5 



Engineering Properties 



of Laminates 



Basic engineering properties of reinforced laminates, such as, specific gravity, 

 strength, moduli, Poisson's ratio, resistance to impact, etc. are primarily dependent upon 

 the type and direction of the reinforcement, the resin, the molding method and fabrication 

 technique used (1). These properties can be affected after fabrication by extended exposures 

 to unfavorable environmental conditions and long term loading. 



Production methods with good quality controls can consistently produce high quality 

 laminates. Thoughtful design with the judicious selection of basic materials and molding 

 methods can minimize the adverse effects of environment and long term loading. 



DIRECTIONAL PROPERTIES 



One of the important characteristics of glass cloth and woven roving laminates is the 

 variation of their physical properties with direction (1). This variation of properties is 

 analogous to the difference in the physical properties of wood measured with and across the 

 grain. Materials which exhibit this characteristic are referred to as orthotropic as opposed 

 to isotropic materials, such as, steel and aluminum, whose properties are essentially the 

 same in all directions. 



Fiberglass laminates with chopped strands reinforced throughout are isotropic. Fiber- 

 glass laminates with mat reinforcement throughout are also considered as isotropic when 

 the laminates are thin and the applied stresses are in the plane of the laminate. This 

 assumption neglects the difference in properties in the direction perpendicular to the plane 

 of the laminate. When the laminates are thick and when the applied stresses are not in the 

 plane of the laminates, these differences must be considered. 



Figs. 5-1 and 5-2 illustrate the variation in physical properties of glass fabric lami- 

 nates with direction. Directions referred to are all in the plane of the laminate. Fig. 5-1 

 shows the difference in tensile and compressive strengths and moduli for a 10 ounce cloth 

 polyester laminate with changing direction. Fig. 5-2 gives the same information for a 

 25-27 ounce woven roving polyester resin laminate. The figures show only one quadrant, 

 the others being similar. The figures indicate the perpendicular axes of major strength, 

 degrees and 90 degrees, which are aligned respectively with the warp and fill directions 

 of the fabric. The direction of minimum properties is at 45 degrees. 



This orthotropic characteristic of glass cloth and woven roving laminates has great 

 significance in the orientation of the plies of reinforcement in a laminate and in the stress 

 analysis of these materials (2). A detailed discussion of a directional stress analysis pro- 

 blem is given in Chapter 6. 



5-1 



