Crack Growth Speed Research Articles

Atomic simulations of crack propagation in Ni-Al binary single crystal superalloy with a central crack

Nickel (Ni)-based single-crystal superalloys are of great importance in the aircraft industry due to their excellent mechanical properties, and cracks as unavoidable defects may affect the mechanical performances of materials dramatically. In this paper, large scale molecular dynamics (MD) simulations are carried out to understand the deformation mechanisms of Ni-based single crystal with a central crack under tension. Here, the effects of matrixes (γ, γ′ and γ/γ′), strain rates (1 × 109 s−1 ∼ 3 × 109 s−1) and temperatures (300 K∼900 K) on the role of crack propagation are considered. It is observed that dislocations and slip systems in the γ′ model are concentrated near the crack, resulting in the rapid expansion of dislocation, which leads to the fastest crack growth speed and early fracture. While the crack propagation rate of γ and γ/γ′ models are relatively slow, due to the combined action of the Lomer-Cottrell lock and stacking fault tetrahedron structure and Stair-rod dislocation, which hinders crack propagation. In addition, deformation at increased strain rates and/or reduced temperatures, lead to superior yield stress and Young′s modulus for models with a central crack at γ/γ′ interface. On the other hand, high temperature and high strain rate will promote crack propagation in the γ phase, and the higher the strain rate and/or temperature, the faster the crack propagation speed will be. These results will enrich our understanding on the crack propagation and evolution mechanisms in Ni-based single crystal superalloy.

Transonic and Supershear Crack Propagation Driven by Geometric Nonlinearities.

Linear elastic fracture mechanics theory predicts that the speed of crack growth is limited by the Rayleigh wave speed. Although many experimental observations and numerical simulations have supported this prediction, some exceptions have raised questions about its validity. The underlying reasons for these discrepancies and the precise limiting speed of dynamic cracks remain unknown. Here, we demonstrate that tensile (mode I) cracks can exceed the Rayleigh wave speed and propagate at supershear speeds. We show that taking into account geometric nonlinearities, inherent in most materials, is sufficient to enable such propagation modes. These geometric nonlinearities modify the crack-tip singularity, resulting in different crack-tip opening displacements, cohesive zone behavior, and energy flows towards the crack tip.

Rate-dependent augmented finite element method for fast arbitrary crack growth

Experimental observations indicate that fast crack growth often exhibits a rate dependency, which is caused by the viscosity of the material and the dissipation of energy resulting from the branching of microcracks. To address this phenomenon in simulation modeling, this study introduces a rate-dependent augmented finite element method designed for analyzing fast arbitrary crack growth. The method utilizes the maximum principal stress criterion to simulate arbitrary crack growth and predict the path of crack growth. The elements where crack growth is expected are divided into subelements, and they are re-assembled by incorporating rate-dependent cohesive elements between them to construct augmented finite elements. The cohesive elements are based on a rate-dependent trapezoid traction–separation relationship, where the critical traction and fracture toughness are modeled as functions of the separation rate and crack growth speed. The augmented finite elements only contain external nodal degrees of freedom (DoFs), while internal nodal DoFs are condensed. They can be utilized as conventional elements. A numerical algorithm based on the proposed method was developed in this article, and it was implemented in ABAQUS via user element subroutine. The effectiveness of this method and algorithm is validated through an example of dynamic crack growth in a double cantilever beam. The results demonstrate agreement with experimental and computational findings reported in the literature. Additionally, the method was applied to other structures, yielding reasonable outcomes. Analysis of these examples confirms the method’s utility in predicting fast arbitrary crack growth under dynamic load conditions.

