Fracture crack opening displacement

The geometry-independent material characteristic Gic as a function of the initial crack opening displacement (ICOD) is introduced. Typical values of the delamination fracture toughness of 3D glass and carbon composites are presented. 

Fracture toughness, delamination, 3D composite, bridging law, crack opening displacement. 

The additional parameter — the crack opening displacement at the initial precrack (ICOD) — was measured to obtain the bridging law, which is a material characteristic independent of the specimen geometry. It is recommended to use the graphs Gic (ICOD) instead of the traditional / -curves Gic(Afl) for characterizing the delamination fracture resistance of composites with extensive bridging. 

The J-integral suggested by Rice [11] and crack opening displacement [13] are the parameters most often used to characterize non-linear fracture behaviour of cracked pipes [13,14], In the present study the tearing modulus stability criterion is used. 

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),... 

Figure 4.11. Typical strain gage-based crack-opening displacement gage (left), and typical load-displacement traces observed during fracture toughness testing (right) [1].

The energy method of Griffith and the stress intensity factor approach of Irwin are, perhaps, the most basic and often used of the fracture criteria. Therefore, they have been emphasized in this article. However, the reader should be aware that other approaches to the problem have gained considerable popularity in recent years, foremost among them the J-integral (19) and the crack-opening displacement methods (20). These two criteria have been extensively applied to the fracture of metals for which crack tip plasticity is significant. 

Wells [11] first proposed the crack opening displacement as a measure of fracture toughness for tough metals such as the structural steels where LEFM is not valid. The concept became a standard in 1979 in the form of British Standard (BS) 5762 [12], which is now-superseded by BS 7448 Part 1. 1991. Method of determination of Afi. critical CTOD and critical J values of metallic materials [13], More recently. ASTM E1290-9,3, Crack-tip opening displacement (CTOD) fracture toughness measurement [14] was issued. It is stated that this method may be used to characterize the toughness of materials that arc (a) too ductile or fail the size conditions to be tested for or (b) likely to produce unstable crack extension that would invalidate Ji tests in accordance with the requirements of ASTM E81.3 [15]. 

The linear-slope CDCB specimens shown in Figs. 13 and 14 were used for mode-I fracture tests on FRP-wood bonded interfaces under both dry and wet conditions. A typical specimen under mode-I fracture load is shown in Fig. 15 for phenolic FRP wood interface under dry conditions, and the corresponding critical tip load versus crack opening displacement is plotted in Fig. 16. As an illustration, the fracture surfaces of epoxy FRP-wood with HMR and RF primers on wood substrates are shown in Figs. 17 and 18,... 

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. 

COD analysis, based on general yielding fracture mechanics as outlined by Wells [ ] and Burdekin and Dawes [% has been used for more than ten years. The COD test is a slow, three-point bend test where the distance through which the notch faces move is used as a measure of prefracture ductility, rather than the total strain to fracture. This distance is called the crack opening displacement, or COD. 

Load-versus-crack opening displacements also were monitored during the fracture toughness tests using the cracked disk specimen. This specimen geometry was used tor the validation experiments. Also, it provided a means to evaluate whether the analytical model was able to predict load-displacement data under mixed-mode fracture conditions. It may be noted that in actual applications, loading is generally mixed mode in character. 

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... 

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