Results of tests to be used for verification of advanced crack growth prediction models

Results of tests to be used for verification of advanced crack growth prediction models

de Koning, A.U.
Schra, L.

National Aerospace Laboratory NLR

1994-07-07

In this report the experimental procedure is described and results are given of tests for verification of advanced crack growth prediction models. Testing involved constant amplitude fatigue loading at R = 0.1 and R = 0.5. In a similar test series loading was interrupted at regular intervals for application of overloads or overload/underload combinations. Also overloads were applied at different fixed percentages crack length increase in combination with basic fatigue loading at R = 0.1. The tests were carried out on 2 mm thick bare 2024-T3 aluminium sheet. Some of the results of the experimental programme are: •An overload of 1.7 times the maximum fatigue stress (Smax) in basic fatigue loading applied at intervals of 1000 cycles increased the crack growth life by a factor of about 20 for testing at R = 0.1 and a factor of 10 for testing at R = 0.5. When the overload was followed immediately by an underload of -0.85 x Smax the gain in crack growth life was reduced by a factor of about 2 both for testing at R = 0.1 and R = 0.5. •An increasing frequency of overload application generally resulted in increased crack growth lives owing to increased crack closure due to increased interaction of peak loads. However, a very frequent overload application resulted in shorter crack growth lives due to the contribution to crack growth by the overload cycles. •The amount of crack extension associated with the overload cycles increased with increasing crack length.

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Nationaal Lucht- en Ruimtevaartlaboratorium

Campus only

NLR CR 94274 L

Aerospace Engineering Reports

report

(c) 1994 National Aerospace Laboratory NLR