Investigation on interlayer debonding behavior of unidirectional composite laminates under cyclical hygrothermal aging duration effects

AbstractThe composite interlamination is the key part of load transfer and plays a decisive role in the overall mechanical performance of composite structures. The interlaminar property is predominantly influenced by the matrix resin which is easily affected by the hygrothermal environment. This paper delved into the effect of hygrothermal aging duration on the interlayer debonding behavior of unidirectional (UD) laminates. The pre‐cracked specimens were treated by hygrothermal aging with a period of 0, 15, 30, 60, 90, and 120 days. The double cantilever beam (DCB) tests were employed to evaluate the interlaminar property. The effect of hygrothermal aging on load–displacement responses, R‐curves, crack growth speed, and fracture surface topography was analyzed. The results revealed that the load‐bearing capacity of aged specimens presented a reduction of 18.6%–29.0% compared with unaged ones. The interlaminar fracture toughness of specimens subjected to 15 days of treatment decreased significantly compared with that of unaged specimens due to the increase of water absorption caused by hygrothermal aging. With the increase in hygrothermal aging time, the interlaminar fracture toughness peaked at 60 days because of the post‐curing effect, followed by a declining trend influenced by the dominant hygrothermal aging effect. The fiber bridging of aged samples was increasingly obvious, became the most prominent at 60 days, and then gradually reduced. The fiber bridging diminished the crack propagation speed, hindered interlaminar debonding, and contributed to the interlaminar toughening.Highlights The samples underwent hygrothermal aging for 0, 15, 30, 60, 90, and 120 days. Load ability of aged samples was at least 18.6% lower than that of unaged ones. R‐curves rose before the crack length of 79 mm and then tended to stabilize. Fracture toughness first decreased, then increased and reduced with aging time. Fiber bridging could hinder crack growth and improve the interlayer property.

Study of the defect-crack offset distance on the impact fracture behavior of plexiglass

To study the interaction of moving cracks and pre-cracks of plexiglass (PMMA) under impact loading, a three-point bending experiment was carried out using dynamic caustics experimental system. Research showed that when the offset distance between the pre-crack and the defect is 9 mm, the failure mode of the crack is the most complicated. When the offset distance is 0 mm, the crack experiences the attraction force of the defect, the stress intensity factor and the crack growth speed increase gradually, and the peak value is reached when the defect and the crack intersect. When the offset distance is greater than 6 mm, the attraction effect of the defect on the crack is no longer significant, and the crack propagates toward the drop hammer location until it penetrates the specimen. As the crack propagates, the caustic speckle rotates counterclockwise and is subjected to compression and shear.

Stress‐Assisted Erosion of Poly(Glycerol‐Co‐Sebacate) Acrylate Elastomer

AbstractIn this work, a systematic investigation is conducted on stress‐assisted erosion of the photocurable and degradable elastomer poly(glycerol sebacate) acrylate (PGSA). Without external stress, it is confirmed that the elastomer undergoes surface erosion in an aqueous environment. Upon the application of mechanical stress, the results revealed that the surface erosion rate is dramatically accelerated. By studying the stress corrosion cracking (SCC) phenomena, it is demonstrated that the crack growth speed depends on the applied load and is significantly faster than the surface erosion rate of the elastomer. It is further shown that with decreasing the cross‐link density of the elastomer, the crack growth speed during SCC can be slowed down due to the increased viscoelasticity of the material.

Crankshaft High-Cycle Bending Fatigue Experiment Design Method Based on Unscented Kalman Filtering and the Theory of Crack Propagation.

The high-cycle bending fatigue experiment is one of the most important necessary steps in guiding the crankshaft manufacturing process, especially for high-power engines. In this paper, an accelerated method was proposed to shorten the time period of this experiment. First, the loading period was quickened through the prediction of the residual fatigue life based on the unscented Kalman filtering algorithm approach and the crack growth speed. Then, the accuracy of the predictions was improved obviously based on the modified training section based on the theory of fracture mechanics. Finally, the fatigue limit load analysis result was proposed based on the predicted fatigue life and the modified SAFL (statistical analysis for the fatigue limit) method. The main conclusion proposed from this paper is that compared with the conventional training sections, the modified training sections based on the theory of fracture mechanics can obviously improve the accuracy of the remaining fatigue life prediction results, which makes this approach more suitable for the application. In addition, compared with the system's inherent natural frequency, the fatigue crack can save the experiment time more effectively and thus is superior to the former factor as the failure criterion parameter.

