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Title: Stress and damage assessment in woven composite materials by means of thermoelastic stress analysis
Author: Fruhmann, Richard Konrad
ISNI:       0000 0004 2678 1706
Awarding Body: University of Southampton
Current Institution: University of Southampton
Date of Award: 2009
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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 oer high specic strength and stiness, while being well suited to low cost manufacturing techniques. This makes them a cost eective 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 eect 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 elds. 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 eld 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 dened that enables the matrix cracks to be clearly identied. 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 simplied 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 simplied loading methodology is proven. The study thereby represents a signicant step towards an improved understanding of TSA and increasing its application range.
Supervisor: Barton, Janice ; Quinn, Simon Sponsor: Not available
Qualification Name: Thesis (Ph.D.) Qualification Level: Doctoral
EThOS ID:  DOI: Not available
Keywords: TS Manufactures ; TJ Mechanical engineering and machinery ; V Naval Science (General)