Short Crack Lengths Research Articles

Seepage characteristics and mechanism of double damage basalt fiber reinforced concrete under three-dimensional variable stress states

Concrete experiences a three-dimensional variable stress state during its service life. To explore the permeability evolution and mechanism under such conditions, a permeability test was conducted on double damage basalt fiber concrete subjected to loading and unloading of twice confining pressure. Pore structure and microstructure of specimens were analyzed before and after permeability test. The findings reveal that the reduction in permeability after primary cycle of confining pressure loading and unloading is significantly higher than that after secondary cycle, and residual deformation is also more pronounced after primary cycle. Both high temperature and salt freeze-thaw cycles induce initial damage to specimens, consequently altering internal pore structure and subsequently impacting initial permeability. Notably, initial permeability decreases in the case of 10th salt freeze-thaw cycles, while it increases under other conditions. The greater the initial extent of damage to the specimen, the higher the permeability damage coefficient and the permeability damage rate. As the confining pressure decreases, the attenuated permeability increases, with the permeability decay rate reaching a significant 88.53 %. Plastic deformation of pores during loading and unloading process cannot be fully reversed, leading to the development of macropores and mesopores into smaller pore sizes, resulting in a remarkable 87.17 % increase in the proportion of micropores. Microscopic observations reveal smaller pores within the matrix, shorter crack lengths, and narrower crack widths in the specimen. The application of confining pressure reduces permeability channels within specimens, ultimately causing a decline in permeability.

Modelling the anisotropic fracture behaviour of cold spray additive manufactured 6061 aluminium alloy deposits based on the phase field method

Cold spraying has been widely accepted as a promising additive manufacturing technique to produce large metallic structures. The fracture behaviour of cold spray additive manufactured deposits is complex because of their inherent anisotropy. In order to study the anisotropic fracture, a multiple-particle impacting model was established in a Lagrangian frame. Then, an image recognition program was developed to convert the simulated deposit image into a finite element discretisation scheme. In finite element framework, a phase field model was proposed to simulate the fracture process of deposits under tensile loading along (0°) and perpendicular (90°) to the particle impacting direction. The deposit anisotropic fracture behaviour for two impact velocities, at 600 m/s and 800 m/s, was investigated. It is shown that the interfacial fracture toughness is the most important parameter that affects the tensile properties of deposits. The tensile ultimate load of the deposit increases thrice when the interfacial fracture toughness increases by an order of magnitude. For the 800 m/s deposit, the ultimate load and fatigue life in 90° loading direction are about 11.3% and 39.2% higher than those in 0° loading direction, respectively. The fracture anisotropy relates to the degree of flattening of the deformed particles. The higher interfacial fracture toughness of mode II can lead to more transparticular cracks, shorter crack length and smoother crack path. The comparison between simulations and published experimental results suggests that the proposed phase field model can capture the complex anisotropic fracture behaviour of cold sprayed deposits.

Experimental study on fracture propagation in anisotropy rock under cyclic hydraulic fracturing

Hydraulic fracturing is commonly used to enhance the hydraulic conductivity of geothermal, oil, and gas reservoirs, particularly those situated in enclosed and unconventional formations. Shale oil and gas reservoirs exhibit distinct characteristics in rock mechanics parameters, morphology, natural fracture geometry, and geological features. Structural anisotropy in materials generally arises from changes in grain type and size, weak layering, and variations in layering angles. This study aims to investigate the impact of two key factors, namely cyclic injection and structural anisotropy, on the hydraulic fracturing process. To achieve this goal, the phenomenon of hydraulic fatigue in rock was examined through various cyclic injection methods, and true-triaxial hydraulic fracturing tests were conducted on cubic samples composed of different materials and layers. To evaluate and analyze the fracture morphology of these samples, CT scan X-ray imaging technology was employed. The results revealed that samples subjected to cyclic injection exhibited an average breakdown pressure approximately 30% lower than those subjected to monotonic injection. Fracture geometry in the cyclic injection samples displayed a network pattern with multiple crack branches, while monotonic injection primarily resulted in a single-surface pattern. In samples with differences in horizontal stress, the anisotropy of the sample did not affect the fracture, and the single fracture surface extended in the direction of maximum stress. The energy generated by cyclic injection using the damage-controlled method (cycle duration: 16 s) was concentrated around the borehole, creating a complex fracture network with shorter crack lengths. Conversely, cyclic injection using the ramp signals method (cycle duration: eight seconds) produced more straightforward cracks with greater expansion and penetration. Examination of CT scan images of the fractured samples revealed average tortuosity and fracture volume fraction parameters of 1.117 and 0.027 for cyclic injection, respectively, in contrast to 1.026 and 0.008 for monotonic injection. Furthermore, the S-N damage model based on the results of true-triaxial hydraulic fracturing was introduced to describe rock damage behavior and the concept of fatigue under cyclic loads.

