Issue 49

R.V Prakash et alii, Frattura ed Integrità Strutturale, 49 (2019) 536-546; DOI: 10.3221/IGF-ESIS.49.50 539 cycled specimens was quantified through the image processing of the planar section 2D X-ray CT images before the tensile tests. The intra-laminar damage which is mainly responsible for the strength or stiffness reduction due to impact and subsequent fatigue loading, is measured and quantified in terms of damage volume through the average area method. Details of damage quantification by average area method are available in Ref. 12. Tab. 2 presents the results of stiffness reduction and damage volume fraction estimated for various specimens. It can be seen that the stiffness at the end state of the tested specimens is proportional to the extent of impact and the fatigue loading which is estimated through damage volume. In one case (23 J Lo-Hi) the residual strength is not proportional to the damage contained in the specimen. The reason for that could be the multiple major ply breaks that happened in this case as seen in F-d diagram (Fig. 2 (a)) as well as observed through the passive thermography (to be discussed in the later section – also Fig. 5). Earlier studies on this test case suggested that there is a minimum threshold in impact damage that can cause damage progression during fatigue and 23 J happens to be on the threshold level of impact damage. Table 1 : Test matrix for mechanical and IR thermography experiments. (a) (b) Figure 2 : (a) The load-displacement diagram of the specimens during tensile test; and (b) The stiffness estimated for the pristine as well as post-impacted fatigue loaded specimens from the tensile test. Infrared Thermography - Passive thermography The change in temperature of the specimen due to the mechanical loading (referred to as thermo-mechanical response) was captured online by the IR camera during the tensile loading for the un-impacted specimens as well as post-impact, post-fatigue specimens. The temperature variation due to thermo-mechanical effect during tensile loading at three area segments (A1, A2 and A3) of equal sizes 24 mm x 34 mm each for the pristine tensile specimen of 25 mm uniform width is plotted in Fig. 3 (a). For the impacted specimens, three area segments (A1, A2 and A3) of equal size 24 mm x 34 mm (width of hour glass specimen at critical section) were chosen for temperature measurements. The temperature increase was significant for unimpacted specimen as seen in Fig. 3(a) while the temperature variation was relatively small in the case of post-impact, post-fatigue cycled specimens. Further, the thermo-elastic response (where the tensile loading in Sl. No. Specimen Impact Condition Fatigue Passive Thermograhy Active Thermograhy (for all the four experimental configurations) 1 Pristine - Carried out for all the specimens during static tensile tests. Done 2 23 J Impacted CA, VA (Lo-Hi) - 3 35 J Impacted VA (Lo-Hi and Hi- Lo) Done for both Lo-Hi and Hi-Lo 4 51 J Impacted VA (Lo-Hi and Hi- Lo) Done for Lo-Hi