Crack Propagation Mode Research Articles

Performance of 3D-Printed Bionic Conch-Like Composite Plate under Low-Velocity Impact.

Biological armors can provide an effective protection against predators. In this study, inspired by conch shell, beetle exoskeleton, and nacre, three different types of bionic composites plates were fabricated: Bio-S, Bio-B, and Bio-N, as well as an equivalent monolithic plate formed from the same stiff material designed and manufactured by additive manufacturing, respectively. Low velocity impact tests using drop tower were conducted to study their impact resistance. Experimental findings indicated that the Bio-S composite had superior impact resistance compared with the other bionic composites and the monolithic plate. Furthermore, the influence of the ply angle on the impact resistance of the Bio-S composite plate was investigated. The (0°/30°/0°/30°) arrangement was able to provide the highest impact resistance. Finally, the crack propagation mode in Bio-S composites plates was analyzed, enhancing our understanding of the underlying mechanisms during impact. Such findings may lead to the development of superior lightweight protective structures with improved anti-impact performance.

A Numerical Study on the Crack Propagation of Homogenized Micro-Crack Crushing for Concrete Pavement

Homogenized micro-crack crushing is a new method of concrete pavement rehabilitation that makes full use of the bearing capacity of the original concrete pavement, whereby the treated pavement can be directly overlaid with hot mixed asphalt concrete. In order to solve the problem of longitudinal crack extension of a cracked core in the weak position of local pavement, the crack propagation mode of concrete slabs under low-velocity impact from different numbers and distributions of impact heads was studied using numerical simulation. Moreover, field tests were also carried out to determine the optimal layout of drop hammer impact heads, and the numerical simulation results were compared with the experimental data. The results show that, when a concrete slab is impacted by a single impact head, the bottom of the slab first cracks and then the crack develops upward until the main crack runs through the concrete slab. However, when the concrete slab is impacted by multiple-impact heads, including three or four impact heads, the concrete slab forms a triangular or quadrilateral fracture core, respectively. The numerical simulation results are in good agreement with the laboratory experiments. Through the optimized arrangement of the hammer’s impact heads, the problem of longitudinal crack extension of a cracked core can be effectively solved, and the probability of reflection cracks can be reduced.

Investigation of Anisotropic Behavior and Statistical Evolution of Acoustic Emission Energy during the Deformation of Layered Sandstone

To further understand the anisotropic behavior of layered rock and the precursor characteristics of rock mass instability, a series of uniaxial compression experiments using a loading system and an acoustic emission system was conducted on sandstone specimens. The influence of bedding on the mechanical parameters and failure modes and the statistical evolution of the acoustic emission energy were successively discussed. The results of axial stress-strain curves and crack propagation modes showed that the existence of a bedding plane increased the anisotropy of the rocks, and the magnitude of the bedding inclination also exerted certain influence on this anisotropy. Furthermore, we used the least squares method and the maximum likelihood method to analyze the b value and power-law exponent, respectively. The results of statistical evolution of acoustic emission energy showed that the b value, the effective power-law exponent, and the optimal exponent could be used as monitoring indexes for the rock mass stability. With the progress of the experiment, the following phenomena pertaining to acoustic emission activities occurred, which may indicate imminent danger of collapse: (1) the crackling noises increased significantly; (2) the variation of the b value exhibited a significant downward trend; and (3) the effective power-law exponent and the optimal exponent changed in different stages and gradually decreased as the final failure was approached. The findings in this paper may provide a theoretical basis for predicting the collapse and instability of rock mass structures.

Theoretical study on the hydraulic sealing integrity of cement sheath in horizontal shale oil wells under fracturing: Crack propagation mode and mechanism

The cement sheath-formation interface is a weak link in wellbore isolation systems, which is prone to the failure of sealing integrity during fracturing, affecting the fracturing effect, and wellbore safety. In this study, we establish a new model to evaluate the failure mode and crack propagation of cement sheath-formation interface. Using the proposed model, we predicted the propagation direction and length of the interface and zigzag cracks caused by the interface stress, fracturing load, and stress intensity factor (SIF) under temperature and pressure in the shale oil well X in the eastern China oil field. The prediction results were consistent with the distribution law of measured microseismic events. In addition, we analyzed the influence of the internal casing pressure, mechanical properties of cement sheath, and formation on the sealing integrity failure and cracks. At the same time, we discussed the influence of these mechanical properties on the propagation length of interface and zigzag cracks and the propagation direction of the zigzag crack in detail, and proposed the optimal mechanical properties design to avoid the crack propagating to the cement sheath and causing its failure. The results can provide effective guidance for sealing integrity prediction and cement sheath mechanical performance design.

