"FATIGUE FAILURE OF A COMPOSITE SANDWICH BEAM CONTAINING A"
FATIGUE FAILURE OF A COMPOSITE SANDWICH BEAM CONTAINING A CENTRAL NOTCH David T. Fishpool, Emmanuel Guidon, Botshelo H. Maedza, Stephen L. Ogin, Anthony M. Thorne and Brian Le Page Faculty of Engineering and Physical Sciences, University of Surrey, Guildford, GU2 7XH, United Kingdom KEYWORDS: Composite sandwich beam, aluminium honeycomb core, fatigue, flexure INTRODUCTION Composite sandwich beams with honeycomb or foam cores and composite material faces represent a class of materials with particularly good strength-to-weight and stiffness-to-weight properties. The faces can be adhesively bonded to the core or, as in the aluminium honeycomb-core material used for this study, the bond between the faces and the core can be formed by co-curing. While the quasi-static properties of such beams have received much attention, it is only relatively recently that the fatigue properties have been investigated (e.g. [1-3]), sometimes for both undamaged beams and for beams with deliberately introduced pre- damage (e.g. in the form of skin-core delaminations). In this work, the effect of a through- thickness notch on the development of fatigue failure has been investigated. EXPERIMENTAL METHODS AND RESULTS Flexural fatigue tests were carried out on samples of a sandwich panel consisting of GFRP faces (woven glass/epoxy resin) co-cured with a hexagonal aluminium honeycomb core, manufactured by Technical Resin Bonders, Ltd. The specimens had dimensions of 500 mm x 70 mm x 26 mm, with a central through-thickness circular notch having a diameter 15 mm (approximately 3 cell diameters). Samples were fatigue tested under four-point loading at a range of loads with R = 0.1 (R = Pmin/Pmax) and a frequency of 2 Hz. 120 Percentage of quasi-static 100 failure load 80 60 40 20 0 0 1 2 3 4 5 6 Log10 (number of cycles to failure) 90% 85% 80% 75% 70% 65% 60% 100% Figure 1: A plot of the peak fatigue load as a percentage of the static failure load, against number of cycles to failure The S-N curve for the failure of the notched sandwich beams (Figure 1) shows two failure regimes: at peak fatigue loads above 70% of the quasi-static failure load, the failure of the samples occurred by the slow fatigue crack growth of cracks which initiated from both edges of the notch and grew on the compressive face of the panel. At loads below 70% of the quasi- static failure load a ‘sudden-death’ type of failure occurred, with fracture of the skin on the tensile face of the panels, with no obvious prior damage being visible. Failure of the beams with peak fatigue loads above 70% occurred in two stages: slow crack growth followed by catastrophic failure. Figure 2 shows an example of the increase in the combined length of the cracks growing from the notch edges (on the compressive face of the panel) as a function of cycle number. Catastrophic failure of the compressive face of the sandwich panel occurred after the combined crack length was about 20mm – representing a total flaw size of 35 mm when the notch is included (note that the beams were 70 mm wide). Examples of hysteresis loop shape changes, measured with a strain gauge bonded to the tensile face of the panel, are shown in Figure 3. 25 Crack Length (mm) 20 15 10 5 0 0 200 400 600 800 1000 1200 1400 1600 Cycle No. Figure 2: Combined length of the cracks growing from both edges of the notch 1800 1800 1600 1600 1400 1400 1200 1200 load (N) load (N) 1000 1000 800 800 600 600 400 400 200 200 0 0 -200 0 500 1000 1500 2000 2500 3000 3500 -200 0 500 1000 1500 2000 2500 3000 3500 strain (µε) strain (µε) (a) (b) Figure 2: Hysteresis loops for a sample fatigued at 2Hz with a peak load 80% of the quasi- static failure load at a) 0 cycles, and b) 1500 cycles; the beam failed after 1560 cycles). The paper will discuss the interrelationship between the slow crack growth on the compressive face of the beam, the flexural stiffness reduction and the hysteresis loop shape changes. REFERENCES 1. Burman M and Zenkert D “Fatigue of foam core sandwich beams -2: effect of initial damage”. International Journal of Fatigue, Vol 19, No 7, pp563-578, 1997. 2. Kulkarni N, Mahfuz H, Jeelani S and Carlsson L A “Fatigue crack growth and life prediction of foam core sandwich composites under flexural loading”. Composite Structures, Vol 59, pp499-505, 2003 3. Beligardi G, Martella P and Peroni L “Fatigue analysis of honeycomb-composite sandwich beams”. Composites Part A, Vol 38, pp1183-1191, 2007