5 " 38 ENGINEERING PROPERTIES OF LAMINATES 



results in a weight saving and reduced cost. Maintenance costs for special linings and 

 painting are not required for fiberglass tanks. 



Long Term Loading 



As is the case with other structural materials, fiberglass reinforced laminates will fail 

 under long term continuous loading at stresses below the ultimate stress for short term 

 loading (18). Figs. 5-17, 5-18 and 5-19 give tensile, flexural and shear long term loading 

 characteristics for mat, woven roving and cloth reinforced polyester laminates in the wet 

 condition. These graphs show the per cent of short term ultimate stress at which the lami- 

 nates will fail in a given time. For example; a 10 ounce cloth polyester laminate, M3, when 

 continuously loaded at 40 per cent of the short term ultimate tensile stress will sustain this 

 stress for 10,000 hours, Fig. 5-17. 



As indicated by these graphs, mat polyester laminates have higher tensile and flexural 

 strength retention than both cloth and woven roving laminates under long term contin- 

 uous loading. 



The effect of periodic or non-continuous loading has not been investigated. It is felt 

 that periodic loading will not be cumulative and therefore will not have the same effect as 

 continuous loading if the working stress level selected is approximately 20 to 30 per cent of 

 the ultimate stress. 



When selecting fiberglass laminates, designers must carefully consider the loading 

 time intervals as well as the direction and magnitude of the applied loads. The judicious 

 selection of working stress levels to allow for extended periods of loading is necessary to 

 provide an adequate structure. 



The nature and time interval of boat hull loads is such that the effect of long term con- 

 tinuous loading will be negligible in most cases. When extended periods of storage are 

 contemplated for a boat hull, careful distribution of its weight on the supporting structure is 

 necessary to minimize the effect of long term loading. Maximum design loads usually occur 

 only momentarily or for short durations in the life span of a boat. Assuming a straight line 

 extrapolation of these curves, it is believed that for normal design loads which may be 

 periodic, the factors of safety suggested in Chapters 2 and 6 for fiberglass construction will 

 allow working stress levels that will not have a long term loading effect on the laminate or 

 reduce the maximum useful life expectancy of practically all boats. 



PROPERTIES OF LOW DENSITY CORE MATERIALS 



As previously discussed in Chapter 4, numerous types of core materials for use in 

 closed frames and sandwich panels are available. The selection of a core material for boat 

 hull construction depends on its physical and mechanical properties, its location in the hull 

 and its specific function. Different core materials can be used satisfactorily in similar 

 applications with minor variations in performance. In many applications the final selection 

 is based on ease in handling and costs. 



Table 5-17 gives physical and mechanical properties data for the most commonly used 

 low density core materials (23-29). Additional research and development should result in 

 improvement of these core materials. 



Fig. 5-20 present SN or fatigue shear strengths as percentages of ultimate shear 

 strengths, for the various commonly used core materials (30). 



