Sides Of Crack Research Articles

Effects of the microstructure on the fatigue fracture of friction stir lap welded Al-clad Al and Al-clad steel sheets

The microstructural characteristics and mechanical properties of friction stir lap welding (FSLW) of thin Al-clad Al (1.5 mm) and ultra-thin Al-clad mild steel (0.7 mm) sheets are experimentally investigated. Optical microscopic observation confirms that the FSLW joints are successfully fabricated without noticeable joining defects, such as cracks and hooks. The material flow is correlated with the silicon (Si) element distribution in the stir zone (SZ). The distribution of Si indicates that material flow is asymmetrical between the advancing side (AS) and the retreating side (RS) of the SZ. The tensile test result shows that all the FSLW joints fracture from the Al-clad Al side of the joints. During the fatigue tests, two different crack propagation modes (vertical crack in the Al-clad Al loading side; horizontal crack in the Al-clad mild steel loading side) are observed simultaneously due to the combined effect of the asymmetric geometry of the FSLW joint and the different loading configurations. The results of fatigue tests suggest that the asymmetric microstructure of the joint retards the propagation of a vertical crack in the Al-clad Al loading side for the RS-loaded FSLW joint. As a result, the fatigue life of the RS-loaded FSLW joint becomes clearly higher than that of the AS-loaded FSLW joint.

Aggregate effect on the mechanical and fracture behaviours of concrete

Mechanical properties of concretes highly depend on aggregate inclusions. In-depth understandings of concrete fracture mechanisms and how aggregates influence the behaviours of concretes are crucial to applications. This paper highlights a systematic investigation on the effects of aggregate size and volume fraction on concrete fracture and mechanical behaviours by means of discrete element modelling (DEM). The present revealed the relations between the observable cracking in experiment with the interior stress distributions and crack growth, explored the fracture mechanism in terms of crack growth rate, and clarified the effect of aggregate size and volume fraction on the mechanical and fracture behaviours of concretes. The results of this study will subsequently navigate the design of fracture-resistant concrete materials.

AN APPROACH TO SUBPIXEL ACCURACY WIDENING CRACK WIDTH DETERMINATION IN IMAGE SEQUENCES

Abstract. As an extension to existing work on crack detection and subpixel accuracy crack width determination as a tool for civil engineering material testing, the paper shows an algorithmic approach to handle widening cracks with relative rotations between related crack borders. In the first time step under zero-load, a set of points to be tracked through consecutive frames of an image sequence is defined. Then, subpixel-precise displacement fields are computed for the image data of the following time steps using an 8-parameter least-squares matching approach. The points are triangulated into a mesh, and the changes of the inner geometry of the triangles are considered with a mathematical model assuming a split of each triangle. With this model, subpixel-precise deformation vectors are derived. Crack candidates are determined by a thresholding applied to the vectors’ lengths. After an estimation of the crack normal, a decomposition of the deformation vectors is applied, allowing to compute crack widths and shear movements. As a novel contribution to the technique, a model extension is proposed for the case of a relative rotation between the crack borders in order to reduce systematic errors. The model includes two separate rigid transformations for each crack side.

Multiscale analysis of interior cracking behavior of Ni-based superalloy fabricated by selective laser melting under very-high-cycle-fatigue at high-temperature

The fatigue test at high-temperature of selective laser melting Ni-based superalloy was performed to reveal the mechanism of interior cracking behavior by multiple analysis methods in the very-high-cycle-fatigue regime. Due to the inhibition effect of oxide layer, the interior multi-defects assisted facetted cracking becomes a significant failure mode. Large grain deformation mainly occurs on the fracture surface, and slip systems with high Schmid factor are activated by high-temperature. The precipitates in the matrix hinder the dislocation movement, resulting in dislocation accumulation and stress concentration increase, which makes it easier for microcracks initiation. The initiated microcracks with crack deflection caused by high angle grain boundaries propagate transgranularly to form a nearly circular region and undergo fast unstable growth, eventually leading to fracture. • The microcracks propagate transgranularly along the maximum shear stress plane. • {111}<112> slip system with high Schmid factor is activated by high-temperature. • Interior multi-defects assisted facetted cracking is the predominant failure mode.

