Issue 49

R.V Prakash et alii, Frattura ed Integrità Strutturale, 49 (2019) 536-546; DOI: 10.3221/IGF-ESIS.49.50 537 visible, effective in-situ NDT methods are important; equally important is the need to understand damage progression in such materials under fatigue loading for effective life prediction [1]. In order to provide practical information on the damage state of FRP composite materials, a number of non-destructive methods that are well-established can be employed [2-4]. The use of full field non-contact measurement technique, such as, the infrared thermography (IRT) for the in-situ detection of fatigue damage in materials has recently been established as a valid non-destructive evaluation (NDE) technique. It is based on the fact that fatigue damage is an energy dissipative process that is accompanied by the temperature variation. The measurement of temperature variation during mechanical loading is referred to as passive thermography as no external thermal activation is provided to the specimen. If an external heating is invoked and the temperature variation with time is studied, it is referred to as active thermography. John Montesano et al. (2013) conducted fatigue studies on carbon fiber-polyimide resin composite and stated that the high cycle fatigue strength determined using the thermographic approach showed excellent correlation with that of conventional stress-life curve. The authors state that IRT is quite successful in detecting damage initiation and growth; hence, it is an effective technique for assessing the fatigue damage progression in components made of composite materials [5]. Kristine M. Jespersen et al. (2016) used multi-scale 3-D X-ray Computed Tomography (CT) technique for understanding the damage evolution under tension-tension fatigue loading for a uni-directional (UD) composite made from a non- crimped glass fiber fabric used for wind turbine blades. They noted that 3-D tomography technique can be used to detect certain damages, which cannot be determined from 2-D destructive analyses [6]. The recent advances in the X-ray CT aid the researchers in detecting and characterizing damages in the fiber reinforced composite materials in the three dimensions [7,8]. In this study, the post-impact, post-fatigue damage is evaluated using active IR thermography and the cooling response is compared with damage quantified through X-ray CT images as well as with the loss in stiffness of the specimens due to the impact and fatigue loading. E XPERIMENTAL METHODOLOGY FRP laminates of size 300 mm x 300 mm with 4.5 mm thickness were prepared by the hand layup process coupled with compression molding technique using carbon woven roving mat as reinforcement and epoxy as the matrix with the quasi-isotropic stacking sequence of [0#90, ±45, 0#90, ±45]s. The carbon fiber mat with density of 480 g/m 2 (gsm) had 90 numbers of yarns for 300 mm of length and which is equal in both warp and weft directions. The carbon fiber had a tensile strength of 3450 MPa and a co-efficient of thermal expansion of -0.4 µm/m/ o C. A commercially available Araldite® LY556 (an unmodified liquid epoxy resin based on Bisphenol-A) is chosen as matrix and the curing agent is Aradur® HY951 (an unmodified aliphatic polyamine – tri-ethylene tetra amine). The specific gravity of the carbon fiber and the epoxy resin used for the study are 1.8 and 1.3 respectively. It may be noted that this resin in general, provides high stiffness and strength to the CFRP laminate, but, it often behaves in an undesirably brittle manner when the plastic deformation is constrained, like for example, an impact loading. The tensile specimens as per the ASTM 3039 standard having a nominal dimension of 25 mm x 250 mm were extracted from the laminate. The fatigue specimens having nominal dimensions of 45 mm x 250 mm as well as ones with reduced width of 35 mm at the hour-glass section were cut using a CNC router. The use of hour-glass specimen ensures concentration of damage and stress at the central region of the specimen during loading. Fig. 1 presents the photograph of one such CFRP laminate prepared and some of the specimens prepared thereafter from the laminate. The tensile and fatigue tests were conducted using a 100 kN MTS servo-hydraulic testing machine. Prior to fatigue testing, the tensile tests as per ASTM D 3039 standard were conducted on un-impacted and 35 J impacted laminate specimens for determining the load range for the fatigue cycling. The static strength tests for the un-impacted and post-impact and post- fatigue specimens were conducted with a constant cross head speed of 2 mm/min. The eight layer quasi-isotropic specimen had an ultimate tensile strength of 337 MPa (with a standard deviation of 16.5 MPa and coefficient of variation of 4.9 %) and a tensile modulus of 42.53 GPa. The tensile strength for a 35 J impacted quasi-isotropic CFRP specimen reduced to 126 MPa. The fatigue specimens were impacted with drop weight impactor to three different energy levels of 23 J, 35 J and 51 J. Fatigue tests were then conducted under: a) constant amplitude (CA) load cycling with the maximum load of 8.82 kN and the minimum load of 0.882 kN (thus, at a stress ratio of 0.1), and b) under programmed-FALSTAFF spectrum loading [9- 11] which is a variable amplitude (VA) with the maximum load of 8.82 kN. The programmed- FALSTAFF spectrum represents 200 flights of an European Standard fighter aircraft fatigue loading and consists of 1242 cycles in total which includes 18 major cycles. The balance, 1224 cycles are divided into 18 identical blocks called as marker blocks (minor C