Stress intensity factor critical value

Here Kq is the value of the stress intensity factor at the instant of the crack onset, Kc is the stress intensity factor critical value corresponding to the fracture transition into the stage of unstable crack propagation, while the difference Kq — Kq characterizes the residual strength of a specimen with a propagating crack. 

Fig. 1. Surfaces of the stress intensity factor critical values at fracture of alloy D16T (Al-4.2Cu-0.5Si-0.6Mn-l.5Mg).

Kc = stress intensity factor critical value Kq = value of stress intensity factor at the instant of crack onset /o = initial crack length L = instantaneous crack length = determination coefficient W = width of specimen X = variable in regression analysis Xi = coded values of strain rate X2 = temperature, K... 

STiscc MPa mm stress intensity factor, critical value at initiation of stress corrosion cracking (SCC)... 

Several additional, non-microstructural, inputs are required for the fracture model (i) Particle critical stress intensity factor, KIc. Here, the value determined in a previous study (Klc = 0.285 MPa in )[3] was adopted for all four graphites studied. This value is significantly less than the bulk Klc of graphites (typically -0.8-1.2 MPa rn). However, as discussed in the previous section, when considering fracture occurring in volumes commensurate in size with the process zone a reduced value of Klc is appropriate (ii) the specimen volume, taken to be the stressed volume of the ASTM tensile test specimens specimen used to determine the tensile strength distributions and (iii) the specimen breadth, b, of a square section specimen. For cylindrical specimens, such as those used here, an equivalent breadth is calculated such that the specimen cross sectional area is identical, i.e.,... 

In a first testing series, the fracture behavior of the neat, fully crosslinked epoxy network was studied. A fully unstable crack propagation behavior was observed and the critical stress intensity factor, Kj (0.82 MPaxm ), and the critical energy release rate, Gj (0.28 kj/m ), were determined [87]. These are typical values for highly crosslinked epoxy networks prepared with DGEBPA and aromatic or cycloaliphatic diamines. 

Secondly, Irwin 6) found that the stress field around a sharp crack in a linear elastic material could be uniquely defined by a parameter named the stress-intensity factor, K and stated that fracture occurs when the value of K, exceeds some critical value, K C. Thus, K, is a stress field parameter independent of the material whereas Klc, often referred to as the fracture toughness, is a measure of a material property. Again the subscript I is used to denote the tensile-opening mode. 

Fig. 6.2.13. Dependence of crack velocity on applied stress intensity factor. Schematic diagram of crack velocity data indicating three regions of crack growth. Boundaries of these three regions are given by specific values of K, (K0, Kt, K2, KIC) and by a critical value of the crack velocity (Flc). (After Fuller et a/., 1980)...

LEFM specimens yield a stress intensity factor, K, which is a continuous function of the applied stress field and the crack length. At fracture, the stress intensity factor takes on a critical value, symbolized by K,, called the fracture toughness. The Roman numeral subscript identifies the fracture mode e.g., I is tensile opening mode. In this chapter, mode I is assumed, as is the critical condition therefore, the unscripted symbol K will subsequently represent the fracture toughness. 

The transition from slow to rapid crack growth, however, is relatively sharp for many polymers. This is exemplified by PMMA. At the critical value of stress intensity factor, Kj, a jump in crack speed is observed at the end of the slow growth region around 0.1 ms to much higher speeds which may be in the range of 10 to 100 ms , depending on testing conditions (e.g. With this jump is also associated a... 

Irwin (40) gave an alternative formulation to fracture by considering the distribution or field of stresses around a crack in an elastic material. He proposed that such a distribution could be expressed as a function of a parameter K, known as the stress intensity factor, and he established that the fracture would occur when K exceeds a critical value characteristic of each material. Figure 14.33 shows a sharp crack of length 2a in an infinite lamina subjected to a tensile stress ct. The equations defining the local stresses an, a22 < 12 are (42)... 

However, a critical value is found for the applied stress intensity factor, K ic, below which the environmental stress-cracking agent accelerates fracture initiation below this transition value, the slope of the stress intensity factor vs. initiation time plot is higher but still similar for the two materials. 

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