Yield Stress Of Metals Research Articles

Effect of Repetitive High-Density Current Pulses on Plastic Deformation of Copper Wires under Stepwise Loading

High-density electric current pulses increase the plasticity and reduce the yield stress of metals with negligible heat generation. This effect has great potential for the development of energy-saving technologies for processing hard-to-deform metallic materials. Despite the long history of the study of electroplasticity, there is still no consensus on the physical nature of this effect. In this paper, the effect of repetitive pulses applied at the same or gradually increasing tensile stress on the plastic deformation of copper wire is investigated using a home-made experimental setup. The electroplasticity of the wire in the delivery state and after annealing is compared. It is shown that for a constant tensile stress, the incremental plastic deformation of the wire decreases with each successive pulse. This effect is stronger for annealed wires because they have a lower dislocation density and therefore a lower plasticity resource. The Joule heat release in the specimen and the heating due to plastic deformation are evaluated. The results obtained will be used to fit the parameters of the atomistic model being developed to describe the interaction of electron flow with dislocations.

The Production of Three-Dimensional Metal Objects Using Oscillatory-Strain-Assisted Fine Wire Shaping and Joining.

Material shaping and joining are the two fundamental processes that lie at the core of many forms of metal manufacturing techniques, including additive manufacturing. Current metal additive manufacturing processes such as laser/e-beam powder bed fusion and Directed Energy Deposition predominantly use heat and subsequent melt-fusion and solidification to achieve shaping and joining. The energy efficiency of these processes is severely limited due to energy conversion losses before energy is delivered at the point of melt-fusion for shaping and joining, and due to losses through heat transfer to the surrounding environment. This manuscript demonstrates that by using the physical phenomenon of lowered yield stress of metals and enhanced diffusion in the presence of low amplitude high frequency oscillatory strain, metal shaping and joining can be performed in an energy-efficient way. The two performed simultaneously enable a metal additive manufacturing process, namely Resonance-Assisted Deposition (RAD), that has several unique capabilities, like the ability to print net-shape components from hard-to-weld alloys like Al6061 and the ability to print components with a very high aspect ratio. In this study, we show this process's capabilities by printing solid components using aluminum-based metal alloys.

Quantification of the local mechanical behavior in dissimilar metal welds using digital image correlation instrumented cross-weld tensile testing

The local yielding behavior in base metal, heat-affected zone, fusion boundary region, and weld metal of low-alloy steel/Alloy 625 filler metal welds was quantified using digital image correlation instrumented cross-weld tensile test. The tested welds exhibited undermatching, matching, or overmatching weld metal yield strength with significant gradients in the local yielding behavior. An undermatching weld yielded at 69 MPa below the base metal yield stress, accumulating to 0.72% total strain. The base metal in an overmatching weld had 110 MPa lower yield strength than the weld metal. The strong strain hardening response in the Alloy 625 weld metal, within the uniform elongation range, and its constraining effect on the fusion boundary region and heat affected zone, led to extensive strain accumulation, necking, and final failure in the base metal of all tested welds. The yielding behavior of the tested welds was compared to stress-based criteria, utilizing minimum specified and as-delivered yield and ultimate tensile strength, and to strain-based criteria. The capability of digital image correlation instrumented cross-weld tensile testing to quantify local yielding and strain accumulation demonstrates potential application in proving conformity to stress-based and strain-based design criteria of dissimilar and matching filler metal welds.

Exact formula for the effective yield stress of metals with arbitrary stress–strain curves

Sufficiently thick ductile protective targets are perforated by rigid, pointed-nose projectiles in a hole enlargement process. Several recent studies have shown that the (spherical cavitation) effective yield stress YcS is an essential effective material parameter for analytical predictions of ballistic limit velocities. This effective yield stress is also found to be essential in determining ballistic equivalence between two different alloys of the same metal and between two different metals like aluminium and steel. First, the concept of the (spherical cavitation) effective yield stress is presented with comprehensive reference to previous studies, to emphasize the practical importance of this effective material parameter. Thereafter, an exact formula for YcS, which is based on the W−1(x) branch of the Lambert function, is derived and eliminates the need for solving the exact value of YcS by a numerical procedure. Moreover, while approximate expressions of YcS are exist for several stress–strain curves with definite yield stress, the new exact formula is applicable for any stress–strain relation, including stress–strain curves without a definite yield point and curves which take into account thermal softening effect.

