Material fracture toughness

Interestingly, the ductile-brittle transition observed for the MIM system provided an opportunity to assess the material fracture toughness, which was not possible using classical fracture mechanics tests due to the intrinsic brittleness of the MIM system. The measurement of the critical crack length, Lc, in the contact plane at the onset of brittle propagation allows estimation of a fracture toughness K C = a x+JnLc in the order of 0.85 MPa m1/2, i.e. much less than that of a poly(methylmethacrylate) homopolymer (1.20 MPa m1/2). 

Indeed, Bravo-Leon et al. [31] and Sakuma et al. [34] have found that the toughness is higher for samples with yttria concentrations lower than the typical 3 mol% used for this material. Fracture toughness values of 16-17 MPa m1/2 were reached by Bravo-Leon et al. for a lmol% yttria specimen with a grain size of 90 nm and a 1.5mol% yttria specimen with a grain size of llOnm. This can be attributed to the lower stability of the tetragonal phase with low dopant concentrations, which easily transforms to the monoclinic phase upon application of the stress. 

Fig. 4. Comparison of cryogenic plate material fracture toughness.

Qualification of weld procedures control that welds and HAZ have properties at least equivalent to the base material (fracture toughness at the start of upper shelf and at operation temperature). 

VI.26. The purpose of the brittle fracture evaluation process is to ensure that the three parameters of this characterization (material fracture toughness, applied stress and flaw size) satisfy Eqs (VI.2) and (VI.3), or corresponding elastic-plastic treattnents, thereby precluding fracture initiation. 

VI.29. Any safety factors that might be applied to Eq. (VI.3), or to the parameters that make up Eq. (VI.3) and its elastic-plastic extensions, must account for uncertainties in the calculation or measurement of these parameters. These uncertainties might include those associated with the calculation of the state of stress in the package, the examination of the package for defects, and the measurement of material fracture toughness. Thus the overall safety factor required depends on whether the values used for the different input parameters are best estimate (mean) values or upper bounds for loading parameters and postulated defect sizes and lower bounds for fracture toughness. In particular, concern about uncertainty in NDE can be accommodated by appropriate conservatism in the selection of the reference flaw. 

VI.40. The calculated applied stress intensity factor should be shown to be less than the material fracture toughness value in Eq. (VI.3), with appropriate allowance for plasticity effects and factors of safety. The method for determining the material fracture toughness should be selected from three options, all of which are illustrated... 

VI.42. Option 2 should be based on the determination of a lower bound or near lower bound value of the material fracture toughness, = Kjj, as shown in... 

Fig. VI.2. This option would encompass, as a limiting case, the reference material fracture toughness determination for ferritic steels that is prescribed, for example, in...

Second, Irwin found that the stress field surrounding a crack could be defined uniquely by a stress-field parameter termed the stress-intensity factor, K. He postulated that fracture occurs when the value of K exceeds some critical value. K, often referred to as the material fracture toughness. Thus K relates the magnitude of the stress-intensity local to the crack in terms of the applied loadings and the geometry of the structure in which the crack is located. A crack in a solid may be stressed in three different modes as depicted in Fig. 2.18. Mode I opening, and hence the Mode I value for the stress intensity factor Ki, is the most critical situation in bonded joints. 

The instantaneous probability of initiation, cpi, for a specific flaw size and location in a trial vessel at any specified time during a transient event is equal to (pxic (i e. the probabiUty that the material fracture toughness, Ki, is less than the applied stress intensity factor K for the flaw at the specified time). The relationship used to calculate cpi is obtained from Eq. 12.4 as ... 

Fabrication and Processing of Ferritic Materials Fracture Toughness NSSS Components... 

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