Zigzag Crack Research Articles

A three-dimensional model of non-slipping stress corrosion cracking under low loads

Stress corrosion cracking (SCC) of 316L single-crystal austenitic stainless steel subjected to low loads (σnom = 20-40 MPa) in a 45 % boiling MgCl2 solution was studied using synchrotron X-ray computed tomography, finite element analysis and so on. Results show that there was no surface slip band around the nucleation sites and the tips of short cracks. Three-dimensional reconstruction of discontinuous zig-zag surface SCC crack indicates that the crack was continuous inside the specimen. The obtaining through two-surface trace analysis manifests, rather than {1 1 1} slip planes, the cracks extended along {1 0 0} planes with the lowest surface energy. It is considered that microcleavage and local dissolution synergistically led to the SCC advance, and microshear was also one of the primarily microscopic SCC mechanisms under the high load. The three-dimensional SCC model was created at the low stress level, where a main SCC crack grew along the MPD due to anodic dissolution, and a microdefect was formed on the crack front. When the microdefect enlarged to a critical size, secondary microcracks nucleated at the stress-concentrated microdefect shoulders. Then, the microcracks propagated to two sides of the MPD through anodic dissolution, microcleavage or mciroshear, resulting in the formation of the river-like fractography and the discontinuous surface SCC cracks with or without surface slipping.

Tactile Sensing System Based on Arrays of Graphene Woven Microfabrics: Electromechanical Behavior and Electronic Skin Application.

Nanomaterials serve as promising candidates for strain sensing due to unique electromechanical properties by appropriately assembling and tailoring their configurations. Through the crisscross interlacing of graphene microribbons in an over-and-under fashion, the obtained graphene woven fabric (GWF) indicates a good trade-off between sensitivity and stretchability compared with those in previous studies. In this work, the function of woven fabrics for highly sensitive strain sensing is investigated, although network configuration is always a strategy to retain resistance stability. The experimental and simulation results indicate that the ultrahigh mechanosensitivity with gauge factors of 500 under 2% strain is attributed to the macro-woven-fabric geometrical conformation of graphene, which induces a large interfacial resistance between the interlaced ribbons and the formation of microscale-controllable, locally oriented zigzag cracks near the crossover location, both of which have a synergistic effect on improving sensitivity. Meanwhile, the stretchability of the GWF could be tailored to as high as over 40% strain by adjusting graphene growth parameters and adopting oblique angle direction stretching simultaneously. We also demonstrate that sensors based on GWFs are applicable to human motion detection, sound signal acquisition, and spatially resolved monitoring of external stress distribution.

Synergy of multi-scale toughening and protective mechanisms at hierarchical branch-stem interfaces.

Biological materials possess a variety of artful interfaces whose size and properties are adapted to their hierarchical levels and functional requirements. Bone, nacre, and wood exhibit an impressive fracture resistance based mainly on small crystallite size, interface organic adhesives and hierarchical microstructure. Currently, little is known about mechanical concepts in macroscopic biological interfaces like the branch-stem junction with estimated 1014 instances on earth and sizes up to few meters. Here we demonstrate that the crack growth in the upper region of the branch-stem interface of conifer trees proceeds along a narrow predefined region of transversally loaded tracheids, denoted as sacrificial tissue, which fail upon critical bending moments on the branch. The specific arrangement of the tracheids allows disconnecting the overloaded branch from the stem in a controlled way by maintaining the stem integrity. The interface microstructure based on the sharply adjusted cell orientation and cell helical angle secures a zig-zag crack propagation path, mechanical interlock closing after the bending moment is removed, crack gap bridging and self-repairing by resin deposition. The multi-scale synergetic concepts allows for a controllable crack growth between stiff stem and flexible branch, as well as mechanical tree integrity, intact physiological functions and recovery after the cracking.

Length-scale-dependent cracking and buckling behaviors of nanostructured Cu/Cr multilayer films on compliant substrates

