Band Crack Research Articles

Quantitative investigation on the deformation modes and cracking behavior during cyclic torsional loading of extruded pure Mg and Mg‐3Y alloy

AbstractThis study explores the effect of adding 3 wt.% Y to pure magnesium (Mg) on its mechanical behavior under cyclic torsional loadings at room temperature. The research examines deformation and cracking modes in both pure Mg and Mg‐3Y samples. Deformation modes are monitored using quasi‐in‐situ EBSD observations coupled with slip trace analysis. The findings reveal that basal slip dominates the cyclic deformation throughout the fatigue life of the pure Mg sample, while both basal and pyramidal slip dominate the cyclic deformation in the Mg‐3Y sample. Intergranular cracking is the primary cracking mode for both samples under cyclic torsional loadings. Basal and pyramidal slip persistent slip band (PSB) cracking serves as a primary transgranular cracking mode in the pure Mg and Mg‐3Y samples, respectively. The study also investigates the underlying mechanism governing the activity of various deformation modes, cracking modes, and mechanical behavior.

On the low temperature fatigue crack growth behavior of AA7075-T651 in ultra-high vacuum environments

It is well-established that the fatigue crack growth behavior of aerospace aluminum alloys in high altitude applications is sensitive to both temperature and water vapor pressure. To isolate the role of temperature, fatigue crack growth experiments were performed on AA7075-T651 under ultra-high vacuum conditions (<6x10−6 Pa) at temperatures ranging from 23 °C to −65 °C. All da/dN–ΔK relationships exhibit the following: (1) a temperature-independent Paris regime at high ΔK, (2) a transition from the Paris regime to a sharply reduced da/dN, which occurs at higher ΔK as the temperature decreases, and (3) a coalescence in da/dN for all temperatures in the threshold region. Interestingly, the point of deviation from the Paris regime is found to correlate with a transition from a flat, transgranular fracture morphology to slip band cracking (SBC) across all conditions. The likely mechanisms governing the observed behaviors are discussed, focusing on the role of slip.

Short-range ordering plays a determining role in the low-cycle fatigue life improvement of fcc metals: A conclusive study on low solid-solution hardening Ni-Cr alloys

The effect of short-range ordering (SRO) on the low-cycle fatigue (LCF) behavior of low solid-solution hardening Ni-Cr alloys with high stacking fault energies (SFEs) was systematically studied under cycling at constant total strain amplitude (Δεt/2) in the range of 0.1%–0.7%. The results show that an inducement of SRO structures can notably improve the fatigue life of the alloy regardless of Δεt/2, and several unique fatigue characteristics have been detected, including the transition of fatigue cracking mode from intergranular cracking to slip band cracking, the non-negligible evolution from non-Masing behavior in pure Ni to Masing behavior in the Ni-40Cr alloy, and the secondary cyclic hardening behavior in the Ni-10Cr and Ni-20Cr alloys. All these experimental phenomena are tightly associated with the transformation in cyclic deformation mechanisms that is induced by SRO based on the “glide plane softening” effect. Furthermore, a comprehensive fatigue life prediction model based on total hysteresis energy has been reasonably proposed, focusing on the analyses of the macroscopic model parameters (namely the fatigue cracking resistance exponent β and the crack propagation resistance parameter W0) and microscopic damage mechanisms. In brief, on the premise that the effects of SFE and friction stress can be nearly ignored, as in the case of the present low solid-solution hardening Ni-Cr alloys with high SFEs, an enhancement of SRO in face-centered cubic metals has been convincingly confirmed to be an effective strategy to improve their LCF performance.

Evolution of cracks in the shear bands of granite residual soil

The evolution of shear bands and cracks plays an important role in landslides. However, there is no systematic method for classification of the cracks, which can be used to analyze the evolution of cracks in shear bands. In this study, X-ray computed tomography (CT) is used to observe the behavior of granite residual soil during a triaxial shear process. Based on the digital volume correlation (DVC) method, a crack classification method is established according to the connectivity characteristics of cracks before and after loading. Cracks are then divided into six classes: obsolete, brand-new, isolated, split, combined, and compound. With evolution of the shear bands, a large number of brand-new cracks accelerate the damages of materials at the mesoscale, resulting in a sharp decrease in strength. The volume of brand-new cracks increases rapidly with increasing axial strain, and their volume is greater than 50% when the strain reaches 12%, while the volume of compound cracks decreases from 54% to 21%. As cracks are the weakest areas in a material, brand-new cracks accelerate the development of shear bands. Finally, the coupling effect of shear bands and cracks destroys the soil strength.

