AL-6XN Stainless Steel Research Articles

Parameter characterization of the PBM-DEM method in numerical simulation of shallow buried mine explosions

Abstract To investigate the impact of various parameters on the numerical simulation of shallow buried mine explosions through the particle explosion method and discrete element method, this paper adopts a coupled method of particle blast method and discrete element method, namely the PBM-DEM method, to simulate the response of AL-6XN stainless steel plate under shallow buried mine explosions. This article first combines the experimental results of Børvik to validate the precision of the PBM-DEM approach. Then, the influence of particle quantity, interaction parameters between sand particles, and interaction parameters between sand particles and plate on the outcomes of the numerical simulations are analyzed. The analysis reveals that as the quantity of explosive particles rises, the central displacement and maximum kinetic energy of the plate escalate. In contrast, these values decrease as the number of sand particles increases. The interaction parameters between sand particles have almost no effect on the numerical simulation results. The damping and stiffness parameters between the sand particles and the plate have a pronounced effect on the numerical simulation results, and the center displacement and peak kinetic energy of the plate rise as the damping and stiffness parameters between the sand particles and the plate increase.

Strain Measurement during Quasi-Static and Cyclic Loads in AL-6XN Material Using Digital Image Correlation Technique.

A customized digital image correlation (DIC) system was implemented to monitor the strain produced in a cold-rolled AL-6XN stainless steel plate, 3.0 mm thick, subjected to quasi-static and cyclic loading tests. A comparison of the DIC strain measurements was made against those provided by conventional extensometers. Furthermore, the DIC system was used to monitor the fatigue crack initiation in low-cycle fatigue tests. The true stress-strain behavior for the AL-6XN material was properly captured by the DIC measurements. For low-cycle fatigue tests (strain control), the strain mapping generated by DIC allowed for identifying zones with higher strain than the nominal strain amplitude applied (εa) since the first stages of the fatigue life (FL). These zones become potential fatigue crack initiation sites.

Multiscale mechanical fatigue damage of stainless steel investigated by neutron diffraction and X-ray microdiffraction

Mechanical fatigue behavior of AL6XN stainless steel as a typical type of planar slip alloy was investigated by in situ neutron diffraction and synchrotron-based X-ray microdiffraction methods. Under cyclic loading at a high strain amplitude (±0.8%), the fatigue damage originated mainly from the accumulation of statistical stored dislocations, as clearly evidenced from a continuous increase in diffraction peak width with increasing the number of load cycles. However, under cyclic loading at a low strain amplitude (±0.3%), the density of statistical stored dislocations became saturated just after a hundred loading cycles and the fatigue damage was mainly dominated by the accumulation of persistent Lüders bands (PLBs) and the complex interactions among various PLBs as evidenced through X-ray microdiffraction measurements. It was further found that there exists obvious grain-orientation-dependent local damage in the low-strain-amplitude fatigued sample. In particular, fatigued grains orientated with [001] paralleling the loading direction are subjected to compressive stress and contain a large number of broad PLBs in boundaries arraying the edge dislocation pile-ups, which generate a large stress gradient leading to local plastic instability. The highly localized stress field at PLBs in the cyclically-deformed sample at a low strain amplitude may explain the obvious cyclic stress softening.

Evolution of dislocation loops in austenitic stainless steels implanted with high concentration of hydrogen

It has been found that under certain conditions, hydrogen retention would be strongly enhanced in irradiated austenitic stainless steels. To investigate the effect of the retained hydrogen on the defect microstructure, AL-6XN stainless steel specimens were irradiated with low energy (100 keV) H2+ so that high concentration of hydrogen was injected into the specimens while considerable displacement damage dose (up to 7 dpa) was also achieved. Irradiation induced dislocation loops and voids were characterised by transmission electron microscopy. For specimens irradiated to 7 dpa at 290 °C, dislocation loops with high number density were found and the void swelling was observed. At 380 °C, most of dislocation loops were unfaulted and tangled at 7 dpa, and the void swellings were observed at 5 dpa and above. Combining the data from low dose in previous work to high dose, four stages of dislocation loops evolution with hydrogen retention were suggested. Finally, molecular dynamics simulation was made to elucidate the division of large dislocation loops under irradiation.

