Fracture Of Metals Research Articles

Effect of inclination of weld faying surface on joint strength of friction welded joint and its allowable limit for austenitic stainless steel solid bar similar diameter combination

This paper describes the effect of the inclination of the weld faying surface on joint strength of friction welded joint and its allowable limit for austenitic stainless steel (SUS304) solid bar similar diameter combination. In this case, the specimen was prepared with the inclination of the weld faying surface pursuant to the JIS Z 3607, and the joint was made with that diameter of 12 mm, a friction speed of 27.5 s−1, and a friction pressure of 30 MPa. The initial peak torque decreased with increasing inclination of the weld faying surface, and then the elapsed time for the initial peak increased with increasing that inclination. However, the steady torque was kept constant in spite of the inclination of the weld faying surface increasing. The joints without the inclination of the weld faying surface, which were made with friction times of 1.5 and 2.0 s with a forge pressure of 270 MPa, had achieved 100% joint efficiency with the base metal fracture. Those joints had 90° bend ductility with no crack at the weld interface. The joints with the inclination of the weld faying surface of 0.3 mm (gap length of 0.6 mm), which were allowable distance, was also obtained the same result with this condition. Furthermore, those joints with a friction time of 2.5 s obtained the same result. On the other hand, the joints with the inclination of the weld faying surface of 0.6 mm (gap length of 1.2 mm), which were twice inclination of the allowable distance, also obtained the same result in a friction time of 2.5 s. However, the joints without the inclination of the weld faying surface at this friction time did not obtain the base metal fracture, although those achieved 100% joint efficiency. In conclusion, to obtain 100% joint efficiency and the base metal fracture with no cracking at the weld interface, the joint must be made with the inclination of the weld faying surface, with allowable distance pursuant to the JIS Z 3607.

Ultimate strength of austenitic stainless steel fillet-welded connections with weld metal fracture

This study investigates ultimate behaviors such as ultimate strength and weld metal fracture mechanism of austenitic stainless steel (304 type) fillet-welded connections with TIG (tungsten inert gas) welding through experimental method. Main variables of test specimens are weld length and welding direction against applied force. Specimens are fabricated with transverse fillet weld (TFW series), longitudinal fillet weld (LFW series) and full fillet weld FW series against loading direction. Specimens of TFW series, LFW series and FW series failed by tensile fracture, shear fracture and block shear fracture in the weld metal of connections. Test ultimate strengths were compared with predictions based on the design specifications, AISC (American institute of steel construction) and ASCE (American Society of Civil Engineers) and existing researcher's equations. Current design equations underestimated excessively the strength of welded connections with weld metal fracture. Consequently, modified design formula is suggested considering actual fracture path and the effect of stress triaxiality on welded connections and its validation was verified in this paper.

Characterization of forming limits at fracture with an optimized cruciform specimen: Application to DP600 steel sheets

In order to identify experimental limit strains at fracture for metallic sheets under in-plane biaxial loadings, a specific specimen shape has to be defined. Ideally, the proposed geometry must lead to fracture at the specimen center whatever the strain path imposed by the experimental device. If such condition is ensured, the whole strain paths ranging from equi-biaxial to uniaxial could be experimentally investigated. Based on the literature review, four in-plane cross specimen shapes have been selected owing to strong potential to reach large strain and fracture in their central point. From finite element simulations, the behavior of these different shapes is evaluated. Based on these numerical results, a new shape is proposed and optimized for determining the forming limits at fracture of DP600 sheet metal with a thickness of 2mm. The forming limit strains at fracture are identified by a time-dependent method and the forming limit strains at necking are determined by a position-dependent method. The experimental forming limit strains at fracture of DP600 sheet metal are fitted by a forming line. Three ductile fracture criteria are then calibrated with experimental points. The Oyane criterion with two parameters gives the best prediction.