Multiscale Mechanics of Thermal Gradient Coupled Graphene Fracture: A Molecular Dynamics Study

The thermo-mechanical coupling mechanism of graphene fracture under thermal gradients possesses rich applications whereas is hard to study due to its coupled non-equilibrium nature. We employ non-equilibrium molecular dynamics to study the fracture of graphene by applying a fixed strain rate under different thermal gradients by employing different potential fields. It is found that for AIREBO and AIREBO-M, the fracture stresses do not strictly follow the positive correlations with the initial crack length. Strain-hardening effects are observed for “REBO-based” potential models of small initial defects, which is interpreted as blunting effect observed for porous graphene. The temperature gradients are observed to not show clear relations with the fracture stresses and crack propagation dynamics. Quantized fracture mechanics verifies our molecular dynamics calculations. We provide a unique perspective that the transverse bond forces share the loading to account for the nonlinear increase of fracture stress with shorter crack length. Anomalous kinetic energy transportation along crack tips is observed for “REBO-based” potential models, which we attribute to the high interatomic attractions in the potential models. The fractures are honored to be more “brittle-liked” carried out using machine learning interatomic potential (MLIP), yet incapable of simulating post fracture dynamical behaviors. The mechanical responses using MLIP are observed to be not related to temperature gradients. The temperature configuration of equilibration simulation employing the dropout uncertainty neural network potential with a dropout rate of 0.1 is reported to be the most accurate compared with the rest. This work is expected to inspire further investigation of non-equilibrium dynamics in graphene with practical applications in various engineering fields.

Failure Analysis of Damaged High-Strength Bolts under Seismic Action Based on Finite Element Method

Damage of high-strength bolt (DHSB) is inevitable during long-term use. Such damage is irreversible that may not be replaced in time, and the potential danger is proven by the changes of the mechanical properties of DHSB. To investigate the mechanical properties of DHSB, this paper uses the software ABAQUS to simulate the stress of various types of DHSB under earthquake, and compares with undamaged high-strength bolts (UDHSB). The results show that the most unfavorable position of the crack is at the bottom of the second ring thread. The model with shorter crack length will have greater stress concentration and displacement deformation. The more the number of cracks, the greater the concentrated stress value. The concentrated stress generated by the tooth deformation is at the top of the thread tooth, and the concentrated stress generated by the crack is at the bottom of the thread tooth. Changing the tooth shape in the appropriate position is beneficial to reduce the degree of bolt damage. Bolt damage will double the harm, and timely replacement of damaged bolts is very time-sensitive and necessary.

Bondline thickness effect on fracture and cohesive zone model of sintered nano silver adhesive joints under end notched flexure tests

AbstractBondline thickness effect on fracture behaviors and cohesive zone model (CZM) is an important topic for adhesive joints. In this paper, the fracture and CZM of sintered nano silver adhesive joints are investigated based on end notched flexure (ENF) test. Results show that load versus displacement curves of sintered silver adhesive joints is sensitive to the bondline thickness. With compliance‐based beam method (CBBM), the shearing fracture toughness of sintered silver bonded joints heightens with the increase of bondline thickness, which indicates that shearing fracture toughness is highly dependent on bondline thickness. R curves increase steeply for short equivalent crack length and become plateau for long equivalent crack length. The cracking morphology shows that the cracking paths are mainly interfacial and interlayer types. The bilinear CZM is also presented. The bondline thickness effect on CZM parameter is presented and discussed. The CZM given in this paper can be adopted for reliability analysis in power devices.

