Stress-strain curves epoxies

Shear-stress-shear-strain curves typical of fiber-reinforced epoxy resins are quite nonlinear, but all other stress-strain curves are essentially linear. Hahn and Tsai [6-48] analyzed lamina behavior with this nonlinear deformation behavior. Hahn [6-49] extended the analysis to laminate behavior. Inelastic effects in micromechanics analyses were examined by Adams [6-50]. Jones and Morgan [6-51] developed an approach to treat nonlinearities in all stress-strain curves for a lamina of a metal-matrix or carbon-carbon composite material. Morgan and Jones extended the lamina analysis to laminate deformation analysis [6-52] and then to buckling of laminated plates [6-53]. 

Typical stress-strain curves are shown for the commonly used fiber-reinforced materials fiberglass-epoxy, boron-epoxy, and a representative graphite-epoxy. These curves are not accurate enough for design use ... 

The curves for 3M XP251S fiberglass-epoxy are shown in Figures C-1 through C-5 [C-1]. Curves are given for both tensile and compressive behavior of the direct stresses. Note that the behavior in the fiber direction is essentially linear in both tension and compression. Transverse to the fiber direction, the behavior is nearly linear in tension, but very nonlinear in compression. The shear stress-strain curve is highly nonlinear. The Poisson s ratios (not shown) are essentially constant with values v.,2 =. 25 and V21 =. 09. 

Figure C-5 Shear Stress-Strain Curve for 3M XP251S Fiberglass-Epoxy (Adapted from [C-1])...

Fig. 7. Comparison of stress-strain curves for carbon-fiber-reinforced epoxy composites of different thermal histories. Error rectangles were drawn to indicate a 95% confidence level for both stress and strain...

Fig. 28. Dependence of the compressive stress-strain curves on strain rate of silica filled epoxy [46]...

Kevlar 29 and versions thereof (K 129 and Kl j.) are also used extensively in lightweight body armor as well as composite liners (with vinylester, polyester or epoxy as the matrix). A quick look at the properties of different Kevlar aramid fibers in Table 4.2 shows why K29 is better than K49 for lightweight body armor applications. K29 has a higher strain to failure than K49. That means that the total work of fracture, i.e. the area under the stress-strain curve, is larger for K29 than K49. Hence, the energy absorbed in the fracture process is higher for K29... 

Families of tensile stress-strain curves have been generated for strain rates in the range of 10 - 10 s " at 23°C, for both the epoxy adhesives. These are illustrated in Fig. 4 (a) and (b). The tensile properties were found to increase progressively with the increasing the rate. Calculated mechanical properties are summarised in Table 3. The properties of the aluminium alloys are not significantly affected by the rates considered and may be regarded as rate independent [13], The mechanical properties of the aluminium alloys used in the current research are summarised in Table 4. 

Figure 4. Stress/strain curve for an aligned poly DCHD/epoxy composite.

Figure 12. Predicted elastic-plastic, stress-strain curves for the epoxy annulus. The volume fraction of voids is 20%.

Figure 2.2 Tensile stress-strain curves for different fiber/epoxy and aluminum and steel materials...

The tensile strength of the reinforcement within the concrete is of basic interest for the design of members. Tensile tests on dog-bone shaped specimens were performed to determine tensile strength as well as crack spacings and widths. In Fig. 2, the stress-strain curves are shown for fabric A and B, both coated with an epoxy resin, as well as fabric C which was applied uncoated (Table 2). 

It should be pointed out that no strong evidence exists for chain scission in unoriented polymers or in oriented polymers in, say, compression or transverse tension or compression. Ito proposed an explanation of the yield drop observed in many polymers as due to the stress required to break secondary bonds superposed on a rubber-like stress-strain curve. However, Steg and Ishai in a study of a plasticised epoxy system concluded that there was no significant change in crosslink density on passing through )rield. 

Figure 12.18. Area under flexural stress-strain curves vs. volume percent glass for epoxy composites (Sahu and Broutman, 1972). ( ) CP02 (A) untreated ( ) DC-20 ( ) Z-6076. Code numbers refer to surface treatments used for the glass CP02, a mold release agent DC-20 and Z-6076, coupling agents for improving adhesion.

For the measurement of tensile properties, dumb-bell specimens of the form shown in Fig. 2.13 are suggested. Tensile modulus. Poisson s ratio and elongation at failure may be measured with appropriate strain monitoring equipment and a set of stress/strain curves for a typical range of epoxies is given in Fig. 2.14. Similar... 

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