International Journal of Fracture最新文献

The present study aims to configure and train a data-driven geometry-specific surrogate model (DD GSM) to simulate the load–displacement behavior until fracture in cylindrical notched specimens subjected to uniaxial monotonic tension tests. Plastic strain hardening that governs the load–displacement behavior and ductile fracture in metals are history-dependent phenomena. With this, the load–displacement response until ductile fracture in metals is hypothesized as time sequence data. To test our hypothesis, a long short-term memory (LSTM) based deep neural network was configured and trained. LSTM is a type of neural network that takes sequential data as input and forecasts the future based on the learned past sequential trend. In this study, the trained LSTM network is referred to as DD GSM as it is used to forecast the load–displacement behavior until ductile fracture for the cylindrical notched specimens. The DD GSM is trained using the load–displacement data until fracture, extracted from the finite element analyses of notched cylindrical test specimens made of ASTM A992 steel. The damage leading to fracture was captured using the Gurson–Tvergaard–Needleman (GTN) model. Finally, the trained DD GSM is validated by predicting the overall load–displacement behavior, fracture displacement, and peak load-carrying capacity of cylindrical notched ASTM A992 structural steel specimens available in the literature that are not used for training purposes. The DD GSM was able to forecast some portions of the load–displacement curve and predict the fracture displacement and peak load-carrying capacity of the notched specimens. Furthermore, the geometric sensitivity of the trained DD GSM was demonstrated by simulating the load–displacement response of an ASTM A992 steel bar with a central hole.

Within the context of linear elasticity, approximate analytical solutions are developed for the energy release rate for axisymmetric planar cracks in elastic thin layers sandwiched between two rigid plates. These solutions are validated by comparing them with finite element solutions, and they are applicable to cracks in constrained thin layers made of compressible, nearly incompressible, or incompressible materials. These analytical solutions provide insights into the effects of geometry and material compressibility on fracture of thin layers. In particular, stability of crack growth is discussed under both displacement and force-controlled loading conditions, summarized in stability maps. Remarkably, it is found that, under force-controlled conditions, stable crack growth is possible in incompressible or nearly incompressible layers, but not in compressible layers. We compare the energy release rates for embedded and interfacial cracks, showing that they differ when the cracks are small but become approximately equal for large cracks. The analytical approach is further extended to non-axisymmetric planar cracks in compressible thin layers. However, a similar extension does not apply for cracks in incompressible or nearly incompressible layers.

Crack-tip dislocation emission is often considered to be the key mechanism that controls the so-called “intrinsically ductile” fracture behaviour. Yet, high fracture toughness and ductility in metals are determined by extensive plastic deformation that dissipates much more energy than solely due to the crack-tip emission process. Thus, there is a gap between intrinsically ductile behaviour and large toughness. Here, we implement the dislocation emission process within a 2D discrete dislocation plasticity (DDP) framework. The framework, which includes anisotropic elasticity and a cohesive-zone model to simulate crack propagation, enables to investigate the interplay between dislocation emission and near-crack-tip plasticity associated with activation of dislocation sources. Guided by dimensional analysis and a sensitivity study, we identify the main variables controlling the fracture process, including dislocation source and obstacle density, dislocation emission strength and the associated dwelling time-scales. DDP simulations are conducted with a range of parameters under mode-I loading. The initiation fracture toughness and the crack-growth resistance curve (R-curve) are calculated accounting for the statistics of dislocation and obstacle distributions. Comparison is performed with cases where no dislocation emission is enabled. Our findings show that dislocation emission can slow down crack growth considerably, resulting in a significant increase in slope of the R-curve. This phenomenon is due to crack-tip shielding caused by the emitted dislocations. Thus, intrinsic ductility can enhance crack-growth resistance and fracture toughness. However, we find that the extent of shielding can also be negligible for some emission planes, making the connection between intrinsic ductility and fracture toughness not straightforward.

Hydraulic fracturing has been widely applied in underground mines for disaster prevention. The effectiveness is highly depended on the morphology of hydraulic fracture (HF), which, however, greatly affected by mining activities. Revealing the propagation behaviors of HF is of profound significance to understand the coupling of coal mining and hydraulic fracturing. In this paper, HF propagation was conducted based on lattice-spring method (LSM) after the analysis of mining-induced triaxial stress. The effects of mining stage, injection rate, and fracturing interval on HF propagation are investigated. The results show that mining-induced triaxial stress is beneficial to the formation of HF network. Once beyond rock failure pressure, increasing injection rate emphasized the stress shadow effect regardless of high or low stress state, resulting in crossing and diversion HFs. However, this effect was mitigated by broadening fracturing intervals which beneficial for vertical cessation HFs. Due to the complicated stress state in severely-affected stress region, the propagation length and angle of HF were influenced significantly, contributing to the occurrence of distortional HFs, HFs connection, and cross-layer phenomenon. Therefore, in the severely-affected stress region, an interlaced fracture network is likely to emerge, particularly in the case of narrow fracturing intervals and high injection rates. The research is committed to provide constructive suggestions for optimizing hydraulic fracturing under mining.

