ENGINEERING PROPERTIES OF LAMINATES 



5-27 



CYCLES TO FAILURE 



Fig. 5-8. Flexural Fatigue Strength of Fiberglass 

 Polyester Laminates 



Laminates exposed to elevated temperatures and extreme weathering conditions or 

 immersed in water will have reduced fatigue strengths. In most cases, notched laminates 

 fail at lower stress levels, for a given number of cycles, than unnotched laminates. 



Further investigation is required to ascertain whether the effect of fatigue on the 

 strengths of mat, 10 ounce cloth and woven roving laminates for boat hull construction will 

 be of greater or lesser magnitudes due to their lower moduli. 



For design purposes, when fatigue strength data for a specific application is not availa- 

 ble, a low enough stress should be selected to insure that the laminate will withstand the 

 applied loads for the required number of cycles to be expected in the normal life span of 

 the structure. 



Creep 



Creep or deformation of fiberglass reinforced laminates under constant stress is de- 

 pendent on time and temperature. Laminates made of different fiberglass reinforcements 

 and resins will exhibit different creep characteristics (17, 18). 



Figs. 5-9 to 5-14 illustrate tensile and flexural creep relationships of mat, woven roving 

 and cloth polyester laminates at various percentages of the short term ultimate strengths. 

 These curves are for laminates in the wet condition tested at 73 degrees F in water except for 

 the tensile tests on the mat laminates which were in the dry condition and taken at 73 degrees 

 F and 50 per cent relative humidity. Figs. 5-9 to 5-14 indicate that creep increases at high 

 percentages of ultimate stress and with duration of continuous loading. Cloth laminates have 

 lower creep characteristics than either mat or woven roving laminates. 



