Epoxy resins stress-strain curves

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]. 

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). 

Epoxy resins. Figure 6 shows the stress-strain curves of Epikote 628 hardened by (a) K61B and (b) polyamide. The curves for preirradiation are indicated by 1. The curves of the specimens after a neutron irradiation of 1.2 x 10 nvt with a y dose of 3.3 X 10 R and of those warmed up to room temperature after a neutron irradiation of 3.3 x 10 nvt with a y dose of 8.8 x 10 R are shown by 2 and 3, respectively. 

Van de Voorde found the strength of Epikote 828 decreases sharply after low temperature irradiation of 10 rad. Nishijima and Okada also studied the mechanical properties of epoxy resin after neutron irradiation and y-ray irradiation at low temperature. The present experiments show that the stress-strain curves after irradiation are strongly dependent on the kind of hardener. The strength of epoxy resin hardened by polyamide shows a remarkable decrease after low temperature irradiation. 

The advantage of including the adhesive s non-linear constitutive stress-strain model, in terms of the accuracy of the FE stress predictions, can be clearly demonstrated with reference to Fig. 10.16. This figure represents the stress-strain curve for a specific resin (i.e. Araldite 420 epoxy) bulk coupon tested experimentally in direct tension inside an environmental chamber where the temperature of the coupon reached 0°C. Path OAB represents the actual measured nominal stress-strain curve, which is characterized by two parts the first part (i.e. OA) is linear, whereas the second part (AB)... 

Figure 6. Typical stress-strain curves for expanded epoxy resin...

Tg especially wl en deformed under the influence of an overall hydrostatic compressive stress. This behaviour is illustrated in Fig. 5.37 where true stress-strain curves are given for an epoxy resin tested in uniaxial tension and compression at room temperature. The Tg of the resin is 100°C and such cross-linked polymers are found to be brittle when tested in tension at room temperature. In contrast they can show considerable ductility in compression and undergo shear yielding. Another important aspect of the deformation is that glassy polymers tend to show strain softening . The true stress drops after yield, not because of necking which cannot occur in compression, but because there is an inherent softening of the material. 

Fig. 5.37 Stress-strain curves for an epoxy resin deformed in tension or compression at room temperature.

The shifts in the peak position of the 1610cm aramid Raman band, shown in Figure 8.4, can be used as calibration curves to monitor the ddbrmation of fibres in a composite under any state of stress or strain. Previous studies have shown [77-81] that it is possible to map out the distribution of stress or strain along a single short, discontinuous fibre in a low-modulus epoxy resin. This is described in detail next. 

The superposition approach can be used to produce a constitutive equation which expresses the creep compliance (Ca) of the adhesive in terms of a reference creep compliance (O and shift factors for stress (a ), temperature (at) and resin content (Cv) such that Ca = Cr X a X Ut X Oy X f". The method has been used by Dharmarajan et al. (30) to characterise the creep behaviour of epoxy, polyester and acrylic mortars in the form of prism specimens under 3 point loading. From relatively short-term tests, strain v. time curves such... 

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