Fiberglass composite tensile fatigue resistance : fiber surface damage analysis and fatigue resistant fiber coating

Glass fibers are the lowest cost and most widely used man-made reinforcement for composite materials . As a material for structural applications such as wind turbine blades, a major drawback is poor tensile fatigue resistance. This, study has been concerned with identifying the cause of fatigue failures and with finding fiber coatings to improve the fatigue resistance. As part of the study, a test method for determining the fatigue resistance of small strand composites was developed which allows rapid assessment o f the effects of material parameters such as fiber coatings. The fatigue sensitivity obtained with this test was similar to literature values obtained at lower frequencies with larger specimens. The most important feature of the test method is a silicon rubber coated tab which reduces the problem of failures near the edge of the tab. Previous studies of the glass fiber composite tensile fatigue mechanism have postulated that the poor fatigue resistance was due to fiber contact damage. In the current study, Atomic Force Microscopy has been used to analyze glass fiber surfaces. Fibers from composites subjected to fatigue were found to have scratches of significant depth in most cases; no such scratches were detected on fibers from static tensile tests. This result strongly supports the contact damage mechanism. In an attempt to reduce fiber surface damage in fatigue, a graphite particle fiber coating procedure was developed. E-glass fiber composites with graphite particle coatings showed the same static strength but improved fatigue resistance relative to control samples. In the relationship S/So=l-b*Log N, where S is the maximum fatigue stress. So the static ultimate tensile Strength5 and N is the cycles to failure, the fatigue coefficient, b , for strand composites decreased from OTO for as-received fiber strand to 0.071 for coated strand. For fabric reinforced composites, the b value for as-received material was about 0.13 while that for the coated fabrics was about 0.089. In both cases, the coated fiber composite cycles to failure increased by about 100 times as compared with asreceived fiber composites when SZSo=zO.5. Thus, the graphite particle coating was effective in increasing the fatigue resistance without reducing the ultimate tensile strength. The fiber/matrix bond strength was also unaffected, so that material properties dominated by the matrix/interface should not be changed significantly.