Crack Arrest Research Articles

Study of collinear cracks in a composite medium subjected to time‐harmonic wave disturbance

AbstractThe article deals with the collinear Griffith cracks under the time‐harmonic waves, situated in an orthotropic strip sandwiched between two identical half‐planes. The outer cracks are considered to be situated on either sides of the central crack. The mathematical model is solved by applying the Fourier transform and then the unknown functions have been determined using Schmidt method. The approximate analytical expressions of the stress intensity factors (SIFs) have been obtained at the tips of the cracks. The values of the SIFs for the considered composite with Graphite epoxy material as the strip and E‐glass epoxy as the half‐plane are computed numerically for different crack lengths and wave numbers, and the obtained results are displayed graphically. The salient feature of the article is the pictorial presentations of the possibilities of arrest of the central crack due to the presence of outer cracks through finding the stress magnification factor for different particular cases.

The Working Principles of a Multifunctional Bondline with Disbond Stopping and Health Monitoring Features for Composite Structures

In comparison to bolted joints, structural bonds are the desirable joining method for light-weight composite structures. To achieve a broad implementation of this technology in safety critical structures, the issues of structural bonds due to their complex and often unpredictable failure mechanisms have to be overcome. The proposed multifunctional bondline approach aims at solving this by adding two safety mechanisms to structural bondlines. These are a design feature for limiting damages to a certain size and a structural health monitoring system for damage detection. The key question is whether or not the implementation of both safety features without deteriorating the strength in comparison to a healthy conventional bondline is possible. In previous studies on the hybrid bondline, a design feature for damage limitations in bondlines by means of disbond stopping features was already developed. Thus, the approach to evolve the hybrid bondline to a multifunctional one is followed. A thorough analysis of the shear stress and tensile strain distribution within the hybrid bondline demonstrates the feasibility to access the status of the bondline by monitoring either of these quantities. Moreover, the results indicate that it is sufficient to place sensors within the disbond stopping feature only and not throughout the entire bondline. Based on these findings, the three main working principles of the multifunctional are stated. Finally, two initial concepts for a novel multifunctional disbond arrest feature are derived for testing the fundamental hypothesis that the integration of micro sensors into the disbond stopping feature only enables the crack arrest and the health monitoring functions, while reaching the mechanical strength of a conventional healthy epoxy bondline. This work therefore provides the fundamentals for future investigations in the scope of the multifunctional bondline.

STRESS MECHANICS OF REINFORCED POLYESTER COMPOSITES

This research investigates the stress mechanics of reinforced polyester composites when exposure to a sudden impact force, using experimental and analytical methods based on impact and Linear Elastic Fracture Mechanics (LEFM) test procedures. In this study, the stress distribution around crack tip and zone for 14 test samples with geometry 210 mm x 150 mm were investigated. KII stress intensity factors, critical stress, σC, shear stress, τnt the impact energy, E and impact strength, U were determined for each specimen. The mode I fracture toughness, KIC was found to be to be 4.97 MPa.m1/2 at a critical stress of 13.53 MPa while the mode II fracture toughness, KIIC was 1.31 MPa.m1/2 at a shear stress, τnt of 3.71 MPa. The effective thickness was found to be in the range of 80-100mm at fibre volume fraction, Vf of within 0.35-0.50. This was largely found to be as result of fibre bridging and crack arrest mechanisms. This mechanism prevented crack growth direction in specimens containing woven roving not to propagate along the original direction, but change the direction to an inclined path till failure with the exception of those containing soft and hard mat in which the crack grew in the original crack direction as the stress intensity increased. During the impact test, fibre stacking sequence played a vital role, thereby making specimens containing woven roving to resist impact damage and failure, and this resulted in fibre pull-out during fracture. The stress could be seen to concentrated at the crack tip and around the loading pins on a smaller level, compared to the level at the crack tip.