A rate-dependent cohesive zone model for simulating fast crack evolution and growth

This paper proposes a rate-dependent cohesive zone model for simulating the evolution and growth of cracks under dynamic loading. This model takes into consideration the effects of cohesive separation rate and crack growth speed on the cohesive traction-separation relationship to reflect the rate-dependent characteristics of both material viscosity and energy dissipation of micro-crack branches. The evolution algorithm of the cohesive law is presented, wherein separation rate and crack growth speed are evaluated using the backward difference method explicitly in incremental computation, leading to the determination of rate-dependent critical traction and fracture toughness. The proposed model was implemented in ABAQUS software via UEL subroutine and utilized in an example of interfacial crack growth simulation, with satisfactory agreement found among the present results, experimental data, and numerical results from the literature. The effectiveness of the rate-dependent model and numerical algorithm in simulating high speed crack growth is validated through further numerical tests.

In-situ SEM investigation of low-cycle fatigue behavior and microstructure evolution of CoCrNi medium entropy alloy

We performed an in-situ fatigue test by in-situ scanning electron microscope (SEM) to elucidate the deformation and failure behavior of equiatomic CoCrNi medium entropy alloy under cyclic loading. The SEM results indicate that the crack growth speed is slow at the initial stage, and the growth direction is perpendicular to the loading direction. As the cyclic loading continues, small cracks are generated in the material, which changes the direction of the main cracks and accelerates the growth of the main cracks. The deformation twins and elongated grains induced by cold rolling are frequently observed in samples before cyclic deformation. These grains are changed from nearly parallel elongated grains to irregular during cyclic deformation. However, deformation twins disappeared or became thinner after cyclic deformation. Molecular dynamics simulations are carried out to reveal the microstructure evolution mechanisms during cyclic deformation. The results show that the grain growth occurred after cyclic deformation in the model without deformation twins. However, the de-twinning, re-twinning and grain growth are observed in the models containing deformations twins. Molecular dynamic simulations also indicated that twin boundaries lead to higher strain energy density and significantly improve the material's fatigue resistance.

Study on Defect Evolution of Steel Strip under High-speed Cold Rolling

In this paper the strain rate effect of the material was quantitatively tested by INSTRON tensile testing machine and Zwick/Roell HTM5020 high-speed tensile testing machine. The dimple size of the fracture surface of the tensile specimen was determined and analyzed by using the scanning electron microscope. Based on the Cowper-Symonds constitutive model, the parameters in the dynamic constitutive model and the true stress-strain curve under quasi-static condition were obtained by combining the results of quasi-static and dynamic tensile tests and the finite element numerical analyses. Considering the strain rate effect of the material the effects of cold rolling speed on the evolution of different types of defects were analyzed by the ANSYS/LS-DYNA dynamics code. The results show that with the increasing of crack length and decreasing of crack width, the critical rolling speed decreases. But if the crack length is less than 5 mm and the aspect ratio of crack length to crack width is larger than 5, the critical rolling speed of crack growth can be much large.

Influence of impact loading orientations on the mechanical behaviour of rocks around a tunnel

The surrounding rock of tunnels may suffer from dynamic disturbances caused by impact loading or blasting at different locations nearby, leading to the instability of fractured rock masses. To investigate the mechanical behaviour of the surrounding rock subjected to various impact loading directions, an improved drop weight system is introduced for experimental study. To better understand the mechanisms in fractured rock mass, the extended peridynamic (XPD) method is adopted for auxiliary analysis. Both the numerical and experimental results consistently show that the initiation time, initiation toughness and fracture toughness (KI and KII) of precracks are notably affected by the loading directions. As the crack propagates towards the tunnel, the crack growth speed gradually decreases and is inversely proportional to the fracture toughness. The average errors between the test results and numerical results for crack fracture time, average propagation toughness and crack propagation velocity are 4.85%, 3.76% (KI), 4.08% (KII) and 6.79%, respectively, demonstrating the effectiveness of the XPD model in experimental prediction. Moreover, by using the XPD model, the distribution of deformation is analysed and the failure trend of tunnels on the shadow side can be identified. The precracks exhibit a tensile failure mode shortly after initiation, but then change to a mixed tensile–shear failure mode because of the influence of different loading directions. When the cracks break through the upper side of the tunnel, the crack initiation points of the shadow side are notably different for different loading directions, mainly occurring at the corner (E2), roof (A), spandrel (C2) and tunnel arc-side 45° (B2), providing a valuable reference for tunnel engineering.

