Prediction of Onset of Mode I Delamination Growth under a Tensile Spectrum Load

In this investigation, the onset of mode I delamination growth under a standard aircraft spectrum load
sequence, mini-FALSTAFF truncated to contain only tension-tension fatigue cycles is predicted. The
study was carried out on a standard Double Cantilever Beam (DCB) test specimen of Uni Directional
(UD) layup Carbon Fiber Composite (CFC) IMA/M21. Finite element modeling and analysis was
carried out using ABAQUS standard to determine Strain Energy Release Rate (SERR) by Virtual
Crack Closure Technique (VCCT). Both two dimensional (2D) and three dimensional (3D) models
were studied. Using the FE analysis results, an empirical equation was derived for SERR (G) and load
(P) relationship. Further, the load cycles in the spectrum sequence were rain flow counted to separate
individual cycles. For each of the counted cycle, the SERR was obtained and corresponding N onset for
that cycle was estimated from the Constant Onset Life Diagram (COLD) of the material. Linear
damage accumulation law was used to predict the number of spectrum load blocks required for onset
of mode I delamination growth. The prediction was carried out for various reference loads (P ref) and
the corresponding reference SERR (G ref) of the spectrum sequence. Predicted results show that
decreasing the reference stress increases the mode I delamination onset life under spectrum loads.