Primary Crack Research Articles

Crack width prediction of RC beams by artificial neural networks

In this paper, artificial neural networks are utilised to evaluate the cracking that occurs in reinforced concrete beams while simultaneously maintaining a consistent deformation pattern. Artificial neural networks are stimulation-based computing systems that simulate biological neural networks. Here, data from several sources is used to form the ANN model and predict the cracking behaviour of RC beams. The model ANN-(A) inspects eight different parameters that comprise height, width, effective height, cover, AS1, AS2, stress, concrete compressive strength, and primary crack spacing, respectively. The findings demonstrate that artificial neural networks (ANNs) make reliable crack width predictions, which are supported by training, testing, and validation outcomes. This crack prediction will assist in predicting the crack width of the RC beams when used with similar parameters.

Application of soft computing in estimating primary crack spacing of reinforced concrete structures

Abstract The investigation related to the serviceability analysis, particularly in terms of crack spacing prediction, has remarkably increased recently. In addition, the prediction of serviceability analysis is highly dependent and influenced by different physical and material factors that contribute to the crack spacing of reinforced concrete (RC) structures. As a result, the cracking phenomenon has not been fully grasped due to these factors’ wide variety and complexity. Recently, soft computing techniques have gained considerable popularity due to their capability of learning and producing generalized solutions and exhibiting desirable performance in terms of time, effort, and cost. However, the literature on crack spacing prediction using various machine learning approaches is limited and insufficient. Therefore, this article is dedicated to estimating the primary crack spacing of RC structures using different machine learning methods. As a part of the study, the findings of these approaches will be computed and compared to the benchmark experimental results. Besides, the results of the developed models will be compared against that of available approaches in the literature to highlight their reliability. Furthermore, a parametric assessment will be conducted to emphasize the most influencing input parameter on the primary crack spacing of RC structures.

Study on the sliding wear map of cylinder liner − piston ring based on various operating parameters

The wear map can directly reflect the wear state and mechanism of the friction pair under different conditions and is widely used in wear prediction and fault diagnosis of steel materials. However, there are few reports on the wear map of the engine cylinder liner and its materials. In this study, the operating parameters were obtained through an engine bench test. The parameters near the cylinder liner's top dead center （TDC） were selected as wear test parameters. Thousands of hours of reciprocating lubrication wear tests were carried out on a multi-function wear tester. By analyzing the wear rate variation under different test parameters, the cylinder liner's wear state was divided into mild, medium, and severe wear. The morphology of the wear scar surface and cross-section of the cylinder liner was analyzed by scanning electron microscope (SEM) and then drawing the wear mechanism map. The results show that the primary wear mechanism under slight wear is spalling wear, accompanied by the plastic deformation phenomenon. Delamination wear is the primary mechanism in the medium wear state, accompanied by a small amount of massive spalling wear. The delamination wear is mainly caused by the crack generated by graphite in or below the deformation layer. The primary wear mechanism of the severe wear state is massive spalling. At this time, the graphite at the bottom of the deformation layer plays the role of primary cracks, which causes the plastic deformation layer to fall off.

In-Situ Synchrotron HEXRD Study on the Micro-Stress Evolution Behavior of a Superalloy during Room-Temperature Compression.

The residual stress generated during heat treatment of nickel-base superalloys will affect their service performance and introduce primary cracks. In a component with high residual stress, a tiny amount of plastic deformation at room temperature can release the stress to a certain extent. However, the stress-releasing mechanism is still unclear. In the present study, the micro-mechanical behavior of the FGH96 nickel-base superalloy during room temperature compression was studied using in situ synchrotron radiation high-energy X-ray diffraction. The in situ evolution of the lattice strain was observed during deformation. The stress distribution mechanism of grains and phases with different orientations was clarified. The results show that at the elastic deformation stage, the (200) lattice plane of γ' phase bears more stress after the stress reaches 900 MPa. When the stress exceeds 1160 MPa, the load is redistributed to the grains with their <200> crystal directions aligned with the loading direction. After yielding, the γ' phase still bears the main stress.