Abstract Cu/Cr nanostructured multilayer films (NMFs) with modulation periods ( λ ) ranging from 250 nm to 10 nm and modulation ratios ( η ) ranging from 0.1 to 2.0 were prepared on flexible polyimide substrates by using magnetron sputtering. Upon uniaxial tensile testing, the critical cracking strain ( e c ), critical buckling strain ( e b ), and fracture toughness ( K IC ) of the NMFs were experimentally measured and all of the mechanical properties showed remarkable λ - and η -dependences. The cracking and buckling behaviors of the Cu/Cr NMFs were systematically investigated and both were found to depend strongly on the length scale. Based on an energy balance model, the interfacial adhesion energies ( Γ ) were determined using the measured buckle dimensions. Cracking maps and buckling maps were constructed from the experimental data to summarize the effects of λ and η on the cracking and buckling modes, respectively. In the cracking map, two regimes can be identified: one is brittle fracture with straight cracks and the other is ductile fracture with zigzag cracks. The ductile fracture regime is located in the region where λ ∼ 40 ± 20 nm and simultaneously η K IC ∼ 12.5 MPa m1/2. In the buckling map, four regimes are distinguished: cracked rectangular buckles, uncracked rectangular buckles, cracked triangular buckles, and uncracked triangular buckles. The effects of the length-scale-dependent deformation and adhesion energies on the buckling behaviors were discussed. A combined dimensionless parameter of K IC / E f Γ ( E f : elastic modulus of the NMF) was proposed to assess the buckling behaviors, and the K IC / E f Γ contours coincided well with the boundaries dividing the four buckling regimes.

Fracture in a thin film of nanotwinned copper

Nanotwinned Cu exhibits an unusual combination of ultra-high strength and high tensile ductility. However, its fracture behavior and associated microscopic mechanisms remain largely unexplored. Here we study the fracture in a free-standing thin film of nanotwinned Cu using molecular dynamics (MD) simulations. For a pre-crack inclined to the twin boundary, MD simulations show a characteristic fracture mode of zigzag cracking, which arises due to periodic deflections of the crack path by twin boundaries. The mechanism of fracture involves the screw dislocation-mediated local thinning ahead of the crack, instead of cleavage fracture. Importantly, MD simulations show a unique fracture footprint of 〈110〉-oriented crack edges, consistent with the previous experimental observation from in situ transmission electron microscopy. Our results reveal the toughening mechanisms by nanotwins and also have broader implications for understanding the mechanical failure of metallic thin films.

A molecular dynamics study of nanofracture in monolayer boron nitride

In this paper, we use molecular dynamics (MD) modeling to study the fracture properties of monolayer hexagonal boron nitride (h-BN) under mixed mode I and II loading. We investigate the impact of crack edge chirality, crack tip configuration and loading phase angle on the crack propagation path and critical stress intensity factors. The MD results predict that under all the loading phase angles cracks prefer to propagate along a zigzag direction and the critical stress intensity factors of zigzag cracks are higher than those of armchair cracks. Under mixed mode loading, the h-BN sheets can undergo out-of-plane deformations due to the buckling induced by compressive stresses. The out-of-plane deformations can be significant when mode II loading is dominant. An excessive amount of out-of-plane deformation can induce buckling cracks. Depending on the loading phase angle and crack configurations, buckling cracks can nucleate before or after the propagation of the original cracks.

3D simulation of short fatigue crack propagation by finite element crystal plasticity and remeshing

An iterative method to simulate 3D fatigue crack propagation in crystalline materials is proposed and demonstrated on simple test cases. The method relies on the computation of a damage indicator based on plastic activity around the crack tip. By post-processing this quantity after a given loading sequence, local crack direction and growth rate are estimated along the crack path. Dedicated remeshing routines allow the crack to propagate in 3D and the iterative process can continue with the updated crack geometry. Two examples in a BCC single crystal featuring different slip systems demonstrate the crack propagation simulations under cyclic loading over distances comparable to the crystal size and with non-regular crack shapes such as zig-zag crack paths. The effect of transferring plastic fields between propagation steps is also analysed.

Fracture mechanics of monolayer molybdenum disulfide

Molecular dynamics (MD) modeling is used to study the fracture toughness and crack propagation path of monolayer molybdenum disulfide (MoS2) sheets under mixed modes I and II loading. Sheets with both initial armchair and zigzag cracks are studied. The MD simulations predict that crack edge chirality, tip configuration and the loading phase angle influence the fracture toughness and crack propagation path of monolayer MoS2 sheets. Furthermore, under all loading conditions, both armchair and zigzag cracks prefer to extend along a zigzag path, which is in agreement with the crack propagation path in graphene. A remarkable out-of-plane buckling can occur during mixed mode loading which can lead to the development of buckling cracks in addition to the propagation of the initial cracks.