Fracture and electric‐field‐induced crack growth behavior in NBT‐6BT relaxor ferroelectrics

AbstractThe fracture properties of 0.94(Na0.5Bi0.5)TiO3‐0.06BaTiO3 (NBT‐6BT) relaxor ferroelectrics were investigated using the Vickers indentation method and computation of crack tip opening displacement. It was found that an unpoled sample had a fracture toughness of around 1.35 MPa m1/2. In contrast, an electrically poled sample exhibited anisotropy with a lower fracture toughness perpendicular to the poling direction and a higher value in the parallel direction, as compared to the unpoled sample. Upon cyclic electrical loading (with applied electric field amplitudes between 0.73EC and 1.4EC), the indented surface crack was found to propagate. In general, the crack grew rapidly during the initial cycles followed by crack arrest, and the principal driving force for crack growth was proposed to be residual stress around the indentation, as evidenced by the limited field dependence of crack growth. There was also a contribution from the electromechanical strain, which played a role at high cycles (>100 cycles) and high fields (>1.3 EC). Evidence of a saturation threshold of crack propagation is an advantage for the electromechanical reliability of relaxor ferroelectrics in devices.

A unified J-Integral-based procedure to investigate at different loading regimes the fracture by FEM simulations and image analysis

This paper presents a robust methodology aimed at assessing the fracture toughness of materials undergoing quasi-static or dynamic loads leading to crack propagation. The proposed implementation relies on the J-Integral concept and it identically operates with data fields obtained by Finite Element modeling or fields experimentally obtained by Digital Image Correlation technique. The capability of the methodology to extract information is put into practice by simulating a FEM-based Single-Edge Notched test specimen undergoing different loading scenarios, from a simple quasi-static linear load increase up to a very aggressive loading pulse that leads to crack propagation and total failure. To do that, the simulated specimen is modeled by an isotropic damageable and pressure-dependent material model that represents a sample made of epoxy resin. The configuration parameters of the J-integral calculation algorithms and the simulation tests are subjected to an extensive parameter study to evaluate the reliability of the calculated static and dynamic energy release rate, with and without crack propagation. From this analysis, a detailed correlation between the instantaneous evolution of the fracture energy and the stress field at every material point is presented and discussed. The proposed method makes it possible to readily track the sequence of propagation and arrest of dynamic cracks.

Improved Fracture Toughness and Crack Arrest Ability of Graphene–Alumina Nanocomposite

In this work, high fracture toughness graphene–alumina composite was developed through a novel chemical method using boehmite and graphene, which is followed by extrusion and consolidation. The mixed precursors were consolidated by sintering at 1550 °C in a nitrogen atmosphere. The plate-like structures of boehmite form α-alumina; meanwhile, graphene particles at the grain boundaries hinder the growth of alumina grains. The graphene reinforcement was bonded to α-alumina matrix by van der Waals forces. The XRD pattern reveals the presence of graphene with a plane (002) along with α-alumina. Properties such as fracture toughness (5.6 ± 0.01 MPa m0.5), Vickers hardness (1872 ± 25 kgf/mm2) and true density (3.8 g/cm3) were achieved in 0.5 wt.% graphene–alumina composite when compared to α-alumina with fracture toughness (5.3 ± 0.1 MPa m0.5), Vickers hardness (1984 ± 28 kgf/mm2) and true density (3.91 g/cm3). The bridging and deviation of cracks in 0.5 wt.% graphene–alumina composite are attributed to the anchoring and dissipation of energy during crack growth, which enhances the fracture toughness, whereas α-alumina exhibits failure caused by linear crack propagation. Meanwhile, the slight decrease in Vickers hardness and true density of 0.5 wt.% graphene–alumina composite is due to the tribological and low-density properties of graphene. The obtained properties of composite could be suitable in high-temperature, wear-resistant applications such as crucibles, bearings, etc.