Stress corrosion cracking failure of friction stir welded nuclear grade austenitic stainless steel

Stress Corrosion Cracking behavior of Austenitic Stainless Steels (SS) in a chloride environment has been investigated by several methods. In this investigation failure of emerging structural material for nuclear reactor (316LN grade austenitic stainless steel) and its Friction Stir Welded (FSW) joint due to stress corrosion cracking were analyzed in a chloride environment. FSW joint was fabricated between 316LN plates with optimized process parameters. Initially, hardness and tensile properties were analyzed across the weld zone to evaluate the mechanical properties of the welded joint. Microstructural characterization of base metal (BM) and weld zone were done by Optical Microscope (OM), and Scanning Electron Microscope (SEM). Grain boundary analysis was done by Transmission Electron Microscope (TEM). Microstructural evolution revealed that the FSW process is capable to refine and reduce grain size nearby 12 times that of the base metal by Continuous Dynamic Recrystallization mechanism (CDRX). Improved strength and hardness are also recorded at the weld zone compared to the base metal. Boiling 45 wt% MgCl2 solution at constant load condition as per ASTM standard G36 was used to assess SCC failure on base metal and FSW joint. Failure due to Chloride Stress Corrosion Cracking (CSCC) on weld samples were investigated and compared with its base metal samples at different tensile stress conditions (20%, 40%, 60%, 80%, 100% of base metal yield stress values). From the experimental result steady-state elongation rate (ISS), Transition time (tss) and time to failure (tf) were evaluated and generalized equations to predict time to failure of base metal and FSW weldment was generated. The impact of microstructural morphology on SCC failure of the FSW joint was investigated. The fracture location of the weld joint, nature of crack advancement of both base metal and FSW joint was analyzed by the SEM study. A study of fracture surface displayed brittle nature transgranular failure for base metal as well as FSW samples.

Thermofluctuational Fracture Mechanism in Solids in the Presence of an Active Medium

The paper addresses an approach to the physical notion of failure of solids in the presence of an active medium (a destructive compound), which is based on two successive steps. The first one is to go from considering a body as an elastic or viscoelastic continuum to studying it as an atomic-molecular system. The second step that follows the investigation of the atomic structure is to take into account the influence of atomic motion in a solid during the study of mechanical properties of solids (at high temperatures, in a wide range of loading speeds, under cyclic and vibrational loads), which is based on the experimental data on ultimate strength and yield stress of metals. A mechanism of crack initiation in a loaded metal under the action of destructive compounds is discussed. The capillary and diffusional propagation of a molten liquid-metal compound along grain boundaries and structural defects in a solid metal in combination with the viscous flow of the melt over its surface are shown to dictate the crack propagation into metal structure. The pattern of the affecting destructive factor has been clarified.

An experimental and numerical study of forces and residual stresses in AISI 316L stainless steel joints due to conventional and pulse gas tungsten arc welding

This paper aims to present the results of a numerical and experimental study of the temperature field, internal forces and the residual stresses in 2 mm thick autogenous welds of AISI 316L stainless steel produced by continuous and pulse current gas tungsten arc welding. A special experimental device was used to measure the temperature and the internal forces due to the welding. The welds were qualified for internal and external weld imperfections according to ISO 15614-1. FEM software ANSYS® Multyphysics™ was applied in order to solve the thermal and mechanical problems. Normal residual stresses were measured by the hole-drilling strain gauge method in the continuous current weld. The peak value of the longitudinal stress was 80 % of the base metal yield stress. The magnitude of the numerically obtained residual stress values was found to be 16 % to 19 % above the measured one in the longitudinal and transverse direction, respectively. The experimental device used in this study allowed for a real time measurement of forces far from the weld seam. On the basis of the correspondence between the calculated and measured forces the numerical results were verified. Therefore, this device might open up new possibilities for determining thermo-mechanical material data.