A multi-scale correlative investigation of ductile fracture

The use of novel multi-scale correlative methods, which involve the coordinated characterisation of matter across a range of length scales, are becoming of increasing value to materials scientists. Here, we describe for the first time how a multi-scale correlative approach can be used to investigate the nature of ductile fracture in metals. Specimens of a nuclear pressure vessel steel, SA508 Grade 3, are examined following ductile fracture using medium and high-resolution 3D X-ray computed tomography (CT) analyses, and a site-specific analysis using a dual beam plasma focused ion beam scanning electron microscope (PFIB-SEM). The methods are employed sequentially to characterise damage by void nucleation and growth in one volume of interest, allowing for the imaging of voids that ranged in size from less than 100 nm to over 100 μm. This enables the examination of voids initiated at carbide particles to be detected, as well as the large voids initiated at inclusions. We demonstrate that this multi-scale correlative approach is a powerful tool, which not only enhances our understanding of ductile failure through detailed characterisation of microstructure, but also provides quantitative information about the size, volume fractions and spatial distributions of voids that can be used to inform models of failure. It is found that the vast majority of large voids nucleated at MnS inclusions, and that the volume of a void varied according to the volume of its initiating inclusion raised to the power 3/2. The most severe voiding was concentrated within 500 μm of the fracture surface, but measurable damage was found to extend to a depth of at least 3 mm. Microvoids associated with carbides (carbide-initiated voids) were found to be concentrated around larger inclusion-initiated voids at depths of at least 400 μm. Methods for quantifying X-ray CT void data are discussed, and a procedure for using this data to calibrate parameters in the Gurson-Tvergaard Needleman (GTN) model for ductile failure is also introduced.

Effect of friction welding condition on joining phenomena and mechanical properties of friction welded joint between 6063 aluminium alloy and AISI 304 stainless steel

Dissimilar metal joints have some advantages such as high functionality characteristics for the industrial usage. This paper describes the effect of friction welding condition on joining phenomena, tensile strength, and bend ductility of friction welded joints between Al-Mg-Si alloy (AA6063) and austenitic stainless steel (AISI 304). When joints were made at a friction pressure of 30MPa with a friction speed of 27.5s−1, the upsetting (deformation) occurred at the AA6063 side. The temperature on the weld interface increased with friction time, and it reached to 623K or over at a friction time of 1.5s or longer. When joints were made with a friction time of 1.5s and a forge pressure of 240MPa, all joints had the joint efficiency of approximately 100% and the fracture in the AA6063 base metal. Furthermore, those joints had the bend ductility of 90° in a single direction with no crack at the weld interface and did not have the intermetallic compound (IMC) interlayer on the weld interface. To obtain 100% joint efficiency with good joint, the joint should be made with the following conditions: a high forge pressure such as 240MPa, the opportune friction time that the temperature on the weld interface reached to about 623K or higher.

Explosive compact-coating of tungsten–copper alloy to a copper surface

This study proposed a new method for coating tungsten–copper alloy to copper surface. First, the tungsten–copper alloy powder was pre-compacted to the copper surface. Then, the powder in the hydrogen atmosphere was sintered, and the pre-compacted powder was compacted by explosive compact-coating. Finally, diffusion sintering was conducted to improve the density of the coating layer. The theoretical density of the coating reached 99.3%. Microstructure characteristics indicated that tungsten and copper powders were well mixed. Tungsten particles were larger than copper particles. Scanning electron microscope (SEM) fracture surface analysis was different from the traditional fracture of metals. Coating and substrate joint surfaces, which were analyzed by SEM, indicated that the tungsten–copper alloy was sintered on the copper surface. The hardness of the coating layer was 197.6–245.2 HV, and the hardness of the substrate was approximately 55 HV.

Influence of the electric double-layer capacitance at the rate of corrosion at the phase interface

AbstractThis article covers the basic regularities of marine corrosion at the waterline and its relationship with the immersion potential (Billiter potential). It offers a model of an electrochemical metal fracture at the interface, which allows to explain the experimentally observed type of corrosion curve. We focus our attention in this model on the influence of the electric double-layer capacitance on the corrosion rate in the waterline spray zone. Within the framework of the adopted concept, the capacitance change at the interface results in a corresponding capacitive (anode) electrical current that causes dissolution when the metal surface is wetted. This phenomenon is associated with some protection (cathode) current on the already wetted surface of the metal.

Damage evolution on dynamic tensile fracture of ductile metals

Dynamic tensile failure, e.g. spallation, of ductile metals is a complex phenomenon, due to its multiple tempo-spatial scales and structural levels involved on one hand, and due to its irreversible and nonlinear features as a non-equilibrium process on the other. Activated/Stimulated by tensile stress, non-uniform mesoscopic structures of materials evolve, leading to internal damage nucleation, growth, coalescence and final fracture, which consist the basic physical processes of dynamic tensile failure of ductile metals. Damage evolution on dynamic tensile fracture of ductile metals has been reported elsewhere, however, our ability to understand the mechanism of damage evolution in micro-scale and to accurately predict fracture in macro-scale is still limited. In this paper, we present investigations on macroscopic response, microscopic mechanism and physical models of dynamic tensile failure, based on our previous works. Current difficulties and challenges encountered when studying such a complex, multi-scale phenomenon are also discussed.

