Propagation cracking

Elastic energy release due to subcritical crack growth is one recognized source of structure-related AE within its acknowledged lunitations, AEBIL provides a viable means of early on-line deteetion and localization of stable crack propagation. 

Fig. 2. Illustrations of forces to which adhesive bonds are subjected, (a) A standard lap shear specimen where the black area shows the adhesive. The adherends are usually 25 mm wide and the lap area is 312.5 mm. The arrows show the direction of the normal apphcation of load, (b) A peel test where the loading configuration, shown by the arrows, is for a 180° peel test, (c) A double cantilever beam test specimen used in the evaluation of the resistance to crack propagation of an adhesive. The normal application of load is shown by the arrows. This load is appHed by a tensile testing machine or other...

This optimum condition is designed to ensure that the botes of all components yield at the same time. If the cylinder is subjected to fatigue conditions, it has been suggested (39) that a better design criterion would arrange for the maximum normal stress, which controls fatigue crack propagation, to be the same in each component. 

In the case of the fibrous laminate not much work has been done, but it has been observed that a significant loss of stiffness in boron—aluminum laminate occurs when cycled in tension—tension (43,44). Also, in a manner similar to that in the laminated PMCs, the ply stacking sequence affects the fatigue behavior. For example, 90° surface pHes in a 90°/0° sequence develop damage more rapidly than 0° pHes. In the case of laminates made out of metallic sheets, eg, stainless steel and aluminum, further enhanced resistance against fatigue crack propagation than either one of the components in isolation has been observed (45). 

Fig. 16. Self-similar crack propagation in an isotropic material. The crack propagates in a direction perpendicular to the cycHc loading axis (mode I loading).

Practure toughness is another way to characterize the strength of a material. It measures how well a material resists crack propagation and is expressed as the stress needed to enlarge a crack of a specific size. The room temperature fracture toughness of clear, vitreous sihca is approximately 0.75 - 0.80 MPa-mT2 (87,163). 

The total area under the curve A—D, shown as shaded in Figure 1, is the strain energy stored in a body. This energy is not uniformly distributed throughout the material, and it is this inequaUty that gives rise to particle failure. Stress is concentrated around the tips of existing cracks or flaws, and crack propagation is initiated therefrom (Fig. 2) (1). 

Impact and Erosion. Impact involves the rapid appHcation of a substantial load to a relatively small area. Most of the kinetic energy from the impacting object is transformed into strain energy for crack propagation. Impact can produce immediate failure if there is complete penetration of the impacted body or if the impact induces a macrostress in the piece, causing it to deflect and then crack catastrophically. Failure can also occur if erosion reduces the cross section and load-bearing capacity of the component, causes a loss of dimensional tolerance, or causes the loss of a protective coating. Detailed information on impact and erosion is available (49). 

Fatigue Crack Propagation, ASTM STP 415, American Society for Testing and Materials, Philadelphia, Pa., 1967. 

The transverse orientation of the cracks, coupled with their predominance along one side, reveals that bending of the tube induced the responsible stresses. The rapidity of crack propagation and the high crack density indicate that the stress level was high. 

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