Effect of metal inert gas welding on the behaviour and strength of aluminum stiffened plates

This paper investigated the effect of a 3D simulated metal inert gas (MIG) welding induced heat affected zone (HAZ), residual stress and distortion fields on the behaviour of aluminum stiffened plates under compressive loading. A two-step, thermo-structural finite element model was developed for the simulation of the welding process and the model was verified using the available experimental results. The welding induced HAZ, residual stress and distortion were then studied for tee-bar stiffened aluminum plates of various geometries. The effect of these welding induced imperfections on the load vs. shortening curves, buckling mode, and the post-buckling behaviour of each stiffened plate geometry were investigated. It was found that welding induced tensile and compressive residual stresses ranged from 72 to 77% and 18–36% of the base metal yield stress, respectively. The width of the HAZ around the weld line increased as the plate slenderness increased. The reduction in buckling strength of the tee-bar aluminum stiffened plates due to the presence of the HAZ and residual stress was as much as 10% and 16.5% respectively.

A physically based yield criterion I. Determination of the yield stress based on analysis of pre-yield dislocation behaviour

In this paper a physically based method to determine the yield stress of metals from tensile curves is proposed. The criterion is based on the identification of the transition in dislocation behaviour from limited reversible glide in the pre-yield stage, without essential changes in the dislocation structure, to dislocation multiplication in the post-yield stage. This transition point is a clearly distinguishable feature in a plot of the hardening rate vs. the flow stress (a Kocks-Mecking plot). Literature shows that this is a common feature shared by metallic alloys under different circumstances. The present paper shows, on the basis of stress-cycling experiments, that the pre-yield behaviour, i.e. at stresses below the observed transition point, is explained by reversible dislocation behaviour. Schoeck's classical theory of this anelastic deformation is extended to give a quantitative description of dislocation behaviour in the pre-yield stage. The extended theory provides a very accurate description of pre-yield deformation and can be used to determine structural parameters that characterise the initial dislocation structure.

Use of the Richtmyer-Meshkov Instability to Infer Yield Stress at High-Energy Densities

We use the Richtmyer-Meshkov instability (RMI) at a metal-gas interface to infer the metal's yield stress (Y) under shock loading and release. We first model how Y stabilizes the RMI using hydrodynamics simulations with a perfectly plastic constitutive relation for copper (Cu). The model is then tested with molecular dynamics (MD) of crystalline Cu by comparing the inferred Y from RMI simulations with direct stress-strain calculations, both with MD at the same conditions. Finally, new RMI experiments with solid Cu validate our simulation-based model and infer Y~0.47 GPa for a 36 GPa shock.

EFFECT OF WELD PARAMETERS ON MECHANICAL PROPERTIES OF THE FRICTION STIR WELDING AA6063-T5

Effect of tool rotation speed and welding speed on the mechanical properties of the FSW joint of AA6063-T5 was investigated. The influences of the ratio of rotation speed and weld speed on the weld thermal cycle, hardness distribution, and tensile strength of the FSW joint were clarified. The experimental results showed the lower the ratio of rotation speed and weld speed, the lower weld heat input, the narrower the softened zone and the higher tensile strength were. In all cases, the tensile fracture located outside the stirred zone, in the retreating side or advancing side, where the hardness was lowest. It was also found that the residual stress in and around the welded zone was quite low, in range of ten percent of base metal yield stress.

