Compressive-failure strain

In all the S4/0 specimens the delamination crack stops after only 5 mm of propagation in the pinned block. The failure mode then changes from delamination crack propagation to a bending failure of the arms. The toughness at the time of failure is 7000 J/m. TTie calculated bending stress in the beam is (Tb=l lOOMPa. The local curvature leads to a compressive failure strain in the outside layers of 1.3%. 

Figure 9.6 Influence of delamination width on compression failure strain.

The ratio of the applied preload strains to the material compressive failure strain. 

Mesophase pitch-based carbon fibers generally have the highest stiffness of all carbon fibers with modulus of elasticity up to 965 GPa (140 Msi), considerably higher than PAN-based fibers. The tensile strength however is much lower, averaging only half. As mentioned in the previous section, the compressive-failure strain is low. 

DIF values vary for different stress types in both concrete and steel for several reasons. Flexural response is ductile and DIF values are permitted which reflect actual strain rates. Shear stresses in concrete produce brittle failures and thus require a degree of conservatism to be applied to the selection of a DIF. Additionally, test data for dynamic shear response of concrete materials is not as well established as compressive strength. Strain rates for tension and compression in steel and concrete members are lower than for flexure and thus DIF values are necessarily lower. 

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... 

Figure 10.7 (a) Failure lines for grouted and ungrouted granular soils, (b) Drained triaxial test results for silicate grouted coarse and medium sands. (From Ref. 11.15.) (c) Typical stress-strain curve from unconfined compression test on chemically grouted sand, (d) Compression versus time data for creep test on chemically grouted sand, under constant load, (e) Failure time versus percent of unconfined compression failure load. (+) indicates unconfined compression tests, and ( ) indicates triaxial tests with S3 = 25% of Si. 

Polyacrylonitrile (PAN) carbon fibers up to 6% (w/w) were used to reinforce PC. In compression, it increased the failure strain, but the strength and modulus decreased. In tension, the addition of carbon fibers increased failure strain, strength, and modulus [9],... 

The stress and strain relationships for a ceramic specimen are more often determined by bending a bar, plate or cylinder of material (Figure 10.3). In this test, the lower part of the ceramic is under tension, and the upper surface is under compression. As ceramic materials are generally much stronger in compression, failure is initiated on the surface under tension. The maximum stress in the upper surface of a deformed sample, (T , is given by ... 

Figures 13.16 and 13.17 are plots of the compressive stress-strain data for two amorphous and two crystalline polymers, respectively, while Figure 13.18 shows tensile and compressive stress-strain behavior of a normally brittle polymer (polystyrene). The stress-strain curves for the amorphous polymers are characteristic of the yield behavior of polymers. On the other hand, there are no clearly defined yield points for the crystalline polymers. In tension, polystyrene exhibited brittle failure, whereas in compression it behaved as a ductile polymer. The behavior of polystyrene typifies the general behavior of polymers. Tensile and compressive tests do not, as would normally be expected, give the same results. Strength and yield stress are generally higher in compression than in tension.

Figure 4.30P 1 shows the compressive stress-strain curves for latex-modified mortars. Generally, the maximum compressive strain at failure increases with rising polymer-cement ratio, even though there is no pronounced change in the modulus of elasticity in compression. The maximum compressive strain at a polymer-cement ratio of 20% increases to 2 to 3 times that of unmodified mortar. 

The denominators in the first two of Eqn (6.34) are the tensile or compressive ultimate strains in the corresponding direction depending on whethCT or ey are tensile or compressive, respectively. It should be noted that for a material that is linear to failure, the predictions from the maximum strain criterion would differ from those from the maximum stress criterion only by a factor depending on the Poisson s ratios and Vyx- As with the maximum stress criterion, the maximum strain criterion provides some feedback on the type of failure mode occurring. 

At present, it is recommended for multiaxial failure analysis that the tensile-compressive-shear-stiength asymmetry be recognized, as well as the strength anisotropy. If properties are not available for a specific site under analysis, failure strains should be used from sites that have a similar density range and architecture. 

The SEM observations revealed that PAN based fibers failed by a buckling mechanism, whereas MP based fibers failed by shear. The TIOOO fiber exhibited the highest compressive failure strength and tensile strain to break. Compressive strengths of carbon fibers are related to the ability to absorb energy, as shown in Figures 20.20 and 20.21. 

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