Crack opening stretch

In a thermoplastic material it is, therefore, important to distinguish between the crack opening stretch (= COS) and the maximum craze width 2v. To characterize plastic deformation and fracture behavior of a thermoplastic material the maximum length of stretched fibrils and hence the maximum craze width 2v is a more fundamental parameter than the crack opening stretch. The latter, in addition, depends on the relaxation behavior of the broken remnants on the fracture surface. 

The fatigue lines have been assumed recently to be stretch zones created at arrest of the jumping crack, and their width was correlated to the maximum craze width. According to the assumption the stretch zone being half the crack opening stretch and, hence, approximately also half the craze width, the line widths in rigid... 

Fig. 14 Stress distribution in a cracked film. The dotted lines correspond to the elastic behavior of the film/substrate system, the solid lines to the presence of plasticity in the substrate at the crack/interface intersection (a) crack opening in a film deposited on a substrate which is uniaxially stretched, (b) longitudinal strain distribution In the film, (c) normal stress distribution in the film, and (d) shear stress distribution In the film at the Interface.

Figure 3.300. Changes in fiber structure under the conditions of mechanical solicitation, by stretching, until the failure [878] a) before solicitation b) apparition of a superficial crack c) crack opening d) F-shape growing of the opened crack e) final fracture.

There are two sources of stress concentration from the fuel concentrations near radial cracks and concentrations near pellet ends. Gittus has analyzed the interaction of opening and closing cracks in frictional contact with the clad (63). Fuel crack width calculations assume that the fuel cracks homogeneously, so the number of cracks in the fuel must be estimated for an actual crack width to be found. As power is cycled the clad adjacent to the fuel crack is stretched when the crack opens and compressed when it closes. Local clad deformations have been produced in out-of-pile experiments and confirm Gittus predictions (13). 

Figure 2.19 Failure of a connecting rod bolt in a diesel engine. In (a), the failed bolt is the upper one, having necked down in a nominally larger cross-sectional area. The lower bolt is another removed from the engine in unstretched condition. In (b), the stretched region of the bolt is shown, with cracks that have opened up during bolt tensioning and subsequent operation. (Reprinted with permission from ASM International. All rights reserved www.asminternational.org)...

After the load has been fully applied, the phase of the actual fiacture initiation follows. The craze continues to move further inside the material and slowly opens up (Fig. 5.19d). The COD value S[t) gradually increases. After a certain crack initiation time ts, after which the craze has fully developed, the fibrils tear in this strongly stretched zone (Fig. 5.19e). The crack starts to move now ever more rapidly through the material and the COD value increases over-propoitionally (Fig. 5.19f) until the remaining thickness finally becomes so small that the entire material in the remaining cross-section is stretched and tears. Figure 5.20 shows the COD value as a function of time. Time needed until failure is the actual time-to-failure tp-. 

The corrosion of steel fibres in cement matrix was a subject of concern in the first period of appUcation of SERC, but later it was proved that only short sections 2-3 mm long that stretch out of the concrete surface corrode and disappear in a short time. Cracked specimens maintained during 18 months in four different environments proved that no decrease of strength could be found in the non-cracked concrete and rusty fibres were found only in the immediate surface in the carbonated zone (Nemeeger et al. 2003). The test carried on by Granju and Balouch (2005) have confirmed that even fibres which bridged open cracks of 0.5 mm wide and maintained a year in a marine environment exhibited only minor traces of corrosion that did not progress inside the concrete matrix. 

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