Thermomechanical Stresses in Copper Films at Elevated Temperature

Thermomechanical stresses in metallic films are a root cause for material fatigue which limits the lifetime of electronic devices. Since the yield stress of metals is temperature dependent, plastic deformations during thermal cycling are increased at elevated temperature. This effect reduces the reliability of electronic parts. In order to investigate this problem, a 20 μm thick copper film was deposited on a silicon wafer. After annealing at 400°C, the sample was exposed to thermal cycles in the temperature range between room temperature and 600°C. The different values for the coefficient of thermal expansion of copper and silicon lead to a curvature of the sample. The wafer curvature was measured by a multilaser beam method. On the basis of the experimental results, a new theoretical model was developed that describes the stress evolution in the film during thermal cycling. In this investigation, the relation between wafer curvature and film stress is calculated by analogy to a model by Freund which is an improvement to the well-known Stoney formula. In addition to the elastic response, the new model considers plasticity of the copper film as well as temperature dependence of creep. It is demonstrated that the model can describe the experiment well and thus thermomechanical stress in copper films.

球状圧子を用いた微小押込み試験による金属材料の降伏応力の同定法

球状圧子を用いた微小押込み試験による金属材料の降伏応力の同定法

Thermomechanical Stresses in Copper Films at Elevated Temperature

Thermomechanical stresses in metallic films are a root cause for material fatigue which limits the lifetime of electronic devices. Since the yield stress of metals is temperature dependent, plastic deformations during thermal cycling are increased at elevated temperature. This effect reduces the reliability of electronic parts. In order to investigate this problem, a 20μm thick copper film was deposited on a silicon wafer. After annealing at 400°C, the sample was exposed to thermal cycles in the temperature range between room temperature and 600°C. The different values for the CTE of copper and silicon lead to a curvature of the sample. The wafer curvature was measured by a multi-laser beam method. On the basis of the experimental results, a new theoretical model was developed, which describes the stress evolution in the film during thermal cycling. In this investigation, the relation between wafer curvature and film stress is calculated by analogy to a model by Freund [1] which is an improvement to the well known Stoney formula. In addition to the elastic response, the new model considers plasticity of the copper film as well as temperature dependence of creep. It is demonstrated that the model can well describe the experiment and thus thermomechanical stress in copper films.

137 球状圧子を用いた微小押込み試験による金属材料の降伏応力の定量的固定手法(材料力学II,一般講演)

137 球状圧子を用いた微小押込み試験による金属材料の降伏応力の定量的固定手法(材料力学II,一般講演)

Effect of Weld Quality and Postweld Improvement Techniques on the Fatigue Resistance of Extra High Strength Steels

High strength steel (HSS) used in lifting equipment structure and metallic framework are known to have high static strength, good stiffness and very good weldability however knowledge of fatigue behaviour of HSS in welded structure is not sufficient in order to have representative and probabilized S-N curves to modify the design codes. In this study, a large fatigue test campaign was performed on cruciform and symmetrical butt welded joints of high and very high strength structural steel plates. Numerous fatigue tests were carried out for each configuration in as welded condition. To improve the fatigue performance, post-welding improvement techniques (burr grinding and TIG dressing) were performed on the welded specimens. All fatigue results were analysed using a statistical approach defined in the Eurocode 3 standard. It was noticed the impact of weld quality on fatigue performance. The application of post-weld improvement techniques (TIG dressing and burr grinding) leads to an additional improvement of the fatigue strength and reduces the scattering (COV), in comparison with the current welding standards or recommendations. The limited influence of the base metal yield stress on fatigue performance is explained by the local microstructure at the weld toe where fatigue cracks initiate; at this location, the mechanical properties are similar whatever the steel grades.

The role of a stressed state in the formation of stress-corrosion flaws in pipelines

In recent years, as a result of performance of intensive in-tube flaw detection, the integrity and reliable operation of trunk pipelines have been achieved. However, because of an unpredictable development of such tube flaws as stress corrosion, the probability of occurrence of emergencies in pipelines substantially increases. An analysis of the stressed state in pipelines exposed to their main load—the operating pressure—and various types of additional forces is presented. As a result, it is shown that the obtained direction, value, and location of the effective maximum stresses in the tube wall fully correspond to the pattern of statistical formation of stress-corrosion flaws in pipelines. It can be concluded from the data of this study that the standardized tube-strength margin is insufficient; during tube operation, this circumstance may lead to the formation of stresses at the level of the metal yield stress and initiation and growth of stress-corrosion cracks.

