Stress-intensity method

With the stress-intensity method Kis used as the basis for assessing failure for fracture to occur the stress intensity must exceed the critical fracture toughness K. Examples of critical fracture toughness values for some materials are shown in Table 6.2. ... 

Although K and G can be related, the stress-intensity method is less useful for adhesives because K values are more difficult to calculate and also because this method is limited to linear-elastic behavior only. With the strain energy release rate method, the nonlinear effects of viscoelasticity and plasticity can be more readily accounted for in the analysis. 

Lack of accepted stress intensity factors for internally pressurized components has, until recently, limited this appHcation. The factors are a function of the size and shape of both cracks and high pressure components as well as modes of loading (91). Stress intensity factors can be derived analytically for some simple geometries, but most require the appHcation of advanced numerical methods (105—107). Alternatively they may be deterrnined experimentally (108). 

Fracture Mechanics Methods These have proved very usebd for defining the minimum stress intensity K[scc. t which stress corrosion cracking of high-strength, low-ductihty alloys occurs. They have so far been less successful when apphed to high-ductility alloys, which are extensively used in the chemicm-process industries. 

Fig. 28 FCP rates as function of stress intensity factor range for unaged UHMWPE (GUR 4150HP) following different sterilization methods...

Linear elastic fracture mechanics (LEFM) approach can be used to characterize the fracture behavior of random fiber composites. The methods of LEFM should be used with utmost care for obtaining meaningful fracture parameters. The analysis of load displacement records as recommended in method ASTM E 399-71 may be subject to some errors caused by the massive debonding that occurs prior to catastrophic failure of these composites. By using the R-curve concept, the fracture behavior of these materials can be more accurately characterized. The K-equa-tions developed for isotropic materials can be used to calculate stress intensity factor for these materials. 

It must be noted that the fracture mechanics framework described above only applies when plastic deformation of the material is limited. Substantial plastic deformation may accompany propagation of existing defects in structures fabricated from relatively low-strength materials, e.g., carbon steels. In these cases, the linear elastic stress intensity factor, K, does not accurately apply in structural design. Alternately, elastic-plastic fracture mechanics methods may apply. ... 

Standard Test Method for Determining Threshold Stress Intensity Factor for Environment-Assisted Cracking of Metallic Materials, Standard E 1681-03, ASTM International, West Conshohocken, PA, 2003. 

The crack opening can be determined (p = 1), if the fringe order n is known. This method has been used in investigations of stationary cracks to determine static and dynamic stress intensity factors, the latter being induced by a shock wave. 

A third method which recently provided considerable insight into the role of crazes in deformation and fracture of amorphous polymers is the optical interference measurement of crazes (preceding a crack). Since the pioneer work of Kambour, this method has been widely used to determine characteristic craze dimensions and critical displacements. W. Doll gives an overview on recent results and on their interpretation in terms of fracture mechanics parameters (stress intensity factor, plastic zone sizes, fracture surface morphology, fracture energy). 

The crack length values obtained from the compliance method were finally compared with crack length measurements derived from a multispecimen procedure, in which a set of identical specimens was subjected to the same applied stress intensity factor (Kic = 0.48 MPa m ") for different times and then broken open in liquid nitrogen, and excellent agreement was found. 

Nuclear Regulatory Commission and the Electric Power Research Institute to understand and develop tools for managing rapid crack propagation and crack arrest [2,3]. In this effort, methods such as optical caustics and dynamic photoelasticity were used to measure directly the instantaneous stress intensity at the tip of a rapidly propagating and arresting crack. The relationship between crack tip stress intensity and crack velocity (Figure 1) was established as the material property governing crack behavior in a structure. This relationship was later confirmed by Zehnder and Rosakis [4]. 

Dally, J.W. and Sanford, R.J. (1987). Strain-Gage Methods for Measuring the Opening-Mode Stress-Intensity Factor, Kj. Experimental Mechanics 27 4, pp. 381-388. Takahashi, K. and Arakawa, K. A parameter influential in dynamic fracture. 

---