Damage development and post-impact performance of composite overwrapped pressure vessels subjected to low velocity impact

Abstract

In this thesis high pressure Type III gas cylinders, consisting of an aluminium liner overwrapped in carbon fibre reinforced plastic (CFRP) and glass fibre reinforced plastic (GFRP), have been investigated. Whilst these structures are principally designed from a gas containment perspective (i.e. burst pressure), in service they may be exposed to undesired out-of-plane loading events, such as low velocity impact. Such events are of primary concern due to the capacity for impact damage to critically compromise the performance of the structure. It is essential to improve the understanding of the impact response and consequent effects on cylinder performance to support continued improvement of Type III gas cylinder design.

A series of experimental investigations using industrially manufactured full-scale Type III cylinder specimens and micro-focus computed tomography (μCT) have been conducted. This has enabled a level of detailed impact damage assessment that has not been presented in the open literature in these structures previously. It is shown that the impact response is a complex combination of multiple damage mechanisms including: plastic deformation of the aluminium liner, interface failure between liner and CFRP and matrix cracking, delamination and fibre fracture in the composite layers.

Via careful integration of loading and imaging strategies a novel description of the damage initiation and progression in a Type III gas cylinder throughout out-of-plane loading is presented. It is demonstrated that quasi-static loading rates can be used as an analogue for low velocity impact in Type III gas cylinders both in terms of the force-displacement response and the damage developed in the different material systems above the threshold of damage initiation. It is also demonstrated that the structural and damage response under LVI exhibits similar behaviour across the different cylinder sizes tested in this thesis, indicating that with appropriate consideration, smaller cylinder designs may be used as scale models for impact testing.

With respect to the post-impact fatigue performance, the weight of evidence across the experimental study and complementary finite element modelling strongly supports the conclusion that plastic deformation of the liner is the most critical damage mechanism relating to fatigue performance after impact. Accordingly, the prevention of, and reduction in the resulting depth of a liner dent is considered to be most effective approach to improving post-impact fatigue performance in Type III gas cylinders.

Overall this thesis makes a significant contribution to the understanding of the damage development and post-impact fatigue performance of Type III gas cylinders subjected to low velocity impact, providing valuable insight to cylinder designers and the wider gas cylinder industry. It also contributes to the knowledge regarding impact testing and damage inspection practices for Type III gas cylinders.

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ORCID for Philippa Reed: orcid.org/0000-0002-2258-0347

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