Through-Thickness Crack

This header has undergone repair after through-thickness cracking of an assembly weld after 108.000 hours of service, it is presently in operation and periodically submitted to ultrasonic inspection during planned maintenance shutdown. Continuous on-line AE monitoring of the integrity of critical welds is expected to contribute supporting continued and safe operation of the header until its replacement, platmed to take place in 1999. 

FIG. 13.82 An elliptical, through thickness crack in an elastic sheet subject to a stress a in the x2-direction along Oxl as shown Oxford University Press. 

Crack propagation within MIM was also associated with a slight but perceptible decrease in the contact tangential stiffness, which was not the case for the GIM copolymer. As reported above, the contact stiffness is strongly sensitive to the extent of crack propagation. The fact that a drop in contact stiffness is not perceptible for the GIM polymer could therefore be interpreted as evidence of a limited propagation of the surface cracks through the thickness of the specimens. 

Fig. 3 Schematic representation of various types of failure associated with the mechanical instability of a film deposited on a substrate, (a) cracking of a thin film subjected to residual tensile stress, (b) plastic deformation of the substrate at the end of the crack, (c) deviation of the crack at the interface, (d) cracking of the substrate, (e) detachment and buckling (formation of a blister from an interface defect) of a film subjected to residual compressive stress, and (f) deviation of the crack through the thickness of the film (flaking).

Fig. 15 (a) Schematic representation of the stress profiles in an adherent portion of film at a distance x from a free surface of a through-thickness crack. Interfacial shear stress x. and peel stress p correspond to the action of the substrate on region (1 The normal stress parallel to the x axis, is supposed to remain constant through the film thickness h. 

The interfacial debond occurs after the through-thickness cracking of the films. It is activated by transverse contraction of the substrate which induces the buckling of the strips of the cracked films. The analysis of this particular adhesion failure mechanism provides insights concerning the interfacial strength of... 

Pour exceptions where risk-limiting measures may be justified include (1) Por new equipment subject to local laws that mandate conformance to an engineering recommended practice. (2) When through-thickness cracks, even though stable, would release a lethal, flammable, or corrosive... 

Consider, therefore, an infinitely large plate of elastic material of thickness B, containing a through-thickness crack of length 2a, and subjected to uniform biaxial tension (a) at infinity as shown in Fig. 2.5. Let U = potential energy of the system, Uo = potential energy of the system before introducing the crack, Ua = decrease... 

Figure 2.5. An infinitely large plate of elastic material containing a through-thickness central crack of length 2a and subjected to uniform biaxial tension a.

Figure 2.10. Typical load-displacement curve for an ideally brittle material with a through-thickness crack. Displacement is measured across the crack opening.

The case of an infinitely large thin plate, containing a central through-thickness crack of length 2a, subjected to remote, uniform biaxial tension is considered (Fig. 3.3). The boundary conditions are as follows ... 

Figure 3.3. A central through-thickness crack in an infinitely large plate snbjected to remote, nniform biaxial tension.

Figure 4.15. Schematic diagram showing the difference in the evolution of shear lips with crack growth that reflect the loss of through-thickness constraint through differences in material thickness, yield strengths, or fracture toughness [7].

As a conceptual experiment, one might consider a wide plate, with a central, through-thickness crack of length 2a, under a uniformly apphed tensile stress a. One might further consider having both failure modes (fracture and yielding) occur at the same time. For fracture. 

EXAMPLE 1 - THROUGH-THICKNESS CRACK. The case of a center-cracked plate, subjected to constant-amplitude loading, is considered to provide physical insight. 

Structural Analysis For a through-thickness crack of length 2a in a wide plate, subjected to uniform remote tension, ct, perpendicular to the plane of the crack. 

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