An investigation into the behaviour of fibre reinforced natural gas powered vechicle (NGV) pressure cylinders under impact loading
Commonly encountered accidental impact, e.g. due to roadway stone hitting, is detrimental not only because it can produce apparent surface defects, but also because barely visible impact damage (BVID) can be induced inside the material, which is not easy to detect by routine inspection. Reliable prediction of the amount of damage of this type induced under known service conditions is particularly important. Therefore, this type of impact was chosen as the focus of the present investigation. A combination of experimental techniques and finite element modelling was used to explore the behaviour of a fibre reinforced natural gas powered vehicle (NGV) pressure cylinder subjected to a low energy impact. In order to identify the modes of failure and understand the structural response, quasi-static indentation tests were carried out on sections of composite pipes and of a composite pressure cylinder. Delamination and matrix cracking were established to be the two major failure modes induced by indentation. Experimental findings were used as a basis for assessing the validity of the modelling approach. Thick shell and three dimensional finite element models were developed using PAFEC, a general purpose finite element code for dynamic and static analysis. It established that the composite pressure cylinder under this type of impact behaves quasi-statically, i.e. the impact phenomenon predominately excites low frequency response. Repeated impact was considered in order to extend the study to include the impact behaviour of a cylinder with pre-existing damage. It was found that a bulging effect was produced in the pressure cylinder at the impact site, where a weak spot was created due to fibre breakage. A fully three dimensional finite element model with static analysis was developed to investigate the damage and material degradation during the BVID phenomenon. The contact pressure distribution based on the Hertzian contact' relationship was applied. Failure mode identification criteria proposed by Hashin (1980) and Chang and Springer (1986) were used to establish the mode and extent of damage in the composite cylinder under quasi-static loading. The predicted failure modes agreed well with the experimental results. Finally, the present study sets out the methodology allowing systematic design of structures having optimal impact tolerance. Based on the findings of this project, suggestions for the improvement of impact resistance of NGV cylinders were given in Chapters ix.