Critical fracture toughness

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. ... 

Once the critical fracture toughness is found, the healing efficiency is defined as [22,23,27]... 

The Constant Moment (CM) test open up a V-shaped notch in the specimen by forces parallel to the long axis of the V on moment arms to either side. The technique is applicable to porous materials at ambient temperature with slow loading rates, and can measure critical fracture toughness as a function of crack growth rate. 

The materials composition of the workpiece is the major factor that determines the ablation rate. The ablation rate increases with increasing brittleness of the material. The critical fracture toughness is commonly used to characterise the effects of brittle crack formation in a material. Klocke and Hilleke [281,282] provided a relationship between the critical fracture toughness of... 

Table 5.1. Ablation dependent on the critical fracture toughness...

One of the most important appHcations of LEFM is to estimate the critical crack or defect size which causes fast fracture to occur. This occurs when the value of iCin a stmcture becomes equal to the plain strain fracture toughness, of the material the critical crack size, for a given stress and fracture toughness, is then given by equation 31. 

Eor an impact strength of 34 J (25 ft-lbf) the equivalent fracture toughness (150) is approximately 120 MPay. The fracture toughness dictates the critical size of crack above which fast fracture intervenes, so the smaller its value the smaller the critical crack and hence the greater significance of the transverse impact requirement specified by Manning. 

Fracture Toughness. The fracture criterion was defined by a critical value of the crack tip stress intensity, known as the fracture toughness. Ceramics often fail ia pure tension, designated mode I, and Kj replaces ia equation 6. Thus die appHed tensile stress at which fracture... 

The critical strain energy release rate is the energy equivalent to fracture toughness, first proposed by Griffith [Phil. Trans. Royal Soc., A22I, 163 (1920)]. They are related by... 

K,- = lG,. = fracture toughness (sometimes critical stress intensity factor). Usual units MN m ... 

If we take the nominal fracture toughness of IG-11 graphite to be 1 MPayin and the maximum stress in the process zone to be = 60.2 MPa according to the above analysis, we find that rj, = 88 pm. This value is virtually identical to r<, , = 90pm, the process zone dimension determined using Eq. 3. To summarize, the above analysis strongly supports a hypothesis that the maximum critical stress... 

Total interpenetration of chains (X approaches Rg) is not necessary to achieve complete strength when M > M and x < 7). However, a word of caution while complete strength may be obtained in terms of critical fracture measures such as G c and A lc, the durability, measured in sub-critical fracture terms, such as the fatigue crack propagation rate da/AN, may be very far from its fully healed state at r. We have shown that while the weld toughness A lc increases linearly with interdiffusion depth X as K f. X, the fatigue crack propagation behavior of partially healed welds behaves as [1]... 

The double torsion test specimen has many advantages over other fracture toughness specimen geometries. Since it is a linear compliance test piece, the crack length is not required in the calculation. The crack propagates at constant velocity which is determined by the crosshead displacement rate. Several readings of the critical load required for crack propagation can be made on each specimen. 

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