Acoustic emission characteristics and damage law for prefabricated single‐crack sandstone under uniaxial compression

The cracks of rock seriously threaten the safety of building structures and underground space. Uniaxial compression tests were carried out on prefabricated single-crack sandstone samples with different angles and lengths. The results show that the existence of a prefabricated single-crack has a certain influence on the mechanical properties and acoustic emission response characteristics of sandstone samples. A function formula of the effect of uniaxial compression stress on the angle and length of prefabricated crack is established. It is found that there is an obvious moment of acoustic emission energy mutation in the three deformation and failure stages before sandstone failure. The change of angle and length of crack has a great influence on the stress value corresponding to the energy mutation at the stable development stage of the micro-crack. Meanwhile, the acoustic emission waveform characteristic parameters of sandstone samples under various fracture conditions are analyzed. Thus, the crack propagation mode of the sample with small angle can be judged. The damage variables based on real-time acoustic emission energy were defined, and the damage constitutive model of rock material with a single fracture under uniaxial compression was established based on real-time acoustic emission energy. The results show that the new model and method can not only reasonably explain the damage mechanism of prefabricated rock with a single fracture under uniaxial compression but also accurately simulate the full stress-strain process of rock with a single fracture at different angles and lengths, which has good rationality and feasibility.

Failure mechanisms and loading capacity prediction for rectangular UHPC beams under pure torsion

This study presents an experimental investigation of the loading capacity and failure mechanism of ultra-high-performance concrete (UHPC) beams under pure torsion. Twelve rectangular solid beams were fabricated and tested. The investigated parameters include the volumetric fraction of steel fibers (1.5%, 2.5%, and 3.5%), the longitudinal reinforcement ratio (0–1.54%), and the stirrup ratio (0–1.29%). The torsional behavior, including the crack propagation and failure modes, torque–twist curves, torque–strain curves, cracking and ultimate strengths, and effectiveness of steel fibers were evaluated and presented. Considering the UHPC high tensile strength, a formula for UHPC solid beams was proposed and verified by a reliability analysis. The results showed that compared to conventional concrete beams, the UHPC beams exhibited similar suitably reinforced, partially over-reinforced, and over-reinforced failure modes. Additionally, two different failure modes (type-I and type-II barely reinforced) were identified for UHPC beams. The additional steel fibers improve the torsional properties and directly determine the effective reinforcement ratio. Moreover, the steel fibers with 1.5% or higher volume fractions were proven to be sufficient to replace stirrups, and the proposed formula is effective in predicting the torsional strength of UHPC rectangular solid beams.

Comparison of continuum damage models for nonlinear finite element analysis of timber under tension in parallel and perpendicular to grain directions

A reliable and efficient numerical modelling technique is essential to investigate the behaviour of timber and engineered timber products to promote their widespread use in construction. Wood is an anisotropic material and hence its mechanical properties largely depend on grain direction and type of loading i.e., material behaves differently under compression and tension. Material responses under tension parallel and perpendicular to the grain directions have been reported in the literature but the relevant progressive fracture behaviour has been ignored in typical numerical simulations, due to the complexities and uncertainties around modelling as well as lack of reliable test data. Fracture characteristics play a significant role in analysing crack initiation, propagation, and failure modes of timber so that its full potential can be utilised by knowing the post-elastic behaviour. This paper applies and compares four continuum damage mechanics based constitutive material models (MAT-22, MAT54/55, MAT-143 and MAT-261) available in the commercial finite element software LS-DYNA for simulating the post-elastic behaviour of general timber lamella products. Timber was modelled as both orthotropic and transversely isotropic material to simulate the fracture behaviour in tensile load cases. It is shown that the predicted fracture properties correlate well with experimental data. It was observed that all considered built-in continuum damage models in LS-DYNA are able to simulate the elastic response, but MAT-261, which was originally developed for modelling fibre reinforced composite materials, provides a simple yet reliable option for simulating fracture behaviour of timber.