Shear friction and fracture based model for reinforced concrete beams with recycled materials

Shear resistance by beams without stirrups presents the transfer of shear and normal principal stresses across the open cracks within reinforced concrete (RC) beams. The complexity of the shear mechanism is further complicated by the distorted structure of RCA, which is composed of hardened adhered surface mortar and virgin aggregate. Compression field theory (CFT) postulates a series of curves for shear-loaded beams to signify the transfer of tensile and compressive stresses in concrete beams across developed shear cracks. Special attention is given to beams with recycled materials, which do not necessarily give the same strength as virgin aggregate. A numerical shear model is developed that accounted for the fracture characteristics, modes I and II of the beam containing RCA along the primary crack path from dowel action, aggregate interlock, and uncracked compression zone of the beam. The model captures the correlation of shear arm ratio, longitudinal reinforcement ratio, maximum coarse aggregate size, and concrete strength. The study model was validated using experimental results from 139 concrete beams containing RCA without stirrups. The strength ratio resulted in mean of 1.32 with a standard deviation of 0.48, a coefficient of variation of 36.8% and R-squared value of 0.65. The statistical evaluations of the model were compared with the other nine (9) theoretical models by authors and codes of practice. The developed model predicted a relatively higher shear strength value due to the inclusion of contribution from dowel action, aggregate interlock and compression zone.

Influence of notch root radius on high cycle fatigue properties and fatigue crack initiation behavior of Ti-55531 alloy with a multilevel lamellar microstructure

Notch high cycle fatigue (HCF) properties and microcrack initiation behavior of Ti-55531 alloy with a multilevel lamellar microstructure under various notch radii were systematically investigated. Results indicate that the reduction of notch root radius significantly promotes fatigue microcrack initiation, and then dramatically reduces the HCF life and strength of the alloy. Cyclic deformation of the alloy is mainly controlled by the slipping and deformation twinning in α plates. The primary fatigue crack initiation micro-mechanism is α/β interface cracking induced by slipping and twinning at all notch HCF specimens. Moreover, the volume fraction of twinning would increase with decreasing of the notch root radius. Interestingly, when the notch root radius is the smallest (R = 0.14 mm) and the stress concentration factor is the largest (Kt = 4), in addition to slipping and twinning, basal stacking faults promoting the cracking of α/β interface could be another crucial HCF microcrack initiation mechanism of the alloy. Furthermore, with decreasing of notch root radius and increasing of stress concentration factor, the size of cycle plastic deformation zone at notch root gradually reduces to the size of α colony and even the α plate. Therefore, the order of influential degree of three different levels microstructure on the crack initiation mechanism of notched HCF can be arranged as α plate > α colony > β GB.

Dependence of tungsten crack behaviors on heat source parameters under transient heat flow

As a plasma facing material in TOKAMAK, tungsten may suffer plastic deformations, cracking, melting, even ablation induced by transient heat loads, such as Edge Localized Modes (ELMs). Focused on investigation of cracking behaviors of tungsten under transient heat flux and its dependence on pulse parameters, powder metallurgical pure tungsten samples have been tested with 1–100 pulses of intense pulse electron beams (IPEB) at 5 ms and 1 ms pulse duration. Three types of cracks were observed, including primary crack, secondary crack and blunt crack. The primary cracks can be assorted as branch, annular and network cracks according to their patterns. Branch cracks were formed in the irradiation area center, and annular cracks appeared surrounded the irradiation area. The width of annular cracks increases as the pulse number increasing. The initiation and evolution of annular cracks are mainly studies in this paper. Considering the ductile-to-brittle transition property, an ideal elasto-plastic mechanics model is adopted in the thermodynamic calculation model of tungsten under the IPEB irradiation. Irreversible plastic deformation plays an important role, which determined the tensile stress distribution during the temperature decreasing. The formation mechanism of annular crack and its dependence on the heat source parameters are discussed, based on the calculation results of temperature, stress distribution and evolution of tungsten under different pulse parameters with this model. It is found that the formation mechanism of annular crack depends on the high radial gradient of energy density E, and the radius of annular crack obviously depends on the energy density deposited per pulse and the heat source boundary gradient for large volume sample. The influence of pulse number on branch crack and annular crack is different. The widening of the annular crack with the increasing of pulse number is caused by the axial residual plastic strain after pulse irradiation.

