One of the aims of the engineering standards related to NDE (Non-Destructive Examina-tion) is to identify and limit the defects in a structure, The aim of defect acceptance criteria is to determine whether a component containing defects can be accepted, re-paired or rejected. These criteria should be based on scientific understanding. Howev-er, most of the current defect acceptance criteria were made based on empirism. The examined components with defects are easy to be rejected or produce new or more serious defects after repairing. This must bring huge economic losing and hidden some dangerous factors. A new defect acceptance criterion is proposed on the basis of fracture mechanics in this paper. The new criterion should have three levels: the first is the primary de-fect acceptance criterion and the same allowable defect size should be given to all struc-tures and materiais which are considered; the second is the secondary defect accep-tance criterion and the allowable defect size should be varied With materials to all consid-ered structures; the third is the advanced defect accel; Itance criterion and the allowable defect size should be calculated for a given material and structure. For each defect accep. tance level three grades may be divided with respected to the importance of structures.

ln this paper. the relation between the maximum strain at the notch root and the J integra, value has been derired for a notch wfth a semi. elliptical end. It is proposed that the maximum strain energy density or the maximum strain criterion controls the cracking at the notch roots. The relation indicates that notch J. integra, fracture toughness depends not only on the material and the magnitude of the notch root radius of curvature(or notch width 1 but also upon the magnitude of ellipticity. In the same Why. the relation derived in this paper shows that, when the strain concentration at notch root is predicted by the value ofthe 3-integral. the effect ofellipticity should not be neglected, in addition to the direct effect of the magnitude of notch root radius of curvature.

In this paper, the J-integral path-independence of a notch under incrementaltheory. the possibility of measurement of notch J. integral fracture toughness, and the equality in numerical values between the notch and the crack J-integrals. are investigated and demonstrated by means of elastic-plastic finite element analysis. Based on this. the authors deem that the J-integral may also play the role of a main parameter for notch elastic-plastic fracture analysis. Meanwhile. based on the notch J-integral numerical calculation. the authors further establish the notch failure assessment curve, and therefore investigate the dependence of the failure assessment curve on the dimension and geometry profile of the notch end and the feasibility of the R6 (third edition) failure assessment curve in the notch assessment. The method of the failure assessment diagram should be regarded as a simple and convenient engineering failure assessment approach for notch elastic-plastic fracture analysis, which can give the possible failure mode of the notch. The notch fracture toughness, JA, is no more a simple material constant. The engineering estimation method for the notch fracture toughness is expounded in this paper. which should lay the foundation of application of the notch failure assessment diagram.

Although it is recognised that biaxial or triaxial stressing can affect the fracture conditions and stress/strain field of a cracked body, insufficient attention has been paid to how biaxial or triaxial stresses affects the properties of the J-integral. In this paper, two-dimensional incremental and three-dimensional incremental theory elastic-plastic finite element programs have been adopted to analyse the effect of biaxial stressing on J-integral of a finite plate containing a through-thickness crack and subjected to biaxial stresses. Constitutive equations describing the material behaviour were chosen as: (i) the practical stress strain relation of 16MnR; (ii) modified yielding plateau stress strain relationship of 16MnR; and (iii) Ramberg-Osgood representation. The finite element analysis results show that the properties and values of J-integral under biaxial stressing can be significantly different from those under uniaxial stressing. Attention must, therefore, be paid to J-integral calculations when considering it as a fracture parameter to analyse multi-stressing situations. 2000 Elsevier Science Ltd. All rights reserved.

Reliability analysis of structures containing defects carried out directly by practising engineers is quite complex and costly. Therefore. a simple and convenient reliability analysis method should be investigated and established. In this paper. the principle and building procedure of a reliabjlity analysis method are described and the feasibility of the method are also explicitly clarified. Based on the assumptions that: (1) the 1oad is a deterministic variable; (2) the yield strength and fracture toughness sarisfy normal distribution and their variational coecients are constant; (3) the initial crack length remains unchangeable, a set of probabilistic failure assessment diagrams (PFAD) called elementary reliability assessment method are established under the simplified option 1 curve of the 3rd version of CEGB R6. The investigation shows that the distribution of random safety factor and failure probability of a structure only depend on the Coordinates of the expectation of P (Lr, Kr) in the failure assessment diagram of the simplified option l curve and is not influenced by the other factors. The investigation further shows that the failure probability will decrease with the increase ofthe initial crack length if the expectation of P (Lr, Kr) keeps unchanged. A conservative result will be given when using the PFAD as an elementary reliability analysis method to assess the integrity of a structure and only a simple procedure is required Jike the deterministic assessment method given in CEGB R6. When the secondary stress exists. the conservativeness of the result assessed will increase with the increase of parameter P although the PFAD is only established based on considering primary stress.