Crack Propagation Behavior Research Articles

Intrinsic fracture toughness of a soft viscoelastic adhesive

The fracture toughness of inelastic materials consists of an intrinsic component associated with the crack tip fracture process and a dissipative component due to bulk dissipation. Experimental characterization of the intrinsic component of fracture toughness is important for understanding the fracture mechanism and predictive modeling of the fracture behavior. Here we present an experimental study on the intrinsic toughness of a soft viscoelastic adhesive. We first obtained full-field and full-history data of the displacement and deformation fields in pure shear fracture tests using a particle tracking method. By combining these data with a nonlinear constitutive model, we extracted the intrinsic toughness through an energy balance analysis. A two-stage crack propagation behavior was observed in our fracture experiments: under monotonic loading the crack first underwent a slow propagation stage and then suddenly entered a fast propagation stage. We found that the intrinsic toughness was highly scattered for the slow propagation stage, but remained consistent for the fast propagation stage. Further examination of the fracture surface and the onset of fast propagation revealed that transition from the slow to the fast propagation stage was governed by the applied stretch and was likely due to a change in the crack tip fracture process.

Microscopic damage behavior in CFRP cross-ply laminates at cryogenic temperature

For the practical use of cryogenic propellant tanks made of CFRP laminates, experimental elucidation of the laminates’ microstructural damage propagation behavior at cryogenic temperatures is important. This paper presents a newly developed tensile test rig that enables in-situ observation of microscopic damage using an optical microscope at cryogenic temperatures using a Gifford–McMahon refrigerator. In-situ observations of microscopic damage under uniaxial tensile loading were done at room temperature (290 K, 17 °C) and at 30 K (−243 °C) on cross-ply thin-ply CFRP specimens of three kinds with different 90° layer thicknesses. The results demonstrated that the crack propagation behavior is independent of temperature, that the matrix crack density is higher, and that the onset of matrix crack initiation strain is lower at 30 K than at 290 K. Furthermore, the thermal strain within 90° layer at 30 K and at 290 K was estimated using finite element analyses (FEA). The FEA results suggest that the decrease in onset strain of matrix crack initiation at 30 K is mainly attributed to the increase in the thermal strains within the 90° layer.

Effects of single overloads on fatigue crack propagation behaviour of AZ31B magnesium alloy under different temperatures

An experimental study examined the influence of single overload ratio (OLR) on fatigue crack growth (FCG) behaviour in AZ31B magnesium alloy at different temperatures (300 K, 373 K, and 473 K). The results showed that FCG rates and stress intensity factor ranges (ΔK) vary with temperature and overload ratios. After applying a single overload (OL), the FCG rate initially decreased, then increased rapidly, and stabilized at a value consistent with constant amplitude loading (CAL). The fatigue life increased by 86 % to 143 % at 300 K, 75 % to 132 % at 373 K, and 75 % to 134 % at 473 K for OLRs ranging from 1.75 to 2.25 when compared to the fatigue life at CAL. A single overload accumulates energy at the crack tip, forming an overload region with tearing ridges, secondary cracks, dimples, and fatigue striations under varying temperatures.

Fracture behavior of environmentally friendly high-strength concrete using recycled rubber powder and steel fibers: Experiment and modeling

Ultra-high-performance concrete (UHPC) is becoming increasingly popular for its exceptional mechanical properties but suffers from high cost and limited ductility. To overcome these limitations, this study explores the development of cost-effective and ductile Environmentally friendly High-strength Concrete (E-HSC) by incorporating recycled steel fiber (RSF) and rubber powder into UHPC. Fracture properties and crack propagation behavior were examined through three-point bending fracture tests on notched beams with varying rubber powder (0 %, 5 %, 20 %, 35 % and 50 %) and recycled steel fiber (1 %, 2 % and 3 %) content. Digital Image Correlation method was Used to Measure Crack mouth opening displacement and Fracture process zone in the Fracture Process of E-HSC of the E-HSC were analyzed using the digital image correlation method. Results revealed that E-HSC with 5 % rubber powder exhibited superior fracture energy and double-K fracture toughness. The use of rubber powder and RSF enhances the ductility of concrete, with the characteristic length consistently increasing as the content of rubber powder and RSF increases. Additionally, a softening constitutive model was proposed to evaluate the fracture performance of E-HSC based on rubber powder and recycled steel fiber content. This research provides experimental data and theoretical insights for developing resilient E-HSC suitable for applications in significant structures.

