Crack resistance curve

Fig. 16 Craze-crack resistance curves for a constant and a temperature-dependent craze critical thickness...

Fig. 3. Craze-crack resistance curves for isothermal conditions with = 30 MPav /s and temperature dependent stress-displacement fields for ki = 300 MPa- /m/s and Ki = 3000 MPa- /m/s. The triangle indicates the initiation of crazing while the square corresponds to the onset of unstable crack propagation, defining...

Standard Mode I Double Cantilever Beam specimens for delamination testing of a unidirectional (UD) IM7/977-2 composite were Z-pinned with two separate blocks of Z-Fiber reinforcement. The reinforced beam configuration was such as to provoke an unstable delamination, propagating between the two Z-pin blocks. Crack resistance curves for these specific geometry specimens of IM7/977-2 indicate that the unstable delamination cracks are arrested by the second Z-pin block, with the crack propagation resistance being dictated primarily by the Z-pinning density within a block. Acoustic emission analysis is used to interpret visual observations and other test data. 

Fig. 5.3 Principle of recording a crack resistance curve with a single specimen [03Rei, 04Rei].

Not only the specimen thickness is of importance, but also the ratio of the initial notch size a and the specimen width W. In Figure 5.6, crack resistance curves from quasi-static fracture mechanics tests (SSM) are shown. The materials were SBR vulcanizates without (a, c) and with carbon black N330 (b, d). The mixtures of the non-reinforced materials contained different amounts of sulphur so that a different crosslink density could be obtained. For both, the unfilled and the CB filled vulcanizates, the comparison of the crack resistance curves indicates an influence of the a/W ratio. The analysis of the data [05Rei] led to the conclusion that 0.2 is a suitable a/W ratio for such experimental investigations. 

Fig. 5.6 Crack resistance curves of unfilled (a, c) and carbon-black filled (b, d) SBR vulcanizates for various amounts of sulphur (a, c) and carbon black N330 (b, d).

Steinbrech, R.W. and Schenkel, 0. (1988) Crack-resistance curves of surface cracks in alumina. /. Am. Ceram. Soc., 71 (5), C271-C273. 

The experimental results clearly show that the stress corrosion is the key mechanism for crack propagation in alumina ceramics and that a unique crack velocity-crack tip stress intensity factor law can be obtained by taking into account the crack resistance curve in coarse-grain ceramics. 

It should be noted that a mode 11 crack-resistance R-curve may also be obtained for some fiber-matrix systems (Vu-Khanh, 1987). In conjunction with the mode 1 R-curve the additional information of a mode 11 R-curve will be of great use to the composite design engineers. This is increasingly the view of the ESIS task group on delamination crack growth resistance. 

It follows from equation (7.8) that unstable crack growth occurs if the slope of the straight line corresponding to the stress intensity factor at constant applied stress is greater than or equal to the slope of the tangent line to the fracture resistance curve at the same point (Fig. 7.6). Also the applied stress intensity factor becomes higher than the fracture resistance of the material. 

Alternative approaches, termed indentation thermal shock tests , with pre-cracks of known sizes have been used by several authors to assess thermal shock damage in monolithic ceramics. Knoop (Hasselmann et al., 1978 Faber etal, 1981) or Vickers (Gong etal., 1992 Osterstock, 1993 Andersson and Rowcliffe, 1996 Tancret and Osterstock, 1997 Collin and Rowcliffe, 1999, 2000 Lee et al., 2002) indentations were made on rectangular bars, which were then heated to pre-determined temperatures and quenched into water. Crack extensions from the indentations were measured as a function of quench temperature differential, and the critical temperature for spontaneous crack growth (failure) was determined for the material. Fracture mechanics analyses, which took into account measured resistance-curve (7 -curve) functions, were then used to account for the data trends. 

G.E. Hale, A Testing Protocol for conducting J-crack Growth Resistance Curve Tests on Plastics, ESIS Draft Protocol, May 1994. 

Figure 7. Crack growth resistance curve of two initial crack lengths...

The single specimen technique was used to determine the value of the energy J versus crack growth Aa. The crack growth resistance curve is derived from experimental data, according to a power law J = b (Aa) (where b and c are constants). 

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