超微細粒組織を活用した1800MPa級超高強度鋼のき裂伝播挙動（微視組織制御からの破壊制御の構築）

A 1800 MPa class steel bar with an ultrafine elongated grain (UFEG) structure was fabricated by multipass caliber rolling at 500°C. The static three-point bending test was conducted in a temperature range from 100°C to -196°C. The behaviors of crack propagation on the developed steel were studied on the basis of the microstructural features. The conventionally quenched and tempered steel with a martensitic structure showed a catastrophic fracture behavior which fractured with a peak bending load in a temperature range from 23°C to -196°C. The crack propagated directly across the center portion of the test bar. On the other hand, the developed steel exhibited a noncatastrophic fracture behavior with evidence of stepwise load increases beyond the first load drop. The microcrack occurred, from near the notch root, with normal to the loading direction (LD) or with an angle of 45° to the LD, and the crack propagated with many zigzag cracks branching from the zigzag crack along the longitudinal direction of the test bar. The occurrence of such brunching cracks corresponded to the spatial distribution of {100} cleavage planes and boundaries of the elongated grains. Namely, the microstructural damage is not localized but rather is widely distributed over very large dimensions. In the bending load - displacement curve, many load drops appeared, and the bending load did not decrease with an increase in displacement due to two effects: the stress shielding effect associated with the interference of multiple cracks and the effect of the improved plastic deformation associated with grain refinement and texture.

The Heat Treatment Effect on Microstructure for Arc Welding of Dissimilar Steel to copper

The purpose of this paper is to be familiarize with the microscopic structure of the welding specimen, before and after heat treatment in which the pure copper ( OFHC ) has been welded to the low carbon steel ( ASTMA36 ) by the method of fusion welding ( Arc Welding ) using covered welding electrode (SMAW) .A 12mm thick plates of the two metals were used.The plate have been prepared (Single Butt Joint ) where the dissimilar plates copper to steel were welded using ( Ecusu8 ) electrodes . The heat treatment has been carried out to the specimen at 600Co,700Co then cooled down in the oven . The microscopic test for the welded specimen has been carried out before and after the heat treatment .Before heat treatment It was clear that there exist a separating limit from the steel side with zigzag indicating that interaction was happened giving a ( Non-epitaxial ) structure with opening windows accompanied by diffusion of masses from the base metal inside the added metal ,while after heat treatment, it is found that more zigzag and opening cracks in the steel side filled with the added metal by the process of ( Brazing ) and excess clearness of diffusion process with diffused metal taking a tree shapes , While from the copper side, it showed ( Epitaxial Growth ) No existence to any cracks was noticed , but there was a granulated growth especially when heated up to 700Co .

Mode I fracture toughness analysis of a single-layer grapheme sheet

To predict the fracture toughness of a single-layer graphene sheet (SLGS), analytical formulations were devised for the hexagonal honeycomb lattice using a linkage equivalent discrete frame structure. Broken bonds were identified by a sharp increase in the position of the atoms. As crack propagation progressed, the crack tip position and crack path were updated from broken bonds in the molecular dynamics (MD) model. At each step in the simulation, the atomic model was centered on the crack tip to adaptively follow its path. A new formula was derived analytically from the deformation and bending mechanism of solid-state carbon-carbon bonds so as to describe the mode I fracture of SLGS. The fracture toughness of single-layer graphene is governed by a competition between bond breaking and bond rotation at a crack tip. K-field based displacements were applied on the boundary of the micromechanical model, and FEM results were obtained and compared with theoretical findings. The critical stress intensity factor for a graphene sheet was found to be KIC = 2.63 ~ 3.2MPa\(\sqrt m \) for the case of a zigzag crack.

Effects of Pretreatment on Properties of Gelatin from Perch (Lates Niloticus) Skin

Fish gelatins obtained from perch fish skin pretreated with various solutions containing acetic acid, sodium hydroxide (NaOH) and sodium chloride (NaCl) were successfully characterized for their nanostructure pattern using field emission scanning electron microscopy. Each pretreatment transformed collagen to gelatin with fibril, zigzag cracks, straight rods, and cross-linked rods nanostructure patterns. Pretreatment solutions also affect the gel yield, gel strength, amino acid profile, and functional groups in perch gelatin as analyzed by Fourier transform infrared spectroscopy. Samples pretreated with NaCl, NaOH, and acetic acid solution showed the highest gel yield (22.84%) and gel strength (179.84 g). Fourier transform infrared spectra for perch gelatins also revealed weak C–N amide II and III bond stretches as well as weak C=O bond stretch.

