Failure Assessment of Test Coupons: Comparison Between Model Predictions and Test Results

A common requirement for engine structures designed in the aviation industry is the ability to withstand the limit and ultimate loads with a flaw of certain size. Thus, reliable prediction of failure load is of utmost importance. In this paper, predictions from several applicable failure criteria are compared with test results. The tests are carried out on simple coupons with a rectangular cross-section and containing a surface crack. Majority of the tests are carried out under tensile loads and a few under bending loads. The tests correspond to two titanium alloys and two nickel-based super-alloys. The tests span a wide range of temperature (room temp to 700° C), and two forms, namely, cast and forged. In total over 200 test results are obtained and compared with predictions. The predictive models include (i) elasto-plastic J-integral analysis through FEA (ii) Failure Assessment Diagram (R6v3 and SINTAP) (iii) elastic stress intensity factor (LEFM) and (iv) average stress over remaining cross-section. The comparisons demonstrate that the R6v3 FAD method provide a reasonable estimate of the failure load for the test coupon geometry with surface cracks. The average stress approach, though works well for purely tensile loading, cannot cope with bending-dominated loads. On the other hand, predictions from LEFM approach can be non-conservative by up to a factor of two.