LONG-TERM HYGROTHERMAL EFFECTS ON DAMAGE TOLERANCE OF HYBRID COMPOSITE SANDWICH PANELS

A sandwich construction, composed of hybrid carbon-glass fibre-reinfor ced plastic skins and a syntactic foam core, was selected as the desig n concept for a wind tunnel compressor blade application, where high d amage tolerance and durability are of major importance. Beam specimens were prepared from open-edge and encapsulated sandwich panels which h ad previously been immersed in water at different temperatures for per iods of up to about two years in the extreme case. Moisture absorption and strength characteristics, as related to time of exposure to hygro thermal conditions, were evaluated for the sandwich specimens and thei r constituents (skins and foam). After different exposure periods, low -velocity impact damage was inflicted on most sandwich specimens and d amage characteristics were related to impact energy. Eventually, the r esidual compressive strengths of the damaged (and undamaged) beams wer e determined flexurally. Test results show that exposure to hygrotherm al conditions leads to significant strength reductions for foam specim ens and open-edge sandwich panels, compared with reference specimens s tored at room temperature. In the case of skin specimens and for beams prepared from encapsulated sandwich panels that had previously been e xposed to hygrothermal conditions, moisture absorption was found to im prove strength as related to the reference case. The beneficial effect of moisture on skin performance was, however, limited to moisture con tents below 1% (at 50 degrees C and lower temperatures). Above this mo isture level and at higher temperatures, strength degradation of the s kin seems to prevail.