Microstructural investigation and model of fatigue short crack in EH36 steels

AbstractFor better revealing the behavior and mechanism of fatigue short crack (SC) propagation, five different kinds of notched samples were designed. Based on the crack growth rate‐SC length map, finite element method and electron backscatter diffraction (EBSD) pattern, the evolution theory of geometrically necessary dislocation density (ρGND) versus SC length was proposed to distinguish the physically short crack (PSC) length; Schmid factor was utilized to define the length of microstructurally short crack (MSC). Thus, the whole SC growth was divided into three parts: MSC, transition zone and PSC. Finally, an original piecewise model containing two parameters was proposed to describe the whole SC growth of EH36 steels.

Characterization of rheological properties and aging performance of bitumen modified by bio-oil from bamboo charcoal production

As a waste stream from charcoal production, the liquid bio-oil has not been properly used, leading to both environmental and economic issues. In this study, this deleterious waste was explored as a promising additive for petroleum-based bitumen. Raw bio-oil was pretreated through distillation under different temperature to remove undesirable components. The resulting medium and heavy bio-oil fractions were blended with petroleum-based bitumen by weight ratios up to 12% to obtain bio-bitumen. The rheological and aging properties of the bio-bitumen were further evaluated with various test means. The results showed that the addition of bio-oil had softening effect on bitumen. Bio-bitumen performed better brittleness resistance with smaller Glover-Rowe parameters and better fatigue crack resistance at shorter crack length than that of base bitumen. The lower non-recoverable creep compliance of the heavy fraction modified bio-bitumen indicated its better rutting resistance, compared to that of the base bitumen. Characterization of the carbonyl and sulfoxide functional groups in the aged bitumen showed comparative aging performance of the bio-bitumen modified with heavy fraction. The findings of this study indicate that the promising potential of bio-oil heavy fraction to replace petroleum-based bitumen for generating sustainable road paving material.

Calculations on compact disc cracking.

The Griffith equation for brittle cracking has three problems. First, it applies to an infinite sheet whereas a laboratory test sample is typically near 100 × 100 mm. Second, it describes a central crack instead of the more dangerous and easily observable edge crack. Third, the theory assumes a uniform stress field, instead of tensile force application used in the laboratory. The purpose of this paper is to avoid these difficulties by employing Gregory's solution in calculating the crack behaviour of PMMA (Poly Methyl Meth Acrylate) discs, pin loaded in tension. Our calculations showed that axial disc loading gave nominal strengths comparable with Griffith theory, but the force went to zero as the crack fully crossed the disc, fitting experimental results. Off-axis loading was more interesting because the predicted strength was lower than in axial testing, but also gave unexpected behaviour at short crack lengths, where nominal strength did not rise indefinitely but dropped as crack length went below D/10, quite different from Griffith, where strength rose continuously as cracks were shortened. Such off-axis loading leads to a size effect in which larger discs are weaker, reminiscent of the fine fibre strengthening phenomenon reported in Griffith's early paper (Griffith 1921 Phil. Trans. R. Soc. Lond. A 221, 163-198. (doi:10.1098/rsta.1921.0006)). This article is part of a discussion meeting issue 'A cracking approach to inventing new tough materials: fracture stranger than friction'.

Extended glaze firings for porcelain-veneered zirconia: Effects on the mechanical and optical behavior

ObjectiveThis study evaluated the effect of dwell time (conventional or extended) and cooling protocol (fast or slow) of self-glaze firings on the mechanical (flexural strength and crack propagation) and optical (color and translucency) properties of a porcelain-veneered zirconia system. MethodsBilayer disc-shaped samples were prepared (Vita VM9 + In-Ceram YZ) and divided according to the final thermal treatment: glaze firing followed by slow cooling (furnace opening at 200 °C) (G-S) or fast cooling (furnace opening at 600 °C) (G-F, manufacturer-recommended protocol), extended glaze firing (15 min of dwell time) followed by slow cooling (EG-S) or fast cooling (EG-F), or no thermal treatment (CTRL). Porcelain roughness (Ra and Rz) was measured before and after glaze firings. Color (ΔE00) and translucency (TP00) alteration were also evaluated. Flexural strength was measured with the piston-on-three-ball test and crack propagation analysis was performed after Vickers indentations. Complementary analyzes of crystalline phase and scanning electron microscopy were carried out. ResultsSignificant effect of dwell time was observed, with extended glaze leading to higher flexural strength and shorter crack lengths. Cracks of EG groups were observed to end in clusters of crystals. Color and translucency changed below perceptibility thresholds. All treatments led to a smoother surface and EG groups reached the lowest Rz values. An extra SiO2 peak was revealed in control and EG groups. No effect of cooling protocol was found. SignificanceExtended glaze firing was able to improve the resistance to crack initiation and propagation of porcelain-veneered zirconia without clinically perceptible changes in optical properties.

