Issue 49

R.V Prakash et alii, Frattura ed Integrità Strutturale, 49 (2019) 536-546; DOI: 10.3221/IGF-ESIS.49.50 542 Figure 5 : The temperature response curves for 23 J impacted fatigue loaded specimens. Figure 6 : The temperature response curves for 35 J and 51 J impacted fatigue loaded specimens. Infrared Thermography - Active Thermography The cooling response curves for the post- impact, post fatigue CFRP specimens with impacted surface facing camera (ISF) and impacted surface at back (ISB) with transmission (HB) and reflection (HF) mode were obtained by the active thermography technique. Tab. 2 presents the details of end state damage condition of the hour glass specimens in terms of residual strength, loss in stiffness and damage volume quantified by X-ray Computed Tomography technique. The loss in stiffness due to impact and post-impact fatigue loading was estimated from the end cycle stiffness and is compared with the pristine specimen stiffness. The intra-laminar damage which causes stiffness degradation in the specimens was quantified in terms of damage volume through the digital image processing of the planar 2D image slices that were obtained from the 3D X-ray CT; the damage volume was observed to correlate well with the loss in specimen stiffness. Four such successive images from the impacted surface for the 35 J Hi-Lo specimens from the least width section of hour glass specimen are presented in Fig. 8(a). The distance between successive 2D image slices was chosen as 0.45 mm, which nearly coincides with each layer thickness of the laminate. The cooling response of some of these tested specimens keeping the impact surface at front (ISF) with heating at the back (HB) or heating at the front (HF) configuration is shown in Fig. 8(b). Also shown in the same figure is the cooling response of pristine specimen. The temperature response at the minimum width region (segment A2) is considered for this purpose, as this is the region where the intra-laminar damage is concentrated. Since the specimens were heated to different temperatures for both the transmission and reflection mode, the cooling response was normalized with reference to the initial temperature at the start of cooling and the same is plotted as a function of time. This helps to understand the