Impact of Rice Husk Ash on the Mechanical Characteristics and Freeze–Thaw Resistance of Recycled Aggregate Concrete

With the accelerating growth of infrastructure construction, carbon emission and environmental pollution problems have become increasingly severe. In order to promote the sustainable development of the construction industry, using rice husk ash (RHA) in recycled aggregate concrete has aroused extensive interest. This study aims to investigate the impact of the partial replacement (0%, 10%, 20%, 30% of binder) of ordinary Portland cement (OPC) with RHA by equal mass on recycled concrete’s mechanical characteristics and freeze–thaw resistance. The workability, compressive strength, mass loss and dynamic elastic modulus of recycled concrete were tested, and the hydration products and microstructure were analyzed using scanning electron microscope (SEM) tests. The mechanism of the freeze–thaw damage deterioration of RHA recycled aggregate concrete was revealed. The results indicate that the incorporation of RHA has an adverse effect on the workability of fresh concrete. Its high specific surface area will provide a large number of nucleation sites for the hydration reaction, refining the pore structure in the paste and improving the weak bonding of the interfacial transition zone (ITZ) by enhancing the matrix’s pozzolanic reaction effect and filling effect, thus improving the compressive strength of concrete. Furthermore, the porous structure of the recycled aggregate attached mortar and mesoporous RHA will absorb a lot of water during the freeze–thaw cycles. With the continuous accumulation of expansion pressure, the interior pores and cracks will gradually expand and extend, leading to more severe damage to the concrete, and the degree of freeze–thaw damage deterioration grows as the RHA replacement ratios increase.

Gradient “Single‐Crystal” Li‐Rich Cathode Materials for High‐Stable Lithium‐Ion Batteries

AbstractAs one of the high‐energy cathode materials of lithium‐ion batteries (LIBs), lithium‐rich‐layered oxide with “single‐crystal” characteristic (SC‐LLO) can effectively restrain side reactions and cracks due to the reduced inner boundaries and enhanced mechanical stabilities. However, there are still high challenges for SC‐LLO with diverse performance requirements, especially on their cycle stability improvement. Herein, a novel concentration gradient “single‐crystal” LLO (GSC‐LLO), with gradually decreasing Mn and increasing Ni contents from center to surface, is designed and prepared by combining co‐precipitation and molten‐salt sintering methods, yielding a capacity retention of 97.6% and an energy density retention of 95.8% within 100 cycles at 0.1 C. The enhanced performance is mostly attributed to the gradient‐induced stabilized structure, free of cracks and less spinel‐like structure formation after long‐term cycling. Furthermore, the gradient design is also beneficial to the safety of LLOs as suggested by the improved thermal stability and reduced gas release. This study provides an effective strategy to prepare high‐energy, high‐stability, and high‐safety LLOs for advanced LIBs.

Hot Deformation Behavior and Simulation of Hot-Rolled Damage Process for Fine-Grained Pure Tungsten at Elevated Temperatures.

Fine-grained pure tungsten fabricated by a sol drying reduction low-temperature sintering method and hot isothermal compression tests were performed by using the Gleeble 3800 thermo mechanical simulator at deformation temperatures from 1273 K to 1473 K and strain rates from 0.001 s-1 to 1 s-1. In addition, the constitutive equation was established by least square method combined with the Zerilli-Armstrong model, and the hot deformation behavior was discussed. Moreover, based on constitutive equation, the influence of the rolling process and its parameters on temperature, strain, density and rolling force in the hot rolling process was investigated at elevated temperature by the finite element model (FEM). Furthermore, the form of rolling damage and its formation mechanism were analyzed. Results showed the grains of pure tungsten are dense, irregular polyhedral spherical and very fine, and the average grain size is about 5.22 μm. At a high strain rate, the flow stress increases rapidly with the increase in strain, while the stress-strain curve shows a flattening trend in the tested strain rate range with increasing temperature, and no flow stress peak exists, showing obvious dynamic recovery characteristics. Furthermore, the FEM simulation showed that compared with the rolling temperature, the reduction has a greater influence on the temperature, stress-strain field and its distribution. There are three kinds of damage in the hot rolling process: transverse cracks, longitudinal cracks and side cracks, which are attributed to the competition between additional stress caused by uneven deformation and material strength. Moreover, the control method of hot rolling defects had been preliminarily proposed. These results should be of relevance for the optimum design of the hot rolling process of pure tungsten.