Crack growth simulation for arbitrarily shaped cracks based on the virtual crack closure technique

In this paper, crack growth simulation for arbitrarily shaped cracks was investigated based on the virtual crack closure technique. During simulations, the crack front was represented by an approximated zigzag line which had the same general shape as the given crack. For this approximated zigzag crack front, a modified approach was developed to determine the required nodal forces, virtually closed area and displacement opening. After the strain energy release rate G was calculated, crack growth was governed by the fracture criterion G/GC = 1 at all the crack tip nodes. The important features of the proposed approach are that (i) a simple stationary finite element mesh can be used for arbitrarily shaped cracks and (ii) adaptive re-meshing technique is avoided in studying crack growth. Three cases having different initial crack shapes are presented to assess the validity of this approach and to evaluate the ease of use in tracking crack growth. Reasonable agreement between the present study and other approaches are obtained. The shape changes during crack propagation can also be tracked with ease.

Origin of flaw-tolerance in nacre.

Over the past decades, our understanding of nacre's toughening origin has long stayed at the level of crack deflection along the biopolymer interface between aragonite platelets. It has been widely thought that the ceramic aragonite platelets in nacre invariably remain shielded from the propagating crack. Here we report an unexpected experimental observation that the propagating crack, surprisingly, invades the aragonite platelet following a zigzag crack propagation trajectory. The toughening origin of previously-thought brittle aragonite platelet is ascribed to its unique nanoparticle-architecture, which tunes crack propagation inside the aragonite platelet in an intergranular manner. For comparison, we also investigated the crack behavior in geologic aragonite mineral (pure monocrystal) and found that the crack propagates in a cleavage fashion, in sharp contrast with the intergranular cracking in the aragonite platelet of nacre. These two fundamentally different cracking mechanisms uncover a new toughening strategy in nacre's hierarchical flaw-tolerance design.

Fatigue properties (S–N and da/dN–ΔK) of cast aluminium alloys

In this paper, the fatigue strengths of cast aluminium alloys have been examined using the S–N and da/dN–ΔK approaches on various cast samples having different microstructural characteristics. These were made by several different casting techniques, namely gravity casting, die casting and twin rolled continuous casting. Owing to the different microstructures, the fatigue properties varied. The S–N relations, e.g. the endurance limit, of the twin-rolled casting were apparently high compared to the other cast samples. The high fatigue strength was related to the high tensile strength and high ductility caused by the tiny spherical α-Al phase and fine eutectic structure. On the other hand, the low fatigue strength for the gravity and die cast samples was caused by the high stress concentration caused by the α-Al phase and needle-like eutectic silicon. Unlike the fatigue strength in the S–N approach, the da/dN–ΔK relationships for the twin rolled casting sample, i.e. the resistance to fatigue crack propagation, were low compared to the gravity casting and the die cast samples. Such a difference in the crack growth resistance was influenced by the crack growth characteristics. For example, for the gravity cast and the die cast samples, the crack growth occurs in the grain boundaries of the coarse α-Al phase and along the long eutectic silicon grains (zigzag crack path), which promotes a strong crack closure tendency, leading to high crack growth resistance. The difference in the fatigue strength determined from the S–N relation and da/dN–ΔK relation was interpreted by experimental and numerical examination of the stress–strain distribution.

TRIBOLOGY OF BIO-INSPIRED NANOWRINKLED FILMS ON ULTRASOFT SUBSTRATES

Biomimetic design of new materials uses nature as antetype, learning from billions of years of evolution. This work emphasizes the mechanical and tribological properties of skin, combining both hardness and wear resistance of its surface (the stratum corneum) with high elasticity of the bulk (epidermis, dermis, hypodermis). The key for combination of such opposite properties is wrinkling, being consequence of intrinsic stresses in the bulk (soft tissue): Tribological contact to counterparts below the stress threshold for tissue trauma occurs on the thick hard stratum corneum layer pads, while tensile loads smooth out wrinkles in between these pads. Similar mechanism offers high tribological resistance to hard films on soft, flexible polymers, which is shown for diamond-like carbon (DLC) and titanium nitride thin films on ultrasoft polyurethane and harder polycarbonate substrates. The choice of these two compared substrate materials will show that ultra-soft substrate materials are decisive for the distinct tribological material. Hierarchical wrinkled structures of films on these substrates are due to high intrinsic compressive stress, which evolves during high energetic film growth. Incremental relaxation of these stresses occurs by compound deformation of film and elastic substrate surface, appearing in hierarchical nano-wrinkles. Nano-wrinkled topographies enable high elastic deformability of thin hard films, while overstressing results in zigzag film fracture along larger hierarchical wrinkle structures. Tribologically, these fracture mechanisms are highly important for ploughing and sliding of sharp and flat counterparts on hard-coated ultra-soft substrates like polyurethane. Concentration of polyurethane deformation under the applied normal loads occurs below these zigzag cracks. Unloading closes these cracks again. Even cyclic testing do not lead to film delamination and retain low friction behavior, if the adhesion to the substrate is high and the initial friction coefficient of the film against the sliding counterpart low, e.g. found for DLC.

