Showing papers on "Crack closure published in 2015"

Why do cracks branch? A peridynamic investigation of dynamic brittle fracture

Abstract: In this paper we review the peridynamic model for brittle fracture and use it to investigate crack branching in brittle homogeneous and isotropic materials. The peridynamic simulations offer a possible explanation for the generation of dynamic instabilities in dynamic brittle crack growth and crack branching. We focus on two systems, glass and homalite, often used in crack branching experiments. After a brief review of theoretical and computational models on crack branching, we discuss the peridynamic model for dynamic fracture in linear elastic–brittle materials. Three loading types are used to investigate the role of stress waves interactions on crack propagation and branching. We analyze the influence of sample geometry on branching. Simulation results are compared with experimental ones in terms of crack patterns, propagation speed at branching and branching angles. The peridynamic results indicate that as stress intensity around the crack tip increases, stress waves pile-up against the material directly in front of the crack tip that moves against the advancing crack; this process “deflects” the strain energy away from the symmetry line and into the crack surfaces creating damage away from the crack line. This damage “migration”, seen as roughness on the crack surface in experiments, modifies, in turn, the strain energy landscape around the crack tip and leads to preferential crack growth directions that branch from the original crack line. We argue that nonlocality of damage growth is one key feature in modeling of the crack branching phenomenon in brittle fracture. The results show that, at least to first order, no ingredients beyond linear elasticity and a capable damage model are necessary to explain/predict crack branching in brittle homogeneous and isotropic materials.

Fracture Toughness and Fatigue Crack Growth Behavior of As-Cast High-Entropy Alloys

Abstract: The fracture toughness and fatigue crack growth behavior of two as-vacuum arc cast high-entropy alloys (HEAs) (Al0.2CrFeNiTi0.2 and AlCrFeNi2Cu) were determined. A microstructure examination of both HEA alloys revealed a two-phase structure consisting of body-centered cubic (bcc) and face-centered cubic (fcc) phases. The notched and fatigue precracked toughness values were in the range of those reported in the literature for two-phase alloys but significantly less than recent reports on a single phase fcc-HEA that was deformation processed. Fatigue crack growth experiments revealed high fatigue thresholds that decreased significantly with an increase in load ratio, while Paris law slopes exhibited metallic-like behavior at low R with significant increases at high R. Fracture surface examinations revealed combinations of brittle and ductile/dimpled regions at overload, with some evidence of fatigue striations in the Paris law regime.

Microstructural mechanisms of cyclic deformation, fatigue crack initiation and early crack growth.

Abstract: In this survey, the origin of fatigue crack initiation and damage evolution in different metallic materials is discussed with emphasis on the responsible microstructural mechanisms. After a historical introduction, the stages of cyclic deformation which precede the onset of fatigue damage are reviewed. Different types of cyclic slip irreversibilities in the bulk that eventually lead to the initiation of fatigue cracks are discussed. Examples of trans- and intercrystalline fatigue damage evolution in the low cycle, high cycle and ultrahigh cycle fatigue regimes in mono- and polycrystalline face-centred cubic and body-centred cubic metals and alloys and in different engineering materials are presented, and some microstructural models of fatigue crack initiation and early crack growth are discussed. The basic difficulties in defining the transition from the initiation to the growth of fatigue cracks are emphasized. In ultrahigh cycle fatigue at very low loading amplitudes, the initiation of fatigue cracks generally occupies a major fraction of fatigue life and is hence life controlling.

Effects of stress ratio on high-cycle and very-high-cycle fatigue behavior of a Ti-6Al-4V alloy

