Simulation of Deformation Process Failure of Jointed Rock Masses Based on the Numerical Manifold Method

On the basis of the numerical manifold method, this work introduces the concept of stress intensity factor at the crack tip in fracture mechanics and proposes the utilisation of artificial joint technology to ensure the accuracy of joint geometric dimensions in the element generation of the numerical manifold method. The contour integral method is used to solve the stress intensity factor at the joint tip, and the failure criterion and direction of crack propagation at the joint tip are determined. Element reconstruction and crack tracking are implemented in crack propagation, and a simulation programme of the entire process of deformation, failure, propagation and coalescence of jointed rock masses is developed. The rationality of the proposed method is verified by performing the typical uniaxial compression test and direct shear test.

Integrated modelling method of bonding strength and residual life prediction for selective repair structure of aluminium alloy

To accurately describe and predict the overall strength and residual life of selective repair bonded structures, an integrated simulation model of crack propagation including bonding strength is established. Based on two methods, an integrated simulation model including a cohesive zone method model for predicting the residual life of a selective repair structure is established. By comparing the computational efficiency and accuracy of both the stress intensity factor and residual life of selective repair structures using different calculation methods, the modelling scheme is optimised. Based on this optimised scheme, the effect of adhesive thickness on the stress intensity factor and residual life of the repair structure is analysed. FM94 adhesive measuring 0.2–0.4 mm thickness is used to decrease the stress intensity factor and improve the remaining life such that material utilisation efficiency is guaranteed.

Strain Rate Effect on Mechanical Properties of Cemented Backfill under Dynamic and Static Combined Loading

In order to study the influence of pillar stopping blasting on the stability of cemented backfill, the dynamic impact test under low strain rate (61.1∼86.8 s−1) was conducted on cemented backfill with two kinds of strength using three-dimensional coupled static-dynamic SHPB equipment. At the same time, the strain rate effect of failure mode, dynamic strength factor, and energy transfer of backfill were analyzed. The results show that when the cemented backfill was loaded under different strain rates in the initial three-dimensional static pressure environment, the pore compaction process was no longer obvious but directly entered the elastic deformation stage. Within the range of strain rates, the extreme value of dynamic intensity factor (DIF) of CTB230 was 6.8, while the extreme value of dynamic intensity factor of CTB310 specimen did not appear within the range of strain rates due to the improvement of the internal cementation force between particles. The fracture surfaces of specimens were perpendicular to the direction of load, and the failure mode was mainly the axial tensile failure, and the fracture surfaces were mostly close to the loading end. According to energy calculation, reflected energy accounts for 80.4%∼86.6% of incident energy; dissipated energy, 5.5%∼14.3%; transmitted energy, 5.3%∼7.9%.

Mode I stress intensity factor with various crack types

Presence of cracks in mechanical components needs much attention, where the stress field is affected by cracks and the propagation of cracks may be occurred causing the damage. The objective of this paper is to present an investigation of crack type effect on crack severity in a finite plate. Three cases of cracked plate with three different types of cracks are assumed in this work, i.e., single edge crack, center crack and double edge crack. 2D numerical models of cases of cracked plate are established in finite element analysis (FEA), ANSYS software by adopting PLANE 183 element. Values of FEA mode I stress intensity factor SIF and Von-Mises stress at crack apex are determined for cases of cracked plate under tensile stress with different values. To identify the crack severity, the comparison of FEA results for different cracked cases is made. The comparison showed that, single edge cracked plate (SECP) has the maximum values of mode I SIF and Von-Mises stress at crack apex, i.e. the greatest crack severity is considered. Also, values of FEA Von-Mises stress at crack apex for center cracked plate (CCP) are moderate and for double edge cracked plate (DECP) are the minimum. Besides, in case of high crack lengths, it is found that, FEA results of mode I SIF in case of (CCP) are higher than those of in case of (DECP). Consequently, crack severity is considered as moderate in case of (CCP) and the minimum in case of (DECP). Empirical formulas are used to approximately estimate mode I SIF for all the case studies of cracked plate in this study and the results are compared to those of FEA. A good agreement between analytical and FEA results has been showed by this comparison.

A Macro–Micro Damage Model for Rock under Compression Loading

Rock damage caused by its microcrack growth has a great influence on the deformation and strength properties of rock under compressive loading. Considering the interaction of wing cracks and the additional stress caused by rock bridge damage, a new calculation model for the mode-I stress intensity factor at wing crack tip was proposed in this study. The proposed calculation model for the stress intensity factor can not only accurately predict the cracking angle of wing crack, but can also simulate the whole range of variation of wing crack length from being extremely short to very long. Based on the modified stress intensity factor, a macro–micro damage model for rock materials was also established by combining the relationship between microcrack growth and macroscopic strain. The proposed damage model was verified with the results from the conventional triaxial compression test of sandstone sample. The results show that the proposed damage model can not only continuously simulate the stress-strain curves under different confining pressures, but also can better predict the peak strength. Furthermore, the sensitivities of initial crack size, crack friction coefficient, fracture toughness, initial damage and parameter m on the stress-strain relationship are discussed. The results can provide a theoretical reference for understanding the effect of microcrack growth on the progressive failure of rock under the compressive loading.

Study of the destruction of the chassis main cross member made of VT22 alloy

The reasons for the destruction of the chassis main cross member made of alloy VT22 are considered and analyzed in bench test conditions. The chemical composition, mechanical properties, as well as macro- and microstructure of the material were studied. The tests of the cross-arm material for crack resistance and low-cycle fatigue (LCF) with the determination of the durability were carried out. The results of analysis proved that material meets the declared performance characteristics. A fractographic study of the traverse fracture showed that the fracture occurred from several foci according to the fatigue mechanism. The length of the longest fatigue crack was 1.7 mm and the critical stress intensity factor KIc was thus attained. Proceeding from the dimensions of the part at the site of fracture, the maximum crack length and the value of the critical stress intensity factor obtained experimentally KIc = 56.5 MPa • m1/2, we have calculated the nominal tensile stress at the moment of fracture. The calculated value of the nominal stresses is 1022 MPa, which is comparable to the yield strength of the material (1100 MPa). A high level of tensile stresses in the loading cycle is considered the most probable reason for the destruction of the chassis main cross member in the conditions of bench tests.

Interpretation of the generalized parameter of the probability of failure through the plastic stress intensity factor

The main purpose of this work is to statistically analyze the fracture toughness of compact specimens made of S55C steel in terms of elastic and plastic stress intensity factors. The fracture toughness tests results at three-point bending were used for a comparative statistical analysis of the fracture parameters. Five type of specimen configuration with various thicknesses were tested at a constant ratio between crack length and specimen width. The critical loads were obtained as a tests result for various combinations of crack length and specimen thickness. In addition, uniaxial tensile tests were carried out to determine the main mechanical properties of the material. Obtained material properties were used in numerical calculations. Numerical calculations were carried out to determine the elastic and plastic stress intensity factors. Three-dimensional finite element analysis was performed on the basis of the experimental data on curvilinear crack front positions in tested specimens. The crack tip stress-strain fields were obtained for each of the tested samples as a result of numerical calculations. These fields were used to calculate the values of the plastic intensity factors along the curvilinear crack fronts. A statistical analysis of the fracture toughness of compact specimens made of S55C steel was carried out based on the obtained critical values of elastic and plastic stress intensity factors. The advantages of using the plastic stress intensity factor as a generalized parameter for the fracture probability are demonstrated. In addition, the sensitivity of the plastic stress intensity factor to constraint effects avoids the introduction of additional parameters into the statistical models.