Fracture Of Metals Research Articles

Effects of Temperature and Strain Rate on the Ductility of an API X65 Grade Steel

In the last few decades, great effort has been spent on advanced material testing and the development of damage models intended to estimate the ductility and fracture of ductile metals. While most studies focused on static testing are applied at room temperatures only, in this paper, multiaxial tests have been executed to investigate the effects of dynamic action and temperature on the mechanical and fracture behavior of an API X65 steel. To this end, a Split Hopkinson Bar (SHB) facility for dynamic tests, and a uniaxial testing machine equipped with a high-temperature furnace, were used. Numerical simulations of the experiments were setup for calibration and validation purposes. Based on the experimental results, the Johnson–Cook and Zerilli–Armstrong plasticity models were first tuned, resulting in a good experimental–numerical match. Secondly, the triaxiality and Lode angle dependent damage models proposed by Bai–Wierzbicki and Coppola–Cortese were also calibrated. The comparison of the fracture surfaces predicted by the damage models under different loading conditions showed, as expected, an overall significant increase in ductility with temperature; an appreciable increase in ductility was also observed with the increase in strain rate, in the range of low and moderate triaxialities.

Revisiting the fracture forming limits of bulk forming under biaxial tension

The formability limits of bulk metal forming in principal strain space and in the effective strain vs. stress-triaxiality space are characterized by an uncertainty region in which cracks may be triggered by tension (mode I of fracture mechanics) or by out-of-plane shear (mode III). The problem in obtaining experimental data in this region has been known for a long time and the main objective of this paper is to present a new upset formability test geometry that can effectively contribute to the characterization of the formability limits of bulk metal forming parts subjected to biaxial tension. Alongside with this objective, this paper also presents an analytical expression for converting the fracture forming limit line corresponding to crack opening by mode III in principal strain space into a hyperbolic fracture limit curve in the effective strain vs. stress-triaxiality space. The overall methodology employed by the authors combines experimentation along with analytical and numerical modelling, and the contents of the paper is a step towards diminishing the actual lack of knowledge regarding failure by fracture in bulk metal forming parts subject to stress-triaxiality values beyond uniaxial tension. Results show that a new uncoupled ductile fracture criterion built upon combination of the integrands of the Cockcroft-Latham and McClintock criteria can be successfully used to model the physics of the bulk metal forming limits for the entire range of stress-triaxiality values corresponding to cracking on free surfaces.

Friction stir welding of AA3003-clad AA6013 thin sheets: Microstructural changes related to tensile properties and fatigue failure mechanism

The microstructure, tensile properties, and fatigue behavior of friction stir welding (FSW) joints of multilayer AA3003-clad AA6013 are experimentally investigated. Linear butt welding is performed using a concave tool equipped with a columnar threaded pin at a rotating speed of 600 rpm and a transverse speed of 200 mm/min. The microstructures of FSW joints were observed using a field emission scanning electron microscope equipped with an electron backscatter diffraction system and an energy dispersive spectrometer. The microstructural characterization in the stir zone (SZ) reveals grain refinement, precipitate refinement, zigzag line, and the AA3003-clad layer fragments due to material mixing. The tensile test result of all-weld joints shows that the tensile strength of the SZ is significantly higher than that of base metal, while the cross-weld joints show a typical ductile fracture in the base metal. The result of fatigue tests shows that all the cross-weld joints fracture from the SZ. Analysis of the fatigue failure mechanism indicates that the crack causing the fatigue fracture originates from the AA3003-clad layer fragments in the advancing side of the SZ. The fatigue analysis also confirms that surface roughness and zigzag line are not the cause of the final fatigue fracture of the cross-weld joint for the present study.

