fatigue rsistance test

Topic

Fatigue Resistance Test

2

What IS a FATIGUE ?

Fatigue is a form of failure that occurs in structures subjected to dynamic and fluctuating stresses (e.g., bridges, aircraft, machine components).

Fatigue

This type of failure normally occurs after a lengthy period of repeated stress or strain cycling.

The process occurs by the initiation and propagation of cracks, and typically the fracture surface is perpendicular to the direction of an applied tensile stress.

Cyclic Stresses

The applied stress may be axial (tension–compression), flexural (bending), or torsional (twisting) in nature.

Three different fluctuating stress–time modes are possible.

Cyclic Stresses reversed stress cycle repeated stress cycle randomly Stress Cycle

Reversed Stress Cycle:

The amplitude is symmetrical

about a meanzero stress

level.

Repeated Stress cycle:The maxima and minima

are asymmetrical

relative to the zero stresslevel.

Random Stress Cycle:

The stress level may

vary randomly in amplitude

and frequency

Reversed Stress Cycle

Repeated Stress Cycle

Random Stress Cycle

Fatigue Resistance Testing

The sequence of stress amplitude. The nature of the test-piece.

Constant-amplitude test

Variable-amplitude tests

Specimens Component

Routine life testShort

life test

Long life test

Fatigue Testing Machines:

Rotating Bending Testing Machine

Axial loading (push-pull) type fatigue tester

Rotating Bending Testing Machine

Rotating Bending Testing Machine

• This type of machine gives S-N curve• The motor is turning at a constant

revolution per minute or frequency• To create a failure on the specimen, a

constant-stationary force is applied on the specimen, which creates a constant bending moment.

Working Principle

A stationary moment applied to a rotating specimen causes the stress at any point on the outer surface of the specimen to go from zero to a maximum tension stress, back to zero and finally to a compressive stress.The process is repeated.

Axial loading (push-pull) type fatigue tester

Axial loading (push-pull) type fatigue tester

In this type the specimen is not exposed to bending but to pure axial (tensile or compressive) loading. Specimen is held at two ends and loaded cyclically between two extreme (maximum and minimum) values.

Fatigue Test for LDPE

Fatigue Test for LDPE

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