DESIGN OF LAMINATES 



6-5 



REFERENCE 7 



INI Tl AL MODULUS LI NE 

 PRELOAD MODULUS LINE 

 SECONDARY MODULUS LINE 



OAD POINT 



. 002 



STRAIN - INCHES PER INCH 



a. SINGLE PROPORTIONAL LIMIT 



.002 

 STRAIN - INCHES PER INCH 



b. DUAL PROPORTIONAL LIMITS 



Fig. 6-3. Typical Tensile Stress-Strain Curves 

 for Fiberglass Reinforced Laminates 



tension, less noticeable for flexure and practically nonexistent for compression. Parallel 

 laminates made with unidirectional reinforcement do not appear to indicate this dual 

 characteristic when stress is applied parallel to the warp. 



Preloading above the initial proportional limit modifies the lower' straight line portion 

 of the curve to the point of preload, P 1 , Fig. 6-3b, and the slope is altered to an inter- 

 mediate value between the initial and secondary slopes of the preload curve. As the pre- 

 loading is increased to the second proportional limit the initial straight line portion of the 

 curve disappears and a new single straight line portion is obtained to the secondary propor- 

 tional limit. Repeated preloading to the same stress level appears to cause no addi- 

 tional change. 



This dual proportional limit characteristic, similar to the clad aluminum alloy pre- 

 viously discussed, is believed to be due to the difference in the individual strengths of the 

 combined materials. The initial break in the curve probably occurs at failure of the resin 

 and the entire load is transferred to the fiberglass reinforcement. The effect of preloading 

 beyond the initial break may then simply be the behavior of the fiberglass reinforcement 

 with additional failure or cracking of the resin as the stress level is increased. 



If a 0. 2 per cent offset line is drawn to obtain an apparent yield stress for fiberglass 

 laminates, Figs. 6-3a and 6-3b, it will intersect the curve well above the proportional limit 

 and for most laminates will be very close to or beyond the ultimate stress. It is obvious 

 that establishing a yield stress at 0.2 per cent strain offset as a basis for design safety 

 factors is quite impractical and should not be done. Therefore it is recommended that 

 factors of safety be based on the ultimate stress of the laminate. 



Directional Characteristics 



Homogeneous materials such as steel and aluminum can be assumed as isotropic and 

 as having the same physical characteristics at any angle or direction. This is not true for 

 all fiberglass laminates; the exception being mat reinforced laminates which are considered 

 homogeneous as discussed in Chapter 5. Fiberglass cloth and woven roving when used as 

 reinforcement in laminates generally produce orthotropic materials. These laminates will 

 have different ultimate strengths at various angles to the warp direction and must be analyzed 