Behaviour of stub columns with initial crack defects under axial compression

An experimental investigation was conducted to evaluate the axial compressive behaviour of circular hollow section (CHS) stub columns with initial crack defects. A total of 96 specimens – 12 stub columns with one crack, 48 stub columns with two cracks, and 36 stub columns with three cracks – were tested under axial compression loading. The failure modes, load-carrying capacities, overall deflections, and strain intensities of all the specimens were examined. The design parameters were the tube thickness, long crack length, short crack length, crack spacing, and crack number. Additionally, a finite-element analysis was performed using the verified models in combination with the experimental data to evaluate the effects of the main influential factors. The initial cracks formed in the circular thin-walled stub columns accelerated the local buckling and changed both the bearing capacity and deformation capacity. The ultimate loads of the stub columns increased with the tube thickness, decreased slightly with an increase in the long crack length, and decreased with an increase in the crack spacing. The effects of the short crack length and crack number on the bearing capacity were complex and were coupled with the tube thickness. An adjustment coefficient was introduced into the predictive formula to estimate the ultimate bearing capacity. Through this, the ultimate bearing capacity can be accurately predicted.

A study on strength and corrosion protection of cement mortar with the inclusion of nanomaterials

Nanomaterials are highly efficient additives for the modification of microstructure, resulting in better mechanical properties and durability of concrete/mortar. This study investigates the effect of three different dosages (1%, 3% and 5%) of nano-SiO2 and nano-TiO2 on workability, mechanical strength, and corrosion resistance of fly ash-based cement mortars. It was found that the workability of mortar decreases with increase in percentage of nanoparticles, and it was more pronounced for the mortar mix made with 5% nano-SiO2. In contrast, yield stress and viscosity were significantly higher compared to the control mix. Similarly, higher compressive strength was observed for the mortar made with both nano-SiO2 and nano-TiO2 compared to the control mortar. For instance, at 56 days, the compressive strength increased by 50% and 26% for mortar made with 3% nano-SiO2 and 1% nano-TiO2, respectively. The flexural strength result shows an increase at 1% nano-SiO2 and nano-TiO2 content. Beyond this level, the flexural strength decreases, especially for nano-SiO2 mortar. Furthermore, the inclusion of nanoparticles in cement mortar enhances the corrosion resistance of the mortar compared to control samples. Lower mass loss, lower pitting depths of the rebar, and shorter crack length were observed. Scanning electron microscopy and energy-dispersive x-ray measurements show better performance of mortar mixes made with nanoparticles compared to the control mix due to the denser matrix, more C-S-H gel and low Ca/Si ratio.

Fracture toughness evaluation for Zr-4 clad tubing structure with pellet inserts

Applying fracture mechanics approach to spent nuclear fuel (SNF) system reliability investigation is warranted due to the inherent flaws and hydride structures existed in a SNF system after nuclear reactor operation. However, none of the existing fracture toughness data deal with fuel cladding specific geometry or spent fuel material conditions, such as cladding structure with the pellet-inserts and the associated pellet clad mechanical interactions induced mixed-mode damage mechanisms. Thus, the development of an intrinsic fracture mechanics approach that is suitable for SNF materials is needed. Due to thin wall and small dimension of clad tubing structure, the spiral notch torsion test (SNTT) method of small specimen approach was used to estimate the clad tubing structure fracture toughness. The estimated fracture toughness for Zr-4 cladding with alumina-pellet inserts are presented in this report. For SNTT samples with a short or medium crack length, between 5.4-mm and 8-mm, the estimate JIQ upon fracture initiation for the baseline Zr-4 cladding is at 50 kJ/m2 with 2-sigma uncertainty of 3.26 kJ/m2, and the associated KIQ is at 67.46 MPa√m. For SNTT samples with a long crack length, around 13-mm, the crack initialization is deviated from that of the Mode-I tensile fracture and appears to be a mixed-mode fracture of Mode I – tensile stress and Mode III – out of plane shear stress; the estimated JMQ is at 18.9 kJ/m2, the associated KMQ is at 41.4 MPa√m.