Digital-image-correlation observation of cyclic plastic strain field during the damage-accumulation mode of fatigue crack propagation under pure cyclic mode II loading for cold-rolled SUS430 steel

Fatigue cracks propagate intermittently by the cyclic initiation and coalescence of voids in front of the fatigue crack tip during fatigue crack propagation in the damage accumulation mode. In this study, digital image correlation is used to measure the evolution of the measured plastic strain distribution near the crack tip with loading and cyclic loading during one cycle. The evolution is estimated based on strain distribution measurements results and mechanical considerations. The strain is set at the unloading point because results indicate that the crack propagated in the region where the experimental shear plastic strain is the maximum in the first quarter cycle (at the first maximum load); the measured plastic strain is almost constant in the region where the crack propagates, and the crack is initiated by a subsequent cyclic strain localization and coalescence with the main crack. This study clarified the damage accumulation mode fatigue crack propagation mechanism with the measured strain behavior, and presents a method to use continuum plastic strain at the crack tip as a mechanical driving force for this mode.

Crack interaction and fracturing of geomaterials with multiscale cracks

Geomaterials contain numerous cracks whose interaction affects the macroscopic mechanical properties of the materials significantly. We propose a mechanical model of cracks of 3 scale levels to investigate the pattern of crack propagation and possible failure modes produced. For modelling crack interaction between different scale levels, we model the unloading domain of influence of a single crack as the sphere with the diameter being the crack size drawn around the crack center. When the tip of a lower scale crack propagates into the influence domain of a crack of a higher scale level, the propagating crack of a lower scale will be unloaded and never grow from this tip. Propagation and coalescence of cracks of different scale levels for different initial crack concentration result in different patterns of cracks propagation and failure. For relatively lower initial crack concentrations, the interaction of cracks of the first scale level dominates, producing failure due to the coalescence of cracks of the first scale level. With the increase of the initial crack concentration, the coalescence of cracks of the third scale level dominates leading to failure at a higher initial concentration. With the further increase of the initial crack concentration, the failure is produced by the coalescence of cracks of the second scale level. At even higher initial crack concentrations the failure is again produced by the coalescence of cracks of the first scale level. The dynamic strength decreases with the increase in the initial crack concentration. • A model of interaction, growth and coalescence of self-similar cracks of 3 scale levels is proposed. • The unloading effect of higher scale level cracks on the propagation of lower scale level cracks is considered. • Degradation of the effective deformation modulus is considered in solving crack-tip equations of motion. • The discreteness of crack propagation at the accelerating stage of loading is discovered. • At certain intervals of the initial crack concentration cracks of the lowest scale level will coalesce first.

The anisotropic fatigue properties of extruded Al–Zn–Mg–Cu alloy

The effect of specimen orientation on the fatigue properties of extruded Al–Zn–Mg–Cu alloy was investigated, and the relationship between tensile properties and fatigue strength was established. The results show that the fatigue properties of specimens vary largely with the orientation. The specimens with 0° orientation (relative to extrusion direction) have the highest fatigue strength, i.e. 301.3 MPa, whereas the fatigue strength of specimens with 45°and 90° orientation was reduced by 13.3% and 33.5%, respectively. The fatigue strength is linearly proportional to the elongation. It was found that the crack propagation modes vary with specimen orientation, a result of the grain orientation and intergranular coarse second phase particles, leading to the difference in fatigue lives. In addition, Basquin equations for specimens in the three directions were proposed for fatigue life prediction.

