5-34 ENGINEERING PROPERTIES OF LAMINATES 



When exposed to unfavorable environmental conditions fiberglass laminates can be ad- 

 versely affected. To minimize these effects, fiberglass laminates should be properly 

 selected and adequately protected with a suitable gel coat. 



Fiberglass laminates under long term continuous loading will exhibit a reduction in 

 strength properties. This is a characteristic of the material that must be considered in 

 design and proper stress levels must be selected to assure satisfactory long term service. 



Shop Practice 



The physical and mechanical properties of a laminate can be considerably reduced by 

 fabrication defects such as voids, wrinkles, delamination, washing, resin dryness or 

 richness, crazing and foreign inclusions (7). Also shrinkage of the resin and temperature 

 changes that occur during curing cause residual stresses in fiberglass laminates affecting 

 their mechanical properties. These effects can be minimized with properly controlled 

 curing cycles. 



Voids: The causes and effects of voids in laminates have been previously discussed in 

 this Chapter. Excessive void content can be eliminated by careful lay up and handling 

 technique, particularly in contact or hand lay up molding. 



Wrinkles: Wrinkles in a laminate are caused by the careless handling of the plies of 

 reinforcement during the lay up and molding process. A wrinkle between the plies of a 

 laminate causes a weak area in the interlaminar bond and reduces the mechanical strength 

 properties of the laminate. A wrinkle in the reinforcement causes a change in direction of 

 an applied stress which is detrimental to the over-all strength of the laminate. 



Delaminations: Lack of proper contact between adjacent plies in a laminate during cure 

 results in interlaminar separation. The area of separation can be a void space or can be 

 filled with excess resin. Either of these conditions results in a weak spot in the laminate. 

 This type of defect does not readily occur in laminates cured under pressure but can result 

 from careless lay up in the contact method. 



Washing: The equal distribution of fiberglass mat, preform or chopped strand reinforce- 

 ment can be disturbed by the relative movement of matched mold surfaces. This will cause 

 weak spots in the laminate due to the separation and uneven distribution of the reinforcement. 

 The effect of washing can be largely eliminated by careful mold design and handling of rein- 

 forcements during the molding process. 



Resin Dryness: Laminates are resin dry when made with insufficient or unequally 

 distributed resin. Resin dryness results in inadequate bonding of the fiberglass reinforce- 

 ment causing excessive voids or porosity in the laminate resulting in low wet strength retention. 



Resin Richness: Excessive amounts or uneven distribution of resin in a laminate can 

 cause resin rich areas. These resin rich areas are subject to cracking and reduce the 

 physical properties in a laminate due to the lack of adequate reinforcement in these areas. 



Crazing: Crazing is the formation of tiny cracks through the body of a resin due to rapid 

 or excessively hot curing conditions. Resin rich areas or heavy unreinforced gel coats of a 

 laminate are subject to crazing. Gradual deterioration in crazed areas of a laminate can 

 occur when subjected to weather and moisture. Laminates made with rigid polyester resins 

 have a greater tendency to craze than laminates made with semi-rigid resins. 