Novel strength reduction numerical method to analyse the stability of a fractured rock slope from mesoscale failure

The strength reduction method is a suitable numerical method for analysing the stability of complex rock slopes. However, in the determination of the safety factor of a slope, a critical limitation to this approach is the definition of the critical (or limit equilibrium) state of the slope. To overcome this problem, the analysis of the development of the crack tip plastic zone for preexisting cracks in a fractured rock slope is used as the basis for a proposed new method that defines the critical state of the slope by considering mesoscale failure. Thus, using the eXtended Finite Element Method as a tool and based on this new definition, a new strength reduction numerical method to analyse the stability of a fractured rock slope is proposed. Finally, the new method is verified by performing a comparison with the traditional strength reduction method for the numerical study of a rock slope that has four unequal preexisting cracks. The results show that a value of 0.3 for the ratio of the crack space to the short crack length can be considered the new definition of the critical state for a slope. Furthermore, because mesoscale failure appears before macroscale failure, the safety factor corresponding to the new method is smaller than that derived by the traditional method, which considers macroscale failure. Therefore, the proposed method is beneficial for engineering safety.

Effects of 3D microstructural distribution on short crack growth behavior in two bimodal Ti–6Al–4V alloys

In situ X-ray micro-tomography was performed to analyze the short crack growth in the 3D microstructure of two bimodal Ti–6Al–4V alloys with predominant α+β and α phase volume fractions. Short crack strongly interacts with the local microstructure and it grows into the predominant phase above the average microstructural fraction in each alloy. The higher volume fraction and size of the α grains in the microstructure induce crack growth with lower variations and larger deflection lengths inside the α grains. As the short crack length increases, the larger deflections left behind the crack front induces higher crack closure that decreases the crack driving forces. As a result, the short crack remains sensitive to the local microstructural features at higher lengths.

Experimental investigation on the flexural behavior of pervious concrete beams reinforced with geogrids

Abstract Pervious concrete has been widely used due to the environmental and stormwater management benefits. However, its susceptibility to cracking and low flexural strength limited the widespread use for vehicular traffic pavement. This paper focuses on the flexural behavior of pervious concrete beams reinforced by geogrids with different layer number at various positions. The influences of the embedded geogrids on effective porosity, strength and cracking behaviors are studied. The load-vertical displacement curves are obtained by four-point bending tests to describe the toughness and post-cracking performance of pervious concrete beams. Meanwhile, the acoustic emission (AE) and digital image correlation (DIC) techniques are used to monitor the internal fracture characteristics, strain fields and crack propagation. The experimental results indicate that the compressive strength and flexural strength of pervious concrete can be improved owing to the embedded geogrids. Notwithstanding an increased porosity is induced, the reinforced beams demonstrate significantly superior post-crack performance. Furthermore, the embedded geogrids in pervious concrete restrain the generation and propagation of the crack, resulting in a low strain field level as well as showing a short crack length and a small crack mouth opening displacement (CMOD). Embedding the geogrids at both one-third depth and two-thirds depth of the thickness provides the optimal performance.

Analysis of crack geometry and location in notched bars by means of a three-probe potential drop technique

It is often necessary to define the crack initiation life of a fatigue tested component, generally at a given (short) crack length. The size of an initiated crack can be estimated by employing different experimental methods, one of which is the direct current potential drop (DCPD) technique. In the case of notched bars subjected to fatigue loadings, the crack configuration (i.e. circumferential or semi-elliptical) and location cannot be singled out by means of the potential drop method (PDM) operating with a single potential probe. In the present contribution, three potential probes are adopted to overcome this issue. The calibration curves reporting the three potential drops as a function of the crack size are derived by means of 3-dimensional electrical FE analyses. Two different crack configurations are analyzed: (i) circumferential and (ii) semi-elliptical surface cracks. The calibration curves have been validated by systematic comparison with experimental results, generated by fatigue testing of sharp as well as blunt notched specimens made of steel and a titanium alloy under pure axial loading. Finally, a procedure to assess the area, the configuration and the location of the initiated fatigue crack starting from the experimentally measured potential drops is discussed.