ДОСЛІДЖЕННЯ ТРІЩИНОСТІЙКОСТІ ДЕТАЛІ ПІД ДІЄЮ ТЕРМОІМПУЛЬСНОГО НАВАНТАЖЕННЯ

The cracking problem of aviation unit part under the influence of the thermal pulse loading were considered. The problem of thermal plasticity during thermal pulse processing were solved in CAE system Ansys; internal stresses were calculated and stress concentration zones in the part were defined. A numerical simulation of crack formation in the zone of greatest stress concentration allowed to determine the stress intensity factors for the case of normal separation, transverse and longitudinal shift and J-integral. This study allows to estimate the defect behavior during thermal pulse part treatment. The reliability of obtained results is confirmed by the adequacy of used models. In subsequently the study of metal fracture toughness allows to improve the method of setting modes for thermal pulse parts treatment taking into account the strength limit.

New biaxially loaded specimens for the analysis of damage and fracture in sheet metals

The paper deals with development of new experiments based on performance of corresponding numerical simulations to study the effect of stress state on damage and fracture behavior of ductile metals. Different stress-state-dependent mechanisms of ductile damage acting on the micro-level are taken into account and, therefore, details of the stress state characterized by stress triaxiality and the Lode parameter in the critical parts of the specimens are of interest. Based on detailed numerical analysis of standard cruciform specimens, newly designed specimens are developed and numerically studied. Specimens of the most promising geometries have been manufactured and tested. Strain fields in critical regions of the specimens have been evaluated by digital image correlation (DIC) technique and compared with the results of finite element simulations. Considering different biaxial loading conditions it is possible to cover a wide range of stress triaxialities and Lode parameters in tension, shear and compression domains and consequently the newly proposed specimens facilitate a controlled study of damage and fracture at different stress states. Experiments with these new specimen geometries are recommended for experimental programs to analyze safety and lifetime of metal structures.

Modeling of ductile fracture from shear to balanced biaxial tension for sheet metals

A ductile fracture model is proposed to describe shear fracture of sheet metals from shear to balanced biaxial tension via uniaxial and plane strain tension. The fracture criterion models plastic damage as strain-induced void nucleation, triaxiality-governed void enlargement, Lode-controlled void torsion, and shear-restrained coalescence of voids. Its flexibility is investigated by a parameter study of the ductile fracture model proposed. The fracture model is employed to describe ductile fracture behavior of an aluminum alloy AA6082 T6 (thickness: 1.0mm). Dogbone specimens are strained to characterize the strain hardening properties, while another four different specimens are tested to characterize fracture behavior in shear, uniaxial tension, plane strain tension and balanced biaxial tension. The loading processes are analyzed numerically with the stress invariant-based Drucker yield function which is for the first time specified for body-centered cubic and face-centered cubic metals. Fracture strains in various loading conditions are measured with a hybrid experimental-numerical approach. The measured fracture strains are then used to calibrate the ductile fracture model proposed. The ductile fracture model calibrated above is employed to predict the onset of ductile fracture for these four specimens. For the purpose of comparison, the predicted fracture strokes of these four loading conditions are compared with those predicted by the modified Mohr–Coulomb model (Bai and Wierzbicki, 2008), and two micromechanism-inspired criteria proposed recently (Lou et al., 2012, 2014). The comparison reveals that the proposed model predicts the fracture behavior in much better agreement compared with experimental results from shear to the balanced biaxial tension. Accordingly, the proposed ductile fracture criterion is recommended for the prediction of ductile fracture in sheet metal forming processes, optimization of forming parameters and design of tools for both solid elements and shell elements. Besides, the ductile fracture model proposed can also be applied in various bulk metal forming processes in case that the model is calibrated by proper sets of experiments.

Identification of forming limits at fracture of DP600 sheet metal under linear and unloaded non-linear strain paths

Abstract The in-plane biaxial tensile test with a new cruciform specimen is used to investigate the forming limit strains at fracture of DP600 sheet metal. The experimental forming limits are determined under linear and non-linear strain paths without unloading provided by four independent dynamic actuators. The Oyane ductile fracture criterion is used to predict the experimental results under different strain paths.

Repair of Cracks in Metals: A Review

Cracks are surface or subsurface fissures that develop in a material. Propagation energy derived from mechanical, thermal, chemical, and metallurgical effects, or a combination of these may influence crack initiation and growth. Various types of cracks exist in metals and can be categorised as cooling, solidification, centreline, crater, grinding, pickling, heat treatment, machining tears, plating, fatigue, creep, stress corrosion and hydrogen cracks. Cracks can grow and lead to complete fracture of the component posing significant threats to component life and may lead to serious injuries or loss of life. Brittle fracture in metals occurs with little or no visible warning. Discovery of any cracks warrants immediate interventions to arrest the cracks before they propagate to the point of fracture. Several crack detection and repair methods in metals have been developed, characterised and validated through research. This paper reviews the repair techniques of cracks in metals.