Evaluation of cracking damage in electrode materials of a LMO/Al-Lix lithium-ion battery through analysis of acoustic emission signals

Damage due to the internal degradation of electrode materials in lithium-ion batteries (LIBs) during charge-discharge cycles can cause capacity fading and safety issues. Therefore, it is essential to assess damage to LIBs using nondestructive methods. In this study, the relationship between the damage mechanism and acoustic emission (AE) activity of a commercial lithium manganese oxide (LMO)/Al-Lix alloy battery is systematically investigated. Microstructural observations of the electrodes revealed that the damage mechanisms of the LMO cathode and Al-Lix anode during accelerated charge-discharge were identical to those of microcracking. Energy, amplitude, duration, rise time, and peak frequency were considered as possible AE parameters for real-time damage detection. The generation of AE hits was prominent during the later stages of the charging and discharging processes. The AE signals produced by damage to the Al-Lix alloy anode during charging had a lower amplitude (0–60 dB) and energy (0–10 aJ). In comparison, those produced by damage to the LMO cathode during charging had a relatively high amplitude (100 dB) and energy (120 aJ). Additionally, the energy levels and amplitudes of the AE signals obtained during discharging decreased with an increasing number of cycles, indicating that they can be used as indicators for evaluating the damage progression of the primary, secondary, and tertiary cracks in LMOs. The AE technique offers the possibility of monitoring and evaluating the damage to the anode and cathode of a commercial full cell separately, as well as predicting the remaining capacity of the battery by monitoring the AE hits.

Experimental study on the dynamic splitting tensile properties of polyvinyl-alcohol-fiber-reinforced cementitious composites

In this study, the dynamic splitting performance and energy absorption capacity of polyvinyl- alcohol-fiber-reinforced engineered cementitious composites were studied using a split Hopkinson pressure bar system. By using a high-speed camera, changes in the surface strain fields and crack expansion patterns of specimens under different striking pressures were observed and validated, and experimental results for four types of fiber-reinforced concrete and one type of plain concrete without PVA fibers were obtained. The strain-rate effect on the dynamic tensile strength of fiber-reinforced concrete and plain concrete was confirmed. Digital image correlation measurement results indicate that microcracks were present near the radial primary cracks in the fibrous concrete, where strain concentration zones were formed. Additionally, PVA fibers can effectively reduce the rate of decline following the peak in the localization factor, and the PVA fibers begin to play a bridging role following the generation of microcracks, which slows the damage development of specimens, allowing them to absorb more impact energy.

Coolant leak rate through fatigue cracked pipe bend: An experimental and design standard based investigation

Leak-Before-Break (LBB) requirement plays a vital role in demonstrating the structural integrity of sodium-cooled Fast Breeder Reactor (FBR) piping system. Crack size at penetration (2CS), crack size that corresponds to detectable coolant leak rate (2CL), and unstable crack size (2CG) are the primary crack parameters associated with the LBB design of FBR piping system. The detectable leak rate through a piping system is directly related to the differential pressure (ΔP) and crack size. The design pressure of FBR piping systems is relatively low, resulting in a high value of CL. Considering the need for an optimum LBB design of the FBR piping system, an experimental approach is adopted to arrive at a realistic estimate of CL. Based on the experiments, a power-law type correlation can be derived to predict the value of CL as a function of ΔP. The evidence from this study points towards the conservatism present in the current FBR LBB design philosophy as per RCC-MRx A16. These observations have several implications for achieving an optimum LBB design criterion for an FBR piping system.