Autofrettage of component-like ultra high Strength Steel Specimens with intersecting Holes

This work is primarily concerned with the fatigue life of high-pressure-bearing components with intersecting holes, typically used in Diesel engine fuel injection systems. The investigation focuses on specimens with intersecting holes that have undergone the process of Autofrettage (single mechanical overload), which is typically used to extend the fatigue life of components loaded by cyclic internal pressure. The resulting residual stress distribution thus influences the fatigue failure and especially the crack propagation behaviour of the components. In previous works, results showed that besides crack initiation, crack arrest behaviour has to be taken into account when calculating fatigue lives of autofrettaged specimens as the endurance limit is otherwise underestimated. In order to achieve reliable results, material testing with samples made of the ultra high strength steel W360 was performed. The resulting test data were used to simulate the Autofrettage process with finite-element analysis. Calculated residual stress distributions were used to determine at which levels of subsequent cyclic loading crack initiation would occur. For predicted crack initiation, the simulated residual stress distribution was used to investigate the crack propagation behaviour with fracture mechanics based approaches of different complexity in order to identify possible crack arrest or crack propagation. Calculated results were compared to experimental test data from component-like specimens. The comparison showed that the fracture mechanics based approaches are capable of describing the crack arrest and propagation behaviour reliably.

Crack arrest and propagation in impact loaded shock resistant PMMA: mesh-free numerical simulation

The use of shock resistant RT-PMMA in engineering structures potentially subject to accidental overloading requires an evaluation of its crack arrest capability under impact loading. Based on experimental results obtained from a series of Kalthoff and Winkler (KW)-type impact tests, the present study aims at numerically reproducing the conditions for brittle-like crack initiation and propagation in impact-loaded RT-PMMA. For that purpose, three-dimensional SPH numerical simulations were conducted and the performance of various failure criteria was evaluated. The numerical model together with a combination of stress- and strain- dependent failure criteria were shown to fairly reproduce the experimental results in terms of finite crack advance and orientation.

Dynamice fracture in a semicristalline polymer: an analysis of the fracture surface

The fracture behaviour of a specific material, a semi-crystalline biobased polymer, was here studied. Dynamic fracture tests on strip band specimens were carried out. Fracture surfaces were observed at different scales by optical and electron microscopy to describe cracking scenarios. Crack initiation, propagation and arrest zones were described. Three distinct zones are highlighted in the initiation and propagation zone: a zone with conical markings, a mist zone and a hackle zone. The conical mark zone shows a variation in the size and density of the conical marks along the propagation path. This is synonymous with local speed variation. Microcracks at the origin of the conical marks in the initiation zone seem to develop from the nucleus of the spherulites. In the propagation zone with complex roughness, the direction of the microcracks and their cracking planes are highly variable. Their propagation directions are disturbed by the heterogeneities of the material. They branch or bifurcate at the level of the spherulites. In the arrest zone, the microcracks developed upstream continue to propagate in different directions. The surface created is increasingly smoother as the energy release rate decreases. It is shown that the local velocity of the crack varies in contrast to the macroscopic speed.

Enhancing low-cycle fatigue life of commercially-pure Ti by deformation at cryogenic temperature

In this study, the low-cycle fatigue behavior of a cryogenic-rolled commercially pure titanium alloy was investigated, and compared with those of undeformed and room-temperature rolled ones. The amounts of deformation twins increased to >69.8% after cryogenic temperature rolling, and to 27.5% after room temperature rolling. Strain-controlled low-cycle fatigue tests were performed at total strain amplitudes 0.4%≤Δεt2≤ 1.2%. The Coffin-Manson and hysteresis energy-based models confirmed that low-cycle fatigue resistance was remarkably improved with increasing the volume fraction of deformation twins by pre-deformation. As the proportion of deformation twins in the microstructure increased, the hysteresis loop area decreased, the striation spacing decreased, and the severity of crack-deflection behavior increased. At low Δεt2 = 0.4%, dislocation recovery was suppressed in the pre-deformed microstructure, so cyclic behavior was stable. However, at Δεt/2 ≥ 0.8%, the recovery became the predominant mechanism, so well-defined cell structures formed and cyclic softening occurred. The smaller dislocation cells formed in RTR and CTR were considered to cause more severe crack arrest than in AR.

