Development and validation of a high-velocity soft-body impact test method for through-thickness reinforced composite structures

• Main Author: Cochrane, Alexander D
• Format: Dissertation
• Language: English
• Published: University of Bristol 2020
• Subjects: delamination; gelatine; high-velocity; impact; through-thickness; Z-pins

Laminated composite structures manufactured from pre-impregnated carbon fibre preforms have very high in-plane strength and stiffness. These properties make carbon-fibre composites ideal for withstanding the adverse loading conditions experienced by many aerospace primary load-carrying structures under normal operating conditions. However, such structures are very susceptible to delamination failure in the event of high-velocity impact by a foreign object. This is particularly important for aerospace components where foreign-object damage (FOD) can lead to catastrophic failure of a component due to the low composite material interlaminar fracture toughness. This property depends on the strength of the interfacial layers between plies, which for an unreinforced laminate is governed by the relatively low strength of the highly brittle matrix resin. Lack of fibrous material in the through-thickness direction means this strength is easily overcome by the large interlaminar shear stresses induced by a high-velocity impact event. Z-pin through-thickness reinforcement is an effective solution to this problem by adding fibrous material in the laminate Z-direction to enhance through-thickness impact performance. The ability of Z-pin reinforcement in enhancing the interlaminar fracture toughness of carbon-fibre composites has been well-documented for smaller scales and low strain-rates. However, Z-pins must encounter a delamination crack that is of sufficient scale to invoke large-scale bridging to have maximum efficacy in arresting crack propagation. Existing small-scale test methods do not produce this large-scale bridging effect, and full-scale Z-pinned component testing produces complex failure modes and is often prohibilitively expensive. In this thesis, a new sub-element scale test has been developed which produces large-scale Mode II crack-bridging at high strain-rate in a field of Z-pins. The test was designed using a design-by-analysis approach with the explicit finite element solver LS-DYNA R7.1.3. A cantilevered, tapered plate manufactured from IM7/8552 pre-impregnated carbon-fibre unidirectional material is subjected to impact by a gelatine projectile using a gas-gun. The specimens are produced using a combination of hand layup and autoclave curing with one-sided taper and soft tooling only for simple laminate manufacture. Undesirable additional failure modes such as fibre-failure near the fixture have been completely avoided. Test conditions, configuration and geometry have been well-scaled to produce a large, single delamination near the mid-plane which is driven in both initiation and propagation by structural bending. Experimental data shows that this large delamination crack may be effectively arrested by embedded, normally-aligned carbon-fibre Z-pins via large-scale bridging. Finite element modelling predictions are compared with experimental results and the modelling approach is developed further to establish some predictive ability in terms of predicting Z-pin large-scale bridging behaviour. Novel Z-pin configurations are then assessed using the predictive model.