Neck retardation and enhanced energy absorption in metal–elastomer bilayers

Retardation of necking under biaxial stretching of bilayer plates comprised of an elastomer layer bonded to a metal layer is studied. Substantial increases in necking limits and consequent energy absorption can be achieved in metal–elastomer bilayers for both quasi-static and dynamic stretching. The phenomena is tied to the fact that under stretching the incremental modulus of the elastomer remains essentially unchanged, or increases, while the incremental modulus of the metal steadily decreases. The effective incremental modulus of the bilayer decreases with stretching but at a lower rate than the metal itself. Since necking instabilities are associated with an erosion of the incremental modulus, necking in the bilayer is delayed to larger strains. Although the strength of a bilayer having the same mass/area as an all-metal plate is reduced, it can nevertheless absorb more energy than the metal plate if the ratio of the elastomer modulus to metal yield stress is sufficiently large. The first part of the paper derives necking limits and energy absorption capacities for bilayers under quasi-static biaxial stretching. The second part of the paper analyses axisymmetric neck development in clamped circular bilayers subject to impulsive pressure loads. The ability of the bilayer to sustain intense impulses is compared to the performance of metal plates of the same material and total mass. Outstanding issues requiring further study are discussed.

Deformation Behavior of Ultrafine Grained Iron

Work hardening behavior and microstructure development during deformation by cold rolling were investigated in iron with different grain size. Grain refinement makes the introduction of dislocation easier. For instance, under the same deformation condition (5% reduction in thickness), dislocation density is the order of 1014m-2 in a coarse grained material (mean grain size; 20μm), while it reaches 7×1015m-2 in an ultrafine grained material (0.25μm). It is well known that the yield stress of metals is enlarged with an increase in dislocation density on the basis of the Bailey-Hirsch relationship. However, it should be noted that the ultrafine grained material never undergoes usual work hardening although the dislocation density is surely enhanced to around the order of 1016m-2: 0.2% proof stress is almost constant at 1.4 ~ 1.5GPa regardless of the amount of deformation. The dislocation density of 1016m-2 is thought to be the limit value which can be achieved by cold working of iron and the yield stress of iron with this dislocation density (ρ) is estimated at 1.1GPa from the Bailey-Hirsch relationship; σd [Pa] = 0.1×109 + 10 ρ1/2. On the other hand, yield stress of iron is enhanced by grain refinement on the basis of the Hall-Petch relationship; σgb [Pa] = 0.1×109 + 0.6×109 d-1/2 as to the grain size d [μm]. This equation indicates that the grain size of 0.35 μm gives the same yield stress as that estimated for the limit of dislocation strengthening (1.1GPa). As a result, it was concluded that work hardening can not take place in ultrafine grained iron with the grain size less than 0.35 μm because dislocation strengthening can not exceed the initial yield stress obtained by grain refinement strengthening.

Simulation of hydride-induced steady-state crack growth in metals ? Part II: general near tip field

Hydride-induced steady-state crack propagation in metals is investigated under conditions of constant temperature, plane strain, small-scale yielding and small-scale hydride precipitation, by taking into account the coupling of the operating physical processes. It is shown that the near-tip field depends on a normalized stress intensity factor, which incorporates both effects of the applied stress intensity factor and the crack velocity. According to Part I of the present study, when the normalized stress intensity factor tends to zero, the crack-tip field near the threshold stress intensity factor is produced, which is characterized by a constant hydrostatic stress in the hydride precipitation zone. As the value of the normalized stress intensity factor increases, the evolution of the near-tip field for crack propagation from stage-I to stage-II regime is produced: the actual size of the hydride precipitation zone decreases, the hydrostatic stress increases, deviating from the level of the plateau, and the near-tip field tends to that of a hydrogen-free metal. The near-tip field depends strongly on hydrogen concentration, far from the crack tip. The stage-II crack growth velocity is predicted and the experimentally observed effect of metal yield stress and temperature on crack velocity is confirmed.