Felicity ratio to assess strengths of GRP composites

Abstract

Acoustic emission is stress wave emitted by a material when it is deformed either by an external stress or internal process (1). This is a highly potential technique for online monitoring appears to be partially well suited for this type of non-destructive testing. However, before using this technique effectively, it is necessary to establish correlation between measured AE parameters and mechanical strength of the materials, i.e. AE are not generated during one of the important properties of AE is the irreversibility the reloading of a material, until the stress level exceeds its previous value. This phenomenon is widely known as Kaiser effect (2). It has however been found that Kaiser effect is not fully operative specially in case of composites(3.4). The Kaiser effect was really established for metals, but the glass fibre composites having different microstructures and deformation mechanisms, do not behave in the same fashion as isotropic materials (5). If a composite is not held at load for sufficient duration and is unloaded, emission will occur at a load lower than the previously attained. This phenomenon is known as the modified Kaiser effect or Felicity effect (6) and the ratio of load at which AE reoccur compare to the previously attained maximum load is known as felicity ratio(7). Fowler(8) was the first to introduce the term FR although a similar method based on the AE during the loading and reloading has been utilised for the inspection of composite pressure vessels(9). In the present work, AE count was obtained under tensile loading and 3 point bending conditions. The FR was evaluated to establish a relationship between FR and tensile/flexural strengths of GRP composites laminates. The purpose of this work is to assess the strengths of GRP composites, utilising the FR.