Rock dynamic fracture of a novel semi-circular-disk specimen

The relationship between velocity, acceleration, and dynamic fracture toughness during rapid crack propagation is difficult to effectively capture. In this study, a novel semi-circular holed disc bend is proposed to achieve a long fracture path. Hopkinson pressure rods with crack growth and strain gauges are used to impact the semi-circular holed disc bend specimens and to monitor the cracking process of a mode-I fracture. The experiments show that the crack growth speed increases to a critical value after crack initiation and then oscillates as a result of the reflected stress wave. The dynamic propagation fracture toughness ( K IC d ) increases with increasing crack velocity for steady crack propagation; however, this does not apply for unsteady propagation. An experimental–numerical–analytical method is developed to determine the dynamic stress intensity factor , which initially increases with time, then decreases, and ultimately increases again. The acceleration ( v ˙ ) can be used to investigate the unsteady crack propagation process. K IC d values for v ˙ < 0 are greater than those for v ˙ > 0. An energy release rate formula related to v ˙ and the crack propagation velocity ( v ) is proposed and verified by the experiments. A thorough understanding of v , v ˙ , and K IC d is important to analyze high-speed fault rupture processes.

Influence of Deposition Plane Angle and Saline Corrosion on Fatigue Crack Growth in Maraging Steel Components Produced by Laser Powder Bed Fusion

Maraging steels are used in several industries, namely in the molds industry. The determination of fatigue crack propagation resistance in 18Ni300 maraging steel at the Paris regime is a vital issue for safety-relevant components, which are designed to work for a large number of loading cycles before periodic inspections. The main goal of this work is to analyze the influence of the deposition plane angle and saline corrosion on fatigue crack growth in maraging steel samples produced by Laser Powder Bed Fusion (LPBF). The crack closure parameter was used in order to analyze the different fatigue crack growth behaviors, as well as the metallographic, hardness, fractography and corrosion/oxidation analysis. From this work, the main achievement was that the deposition plane angle did not reveal a notable influence in the fatigue crack growth behavior for the fatigue tests unsubmitted to saline corrosion. On the other hand, the fatigue crack growth behavior for the tests under saline corrosion showed an increase in the crack closure parameter due to the appearance of the crack closure induced by oxides, which reduced the fatigue crack growth speed. This phenomenon depends on the deposition plane angle, which controls the martensite amount and consequently controls the level of corrosion/oxidation.

Fatigue crack growth under mixed mode I + II in Ti-6Al-4V specimens produced by Laser powder Bed fusion

Mode I and mode I + II fatigue crack growth experiments are performed using compact tension shear specimens made of Ti-6Al-4V by additive manufacturing for various loading angles. The performance of various methodologies used to determine the equivalent stress intensity factor were studied, obtaining good results in simpler methods when the crack deflection angle was also included in the algorithm. The crack growth speed didn’t suffer significant alterations due to similar values of crack deflection angle, mode I loading predomination and similar crack closure effect. Fractured surfaces presented irregular regions consequence of intergranular separation between α phase and agglomerations of β phase.

Prefabricated crack propagation in translucent alumina ceramic sheets during flame thermal shock

The prefabricated cracks propagation of ceramic under flame thermal shock was studied by real-time observation. The experimental results show that wing cracks are generated from both tips of the prefabricated crack, and the complete failure of the samples is a high-speed process. The crack propagation speed is fast at first and then gradually slows down. The maximum propagation speed of the near flame tip increases with the prefabricated crack distance to the heated surface, while an inverse trend is observed for the far flame tip. The effect of prefabricated crack angle on thermal shock crack propagation was also considered. In addition, numerical simulations based on dynamic fracture mechanics were carried out to reproduce the entire cracking process. To this end, the relation between the dynamic stress intensity factors and the crack growth speed in the speed range of 100 m/s to 2600 m/s was identified by combing experiments and numerical simulations. The simulation results and the above-mentioned relation are consistent with the experimental observations. The numerical model can faithfully reproduce the crack evolution process of the thermal shock, which is difficult to observe in an experiment due to the high speed of the cracking process.