Fatigue crack propagation life analysis of propeller

Fatigue crack fracture is a common failure mode in structures. For the propeller with initial crack, the hydrodynamic effect cannot be ignored when studying the fatigue crack propagation behavior. Taking the full-size KP505 propeller as the research object, its hydrodynamic performance is simulated and verified. By using the fluid-structure interaction (FSI), the pressure distribution and stress distribution of propeller under different working conditions are analyzed comprehensively, and the position of propeller hot spot stress is obtained. According to the principal stress and its direction of the hot spot stress, the location and orientation of possible macroscopic crack initiation are determined. By using the linear elastic fracture mechanics and finite element method (FEM), the propeller fatigue crack propagation law and life are calculated. The analysis results show that the Forman-Newman-de Koning (FNK) model predicts more complete fatigue crack propagation life than Paris model. The fatigue crack propagation of propeller has two stages, steady propagation stage, and accelerated propagation stage. The crack breaking blade thickness is the dividing point between the two stages. In addition, the effects of the advance coefficient, stress ratio, and initial crack size on the computed results are also analyzed, which provide reference for ship driving and propeller maintenance.

Simulation of Rock Crack Propagation and Failure Behavior Based on a Mixed Failure Model with SPH

Simulation of Rock Crack Propagation and Failure Behavior Based on a Mixed Failure Model with SPH

Experimental and numerical investigation on crack propagation in biomimetic nacreous composites with gradient structures

Combining interlocking structures and gradient design, this paper proposes a 2D biomimetic nacreous composite material with a gradient interlocked structure. Experimental and numerical simulation methods are employed to investigate the initiation and propagation behavior of cracks in the regular interlocking and gradient interlocked biomimetic nacreous composite materials. Samples of biomimetic nacreous composite materials with pre-existing cracks are manufactured using 3D printing technology, and uniaxial tensile tests are conducted to explore their crack propagation behavior. For biomimetic nacreous composite materials with periodic interlocking structures, the interlocking angle is found to play an important role in the toughening mechanism of composites. In biomimetic composite materials with large interlocking angles, crack inhibition effects are achieved through structural gradient design, effectively preventing catastrophic failure. Within the numerical analysis framework, a cohesive zone modeling approach is employed to represent the softer phase of the material, and the numerical simulation are validated by direct comparison with experimental results. In addition, a comprehensive numerical analysis is carried out to comprehend how the structural gradient affects the spread of cracks in these nacreous composites. This research not only illuminate the underlying deformation and toughening mechanisms intrinsic to interlocking nacreous composites but also pave the way for innovative strategies in the design of biomimetic materials through the incorporation of structural gradients.

Heterogeneity induced fracture characterization of concrete under fatigue loading using digital image correlation and acoustic emission techniques

Understanding the fatigue failure mechanisms in concrete is complex and remains an area of ongoing investigation. Material heterogeneity, along with the size effect, plays a significant role in specifying the crack propagation behaviour and fatigue life in case of repetitive loading conditions. In this study, the influence of material heterogeneity on the fatigue behaviour of concrete has been examined. Three different-size beam specimens with varying heterogeneity (aggregate sizes) have been considered to investigate various fracture characteristics under repetitive load conditions. Acoustic emission and digital image correlation techniques have been employed to analyse crack growth behaviour. The results revealed that stiffness degradation and crack propagation rates are faster for the specimens with smaller aggregate sizes compared to the larger aggregate size specimens of the same depth. Moreover, the fatigue life and effective crack length at failure are also higher in the case of larger aggregate sizes compared to smaller ones. Therefore, a heterogeneity-adjusted, analytical model of fatigue crack growth has been proposed. Further, the AE parameters have been utilized to characterize the tensile and shear crack dominance in different stages of fatigue crack propagation. The outcome of this study can be utilized to anticipate the crack propagation behaviour and residual fatigue life for any combination of specimen size and aggregate size.

Low-cycle fatigue property of Inconel 617 alloy at 700 °C: Mechanisms of cyclic deformation, precipitation and cracking behavior

Effects of strain amplitude ranging from 0.2 % to 1.0 % on the low-cycle fatigue properties of Inconel 617 alloy at 700 °C were studied. Much attention was paid to reveal the physical mechanisms of the cyclic deformation, γ′ phase precipitate and cracking behaviors based on the methods of flow stress partitioning and microstructure evolution characterization. Results showed that the material demonstrated a two-slope cyclic hardening behavior with different hardening rates. The strain amplitude of 0.4 % was found to be the threshold value in terms of both the precipitation behavior and dynamic strain aging (DSA). The diameter of γ′ phase precipitate was decreased from 28 nm to 12 nm when the strain amplitude increased from 0.2 % to 0.4 % and became disappeared at the larger strain amplitudes. The DSA activity characterized by the DSA strain range and maximum stress drop got significantly intensified with the increasing strain amplitude, about five times larger at 1.0 % than that at 0.4 %. Further, the mutual interaction between precipitation behavior and DSA was thoroughly analyzed. The number of crack initiation sites was increased with the increasing strain amplitude, and the dominant damage mechanism responsible for the crack propagation behavior was elucidated.

