6-2 DESIGN OF LAMINATES 



In the elastic design of steel, the assumption is made that the material is never stressed 

 beyond the proportional limit. Within this range, the material obeys Hooke's Law of Pro- 

 portionality, i.e. the unit stress is directly proportional to the unit strain. Fiberglass 

 laminates are complex structurally and do not always behave according to Hooke's Law, but 

 in the majority of cases this assumption can be made without appreciable error in the results. 



Therefore in the design procedures and theories for laminates or structures composed 

 of fiberglass and resin combinations, the following basic assumptions are considered valid: 



The fiberglass and resin act as a single unit and have equal strains under 

 all loading conditions. 



The material is considered elastic and obeys Hooke's Law. This assumes 

 that the stress is directly proportional to the strain and that the material 

 will return to its original shape when the load is removed. 



Most fiberglass reinforced laminates, because of their layered construction, are not 

 homogeneous. Laminates are built up of a number of layers of the same or different type 

 of reinforcement and each layer may have different physical properties in different direc- 

 tions. The physical properties and structural behavior of a laminate are primarily depen- 

 dent upon the type and orientation of the reinforcement, the resin and the molding method 

 used to fabricate the laminate. 



Since many types of reinforcements and resins are available, there can be many satis- 

 factory types of fiberglass laminates. Further variations in laminates are due to fillers, 

 glass and resin content and laminate thickness. These variable factors can be controlled in 

 some cases by the designer or fabricator. 



The factors under control of the designer include the choice of fiberglass reinforce- 

 ments, resins, dimensions, form and molding method. This gives the designer considerable 

 latitude in designing boat hulls or other structures. In addition, there are a number of 

 factors that cannot be controlled to any extent by the designer. 



The factors beyond the control of the designer include variability of resins and other 

 materials from one lot to another and variations in workmanship and technique. The degree 

 of variability due to these factors may be established by testing methods and allowed for by 

 the designer. The development of quality control methods by resin manufacturers and the 

 use of better fabrication techniques are gradually reducing the effect of these variables. 



The strength of fiberglass reinforced plastics is basically dependent upon the adhesion 

 and the greater frictional resistance between the resin and glass fiber (1). The greater the 

 adhesion and frictional resistance the greater the strength. The maximum theoretical 

 strength that can be developed occurs when the adhesion strength plus the frictional resistance 

 between the resin and glass fiber equals the cohesion strength of the resin. To assist in 

 developing this maximum strength, sizes and finishes are applied to the glass fibers which 

 react chemically with the glass when applied, and later react with the resin during molding. 

 The frictional resistance is developed by the shrinkage of the resin during curing. 



Stress-Strain Relationship 



The mechanical behavior of a material under load can generally be predicted from ob- 

 servations of its stress-strain curve (2,3). The stress-strain curve is obtained by gradually 



