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

Fruhmann, Richard Konrad (2009) Stress and damage assessment in woven composite materials by means of thermoelastic stress analysis. University of Southampton, School of Engineering, Science and Mathematics, Doctoral Thesis , 131pp.


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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.

Item Type: Thesis (Doctoral)
Subjects: T Technology > TS Manufactures
T Technology > TJ Mechanical engineering and machinery
V Naval Science > V Naval Science (General)
Divisions : University Structure - Pre August 2011 > School of Engineering Sciences > Engineering Materials & Surface Engineering
ePrint ID: 72146
Accepted Date and Publication Date:
1 September 2009Made publicly available
Date Deposited: 25 Jan 2010
Last Modified: 27 Mar 2014 18:51

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