International Journal of Fracture最新文献

This paper investigates the stress singularity at the crack tip in a two-dimensional Neo-Hookean hyperelastic material, with a focus on how far-field strain influences the crack tip field distribution. The study demonstrates that, under small far-field strains, the crack tip field can be generally divided into three regions: Region I—the asymptotic neo-Hookean crack tip field as (rrightarrow 0); Region II—a finite-radius nonlinear neo-Hookean zone; and Region III—an outer linear elastic region. Within Region III, a subregion may still obey the asymptotic linear elastic solution when the radial distance is sufficiently small. As the far-field strain increases, both the asymptotic linear subregion and the broader linear region shrink and eventually vanish, leaving only the nonlinear zones. This multiscale structure reflects the principle of small-scale nonlinearity, wherein nonlinear effects are confined to an inner core. The inner core consists of Region I, where asymptotic neo-Hookean fields dominate, and Region II, where general nonlinear effects prevail. Initially, this inner core is nested inside Region III. At sufficiently small far-field strains, Region III itself contains an inner core that follows asymptotic linear elastic crack tip fields. As loading intensifies, Regions I and II expand, and Region III—first its asymptotic core, then the broader linear zone – .- progressively diminishes, culminating in a large-scale nonlinearity regime. We also identify and quantify the characteristic length scales over which each region exists and dominates—nonlinear fields in Regions I and II and linear elastic behavior in Region III. An important point is that at the crack tip, Region I always governs the local field, although its extent may be small under small far-field strains, making it difficult to capture in computational simulations. To address this, we introduce a rescaling method to better resolve this near-tip behavior.

This paper presents a novel failure evaluation method for fracture instability analysis of ring-stiffened titanium alloy cylinders under hydrostatic pressure. The quantification of fracture instability behavior for a titanium alloy cylinder remains unclear. In this work, we propose a post-buckling mode analysis for fracture instability, considering the relationship between plastic strain and structural deformation. Further, a novel fracture failure criterion involving material properties and structural parameters is established, bringing the proposal of a structural failure range evaluation method for titanium alloy cylinders. By incorporating the strain rate effect into the constitutive model of the two-phase (alpha +beta ) titanium alloy Ti-6Al-4 V, we developed a dynamic finite element model for studying the fracture instability of the titanium alloy cylinder. The theoretical results and finite element simulation results are finally verified by the collapse experiment of a titanium alloy cylinder. The findings can provide a fundamental basis for designing and service safety evaluation of titanium alloy cylinders.

The tensile strength and fracture properties of concrete play a vital role in the design of large concrete structures. These properties are evaluated from indirect tests on small-sized specimens. As concrete is known to exhibit a strong size effect, it is important to evaluate these properties from large-sized specimens through a direct tension test. In this research, we perform direct tension tests on large-sized concrete specimens under displacement control. The conditions under which the specimens have to be prepared and tested, and the challenges faced are highlighted. It is seen that the tensile strength is almost half the values that are suggested in major codes of practice. Important elastic and fracture properties under direct tension, including tensile strength, fracture toughness, fracture energy, critical crack mouth opening displacement, critical crack length, and softening law parameters, which find application in non-linear finite element models, are evaluated.

In the first part of the companion paper, experimental procedures for conducting direct tension tests on large concrete specimens were discussed, together with the fracture properties and size effects. In this paper, we delve into the details of micromechanisms of fracture and failure in concrete under direct tension using digital imaging and acoustic emission (AE) techniques. The surface cracking characteristics are obtained from imaging, while the evolution of microcracks in the fracture process zone (FPZ) are predicted through AE events and energy. The differences in microcrack initiation, and their coalescence to form macrocracks for different sizes of specimens are explained, and the size of the FPZ is estimated. It is concluded that in large-size specimens, the size of FPZ is relatively smaller with the formation of microcracks, their coalescence to form macrocracks, and the propagation of the final crack taking place almost simultaneously, leading to a brittle failure.

In practical projects, concrete members are often subjected to multiple axial stress states, and the actual fracture process is more complex. This study utilizes a novel gap tensile test method and two-dimensional random circular aggregate mesoscopic numerical simulation to investigate notched concrete fracture characteristics. On the basis of Bažant's Type II size effect law (SEL) and linear elastic fracture mechanics (LEFM), fracture parameters such as fracture energy, notch tensile strength, and fracture toughness of concrete under biaxial tensile stress were obtained. The results indicate, compared with uniaxial loading, that crack–parallel tensile stress significantly affects the key fracture parameters of concrete. As the crack–parallel tensile stress increases, the peak fracture load of all the concrete samples tend to monotonically decrease. Compared with that when the normalized crack–parallel tensile stress η = 0, the peak load decreases by approximately 5, 20, and 40% when η is 0.19, 0.38, and 0.57, respectively, for different specimen dimensions. When the normalized crack–parallel stress increases to 0.57, the fracture energy, fracture toughness, and notch tensile strength decrease by approximately 30–80%. The crack–parallel tensile stress induces predamage in the weak interfacial transition zone (ITZ) of the concrete, leading to a reduction in the load-bearing capacity. Likewise, it can be expected that this degradation effect will be even more severe in low-strength concrete.