Seismic Performance of Reinforced Concrete Beams Susceptible to Single-Crack Plastic Hinge Behavior

Following the 2010/2011 Canterbury and 2016 Kaikoura earthquakes, a number of reinforced concrete (RC) beams in high-rise structures developed a single primary crack at the beam-column interface without the formation of distributed secondary cracks along the beam length. Detailed assessments showed that these beams have conforming longitudinal steel ratios and the single-crack mechanism may be due to design and/or construction practices for beam-column joints in the 1980s. In order to investigate the seismic behavior of reinforced concrete beams with detailing that inhibited the spread of flexural yielding, an experimental program was carried out on RC beam specimens, having similar reinforcement detailing to that of beams that developed a single crack at their ends during the Kaikoura earthquake to understand their seismic behavior, postearthquake repairability, and residual low-cycle fatigue life. Experimental results showed that the beams were able to undergo significant inelastic drift demands without loss of lateral resistance and have sufficient residual drift capacity following moderate and large earthquake demands. The response of the beam specimens was dominated by hinge rotation via the bond-slip mechanism. Comparisons showed that the measured drift capacities of the beams exceeded the predicted drift capacities computed using state-of-the-practice procedures.

Effect of the in-situ formed CuO additive on the fracture behaviour and failure mechanism of Ag-SnO2 composites

Oxide additives can significantly improve the mechanical properties of ceramic particle-reinforced metal matrix composites. In this study, the fracture behaviour and failure mechanism of the Ag-SnO2 composites without and with in-situ formed CuO additive are investigated using in-situ scanning electron microscope (SEM) and numerical simulation. The results show that the CuO additive significantly enhances the fracture toughness of the Ag-SnO2 composite. The initiation and propagation of secondary cracks near the primary crack tip are effectively inhibited, and large dimples are formed around SnO2 particles. The improvement in the mechanical properties is attributed to the stress concentration relief in the Ag matrix and enhanced interfacial strength. Interface debonding at the sharp corners of the SnO2 particles initiates the formation of cracks in the Ag-SnO2 composite without CuO. The initiated cracks propagate through the high-stress concentration region at an angle of 45° to the loading direction, resulting in a matrix fracture. For the Ag-SnO2 composite with CuO nanoparticles, the propagation rate of the primary cracks reduces significantly, and microcracks are primarily induced near clustered SnO2 particles. Moreover, the damage initiation shifts from the SnO2/Ag interface to the Ag metal matrix with reduced SnO2 particle aggregation in the Ag-SnO2(CuO) composite.

Laboratory study on shrinkage and cracking behavior of historic earthen plaster

Grottoes are widely distributed along the ancient Silk Road in Northwest China, and wall paintings are among the most precious cultural relics in these grottoes. On-site investigation of some grottoes has revealed that these wall paintings are painted on earthen plaster and that most of them have a double-layer structure of plaster, i.e., a base coarse plaster layer (with wheat straw added) and a surface fine plaster layer (with hemp fibers added). Meanwhile, studies regarding the improvement in shrinkage and cracking behaviors via ancient plaster craftsmanship have been scarce. To investigate the specific effects of different material compositions on the shrinkage and cracking properties of plaster during desiccation, a series of laboratory shrinkage and cracking tests is conducted on plaster specimens composed of Dengban clay and quartz sand, vegetal fibers, and a potential restoration material (i.e., calcined ginger nut), respectively. The results show that adding sand and vegetal fibers can significantly increase the shrinkage limit of Dengban clay and inhibit its volume shrinkage and that vegetal fibers play an important role in the inhibition of surface cracking on plaster. Sand inhibits shrinkage by forming a skeleton in soil and reducing the formation of primary cracks. Fibers inhibit soil shrinkage by overcoming the suction between soil particles and preventing the formation of primary cracks. However, wheat straw cannot deform with the shrinkage of soil (e.g., hemp fibers); consequently, many subcracks appear around the wheat straw on the surface of coarse plaster. As a typical lime-based restoration material, calcined ginger nut paste presents slight shrinkage and a low surface crack ratio during desiccation, which demonstrates its good application prospect in plaster restoration.

Identifying Critical Defect Sizes From Pore Clusters in Nickel-based Superalloys Using Automated Analysis and Casting Simulation

