Infrared techniques for quantitative evaluation of interfacial fracture behaviour and damage tolerance in sandwich structures

Abstract

An important failure mode of foam cored composite sandwich structures is the debonding between the face sheet and core. The challenges in establishing and improving the damage tolerance of sandwich structures mainly arise from the lack of reliable methods to characterise the fracture behaviour at face sheet/core interface. It has been demonstrated that the inclusion of crack arresting devices can improve damage tolerance. Characterisation of the fracture behaviour at tri-material interfaces is essential to evaluate the efficiency of such crack arresting devices, which are embedded in the foam core. Thereby, the research reported in the thesis seeks to establish optical measurement methods for characterising the stress fields at the crack tip and the crack tip parameters (i.e. fracture toughness) for foam cored composite sandwich structures, to provide a better understanding of the fracture behaviour at bi/tri-material interfaces. Thermoelastic stress analysis (TSA) is used to establish the stress state at interfacial cracks. In the vicinity of growing interfacial cracks large and discontinuous motion occurs that has a deleterious effect on TSA, therefore a new motion compensation (MC) method is required. The thesis describes how the MC was developed and demonstrates the technique is essential for quantitative evaluations in the neighbourhood of a growing crack. Thus, the full-field stress state at an interfacial crack is obtained reliably and with a high spatial resolution.

An experimental method based on high speed infrared (IR) thermography is defined that enables the characterisation of fracture toughness by measuring the increase in temperature at the crack front (?T) during crack propagation. It is demonstrated that IR thermography with 15 kHz frame rate can be used to conduct a quantitative measurement of the crack front temperature associated with the crack growth. A constant of proportionality, ?, is derived that links the temperature change per unit area at the crack front to the fracture toughness. It is shown that the ? values obtained from specimens with the same interface are identical, even though the specimen dimensions and loading mode-mixities are different. It is demonstrated that by determining the values of ?, the fracture toughness can be determined in any loading configuration by a direct temperature measurement.

The efficiency of crack arresting devices or ‘peel stoppers’ is established using TSA. The efficiency is defined as the ability of the peel stopper to deflect the crack from the face sheet/core interface and arrest its growth. To enable effective crack deflection away from the interface, modifications of the initial peel stopper design are proposed. TSA identified the local stress concentrations introduced by the different peel stoppers and established the mechanisms that control crack propagation in the vicinity of the peel stoppers. Thereby defining a configuration that contains the face sheet detachment in mode I dominated loading. The work described in this thesis develops the optical measurement methods, based on imaging, to address the challenges in understanding interfacial fracture mechanisms and the determination of the crack tip parameters. This provides an important contribution to the understanding of the mechanics of interfacial fracture in foam cored composite sandwich structures.

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