Prediction of the Fatigue Strength Defining Mechanism of Nodular Cast Iron Based on Statistical Microstructural Features

Nowadays, casting simulations can predict local properties, such as matrix ferrite-pearlite ratio and nodule count. Thus, there is a need for a model capable of using these to predict the fatigue of nodular cast irons. In this paper, we derive the necessary methods for predicting the fatigue of nodular cast irons, where graphites act as crack initiating defects. The graphite and ferrite size distributions were derived based on elementary inputs using stochastic processes to emulate the microstructure patterns. The √area model was used to check crack arrest at the ferrite-pearlite interface of mixed grades, with respective microhardness. We address ferrite and graphite clustering by stochastic processes that have not been addressed before properly. Furthermore, we provide a solution to an important problem that has not been raised in the literature: the largest ferrite defect containing a crack initiating graphite. We propose a model to take into account changes in ferrite hardness by solid solution strengthening. The model predictions were compared to a large amount of literature data with various parameters. Finally, an in-depth analysis of the mechanisms was performed to provide a clear overview of the problem. The method was compared to some of the other methods proposed in the literature.

3D fluorescent mapping of invisible molecular damage after cavitation in hydrogen exposed elastomers.

Elastomers saturated with gas at high pressure suffer from cavity nucleation, inflation, and deflation upon rapid or explosive decompression. Although this process often results in undetectable changes in appearance, it causes internal damage, hampers functionality (e.g., permeability), and shortens lifetime. Here, we tag a model poly(ethyl acrylate) elastomer with π-extended anthracene-maleimide adducts that fluoresce upon network chain scission, and map in 3D the internal damage present after a cycle of gas saturation and rapid decompression. Interestingly, we observe that each cavity observable during decompression results in a damaged region, the shape of which reveals a fracture locus of randomly oriented penny-shape cracks (i.e., with a flower-like morphology) that contain crack arrest lines. Thus, cavity growth likely proceeds discontinuously (i.e., non-steadily) through the stable and unstable fracture of numerous 2D crack planes. This non-destructive methodology to visualize in 3D molecular damage in polymer networks is novel and serves to understand how fracture occurs under complex 3D loads, predict mechanical aging of pristine looking elastomers, and holds potential to optimize cavitation-resistance in soft materials.

SIFCON-A High Performance Concrete - Experimental Investigations

This paper addresses the development of High Performance Concrete in the past along with the experimental work on properties of Slurry Infiltrated Fibrous Concrete (SIFCON). Slurry infiltrated fibrous concrete has high strength, large ductility and good performance under seismic excitations. There is an increase in numbers of structures testified by the recent symposiums and conferences held on HPFRC. Now presently investigations are being carried out to suggest recommendations for the design of HPFRC structures. SIFCON consist of cement slurry and different types of steel fibers. It is observed that with the increase in percentage of steel fibers there is an improved static, dynamic and mechanical properties of concrete. Steel fibers also act as crack arrester and improve durability. In the present study, it is observed that at 9% fiber content the compressive strength, split tensile strength and flexural strength is optimum.

A Study of Brittle Crack Arrestability Test Method

This study evaluated the brittle crack arrest temperature (CAT) through an isothermal crack arrest test. The concept of evaluating CAT by an isothermal crack arrest test was proposed by C. Wiesner for application to low temperature steels for LPG tanks. The proposed test method consists of arresting a double-tension-type brittle crack with EB melt-run, which acts as a crack initiator and crack runway. There are some disadvantages when evaluating CAT with electron beam welding (EBW), which requires a local brittle zone to initiate a brittle crack. A local temperature gradient (LTG) system is developed to overcome the limitations of the EBW method. LTG systems do not require additional EBW to develop a brittle zone to initiate brittle crack. Recently, brittle crack arrest (BCA) steels are being used in large container ship such as in the upper deck or hatch side coaming of 22,000 twenty-foot equivalent units to prevent fatal brittle fracture accident of those ships. The International Association of Classification Societies (IACS) defined BCA steels using Kca requirement or CAT requirement in their unified rule (UR) S33. Therefore, when a shipyard uses BCA steel in the construction of large container ships, CAT or Kca should be evaluated. In this study, a new CAT test method is introduced to evaluate brittle crack arrest temperature more easily. Furthermore, both test methods were compared for the same specimens to verify the validity of the test method. Key words: Brittle fracture, Crack Arrest Temperature (CAT), Electron Beam Welding (EBW), Embrittled zone, Local Temperature Gradient (LTG)