On the calibration strategies of the Johnson–Cook strength model: Discussion and applications to experimental data

The present paper aims at assessing the various procedures adoptable for calibrating the parameters of the so-called Johnson–Cook strength model, expressing the deviatoric behavior of elastoplastic materials, with particular reference to the description of High Strain Rate (HSR) phenomena. The procedures rely on input experimental data corresponding to a set of hardening functions recorded at different equivalent plastic strain rates and temperatures.After a brief review of the main characteristics of the Johnson–Cook strength model, five different calibration strategies are framed and widely described. The assessment is implemented through a systematic application of each calibration strategy to three different real material cases, i.e. a DH-36 structural steel, a commercially pure niobium and an AL-6XN stainless steel. Experimental data available in the literature are considered. Results are presented in terms of plots showing the predicted Johnson–Cook hardening functions against the experimental trends, together with tables describing the fitting problematics which arise in each case, by assessing both lower yield stress and overall plastic flow introduced errors.The consequences determined by each calibration approach are then carefully compared and evaluated. A discussion on the positive and negative aspects of each strategy is presented and some suggestions on how to choose the best calibration approach are outlined, by considering the available experimental data and the objectives of the following modeling process. The proposed considerations should provide a useful guideline in the process of determining the best Johnson–Cook parameters in each specific situation in which the model is going to be adopted.A last section introduces some considerations about the calibration of the Johnson–Cook strength model through experimental data different from those consisting in a set of hardening functions relative to different equivalent plastic strain rates and temperatures. In particular, the opportunity of using experimental data coming from Taylor impact tests is assessed, together with an evaluation of the possibility of using other less popular and somehow innovative ways for obtaining the Johnson–Cook strength model parameters.

Corrosion of a stainless steel and nickel-based alloys in high temperature supercritical carbon dioxide environment

Corrosion of four alloys has been studied in supercritical carbon dioxide at 650°C and 20MPa, specifically AL-6XN stainless steel and three nickel-based alloys, PE-16, Haynes 230, and Alloy 625. The tests were performed for exposure durations of up to 3000h with samples being removed for analyses at 500h intervals. The corrosion performance of the alloys was evaluated by weight change measurements, and the surface oxide layers were characterized by scanning electron microscopy, X-ray diffraction and X-ray photoelectron spectroscopy. Weight gain measurements showed that the Al-6XN stainless steel exhibited the least corrosion resistance while the weight gains were nearly similar for the other alloys. The oxide layer in AL-6XN stainless steel was composed of large equiaxed grained outer layer of Fe3O4 (magnetite) and an inner layer of FeCr2O4. Oxide spallation was observed in this stainless steel even after 500h exposure. In all alloys, Cr-rich oxides phases of Cr2O3 and Cr1.4Fe0.7O3 were identified as the protective layers. In alloy PE-16 a thin layer of aluminum oxide formed that promoted the corrosion resistance of the alloy. Cr2O3 was identified as the main protective oxide layer in nickel base alloys Haynes 230 and 625.

Localization in stainless steel using microstructural based viscoplastic model

The plastic deformation behavior of AL-6XN stainless steel over a wide range of strain rates and temperatures is modeled using a combination of recently developed constitutive models VA-BCC [3,52] and VA-FCC [4,53]. In this regard, a general consistent thermodynamic framework for thermo-viscoplastic deformations of steel alloys is presented here along with an appropriate definition of the dissipation potentials and free energy, after considering the strain rate effect imbedded through the flow stress and hardening definition. Moreover, the additive combination of thermal and athermal yield function definitions are utilized in this framework for dynamic deformations of AL-6XN stainless steel.Finite element simulations are performed by implementing the proposed viscoplasticity constitutive models in the commercial finite element program ABAQUS. Numerical implementation for a simple tensile problem is used for validating the material parameters of the AL-6XN under low and high strain rates and temperatures. The numerical results of the adiabatic true stress–true strain curves compare very well with the experimental data. The effectiveness of the present approach is also tested by studying strain localization in plane strain problems. Results indicate excellent performance of the present framework in describing the strain localization problem and in obtaining mesh-independent results.