Study on the effects of H on the plastic deformation behavior of grain boundaries in nickel by MD simulation

It is generally believed that the influence of hydrogen on plastic deformation of grain boundaries should be considered when analyzing hydrogen-induced intergranular fracture in polycrystalline metals. In this paper, the equilibrated H distribution around GBs was firstly investigated by employing the grand canonical Monte Carlo method. Then, MD simulations were performed to study the plastic response of GBs under uniaxial tensile loads in different directions, with various bulk H concentrations considered. The results indicate that the influence of H on dislocation nucleation from GB depends on both tensile directions and characteristics of GB structures. Specifically, two dislocation nucleation mechanisms, called dislocation dissociation nucleation (DDN) and heterogeneous dislocation nucleation (HDN), are identified. Careful analyses show that H segregation can increase the energy barrier of DDN, which results in H-inhibited dislocation nucleation. In contrast, the HDN mechanism involves H-enhanced or H-insensitive dislocation nucleation, which mainly depends on the influence of H on GB stress.

Stud shape and joint strength for low carbon steel joints fabricated by friction stud welding with low load force requirement

This paper describes the stud shape and joint strength of low carbon steel joints fabricated by friction stud welding with low load force requirement. To reduce the load force during the welding process, the stud side with the circular hole at the weld faying surface part was used. The outer diameter of a cylindrically shaped stud side had 12.0 mm and that was welded to the circular solid bar with a diameter of 24.0 mm as the work side. The joint was made with a friction speed of 27.5 rps, a friction pressure of 60 MPa, and a forge pressure of 60 MPa, which was determined as the low force condition for obtaining good joint in the previous study. When joints were made by a cylindrically shaped stud with a hole diameter of 6.0 mm and its depth of 0.5 mm, all joints at a friction time of 0.6 s, i.e. the friction torque reached to the initial peak, had the same tensile strength as that of the base metal with the base metal fracture. All joints with flash from the initial weld interface had the fracture on the base metal, the bend ductility of over 15° with no cracking at the initial weld interface through an impact shock bending test, and a high fatigue strength of the base metal. That is, the sound joint could be successfully achieved, and that could be obtained with the same friction stud welding condition of the circularly shaped solid stud. As a conclusion, the joining technique for the friction stud welding method with low load force requirement was proposed in accordance with using a cylindrically shaped stud that has the circular hole with the shallow depth at the weld faying surface part.

Grain-boundary fracture mechanisms in Li[formula omitted]La[formula omitted]Zr[formula omitted]O12 (LLZO) solid electrolytes: When phase transformation acts as a temperature-dependent toughening mechanism

Garnet-type, solid electrolytes, such as Li7La3Zr2O12 (LLZO), are a promising alternative to liquid electrolytes for lithium-metal batteries. However, such solid-electrolyte materials frequently exhibit undesirable lithium (Li) metal plating and fracture along grain boundaries. In this study, we employ atomistic simulations to investigate the mechanisms and key fracture properties associated with intergranular fracture along one such boundary. Our results show that, in the case of a Σ5(310) grain boundary, this boundary exhibits brittle fracture behavior, i.e. the absence of dislocation activity ahead of the propagating crack tip, accompanied with a decrease in work of separation, peak stress, and maximum stress intensity factor as the temperature increases from 300 K to 1500 K. As the crack propagates, we predict two temperature-dependent Li clustering regimes. For temperatures at or below 900 K, Li tends to cluster in the bulk region away from the crack plane driven by a void-coalescence mechanism concomitant a simultaneous cubic-to-tetragonal phase transition. The tetragonalization of LLZO in this temperature regime acts as an emerging toughening mechanism. At higher temperatures, this phase transition mechanism is suppressed leading to a more uniform distribution of Li throughout the grain-boundary system and lower fracture properties as compared to lower temperatures.

Fatigue fracture morphology of AISI H13 steel obtained by additive manufacturing

The paper focuses on researching the effect of fatigue loading on metallic structure, lifetime, and fracture surface topographies in AISI H13 steel specimens obtained by selective laser melting (SLM). The topography of the fracture surfaces was measured over their entire area, according to the entire total area method, with an optical three-dimensional surface measurement system. The fatigue results of the SLM 3D printed steel specimens were compared with those reported for conventionally manufactured 13H steel. The investigation also considers the roughness of the specimens’ side surface. Moreover, the fractographic evaluation conducted using scanning electron microscopy confirms that the predominant fracture mechanism is transgranular fracture. Microtomography done after mechanical loading also showed the influence of the stress level on the porosity distribution. Both fractographic and Micro-CT investigations confirm that higher stresses result in coarser and much more uniform porosity observed in fractured samples. These comprehensive quantitative and qualitative fracture analyses are beneficial to predict the failure conditions of SLM steel parts, especially in the case of fatigue damage. From the quantitative analysis of the H13 SLM-manufactured fracture surface topography, it was possible to conclude that the larger the loadings acting on the specimen, the rougher the fracture surface because the ductile fracture mode dominates. It has also been proven that the porosity degree changes along the length of the sample for the most stressed specimens.