Behaviour of self compacting concrete with hybrid fibers in hollow beams

Self-Compacting-Concrete (SCC) is easily flowable, steady and segregation free without any bleeding. The concrete flows freely due its self weight and fills the formwork in congested reinforcement, without any mechanical consolidation. The fibers are added in SCC which acts as crack arresters and enhances the tensile strength and toughness of concrete by reducing crack propagation. In this paper micro steel fibers(S) and nylon fibers (N) are used in different combinations. Micro steel fibers are used in the different percentages of 0, 0.1, 0.2, 0.3, 0.4 by volume of concrete and Nylon fibers are kept constant as 0.1% by volume of concrete. The mix proportions are S0/N0.S 0.1/N0.1,S0.2/N0.1,S0.3/N0.1 and S0.4/N0.1.To assess the filling and passing ability, slump flow and J ring tests were conducted on fresh properties of SCC mixes and the results obtained were checked against the specifications given by EFNARC guidelines. The compressive strength of SCC is obtained for the fiber combinations stated above and the maximum strength is attained for S0.3/N 0.1 fiber combination. Hollow beams with circular openings of 50 mm diameter along neutral axis are casted with SCC containing different combinations of fibers as mentioned above and the beams are tested under flexure to ascertain the bending or flexural strength. It is found that the beams with 0.3% of micro steel fiber content along with 0.1% nylon fiber content (S0.3/N0.1) shown the maximum strength both in compression and flexure when compared to solid beam (without fibers).

Microstructural evolution and mechanical properties of as-cast and annealed Nb–Si–Mo based hypoeutectic alloys with quaternary additions of Ti or Fe

The effect of quaternary addition of 20 at% Ti and 4 at% Fe on microstructural evolution, dynamically measured Young's modulus and Poisson's ratio, hardness, indentation fracture toughness (IFT), and compressive strengths of arc-melted hypoeutectic Nb–12Si–5Mo in as-cast or annealed (1500 °C for 100 h) condition have been investigated. Microstructural studies have shown the presence of primary niobium-rich solid solution (Nbss), and eutectic comprising Nbss+(β or α) (Nb, × )5Si3, where × = Mo, Ti, or Fe. Additionally, β-Tiss and Nb4FeSi have formed in Ti and Fe containing alloys, respectively. Annealing has led to conversion of partially lamellar to non-lamellar eutectic morphology, besides compositional alterations in phases. The dynamic Young's modulus being influenced by Mo-content of Nbss, and phase volume fractions, is found to peak in Ti-containg alloy (~150 GPa), with further increment on annealing (~162 GPa) due to β → α (Nb, × )5Si3 transformation. Whereas microhardness of Nbss (6–7 Hv) is influenced by solute content, that of eutectic (16–17 GPa) is also affected by (Nb, × )5Si3 content. Further, both hardness and compressive strength being influenced by phase volume fraction, and eutectic morphology, have decreased in Nb–12Si–5Mo and Nb–12Si–5Mo–20Ti alloys, but increased marginally in Nb–12Si–5Mo–4Fe alloy on annealing. The IFT appears to be the highest (~22 MPa√m) in the Ti-containing alloy, with toughening contributions from bridging and arrest of cracks by the ductile Nbss phase.

Evolution of stress fields during crack growth and arrest in a brittle-ductile CrN-Cr clamped-cantilever analysed by X-ray nanodiffraction and modelling

