Composites stress-strain behavior

Several experiments will now be described from which the foregoing basic stiffness and strength information can be obtained. For many, but not all, composite materials, the stress-strain behavior is linear from zero load to the ultimate or fracture load. Such linear behavior is typical for glass-epoxy composite materials and is quite reasonable for boron-epoxy and graphite-epoxy composite materials except for the shear behavior that is very nonlinear to fracture. 

Robert M. Jones ar Harold S. Morgan, Analysis of Nonlinear Stress-Strain Behavior of Fiber-Reinforced Composite Materials, AIAA Journal, December 1977, pp. 1669-1676. 

Harold S. Morgan and Robert M. Jones, Analysis of Nonlinear Stress-Strain Behavior of Laminated Fiber-Reinforced Composite Materials, Proceedings of the 1978 International Conference on Composite Materials, Bryan R. Noton, Robert A. Signorelli, Kenneth N. Street, and Leslie N. Phillips (Editors), Toronto, Canada, 16-20 April 1978, American Institute of Mining, Metallurgical a Petroleum Engineers, New York, 1978, pp. 337-352. 

Let s address the issue of nonlinear material behavior, i.e., nonlinear stress-strain behavior. Where does this nonlinear material behavior come from Generally, any of the matrix-dominated properties will exhibit some degree of material nonlinearity because a matrix material is generally a plastic material, such as a resin or even a metal in a metal-matrix composite. For example, in a boron-aluminum composite material, recognize that the aluminum matrix is a metal with an inherently nonlinear stress-strain curve. Thus, the matrix-dominated properties, 3 and Gj2i generally have some level of nonlinear stress-strain curve. 

Hydrogen effect on the mechanical properties discussed below was studied on several a and a+fi alloys with the following nominal composition of metallic components (Russian trade marks given in parentheses) commercial titanium of nominal purity 99.3% (VTl-0), Ti-6Al-2Zr-1.5V-lMo (VT20), Ti-6A1-4.5V (VT6), Ti-6Al-2.5Mo-2Cr (VT3-1), Ti-4Al-1.5Mn (OT4), Ti-6.5Al-4Mo-2Sn-0.6W-0.2Si (VT25u) and others. The main features of their stress-strain behavior due to hydrogenation were much similar, but some individuality was characteristic of each alloy. 

The influence of ambient aging at 70°F and accelerated aging at 160°F on the stress-strain behavior of carboxy-terminated polybutadiene, polybutadiene-acrylic acid, polybutadiene-acrylic acid-acrylonitrile, and hydroxy-terminated polybutadiene composite propints is shown in Figures 10 and 11. The elastomers and curative agents for these formulations are listed below... 

At the end of Section 5.4.2.5, the statement was made that most continuous, unidirectional fiber-reinforced composites are used to produce layers that are subsequently assembled to form laminate composites. In this section, we expound upon this statement by examining the mechanics of laminate composites, first through a generalized description of their mechanics, then with some specific stress-strain behavior. 

In cross-ply laminates, the stress-strain behavior is slightly nonlinear, as illustrated in Figure 5.123. The stress-strain behavior of a unidirectional lamina along the fiber axis is shown in the top curve, while the stress-strain behavior for transverse loading is illustrated in the bottom curve. The stress-strain curve of the cross-ply composite, in the middle, exhibits a knee, indicated by strength ajc, which corresponds to the rupture of the fibers in the 90° ply. The 0° ply then bears the load, until it too ruptures at a composite fracture strength of ct/. 

Fig. 10 Stress-strain behavior of CR gum, CR-clay (CRLDH and CRMMT), CR-modified clay ( CROLDH and CROMMT), and CR-carbon black (CRN220) composites...

Figure 17. Longitudinal vs. transverse stress-strain behavior for 50-volume rayon composite...