Formation mechanisms of surface blistering in AA6xxx rolled products: microstructure characterisation, ultrasonic analysis, and rolling tests results

ABSTRACT The source of surface blister defect in 6xxx series Al rolled products was studied. A blister defect, aligned with the rolling direction, was observed on the surface of the AA6xxx Al rolled sheets. It was indicated that the porosities and small microstructural inhomogeneities inside the cast ingots, identified by ultrasonic tests and metallography analysis, were not the origin of these surface defects. Pilot scale rolling trials indicated that the blister defects were correlated with the longitudinal internal casting cracks. The short side cracks in the cast ingots, covered by a thin Al oxide layer, were not healed and remained partially open after the rolling process. The unhealed cracks served as the nucleation sites for H2 precipitation during the later stages of the thermomechanical process, which were subsequently resulted from surface blisters on the AA6xxx Al rolled sheets.

Numerical investigation of creep crack growth behavior of UNS N10003 alloy based on the creep damage model

Creep crack growth (CCG) analysis is extremely important for integrity evaluation of the defective structures at elevated temperatures in a thorium molten salt reactor (TMSR). However, a numerical method of CCG analysis that can be applied to the TMSR has not yet been developed. In this study, the CCG behavior of a UNS N10003 alloy was investigated based on the Liu and Murakami (L-M) creep damage model. First, the material constants were obtained according to the uniaxial creep test data. Second, finite element analysis (FEA) of CCG for compact tensile (CT) specimens was conducted, and the influence of factors such as element meshing, crack tip radius, side groove, and fillet was analyzed. The results indicated that the crack tip radius and fillet at the side groove demonstrated a significant effect on the accuracy and rationality of the FEA model. Moreover, the multi-axial parameter α in the L-M model was obtained by comparing the calculated results with the CCG test data. The calculated results were found to be more conservative than the experimental results. Finally, the proposed numerical analysis method was applied to the CCG life-span prediction of the pressure vessel in the TMSR. Thus, this study suggests an effective, reliable numerical method for CCG behavior prediction of the UNS N10003 alloy, which has important engineering significance for structural integrity evaluation at elevated temperatures in the TMSR.

Experimental Investigation on Compressive Strength, Ultrasonic Characteristic and Cracks Distribution of Granite Rock Irradiated by a Moving Laser Beam

Efficient fracturing is the key issue for the exploitation of geothermal energy in a Hot Dry Rock reservoir. By using the laser irradiation cracking method, this study investigates the changes in uniaxial compressive strength, ultrasonic characteristics and crack distributions of granite specimens by applying a laser beam under various irradiation conditions, including different powers, diameters and moving speeds of the laser beam. The results indicate that the uniaxial compressive strength is considerably dependent on the power, diameter and moving speed of the laser beam. The ultrasonic-wave velocity and amplitude of the first wave both increase with a decreased laser power, increased diameter or moving speed of the laser beam. The wave form of irradiated graphite is flattened by laser irradiation comparing with that of the original specimen without laser irradiation. The crack angle and the ratio of the cracked area at both ends are also related to the irradiation parameters. The interior cracks are observed to be well-developed around the bottom of the grooving kerf generated by the laser beam. The results indicate that laser irradiation is a new economical and practical method that can efficiently fracture graphite.

