Varvani-Farahani, A *Ph.D. studentfull professor, University of Waterloo, Waterloo, Ontario*

Topper, TH *Ph.D. studentfull professor, University of Waterloo, Waterloo, Ontario*

Pages: 17 Published: Jan 1999

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**Source: **STP1343-EB

A comparison of the growth of fatigue cracks under constant-amplitude straining and under strain histories having periodic compressive overstrains revealed that the fracture surface near the crack tip and the crack growth rate changed dramatically with the application of compressive overstrains. When the magnitude of compressive overstrains was increased, the height of the fracture surface irregularities was reduced as the increasing overstrain progressively flattened fracture surface asperities near the crack tip. The reduced asperity height was accompanied by a lower crack closure stress and a higher crack growth rate.

The fatigue strength was reduced by a factor that ranged from 1.24 at short lives (10^{4} cycles) to 3.37 at long lives (10^{7} cycles) when periodic compressive overstrains of near yield point magnitude were applied in uniaxial tests. The corresponding reductions in the fatigue strength for shear tests varied from 1.40 to 1.70.

A model of the plastic deformation of the fracture surface asperities at the crack tip (under periodic compressive overstrains) was developed to relate the crushing of the asperities to crack closure and crack growth rate. The model correlates the magnitude of the periodic compressive overstrain, the fracture surface asperity height, and the plastically flattened area to the fully effective strain intensity factor range (Δ*K*^{eff}) and its ratio to the range of strain intensity factor (Δ*K*(^{ε})) obtained from constant-amplitude straining, *U* =Δ*K*^{eff}/Δ*K*(^{ε}).

For both uniaxial and shear fatigue straining, the strain range at a fatigue life of 10^{7} cycles for constant-amplitude straining (Δε^{fl}) and the effective strain range, (Δε^{eff}.^{fl}) at 10^{7} cycles in the strain-equivalent life curve, obtained from strain histories containing periodic compressive overstrains, were used to calculate the *U* ratio (*U=* Δε^{eff}.^{fl} /Δε^{fl}). These values were close to those obtained from the model.

**Keywords:**

fatigue crack growth rate, crack closure, periodic compressive overstrain, fracture surface asperity, effective strain intensity factor range, the ratio of effective strain intensity factor

**Paper ID:** STP15764S

**Committee/Subcommittee:** E08.06

**DOI:** 10.1520/STP15764S