Edwin Smith. 28 July 1931 — 4 July 2010

Ted Smith is best known for his contributions to the analysis of continuous dislocations in deformed crystals and the application of this to understanding the conditions leading to plastic flow and fracture in metals. He applied his knowledge to a range of practical problems, particularly ones concerned with the structural integrity of key components in the nuclear power generation industry. His career spanned both industry and academia, including 20 years as Professor of Metallurgy at the University of Manchester, during which time he helped oversee the joint operation of the Departments of Metallurgy at both the University and UMIST and served at senior levels in the University administration. His research was frequently motivated by interactions with industry in consultancy work. He published over 500 papers, the great majority of which were of his sole authorship.

Flow unit model in metallic glasses

Metallic glass is a promising metallic material with many unique properties, and also considered as a model system to study the mysteries of amorphous materials. Recently, many experimental and simulation results supported the existence of “flow unit” in metallic glass. In this paper, we review the background, the theoretical and experimental evidences of flow unit model. Flow units are considered as those loosely packed regions embedded inside the elastic matrix and behave like viscous liquid. Compared with the matrix, flow unit regions have low modulus and strength, low viscosity, high atomic mobility and stand in the saddle points on energy landscape. Therefore, flow units can be treated as dynamical defects in metallic glass. The feature, activation and evolution process of flow unit region in metallic glass as well as their correlation with property in metallic glass are also reviewed. Through dynamical mechaincal methods like dynamical mechanical spectra and stress relaxation, flow unit region and its properties can be distinguished and studied. A three-parameter physical model is proposed to describe the mechnical behaivors of flow units. The activations and evolutions of flow unit under different temperature and strain conditions are studied. A three-stage evolution process is found and the relation with mechanical performance and relaxation behavior is established. The characteristics of flow units are also related to various properties of metallic glass, like plasticity, strength, fracture and boson peaks. By using the thermal, mechanical and high pressure aging procedues, the properties of metallic glass can be manipulated as desired through adjusting the density of flow units. We show that the flow unit model not only helps to understand the mechanism behind many long-standing issues like deformation, glass transition dynamic relaxations, and the connection between strucutre and properties and performance of metallic glasses, but also is crucial for tuning and designing the properties of metallic glasses.

DESTRUCTION OF LARGE-DIAMETER STEEL PIPES AT ROLLED BURNTON DEFECT

In the domestic pipelines the large-diameter pipes of different methods of manufacturing are used − the straight-line-seam welded pipes, the one- and double-seam welded pipes and the spiral-seam welded pipes. The diameters of pipes are up to 1420 mm, the strength class of pipes is up to K65 per the API standard. The world’s latest innovative technologies for the production of the large-diameter straight-line-seam single-joint welded pipes (the diameters are 1020 mm, 1220 mm and 1420 mm, the grade strengths of steel are K38 − K65 and X42 − X80, the wall thickness is up to 52 mm, the length is up to 18 m and the working pressure is up to 22.15 MPa) are the processes developed by the German company SMS Meer and based on the step-by-step process of press forming according to the scheme JCOE. The SMS Meer technologies are widely used by the Russian pipe plants − JSC “Vyksa Steel Works”, JSC “Izhora Pipe Mill”, PJSC “Chelyabinsk Pipe-Rolling Plant”, as well as the plants in Germany, China and India. However, the accident statistics of Russian pipelines shows that the stress corrosion of metal of the pipe’s wall occurs mainly on the pipelines with the large diameter 700 − 1420 mm. With more than 80 % of the destruction of pipelines with the signs of stress corrosion are observed on the pipelines with the diameters of 1020 – 1420 mm. The main cause of the corrosion-mechanical cracking of metal of pipe’s wall is the combined effect of three factors: 1) low steel-smelting quality of metal and the manufacturing defects of pipes (the large residual stresses, the microcracks and microexfoliation of metal after the pipe blanks’ forming, the corrugation and hairlines defects, the rolled burnt-on defects, the faulty fusion of weld seam and so on); 2) presence of corrosive-active environment and its access to the metal surface; 3) high-cycle fatigue and fracture of metal due to the pulsations of the in-tube working pressures and hydroblows. On the domestic pipelines pipes’ ruptures are almost two times more frequent than in the United States and Europe due to the manufacturing defects and the construction-installation defects. Therefore, it is necessary to study carefully the causes of the known cases of pipelines’ rupture due to the manufacturing defects. In this work, the pipe with the rolled burnt-on and the hairline defect on the outer surface of pipe is considered. The mathematical criterion for determining the critical in-tube pressure at which the elastic-plastic rupture of the pipe’s wall is taken place is obtained. The results of the investigation can be used in the diagnostics of the rupture’s causes of the steel major- and medium-diameter pipes on the main and interfield pipelines.

