Variability of the flexural strength of sheet moulding compounds

Abstract

Sheet moulding compound (SMC) is a composite material consisting mainly of unsaturated polyester resin and long discontinuous glass fibres. SMC combines good mechanical properties with rapid compression moulding processability. Recently, improved grades with an increased glass fibre content have been introduced. They offer even better mechanical properties and therefore have a large potential for application in load-bearing structural components. The safe application of SMC in critical structures requires insight into the variability of the material strength in order to guarantee a sufficiently low risk of failure, even for large numbers of constructions. In this paper, the variability of the strength of SMC is investigated in great detail. Several sources of strength variability observed in a large number of specimens have been recognised and quantified. All specimens were produced in a manner similar to that used in commercial industrial practice. However, an additional registration of panels and specimens, and post mortem observation of tested specimens allowed conclusions on several special contributions to the failure processes (special causes). The results are discussed in terms of mechanics and statistics. The intrinsic scatter of the SMC material strength was estimated. Consequently, it could be concluded that high design stress levels are potentially allowable if such special causes for contributions to the failure process are excluded. Furthermore, it was observed that specimens with a larger width exhibit a significantly larger strength and lower scatter. This positive effect of size on strength is contrary to conventional expectations. However, it can be explained qualitatively in terms of mechanics. Moreover, it is a favourable trend towards the behaviour of large practical structures. All together, it appears therefore, that the potential of SMC as a high-performance construction material is significant.