Behaviour of continuous steel–concrete composite beams strengthened with CFRP sheets at hogging-moment region

Although the use of carbon fibre reinforced polymer (CFRP) materials for strengthening reinforced concrete structures and steel structures is well established, relatively few works have been reported for the strengthening of steel–concrete composite structures, especially at their hogging-moment region. In this paper, an experimental program consists of eight beam specimens with different degrees of shear connection, numbers of layers of CFRP sheets and strengthening methods was conducted. The experimental results show that adding CFRP sheets on the top of the concrete slab at hogging-moment region has a great effect on the structural performance in terms of ultimate loading capacity, load–deflection and load-slip responses, crack propagation, ductility, and failure mode, etc. The hybrid bonded FRP (HB-FRP) technique is firstly applied for the strengthening of steel–concrete structures at hogging-moment region in this study, and the advantages of the HB-FRP technique is proved through the comparisons with the externally bonded (EB) method. Based on the experimental observations, an analytical method for estimating the ultimate moment capacity of the strengthened composite beam at hogging-moment region is developed, for which the validity is verified against the experiment results.

Mode I and II R-curves characterization of the Maritime Pine and Spruce under the same geometry

This study provides detailed data on the quasi-brittle behaviour of two softwood species, the Maritime Pine (Pinus pinaster.) and the Spruce (Picea abies.), by using R-curves. The methodology of characterization of the R-curve used in this article is an improvement of the method presented by Lespine (2007). This method is automated and based on the measurement error made on GR(Δau). And the same geometry to characterize the crack propagation in mode I and in mode II. The influence of the crack plane orientation and the density on the fracture mechanisms is investigated for both species and shows a dependency of the R-curve parameters of the Maritime Pine in mode I to the crack plane orientation. The differences between Spruce and Maritime Pine are highlighted. This demonstrate the importance to characterize the quasi-brittle behaviour of each wood species in order to provide a useful data base for engineers.

An improved smoothed particle hydrodynamics method for simulating crack propagation and coalescence in brittle fracture of rock materials

A new meshless numerical method, namely the improved smoothed particle hydrodynamics (ISPH) method, is proposed to simulate the fracture process of rock samples with pre-existing flaws under uniaxial compression. By improving the kernel function of SPH method, the brittle fracture characteristics of materials are presented. In ISPH, a prefabricated node segment (PNS) method is used to generate arbitrarily complex flaws, which avoids the division and distortion of traditional particles. In order to further improve the numerical accuracy, a new solid boundary treatment method called coupled dynamic buffer SBT algorithm is applied to ISPH, which is verified to be effective and robust via two benchmark samples. The effect of the length of a pre-existing flaw on the fracture mode and compressive strength of the rock sample is studied by using the elastic-brittle constitutive model. The numerical prediction results are in good agreement with the experimental observations. In addition, the effect of the unconnected rate on the crack propagation and coalescence mode of a rock sample with two pre-existing flaws under uniaxial compression is also studied. The numerical simulation results are in good agreement with the experimental results, which further verify the accuracy and superiority of the ISPH method.

Effect of ultra-high temperature treatment on microstructures and mechanical properties of TRISO particles

Tristructural-isotropic (TRISO) fuel particles have long been widely used in high temperature gas-cooled reactors (HTGR), where their high temperature mechanical integrity and ability to retain fission products even in the accident conditions are the critical safety issues. In the present work, TRISO particles were subjected to ultra-high temperature thermal treatments at 1900, 2000, and 2200 °C, respectively, which were much higher than the projected accident temperature (1620 °C). Phase compositions and microstructures of the samples before and after thermal treatments were studied by X-ray diffraction (XRD), Raman spectroscopy, and Scanning Electron Microscopy (SEM). Nano hardness and Young's modulus of the SiC layer in TRISO particles were studied by nano indentation technique, and crushing strength of the TRISO particles was evaluated by mechanical crush tests. The results show that, there were no obvious temperature effects on phase composition and microstructure, except for SiC layer in the sample thermal treated at 2200 °C, in which many pores caused by SiC decomposition were observed. The nano hardness and Young's modulus values of SiC layer in the as-received and thermally treated TRISO particles were similar within the experimental error. On the other hand, the crushing strength of the thermally treated TRISO particles decreased greatly as compared with the as-received particles, but the crack propagation mode changed from through-the-particle to deflection inside the buffer PyC layer or between buffer PyC and inner PyC (IPyC) layers. The change of crack propagation mode could probably due to the shrinkage of PyC layers due to anisotropy at high temperature and ordering of graphitization of IPyC layers, which probably weakened the interfacial bonding between the IPyC and the buffer PyC layers. The results of this study provide a database reference for the performance of TRISO particles under ultra-high temperature operation and even accident conditions.