Interior defect-induced crack initiation mechanism and initial growth behavior for Ti6Al4V alloy fabricated using laser powder bed fusion

The interior defect-induced crack initiation mechanism and early growth behavior of Ti6Al4V alloy fabricated by laser powder bed fusion (LPBF) has been investigated in very high cycle fatigue (VHCF) regime. P–S–N curves under 10% and 90% failure probabilities are obtained in VHCF regime. The cracks inside the early stages of fine granular area (FGA) formation are driven by the maximum shear stress and propagate as Mode II + III mixed cracks. It can be found that the FGA region is composed of many discontinuous nanograins for Ti6Al4V alloys manufactured by LPBF, which are responsible for grain refinement. Grain refinement is associated with dislocation movement within the martensite laths. Dislocation pileup and rearrangement in martensitic laths form dislocation cells, which further develop into nanograins and low angle boundaries. Besides, both the fatigue loading process and the LPBF process form their respective microvoids, which merge and aggregate with each other, thus accelerating the microcrack extension.

High cycle and very high cycle fatigue properties and microscopic crack growth modeling of Ti‐6.5Al‐3.5Mo‐1.5Zr‐0.3Si titanium alloy at elevated temperatures

AbstractPulsating tension fatigue tests were conducted at 25°C, 150°C, and 250°C to investigate the effect of temperature on high cycle and very high cycle fatigue behavior of titanium alloy. The fatigue strength decreases with the increase of temperature, and the surface failure is more sensitive to temperature than interior failure. Afterwards, the fatigue fracture morphology, microstructure, and texture of titanium alloy were characterized. The results show that the fracture of larger αp phase with the maximum shear stress and higher strain concentration is responsible for the facet formation. The formation of facets leads to the interior failure at different temperatures. Finally, combining with the definition of interior crack growth rate, an assessment approach was proposed to predict the crack growth rate within the inhomogeneous microstructure area (IMA), which takes crack growth acceleration caused by the increase of temperature into consideration.

Colloidal Nanosilica Treatments for Sealing Cracks in Mortar.

Presence of microcracks in concrete can diminish the service life of a structure. The injection of materials for filling the crack is proposed for facing this problem. The traditional materials used for sealing cracks present some drawbacks, such as the difficulties of inorganic materials for flowing to all the depth of the crack and the lack of compatibility with the cementitious matrix in the case of organic materials. In this work, the injection of colloidal nanosilica dispersed in water is proposed for filling microcracks in mortars. The effect of the injection procedure on the sealing performance of the colloidal nanosilica has been assessed. The ability of colloidal nanosilica for penetrating through the crack and its posterior gelification-solidification inside the crack after a curing period have been confirmed. The microscopic analysis of a cross-section of the crack indicates that the sealing ability of the nanosilica seems to be not only due to the filling of the crack but also to chemical interactions with the cementitious phases of the surrounding crack sides.

ANALISIS KERUSAKAN JALAN DENGAN METODE PCI, SDI, DAN BINA MARGA SERTA ALTERNATIF PENANGANAN KERUSAKAN

Poros Gresik– Lamongan Highway KM 36-39 is a national road that connects the provinces of East Java and Central Java, the road is often traversed by high-loaded vehicles, resulting in road damage that can interfere with the safety and comfort of users. This study aims to determine the condition of the pavement layer according to the type and level of damage that occurs and can determine how to maintain it. The research was conducted by means of a survey to obtain primary data so that it can be used as a reference for the calculation and maintenance of road pavements. The method used is the Bina Marga method, PCI (Pavement Condition Index), and SDI (Surface Distress Index). On the Jalan Raya Poros Gresik – Lamongan KM 36-39, the types of damage include grain release, holes, box cracks, side cracks, patches, curls, crocodile cracks, longitudinal cracks, skid cracks, joint cracks and transverse cracks. The Bina Marga method obtained a priority order value of 7 which is included in the routine maintenance program. The PCI method obtained an average value of less than 60, and rehabilitation can be carried out for damage. The SDI method got a value of 45 which means it is in good condition. Handling carried out refers to the Practical Guide to Routine Road Maintenance (1992) in the form of Asphalt (P2), Patching (P5), and Leveling (P6).

