Stress and damage assessment in woven composite materials by means of thermoelastic stress analysis

Abstract

The work described in this thesis considers the application of thermoelastic stress analysis (TSA) to assess stresses and damage in woven composite materials. Woven composite materials offer high specific strength and stiffness, while being well suited to low cost manufacturing techniques. This makes them a cost effective material for weight critical structural applications. The weave, however, introduces stress concentrations at the meso-scale which are critical to damage initiation. Experimental techniques are therefore required to assess the severity of stress concentrations and damage.
TSA is an infrared (IR) technique which uses the thermoelastic effect to obtain measurements related to the stresses within a material. The non-contacting nature of TSA make it ideal for studying components with non-uniform stress fields. In this work, a new IR detector system for TSA is introduced which provides radiometric calibration, high frame rates and a motion compensation routine, essential for studying the thermoelastic response at small scales. This has enabled TSA to be conducted at the scale of the individual yarns in woven composites.
A simple model has been used to predict the thermoelastic response from individual yarns. This has revealed that careful determination of the material properties is critical for accurate predictions, and that the use of literature values, as has been done in the past, can lead to misleading results. Thus it is shown that the response from a woven composite originates from the yarns, rather than a surface resin layer, and that the non-uniform strain field manifests itself strongly in the TSA data.
The work then investigates the development of fatigue damage in woven composites. This has shown that damage can initiate at stress levels as low as 10% of the ultimate failure stress in single ply composites. Using the high resolution optics and motion compensation it has been possible to follow the development of matrix cracks in individual yarns. A signature pattern in the TSA data is defined that enables the matrix cracks to be clearly identified.
For TSA to be applied as a tool for non-destructive testing of in-service structures, it is essential that simple procedures are developed and that the equipment is portable.To facilitate the more widespread uptake of TSA, the feasibility of using a simplified means of introducing a load into a component was investigated. It was demonstrated that a single transient excitation can be used to obtain a TSA measurement.
The work described in this thesis thereby demonstrates that TSA can be applied to study stresses and damage in inhomogenous materials. The feasibility of using a simplified loading methodology is proven. The study thereby represents a significant step towards an improved understanding of TSA and increasing its application range.

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ORCID for S. Quinn: orcid.org/0000-0002-9727-5080

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