Ductile failure analysis of epoxy resin plates containing multiple circular arc cracks by means of the equivalent material concept

Ductile failure of polymeric samples weakened by circular arc cracks is studied theoretically and experimentally in this research. Various arrangements of cracks with different arc angles are considered in the specimens such that crack tips experienced the mixed mode I/II loading conditions. Fracture tests are conducted on the multi-cracked specimens and their fracture loads are achieved. To provide the results, the equivalent material concept (EMC) is used in conjunction of dislocation method and a brittle fracture criterion such that there is no necessity for performing complex and time-consuming elastic-plastic damage analyses. Theoretical and experimental stress intensity factors are computed and compared with each other by employing the fracture curves which demonstrate the appropriate efficiency of proposed method to predict the tests results.

ABOUT THE PRE-OUTBURST STATE OF THE COAL AND SANDSTONE AND THE POSSIBLE MECHANISM OF COAL AND GAS OUTBURST PROCESS

The paper aims to examine the suitability of A. S. Balankin brittle fracture criterion in predicting the of large-scale coal and gas outbursts in underground coal mines under compression load. Formation of dissipative structures as cracks and clusters of local microcracking occurs under the influence of the rock pressure. The formation of microdefects in dissipative structures occurs mainly on local anisotropic inclusions. Mechanism of pre-outburst spalling of gas-saturated coal is analyzed. Destruction (primary outburst) occurs when the brittle fracture conditions are met. And also in the case of such a ratio of “coal seam thickness – the size of micro defects aggregations”, which provides reduction in coal strength to a minimum. Further destruction occurs by known scenario.

Structural Integrity Assessment of RPV With Brittle Fracture Criterion: Issues and Advanced Approaches

A key input to calculation of the structural integrity of the RPV by the brittle fracture criterion is known to be the fracture toughness of a material. When calculating resistance of RPV to brittle fracture it was taken earlier that fracture toughness, KJC, depends only on temperature, and does not depend on thickness of specimens if this thickness is sufficiently large so that the plane strain condition is satisfied. To date it has become clear by many researches that KJC depends on a crack front length. This follows from the statistical nature of brittle fracture in steels. It means that for adequate assessment of RPV structural integrity, the dependence of KJC on the postulated flaw size has to be taken into account. Moreover, it has been shown that the KJC values for specimens with shallow cracks are larger than for specimens with deep cracks, and the KJC values obtained from small-size fracture toughness specimens (for example, pre-cracked Charpy specimens) are larger than the values for CT specimens with thickness of 25 mm, and KJC values may be affected load biaxiality. These findings are explained by the loss of constraint. Thus, for adequate assessment of RPV structural integrity by the brittle fracture criterion, the scale factor has to be taken into account, with two causes being responsible: the physical cause connected with the statistical nature of brittle fracture and the mechanical one related to the difference in constraint for specimens of various geometries. This analysis is important for assessment of RPV structural integrity because i) for most cases the postulated flaws in RPV, due to their size and location, may be considered as shallow cracks; ii) the KJC values for irradiated RPV steels are usually obtained from small-size surveillance specimens that are as a rule, pre-cracked Charpy specimens; iii) for a postulated flaw in RPV, a load acts both perpendicular and along a crack front. i.e. loading is biaxial. For adequate assessment of RPV structural integrity by the brittle fracture criterion it is also necessary to take into account that distribution of the stress intensity factor along a crack front in RPV is heterogeneous, and loading is non-monotonic and non-isothermal. In the present report, advanced approaches are considered for assessment of RPV structural integrity that allow solving the above problems. The considered approaches have been included in Russian Standards for assessment of RPV structural integrity.