Numerical Simulation Study on the Strengthening Mechanism of Rock Materials under Impact Loads

Based on an analysis of real stress in rock specimens, the mechanisms of inertia and cracks on the dynamic properties of concrete material are discussed in this paper. A dynamic enhancement factor is introduced on the basis of static mesodamage constitutive theory, and a rate-dependent three-dimensional rock dynamic constitutive model is established. The orthogonal design and range method are used to analyze the sensitivity of the dynamic damage constitutive parameters, and a differential evolution algorithm is used to invert the relevant parameters of the constitutive model. Based on the drop hammer impact model established by the rock dynamic damage constitutive model, the AE distribution, crack propagation mode, and load-displacement curve of the rock sample are obtained. The numerical simulation results show that rock specimens under dynamic loading have different failure forms, mechanical response mechanisms, and energy evolution than those under static loading. Under dynamic loading, the rock specimen forms more cracks and releases more elastic energy. Furthermore, it shows greater strength characteristics. Inhomogeneities and inertial effects of rock materials are the fundamental reasons for the increase in the dynamic strength of rock materials. A reduction in inertial effects is the main reason for strength reduction in rock samples with hole defects.

Failure analysis of cracking in the welded joints of hydrogen reformer outlet pigtail tubes

Cracking failure of Incoloy 800H welded joints used for about eight months occurred in hydrogen reformer outlet pigtail tubes. Visual inspection, microscopic examination, chemical composition and microhardness testing were performed to find out the failure mechanism. The results indicated that microcracks were discovered in the fusion zone and heat affected zone (HAZ). The crack propagation mode was intergranular and irregular. The cracked regions were occupied with some oxides in the fusion zone. The Cr content at the grain boundaries was abnormal and intergranular oxidation was found in the HAZ. The microcracks in the outlet pigtail tubes were caused by stress relaxation, and oxidation at grain boundaries. The intergranular oxidation caused the embrittlement at the grain boundaries during long-term service. From the investigations, it was believed that the failure of welded joints in the outlet pigtail tubes originated from stress relaxation cracking during high temperature service.

Distribution of cracks in an anchored cavern under blast load based on cohesive elements

To explore the distribution of cracks in anchored caverns under the blast load, cohesive elements with zero thickness were employed to simulate crack propagation through numerical analysis based on a similar model test. Furthermore, the crack propagation process in anchored caverns under top explosion was analyzed. The crack propagation modes and distributions in anchored caverns with different dip angles fractures in the vault were thoroughly discussed. With the propagation of the explosive stress waves, cracks successively occur at the arch foot, the floor of the anchored caverns, and the boundary of the anchored zone of the vault. Tensile cracks are preliminarily found in rocks that surround the caverns. In the scenario of a pre-fabricated fracture in the upper part of the vault, the number of cracks at the boundary of the anchored zone of the vault first decreases then increases with the increasing dip angle of the pre-fabricated fracture. When the dip angle of the pre-fabricated fracture is 45°, the fewest cracks occur at the boundary of the anchored zone. The wing cracks deflected to the vault are formed at the tip of the pre-fabricated fracture, around which are synchronous formed tensile and shear cracks. Under top explosion, the peak displacement and the peak particle velocity in surrounding rocks of anchored caverns both reach their maximum values at the vault, successively followed by the sidewall and the floor. In addition, with the different dip angles of the pre-fabricated fracture, asymmetry could be found between the peak displacement and the peak particle velocity. The vault displacement of anchored caverns is mainly attributed to tensile cracks at the boundary of the anchored zone, which are generated due to the tensile waves reflected from the free face of the vault. When a fracture occurs in the vault, the peak displacement of the vault gradually decreases while the residual displacement increases.

