Strength of a laminate

To put things in perspective, the effect of a hole on the tensile or compressive strength of a laminate is discussed here in more detail. This is the case of an open hole, that is, a hole that does not have a fastener in it. Such holes are... 

The strength of a laminate is primarily determined by the type and amount of reinforcing used. The specific polyester resin will determine operating temperatures and corrosion resistance. 

The following steps may be used to determine the strength of a laminate ... 

Lamina and laminate strengths are dealt with simultaneously in this document. The strength of a laminate is a function of the strengths of its laminae. 

These values are determined by experiment. It is, however, by no means a trivial task to measure the lamina compressive and shear strengths (52,53). Also the failure of the first ply of a laminate does not necessarily coincide with the maximum load that the laminate can sustain. In many practical composite laminates first-ply failure may be accompanied by a very small reduction in the laminate stiffness. Local ply-level failures can reduce the stress-raising effects of notches and enhance fatigue performance (54). 

Ciassicai lamination theory is derived in Section 4.2. Then, special stiffnesses of practical interest are classified and examined in Section 4.3. Next, the theoretical stiffnesses obtained by classical lamination theory are compared with experimental results in Section 4.4. In Section 4.5, the strengths of various laminates are predicted. Finally, the stresses between the laminae of a laminate are examined in Section 4.6 and found to be a proba lei causg of delamination of some laminates. 

Figure 4-43 Strength of a Cross-Ply Laminate with M =. 2 (After Tsai [4-10]) Strength and Stiffness for Other Cross-Ply Ratios...

The macromechanical behavior of a lamina was quantitatively described in Chapter 2. The basic three-dimensional stress-strain relations for elastic anisotropic and orthotropic materials were examined. Subsequently, those relations were specialized for the plane-stress state normally found in a lamina. The plane-stress relations were then transformed in the plane of the lamina to enable treatment of composite laminates with different laminae at various angles. The various fundamental strengths of a lamina were identified, discussed, and subsequently used in biaxial strength criteria to predict the off-axis strength of a lamina. 

Fibers are often regarded as the dominant constituents in a fiber-reinforced composite material. However, simple micromechanics analysis described in Section 7.3.5, Importance of Constituents, leads to the conclusion that fibers dominate only the fiber-direction modulus of a unidirectionally reinforced lamina. Of course, lamina properties in that direction have the potential to contribute the most to the strength and stiffness of a laminate. Thus, the fibers do play the dominant role in a properly designed laminate. Such a laminate must have fibers oriented in the various directions necessary to resist all possible loads. 

One of the ways to increase the bearing strength of a joint is to use metal inserts as in the shimmed joint of Figure 7-40. Another way is to thicken a section of the composite laminate as in the reinforced-edge joint in Figure 7-40. 

The highest mechanical strengths are usually obtained when the fibre is used in fine fabric form but for many purposes the fibres may be used in mat form, particularly glass fibre. The chemical properties of the laminates are largely determined by the nature of the polymer but capillary attraction along the fibre-resin interface can occur when some of these interfaces are exposed at a laminate surface. In such circumstances the resistance of both reinforcement and matrix must be considered when assessing the suitability of a laminate for use in chemical plant. Glass fibres are most commonly used for chemical plant, in conjunction with phenolic resins, and the latter with furane, epoxide and, sometimes, polyester resins. 

In general, the compressive strength of a non-reinforced plastic or a mat-based RP laminate is usually greater than its tensile strength. The compressive strength of a unidirectional fiber-reinforced plastic is usually slightly lower than its tensile strength. Room-temperature compressive stress-strain data obtained per ASTM for several plastics are shown in Table 2-5. 

Effect of coupling agents on the flexural strength of a structural epoxy-fiberglass laminate... 

A technique generally applied to characterize and prevent the capping and lamination of a material intended to be compacted is using the brittle fracture index (BFI). The BFI was designed by Hiestand et al. [31] and measures the ability of a material to relieve stress by plastic deformation around a defect. It is obtained by applying Equation (8) and compares the tensile strength of a tablet with a hole in its center (To), which acts as a built-in stress concentrator defect, with the tensile strength of a similar tablet without a hole (T), both at the same relative density ... 

In some instances the heat seal ply of a lamination has to be peelable. These should be checked for peel strength over the shelf life of the product as in some instances the peel strength may reduce and so jeopardise the seal or increase it to a point where a permanent non peelable seal is obtained. The use of pattern lacquers also assists peelability. 

It is important to avoid the use of materials that are intrinsically prone to the occurrence of pinholes. Thinner materials are more prone to pinholes than thicker materials. Single-ply films are of greater risk than laminated or coated films because coatings fill in pinholes and because the probability of pinholes coinciding between two laminants is remote. Lamination may also add to the strength of a material so that pinholes do not occur during movement or as a consequence of pressure differentials in sterilization or transportation. 

Determining the strength of a single ply is useful for two reasons. First, because in limited cases, laminates consisting of a single ply that repeats are used (typically fabric... 

Finally, of great importance are finite width effects. The strength of a composite laminate in the presence of a hole, a crack, or impact damage is drastically reduced as the size of the notch covers a significant percentage of one of the dimensions of the structure. Finite width correction factors have been obtained by Tan for holes [19]. 

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