Effect of vent hole size on combustion and explosion characteristics during cook-off tests

In this laboratory, a cook-off device was designed and used to analyse combustion and explosion characteristics in fast and slow cook-off tests with different vent hole diameters. Using JO-9C as the experimental material, we measured cook-off reaction temperatures and analysed damage of restraint structures. Through the analysis of the crystal phase of the metal fracture surface, according to the characteristics of the crystal phase fracture surface of different fracture modes, the different conditions and different positions of the shell crushing load are analysed, combined with the change law of the reaction temperature, a reasonable explanation is given to the changing law of the response level. The results showed that the reaction temperature increased continuously with increasing vent hole diameter under fast cook-off conditions, and the reaction temperature difference between the sealed condition and the 7.0 mm vent hole was 45.4 °C. Under slow cook-off conditions, influenced by thermal convection, the diameter of the vent hole had little effect on the reaction temperature. For vent hole diameters that were smaller and larger than 16.0 mm, average reaction temperatures were 272.9 °C and 278.6 °C, respectively. The severity of the reaction first increased and then decreased with increasing vent hole diameter. The fast and slow cook-off reactions were the most violent when the vent diameters were in the ranges of 2.0–4.3 mm and 4.4 mm, respectively. Fragmentation of restraining shells and deformation of upper covers and restraining bolts were comprehensively evaluated. For fast cook-off tests, when the diameter was 5.5 mm, the reaction changed from partial detonation to deflagration. Under slow cook-off conditions, when the vent hole diameter was greater than 16.0 mm, the reaction changed from partial detonation to deflagration. When the vent hole diameter was greater than 20.0 mm, the reaction changed from deflagration to slow combustion.

Nickel-Titanium peripheral stents: can fracture mechanics shed light on their fatigue failure?

The major concern about Nickel-Titanium (Ni-Ti) stents, which are the gold standard in the treatment of occlusive peripheral disease, is fatigue and the consequent fracture in vivo. Indeed, their failure might be responsible for severe drawbacks, among which is the re-occlusion of the treated artery. Although many phenomenological approaches have been proposed to study this topic, the current literature lacks extensive knowledge on the Ni-Ti local damage mechanisms produced by the cyclic loads that promote crack nucleation and lead to the failure of thin struts, such as those of stents. Moreover, due to the super-elastic property of the alloy, the standard approach for interpreting the fracture of metals might be not accurate for this case. This work aims at increasing awareness of fatigue failure in superelastic Ni-Ti thin struts, such as those of stents. To do so, multi-wire specimens, sharing the same dimensions and thermo-mechanical treatment of the stent struts, were fatigue tested under different strain levels and the number of cycles to failure was recorded for each sample. Numerical simulations corroborated the experimental results to gain information on the local stress and strain fields during the fatigue cycles. A fracture mechanics-based fatigue model adopting the cyclic J-integral was here proposed, giving promising results for the interpretation of such failures.

Effectiveness of an additively manufactured porous layer in dissimilar solid-state bulk joining of additively manufactured maraging steel and conventional AISI410 steel