Numerical simulation of fatigue fracture in gradient high-strength steel: effects of carbides and gradient structure on stress–strain response and crack propagation behavior

Numerical simulation of fatigue fracture in gradient high-strength steel: effects of carbides and gradient structure on stress–strain response and crack propagation behavior

Effect of binder content and sintering time on the microstructure and mechanical properties of gradient cemented carbides with Ni binder

Replacement of Co by alternative Ni binder in gradient cemented carbide is an effective way to reduce the health concerns associated with Co powders. In the present work, gradient cemented carbides with different Ni contents and gradient sintering times were prepared by traditional powder metallurgy with a two-step sintering method. The microstructure of cemented carbide including the gradient layer formation was analyzed by XRD, SEM, EDS, TEM, and EBSD. The thickness of the gradient layer is almost proportional to the square root of sintering time, indicating that the gradient layer formation is a diffusion-controlled process. The atomic migration can be accelerated to form a thicker gradient layer with the increase of the binder content. The coherent interface of the Ti(C, N) core-rim structure for the gradient cemented carbides with Ni binder was revealed. With the increase in binder content, the average grain size of the WC phase is increased. The hardness and fracture toughness of the cemented carbides were also studied and analyzed according to the crack propagation behavior and the distribution of dislocation and stress indicated by KAM and GND. The variation of the hardness and the fracture toughness with the sintering time is mainly related to the density and the grain size of hard phases, while the change of the mechanical properties along with the Ni content probably results from the local dislocation density and the stress in the cemented carbides. The cemented carbide can obtain a good combination of hardness and fracture toughness with 10 wt% Ni gradient sintered for 2h.

Enhanced corrosion fatigue strength of additively manufactured graded porous scaffold-coated Ti-6Al-7Nb alloy

Current modifications of Ti-based materials with porous scaffolds for achieving biological fixation often decrease corrosion fatigue strength (σcf) of the resultant implants, thereby shortening their service lifespan. To resolve this issue, in the present, a step-wise graded porous Ti-6Al-7Nb scaffold was additively manufactured on optimally surface mechanical attrition treated (SMATed) Ti-6Al-7Nb (specifically denoted as S-Ti6Al7Nb) using laser powder bed fusion (PBF) technology. The microstructure, bond strength, residual stress distribution, and corrosion fatigue behavior of porous scaffolds modified S-Ti6Al7Nb were investigated and compared with those of mechanically polished Ti-6Al-7Nb (P-Ti6Al7Nb), S-Ti6Al7Nb, and porous scaffolds modified P-Ti6Al7Nb. Results showed that corrosion fatigue of porous scaffolds modified Ti-6Al-7Nb was propagation controlled. Moreover, the crack propagation behavior in the PBF scaffold's fusion zone (FZ) and heat-affected zone (HAZ), exhibiting insensitivity to the microstructural configurations characterized by columnar prior-β grain (PBG) boundaries and acicular α′ martensites, coupled with the PBF-induced residual tensile stresses in these regions, resulted in a considerable decrease in σcf for porous scaffolds modified P-Ti6Al7Nb compared to P-Ti6Al7Nb. In contrast, step-wise graded porous scaffold-modified S-Ti6Al7Nb demonstrated an improved σcf which was even higher than that of P-Ti6Al7Nb. Such an advancement in corrosion fatigue strength is primarily attributed to the presence of residual compressive stresses within the underlying S-Ti6Al7Nb substrate, extending beyond FZ and HAZ. These stresses increased the crack propagation threshold, leading to crack deflection/branching and increased crack-path tortuosity, thereby synergistically markedly enhancing the crack propagation resistance of porous scaffolds modified S-Ti6Al7Nb.

