Fracture Of Metals Research Articles

Micromorphic elasticity for an axisymetrically loaded cylindrical thick-walled structure under plane strain conditions

In this paper we derive a closed-form solution for the problem of a second gradient elastic circular thick-walled cylinder subjected to some axisymmetric loading conditions and deformed in plane strain. The second gradient elastic model used is the one of Gologanu, Leblond, Perrin and Devaux proposed some years ago and used in the context of the numerical implementation of a micromorphic model for ductile fracture in porous plastic metals. The ordinary Cauchy stress and 'hyperstress' tensors are computed as a function of the displacement field which is obtained via a suitable combination of the balance equations, compatibility, stress-strain/hyperstress-gradient of the strain relationships, and appropriate boundary conditions. The first gradient solution is recovered when the second gradient effects are negligible. This study demonstrates that the first gradient elastic solution may not be accurate, particularly in elastic materials exhibiting substantial microstructure dependence. Future works will use the newly derived solution to interpret the size effects experimentally observed, for instance, in pressurised sandstone hollow cylinders.

Self-Expanding Metallic Stent Fracture in the Treatment of Malignant Biliary Obstruction.

Background Palliative therapies for malignant biliary obstruction (MBO) include choledochojejunostomy and self-expanding metallic stent (SEMS) insertion. Fractures following SEMS insertion in MBO treatment are scarce. Objective To assess the clinical features of biliary stent fractures and evaluate associated factors. Methods One hundred fifty-six consecutive patients who underwent biliary SEMS placement for MBO treatment at Beijing Chaoyang Hospital affiliated to Capital Medical University, in 2010–2015, were evaluated retrospectively. Demographics, clinical features, stent parameters and patency times, and survival times were collected. Across the ampulla of Vater, balloon dilatation, number of stents, stent patency time, and survival time were compared between the stent and nonstent fracture groups. Results There were 168 biliary metallic stents inserted in 156 patients, including 144 and 12 patients with one and 2-3 stents, respectively. Pre- and/or postballoon dilation was performed in 107 patients. Stents across and above the duodenal papilla were used in 105 and 51 patients, respectively. Six cases (3.8%) with stent occlusion had stent fractures. Single- and multiple-stent fracture rates were 4/144 (2.8%) and 2/12 (16.7%), respectively. Fracture times after stent deployment were 126.8 ± 79.0 (median, 115.5) days. Stent patency times in the stent and nonstent fracture groups were 151.8 ± 67.8 (median, 160.5) days and 159.3 ± 73.6 (median, 165.5) days, respectively. Overall survival times in the stent and nonstent fracture groups were 399.7 ± 147.6 (median, 364.0) days and 283.7 ± 126.1 (median, 289.0) days, respectively. Conclusion Stent fractures following MBO treatment constitute a relatively rare long-term complication. Though there were no factors found to be significantly associated with SEMSs fracture, a trend could be observed towards more fractures in multistent, transpapillary, and balloon dilation groups.

Relation between structure of metallic materials and fracture properties under conditions of solid particle erosion

The fraction of the ductile component of the fracture surface after solid particle erosion for different materials was investigated. The minimum amount of viscous fracture on the fracture surface was observed in aluminum samples with the FCC lattice and very high stacking fault energy, and the maximum was found in copper samples with the same FCC lattice but small stacking fault energy. The percentage of viscous fracture in BCC steel samples is close in magnitude to titanium alloy with HCP + BCC lattice and they occupy an intermediate position between copper and aluminum samples.

Bi-dimensional Empirical Mode Decomposition and Nonconvex Penalty Minimization L q (q = 0.5) Regular Sparse Representation-based Classification for Image Recognition

This paper reports an innovative pattern recognition technique for fracture microstructure images based on Bi-dimensional empirical mode decomposition (BEMD) and nonconvex penalty minimization L q (q = 0.5) regular sparse representation-based classification (NPMLq-SRC) algorithm. The detailed procedures of this work can be divided into three steps, i.e., the preprocessing stage, the feature extraction stage and the image classification stage. We test and validate the proposed method through real data from metallic alloy fracture images. The case verification results show that our proposal can obtain a much higher recognition accuracy than the conventional Back Propagation Neural Networks (BPNN for short), the L1-norm minimization sparse representation-based classification (L1-SRC) and the BEMD combined with L1-norm minimization sparse representation-based classification (BEMD+L1-SRC) methods, respectively. Specifically, the proposed BEMD+NPMLq-SRC (q = 0.5) method outperforms the BEMD+L1-SRC method by 3.33% improvement of the average recognition accuracy, and outperforms L1-SRC method by 14.06% improvement of the average recognition accuracy, respectively.

