ENGINEERING PROPERTIES OF LAMINATES 5-31 



Since the initial strains and total strains indicated in the Figures are relatively small 

 for the lower stress percentages at room temperatures, it appears that creep in fiberglass 

 laminates may be negligible when the load percentages are further reduced to normal design 

 stress levels of 20 to 30 per cent of ultimate stress. When fiberglass laminates are exposed 

 to higher temperatures, creep may be of considerable importance. 



Further investigation of the creep characteristics of commonly used boat hull laminates 

 is required for normal design stress levels and slightly higher temperatures. 



Rigidity 



The flexural rigidity of a material is dependent on the moment of inertia of the section 

 and the modulus of elasticity of the material. With a constant moment of inertia, the 

 flexural rigidity increases with increased modulus of elasticity. Although the modulus of 

 elasticity of fiberglass alone is 10.0 x 10^, present fiberglass reinforced plastics can only 

 attain moduli of elasticity between 0. 5 x 10^ to 5. x 10^. The range of values is dependent 

 on the type of reinforcement, resin and the molding method used. Lack of stiffness when 

 compared to steel and aluminum can be of considerable importance when maximum rigidity 

 is required. But low stiffness can be advantageous where flexibility is desired for impact. 

 Lightweight sandwich construction with fiberglass laminate facings can be utilized where 

 maximum rigidity is necessary. 



SUPPLEMENTARY TEST PROGRAM TO OBTAIN PROPERTIES 

 Purpose 



The types of laminate constructions evaluated by the basic test program do not include 

 all of the many new types recently developed for boat hull construction. This supplementary 

 test program was conducted to determine engineering properties of some of these new types 

 of laminate constructions considered to be the most typical of those employed in the boat 

 industry. Table 5-15 has been developed from the results of this test program. For ease 

 of comparison both physical and mechanical properties are included. 



The values given in Table 5-15 are intended for guidance only and each fabricator should 

 conduct the necessary tests to establish similar properties representative of the laminates 

 made in his plant. 



Materials and Method of Fabrication 



The materials and method of fabrication used were essentially the same as those used 

 in the basic program except for the weights of the mats which were 3/4 and 1-1/2 ounces 

 instead of 2 ounces. The laminates were made by the contact or hand lay up method with a 

 polyester resin and cured at room temperature. The resin was formulated specifically for 

 the contact molding of boat hulls and to provide some elasticity in the cured laminate. The 

 catalyst system used was methyl ethyl ketone peroxide. No special post cure by heat was 

 used to produce improved panels. 



Types of Laminates 



M6 = 1 ply 10 ounce cloth, 1 ply 1-1/2 ounce mat, 1 ply 25-27 ounce woven roving. 



M7 = 1 ply 3/4 ounce mat, 1 ply 10 ounce cloth, 1 ply 25-27 ounce woven roving. 



