- increase in sy and TS

- decreases %EL

Why?

A rapid loading gives less time for dislocations to

move past obstacles: thus hardens the material

So, The response to sudden loading is also a

measure of: • Lack of Ductility and

• Toughness

Very low toughness:unreinforced polymers

Engineering tensile strain, e

Engineering

tensile

stress, s

Low toughness: ceramics

High toughness: metals

Impact Resistance: Toughness

• Energy to break a unit volume of material:

Energy is absorbed for movement of dislocation

• The lesser the energy absorbed the poorer the toughness

Impact Tests

Impact test conditions are chosen to represent

severe conditions for fracture:

1. Deformation at a relatively low temperature: less

opportunity for movement of dislocations

2. Very rapid deformation: less opportunity for

movement of dislocations

3. Presence of a notch: existing crack initiation site

• Charpy (common)

• Izod (for non-metallic)

Two Standard Testing Methods:

Heavy pendulum:

• starting at height

h, swings through

its arc

• strikes and

breaks the

specimen

• reaches a final

elevation, h'

• h' is a measure

of the ability of

the material to

withstand the

impact (energy

absorbed)

Ductile-Brittle Transition (DBT) Temperature

Impact Energy vs DBT Temperature

DBT Temp for Steel: Role of Carbon

Creep

Deformation of materials at elevated temperatures

• plastic deformation,

• time-dependent,

• for metallic materials creep temperature > 0.4 Tm

(Tm = melting point on absolute temp. scale, K),

• life limiting factor for several critical high

temperature components

Examples of in-service creep:

• steam generators in thermal power plants

• turbine rotors in jet engines

Assessment of Creep

Measurement of deformation or strain as a function of time at:

• Constant temperature and

• Constant load or stress

Constant Load Creep Behaviour

Instantaneous Deformation

On loading, instantaneous elastic deformation

Constant Load Creep: Primary RegionPrimary or Transient Creep Region

Continuously decreasing:

• creep strain with time

• creep/strain rate with time, the slope decreases with time

Indicates creep resistance: Strain Hardening

Constant Load Creep: Secondary Region

Secondary or Steady-state Creep region:

Creep or strain rate is constant with time

• the plot takes almost a linear shape

• the region of longest duration

• for practical applications, the regions should be as long

as possible

The constancy of creep

rate is attributed to the

balance between

• Strain hardening

• Recovery (a softening

process)

Constant Load Creep: Secondary Region

Recovery:

stored strain energy is removed by virtue of dislocation motion and enhanced atomic diffusion at elevated temperatures, resulting in annihilation (vanishing off) of dislocations and improved ductility

Strain Hardening:

ductile material becoming harder when plastically

deformed as a result of extensive generation of

dislocations and straining in the crystalline structure.

Also, called work hardening (cold working)

Constant Load Creep: Tertiary Region

Tertiary Creep region / Creep Rupture:

• Rapid acceleration of creep rate, leading to failure/rupture

• Rupture results from microstructural changes:

• grain boundary separation

• internal cracks, cavity cracks, leading to decrease in the

effective load-bearing area

• For practical applications,

the onset of tertiary creep

should be delayed as long

as possible

Creep Parameters and

Design of High Temperature Components

For long-life components (e.g., steam generators):

• Steady-state creep rate (e/t) is the design parameter

• The creep tests need not last to failure

For short-life components (e.g., rocket motor nozzle):

• Time to rupture (creep-rupture life) is the design parameter

• Creep rupture tests required

Design data for creep is commonly represented as

log stress

vs

• log Steady-state Creep Rate (e/t) or

• log Rupture Time

Creep Data Extrapolation

For long-life components, generating actual creep

data under actual conditions (temperature/stress)

would require impractically long-term tests

Solution:

• Short-term accelerated tests at higher

temperatures and comparable stresses

• At different temperatures, determine Larsen-

Miller parameter

• T (C + log tr): Larsen Miller Parameter

(T: temp. in K, C: constant (~20), tr: rupture

time in hours)

• Extrapolation of the data

Creep Data Extrapolation

Larson-Miller

Parameter

vs

log stress

Cyclic LoadingFluctuations in:

Temperature (inside and outside)

Loading

Pressure

Hip implant:

Cyclic loading

during

walking

Examples of Cyclic Loading

Fatigue

• Damage or failure caused due to dynamic and

fluctuating stress

(e.g., bridges, aircraft and machine components)

• Failure may occur at a stress level considerably

lower than the tensile or yield strength

• Failures normally occur after a lengthy period of

repeated stress or strain cycling

• 90% of metallic components fail due to fatigue

• Failures are sudden

S-N Curve: Fatigue Parameters

Fatigue Life: Number of cycles to cause fatigue failure at a

given stress level

Fatigue Strength: Maximum stress at which no fatigue

failure occurs for a given number of cycles (say, 107)

S-N Curve: Fatigue Parameters

Fatigue Limit: Stress below which no fatigue failure occurs