Solid-state bulk joining of additively manufactured maraging steel and commercially available martensitic stainless steel (AISI410) specimens in a cylindrical shape is accomplished through electrically assisted pressure joining (EAPJ). The cylindrical maraging steel specimen is fabricated to have a porous layer on the joining side by selective laser melting additive manufacturing. During EAPJ, the porous layer, which serves as an interlayer with locally increased electrical resistance due to geometrically induced defects (pores), significantly and locally increases the maximum temperature while the joining load is dramatically decreased. The microstructure evolution suggests that grain refinement occurs on both the maraging steel and AISI410 sides due to recrystallization. The high residual stress induced during additive manufacturing of maraging steel specimens is significantly released during EAPJ. Martensite formation in the AISI410 steel and the reverted austenite in the maraging steel are characterized by the grain average image quality. Tensile tests show that the fracture always occurs in the transition region between the heat-affected region and the unaffected base metal region. The present study demonstrates that bulk joining of additively manufactured components and conventional components can be more easily and effectively achieved with the use of an additively manufactured porous layer, even for dissimilar material combinations. • AMed maraging steel and conventional AISI410 steel were solid-state joined. • The temperature was locally controlled by the AMed porous layer. • The high residual stress of AMed maraging steel was released during joining. • Microstructure evolution during joining was characterized by EBSD. • Base metal fracture in AISI410 side occurred during tension test.

Fracture prediction in transverse fillet welded joints of high strength structural steel

Due to the huge property difference between high strength structural (HSS) steels and mild steels after welding, mechanical behavior and failure mode of fillet welded joints of HSS steels should be studied further. Numerical simulations can provide increased insight into this topic. This paper concerns finite element simulations of component tests of two types transverse fillet welded joints of HSS steels: lap-welded transverse fillet welds and cruciform type transverse fillet welds. In order to investigate the influences of triaxial stress states on fracture ductility of weld metals, a series of tests on specimens with various initial geometries are carried out. By comparing three typical uncoupled fracture models (stress triaxiality-dependent ductile fracture model (VGM), Lode angle-dependent ductile fracture model (MSSM) and combination of stress triaxiality and Lode angle ductile fracture model (BWM)), BWM gives precious prediction of ductile fracture of weld metals. The behaviors in terms of load-displacement and failure modes of transverse fillet welds are all well captured by the numerical simulations with calibrated ductile fracture models. Specifically, the location of fracture initiation and propagation of cracks obtained from simulations are both identical with real responses.

Microstructural evolution and fatigue performance of dissimilar solid-state joints of SUS316L and SUS410

Dissimilar stainless steels 316L (SUS316L) and SUS410 in a cylindrical butt configuration are solid-state joined along the cylinder axis by electrically assisted pressure joining (EAPJ). In EAPJ, different electric current densities are applied during compressive plastic deformation to investigate the effect of electric current density on joint performance. The microstructure analysis confirms that solid-state joints without macro- and micro-defects are fabricated through elemental diffusion and recrystallization during EAPJ. The effect of electric current density on the formation of martensite and subsequently on the variation of hardness is revealed by the grain average image quality maps and their relevant area distribution. The results of quasi-static tensile tests show that the fracture behavior of the joint is strongly affected by the applied electric current density. The fatigue behavior of joints with a higher tensile strength (SUS410 base metal fracture) is also evaluated by the S–N curve under a certain confidence level using two-parameter Weibull distribution. Finally, the fracture mechanism is investigated by examining the fracture surfaces from fatigue failure. The fracture morphology under high stress level exhibits a mixed fracture mode of base metal and interface fracture, while the fracture under low stress level exhibits interface fracture.

Design and development of aluminum alloy 6061-T6 pressure vessel liner for aerospace applications: A technical brief

This work mainly focuses on designing a novel aluminum alloy 6061-T6 pressure vessel liner intended for use in launch vehicles. Fabrication of custom-made welding fixtures for the assembly of liner parts, namely two hemispherical domes and end boss, is illustrated. The parts of the liner are joined using the cold metal transfer welding process, and the welding trials are performed to arrive at an optimized parametric range. The metallurgical characterization of weld joint reveals the existence of dendritic structures (equiaxed and columnar). Microhardness of base and weld metal was 70 and 65 HV, respectively. The tensile strength of base and weld metal was 290 and 197 MPa, respectively, yielding a joint efficiency of 68%. Finite-element analysis of a uniaxial tensile test was performed to predict the tensile strength and location of the fracture in base and weld metal. The experimental and predicted tensile test results were found to be in good agreement.

