Crack Opening Displacement COD

A further use for this model is in providing a fracture criterion. Many processes in polymers are governed by strain rather than stress criteria as might be expected from consideration of molecular measurements, and for fracture both indirect and direct measurements confirm that a critical crack opening displacement (COD),... 

The crack-opening displacement (COD) at any point along the crack is defined as twice the displacement of the crack surface at that location, i.e., COD = 2v -r, 0). The quantity crack-tip-opening displacement (CTOD), however, is given a special designation as the opening displacement at the actual crack tip, which is assumed to be correctly located at a distance x = -Vy from the effective crack tip based on Irwin s approximation. As such, from Eqns. (3.28), (3.37), and (3.49), one obtains ... 

In general, it is not possible to measure CTOD, but rather the crack-opening displacement (COD). The latter quantity can be determined at the outer end of the notch with a suitable clip gauge. Thus, for a notched three-point-bend specimen, it was shown that a plastic hinge can form around the tip of the crack [Brostow and Comeliussen, 1986]. If the center of rotation is known, the CTOD can be calculated from the measured COD. A Standard has been published [BS 5762]. 

Figure 9.46a and b shows the crack growth initiation stress intensity factor, Ki, and SICC growth rate, da/disicc> function of the crack opening displacement (COD)... 

No crack instability developed in the tests of load vs. crack opening displacement (COD), and all specimens fractured in a ductile manner after general yielding. Neither the concept of linear elastic fracture mechanics (K) nor that of nonlinear fracture mechanics (COD, /-integral) for unstable fracture can be applied to the phenomenon such as a stable fracture of 5083-0. 

Table IIL Crack Opening Displacement (COD) of Cryogenic Steel Base Plates and Weldments ... 

Methods for Crack Opening Displacement (COD) Testing, British Standards Publication DD19, British Standards Institution, London (1972). 

Ultimate crack opening displacement (COD), m 1 X 10- Dawding et al. (1985) Tested value... 

The cohesive zone stress can be extracted from experimental data. To determine the cohesive stresses, the J-integral as a function of crack opening displacement (COD) can be determined experimentally for a DCB specimen using the following relationship [175],... 

The crack formation in non-britde materials, i.e. fracture with relevant plastic deformation in the notch or crack tip, can similarly be parameterised using an appropriate parameter for plastic deformation. A considerable opening of the notch or crack due to craze formation and plastic deformation in the notch or crack tip is usually observable for these materials. A crack opening displacement (COD) as a measurable quantity, denoted here by d, is connected with this opening (see Fig. 5.19) and is... 

Fig. 5.20. Crack opening displacement (COD)

Figure 12.26 Crack opening displacement (COD) for polyethylene copolymer at a bulk stress of 3 MPa. (Reproduced with permission from O Connell, Bonner, Duckett, et ai. Polymer, 36, 2355 (1995))...

The specimens were sawn out of large slabs after hardening of the concrete and were stored in constant laboratory conditions of +18 1 °C and 90 2% RH. After two years curing, the specimens were notched and subjected to four-point bending in an Instron testing machine with the head displacements control. Load, central deflection and crack opening displacement (COD) were measured and recorded to calculate the characteristics of the cracking process. 

Another method for characterizing the behaviour of composite elements under load is based on the concept of crack opening displacement (COD). It consists of the measurement of displacements at an initial notch subjected to Mode I of cracking. The characterization of cracking behaviour of materials by COD is possible in LEFM as well as beyond the linear relation for slow crack propagation. The tests are undertaken in three- or four-point bending in displacement-controlled testing machines. However, the COD method does not allow the effective stress concentration to be calculated and may only be used as a valuable comparative measure. 

In the Dugdale model the stresses in the cohesion zone are assumed to have a constant value CTq over a length d, so that at equilibrium G = where bj is the crack opening displacement (COD) beyond which interactions disappear. This model is used for elastoplastic materials where CTq is the yield stress and d the plastic zone length, and for polymers where ag is the craze stress and d the craze length it can also be applied to liquid bridges where (Jq is the Laplace pressure (see Section 6). 

Figure 8. Crack opening displacement (COD) measured with an AFM in the topography mode. The figures display the width of the COD measured a few hours after the indent was made. The measurement was repeated after two days leading to a slight increase in the stress intensity factor. The data shown in Fig. 8a were obtained on the sample shown in Fig. 7a, and Fig. 8b corresponds to Fig. 7b.

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