Porosity can form during investment casting as a result of the solidification conditions. Significant porosity can result in agglomerations (pore clusters) which are a primary crack initiation source and can result in reduced fatigue life of Nickel-based superalloys. Such clusters of porosity are today conventionally measured using 2D micrographs. An approach to accurately predict and analyze casting porosity from 2D cuts while facing in reality a 3D shape problem is currently missing. In this work, an approach that combines automation and simulation to assess the representative pore dimension, their interconnectivity and the porosity position is presented. On a LPT blade made of IN100, the porosity percentage and Feret diameter have been measured in multiple cuts and the porosity was found to be in the range of 0.88 to 5.4 pct. From the same micrographs the critical defect sizes were estimated with an automated tool to be in the range of 200 to 1800 mum. The simulated shrinkage porosity for the same part, was predicted to be in the range of 1.67 to 2.33 pct. This study shows that the scaling factor between the pore Feret diameter and the critical pore size is equal to 2.9, in accordance to the proportionality of critical pores size clusters calculated from a 2D and 3D dataset. Finally, the simulated casting porosity was compared to that measured on cast blades in critical regions and the predictive accuracy is discussed in detail.

Mixed mode I/II fracture behavior and surface morphology of hard rock under dynamic loading

To investigate the dynamic mixed mode I/II fracture properties of hard rock, split-Hopkinson pressure bar (SHPB) was utilized to conduct dynamic fracture tests on cracked straight through Brazilian disc (CSTBD) diorite specimens with different mode mixities. The dynamic fracture load and fracture toughness for different crack angles or mode mixities were determined. The dynamic fracture process and crack mouth opening displacement (CMOD) of CSTBD specimens were monitored by high-speed camera and digital image correlation (DIC) technique, and the fractal and morphology characteristics of fracture surfaces were further analyzed using laser 3D scanner and scanning electron microscope (SEM). The test results indicate that the dynamic CMOD behavior is interrelated closely to the force acting on the crack surface, and the allowable CMOD corresponding to the dynamic peak load decreases with increasing crack angle. In addition, the stress-based and strain-based fracture criteria were introduced to analyze the mixed mode fracture mechanism of diorite. It is found that the GMTS criterion with small fracture process zone (FPZ) size and the EMTSN criterion have the best prediction effect for fracture toughness and crack initiation angle respectively. Observation results show that the macro fracture trajectories of primary cracks and secondary cracks are controlled by the prefabricated crack angle, while the variation in mode mixity has little impact on the meso-roughness and micro-morphology of the fracture surface. The fracture surface of CSTBD diorite specimen under dynamic load exhibits the SEM morphology features of mixed tensile-shear cracks.

Three-dimensional characterization of cracks in undisturbed Mile expansive soil using X-ray computed tomography

The distribution of cracks strongly affects the hydro-mechanical behavior and strength of soil. In order to characterize the distribution of cracks in undisturbed expansive soil, X-ray computed tomography (X-ray CT) tests were carried out for one type of undisturbed expansive soil from Mile City, China. Seven undisturbed specimens retrieved from depths between 1 and 4.7 m were used to study the evolution of cracks along the drying path. The characteristics of the cracks were quantitatively studied by 3D parameters extracted from CT images by commercial software. The test results suggested that, with an increase in suction, the total volume of the cracks gradually increased, and one large crack formed due to the merging of sub-cracks before the shrinkage limit. When the suction was larger than the suction corresponding to the shrinkage limit, degradation of the cracks caused by the collapse of the soil structure was observed. Meanwhile, due to the development of sub-cracks, the crack anisotropy caused by the typically oriented primary cracks decreased along the drying path. Finally, a conceptual model of crack development for undisturbed Mile clay was proposed based on the evolution of crack volume, number, shape, and orientation.

Strain compliance crack model for RC beams: primary versus secondary cracks

This study proposes the Strain Compliance Crack (SCC) model to capture, as reported by tests, the specifics of various cases of crack spacing and width analysis involving primary and secondary cracks in RC beams. The model dwells on calculating crack spacing based on the reinforcement strain profile. As part of the model, a new concept of the limit effective depth, dlim, is proposed to classify crack patterns. When d ≤ dlim (small beam), the pattern consists of primary bending cracks only; when d > dlim (large beam), both primary and secondary cracks are taken into account. Being dependent on the reinforcement ratio, modular ratio, cover, and bar diameter, for most practical cases, dlim ranges from 0.12 to 0.64 m. While the model can predict mean spacing between primary and secondary cracks, only primary cracks are considered for the crack width analysis. A linear crack width profile is assumed for small beams. A bilinear shape (including a fish shape) of the width profile is taken for primary cracks of large beams: between the tips of primary and secondary cracks, width is governed by the primary crack spacing, whereas the remaining part is also affected by the secondary cracks. Crack spacing and width predictions by the proposed model agreed well with the tests.