Characterization of hardness, elastic modulus and fracture toughness of RB-SiC ceramics at elevated temperature by Vickers test

In this paper, mechanical properties of RB-SiC ceramics, such as hardness, elastic modulus and fracture toughness, are characterized through indentation technique using a Vickers indenter at elevated temperatures ranging from room temperature to1200 °C realized by laser heating. The indentation size effect, load-displacement curves and relationship between crack length and applied load are studied in order to determine hardness, elastic modulus and fracture toughness accurately. The results show that the Meyer’s index and Vickers hardness decrease with the increase temperature. It indicates that the permanent plastic deformation of RB-SiC ceramics is mainly responsible for the indentation size effect and the reduction of hardness at elevated temperature. Both material softening and plastic deformation will contribute to the indentation creep at elevated temperature as shown in the load-displacement curves. The elastic modulus decreases with the increase of temperature due to increase of contact depth as a result of less elastic recovery. In the indentation test for calculating fracture toughness, only radial-median cracks are identified by the relationship between crack length and applied load at all temperatures, although the fracture mode observed at the indent corner changes from transgranular at room temperature to intergranular at elevated temperature. As more energy is consumed by intergranular facture and cracking-healing takes place due to oxidation, only short crack length appears in the indentation test which implies an increase of fracture toughness with the increase of temperature. However, this tendency has an exception at the highest temperature of 1200 °C. This is because the free Si softening in RB-SiC specimen fails to resist crack propagation at extremely high temperature. Consequently, the crack length increases again which leads to the increase of the calculating fracture toughness at the highest temperature. These variations of hardness, elastic modulus and fracture toughness with temperatures will account for the possible change of material removal regimes occurred in some thermal-involved hybrid machining of RB-SiC ceramics.

A micromechanical model to study the closure stress effect on fatigue life of Ti6Al4V subjected to laser shock peening

With the wide application of titanium alloy in various engineering fields, the service life of Ti alloy product has drawn much more attention. It’s been proven that the laser shock peening (LSP) is an effective technology to improve the fatigue performance of titanium alloy by introducing a large and deep compressive residual stress. In this paper, a microstructural fracture mechanics model incorporating the crack closure stress effect is presented to investigate the reinforcement mechanism of laser shock peened (LSPed) titanium alloy. Firstly, a rigid-plastic simplified model is utilized to describe the residual stress characteristics of LSPed Ti6Al4V. Then the extended Navarro-Rios model incorporating crack closure stress effect is established to systematically analyze the crack propagation behavior. Finally, the effect of crack closure stress on fatigue life can be accurately predicted. Result shows that the crack propagation processing is a behavior reflecting the deceleration and acceleration of the crack growth rate. A higher threshold stress is required for LSPed Ti6Al4V to promote the propagation of fatigue crack. The closure stress effect reduces the barrier stress and critical crack length in each grain leading to the highly raised fatigue life of LSPed Ti6Al4V. Besides, the closure stress effect has a greater improvement in fatigue performance for a sample with a shorter crack length and thus can be neglected for a long crack.

Calibration of the potential drop method by means of electric FE analyses and experimental validation for a range of crack shapes

AbstractIn experimental fatigue tests, it may be necessary to identify the onset of crack initiation, which is often defined at a given (short) crack length. Different experimental techniques are available to estimate the initiated crack size, one of which is the direct current potential drop technique. In this paper, the calibration curves reporting the potential drop change as a function of the crack depth have been derived by means of 3D electrical finite element analyses. Driven by previous experimental observations, two different crack shapes have been considered: (i) circumferential and (ii) semi‐elliptical surface cracks. Dealing with circumferential cracks, the effects of the three‐dimensional distribution of the electric current density and the temperature have been investigated. Concerning semi‐elliptical surface cracks, the effects of the crack aspect ratio and the potential probes locations have been analysed. Finally, the obtained calibration curves have been validated against experimental measurements, obtained by fatigue testing notched specimens under a selection of loading conditions.