5-26 



ENGINEERING PROPERTIES OF LAMINATES 



Fatigue 



When certain materials are subject to cyclic or repeated loads, fatigue failure can occur 

 at stresses below the ultimate static strength. Fatigue failure can be due to the reversal of 

 a tensile, flexural or torsional stress in a member. Reversal of stress can occur with or 

 without an initial or "mean" stress. If a mean stress is present in a system that undergoes 

 stress reversals, the stress at failure is further reduced. In other words, when a member 

 is preloaded and then subjected to stress reversals, the fatigue strength of the member is 

 less than the strength of the same member without preload. Fatigue failures are usually 

 propagated by cracks in high tensile areas and can be accelerated by initial cracks, flaws, 

 discontinuities, holes, notches, etc. 



Fiberglass laminates are subject to fatigue failures. However, because of the nature 

 of their composition, it is difficult to pinpoint what type of failure actually predominates, 

 that is, tensile, shear or delamination. 



Figs. 5-7 and 5-8 present SN curves or fatigue strengths as per cent of ultimate 

 strengths for mat-polyester and 181-136 cloth-polyester laminates in the dry condition, 

 tested parallel to warp and at 73 degrees F and 50 per cent relative humidity. Similar data 

 for 10 ounce cloth and woven roving is presently unavailable. These curves indicated that 

 both tensile and flexural fatigue strengths of fiberglass polyester laminates tend to level off 

 at approximately 20 to 30 per cent of their ultimate strengths at 10 million cycles (9-16). 

 This stress level defines the fatigue limit or the value at which the material can undergo 

 stress reversals for an indefinite period. 



CYCLES TO FAI LURE 



Fig. 5-7. Tensile Fatigue Strength of Fiberglass 

 Polyester Laminates 