New data on the kinetics and governing factors of the spall fracture of metals

This paper presents two examples of significant departures from usual trends of varying the resistance to spall fracture (spall strength) with changing loading history, load duration and peak shock stress. In experiments with vanadium single crystals we observed an important decrease of spall strength when increasing the shock stress. This was interpreted in terms of disruption of the matter homogeneity as a result of its twinning at shock compression. In experiments with 12Kh18N10T austenitic stainless steel we observed a sharp increase of recorded spall strength value when short load pulses of a triangular profile were replaced by shock pulses of long duration having a trapezoidal shape. This anomaly is associated with formation of the deformation-induced martensitic phase.

Metallien virumismurron ja virumisväsymisen mallintaminen

Artikkelissa tarkastellaan metallien korkealämpötilan virumismurron ja virumisväsymisen mallintamista. Aluksi esitetaan lyhyt katsaus virumisen fysikaalisiin perusteisiin ja virumismalleihin. Kaytetyn termodynaamisesti konsistentin materiaalimallin konstitutiiviset yhtälöt esitetaan varsin perusteellisesti. Mallin materiaaliparametrit määritetään 7CrMoVTiB10-10 teräkselle lampötila-alueella 500 - 600 C. Malli on implementoitu ANSYS elementtimenetelmäohjelmiston USERMAT-aliohjelmaksi.

Anisotropic yield function based on stress invariants for BCC and FCC metals and its extension to ductile fracture criterion

It is essential to accurately model the anisotropic plastic deformation and ductile fracture of metals in order to guarantee the reliable numerical analysis and optimization of metal forming. For this purpose, the Drucker function is revisited. Effect of the third stress invariant in the Drucker function is analyzed and calibrated for metals with body-centered cubic (BCC) and face-centered cubic (FCC) crystal systems based on the yielding and plastic flow of both crystal plasticity and biaxial tensile experiments. The calibrated Drucker function is extended into an anisotropic form using a fourth order linear transformation tensor. The anisotropic flexibility is enhanced by two approaches: non-associate flow rule (non-AFR) and the sum of n-components of the anisotropic Drucker function. The proposed anisotropic Drucker function is applied to model the anisotropic behavior of both BCC and FCC metals. The predicted anisotropic behavior is compared with experimental results. The comparison demonstrates that the anisotropy is accurately modeled for both BCC and FCC metals by the anisotropic Drucker function. The anisotropic Drucker function is also implemented into numerical analysis of tension of specimens with a central hole to investigate its computation efficiency under spatial loading compared with the Yld2000-18p function. It is found that the proposed anisotropic Drucker function can reduce about 60% of computation time in case that the Yld2000-18p function is substituted by the anisotropic Drucker function in numerical computation due to its simplicity compared to the Yld2000-18p function. A ductile fracture criterion is also developed by coupling the Drucker function with the first stress invariant. The modified Drucker function is reformulated to investigate the effect of the stress triaxiality and the normalized third invariant on ductile fracture. Comparison of the modified Drucker fracture locus with the experimental results of AA2024-T351 demonstrates that the modified Drucker criterion accurately illustrates the fracture stress of the alloy in wide stress states with the stress triaxiality ranging from −0.5 in plane strain compression to 0.6 in tension of notched specimens. The modified Drucker fracture criterion is expected to be less sensitive to the change of strain path considering that the criterion describes fracture in the stress space. Accordingly, the anisotropic Drucker yield function and the pressure-coupled Drucker fracture criterion are suggested to model anisotropic plastic deformation and to predict the onset of failure for both BCC and FCC metals due to simple implementation in numerical analysis under spatial loading and computation efficiency with brick elements.

The Study on Characteristics of Dissimilar Weld Metals between AA6061-T6 and Silafont-36 by Laser Welding

The characteristics of dissimilar weld metals between AA6061-T6 and Silafont-36 by laser welding were evaluated in terms of bead shape, microstructure, EDS analysis, hardness, and tensile strength. Laser power of 3.0, 3.5, 4.0kW and incident angle of 15, 20° were employed to make dissimilar weld metals. Bead shape of weld metal with 3.5kW, 15° was most stable. Microstructure of dissimilar weld metals consisted of columnar dendrite in area adjacent to fusion line and equiaxed dendrite in center of weld metal. Also, Mg2Si and α-AlFeMnSi were precipitated in weld metal. Microstructure in HAZ of Silafont-36 consisted of eutectic Al-Si, Mg2Si, and β-Al5FeSi. Si contents of weld metal made with 4.0kW, 15° were very high. And Mg contents of weld metals were relatively low because of vaporation of Mg. Hardness of weld metal made with 4.0kW, 15° was very high because Si contents of that were high. In tensile test, Specimens were fractured in Silafont-36 or weld metals. The fracture in base metals was due to low tensile strength of Silafont-36, and the fracture in weld metals was due to undercut or porosity. Key words: AA6061, Silafont-36, Dissimilar weld, Laser welding, Mg2Si