Slip transfer accelerates slip band cracking of high cycle fatigue in high-Mn austenitic steel at 77 K

Slip transfer accelerates slip band cracking of high cycle fatigue in high-Mn austenitic steel at 77 K

Microstructure, Fatigue Properties and Stress Concentration Analysis of 6005 Aluminum Alloy MIG Welded Lap Joint.

This paper studies the microstructure and mechanical properties of MIG (Melt Inert Gas) lap welded 6005 aluminum alloy plates. Microstructure analysis (OM) of the joint showed that 15~30 μm small grains were observed at the fusion line. Mechanical analysis shows that the small grains are broken by shielding gas and molten pool flow force. Hardness test shows that there is a softening zone (41~43 HV) in HAZ much lower than BM and WZ. The low cycle fatigue test showed that the performance of lap joint decreased sharply, and the fatigue strength of weld decreased significantly, which was only 27.34% of the base metal. The fatigue fracture (SEM) of the weld observed slip band cracking and a large number of brittle fracture characteristics. Using the stress concentration factor Kt for analysis, it was found that the cause of brittle fracture was mostly stress concentration. Lap joint stress concentration model appears in two ways: firstly, at the weld toe, the weld is subjected to eccentric force, secondly, there is a small gap between the two plates at the weld root, which cracks along the direction of 45° of the maximum shear stress.

Crack growth behavior of 725 in seawater under cathodic polarization

Hydrogen embrittlement behavior of four heats of alloy 725 (H1, H2, H3, and H4) were characterized by measuring the crack growth rate response over a range of cathodic potentials. Significant grain boundary precipitation in two heats (H1 and H3) may have contributed to higher susceptibility, as evidenced by higher values of crack growth rate. The crack growth rate increased with increasing twin fraction. The crack morphology exhibited evidence of intergranular as well as slip band cracking. The crack growth rate response of the two heats that were less susceptible (H2 and H4) were evaluated in more detail over a range of applied potentials and stress intensity factors. Evidence of nano void formation at the intersection of dislocation slip bands, and grain boundaries suggests that hydrogen stabilized vacancies may play a role in the crack growth mechanism. Crack growth rate exhibited a strong response with applied potential varying by as much as 100 times in the potential range of −1050mV SCE to −850mV SCE. Over the range of K values evaluated, the crack growth rate increased by a factor of 3to 30 with applied K depending on the applied potential. The crack growth rate was related to the water adsorption on fresh metal surfaces of alloy 725 in the anticipated crack tip conditions. A crack tip strain rate based approach was applied to model the measured crack growth response.

Improving the stress-controlled fatigue life of low solid-solution hardening Ni-Cr alloys by enhancing short range ordering degree

The tension–tension fatigue behavior of low solid-solution hardening Ni-Cr alloys with an approximately constant stacking fault energy was systematically investigated under different stress amplitudes. With an enhancement of short range ordering (SRO) degree, the fatigue crack initiation mode is evolved from intergranular cracking to slip band cracking, and the cyclic deformation mechanism changes from wavy to planar dislocation slip. Both the fatigue strength coefficient (σ′f) and exponent (b) can be simultaneously increased by inducing SRO structure that greatly enhances the cyclic strain hardening capacity, deformation homogeneity and slip reversibility, and the fatigue life is thus significantly improved.

