International Journal of Fracture最新文献

This work examines the void growth and coalescence in isotropic porous elastoplastic solids with sigmoidal material hardening via finite element three-dimensional unit cell calculations. The investigations are carried out for various combinations of stress triaxiality ratio (({mathcal {T}})) and Lode parameter (({mathcal {L}})) and consider a wide range of sigmoidal hardening behaviors with nominal hardening rates spanning two decades. The effect of ({mathcal {L}}) is considered in the presence and in the absence of imposed shear stress. Our findings reveal that depending on the nature of sigmoidal hardening the cell stress-strain responses may exhibit two distinct transitions with increasing stress triaxiality (({mathcal {T}})). Below a certain lower threshold triaxiality the stress-strain responses are sigmoidal, while above a certain higher triaxiality they exhibit softening immediately following the yield. Between these threshold levels, the responses exhibit an apparent classical rather than sigmoidal strain hardening. The sigmoidal hardening characteristics also influence porosity evolution, which may stagnate before a runaway growth up to final failure. For a given ({mathcal {L}}), an imposed shear stress adversely affects the material ductility at moderate ({mathcal {T}}) whereas at high ({mathcal {T}}) it improves the ductility. Finally, we discuss the role of material hardening and stress state on the residual cell ductility defined as strain to final failure beyond the onset of coalescence.

The present study investigates in the failure of adhesive bondings with structural silicone sealants. Point connectors of two circular metal adherends bonded with DOWSIL™  TSSA are subjected to tensile loading. We formulate and use a constitutive law that captures volumetric softening owing to the formation of cavities. Therein, cavitation is considered a process of elastic instability which is homogenized with a pseudo-elastic approach. Ultimate failure initiating from the free edges is predicted employing the framework of finite fracture mechanics. The concept requires both a strength-of-materials condition and a fracture mechanics condition to be satisfied simultaneously for crack nucleation. For the former, we use a novel multiaxial equivalent strain criterion. For the latter, we employ literature values of the fracture toughness of DOWSIL™  TSSA . The predicted onset of cavitation and ultimate failure loads are in good agreement with our experiments. The proposed model provides initial crack lengths that allow for the derivation of simple engineering models for both initial designs and proof of structural integrity while simultaneously extending the range of usability of the structural silicone compared to standardized approaches.

Hydride precipitation ahead of a crack is examined under conditions of hydrogen chemical equilibrium, constant temperature and elastic-plastic power-law hardening metal behavior. The limiting conditions are approached via the interaction of the operating physical mechanisms of material deformation, hydrogen diffusion and hydride precipitation. Hydrides are characterized by hydride volume fraction and isotropic transformation strain. Analytical relations are presented for hydride volume fraction and stress, as well as for hydride precipitation zone boundary. It is shown that there is an annulus, within the hydride precipitation zone, where stresses, although vary according to ({left(1/rright)}^{1/n+1}) -singularity, deviate significantly from the well-known HRR-field, being smaller, according to the difference of hydrostatic stress before and after hydride precipitation. Hydride precipitation zone increases with crack-tip constraint, given by triaxiality parameter (Q).

This work reports a selective median crack propagation phenomenon in glass, leading to a novel glass cutting process. We found that by scribing a glass sample to the extent of plastic deformation with a deformation depth of 100–400 nm, followed by inducing an initial crack, a subsurface crack with a depth of ~ 10 μm was propagated backward along the centerline of the scribed region with a speed of 1 μm/s order. The crack depth and propagation speed were increased by increasing the scribing load. We conclude that the propagation direction was determined by the effect of the shear stress caused by a scribing tip sliding motion.

The objective of this work is to investigate the three-dimensional nature of stationary mode I notch tip fields in a basal-textured magnesium alloy. To this end, crystal plasticity based finite element analyses are performed pertaining to a four-point bend fracture specimen for two notch orientations. In the first orientation, the notch and line perpendicular to it are taken parallel to transverse and rolling directions, respectively, while in the second, they are chosen along normal and transverse directions. An additional simulation is performed corresponding to an isotropic plastic solid obeying the von Mises yield condition. The macroscopic results from the simulations agree well with an experimental study conducted pertaining to the first orientation. A pronounced thickness variation in stresses is perceived up to a radial distance of about 0.4 times specimen thickness from the tip. The stresses and plastic strains near the tip on the specimen mid-plane are higher for the ND-TD orientation, whereas on the surface they are more for the TD-RD case. In the former, multiple slip systems along with profuse tensile twinning is observed near the tip, whereas prismatic slip is preponderant for the latter. The strong anisotropy of this alloy manifests in terms of plastic zone shape and size, near-tip plastic strain/slip distributions and plane strain constraint ratio.