Effect of Tempforming Temperature on the Impact Toughness of an HSLA Steel

The fracture behavior of a high-strength low-alloy steel subjected to tempforming at 873 K, 923 K, or 973 K was studied by means of impact and bending tests. A decrease in tempforming temperature promoted the grain refinement resulting in the ultrafine grained lamellar-type microstructure with finely dispersed particles that led to significant strengthening along with an increase in the impact toughness. The tempformed steel samples exhibited Charpy V-notch impact energy well above 100 J at temperatures of 183–293 K due to delamination along the rolling plane during bending, which was attributed to high anisotropy of cleavage fracture stress. Delamination owing to easy cleavage crosswise to the impact direction blunted the primary crack and resulted in the zigzag crack propagation, leading to high impact toughness. Depending on tempforming temperature, three types of the delamination behavior were observed in the V-notch specimens upon three-point bending tests at room temperature. Namely, the early, restrained, and late delaminations took place in the samples after tempering at 873 K, 923 K, and 973 K, respectively. On the one hand, a decrease in the test temperature promoted delamination, and on the other the strengthening by lowering tempforming temperature is accompanied by a suppression of ductile fracture. The sample tempformed at 873 K exhibited the highest impact toughness at room temperature, whereas the samples tempformed at 923–973 K were characterized by the higher impact toughness at 183–233 K.

Effect of Tempforming on Strength and Toughness of Medium-Carbon Low-Alloy Steel.

The effect of tempforming on the strength and fracture toughness of 0.4%C-2%Si-1%Cr-1%Mo-VNb steel was examined. Plate rolling followed by tempering at the same temperature of 600 °C increases yield stress by 25% and the Charpy V-notch impact energy by a factor of ~10. Increasing rolling reduction leads to the reorientation and elongation of grains toward the rolling direction (RD) and the development of a strong {001} <110> (rotated cube) texture component that highly enhances fracture toughness. A lamellar structure with a spacing of 72 nm between boundaries and a lattice dislocation density of ~1015 m-2 evolves after tempforming at 600 °C with a total strain of 1.4. Two types of delamination were found, attributed to crack branching and the propagation of secondary cracks along the rolling plane perpendicular to the propagation direction of the primary crack. Delamination toughness is associated with the nucleation of secondary cracks in RD and their propagation over a large distance. The critical condition for delamination toughness is the propagation of primary cracks by the ductile fracture mechanism and the propagation of secondary cracks by the brittle quasi-cleavage mechanism.

Strain-Based Fatigue Experimental Study on Ti–6Al–4V Alloy Manufactured by Electron Beam Melting

Additive manufacturing (AM) by electron beam melting (EBM) is a technique used to manufacture parts by melting powder metal layer-by-layer with an electron beam in a high vacuum, thereby generating a 3D topology. This paper studies the low-cycle fatigue of Ti–6Al–4V specimens obtained by EBM. Static tests were carried out according to ASTM E8 for a yield stress of 1023 MPa, a fracture stress of 1102 MPa, and a maximum tensile strength of 1130 MPa with a maximum true normal strain at fracture εmax = 9.0% and an elastic modulus of 120 GPa. Then, fatigue tests were conducted at a load inversion rate of R = −1. It was observed that the material exhibited plastic strain softening, which was attributed to the Bauschinger effect. These results were plotted on a strain vs. life (ε−N) curve using the Ong version of the Coffin–Manson rule and the Baumel–Seager and Meggiolaro–Castro rules. The results were compared to forged Ti–6Al–4V alloys. The cyclic stress–strain behavior was described with the Ramberg–Osgood model. Finally, the fracture surface was analyzed using scanning electron microscopy (SEM) to observe the formation of primary cracks. The fracture morphology showed a mixed surface, also known as a “quasi-cleavage”, which is characterized by dimples, cleavage facets, extensive primary cracks with broken slipping planes, and a large number of inclusions. This phenomenon caused a possible brittle behavior in the material.