Influence of microstructure on fatigue crack growth behavior of Ti–6Al–3Nb–2Zr–1Mo alloy: Bimodal vs. lamellar structures

The influence of microstructure (bimodal and lamellar structure) on the fatigue crack growth (FCG) rate of Ti–6Al–3Nb–2Zr–1Mo alloy was investigated using compact-tension (CT) specimens. The da/dN curves showed that the FCG rate of lamellar structure was slower than that of bimodal structure, especially in the early stage of crack growth. The FCG paths in the early stage were observed by optical microscopy and scan electric microscopy. The results revealed that the FCG paths of lamellar structure were more tortuous than that of bimodal structure. The frequency of crack deflection and bifurcation of the lamellar structure is significantly higher than that of bimodal structure. The dominant mode of crack propagation for bimodal structure is crossing βtrans and αp, and for lamellar structure is crossing α colonies. Fatigue crack growth mechanisms at the early stage have been studied by observation of crystalline orientation near the crack propagation path at the crack tip, in combination with the slip trace analyzing. The results show that the fatigue crack growth directions in αp or βtrans for bimodal structure and α colony for lamellar structure are mostly consistent with that of prismatic or basal plane trace. This indicated that the cracks propagation modes of crossing αp or βtrans in bimodal structure and crossing α colonies in lamellar structure are resulting from the slip band cracking of the microstructure at crack tip, as the evidence of {101‾0}112‾0 prismatic dislocation lines were observed in αp or βtrans for bimodal structure and α colony for lamellar structure by transmission electron microscope (TEM) near the fracture surface.

Data rich imaging approaches assessing fatigue crack initiation and early propagation in a DS superalloy at room temperature

Crack initiation and early propagation behavior of the directionally solidified (DS) superalloy CM247LC has been assessed by data rich imaging approaches. These include conventional characterization methods such as replica record analysis, 3D optical surface imaging, optical and scanning electron microscopy (SEM) as well as more recent techniques like digital image correlation (DIC) and synchrotron radiation computed tomography (SRCT). Three modes of secondary crack behaviors were found during evaluation of the fatigue process. The early stages of fatigue damage were controlled by microstructure-induced cracking, mainly consisting of carbide cracking. Fatigue damage was then promoted via slip band cracking and opening mode controlled carbide-cracking. The mechanisms of these different cracking behaviors are associated with the plastic zone of the main crack tip. Even though the early localized strain levels were of the same intensity within slip bands and at the intersection sites with carbides, carbide-induced cracking occurred prior to slip band cracking, characterized by SEM-DIC. This indicated that carbide-induced cracking was more likely to occur in the early stages of the fatigue process. Early crack growth behaviors were further investigated in situ at the microstructural scale via SRCT. The effect of carbides on crack initiation and propagation processes were evaluated in 3D. This revealed the phenomenon around pores, that cracks simultaneously grew on different slip planes in 3D, contrary to previous theories that such cracks tend to grow on a single favourable slip plane (in polycrystalline alloys). The SRCT result demonstrates the importance and necessity of 3D characterization of the crack propagation behavior at sub-surface, which is not fully analyzed by 2D characterization.

Investigation of energy release rate under mode II loading in a rectangular orthotropic plate with a band crack using 3D finite element analysis

A rectangular plate containing a band crack and made of orthotropic elastic material is considered and the energy release rate (ERR) component in mode II at this crack front is investigated. It is assumed that the crack’s edge planes are parallel to the upper and lower face planes of the plate and the edge planes of the crack are loaded with uniformly distributed sliding forces. The corresponding boundary value problem is formulated within the scope of the three-dimensional linear theory of elasticity and the solution to this problem is found by employing 3D finite element method (3D FEM) modeling. Numerical results on the ERR for mode II and on the influence of the mechanical-orthotropic properties of the plate material are presented and discussed. In particular, it is established that the influence of the shear modulus in a plane which is simultaneously perpendicular to the crack’s front and crack’s plane on the ERR is more significant than that caused by the change of another modulus of the orthotropic material.

Calculation and 3D analyses of ERR in the band crack front contained in a rectangular plate made of multilayered material

Abstract An investigation into the values of the Energy Release Rate (ERR) at the band crack’s front in the rectangular plate made of multilayered composite material is carried out for the opening mode. The corresponding boundary-value problem is modelled by using threedimensional linear theory and solved numerically by using 3D FEM (Three Dimensional Finite Element Method). The main purpose of the current investigation is to study the influence of mechanical and geometrical parameters on the Energy Release Rate (ERR) at this crack front. The numerical results related to the ERR, and the effect of the mechanical and other problem parameters on the ERR are presented and discussed.