Research on Fatigue Performance of the Rib Beam Bridge Carriageway Slab Based on Cumulative Damage Theory

The purpose of this paper is to study the development law of fatigue damage of the carriageway slab of reinforced concrete ribbed girder bridge and to provide theoretical support for carriageway slab design. The fatigue test of reinforced concrete rib beam bridge carriageway slabs was conducted. Based on the material fatigue damage theory, the fatigue damage model was established by ABAQUS software. The fatigue performance, failure mechanism, and the effect of fatigue level on the fatigue performance of the carriageway slab were studied. The experimental research results show that when the slab of the rib beam bridge was fatigued, and the radial crack appeared at the bottom of the slab, which belonged to punching shear failure. The fatigue failure process was divided into three stages as follows: in the initial stage of fatigue, the deflection and reinforcement strain increased linearly with fatigue times, and in the stable development stage, the bottom deflection and reinforcement strain increased steady progression. With the increase in fatigue cycles, the fatigue accumulation damage was gradually accumulated, and the stiffness of the carriageway slab gradually decreases. In the rapid development stage, the deflection, reinforcement strain, and crack growth speed were significantly increased. According to the fatigue damage model, the simulation analysis results show that the accumulated damage to the bridge carriageway was more serious with the increase in fatigue load level. The fatigue damage degree was lower when the design fatigue load level is not greater than 0.55.

Fatigue crack growth of TiAl6V4 parts produced by SLM under biaxial mode I/mode II loading

Mode I and mixed-mode fatigue crack growth tests were performed using compact tension shear specimens made of Ti-6Al-4V by additive manufacturing for various loading angles. The accuracy of the methodology used to determine the equivalent stress intensity factor was studied, obtaining good results in a simpler method and particularly when the crack deflection angle was included in the algorithm. The crack growth speed did not suffer significant alterations due to similar values of crack deflection angle, mode I loading predomination and similar crack closure effect. Fractured surfaces presented irregular regions consequence of intergranular separation between β phase and agglomerations of α phase.

Measurement of crack growth speed in concrete by acoustic emission and mechanics of damage methods

The article presents the results of research and experimental studies aimed at measurement of crack growth speed in concrete of different types and composition. The study included complex methods to measure the parameters of concrete destruction under the impact of mechanic loads and aggressive factors. The experiments were performed using mechanics of damage methods and acoustic measurement of such parameters as acoustic emission gross counting and counting rate. The experiments resulted in regression equations showing change of acoustic emission gross counting depending on the speed of a specimen loading. Data on slow growth of cracks in heat-resistant concrete during heating up to 800°C have been obtained by acoustic emission and mechanics of damage methods. The performed studies and statistic processing of the obtained data allowed identifying that parameters of acoustic emission are directly related to the characteristics of mechanics of damage that reveals a considerable capacity of acoustic emission as a method for forecasting destruction of concrete.

The study of the strength of structures weakened by a system of cracks

Strength of Space vehicles is a crucial issue for Space flight safety. The presence of latent defects in the material has a significant effect on the strength at different loads: the speed and direction of crack growth. In this paper, we study a three-dimensional elastic medium weakened by a system of plane cracks and one crack with a bend. As a numerical method, the method of boundary elements was chosen, namely, the method of discontinuous displacements. The code is implemented in C ++. A comparison was made with known analytical results. Bend crack behavior is studied for various loads.

Universal relation between crack-growth dynamics and viscoelasticity in glass-rubber transition for filled elastomers

The characteristic power-law relationship between crack-growth speed (v) and tearing energy (Γ), Γ ~ vα in high speed regime (typically, v/cs > 10−3 where cs is the shear wave speed), is investigated for the filled elastomers with varying systematically the material parameters. The types of rubber, the concentration of filler and cross-linker concentration are widely varied in order to change the viscoelastic spectra of the elastomers. The power-law relationship is correlated with linear viscoelastic spectra, since Γ and the relaxation modulus are similarly dependent on temperature. An universal relation is found between the exponents α and κ of the relaxation modulus G(t) ~ t−κ in the glass-rubber transition regime, independently of the material parameters. The expectations of the existing theories fail to account for the observed relation, in particular for the elastomers with high filler contents or low cross-linker concentrations. The universal α-κ relation found here will contribute to predict the crack speed in high speed regime from the linear viscoelastic spectra of the elastomers, and will give a definite basis for the theoretical development in the crack-growth dynamics of elastomers.