Fracture analysis on involute spline of large tunnel boring machine

The main drive spline shafts of two large tunnel boring machines were broken after service of less than 150 h. The failure located at the section flush with the end of the spline sleeve. Different from the gear fracture occurred at the tooth roots of the working side, the spline fracture started from the tooth roots of the non-working side. The non-working side cracks were wider and longer than these of working side, and their damage were more serious. At the failed roots, there were obvious fan-shaped shellfish lines which these were significant fatigue characteristics. Through chemical composition test and metallographic analysis, the spline shaft material was speculated to be 17CrNiMo6, and quenched and tempered heat treatment. The microstructure was mainly bainite with tempered martensite and retained austenite. The micro-hardness was uneven and fluctuated between 31 and 43HRC. Sulfide inclusion was grade 2 and alumina inclusion was 2.5. The calculation by finite element method (FEM) showed that the maximum stress was located at the tooth root of the non-working side. The safety factors calculated by three methods were less than 1.0. Therefore, the root cause of spline failure is improper design and heat treatment. Failure analysis, theoretical calculation and simulation test show that the failure mode was torsion fracture.

Advanced single-crystal layered Ni-rich cathode materials for next-generation high-energy-density and long-life Li-ion batteries

The studies of Ni-rich cathode materials have been the top priority of research because of the high energy density and fair cycling life. However, suffering from severe crack generations and side reactions, the traditional polycrystal (PC) Ni-rich material displayed structural/electrochemical fade during cycling. Compared with PC, single-crystal (SC) Ni-rich materials exhibited excellent structural stability and cycling performance, benefiting from the limited side reaction and gas generation. In this review, the authors not only compared the structural evolution and electrochemical failure mechanisms between PC and SC, but also summarized the synthesis methods and characterization techniques of SC, which provides universal insights into the development of Ni-rich cathode materials.

Features and Formation Mechanism of the Jiaopenba Landslide, Southwestern Sichuan Province, China

The Jiaopenba landslide, which represents a typical large basalt high-speed remote landslide, is located in Emeishan City, southwestern Sichuan province, China. Based on the field investigation, the characteristics of the Jiaopenba landslide have been revealed. The Jiaopenba landslide occurred on the steep Mount Emei. Landslide materials with a volume of approximately 6.75×108 m3 slid down from a high position, forming a large-scale high-speed remote debris flow, with the farthest sliding distance of about 7.5 km. The landslide area is located in the west wing of the Emeishan anticline, which is cut by faults to form a monoclinal fault-block mountain. Although the shear outlet of the landslide is near the slope toe and does not have good free conditions, there are faults passing through the slope toe of the bedding slope of the fault hanging wall, the landslide is affected by the fault activity, and the integrity of the layered slope is poor. When the foot of the slope is empty, the fault zone is compressed and plastic extrusion, which leads to bedding slip of the slope rock mass and greatly weakens the interlayer bonding force. When coupled with long structural planes on both sides to form side crack planes, a large-scale inclined plate structure is formed. Under the action of a strong earthquake and other external forces, the rock mass near the fault can be damaged by tension, and a large-scale high-position landslide can be formed by the mode of compression-slip-tension fracture.

Crack growth behavior during actuation cycling of hot extruded and annealed Ni50Ti30Hf20 high temperature shape memory alloys

Actuator-type applications of high temperature shape memory alloys (HTSMAs) require better understanding of the crack growth behavior at high temperatures as there are few studies on how cracks propagate and cause fracture with phase transformation via cooling and heating under constant load. In this study, hot-extruded and subsequently annealed Ni50Ti30Hf20 (at.%) HTSMA dog-bone shaped tensile test samples with pre-notches were cooled and heated under uniaxially applied constant load magnitude till fracture. Therefore, the crack growth behavior and effect of annealing heat treatment on the crack growth rate of Ni50Ti30Hf20 (at.%) HTSMA during actuation fatigue were particularly determined. Additionally, fatigue tests were conducted twice on the pre-notched samples to investigate the repeatability of the data obtained from the experiments. It was shown that slower crack growth was observed in the pre-notched annealed samples than in the hot extruded samples because of the possible stress-relieving effect of annealing after the hot extrusion process. Several minor side crack formations and the propagation of these side cracks together with the crack growth behavior of the major crack were realized in the annealed samples. In contrast, the major crack growth rate was found to be higher than that of the growth rate of the minor side cracks. Additionally, the annealing process led to obtain consistent crack growth rate values and actuation behavior in the cooling-heating cycles.