In order to understand the fracture resistance of nanocrystalline thin films, it is necessary to assess nanoscopic multiaxial stress fields accompanying crack growth during irreversible deformation. Here, a clamped cantilever with dimensions of 200 × 23.7 × 40 μm3 was machined by focused ion beam milling from a thin film composed of four alternating CrN and Cr layers. The cantilever was loaded to 460 mN in two steps and multiaxial strain distributions were determined by in situ cross-sectional X-ray nanodiffraction. Characterization in as-deposited state revealed the depth variation of fibre texture and residual stress across the layers. The in situ experiment indicated a strong influence of the residual stresses on the cross-sectional stress fields evolution and crack arrest capability at the CrN-Cr interface. In detail, an effective negative stress intensity of −5.9 ± 0.4 MPa m½ arose as a consequence of the residual stress state. Crack growth in the notched Cr layer occurred at a critical stress intensity of 2.8 ± 0.5 MPa m½. The results were complemented by two-dimensional numerical simulation to gain further insight into the elastic-plastic deformation evolution. The quantitative experimental and modelling results elucidate the stepwise nature of fracture advancement across the alternating brittle and ductile layers and their interfaces.

Controllable fabrication and mechanical properties of C/C–SiC composites based on an electromagnetic induction heating reactive melt infiltration

To regulate the microstructures of carbide ceramic-doped C/C (C/C-ceramic) composites using reactive melting infiltration (RMI) in a controlled manner, an electromagnetic induction heating RMI (ERMI) was proposed and used to fabricate typical C/C–SiC composites herein. Because the tedious heating and cooling regions could be bypassed using ERMI, excessive graphitization and ceramic overreactions of the ERMI-C/C-SiC composites were effectively avoided, which made the interfacial bonding strength (τ) of the ERMI-C/C–SiC composites (∼25.7 MPa) much lower than that of the CRMI-C/C-SiC composites (∼36.1 MPa) (fabricated using conventional RMI (CRMI)). A weaker τ value triggered strengthening/toughening mechanisms such as crack deflection, and crack arrest, which ultimately led to higher flexural strength and displacement of the ERMI-C/C–SiC composites than the CRMI-C/C–SiC composites. The proposed ERMI exhibited relatively good controlling capability to regulate the microstructures of C/C-ceramic composites.

Development of Crack Arrest Temperature Test Method by Using a Local Temperature Gradient

Abstract This study evaluated the brittle Crack Arrest Temperature (CAT) through an isothermal crack arrest test. Generally, the embrittled zone is required in a CAT test. According to conventional studies, the test method consists of arresting a double tension–type brittle crack with an Electron Beam melt-run, which acts as a crack initiator and crack runway. However, conventional CAT test methods have disadvantages in that they need additional processes to create an embrittled zone with Electron Beam Welding (EBW). A Local Temperature Gradient (LTG) system is developed to overcome the limitations of the conventional EBW embrittlement CAT test method. LTG systems do not require additional EBW to develop a brittle zone to initiate a brittle crack. In a LTG system, the LTG method is proposed instead of EBW to create the embrittled zone. The LTG system is free of flaws. In EBW, it is impossible to perform accurate CAT tests because the presence of defects affects the brittle crack arrest. In this study, a new CAT test method is introduced to evaluate brittle CATs more easily. Furthermore, both test methods were compared for the same specimens to verify the validity of the test method.

Plastic strain distribution throughout the microstructure duality during the fracture process of non-combustible Mg products fabricated by selective laser melting

In this study, several Mg-9%Al-2%Ca alloy round-bar specimens were fabricated by the selective laser melting (SLM) process. A fraction of the specimens was submitted to tensile testing. Then, the remaining fraction of specimens were drilled and subsequently submitted to further tensile testing interrupted at 90% of the ultimate tensile strength (UTS). Fracture surfaces were analyzed and an Electron Backscatter Diffraction (EBSD) analysis was performed to observe plastic strain. The results showed a dispersed plastic strain distribution that concentrated at the coarse grain microstructure, specifically at the outer regions of each melt pool formation. Microstructure coarsening is an intrinsic phenomenon of the SLM processes and induces crack arrest during the transition from a stable fracture to an unstable fracture. Accordingly, fractography results showed a large flattened area in the fracture surface, which suggests a stable and propagating plastic strain beneath the fracture surface, but no crack propagation of the corresponding length was found, despite interrupting the test at 90% of the UTS value. Based on this information, an indicator that evaluates plastic strain as continuous or discontinuous could be used in order to explain the high tensile strength and fracture behavior of SLM products in as-built conditions, which have been discussed but not verified to date.