Abstract:Thermoelastic Stress Analysis is used to investigation two damage types in composite materials, namely delamination and fibre breakage. The 'damage' is introduced into the material at the manufacturing stage using PTFE patches to model delamination and by cutting fibres to model breakage. Both Glass Fibre-Reinforced Plastic (GRP) and Carbon FibreReinforced Plastic (CFRP) specimens were tested using an Instron 8800 servohydraulic test machine, and the Deltatherm TSA equipment was used to obtain full-field image… Show more
“…As a consequence, it is possible to qualitatively and quantitatively identify the localized damage status by analyzing the variations of the local stress, dissipation, and temperature signals during fatigue evolution process. 11 Figure 12 illustrates the distribution of the surface intrinsic dissipation and temperature signal for the above specimen under σ anom = 190 MPa. It is obviously shown that the intrinsic dissipation and temperature gradients are very high around the blind hole.…”
Section: Results and Analysismentioning
confidence: 99%
“…All the stress, dissipation, and temperature signals are useful for the assessment of the localized stress state and damage status. 11 By analyzing the variations of these signals, it is possible to rapidly predict fatigue parameters of materials and components. 28 Figure 4.Successive stepwise loading procedures.…”
Section: Methodsmentioning
confidence: 99%
“…10 Through the local stress analysis, it is possible to identify the localized damage status in real time. 11…”
The relationships between the stress concentration factors K t s to the depths h and diameters È of blind holes were investigated based on the finite element method. The E-Mode, built-in the lock-in thermographic technique, was used to determine the stress distribution around blind holes and to predict the variation of K t s. Good predictions were achieved between the thermography and finite element method. The Altair Li software and Altair software were, separately, used to study the variations of the intrinsic dissipation and temperature increment during fatigue process for the purpose of damage status analysis. These two signals were considered as fatigue damage markers to rapidly predict the fatigue limits of components with different blind holes and then to determine the fatigue notch coefficient K f . Theoretical results validate the capability of the lock-in thermography for fatigue evaluation.
“…As a consequence, it is possible to qualitatively and quantitatively identify the localized damage status by analyzing the variations of the local stress, dissipation, and temperature signals during fatigue evolution process. 11 Figure 12 illustrates the distribution of the surface intrinsic dissipation and temperature signal for the above specimen under σ anom = 190 MPa. It is obviously shown that the intrinsic dissipation and temperature gradients are very high around the blind hole.…”
Section: Results and Analysismentioning
confidence: 99%
“…All the stress, dissipation, and temperature signals are useful for the assessment of the localized stress state and damage status. 11 By analyzing the variations of these signals, it is possible to rapidly predict fatigue parameters of materials and components. 28 Figure 4.Successive stepwise loading procedures.…”
Section: Methodsmentioning
confidence: 99%
“…10 Through the local stress analysis, it is possible to identify the localized damage status in real time. 11…”
The relationships between the stress concentration factors K t s to the depths h and diameters È of blind holes were investigated based on the finite element method. The E-Mode, built-in the lock-in thermographic technique, was used to determine the stress distribution around blind holes and to predict the variation of K t s. Good predictions were achieved between the thermography and finite element method. The Altair Li software and Altair software were, separately, used to study the variations of the intrinsic dissipation and temperature increment during fatigue process for the purpose of damage status analysis. These two signals were considered as fatigue damage markers to rapidly predict the fatigue limits of components with different blind holes and then to determine the fatigue notch coefficient K f . Theoretical results validate the capability of the lock-in thermography for fatigue evaluation.
“…The results are shown in Figure 8 for the UD material and Although it has been shown that temperature dissipation between plies in specimens loaded in uniaxial tension can be neglected in E-glass / epoxy specimens even at low loading rates [5], large stress gradients exist not only between plies, but also within the surface ply; the stress in the outer surface of the LAM specimen is 25 % greater than the stress at its inner surface.…”
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 ...
“…In previous experimental studies [10] an approach to simulate delamination damage, with predetermined location and size, has been devised. This method used PTFE inserts introduced in the laminate at the manufacturing stage.…”
The current paper presents a new combined technique for damage detection, localisation
and establishing damage severity. The technique is based on the use of two infrared detection
approaches: Pulsed Phase Thermography (PPT) and Thermoelastic Stress Analysis (TSA). A
methodology is described that allows the technique to be applied to fibre reinforced composites.
The usefulness of the technique is demonstrated on delaminated glass epoxy laminates. A means of
developing controlled delamination damage in such specimens is developed so that real sub surface
damage is evaluated in the paper.
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