Long-term durability of CFRP foam core sandwich structures

Abstract

The work presented in this article is directed towards the application of CFRP foam core sandwich structures as primary structures in commercial aviation. With closed cell rigid foams, it is possible to produce comparatively low priced high-integral sandwich components having a complex geometry in terms of a curved and a variable lateral cut. Sandwich structures are offering a good bending stiffness and strength to weight ratio. Thus, they are suited for using in structures which are at risk to fail by buckling (Herrmann et al. Sandwich structures 7: advancing with sandwich structures and materials, 2005). The investigations are focused on a CFRP sandwich structure with polymethacrylimide (PMI) foam core, named ROHACELL Ò RIST. Besides good structural stability at thermal conditions, the foam is characterized by a good strength and stiffness to weight ratio (Seibert, Reinforced Plast 50(1): [44][45][46][47][48] 2006). Primary structures in aircraft applications are exposed to a superposition of in-service loads and environmental conditions. The typical working loads in combination with environmental conditions were investigated. The structure needs a sizing with respect to large temperature changes and influences of humidity. Thus, the time, temperature, and moisture dependency of the mechanical behavior were studied for the single components of the structure and for the composite itself. Therefore, Finite Element Models on macroscopic level were built with reference to the experiments. For each in-service case, the residual stresses arising during manufacturing have to be regarded and quantified (John et al. ECCM14, 2010). During manufacturing, the sandwich structure is cured at 180°C. Due to the different stiffnesses and coefficients of thermal expansion of the foam and the CFRP face sheets, residual stresses are induced by cooling down to service temperature. Among others, some tests were made at laterally closed CFRP sandwich structures with a storage time up to half a year at certain climate conditions. The aging process is not only controlled by external conditions, but also by a rearrangement of molecules, for example, the relaxation behavior of the PMI foam (Gutwinski et al. Verbundwerkstoffe. Wiley, Weinheim, 2009). Another question of the long-term behavior of the CFRP foam core sandwich structure is the characteristic of delamination of the face sheets from the inner core after an impact has occurred. To describe the crack growth behavior of the sandwich structure fracture mechanical principles can be used estimating the damage tolerance. Sandwich specimens with initial damage were loaded up to 3 million mechanical load cycles.

Effect of temperature and humidity conditions on polymethacrylimide (PMI) foam core material and indentation response of its sandwich structures

“…Moisture content of the wet specimens, however, keeps increasing notably faster than the ambient specimens, and it seems to take much longer to reach the saturation state. One-dimensional Fickian diffusion model [29] can be used to simply estimate the amount of moisture diffused into the material over time [18,21,22]. The model gives the moisture content percentage M t of the foam at any time as where D is the diffusion coefficient of the material, M ∞ is the moisture content at equilibrium, h is the thickness of the specimen and t is time.…”

“…The size of the foam specimens was 25 Â 25 Â 17 mm 3 , which gives a cross-section area of 625 mm 2 that complies with standard ASTM: C365/C365M -11a for compression tests of sandwich core materials and ASTM C297/C297M-04 for tension tests. After cutting, the specimens were dried in an oven at 120 C for 24 h or longer to remove any aging shrinkage [22] and to reach dry condition. Dried foam specimens were then adhered to steel jigs using AF 163-2K (3M Co.) film adhesive and cured in an oven for 2 h at 120 C. A foam test specimen adhered to steel jigs is depicted in Figure 1(a).…”

“…Moisture is primarily absorbed into the PMI or PVC foam core materials by diffusion of water molecules into the polymer cell walls. Even though the core in sandwich panels is often totally covered by the face sheets thus preventing direct contact between the core material and humidity in the environment, it still will not stop the moisture from reaching the core as the water can diffuse through the composite face sheets and finally reach the polymer foam core over time [22]. Experimental tests have shown that carbon fibre-reinforced plastic (CFRP) face sheets only slowed down the diffusion process, but the moisture still could reach the core and raise its moisture content depending on the environmental conditions [19,22].…”