6-4 



DESIGN OF LAMINATES 



Based on practical limitations of permissible deformation in a structure, an arbitrary- 

 strain of 0.2 per cent has been established for most metals and the corresponding stress at 

 this strain is referred to as the yield stress. This yield stress, point Y on the curves, is 

 taken at the intersection of the stress-strain curve with a line drawn parallel to the straight 

 or elastic portion of the curve, OP, through 0.002 strain at zero stress. For metals, 

 factors of safety are usually based on this yield stress. 



The ultimate tensile stress indicated as point U on the curves in Figs. 6- la, 6- lb and 

 6-lc, is simply the stress at the maximum load based on the original cross-sectional area. 

 It is interesting to note that most metals including steel and aluminum have yield strengths 

 considerably lower than their ultimate strengths and have comparatively large deformations 

 between their proportional limits and ultimate strengths. 



The stress-strain curves for plywood, Fig. 6-2a and wood, Fig. 6-2b, similar to the 

 nonferrous metals curve, Fig. 6- lb, do not have yield points but yield gradually after 

 passing the proportional limit. The deformations between the proportional limits and the 

 ultimate strengths, particularly for plywood, are less than for the metals. If a 0. 2 per cent 

 offset line is drawn in Figs. 6-2a and 6-2b to obtain a corresponding yield strength similar 

 to the metals, Fig. 6-1, it will intersect the curve well above the proportional limit and in 

 some cases above the ultimate strength. Therefore, only the proportional limit and ulti- 

 mate strengths are considered when designing with wood. 



0.002 



STRAIN - INCHES PER INCH 

 a. PLYWOOD - PARALLEL TO FACE GRAIN 



REFERENCE 5 



0.002 

 STRAIN - INCHES PER INCH 



b. SOLID WOOD - PARALLEL TO GRAIN 



REFERENCE 6 



Fig. 6-2. Typical Tensile Stress- 

 Strain Curves for Wood 



The stress-strain curves for fiberglass laminates are generally similar to most 

 structural materials since they exhibit a linear portion followed by a nonlinear portion, 

 Fig. 6-3a. Like some of the metals and like wood no yield point exists. It is important 

 to emphasize that for loading in the direction of warp, the deviation of the upper nonlinear 

 portion of the curve from the lower linear portion is usually small at the point of failure. 

 This limited deviation of the curve indicates that the strain deformations are quite small and 

 the material has low ductility. This low ductility of fiberglass laminates does not allow for 

 stress relief around stress concentration raisers such as notches, holes, reduction in area 

 and sharp angles as the more ductile materials do. Allowance for this low ductility should 

 be made by increased factors of safety for such points. 



In some instances stress-strain curves for fiberglass laminates indicate two separate 

 proportional limits and moduli of elasticity, Fig. 6-3b. This behavior is predominant for 



