A new methodology to characterise the elastic properties of polymeric foam core materials at elevated temperatures is proposed.

The thermoelastic response from a woven composite component is complicated because of the variation in material properties across its surface. In this paper the response is interpreted by treating the woven structure as a 'patchwork' of unidirectional cells and considering each in isolation. To make matters as simple as possible, single-ply specimens loaded in uniaxial tension are studied. The experiments show that, even for this simple tensile case, the strain field is non-uniform and the approach fails to produce any correlation with predictions based on treating each cell in isolation. In deriving the predicted response a detailed knowledge of the material properties required for thermoelastic stress analysis (TSA) is necessary. It is demonstrated that the response is sensitive to small variations in certain quantities and this is discussed in detail in the paper. The TSA is carried out using a new system and a description of this together with the motion compensation techniques used in the analysis of the high-resolution data is provided.

Please cite this article as: Fruehmann, R.K., Dulieu-Barton, J.M., Quinn, S., Assessment of fatigue damage evolution in woven composite materials using infra-red techniques, Composites Science and Technology (2010), doi: 10.1016/ j.compscitech.2010 This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting proof before it is published in its final form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain. ACCEPTED MANUSCRIPT 1 ASSESSMENT OF FATIGUE DAMAGE EVOLUTION IN WOVEN COMPOSITE MATERIALS USING INFRA-RED TECHNIQUES ABSTRACTThermoelastic stress analysis (TSA) is used to study the growth of fatigue damage in single and two ply, 2 x 2 twill woven composite materials. Test specimens were subjected to a uniaxial tensile cyclic loading with maximum stresses of 10, 15 and 20% of the ultimate failure stress. The development of fatigue damage locally within the weft yarns is monitored using high resolution TSA. The specimens were subsequently inspected using optical microscopy to evaluate the location and extent of cracks.Cracks were found in the weft fibres, running transverse to the loading direction. It is demonstrated that the lighter weight fabric is more resilient to damage progression. A signature pattern is identified in the TSA phase data that indicates the onset and presence of fatigue damage in the composite material.

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Response to reviewers' commentsWe thank the referees for the time they have spent reviewing our paper and for providing us with some helpful comments which have improved the quality of the paper. We have responded to each comment in turn below and highlighted the manuscript in yellow to indicate where the changes have been made.Reviewer #1: This papers looks at a variation on thermoelastic stress analysis in which instead of using cyclic loading and lock-in amplification either a single step load or an impact load are used. In either case a dynamic stress field is created, and for short durations the associated thermal problem is approximately adiabatic, resulting in measurable temperature field changes. The point of the work is to demonstrate thermoelastic stress analysis as an NDE technique that might complement or compete with flash thermography or high frequency, vibration induced heating of cracks in structures.The paper is OK, but could and should be improved. Figures 3 and 4 should have a time scale on them. 1. Done.It would also be useful to indicate the framing rate of the IR camera for this data.Added at top of page 4.2. There is a difference between the theoretical, TSA, step and impact loading temeperatures, figures 6-9. The authors brush this off as experimental error, but the difference is always there and in the same direction -lower.We do not brush this off as experimental error. Instead we have provided detailed discussion in the original text as follows:On the standard test, this is covered on page 8/9 and centres on the assumed emissivity value. The difference in the scatter in the 2.5 Hz and 20 Hz measurement is explained on page 9.The step loading case in discussed on page 9 and it is highlighted that there is a lower stress limit for which viable readings can be obtained. Response to Reviewer CommentsFor the impulse test (Figs 8 and 9) we say that the difference cannot be attributed to emissivity alone -we highlight potential sources of errors in the clamping arrangement and we mention the possibility of heat dissipation -see bottom of page 10.It is likely that the process is not entirely fast enough to neglect conduction of heat.This does not mean that the method will not work, but perhaps the proposed approach must be coupled to transient heat conduction calculations in order to be quantitative about the measured stress levels.The authors agree with the reviewer's comment that heat transfer is an important consideration when testing at low frequencies, and that this was perhaps not given sufficient emphasis in the original submission. However, in uniaxial tension tests (Figures 6 & 7), where the stress and hence temperature gradients are small (or nominally zero in the UD case), non-adiabatic effects do not provide a sufficient explanation. Hence the experimental errors, for example estimates of the surface emissivity and the material properties (i.e. density, heat capacity, CTE…) are sought to explain the discrepancy (top of page 9). In the bending case (Figures 8 & 9), stress gradients ...