Variability of the flexural strength of sheet moulding compounds

Sheet moulding compound (SMC) is a composite material consisting mainly of unsaturated polyester resin and long discontinuous glass fibres. SMC combin es good mechanical properties with rapid compression moulding processabilit y. Recently, improved grades with an increased glass fibre content have bee n introduced. They offer even better mechanical properties and therefore ha ve a large potential for application in load-bearing structural components. The safe application of SMC in critical structures requires insight into t he variability of the material strength in order to guarantee a sufficientl y low risk of failure, even for large numbers of constructions. In this pap er, the variability of the strength of SMC is investigated in great detail. Several sources of strength variability observed in a large number of spec imens 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 observati on of tested specimens allowed conclusions on several special contributions to the failure processes (special causes). The results are discussed in te rms of mechanics and statistics. The intrinsic scatter of the SMC material strength was estimated. Consequently, it could be concluded that high desig n stress levels are potentially allowable if such special causes for contri butions to the failure process are excluded. Furthermore, it was observed t hat specimens with a larger width exhibit a significantly larger strength a nd lower scatter. This positive effect of size on strength is contrary to c onventional expectations. However, it can be explained qualitatively in ter ms of mechanics. Moreover. it is a favourable trend towards the behaviour o f large practical structures. All together, it appears therefore, that the potential of SMC as a high-performance construction material is significant . (C) 1999 Elsevier Science Ltd. All rights reserved.