Fatigue crack initiation and propagation in plain and notched PBF-LB/M, WAAM, and wrought 316L stainless steel specimens

Additively manufactured (AM) components—either made by laser-powder bed fusion or wire and arc additive manufacturing—typically contain process-related defects on and near surfaces that can be removed by machining. Various studies have shown that post-treatment, such as machining significantly improves the fatigue strength of AM parts. To this day, however, hardly any studies have investigated the fatigue strength of post-treated additively manufactured components with notches. In this study, fatigue tests were performed on plain and notched specimens to determine and compare the crack initiation and crack propagation behavior due to different manufacturing-related effects. Tests were performed on specimens produced by the two aforementioned AM processes and compared to specimens taken from wrought sheets. The fatigue strength of AM materials is influenced by microstructure, defects, residual stress, and notches. PBF-LB/M specimens exhibit the highest fatigue strength in plain, notch-free conditions, attributed to differences in microstructure and static strength affecting fatigue crack initiation. Notched specimens show larger differences among materials, with PBF-LB/M having shorter fatigue crack propagation life related to line-type defect clusters, while the plain PBF-LB/M specimens are less affected as their fatigue strength is primarily determined by fatigue crack initiation.

Effect of grain structure on fatigue crack propagation behavior of 2024 aluminum alloy under different stress ratios

The fatigue load that a material experiences and its microstructure are important factors influencing fatigue crack propagation behavior. This study employed laser scanning microscopy and electron backscatter diffraction (EBSD) technology, along with fatigue crack propagation experiments, to investigate the fatigue crack propagation behavior of 2024 aluminum alloy under varying stress ratios (R). The results showed that the stress amplitude (σp) was the main factor controlling the fatigue crack propagation life (N). Additionally, detailed characterization of the fatigue crack propagation path was conducted using crystal models and EBSD. A fatigue crack propagation model for 2024 aluminum alloy under different R-values was established based on the grain twist angle and Schmid factor. Finally, the impact of R on the crack tip shielding (Ks) was systematically analyzed, elucidating the intrinsic mapping relationship between R, microstructure, and crack propagation characteristics.

Controlling the columnar-to-equiaxed transition and crack propagation behavior of laser welded Al–Li alloy reinforced with TiC nanoparticles

The predominant method for improving the mechanical characteristics of aluminum-lithium (Al–Li) alloys during laser welding is by regulating the transition from columnar crystals to equiaxed crystals. The transition is facilitated through the incorporation of TiC particles, which play a crucial role in augmenting the strength and toughness of the aluminum alloy. The inclusion of TiC particles offers a new opportunity to enhance the utilization of Al–Li alloys in laser welding procedures. The main aim of this study is to control the transition from columnar to equiaxed crystal structures by creating a TiC nanoparticle-reinforced aluminum alloy filler wire. This is intended to improve the properties of laser-welded joints. The study investigates the impact of different TiC particle contents on the microstructural characteristics. The presence of TiC particles is noted to enhance the transformation of columnar crystals into equiaxed crystals in the welded joints, resulting in a refined microstructure. The augmentation of particle content results in a notable reduction in the average grain size, decreasing from 78.25 μm to 37.15 μm. This alteration shifts the directional growth preference from specific crystallographic axes to a stochastic trajectory. Significantly, when the particle content reaches 0.6 wt%, remarkable enhancements in both tensile strength and elongation are evident, resulting in an ultimate tensile strength of 318 MPa and an elongation of 4.8 %. The dispersed configuration of TiC particles plays a pivotal role in hindering the movement of dislocations, consequently increasing the energy necessary for this mechanism. Moreover, the existence of these particles at grain boundaries impedes the propagation of cracks by altering the direction of the crack path, absorbing additional energy, and consequently improving toughness. An evident pattern emerges when examining higher particle concentrations, indicating that a decrease in ultimate tensile strength is concomitant with an increase in elongation.

Fretting fatigue crack initiation and propagation behaviours of Ti6Al4V alloy coated by functionally graded material

Fretting fatigue pertains to the behaviour of engineering components undergoing cyclic loading while in contact with each other. This intricate contact-related phenomenon often results in premature failure compared to conventional fatigue issues. Moreover, when these components operate in high-temperature atmospheres and experience high wear conditions, such as the heat transfer tubes in the nuclear reactor, the application of Functionally Graded Material (FGM) coatings becomes essential. Consequently, it is imperative to investigate how FGM coatings influence the initiation and propagation behaviour of fretting fatigue cracks. This paper employs the Critical Plane (CP) method to calculate the damage parameter. Additionally, Linear Elastic Fracture Mechanics (LEFM), specifically the Extended Maximum Tangential Stress (E-MTS) criterion, is utilized to examine how FGM coatings affect the paths of fatigue crack propagation in the presence of fretting conditions. Meanwhile, due to the unavailability of material properties of FGM coatings, the damage parameter and stress intensity factors are utilized to indirectly assess the effect of FGM coatings on crack initiation and propagation lifetimes. The FGM coating significantly influences tangential stress, thereby affecting the length of the stick zone, while having minimal impact on the distribution of normal stress along the contact surface. Furthermore, it is observed that adjusting the variation in composition of FGM coatings and the elastic modulus of the first FGM coating layer provides a potential technique for enhancing both crack initiation and propagation lifetimes.