A discrete particle approach to simulate the combined effect of blast and sand impact loading of steel plates

The structural response of a stainless steel plate subjected to the combined blast and sand impact loading from a buried charge has been investigated using a fully coupled approach in which a discrete particle method is used to determine the load due to the high explosive detonation products, the air shock and the sand, and a finite element method predicts the plate deflection. The discrete particle method is based on rigid, spherical particles that transfer forces between each other during collisions. This method, which is based on a Lagrangian formulation, has several advantages over coupled Lagrangian–Eulerian approaches as both advection errors and severe contact problems are avoided. The method has been validated against experimental tests where spherical 150 g C-4 charges were detonated at various stand-off distances from square, edge-clamped 3.4 mm thick AL-6XN stainless steel plates. The experiments were carried out for a bare charge, a charge enclosed in dry sand and a charge enclosed in fully saturated wet sand. The particle-based method is able to describe the physical interactions between the explosive reaction products and soil particles leading to a realistic prediction of the sand ejecta speed and momentum. Good quantitative agreement between the experimental and predicted deformation response of the plates is also obtained.

Plastic Deformation and Damage Behavior of AL6XN Super-Austenitic Stainless Steels

Tensile and compressive deformation and damage behaviors of Al6XN super-austenitic stainless steels were examined at different strain rates. The deformation and fracture surfaces were characterized by scanning electron microscopy (SEM). It was found that the uniaxial deformation (tensile or compressive) behaviors of Al6XN stainless steel shows a low strain rate sensitivity over the range of 10-4s-1 - 10-2s-1. The tensile and compressive yield strengths measured are nearly comparable. The steel shows a good tensile plasticity. Dislocation slip deformation is the main characteristic of uniaxial deformation. All fracture surfaces induced by tensile deformation at different strain rates can be divided into two parts, i.e., fibrous zone and shear lip zone. The fibrous zone consists of dimples with a bimodal size.

Diffraction measurements of elastic strains in stainless steel subjected to in situ biaxial loading

The lattice strains in AL6XN stainless steel specimens subjected to in situ biaxial stress states have been measured by neutron diffraction. The biaxial stress states were generated in tubular specimens using a specially designed loading apparatus that is capable of applying axial loads to specimens under internal pressure. Lattice strains in the axial and hoop directions were measured for different levels of stress biaxiality in numerous loaded and partially unloaded states. The results revealed the role of the biaxial stress state in the differences in average lattice strains between various crystallographic fibers. These trends are examined in light of the orientational dependencies of the lattice strains on the stress biaxiality under an assumption of uniform stress. Possible additional factors contributing to the observed trends are discussed.

Fatigue of AL6XN Stainless Steel

Tension-compression, torsion, and axial-torsion fatigue experiments were conducted on the AL6XN alloy to experimentally investigate the cyclic plasticity behavior and the fatigue behavior. The material is found to display significant nonproportional hardening when the equivalent plastic strain amplitude is over 2×10−4. In addition, the material exhibits overall cyclic softening. Under tension-compression, the cracking plane is perpendicular to the axial loading direction regardless of the loading amplitude. The smooth strain-life curve under fully reversed tension-compression can be described by a three-parameter power equation. However, the shear strain-life curve under pure torsion loading displays a distinct plateau in the fatigue life range approximately from 20,000 to 60,000 loading cycles. The shear strain amplitude corresponding to the plateau is approximately 1.0%. When the shear strain amplitude is above 1.0% under pure shear, the material displays shear cracking. When the shear strain amplitude is below 1.0%, the material displays tensile cracking. A transition from shear cracking to tensile cracking is associated with the plateau in the shear strain-life curve. Three different multiaxial fatigue criteria were evaluated based on the experimental results on the material for the capability of the criteria to predict fatigue life and the cracking direction. Despite the difference in theory, all the three multiaxial criteria can reasonably correlate the experiments in terms of fatigue life. Since the cracking mode of the material subjected to pure torsion is a function of the loading magnitude, the prediction of cracking orientation becomes rather challenging.