In the last few decades, the advancements made in material characterisation equipment and physics-based multiscale material modeling have generated vast database in the field of Material Science and Engineering. This has inspired material innovators to attempt predicting mechanical properties of synthesised materials using big-data so as to reduce the cost, time and effort for materials innovation. However, the impact of collinerarity has always been a matter of concern in emperical research, specially in such predictions of mechanical properties. In the present work, we revisit NIMS database for steel and study the effect of multicollinearity on regression based models for predicting fatigue strength for the material. We use an iterative scheme to isolate highly correlated parameters contributing in determination of the fatigue strength of the steel. We then construct a regression model using only the non-correlated parameters to make the model more efficient computationally. Our results show that the regression model built after consideration of multicollinearity of the variables provide better performance in comparison with regression model built without consideration of the same.

Despite the extensive research on crack propagation in brittle solids, numerous unexplored problems still necessitate in-depth study. In this work, we focus on numerical modeling of multi-crack growth, aiming to explore the effect of material heterogeneity and multi-crack interaction on this process. To do this, an improved singular-finite element method (singular-FEM) is proposed with incorporation of heterogeneity and crack interaction. An efficient algorithm is proposed for simulating multi-crack propagation and interaction. Stress singularity near crack tip is reproduced by the singular elements. The singular-FEM is convenient and cost-effective, as the zone far away from crack tips is directly discretized using linear elements, in contrast to the quadratic or transition elements utilized in traditional FEM. Next, the proposed method is validated through benchmark study. Numerical results demonstrate that the superiority of the singular-FEM, which combines the merits of low cost and high accuracy. Then, the mechanics of crack growth are explored in more complex scenarios, accounting for the effects of crack interaction, loading condition and heterogeneity on crack trajectory, stress field and energy release rate. The findings reveal that the combined effect of heterogeneity and crack interaction plays a critical role in the phenomenon of crack growth, and the proposed method is capable of effectively modeling the process.

The study of fracture mechanics is usually within the paradigm of a failure mode that needs to be avoided. However, both in nature and in modern technology, there exist several situations where an ability to fracture is essential. In this work, we consider the problem of machining highly ductile and strain-hardening metals, such as annealed Cu, Al and Ta. These metals are known by the moniker “gummy metals” due to the large forces and poor surface finish associated with machining them. We investigate a chemo-mechanical technique involving adsorption of organic monolayers on the metal surfaces that causes the metals to become relatively brittle. This transition from ductile to brittle results in > 50% drop in the cutting force and an order of magnitude improvement in the surface finish. Molecular dynamics simulations of the phenomenon show the organic monolayers impose a surface stress on the metal surface which results in arresting of the dislocations close to the surface. The results suggest that a deeper understanding of the underlying mechanism has implications in environment-assisted cracking, stress-corrosion cracking and hydrogen embrittlement.

Ductile materials exhibit rate-dependent behaviors when subjected to different loading rates, particularly during impact and explosion events. In order to investigate the high strain rate behaviors of metal materials, a phase field model considered the rate-dependent threshold for effective plastic work is proposed. And the presented model couples the influences of the stress triaxiality and Lode angle parameter on failure behaviors. Later, a single element is modeled to demonstrate the impacts of the model in predicting stress-strain relations under varying loading rates. To illustrate the impacts of the Lode angle parameter on load-displacement responses, rectangular notch specimens are used. Next, the presented model is employed to mimic the shear fracture of hat-shaped specimens at different strain rates based on the split Hopkinson pressure bar tests, and the model parameters are calibrated by comparing the strain waveforms between the simulations and experiments. The numerical results indicate the developed model is capable of accurately reproducing the shear ductile fracture of the hat-shaped specimens under high strain rates.

Micro-crack propagation in polycrystalline materials can strongly depend on the defect size and its ratio to specimen size, and local variation in the microstructural features such as grain orientation, size, etc. While the dependencies are understood heuristically, the use of mechanistic models to capture the effect of various factors influencing micro-crack propagation can enable accurate prediction of fracture properties of polycrystalline materials and their engineering. To this end, a crystal plasticity coupled to damage model for micro-crack propagation on cleavage planes has been developed in this work and is shown to successfully capture the grain orientation dependent growth. In order to identify a suitable integration scheme for the coupled model, a one-dimensional model is developed and a detailed comparative analysis of three different schemes is performed. The analysis shows that the coupled explicit–implicit scheme is the most suitable and is a key finding of this work. Subsequently, a two-scale multi-scale method has been developed to include the interaction between the defect, its surrounding microstructure and the specimen. The two-scale method along with the coupled crystal plasticity-damage model has been applied to perform finite element method based micro-crack growth simulations for a microstructurally short and physically long crack with two different microstructures with random orientation and texture. Such a study comparing microstructural effects on crack growth from pre-existing defects of drastically disparate sizes hasn’t been performed before and is a novelty of this work. The analyses clearly show that though the micro-crack path from long crack is different depending on the orientation distribution, the rates are nearly independent of the local behavior. Moreover, the micro-crack propagation rate from long crack is significantly larger at the initial stages, with the latter showing significant acceleration after a small growth. Overall, the influence of microstructure on the crack growth behavior is stronger for short cracks, which conform with experimental observations and is successfully captured by the proposed model.