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Composite materials have high in-plane mechanical properties, but are susceptible to out-of-plane impact damage. The use of non-destructive evaluation techniques, combined with mechanical testing, was investigated to characterize the progression of post-impact static and fatigue damage of composite laminates. Quasi-isotropic carbon/epoxy specimens were impacted at energy levels of 35 J and 50 J. The initial damage was characterized using ultrasonic C-scan and thermography. Residual strength testing revealed that the compressive static strength of the test specimens was reduced by over 50%. Digital image correlation was used to characterize the growth of damage and the local strain during compression-compression fatigue tests. Initially, no significant statistical trend could be measured when the fatigue data was plotted as a stress-life curve. When stress concentration factors were used to calculate and plot the local stress amplitude, a correlation with fatigue life was observed. The undamaged fatigue data was altered using damage factors which allowed for post-impact fatigue life predictions.

Complicated structures made of fibre reinforced laminates will suffer the failure of delamination which is the main through-thickness failure mode and is usually caused by the combined through-thickness stress state. Three tests, namely the oblique test, the waisted C-specimen test and the notched beam test, have been developed to reveal the failure activities under the combined stress state of in-plane normal stress, through-thickness normal stress and interlaminar shear, and, as a part of the big project, to establish a 3-D failure map in the coordinates of the three stress components. All tests have been analyzed either numerically by FEM or experimentally by Moire interferometry technique. The experimental results have shown that the failure behaviours of laminates are far more complicated than and very different from the traditional metallic materials which can usually be described by phenomenological failure criteria. The phenomenological criteria attempt has been tried and further confirmed that a full experimental understanding is the most important. Moreover, fractographic analysis by SEM has also been conducted to support the test results. Finally the Weibull statistics method has been used to treat the stress gradient effect in the different test geometries while establishing the failure map. It has proved to be a very efficient instrument.

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No description available.