Fatigue Definition

Reduced material resistance underfluctuating stresses or reversals,which may culminate in cracks orfailure after a number of cycles.

• Fatigue is the tendency of a member to fail at stress levels below yield stress when subject to cyclical loading (Truck loading)

Stress – Strain Curve

• Add stress strain curve

Three Stages of Fatigue Failure

1. Crack Initiation Initiation from a point of high stress concentration. Stress concentration can result from weld flaws, out-

of-plane distortion, fabrication details or fatigue prone details

Three Stages of Fatigue Failure

2. Stable Crack Propagation Crack continues to grow under cyclic loading until it

reaches critical size.

Three Stages of Fatigue Failure

3. Fracture• When the crack propagates to the critical size fracture will

occur.

Fatigue Design ParametersNumber of Cycles (75 year life) – 4 considerations• 100,000• 500,000• 2,000,000 • Over 2,000,000 Number of cycles depends on road classification (AASHTO)• Case I more than 2500 ADTT (average daily truck traffic)• Case II less than 2500 ADTTStress Range (thru analysis) • Differences in maximum and minimum live load stresses• Allowable stress range based on number of cycles and

detail type• Note lane loading is less severe than truck loading (single

truck load is the load that causes the worst fatigue condition)

Stress Range• Live Load Stress Range• Fatigue is only considered for tension or stress

reversal situation.• Tensile portion of stress cycle drives or

propagates the fatigue crack, no matter how small the tension component

• No test specimen lost their load-carrying capacity as a result of compression cracks

Fatigue Design ParametersFatigue Detail Types (AASHTO)• Assignment of Stress Categories for various detailsRedundancy• Different allowable for non-redundant and redundant

membersTest were made to determine allowable stress range for

various details and plotted against number of cycles• Allowable Stress v. number of cycles S-N Curves• If computed stress range is less than the allowable stress

range = infinite life for the detail