ENGINEERING PROPERTIES OF LAMINATES 5-9 



Below each average value, a lower limit, LL, is also given. It can be taken as assured 

 that 95 per cent of the panels from a single fabricator who can maintain the average given, 

 will have values of the property above this lower limit. 



Entry into a table's high or low range should be made on the basis of qualification tests. 

 Basic testing of samples from several large panels for tensile, flexural and compressive 

 strengths and moduli, and per cent glass would enable a fabricator to determine his laminate 

 quality. Application of the tables could be made on the basis of a few such fixed points. A 

 fabricator producing laminates with high tensile and flexural strengths and moduli would 

 design from the high range values of the table. Conversely, a fabricator obtaining values 

 in the low ranges would necessarily design to that range. Fabricators with improved shop 

 practices and rigid quality control can undoubtedly produce the higher strength lami- 

 nates consistently. 



PHYSICAL PROPERTIES 



Thickness and Weight 



The importance of laminate thickness and its effect on physical properties has been 

 previously discussed and should be carefully controlled during the molding process. This 

 is particularly true for contact molding where variation in thickness can occur more readily 

 than with pressure molding. Table 5-2 presents the relationship between the number of plies 

 and laminate thickness, and Table 5-3 presents the relationship between the number of plies 

 and laminate weight. Since the variation in laminate thickness and weight for all fabricators 

 was not as significant as the variation in strength properties, over-all averages were con- 

 sidered sufficiently reliable. In addition to the average values, the ranges of average 

 variability in thickness and weight are also given. 



Glass Content and Specific Gravity 



Glass content is controlled by the type of reinforcement and molding method used to 

 make the laminate. The specific gravity of a laminate is dependent upon the glass content 

 and, to a lesser degree, upon the voids that may be present in the laminates. Table 5-4 

 presents per cent glass by weight and volume, and specific gravity for the laminates evaluated. 



Void Content 



Fiberglass laminates often have small voids or bubbles which detract from their 

 strength properties (7). Voids are formed chiefly by entrapment of air during lay up of the 

 resin and reinforcement and by release of volatile components from the resin during cure. 



Laminates made by the contact molding method tend to have a higher void content than 

 laminates molded under pressure. Sufficient pressure on a laminate during molding can 

 force out a large portion of the air bubbles. Although voids cannot be eliminated entirely 

 from laminates made by the contact method, careful fabrication techniques can be used to 

 keep them at a minimum. Laminates that are essentially void free can be made with light 

 cloth reinforcement by the contact method. 



Air bubbles can be worked out of a lay up to a considerable degree by use of rollers 

 and other implements. Inclusion of air bubbles in the resin during mixing or stirring 

 operations should be avoided. Resin cure temperatures should be kept below the point 



