Composite compression damage

Figure 15.2 Compression damage of fiber composites through microhuckling. (a) Undulations of buckled fibers (b) kink band local failure schematic and (c) micrograph of kink hand formation in a T800/924C carbon-fiber composite [6].

It is critical that surface treatment conditions be optimized to composite properties since overtreatment as well as undertreatment will degrade composite properties. Typically composite interlaminar shear strength (ILSS), in-plane shear, and transverse tension ate used to assess the effectiveness of surface treatment. More recently damage tolerance properties such as edge delamination strength, open hole compression, and compression after impact have become more important in evaluating the toughness of composite parts. 

One of the most important properties which control the damage tolerance under impact loading and the CAI is the failure strain of the matrix resin (see Fig. 8.8). The matrix failure strain influences the critical transverse strain level at which transverse cracks initiate in shear mode under impact loading, and the resistance to further delamination in predominantly opening mode under subsequent compressive loading (Hirschbuehler, 1987 Evans and Masters, 1987 Masters, 1987a, b Recker et al., 1990). The CAI of near quasi-isotropic composite laminates which are reinforced with AS-4 carbon fibers of volume fractions in the range of 65-69% has... 

The improvement of damage resistance and tolerance in interlaminar fracture and under impact loading for the toughened matrix composites is at the expense of other important mechanical properties, such as inferior stiffness and hot/wet compressive strength (Evans and Masters, 1987). These trade offs appear to be associated with the reduction in matrix modulus and glass transition temperature (Jordan et al., 1989). 

Some of the best uses of Kevlar are in aircraft composites and flak jackets for military and police uses, which require high tensile strength and high modulus per unit of weight. It also has the property of damage tolerance, as it has a ductile compressive failure mode, which is very different from the brittle failure of carbon fibers. 

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. 

E) is the ordinary primed fuse described above. (F) is sprinkled with blackpowder grains at the pasted end and is better than (E). (G) is split at the end, but there is the risk that the core composition at the end drops away. (H) is primed with paste on (G). Never use too much paste or the core composition may be damaged by the solvent. ( ) is drilled at the side, but it is not always certain. (J) has a piece of match or twisted powder pasted paper passed through the hole, but sometimes there is a misfire due to the low quality of the match or twisted pasted paper. Moreover these additives become inert with strong compression and the fire stops at the compressed point. Therefore the match or paper must be passed through the hole quite loosely. (K) is bored cut at the end into the core... 

To verify the reasonableness of die FGM compositional optimum design, one must also check the stress state at the pure ceramic side since it is usually damaged first. Fig. 3 provides the relationship between die stress at die TiC side and the exponent P. From Fig.3, it is observed that the residual stress at die pure TiC side decreases with increasing of P, and it experiences a transition at P=1.6 from tensile to compressive, where the stress is zero. This is favourable for FGM fabrication. Therefore, P=1.6 is indeed an optimum des for TiC/Ni3Al-Ni FGMsi. 

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