Buckling delamination of a PZT/Metal/PZT sandwich rectangular thick plate containing interface inner band cracks

This work studies the 3D buckling delamination problem around the interface band cracks which are in the PZT/Metal/PZT sandwich rectangular thick plate within the scope of the piecewise homogeneous body model by utilizing the so-called 3D linearized theory of stability loss for piezoelectric materials. It is assumed that the plate is simply supported with respect to mechanical quantities and short-circuit conditions with respect to the electric potential along all its lateral edge-surfaces. On the face planes of the PZT layers, the open-circuit condition with respect to the electrical displacement is satisfied. It is also assumed that on the opposite two ends of the plate which are parallel to the band cracks’ front, uniformly distributed normal compressive mechanical forces act. The critical values of these forces under are determined by employing an initial imperfection criterion. For solution of the corresponding boundary value problems the 3D FEM is utilized and numerical results on the critical forces and on the influence of the problem parameters on these forces are presented and discussed. In particular, it is established that the piezoelectricity of the face layers and the three dimensionality of the problem cause to increase the values of the critical forces.

Fatigue crack propagation of aerospace aluminum alloy 7075-T651 in high altitude environments

Fracture mechanics fatigue testing of 7075-T651 in environmental conditions pertinent to high altitude airframe operation established the fatigue crack growth behavior over a wide span of stress intensity range (ΔK). Two distinct methods were used to control the environmental severity parameter of water vapor pressure over frequency (PH2O/f): (1) at 23°C the PH2O value was controlled by an ultra-high vacuum system into which purified water was leaked, and (2) the testing temperature was decreased to temperatures as low as −65°C allowing an equilibrium PH2O over ice at a given temperature to be maintained. Decreasing PH2O at 23°C (at a constant f) results in a drastic reduction in the fatigue behavior, strongly suggesting that the reduction in moisture is a primary mechanism controlling the retarded fatigue crack growth behavior observed at low termperatures. Additionally, studies conducted at a corresponding PH2O/f with various testing temperatures down to −15°C show that fatigue crack growth rate (FGCR) behavior is consistent with data generated at 23°C. However, at temperatures below −30°C, low T tests exhibit crack growth rates below the 23°C results despite having the same PH2O/f value. This suggests that the strong role of environmental moisture reduction is augmented by a separate temperature dependent effect on either the inherent dislocation behavior and/or a separate aspect of the Hydrogen Environment Embrittlement (HEE) process. Mechanisms based purely on temperature dependent dislocation behavior fail to fully describe the experimental observations. Temperature dependent impacts on the HEE process (apart from the reduction of the PH2O) via influencing the surface reaction process to generate atomic H at the crack tip or the diffusion of the H within the crack tip process zone do not adequately describe the observed experimental data. Furthermore, a dip in the crack growth rate at −30°C suggests that molecular flow of H2O from the bulk to the crack tip is in part responsible for the temperature dependent behavior. Such behavior is hypothesized to be caused by the preferential onset of a rough crack wake morphology (likely slip band cracking) at low temperatures, that impedes molecular flow from the bulk to the crack tip. This behavior strongly suggests an important temperature dependence of the dislocation motion or H-dislocation interactions on the observed FCGR behavior.

Gradual shear band cracking and apparent softening of metallic glass under low temperature compression

Metallic glasses (MGs) usually exhibit synchronously enhanced plasticity and strength with decreasing the testing temperature. Although great efforts have been made, why MGs show better plasticity at low temperature remains unclear. In this work, the shear band cracking and fracture mechanism of a relatively brittle TiZr-based MG was investigated through methods of low temperature compression and 3D X-ray tomography. Pronounced apparent softening and profuse internal shear-band cracks were observed along with the improved plasticity, enhanced yield strength, decreased average softening rate, and reduced area of vein pattern as decreasing the testing temperature. Moreover, the fracture features can be correlated well with the compressive properties; and the true rupture stress that is carried by the still-bonded part of major shear band was found to be very close to the yield strength, demonstrating that the apparent softening should mainly originate from the cracking rather than dilation of shear band. The decreased softening rate can be fitted by a diffusion model, implying reduced atomic mobility and increased cracking resistance. Consequently, the improved plasticity of TiZr-MG at low temperature was attributed to the suppression of instant fracture and the enhanced resistance to shear band cracking, rather than the change of shear band density.