Fracture toughness describes a material’s ability to resist failure in the presence of defects. In case of soft biological tissues, a reliable determination and interpretation of the fracture properties is essential to estimate the risk of fracture after clinical interventions. Here we perform a comparative computational study between soft biological tissues and compliant elastomers to discuss the influence of material non-linearity on the crack tip nearfield. Using detailed finite element simulations, singular near-tip stress fields are obtained, and a so-called nonlinear region is identified. Additional focus is put on the effect of material nonlinearity on the phenomenon of elastic crack blunting, by analysing the deformed crack profile and extracting a radius of curvature at the tip. Through concepts of traditional fracture mechanics, we identify the size of the process zone and nonlinear elastic zone in biological tissues, juxtaposed with that of elastomers, demonstrating the limitations of the traditional metrics in capturing the remarkable defect tolerance of this highly nonlinear material class.

This work focuses on the effects of hydrogen flakes on the fracture toughness of forged ferritic steel under uniaxial or biaxial loading conditions. The fracture toughness under uniaxial loading was investigated on CT specimens and cruciform specimens were used for the biaxial loading conditions representative of a thermal shock in a Reactor Pressure Vessel. The observed decrease in fracture toughness in the flaked material was related to the higher carbon content near the hydrogen flake. Moreover, the nature of the initial defect (fatigue crack or hydrogen flake) did not significantly affect the fracture toughness. The cruciform specimen exhibited higher fracture toughness compared to CT specimens, even in the presence of flakes. This confirmed the conservatism of standard fracture analyses used for structure integrity assessment based on a lower bound fracture toughness curve obtained from CT specimens.

Assuming a scenario of small-scale domain switching, the dimensions and configuration of the domain switching region preceding a clearly defined primarily monoclinic piezoelectric bi-material notch are determined by embracing the energetic switching principle and micromechanical domain switching framework proposed by Hwang et al. (Acta Metall Mater 43(5):2073–2084, 1995. https://doi.org/10.1016/0956-7151(94)00379-V) for a given set of materials, structure, and polarization alignment. The piezoelectric bi-material under consideration comprises piezoelectric ceramics PZT-5H and BaTiO₃. The analysis of the asymptotic in-plane field around a bi-material sharp notch is conducted utilizing the extended Lekhnitskii–Eshelby–Stroh formalism (Ting in Anisotropic elasticity, Oxford University Press. 1996. https://doi.org/10.1093/oso/9780195074475.001.0001). Subsequently, the boundary value problem with the prescribed spontaneous strain and polarization within the switching domain is solved and their influence on the in-plane intensity of singularity at the tip of interface crack is computed. The effects of the initial poling direction on the resulting variation of the energy release rates are discussed.

It is well recognized in the literature that the fracture process of thin metal sheets involves three energy dissipation mechanisms i.e., plasticity, necking and surface separation. However, the complex stress state in thin structures hinders the experimental assessment of these quantities and, consequently, the failure modelling. This work evaluates the contribution of these mechanisms to the ductile damage of a thin advanced high strength steel sheet under different stress triaxiality ranges. The essential work of fracture test was carried out on a set of different notch geometry specimens that cover a wide range of stress states. The experimental trend of these specimens was simulated in ABAQUS/Explicit using a VUSDFLD subroutine. Bai and Wierzbicki uncoupled fracture model, which is a function of fracture plastic strain to stress triaxiality (η) and normalized Lode angle ((overline{theta })), was selected as damage initiation criterion. A quantitative relationship of the fracture energy (G₀) as a function of (η) was proposed in this work and implemented in the model as a damage evolution law. The model captures well the experimental response and the influence of (η) on the softening behavior of the material. It was found that the sensitivity of G₀ to η is significant between 0.7 and 1.5. Above this rage, it seems that (η) has no influence on G₀. The model showed also the relationship between the two local damage parameters (G₀) and the necking (G_n) with respect to the stress state. G₀ represents less than 10% of the total work of fracture, while the largest contribution comes from (G_n).

This work introduces a novel approach for characterizing the residual load bearing capacity of fractured components based on the Phase Field fracture model. The underlying idea involves exploiting this well-established framework for fracturing materials and applying it to mechanically loaded domains in which fracture has already occurred. Hence, the continuous phase field here portrays the smeared representation of known crack patterns, based on which the unilateral contact interactions between the crack lips are enforced through a suitable strain energy decomposition. This allows for a theoretically robust and implicit treatment of the originally discontinuous problem while remaining in a continuum framework. As such, the proposed approach avoids the numerically challenging definition and management of conventional contact pairs, thus proving to be especially promising for its application to domains with multiple fragments. Besides presenting the theoretical foundation and algorithmic convenience of the approach, its accuracy and representativeness are proven against theoretical predictions and numerical results from Finite Element models featuring conventional contact interactions.