Tensile properties of friction welded AISI 4340 joints

Abstract Armor grade Q&T steels very similar to AISI 4340 are used for construction of combat vehicles due to its high hardness, high strength to weight ratio and excellent toughness. The influence of friction welding parameters on microstructure and mechanical properties of such armor steel friction weld joints were evaluated. The microstructural properties of the various weld joint regions were examined by optical microscopy. It was also observed that the fully deformation zone (FDZ), the partially deformation zone (PDZ) and the heat affected zone (HAZ) have a vital effect on the tensile strength of the welded joints. Nearly 83 % of the tensile strength of the parent material (PM) can be achieved in friction welded joints. This is a 17 % higher strength than with the same parent material welded by shielded metal arc welding (SMAW) and flux cored arc welding (FCAW). The respective tensile and impact properties of the welded joints were correlated with the weld metal microstructures and fracture morphologies.

Grain neighbour effects on twin transmission in hexagonal close-packed materials.

Materials with a hexagonal close-packed (hcp) crystal structure such as Mg, Ti and Zr are being used in the transportation, aerospace and nuclear industry, respectively. Material strength and formability are critical qualities for shaping these materials into parts and a pervasive deformation mechanism that significantly affects their formability is deformation twinning. The interaction between grain boundaries and twins has an important influence on the deformation behaviour and fracture of hcp metals. Here, statistical analysis of large data sets reveals that whether twins transmit across grain boundaries depends not only on crystallography but also strongly on the anisotropy in crystallographic slip. We show that increases in crystal plastic anisotropy enhance the probability of twin transmission by comparing the relative ease of twin transmission in hcp materials such as Mg, Zr and Ti.

Elasto-viscoplastic phase field modelling of anisotropic cleavage fracture

A finite-strain anisotropic phase field method is developed to model the localisation of damage on a defined family of crystallographic planes, characteristic of cleavage fracture in metals. The approach is based on the introduction of an undamaged configuration, and the inelastic deformation gradient mapping this configuration to a damaged configuration is microstructurally represented by the opening of a set of cleavage planes in the three fracture modes. Crack opening is modelled as a dissipative process, and its evolution is thermodynamically derived. To couple this approach with a physically-based phase field method for brittle fracture, a scalar measure of the overall local damage is introduced, whose evolution is determined by the crack opening rates, and weakly coupled with the non-local phase field energy representing the crack opening resistance in the classical sense of Griffith. A finite-element implementation of the proposed model is employed to simulate the crack propagation path in a laminate and a polycrystalline microstructure. As shown in this work, it is able to predict the localisation of damage on the set of pre-defined cleavage planes, as well as the kinking and branching of the crack resulting from the crystallographic misorientation across the laminate boundary and the grain boundaries respectively.

Comparison of anisotropy evolution in BCC and FCC metals using crystal plasticity and texture analysis

Abstract The evolution of anisotropy in metals has been studied for many decades both experimentally and theoretically using computation. Although the experimental research on the evolution of anisotropy provided valuable data, there is lack of theories that can be used in macro-scale plasticity. Fracture of ductile metals occurs after going through a large amount of plastic deformation where anisotropy of the material changes significantly as manifested by texture development; processed metal slabs (such as rolled slabs) possess anisotropy due to texture produced by the metal forming process that the slabs went through and initially isotropic metals get to possess anisotropy as well due to the texture formation induced by the large amount of plastic deformation. Therefore, it is critical to take into account the effect of anisotropy evolution to accurately predict fracture characteristics and plastic flow behaviors in the simulation of ductile fracture and plastic flow of metals. The theory of anisotropy evolution of metals in macro-scale plasticity can be derived by quantifying the meso-scale grain-level physics through crystal plasticity simulations. Therefore, this paper investigates the evolution of anisotropy in BCC and FCC metals using crystal plasticity simulations and texture analysis to reveal the physics behind anisotropy evolution and provide comparison of anisotropy evolution between BCC and FCC metals, which can be used to propose the theory of anisotropy evolution in macro-scale plasticity.