Bant Çatlak Içeren Ortotropik Malzemeden Yapılmış bir Dikdörtgen Prizmada, Çatlak Düzlemine Dik ve Çatlak Yüzüne Paralel Kayma Modülünün ERR’ye Etkisi

In this study, a rectangular prism made of an orthotropic material is considered. It is assumed that this prism contains a band crack whose edge-planes are parallel to the upper and lower face planes. It is also assumed that uniformly distributed normal forces are imposed the top and bottom surface of the prism. The aim of this paper is t o a nalyze t he e ffect o f t he s hear m odulus on planes which is perpendicular to the crack’s edge-planes and parallel to the crack’s front on the Energy Release Rate (ERR) for different geometric parameters in a rectangular prism. The mathematical formulation of the corresponding boundary-value problem is carried out within the framework of the 3-dimensional linear theory of elasticity. In order to solve this problem, the 3D finite element method was employed. The numerical results are presented.

Mathematical Modelling and 3D FEM Analysis of the Influence of Initial Stresses on the ERR in a Band Crack’s Front in the Rectangular Orthotropic Thick Plate

Mathematical Modelling and 3D FEM Analysis of the Influence of Initial Stresses on the ERR in a Band Crack’s Front in the Rectangular Orthotropic Thick Plate

Torsion fatigue behavior of pure titanium with a gradient nanostructured surface layer

The effect of surface nanocrystallization (SNC) on torsion fatigue property of pure titanium was investigated. Compared with coarse-grained Ti (CG Ti), the SNC Ti exhibits the longer fatigue life at the same cyclic stress amplitude and the higher fatigue strength limit. Both CG and SNC Ti display cyclic softening during torsional loading. The cyclic softening rate of SNC Ti is lower than that of CG Ti. Microstructural analysis reveals that, fatigue crack initiation is suppressed in the SNC Ti since the surface gradient nano/ultrafine grained layer restrains strain localization. As a result, the mechanism of crack initiation changes from grain boundary and twin boundary cracking to shear band cracking. Parallel dislocation lines, embryonic cells and twins are observed in the fatigued CG Ti. In comparison, no obvious variation of substructures can be distinguished in the SNC Ti before and after torsion fatigue. The relative stable microstructure in the SNC Ti facilitates the improvement of fatigue life. Furthermore, the large residual compressive stress on the surface restrains the fatigue crack propagation in the SNC metals. Therefore, the SNC Ti displays the better torsion fatigue properties than the CG Ti.

Effects of microstructures on fatigue crack initiation and short crack propagation at room temperature in an advanced disc superalloy

Fatigue crack initiation and early short crack propagation behaviour in two microstructural variants of a recently developed Low Solvus, High Refractory (LSHR) disc superalloy at room temperature has been investigated by three-point bending with replication procedure. The results shows that fine gained (FG) LSHR possesses higher fatigue life due to its better crack initiation resistance, limited crack coalescence and comparable Stage I crack propagation resistance to the coarse grained (CG) LSHR, although its resistance to Stage II crack propagation is inferior. Twin boundary (TB) cracking in the relatively large grains dominates the crack initiation process along with occasional crack initiation due to slip band cracking. Activation of the primary slip systems parallel to the TB at matrix and twin and high resolved shear stress associated with high Schmid factor (SF) are required for TB crack initiation. Cracks preferentially propagate along slip bands associated with high SF slip systems after initiation. But cracks also propagate along slip bands associated with slip systems with lower SF if the inclination angle between the slip band ahead of the crack tip and the crack segment of the crack tip is small enough to enable a steady transition (or non-deflected growth) of cracks across the grain boundary.

Study on the Solid Edge Banding Cracking of Panel Furniture

With the development of wood processing technology, panel furniture is getting more and more popular, where solid wood is often used for the edge banding to keep the natural wood grain, feel and colour. However, part of the panel furniture, especially that made in the south but sold to the north, has banding cracking issues due to various reasons. In this paper, on the base of comprehensive reference analysis and in-field exploration, the potential factors leading to banding cracking is studied; the influence of wood moisture content and adhesive on the cracking is specially analyzed; and the suggestions on the drying of the solid wood, glueing, manufacturing environment and process are presented. Thus, a new systematic concept to solve edge banding cracking is established, the study on which will be helpful to the development of panel furniture.