In recent years, composite materials have become irreplaceable in many industry sectors such as automotive, aerospace, high performance sports goods or wind energy. However, their potential for light weight constructions has to be weighed against the high manufacturing cost of these materials. Thus, the damage and failure behavior of composites must be understood well in order to optimize cost-efficient material usage while still complying with safety standards. [1]
Contrary to homogeneous materials, damage in composites often starts as diffuse matrix cracks without a single dominant crack. Only after the material body is saturated with cracks, delaminations are formed, which ultimately lead to specimen failure. In this contribution, optical characterization techniques are utilized to identify this point of saturation, so that critical delaminations may be detected before catastrophic failure. [2,3]
More specifically, a ±45° laminate of glass fibre reinforced epoxy resin was tested under cyclic loading. Periodically, the fatigue loading was interrupted to test the remaining integrity in quasi-static tensile testing. During these tensile tests, digital image correlation (DIC) was used to measure local strains and capture the decay in the Young’s modulus, E, as well as the Poisson’s ratio, v, see Fig 1. At the same time, an automated algorithm was used to monitor the crack density, which was defined as number of cracks per area.
Results show a change in the slope of E and v at approximately 100.000 cycles, as is shown in Fig. 2. This correlated well with the optical pictures, where the crack density reached a maximum at the same number of cycles. After the maximum the value started to decrease due to the coalescence of small cracks to larger cracks. The assumptions made earlier were confirmed, as the formation and growth of delaminations could indeed be observed from the same point onwards.