Figure 17 illustrates the effect of orientation on the stress-strain properties of the rayon composite shown in Figures 10 and 11. The upper curve represents stress-strain behavior for stress applied parallel to the fiber orientation direction. In the lower curve the force is applied perpendicularly. Even a small degree of orientation has a large effect on the anisotropy of the composite. The differences in tensile strength, modulus and elongation at break in the two directions are considerable. 

The stress-strain behavior of heterogeneous block copolymers depends on their chemical composition. Those consisting of a soft rubbery component and a hard glassy component may either be rubberlike or... 

Stress—Strain Curves. The tensile stress-strain behavior of the blends in which PC is the continuous phase (blends with 5, 10, 20, and 25 wt% PST) also has been investigated. Some preliminary results regarding the influence of composition, strain rate, and temperature on the yield and fracture behavior of these blends will be reported. 

In our previous paper (2), we proposed a possible mechanism to interpret the stress-strain behavior of gradient polymers. We perceived the gradient polymer as consisting of infinite number of layers of varying compositions. Upon deformation, the macroscopic strain is the same for the entire sample. Because of the fact that the moduli of the various... 

Figure 4.5. (a) Tensile strength of chitosan/clay nanocomposite as a function of clay content (b) tensile strength of chitosan/CNTs nanocomposite as a function of CNT content (c) stress-strain behavior for neat chitosan, chitosan/0.4% CNTs, chitosan/3% clay, and chitosan/3%clay/0.4% CNTs composites (d) tensile strength of chitosan/clay/CNTs nanocomposite as a function of clay content. Reprinted with permission from ref (42). 

After a time increment dt, the local stresses in the fibers and matrix can be updated to obtain (tr,-, e e,)t at any time t+ dt. Using this information, the composite stress, strain, and strain rate (ac, ec, ec), can be obtained from the constituent parameters (iterative computation, the creep behavior of the composite and constituents can be predicted for any loading history, including cyclic creep. 

Generally, when testing materials with a nonlinear stress-strain behavior, the tests should be conducted under uniform stress fields, such that the associated damage evolution is also uniform over the gauge section where the material s response is measured. Because the stress field varies with distance from the neutral axis in bending tests, uniaxial tension or compression tests are preferred when characterizing the strength and failure behavior of fiber-reinforced composites. 

Fig. 6.2 Room temperature stress-strain behavior of a woven 0°/90° Q/SiC composite. Because of processing-related matrix cracking and progressive fracture near the crossover points of fiber bundles, Stage II behavior (non-linear stress-strain response) is observed from the onset of loading. Above a strain of approximately 0.5% the composite exhibits Stage III (linear) behavior.

Fig. 6.3 Influence of loading rate on the monotonic stress-strain behavior of unidirectional Nicalon SiCf/CAS-II composites tested in air at 20°C. As the loading rate decreases, the proportional limit, ultimate strength and failure strain decrease. After S0rensen and Holmes.39...

Stress Distribution and High Temperature Creep Rate of Discontinuous Fiber Reinforced Metals, Acta Metallurgies et Materialia, 38, 1941-1953 (1990). 26. A. G. Evans, J. W. Hutchinson, and R. M. McMeeking, Stress-Strain Behavior of Metal Matrix Composites with Discontinuous Reinforcements, Scripta Metallurgica et Materialia, 25, 3-8 (1991). 

The strength of the fiber-matrix interface is one of the key parameters responsible for the stress-strain behavior and damage tolerance of ceramic composites. Two different types of tests are available to measure the fiber-matrix interfacial properties in fiber-reinforced ceramic composites. The first is based on an indentation technique to either push the individual fiber into or through the matrix. The second test method relies on pulling a single fiber out of a matrix. These methods have been compared59 to one another for a glass matrix material, and yield similar results. 

Figure 26.11 shows the stress-strain behavior of ES30 filled with various levels of ATH. It can be seen that the yield stress increases with increasing level of ATH while the ultimate elongation is in excess of several hundred percent even for materials with more than 50wt% ATH. The modulus of composite materials can be modeled by the generalized Kemer equation ... 

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