The Evolution of the Weld Metal Microstructures in Dissimilar Titanium Welds Based on Al and Mo Equivalents

CO2 laser welding of Ti-15V-3Cr-3Al-3Sn to Ti-4.5Al-3V-2Fe-2Mo was conducted in the study. The notched fracture and microstructures of the weld metal (WM) with various post-weld aging treatments were studied. In the as-welded sample, the WM comprised only β phase, which was relatively soft and ductile. Moreover, the phase constituent of the as-welded WM was related to the Al and Mo equivalents and further compared with other welds. The microstructural morphologies and microhardness of the WMs of dissimilar welds exhibited significant changes after post-weld aging at distinct temperatures. Increasing the Al equivalent ([Al]EQ) of the WM caused an increased response to age-hardening during post-weld aging treatments. When the aging temperature was increased from 426 to 593 °C, the α precipitates in the WM grew in size, causing a decrease in hardness, but an improvement in toughness.

A correct form of Bai–Wierzbicki plasticity model and its extension for strain rate and temperature dependence

A mistake in the original Bai–Wierzbicki plasticity model (Bai, Y., Wierzbicki, T., 2008. A new model of metal plasticity and fracture with pressure and Lode dependence. Int. J. Plast. 24 (6), 1071–1096.) is revealed. A correct form of the pressure- and Lode-dependent plasticity model is presented and its extension for strain rate and temperature dependence is given. A corresponding associated flow rule, a convexity condition, and return mapping algorithm relations for an explicit solver are derived. Calibration and verification of the pressure- and Lode-dependent part of the proposed plasticity model are performed through numerical simulations of four different types of tests on 2024-T351 aluminum alloy. The accuracy of the return mapping algorithm realized via VUMAT user subroutine in Abaqus/Explicit commercial software is evaluated.

On the description of ductile fracture in metals by the strain localization theory

Numerical simulations based on the bifurcation and imperfection versions of the strain localization theory are used in this paper to predict the failure loci of metals and applied to an advanced high strength steel subjected to proportional loading paths. The results are evaluated against the 3D unit cell analyses of Dunand and Mohr (J Mech Phys Solids 66(1):133–153, 2014. doi: 10.1016/j.jmps.2014.01.008 ) available in the literature. The Gurson porous plasticity model (Gurson in J Eng Mater Technol 99(1):2–15, 1977. doi: 10.1115/1.344340 ) is used to induce strain softening and drive the localization process. The effects of the void growth, void nucleation and void softening in shear are investigated over a large range of stress triaxialities and Lode parameters. A correlation between the imperfection and bifurcation results is established.

Do voids nucleate at grain boundaries during ductile rupture?

In the absence of pre-existing failure-critical defects, the fracture or tearing process in deformable metals loaded in tension begins with the nucleation of internal cavities or voids in regions of elevated triaxial stress. While ductile rupture processes initiate at inclusions or precipitates in many alloys, nucleation in pure metals is often assumed to be associated with grain boundaries or triple junctions. This study presents ex situ observations of incipient, subsurface void nucleation in pure tantalum during interrupted uniaxial tensile tests using electron channeling contrast (ECC) imaging, electron backscatter diffraction (EBSD), transmission Kikuchi diffraction (TKD) and transmission electron microscopy (TEM). Instead of forming at grain boundaries, voids initiated at and grew along dislocation cell and cell block boundaries created by plastic deformation. Most of the voids were associated with extended, lamellar deformation-induced boundaries that run along the traces of the {110} or {112} planes, though a few voids initiated at low-angle dislocation subgrain boundaries. In general, a high density of deformation-induced boundaries was observed near the voids. TEM and TKD demonstrate that voids initiate at and grow along cell block boundaries. Two mechanisms for void nucleation in pure metals, vacancy condensation and stored energy dissipation, are discussed in light of these results. The observations of the present investigation suggest that voids in pure materials nucleate by vacancy condensation and subsequently grow by consuming dislocations.