Experimental study of basalt fiber/steel hybrid laminates under low-velocity impact

Due to their excellent impact resistance, fibre-metal laminates are widely utilised in modern engineering industries such as the construction of aircraft, vehicles and ships. In this study, the low-velocity impact characteristics of basalt fibre/steel hybrid laminates with different lay-up structures were studied with a drop hammer. This was done through a series of low-velocity impact experiments with different hammer head shapes and impact energies. It can be seen from the results that the impact behaviour of hybrid laminates largely depends on the shape of the hammer head, impact energy and lay-up structure. The sharper the hammer head, the smaller the peak impact load and the greater the displacement. When the hammer head penetrates the laminate, apparent shear failure occurs under the impact of the flat hammer head. Petal-like fractures caused by tensile tear failure occur under the impact of the hemispherical and conical hammer head. Low-energy impacts may result in large plastic deformation, delamination and debonding in the layers. High-energy impacts may also lead to fibre and metal fractures as well as perforation damage. Consequently, the laminate absorbs the energy from overall deformation under low-energy impact, but mainly absorbs the energy generated by various fibre and metal fractures under high-energy impact.

Mesh objective simulations of large strain ductile fracture: A new nonlocal Johnson-Cook damage formulation for the Total Lagrangian Material Point Method

We present mesh objective simulations of large strain fracture of metals using a Total Lagrangian Material Point Method. The mesh objectivity is obtained by developing two nonlocal formulations of the well known Johnson-Cook damage criterion: an integral and a gradient enhanced formulation. Unlike previous nonlocal models in which the nonlocal variable is the plastic strain, we adopt the ratio of the plastic strain and the failure strain as the nonlocal variable. Using one simulation we demonstrate that the integral type nonlocal formulation results in incorrect damage initiation (i.e., not at the sample edge but within the sample). We then focus on the gradient enhanced formulation and use it to simulate three common benchmarks for quasi-static large strain ductile fracture of metals and one impact fracture of a metal plate hit by a blunt bullet. All the results exhibit mesh independence and are in good agreement with previous findings and experiments.

Methods for improving joint strength of friction stud welded AA5083 alloy joints

The joint strength and its improvement of AA5083 Al alloy joints fabricated by friction stud welding method were investigated. The diameter of the work and stud side specimens were 32.0 mm and 12.8 mm, respectively, and those were friction welded. The appropriate welding condition for obtaining high tensile strength was established as follows: a friction speed of 17.5 s−1, a friction pressure of 80 MPa, a friction time of 1.6 s, and a forge pressure of 360 MPa. However, all joints fractured between the initial weld interface and the work or stud sides, i.e. the fracture did not occur in the base metal. It could be considered that the initial oxide film on the weld faying surface of the work side was not exhausted as the flash during the welding process. To obtain the joint having the fracture in the base metal, the suitable shape at the weld faying portion of the work side specimen was suggested as the groove shape. The inner diameter of the groove corresponded to the same diameter of the stud side, and that had a groove width of 3.0 mm with a groove depth of 1.0 mm. As a conclusion, AA5083 friction stud welded joint, which had the tensile strength of the base metal and the fracture in the base metal, could make with the appropriate welding condition. Furthermore, the weld faying portion at the work side should be in the suitable shape that urges the extrusion of the flash from this side.

Modified Embedded‐Atom Method Potentials for the Plasticity and Fracture Behaviors of Unary HCP Metals

AbstractModified embedded‐atom method (MEAM) potentials have been widely used in molecular dynamics (MD) simulations to describe the interatomic interactions in metallic systems. However, conventional MEAM potentials are almost exclusively fit to a limited number of key single‐value properties, limiting the predictability of MD simulations, especially for complex hexagonal‐close‐packed (HCP) metals with multiple slip systems. Here, three MEAM potentials for unary HCP metals (Mg, Co, and Ti) are fit to conventional target properties, as well as continuous‐energy curves and their gradients obtained from density‐functional theory calculations. These targets include the lattice parameters, cohesive energy, elastic constants, and surface energies as well as the cohesive energy curve, basal plane decohesion energy curve, and generalized stacking fault energy curves for basal, prismatic, pyramidal I, and pyramidal II planes. These interatomic potentials demonstrate excellent reproduction of relevant properties and enable accurate MD simulations of volumetric and plastic deformations, as well as fracture in Mg, Co, and Ti HCP metals. Importantly, the interatomic potentials demonstrate better performance than all existing EAM/MEAM potentials readily available from the literature.