Abstract: The effects of stress ratio on high-cycle fatigue (HCF) and very-high-cycle fatigue (VHCF) behavior of a Ti-6Al-4V alloy were systematically investigated in this paper. Fatigue tests with ultrasonic frequency (20 kHz) were performed on specimens of a bimodal Ti-6Al-4V alloy with stress ratios of -1, -0.5, -0.1, 0.1 and 0.5. Three types of crack initiation mode were observed on the fracture surfaces of the specimens that failed in the HCF and the VHCF regimes, which were explicitly classified as surface-without-facets, surface-with-facets and interior-with-facets. With the increase of stress ratio from 1 to 0.5, the number of specimens for surface-without-facets decreased, that for surface-with-facets increased, and that for interior-with-facets increased first and then decreased. For the failure types of surface-with-facets and interior-with-facets, the fatigue strength decreased sharply in the VHCF regime, and the S-N curve switched from an asymptote shape to a duplex shape. Then, a new model based on Poisson defect distribution was proposed to describe the effects of stress ratio on the occurrence of different failure types, i.e., the competition of alternative failure types. The observations also showed that there is a rough area at the crack initiation region for interior initiation cases, and the values of the stress intensity factor range for the rough area are within a small range, with the mean value being close to the threshold for the crack starting to grow in vacuum environment of the alloy. The estimated value of plastic zone size at the periphery of rough area is close to the average diameter of the primary cc grains of the alloy. (C) 2014 Elsevier B.V. All rights reserved.

Effect of build direction on the fracture toughness and fatigue crack growth in selective laser melted Ti-6Al-4 V

Abstract: Experimental investigation was conducted to evaluate the fracture toughness and fatigue crack growth characteristics in selective laser-melted titanium 6Al-4 V materials as a follow-on to a previous study on high cycle fatigue. For both the fracture toughness and crack growth evaluation, the compact tension specimen geometry was used. It was found that the fracture toughness was lower than what would be expected from wrought or cast product forms in the same alloy. This was attributed to the rapidly cooled, martensitic microstructure, developed in the parts. At low stress ratios, the crack growth rates were faster than in wrought titanium but became comparable at higher ratios. The fracture toughness appears to be higher when the crack is oriented perpendicular to the build layers. The difference in the average threshold and critical stress intensity values for the crack growth results for the three orientations was within the scatter of the data, so there was essentially no difference. The same was true for the empirically derived Paris Law constants. Residual stresses were likely to have overshadowed any variation in crack growth because of microstructural directionalities associated with build orientation.

Microscopic and nanoscopic observations of metallurgical structures around inclusions at interior crack initiation site for a bearing steel in very high-cycle fatigue

Abstract: In some high-strength steels, a fatigue crack tends to occur at the interior inclusion after a long-term sequence of the cyclic loadings at low stress levels, although the crack takes place at the surface in the usual life region at high stress levels. Thus, we have the duplex S-N curves consisting of the respective S-N curves for usual life region and very high- cycle regime. It is well known that a significant fracture surface having the fine granular morphology is formed around the interior inclusion at the crack initiation site. This sur- face area is sometimes called as 'fine granular area'. In this work, metallurgical structures around the interior inclusion at the fatigue crack initiation site were carefully observed by combining several special techniques such as focused ion beam technique and high- resolution scanning electronic microscopes. Based on the current observation results, it was found that the microstructure around the interior inclusion was changed into the penny-shape fine granular layer from the usual martensitic structure during long-term cyclic loadings. Then, debondings along with the boundaries of the matrix and the fine granular layer have produced the small cracks inside the metallic material, and these in- terior cracks caused the final fatigue fracture after definite loading cycles of the crack propagation.

Evaluation of Orientation Dependence of Fracture Toughness and Fatigue Crack Propagation Behavior of As-Deposited ARCAM EBM Ti-6Al-4V

Abstract: This preliminary work documents the effects of test orientation with respect to build and beam raster directions on the fracture toughness and fatigue crack growth behavior of as-deposited EBM Ti-6Al-4V. Although ASTM/ISO standards exist for determining the orientation dependence of various mechanical properties in both cast and wrought materials, these standards are evolving for materials produced via additive manufacturing (AM) techniques. The current work was conducted as part of a larger America Makes funded project to begin to examine the effects of process variables on the microstructure and fracture and fatigue behavior of AM Ti-6Al-4V. In the fatigue crack growth tests, the fatigue threshold, Paris law slope, and overload toughness were determined at different load ratios, R, whereas fatigue precracked samples were tested to determine the fracture toughness. The as-deposited material exhibited a fine-scale basket-weave microstructure throughout the build, and although fracture surface examination revealed the presence of unmelted powders, disbonded regions, and isolated porosity, the resulting mechanical properties were in the range of those reported for cast and wrought Ti-6Al-4V. Remote access and control of testing was also developed at Case Western Reserve University to improve efficiency of fatigue crack growth testing.

