Fracture process zone

In order to develop such design criteria, one needs to study hydrogen-induced fractures in laboratory specimens that simulate the conditions in the fracture process zone in the actual pipeline environment in terms of the stress, strain, and hydrogen accumulation6 1. We have recently demonstrated6-9, that under hydrogen pressures up to 15 MPa both stress and hydrogen fields ahead... 

The last issue that remains to be addressed is whether the MBL results are sensitive to the characteristic diffusion distance L one assumes to fix the outer boundary of the domain of analysis. In the calculations so far, we took the size L of the MBL domain to be equal to the size h - a of the uncracked ligament in the pipeline. To investigate the effect of the size L on the steady state concentration profiles, in particular within the fracture process zone, we performed additional transient hydrogen transport calculations using the MBL approach with L = 8(/i — a) = 60.96 mm under the same stress intensity factor Kf =34.12 MPa /m and normalized T-stress T /steady state distributions of the NILS concentration ahead of the crack tip are plotted in Fig. 8 for the two boundary conditions, i.e. / = 0 and C, =0 on the outer boundary. The concentration profiles for the zero flux boundary condition are identical for both domain sizes. For the zero concentration boundary condition CL = 0 on the outer boundary, although the concentration profiles for the two domain sizes L = h - a and L = 8(/i - a) differ substantially away from the crack tip. they are very close in the region near the crack tip, and notably their maxima differ by less than... 

Fig, 6.17. Fracture process zone (FPZ) in transverse fracture of unidirectional fiber composite. After... 

Fig. 8.4. Schematic illustrations of the formation of a fracture process zone in front of the crack tip for composites containing (a) a brittle resin matrix and (b) a ductile resin matrix. After Bradley and Cohen (1987). Reproduced by permission of ASTM.

Fig. 10.12 Curves showing the creep crack growth rate as a function of crack tip fracture process zone size for different values of crack length. The abscissa is plotted as a percentage of frontal creep fracture process zone size to the crack length. See text for discussion.

If the transport and surface reaction processes are rapid i.e., not rate limiting), then crack growth would be controlled by the rate of diffusion of the embritthng species into the fracture process zone ahead of the crack tip. For diffusion-controlled crack growth, therefore, the rate equation assumes the following form ... 

To overcome the above drawbacks, a new method based on essential work of fracture concept was introduced [Broberg, 1971, 1975]. In this method, it is proposed that when a cracked ductile solid, such as a toughened polymer blend is loaded, the fracture process and the plastic deformation take place in two different regions, viz. the inner process zone and the outer plastic zone. Much of the fracture work during crack propagation, dissipated in the plastic zone, is not directly associated with the fracture process. Only that work that goes into the fracture process zone is a material constant. Hence, the total fracture work, Wp should be separated into two parts, i.e., the essential work of fracture (i.e., the work required to create two new fracture surfaces, W),... 

Figure 4 (a) Fracture process zone (area of stress concentration) surrounding the area or volume of the bondline immediately ahead of the crack tip when the joint is subjected to cleavage, shear, or shrinkage forces (b) small process zone and high stress concentration with rigid adherend and adhesive (c) large process zone and low stress concentration with flexible adhesive and adherend. 

A prominent mechanism of crack advance in flexible-chain glassy polymers is associated with crazing, where it has been amply demonstrated that the fracture-process zone at the tip of advancing cracks is composed of one or more crazes... 

Ohtsu M, Okamoto T, Yuyama S (1998) Moment tensor analysis of acoustic emission for cracking mechanisms in concrete. ACI Stracmral Journal 95(2) 87-95 Ohtsu M, Ohtsuka M (1998) Damage evolution by AE in the fracture process zone of concrete. J Materials, Cone. Struct. Pavement, JSCE, 599/V-40 177-184 Ohtsu M (2000) Moment tensor analysis of AE and SiGMA code. Acoustic Emis-sion-Beyond the Millennium. Elsevier, pp 19-34 Ouyang C, Landis E, Shah SP (1992) Damage assessment in concrete using acoustic emission. Nondestructive Testing of Concrete Elements and Structures, ASCE, pp 13-24... 

It is well known in concrete engineering that the fracture process zone is created ahead of a notch (crack) in concrete, without revealing the notch sensitivity. Nucleation of micro-cracks in the fracture process zone is clarified as shown in Fig. 10.25 (Nomura, Mihashi et al. 1991). The fracture process zone was ideally introduced in order to explain the tension-softening behavior. Then, as shown in the figure, the presence of the zone is physically confirmed. 

Further, a relation between the area of the zone and the size of aggregate was studied as given in Fig. 10.26 (Ohtsuka, Date et al. 1998). With the increase in the size of aggregate, the fracture process zone grows broadly. 

The fracture process zone in concrete, in Toughening Mechanisms in... 

A finite nonnegligible fracture process zone in front of the crack tip gives rise a deviation from the size-effect law prescribed in Eqs. (3a) and (3b). When the characteristic dimension of the frontal process zone Cf becomes significant compared to the characteristic dimensions of W and ao. Eqs. (3a) and (3b) must include the nonlinear contributions of the microscopic deformation and fracture in the process zone, and be modified through the replacement of the crack length ao with its effective value aeff( = oo + c/). This modification in Eqs. (3a) and (3b) results in the following formulae of failure strength in their intrinsic expressions ... 

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