The role of a shear planar mesodefect in the nucleation of a crack at a grain junction due to athermal grain boundary sliding

Ductile fracture of polycrystalline metals is usually preceded by a long deformation stage, during which grains of polycrystal are gradually divided into mutually misoriented regions (fragments) separated by strain-induced grain boundaries. The size of these regions usually does not exceed 0.2 – 0.4 mm. At such small sizes of fragments, classical models of crack nucleation based on the concept of lattice dislocation pile-ups retarded by grain boundaries become incorrect. In recent years, models have been developed to describe the nucleation of cracks under the action of elastic stress fields of rotational and shear-type mesodefects formed at the grain junctions and grain boundaries due to inhomogeneous plastic deformation in the ensemble of polycrystal grains. In this paper we consider the possibility of the nucleation of microcracks at the grain junction due to athermal sliding along the grain boundary, containing a planar mesodefect of the shear-type induced by strain. It is assumed that a planar mesodefect, represented in the initial state by uniformly distributed glissile components of orientational misfit dislocations, loses its stability when the external stress exceeds a certain threshold value. As a result of the strain-induced grain boundary sliding and plastic shear retarding, a stress concentrator arises near the triple junction of grains, creating conditions for the appearance of a Zener crack. The dependences of the critical external stress for the nucleation of microcracks on the length of the mesodefect, its strength and the threshold stress of athermal sliding are obtained. It is shown that the presence of a mesodefect at the grain boundary can lead to a significant decrease of the crack nucleation stress in comparison with the case of pure grain boundary sliding. It is concluded that the proposed model can be considered as one of the possible mechanisms for the nucleation of microcracks in materials with a fragmented structure.

In Situ TEM tensile testing of bicrystals with tailored misorientation angles

Grain boundaries have complex structural features that influence strength, ductility and fracture in metals and alloys. Grain boundary misorientation angle has been identified as a key parameter that controls their mechanical behavior, but the effect of misorientation angle has been challenging to isolate in polycrystalline materials. Here, we describe the use of bicrystal Au thin films made using a rapid melt growth process to study deformation at a single grain boundary. Tensile testing is performed on bicrystals with different misorientation angles using in situ TEM, as well as on a single crystalline sample. Plastic deformation is initiated through dislocation nucleation from free surfaces. Grain boundary sliding is not observed, and failure occurs away from the grain boundary through plastic collapse in all cases. The failure behavior in these nanoscale bicrystals does not appear to depend on the misorientation angle or grain boundary energy but instead has a more complex dependence on sample surface structure and dislocation activity.

Dual-scale porosity effects on crack growth in additively manufactured metals: 3D ductile fracture models

The presence of microstructural defects resulting in unpredictable failure behavior has limited the use of additively manufactured (AM) metals in structural applications. In addition to the distributed porosity responsible for ductile fracture in conventional metals, larger defects (20–50 μm) can be introduced during additive manufacturing resulting in a dual-scale porosity failure process in AM metals. Here, we subject a three-dimensional, small-scale yielding boundary layer model containing a centerline crack to remote mode I K-field loading to study the effects of such dual-scale porosity on crack-defect interactions in an AM Ti-6Al-4V alloy. We model the background porosity implicitly within the fracture process zone viz. several rows of void-containing computational cell elements governed by the Gurson porous material relation, while the larger AM void defects are discretely represented based on size and porosity distributions of actual AM Ti-6Al-4V specimens characterized using optical and electron microscopy. Results show that AM defects can contribute to an increased apparent toughness of the AM metal over its conventional counterpart by activating isolated and/or clustered damage zones surrounding the crack, which shields and blunts the crack-tip and promotes crack tortuosity. However, the presence of planar clusters of AM defects can also accelerate crack growth and cause premature failure by forming preferential crack paths.