Fatigue degradation and electric recovery in Silicon solar cells embedded in photovoltaic modules

Abstract: Cracking in Silicon solar cells is an important factor for the electrical power-loss of photovoltaic modules. Simple geometrical criteria identifying the amount of inactive cell areas depending on the position of cracks with respect to the main electric conductors have been proposed in the literature to predict worst case scenarios. Here we present an experimental study based on the electroluminescence (EL) technique showing that crack propagation in monocrystalline Silicon cells embedded in photovoltaic (PV) modules is a much more complex phenomenon. In spite of the very brittle nature of Silicon, due to the action of the encapsulating polymer and residual thermo-elastic stresses, cracked regions can recover the electric conductivity during mechanical unloading due to crack closure. During cyclic bending, fatigue degradation is reported. This pinpoints the importance of reducing cyclic stresses caused by vibrations due to transportation and use, in order to limit the effect of cracking in Silicon cells.

Fracture mechanics by three-dimensional crack-tip synchrotron X-ray microscopy.

Abstract: To better understand the relationship between the nucleation and growth of defects and the local stresses and phase changes that cause them, we need both imaging and stress mapping. Here, we explore how this can be achieved by bringing together synchrotron X-ray diffraction and tomographic imaging. Conventionally, these are undertaken on separate synchrotron beamlines; however, instruments capable of both imaging and diffraction are beginning to emerge, such as ID15 at the European Synchrotron Radiation Facility and JEEP at the Diamond Light Source. This review explores the concept of three-dimensional crack-tip X-ray microscopy, bringing them together to probe the crack-tip behaviour under realistic environmental and loading conditions and to extract quantitative fracture mechanics information about the local crack-tip environment. X-ray diffraction provides information about the crack-tip stress field, phase transformations, plastic zone and crack-face tractions and forces. Time-lapse CT, besides providing information about the three-dimensional nature of the crack and its local growth rate, can also provide information as to the activation of extrinsic toughening mechanisms such as crack deflection, crack-tip zone shielding, crack bridging and crack closure. It is shown how crack-tip microscopy allows a quantitative measure of the crack-tip driving force via the stress intensity factor or the crack-tip opening displacement. Finally, further opportunities for synchrotron X-ray microscopy are explored.

Fatigue crack growth in CFRP-strengthened steel plates

Abstract: In this paper fatigue crack growth in steel plates reinforced by using carbon fiber reinforced (CFRP) strips is investigated from the experimental, numerical and analytical point of view. Single edge notched tension (SENT) specimens were strengthened with different reinforcement configurations and tested at a stress ratio R of 0.4. Different initial damage levels were considered and the experimental results showed that the reinforcement application can effectively reduce the crack growth rate and significantly extend the fatigue life. Numerical models (finite elements) were also developed to evaluate the stress intensity factor (SIF) and the crack opening displacement (COD) profile. Based on the numerical results, an analytical model was proposed to predict the fatigue crack growth rate and the fatigue crack growth curves. The analytical results are validated by comparing the fatigue crack growth curves to the experimental ones.

CO2-induced mechanical behaviour of Hawkesbury sandstone in the Gosford basin: An experimental study

Abstract: Carbon dioxide (CO2) sequestered in saline aquifers undergoes a variety of chemically-coupled mechanical effects, which may cause CO2-induced mechanical changes and time-dependent reservoir deformation. This paper investigates the mineralogical and microstructural changes that occur in reservoir rocks following injection of CO2 in deep saline aquifers and the manner in which these changes influence the mechanical properties of the reservoir rocks. In this study, cylindrical sandstone specimens, 38mm in diameter and 76mm high, obtained from the Gosford basin, were used to perform a series of unconfined compressive strength (UCS) tests. Different saturation conditions: dry, water- and brine-saturated sandstone samples with and without scCO2 (super-critical carbon dioxide) injection, were considered in the study to obtain a comprehensive understanding of the impact of scCO2 injection during the CO2 sequestration process on saline aquifer mechanical properties.An acoustic emission (AE) system was employed to identify the stress threshold values of crack closure, crack initiation and crack damage for each testing condition during the whole deformation process of the specimens. Finally, scanning electron microscopy (SEM), X-ray diffraction (XRD) and X-ray fluorescence (XRF) analyses were performed to evaluate the chemical and mineralogical changes that occur in reservoir rocks during CO2 injection. From the test results, it is clear that the CO2-saturated samples possessed a lower peak strength compared to non-CO2 saturated samples. According to SEM, XRD and XRF analyses, considerable quartz mineral corrosion and dissolution of calcite and siderite were observed during the interactions of the CO2/water/rock and CO2/brine/rock systems, which implies that mineralogical and geochemical rock alterations affect rock mechanical properties by accelerating the collapse mechanisms of the pore matrix. AE results also reveal the weakening effect of rock pore structure with CO2 injection, which suggests a significant effect of CO2 on failure mechanisms of the reservoir rock, with CO2 saturation showing a significant influence on crack initiation and crack damage stages.