A coupled thermo-mechanical bond-based smoothed particle dynamics model for simulating thermal cracking in rocks

A smoothed particle dynamics (SPD) method was proposed for crack propagation analysis. The computational efficiency of this method was significantly better than the traditional smoothed particle hydrodynamics (SPH) method. In the SPD method, the motion equations were approximated by a Lagrange kernel determined by the coordinates of the material in the initial configuration. A coupled thermo-mechanical bond-based smoothed particle dynamics (TM-BB-SPD) model was proposed for simulating the thermal cracking process of rocks. In the SPD program, the thermal cracking behaviors of rock, such as crack initiation and propagation, were dynamically tracked by capturing the damage state of virtual bonds. The robustness and accuracy of the proposed SPD method and TM-BB-SPD model were verified using two benchmark examples. The numerical results were in good agreement with the experimental results and analytical solutions. In addition, the effects of the thermal expansion coefficient and homogeneity index on the mode of thermal cracking in the rock disc were also investigated. The numerical results were also in good agreement with previous experimental observations. Moreover, the numerical results showed that with an increase in the homogeneity index, the mode of crack propagation in rock gradually changed from non-smooth to smooth types of fracture.

Crack propagation characteristics and fracture toughness analysis of rock-based layered material with pre-existing crack under semi-circular bending

Fracture initiation, fracture propagation, and their interactions with the interlayer interface in rock-based layered material are important in assessing the stability of rock-based structures such as lined tunnels. A numerical model based on the inserted cohesive zone model (CZM) method is established, and further verified against the semi-circular bending (SCB) test to simulate the complex crack initiation and propagation behavior in a rock-based layered semi-circular disk with pre-existing rock crack at different angles. From the results, three typical crack propagation modes are identified: interface fracture, wing fracture from crack tip and wing fracture not from crack tip. At a small crack inclination angle, interfacial crack is formed, while at a larger crack inclination angle, wing crack which initiates deviating from the crack tip appears. It is found that the crack propagation mode is also controlled by the interface strength, and the interface crack is always formed in SCB specimens with lower interface strength. By increasing the interface strength, the length of interface crack gradually decreases and wing crack presents. When the pre-existing rock crack inclines greater than 45° and the interface strength is larger, the wing crack initiating far away from the crack tip merges with the interface crack to be connected propagating path, and the initiation position of wing crack gradually moves toward the crack tip with the further increasing of interface strength. Moreover, it is also found that the crack length ratio and the support span ratio are important influencing factors for crack propagation mode. Increasing the crack length ratio can suppress the formation of interfacial crack. When the crack inclination angle is more than 45°, the wing crack tends to originate at the tip of pre-existing crack as the support span ratio increases. In addition, the effects of crack length ratio, support span ratio, crack inclination angle and Young’s modulus ratio on the fracture toughness of layered SCB specimen under different loading modes are also investigated and the corresponding results indicate that these factors remarkably control the fracture mode and fracture resistance of rock-based layered material.

Preparation and Mechanical Properties of Ceramic Fiber Reinforced Titanium-Based Hybrid Laminated Composite

Based on traditional titanium-based composites and the design concept of biomimetic laminated structures, the continuous SiC ceramic fiber, ductile metal Ti foil, and intermetallic Ti2AlNb foil were selected as structural components, which were alternately stacked in the sequence of Ti2AlNb-Ti-SiCf-Ti-Ti2AlNb to prepare the continuous SiC ceramic fiber-reinforced titanium-based laminated composite. The methods adopted were vacuum hot-pressing and ceramic fiber braiding. In the prepared state, the composite’s structural components were metallurgically bonded ideally, and the continuous SiC ceramic fiber was equidistantly distributed in the ductile metal Ti matrix. This composite’s phase is mainly composed of α-Ti, β-Ti, SiC, TiC, O, and B2 phases. In addition, along the ceramic SiC fiber lengthwise, the tensile test was performed on this composite at room temperature and a high temperature of 600°C, with the ultimate tensile strength being 948.76 MPa and 526.62 MPa, respectively, and, in this process, the fiber was debonded and pulled out. Meanwhile, the composite’s bending strength was measured to be 1506.21 MPa in a three-point bending test, and, under this bending load, the mode of crack propagation and failure mechanism were analyzed.