Quantifying and visualising change: strain monitoring of tapestries with digital image correlation

A three-year research project at the University of Southampton (2007–2010) investigated whether monitoring techniques commonly used by engineers to assess the strength and durability of materials could be usefully applied to inform the condition assessment of historic tapestries. To date it has not been possible to obtain an objective picture of the overall condition of a tapestry; the study investigated whether it is possible to identify precursors of structural damage. The two techniques, digital image correlation (DIC) and optical fibre sensors, were used to monitor a representative wool fabric, specially woven tapestry samples, a newly woven tapestry, and historic tapestries, both in the laboratory and in situ in a historic house. This study first sets out to answer the question: can DIC be used to monitor and visualize strain in historic tapestries? It is shown that DIC can be used successfully. Secondly, it discusses the map function, a novel development which allowed the monitoring equipment to be moved, so that it could be used in situ in a historic house. Thirdly, it provides further details of the experimental work using optical fibre sensors to confirm the accuracy of the DIC technique

A stress free model for residual stress assessment using thermoelastic stress analysis

Thermoelastic Stress Analysis (TSA) has been proposed as a method of obtaining residual stresses. The results of a preliminary study demonstrated that when Al-2024 plate containing holes that were plastically deformed by cold expansion process to 2% and 4% strain the thermoelastic response in the material around the hole was different to that obtained from a plate that had not experienced any plastic cold expansion (i.e. a reference specimen). This observation provides an opportunity for obtaining residual stresses based on TSA data. In many applications a reference specimen (i.e. residual stress free specimen) may not be available for comparison, so a synthetic, digital bitmap has been proposed as an alternative. An elastic finite element model is created using commercially available software Abaqus/Standard and the resultant stress field is extracted. The simulated stress field from the model is mapped onto a grid that matches the TSA pixel data from a physical reference specimen. This stress field is then converted to a ?T/T field that can be compared to the full-field TSA data. When the reference experimental data is subtracted from the, bitmap dataset the resultant ?T/T field is approximately zero. Further work proposes replacing the experimental reference data with that from specimens that have undergone cold expansion with the aim of revealing the regions affected by residual stress through a departure from zero in the resultant stress field. The paper demonstrates the first steps necessary for deriving the residual stresses from a general specimen using TSA

Application of Thermography for Assessment of Crack Propagation in Composites

This paper presents the preliminary work on the application of Telops Fast m3K infrared camera for strain energy release rate measurement in fibre failure of carbon fibre composites. The technique is shown for quasistatic loading of compact tension specimen and identifying the crack propagation and instantaneous crack lengths using local changes in temperature near the crack tip

COMBINED DIC-INFRARED THERMOGRAPHY FOR HIGH STRAIN RATE TESTING OF COMPOSITES

In this paper, simultaneous high-speed white-light and infrared imaging were conducted to obtain quantitative description of thermomechanical response of the material and to study the formation and propagation of shear localizations from the temperature history

Data-rich characterisation of damage propagation in composite materials

A novel methodology for the synchronised capture of high resolution white-light and infra-red (IR) images during a fatigue test is described. The approach allows digital image correlation (DIC) and thermoelastic stress analysis (TSA) to be applied practically simultaneously without the requirement to pause the cyclic load. The methodology is demonstrated on cross-ply carbon-epoxy specimens that have experienced damage induced by intermediate strain rate loading. Similar undamaged specimens are studied and the results from each compared. Various damage types are identified which include transverse cracking, delaminations and longitudinal splitting. The results are verified using X-ray computed tomography (CT