Transition of crack propagation from a transgranular to an intergranular path in an overaged Al-Zn-Mg-Cu alloy during cyclic loading

The fatigue crack propagation behavior in the overaged Al-Zn-Mg-Cu alloy was characterized by optical microscopy, scanning electron microscopy, transmission electron microscopy and electron backscatter diffraction. The results revealed that a fatigue crack tended to transgranularly propagate in the near-threshold regime, whereas intergranular crack propagation was dominant at the high ΔK regime. The transition of crack propagation from a transgranular to an intergranular path that occurred in the Paris regime was strongly influenced by the misorientation of adjacent grains and precipitate free zones. In addition, a crystallographic model of crack propagation was proposed to interpret the transition. The fatigue short crack propagation on a single slip plane was responsible for the formation of a transgranular propagation path in the near-threshold regime. The fatigue long crack propagation, which was conducted by a duplex slip mechanism in the Paris regime, led to the formation of fatigue striations. The formation of a zigzag crack in the near-threshold regime was ascribed to the high misorientation of adjacent grains.

Influence of the microstructure on the tensile and impact properties of a 14Cr ODS steel bar

A Fe–14Cr–1W–0.4Ti–0.3Y203 ferritic steel bar was characterised by different microstructural techniques in order to evaluate the link between its microstructure and the mechanical properties achieved. This bar was produced by mechanical alloying of a pre-alloyed gas atomised powder with yttria particles, followed by hot extrusion and subsequently annealing. The knowledge of the microstructure of this 14Cr oxide dispersion strengthened (ODS) steel allows for the explanation of the mechanical properties observed, such as the decrease of the ductility found in the transverse orientation of the bar, the existence of zig-zag crack paths in the broken specimens after the impact tests at low temperatures and the appearance of delaminations when the temperature is increased.

A coupled quantum/continuum mechanics study of graphene fracture

A new technique is presented to study fracture in nanomaterials by coupling quantum mechanics (QM) and continuum mechanics (CM). A key new feature of this method is that broken bonds are identified by a sharp decrease in electron density at the bond midpoint in the QM model. As fracture occurs, the crack tip position and crack path are updated from the broken bonds in the QM model. At each step in the simulation, the QM model is centered on the crack tip to adaptively follow the path. This adaptivity makes it possible to trace paths with complicated geometries. The method is applied to study the propagation of cracks in graphene which are initially perpendicular to zigzag and armchair edges. The simulations demonstrate that the growth of zigzag cracks is self-similar whereas armchair cracks advance in an irregular manner. The critical stress intensity factors for graphene were found to be 4.21 MPa\({\sqrt {\rm m}}\) for zigzag cracks and 3.71 MPa\({\sqrt{\rm m}}\) for armchair cracks, which is about 10% of that for steel.

Studies on Mechanism of Zigzag Crack Forming Process in Weld Metal of Narrow Gap Submerged Arc Welding of Thick Plate

For improving the toughness of 2.25Cr-1Mo-0.25V heat resisting steel welding joint,the impact test and the three point bending test are done in order to locate the weakest part of the joint,and much more micro analysis is necessary to find out the reason of that.The toughness of weld metal is lower than the other parts of the joint according to the result of the impact test,so the crack growth process in weld metal is further researched with three point bending test.A zigzag crack is found in weld metal after three point bending test.Dimple and quasi cleavage are both exist on the surface of the fracture through scanning electron microscope(SEM) technic.Zigzag crack appears in the columnar grain zone of weld bead with metallographic observation,and its direction is parallel or vertical to the direction of columnar grains.After dealing with picric acid,it appears that the parts of the zigzag crack which are parallel to the columnar grains are the original austenite grain boundaries,the result of electron backscattered diffraction(EBSD) test also proves that conclusion,and the parts which are vertical to them are layer lines.Because of that,the original austenite grain boundaries and layer lines of submerged arc welding(SAW) weld bead should be paid especially attention.The toughness of the joint can be obviously improved by adjusting the composition of welding wire or changing welding parameters,the number of layer lines is decreased due to it.