Small-angle X-ray scattering investigation of deformation-induced nanovoids in AA6063 aluminium alloy

Small-angle X-ray scattering studies of microstructure in metallic systems are prone to contamination by double Bragg scattering from the crystalline matrix. This is particularly problematic to the study of fracture in ductile metals via the nucleation and growth of nanovoids in response to plastic deformation. We show clear evidence of the presence of these scattering artefacts in the scattering data from representative Al systems and describe a simple method of numerically isolating and removing potentially misleading information to reveal the true small angle scattering response of the sample. This data correction process is used to obtain quantitative measurements of the nanovoid volume fraction in deformed AA6063. The SAXS results yield values comparable to existing predictions of the total vacancy volume fraction obtained from the mechanical stress–strain data.

Effect of Damaging Irradiation on Low-Temperature Embrittlement of Metals

The effect of low-temperature damaging irradiation on the high-temperature embrittlement (cold shortness) in metals was investigated in a previously proposed model. The cold shortness state with possible brittle fracture in metals is not formed in the irradiation process because of the dynamical radiation processes arising.

Combined stability analysis of phase-field dynamic fracture and shear band localization

Dynamic fracture of metals is a fascinating multiphysics-multiscale problem that often results in brittle and/or ductile fracture of structural components and under high strain rates may also suffer from shear banding. While fracture events are mostly driven by tensile loading, shear bands are driven by shear heating caused by inelastic deformations and associated with high temperature rise. These processes are characterized by different failure modes with distinct spatial and temporal scales.A recent unified model developed by the authors attempts to capture dynamic fracture and shear banding failure modes using implicit, mixed finite element formulations. In this model, fracture is modeled with a novel phase field formulation coupled to a set of equations that describe shear bands. While fracture is governed by a strong length-scale that propagates with a fast time scale, shear bands are dominated by a weak length-scale and propagate slower.In this work, we analyze the physical stability of both failure modes and their interaction using a linear perturbation method. The analysis provides insight into the dominant failure mode and can be used as a criterion for mesh refinement.Several numerical results with different geometries and a range of strain rate loadings demonstrate that the stability criterion predicts well the onset of failure instability in dynamic fracture applications.For the example problems considered, if a fracture instability precedes shearbanding, a brittle-like failure mode is observed, while if a shear band instability is initiated significantly before fracture, a ductile-like failure mode is expected. In any case, fracture instability is stronger than a shear band instability and if initiated will dominate the response.

Comprehensive investigation of the metal in drums of boilers at thermal power stations

A comparative investigation of the metal of drums of two TP-100 boilers at the Starobeshevskaya and the Lugansk thermal power stations (TPS) was performed. Their operation time was approximately 300000 hours; the shell of one drum was ruptured during a hydraulic test, and the other drum is in operation. According to the results of the technical diagnostics and a strength analysis, both drums comply with the applicable regulatory requirements. The objects of the investigation were fragments of the ruptured drum and a “plug” cut out of the shell during a scheduled inspection. The investigation was carried out by microscopic metallography methods and the scanning electron microscopy technique. Mechanical tests of metal specimens were performed, and the hydrogen content in these specimens was measured. Prior to the material research, the metal was examined using a magnetic memory method. The investigation yielded specifics of the metal microstructure, mechanical properties, and fracture patterns of the metal specimens at various temperatures. An investigation performed by the method of thermal-desorption mass spectrometry revealed no considerable difference in the hydrogen content in the metal of both drums, thereby excluding the effect of hydrogenation in analyzing the rupture causes. It was established that the drum at the Starоbeshevskaya TPS had been damaged due to its low impact strength at room temperature and high brittle-ductile transition point. Comparison of the metallographic study data with the results obtained using the magnetic memory method suggests that the fracture was caused by local formation of the Widmannstatten pattern at points where accessories are welded to the shell. The prospects are demonstrated of the comprehensive approach to nondestructive examination (NDE) of TPS drums using the magnetic memory technique and metallographic methods.