Self-protected nitrate reducing culture for intrinsic repair of concrete cracks.

Abstract: Attentive monitoring and regular repair of concrete cracks are necessary to avoid further durability problems. As an alternative to current maintenance methods, intrinsic repair systems which enable self-healing of cracks have been investigated. Exploiting microbial induced CaCO3 precipitation using (protected) axenic cultures is one of the proposed methods. Yet, only a few of the suggested healing agents were economically feasible for in situ application. This study presents a NO3- reducing self-protected enrichment culture as a self-healing additive for concrete. Concrete admixtures Ca(NO3)2 and Ca(HCOO)2 were used as nutrients. The enrichment culture, grown as granules (0.5 – 2 mm) consisting of 70 % biomass and 30 % inorganic salts were added into mortar without any additional protection. Upon 28 days curing, mortar specimens were subjected to direct tensile load and multiple cracks (0.1 – 0.6 mm) were achieved. Cracked specimens were immersed in water for 28 days and effective crack closure up to 0.5 mm crack width was achieved through calcite precipitation. Microbial activity during crack healing was monitored through weekly NOx analysis which revealed that 92±2 % of the available NO3- was consumed. Another set of specimens were cracked after 6 months curing, thus the effect of curing time on healing efficiency was investigated, and mineral formation at the inner crack surfaces was observed, resulting in 70 % less capillary water absorption compared to healed control specimens. In conclusion, enriched mixed denitrifying cultures structured in self-protecting granules are very promising strategies to enhance microbial self-healing.

The contribution of intragranular acicular ferrite microstructural constituent on impact toughness and impeding crack initiation and propagation in the heat-affected zone (HAZ) of low-carbon steels

Abstract: We elucidate here the contribution of intragranular acicular ferrite (IAF) on impending crack initiation and propagation in the heat-affected-zone (HAZ) of low carbon steels based on Instrumented Charpy impact test. It was observed that both crack initiation energy and crack propagation energy of the steel in the presence of IAF were higher than steel that did not contain IAF, where the former situation was observed to have a large ductile fracture zone as compared to the latter. An analysis of the statistical distribution of voids in the cross-section of impact fracture indicated that IAFs increase the length of plastic zone at the tip of the main crack and impede crack initiation and enhance crack initial energy because of refined prior austenite grain and superior deformability. Furthermore, EBSD analysis of secondary cracks indicated that IAF impedes crack propagation and increases crack propagation energy by forming high angle grain boundaries with surrounding bainite and its own superior deformability.

Crack propagation in graphene

Abstract: The crack initiation and growth mechanisms in an 2D graphene lattice structure are studied based on molecular dynamics simulations. Crack growth in an initial edge crack model in the arm-chair and the zig-zag lattice configurations of graphene are considered. Influence of the time steps on the post yielding behaviour of graphene is studied. Based on the results, a time step of 0.1 fs is recommended for consistent and accurate simulation of crack propagation. Effect of temperature on the crack propagation in graphene is also studied, considering adiabatic and isothermal conditions. Total energy and stress fields are analyzed. A systematic study of the bond stretching and bond reorientation phenomena is performed, which shows that the crack propagates after significant bond elongation and rotation in graphene. Variation of the crack speed with the change in crack length is estimated. (C) 2015 AIP Publishing LLC.