Influence of Interaction between Microcracks and Macrocracks on Crack Propagation of Asphalt Concrete.

This paper aims to reveal the interaction relationship between microcracks and macrocracks and the influence of the interaction on the crack propagation behavior. A theoretical model of asphalt concrete was established for the interaction between microcracks with different crack densities and a macrocrack. And a meso-structure model of AC-13 dense-graded asphalt concrete was established by combining the Talyor medium method and the DEM (discrete element method). Macro and micro parameters, such as the stress-strain characteristics, crack evolution parameters, and crack tip stress field, were obtained through a semi-circular bend virtual test and used to study the characteristics of crack propagation under the interaction between microcracks and the macrocrack. The results indicate that the interaction has an effect throughout the process of asphalt concrete damage, and shows shielding and acceleration effects as the microcrack density changes. When the microcrack density is low (f3 ≤ 0.8), the crack propagation process, which is affected by the interaction effect, exhibits significant differences, and the interaction effect shows the shielding effect. When the microcrack density is high (f3 > 0.8), the fracture stage is mainly affected by the interaction effect, which shows the acceleration effect. The results provide a predictive theoretical and numerical model for low-temperature cracking of asphalt pavement, and theoretical support for the design, maintenance, and upkeep of long-life pavement.

Numerical simulation of the crack propagation effect on the hysteretic behavior of CHS-X joints

The circular hollow section (CHS) X-joint hysteresis behavior significantly affects the seismic behavior of the steel tubular structure using this type of joint. The hysteresis behavior of this type of joint considering crack growth is studied by tests and numerical simulation. Two CHS X-joints were fabricated and tested under axial cyclic loading until the joints were cracked. The hysteresis curves of the joints, considering the influence of damage and fracture, were obtained, and the hysteresis performance of the joints was evaluated according to the curves. The VUSDFLD subroutine based on the ductile fracture model CVGM (cyclic void growth model) was embedded into ABAQUS, and the FE simulation of the test joints considering crack occurrence and growth was carried out. The test results are compared with the results of FE analysis, revealing that the proposed method accurately simulates the hysteresis behavior of these joints and the crack propagation behaviors under cyclic loads. The difference between the test and simulated crack initiation loads for the two joints is 10.8 % and 5.7 %, respectively.

Thermal fatigue behavior of the ZGH451 Ni-based superalloy fabricated by direct energy deposition in the temperature range of 900–1100 °C

In this study, a novel Ni-based superalloy, ZGH451, has been fabricated using direct energy deposition (DED). The thermal fatigue resistance of ZGH451 is systematically evaluated at 900, 1000, and 1100 °C, primarily focusing on the crack initiation and propagation behaviors. The results indicate that higher peak temperatures lead to earlier initiation and more rapid propagation of cracks. Cracks are initiated at the defects and grain boundaries in the vicinity of the notch, and different crack propagation mechanisms (γ' phase slip shearing, γ' phase distortion shearing, and γ' phase rafting shearing at 900, 1000, and 1100 °C, respectively) are the main reason for the different cracks propagation behaviors under the three temperatures. The main crack propagation paths are oriented at approximately 45° with respect to the build direction, suggesting activation of the {111}<110> slip system. Additionally, oxidation reduces the matrix strength and passivates the crack tips, leading to varying rates of crack propagation. At elevated temperatures, the synergistic effects of thermal stress and oxidative erosion are found to be the primary damage mechanisms of thermal fatigue. Overall, the proposed ZGH451 superalloy demonstrates exceptional thermal fatigue resistance, providing a crucial experimental reference for thermal fatigue in additively manufactured superalloys.

Multiple cracking and strain-hardening criteria of Engineered Cementitious Composites (ECC) with river sand

The inclusion of river sand in ECC alters the crack initiation and propagation behavior. As a result, the multiple cracking and pseudo strain-hardening of the river sand ECC (RS-ECC) are different from that of the typical micro-silica sand ECC. This study investigates the strain-hardening criteria for the RS-ECC by considering the matrix as a three-phase material containing pre-existing flaws along the sand/matrix interface with a given size distribution. Micromechanics is used to impart the new mechanisms in the analytic model for the steady-state crack analysis and the formulation of the strain-hardening criteria of ECC incorporating river sand. A probability-based approach is adopted to assess the strain-hardening potential of RS-ECC and validated with experimental results. Results show that the proposed model can predict the tensile properties of RS-ECC, which can be used to guide ingredients selection and components tailoring of RS-ECC to achieve required tensile strain-hardening performance.