Assessment of Complications Associated with Tooth Supported Fixed Dental Prosthesis amongst Armed Forces Personnel

Introduction: Fixed dental prosthesis is becoming more popular for preservation of badly damaged crowns and endodontically treated teeth and also for replacing lost teeth around the world. Complications of fixed dental prosthesis are also becoming very common. Fixed dental prosthesis assures greater retention and stability in addition to comfort. Fixed dental prosthesis is more acceptable to patients when correctly planned and designed which provide predictable function are also enhances the esthetics and psychological well being of the patients.
 Objective: To assess the complications associated with tooth supported fixed dental prosthesis amongst armed forces personnel reporting to Combined Military Hospital (CMH) Cumilla and Combined Military Hospital (CMH) Dhaka.
 Materials and Methods: An interview based questionnaire was used for 220 patients followed by clinical oral examination of every individual in this cross sectional study conducted during September 2016 to August 2017 in Combined Military Hospital, Dhaka. Patients soft tissues were checked but the main emphasis was given on recording the information regarding the type of prosthesis being used, material of the prosthesis, location of the prosthesis and the complication present including dental caries, coronal or radicular abutment fracture, root canal treatment failure, periapical lesions, periodontal problems and mechanical failures including occlusal interference, no occlusal contact, decementation of the prosthesis, esthetic problems, porcelain and metal fractures. Periapical radiograph was made to assess mentioned complaints; more so periodontal probing was applied to measure the gingival attachment loss.
 Results: Approximately 183(83%) patients were using porcelain fused to metal prosthesis with 133(60%) of prosthesis being used in posterior segments of mouth. Complications like dental caries, coronal abutment fracture, radicular abutment fracture, occlusal interferences, root canal failures and decementations were more significantly associated with crowns than bridges (p=0.001). On the other hand esthetic issues, periapical lesions, periodontal problems, porcelain fractures and metal damage were more commonly associated with bridges (p=0.001). All cases of dental caries reported were associated with acrylic crown and bridges, whereas all coronal abutment fractures were associated with metal prosthesis. A significantly higher number of patients who got their fixed dental prosthesis from other sources i.e. paramedics, technicians, dental assistants or unqualified dentists had periapical lesions, decementations, esthetic issues and periodontal diseases.
 Conclusion: Complications associated with fixed dental prosthesis like root canal failures, decementations, periapical lesions and periodontal disease were more significantly associated with prosthesis fabricated by other sources rather than those acquired from CMHs and private dental care and also in patients using prosthesis for 5 to 10 years.
 Journal of Armed Forces Medical College Bangladesh Vol.13(1) 2017: 75-80

Anisotropic ductile fracture criterion based on linear transformation

An anisotropic ductile fracture criterion is proposed for ductile fracture of lightweight metals. The ductile fracture criterion couples effect of stress triaxiality on void growth, and assumes the shear linking-up of voids governed by the largest shear stress. The criterion is developed based on an isotropic strain rate potential computed from an isotropic damage equivalent strain rate vector, which is mapped from the plastic strain rate vector by a forth order linear transformation tensor. The proposed anisotropic ductile fracture criterion is applied to depict anisotropic ductile fracture of AA 6K21 in shear, uniaxial tension and plane strain tension along different loading directions, and the balanced biaxial tension. The predicted fracture strain and fracture locus are compared with experimental results for the verification of the proposed criterion. The comparison demonstrates that the ductile fracture criterion properly models the anisotropy in ductile fracture under shear, uniaxial tension, plane strain tension and balanced biaxial tension with high accuracy. It shows that the proposed anisotropic ductile fracture criterion can be utilized to predict the onset of ductile fracture in plastic deformation and metal forming of lightweight metals with approximately proportional loading.

Using the lead crack concept and fractal geometry for fatigue lifing of metallic structural components

Design of several metallic structural components, for instance in aerospace applications, should take into account the physically short crack behaviour. By characterising a fracture surface using fractals concepts, crack growth models similar to that proposed by Frost and Dugdale in 1958 and Hartman and Schjive in 1970 can be obtained. For short cracks, these models predict exponential crack growth with respect to the applied load history, and that has led to a practical aircraft lifing approach known as the lead crack framework. The present paper describes the fractality of metallic fracture surfaces and the crack growth behaviour in some fatigue tests.

Multiscale models of metal behaviour and structural change under the action of high-current electron irradiation

We present our models of the tensile fracture of metals in the solid and molten states, the melting and the plastic deformation of the solid metals. Also we discuss implementation of these models for simulation of the high current electron beam impact on metals. The models are constructed in the following way: the atomistic simulations are used at the first stage for investigation of dynamics and kinetics of structural defects in material (voids, dislocations, melting cites); equations describing evolution of such defects are constructed, verified, and their parameters are identified by means of comparison with the atomistic simulation result; finally, the defects evolution equations are incorporated into the continuum model of the substance behaviour on the macroscopic scale. The obtained continuum models with accounting of defects subsystems are tested in comparison with the experimental results known from literature. The proposed models not only allow one to describe the metal behaviour under the conditions of intensive electron irradiation, but they also allow one to determine the structural changes in the irradiated material.