Mean-stress sensitivity of an ultrahigh-strength steel under uniaxial and torsional high and very high cycle fatigue loading.

The influence of load ratio on the high and very high cycle fatigue (VHCF) strength of Ck45M steel processed by thermomechanical rolling integrated direct quenching was investigated. Ultrasonic fatigue tests were performed under uniaxial and torsional loading at load ratios of R = -1, 0.05, 0.3, and 0.5 with smooth specimens and specimens containing artificially introduced defects. Up to 2 × 105 cycles, failure originated from surface aluminate inclusions and pits under both loading conditions. The prevailing fracture mechanisms in the VHCF regime were interior crack initiation under uniaxial loading and surface shear crack initiation under torsional loading. The mean-stress sensitivity and the fatigue strength were evaluated using fracture mechanics approaches. Equal fatigue limits for uniaxial and torsional loading were determined considering the size of crack initiating defects and the appropriate threshold condition for Mode-I crack growth. Furthermore, the mean-stress sensitivity is independent of loading condition and can be expressed by and .

Field Detection and Numerical Simulation Study on Roof Asymmetric Fracture Mechanism of Large-Span Soft Rock Roadway with Discontinuity Surface

Existence of roof discontinuity surface is one of the extremely important factors causing the asymmetric fracture of roadway roof, especially for large-span soft rock roadway. In this paper, a crack density coefficient ( D ) is firstly defined by local thresholding-microcell segmentation method and used to analyze the evolution law of D with roof drilling depth ( d ) (the lower and upper of roof discontinuity are defined as regions I and II, respectively). Then, UDEC numerical simulation is used to investigate the asymmetry evolution law of roof total displacement, maximum principal stress, and crack density with stress release coefficient ( α ) considering the effect of discontinuity surface. Research results indicate that (1) the roof parameters (D) in regions I and II both show a negative logarithmic function decreasing trend with the increase of drilling depth. When d &lt; 4.5 m, the parameter ( D ) in region II is about 1.5 times that in region I; when d ≥ 4.5 m, the parameter ( D ) in region I is almost zero, while the parameter ( D ) in region II maintains a slight decreasing trend. Roof failure presents asymmetric distribution characteristics along both sides of the discontinuous surface. (2) In the initial stage of open-off cutting excavation, the top left and right corners of roof as well as the bottom of discontinuous surface first occurred the tension-shear failure, and then as the α increases, the two side cracks gradually shift to the middle of roof in regions I and II with the appearance of stress concentration in two top corners. Meanwhile, the direction of maximum principal stress is transformed from “direction approximately parallel to the excavation surface” to “direction perpendicular to the discontinuous surface,” of which the location transfers to the deep anisotropically, forming an asymmetric stress loosening zones in the roof of regions I and II. The range of stress loosening zone in region II is significantly larger than that in region I. When the surrounding rock stress is completely released, the roof cracks in region I gradually transition from nonconnected to connected state and form a “quasi-right triangle” loosening zone. In addition, an “isosceles triangle-like” high crack density loosening zone with the roof middle as the axis is also formed in region II. The roadway roof presents a markedly asymmetric caving feature. (3) With the increase of discontinuity surface angles, the roof fracture range gradually decreases in region I and increases in region II as well as the structural features of roof pressure-bearing arch transform from “left-low right-high continuous asymmetric structure” (30°-60°) to “left-low right-high discontinuous asymmetric structure” (90°) to “unilateral partial pressure-bearing arch structure” (120°-150°). Research results can provide an extremely important reference for the optimization and design of support scheme with discontinuity-thick soft rock roof-large span roadways.