We propose a method for quantifying the competition between tunnel and delamination cracking in laminates. Based on the analytical relation between the compliance derivative and the energy release rate, which we refer to as the compliance method, the competition between the two damage mechanisms is investigated based on compliance maps that may be precomputed, thus providing an efficient tool for predicting damage development in laminates without ad hoc fracture simulations. Tunnel and delamination cracking in an off-axis layer, much thinner than the load-carrying layers of a laminate, are used as an example to illustrate the proposed methodology. However, the compliance method is generalizable to any parameterizable cracking mechanism. The cracks are simulated in a specialized two-dimensional off-axis finite element framework, which fully captures the three-dimensional solution. The method is validated against the energy balance method for tunneling cracks and the virtual crack closure technique for delamination cracks. The method has great potential in the context of nondestructive testing, where future damage states can be predicted from precomputed or experimentally determined compliance maps, thus reducing computational and experimental costs.

Shear-box loading could partially separate rock specimen and loading device to keep moment balance, but the influence of the partial separation has not been considered in determining Mode II fracture toughness. In this study, the finite element method was employed to analyze the separation states between rock specimen and loading device in the shear-box tests with various loading angles, and the influences of the separation state on the stress intensity factors and Mode II fracture toughness were investigated by recalibrating the Mode I and Mode II dimensionless shape factors. The results indicate that the separated length increases as the initial crack length or the loading angle increases. If the separation state is not considered in the mechanical analysis, the average relative errors of the resulted Mode I and Mode II stress intensity factors are about 21.66% and 11.06%, and the relative errors of the Mode II fracture toughness determined by the Mohr–Coulomb fracture criterion are greater than 9.3%. Therefore, the influence of the separation state between rock specimen and loading device cannot be ignored in the fracture analysis of the shear-box test.

Two types of crack appearances may be observed in a median crack formed by scribing wheels. A cracked surface with a striped pattern and smooth cracked surface are formed, and a distinct crack-arrest line is formed at the boundary between them. This study explored the mechanisms that control changes in crack surfaces. The median crack caused by wheel scribing propagated within a symmetric stress field composed of axial and shear stresses. Numerical analysis revealed that the direction of the shear stress changed in regions that formed cracks with stripe patterns and smooth cracks. Depending on the combination of the crack propagation direction and symmetry shear stress direction, the crack propagated toward or deviated from the symmetry plane. These crack-propagation characteristics caused differences in the appearance of the crack surface. The existence of an area behind the scribing wheel where the crack temporarily stopped was predicted by estimating the crack behavior based on the stress intensity factors. This temporary crack stop was the cause of the distinct crack arrest lines. This study suggests that crack propagation behavior and surface appearance of cracks can be controlled by generating a symmetrical shear stress field in an appropriate direction.

Ni₂MnGa magnetic shape memory alloys (MSMAs) experience the shape memory effect due to magnetic field-induced or mechanical stress-induced microstructure reorientation. However, crack initiation and propagation, influenced by the evolving twin microstructure under coupled magneto-mechanical loading, can significantly hamper its function in applications. This study presents a semi-empirical approach to evaluate fracture toughness and fracture energy in Ni₂MnGa using Vickers microindentation. An improved analytical expression is proposed, extending the classical indentation-based fracture model to incorporate magneto-mechanical effects and microstructural evolution through a stress and field dependent exponential term. Experimental results confirm that the transverse magnetic field facilitates crack growth, decreasing the fracture energy, while axial compressive stress impedes crack growth, increasing the fracture energy of the alloy. The proposed empirical relationship provides configuration-specific fracture energy values for the alloy and contributes to identifying loading conditions least conducive to fracture initiation and growth in MSMAs.

A novel computational analysis is developed to model mode II fracture of a bimaterial specimen in an End Notched Flexure test considering frictional sliding contact between the crack faces. In the Comninou contact model of interface cracks, the frictional contact zone at the tip of an interface crack between dissimilar linear elastic materials entails a stress singularity, which is weaker than the square root singularity. This weak singularity results in a zero Energy Release Rate (ERR) (G_{II}=0) in such cracks. Therefore, the classical Griffith criterion cannot be used to predict crack growth in this case. To address this challenging issue, a new approach based on the Coupled Criterion (CC) introduced by Leguillon (Eur. J. Mech. A/Solids 21, 61-72, 2002), which adopts the Finite Fracture Mechanics (FFM) hypothesis proposed by Hashin (J. Mech. Phys. Solids, 44, 1129-1145, 1996), is developed. The CC is satisfied when both the stress and incremental energy criteria are satisfied simultaneously. A novel CC implementation is required to address the nonlinearity caused by the frictional contact between the interface crack faces, particularly the frictional dissipation of energy during the growth of such interface cracks. The methodology developed involves Finite Element Analysis (FEA) to compute shear stress and relative displacements along the crack path, the change of the potential energy and the energy dissipated by friction. Finally, the implemented CC provides the critical load and finite crack advance at the initiation of crack propagation. The numerical study presented considers various combinations of isotropic materials and friction coefficients.