An experimental investigation on fatigue crack growth of AL6XN stainless steel

The crack growth behavior of AL6XN stainless steel was experimentally investigated using round compact tension (CT) specimens. The influences of the R-ratio (the ratio of the minimum load over the maximum applied load in a cycle), the tensile and compressive overloads, and the loading sequence on crack growth were studied in detail. The results from the constant-amplitude experiments show a sensitivity of the crack growth rate to the R-ratio. The application of a tensile overload has a profound effect on crack growth, resulting in a significant retardation in the crack propagation rate. A compressive overload (underload) leads to a short-lived acceleration in crack growth. Results from the two-step high–low loading reveal a period of crack growth retardation at the beginning of the lower amplitude step, an effect similar to that of a single overload. A crack driving force parameter together with a modified Wheeler model is found to correlate the crack growth experiments well.

In-Situ Neutron Diffraction Study of the Behavior of AL6XN Stainless Steel Under Biaxial Loading

AbstractIn–situ neutron diffraction has been used to measure lattice strains parallel to two principal stress directions in biaxially-loaded AL6XN stainless steel. A new fixture was developed for loading thin-walled tubular specimens through combinations of internal pressure and axial loading. Under these conditions, the principal directions (σzz and σθθ in a cylindrical r, θ, z coordinate system) remain constant with respect to the initial crystallographic texture regardless of the level of biaxiality, a distinct advantage for diffraction experiments over the traditional tension/torsion tests for which this condition does not hold. Specimens were first pressurized to the level required to obtain a chosen value of σθθ. The axial load was then increased to reach the yield surface at different σθθ/σzz ratios, ranging from uniaxial to balanced biaxial loading (0, 0.4, 0.7, 1 according to Tresca). The {200}, {220}, {222}, and {311} reflections were measured in the axial and hoop directions as a function of axial load. A sequence of axial loading/unloading episodes was applied for different levels of plastic deformation. Under uniaxial tension, the {200} reflection showed the highest axial strains, followed by the {311}, and {220}/{222} reflections. With increasing internal pressure (biaxiality), the axial lattice strains corresponding to a given axial stress tended to decrease, and the responses of the various reflections tended to merge.

The response of clamped sandwich plates with lattice cores subjected to shock loading

The dynamic response of clamped circular monolithic and sandwich plates of equal areal mass and thickness has been measured by loading the plates at mid-span with metal foam projectiles. The sandwich plates comprise AISI 304 stainless steel face sheets and either AL-6XN stainless steel pyramidal core or AISI 304 stainless steel square-honeycomb lattice core. The resistance to shock loading is quantified by the permanent transverse deflection at mid-span of the plates as a function of projectile momentum. It is found that the sandwich plates have a higher shock resistance than monolithic plates of equal mass, and the square-honeycomb sandwich plates outperform the pyramidal core plates. Three-dimensional finite element simulations of the experiments are in good agreement with the experimental measurements. The finite element calculations indicate that the ratio of loading time to structural response time is approximately 0.5. Consequently, the tests do not lie in the impulsive regime, and projectile momentum alone is insufficient to quantify the level of loading.

Correlation of diffraction peak broadening to crystal strengthening in finite element simulations

Trends in the evolution of the distributions of crystal slip system strength and lattice strain were collected from finite element simulations of tensile loading of polycrystals. The loading programmes replicated physical experiments on two metallic alloys (AA5182 aluminum and AL6XN stainless steel) in which Bragg peaks were measured by neutron diffraction. Correlations between the broadening of the Bragg peaks and the evolution of slip system strength are demonstrated.

Plastic deformation modeling of AL-6XN stainless steel at low and high strain rates and temperatures using a combination of bcc and fcc mechanisms of metals

Combination of physically based constitutive models for body centered cubic (bcc) and face centered cubic (fcc) metals developed recently by the authors [Voyiadjis, G.Z., Abed, F.H., 2005. Microstructural based models for bcc and fcc metals with temperature and strain rate dependency. Mech. Mater. 37, 355–378] are used in modeling the plastic deformation of AL-6XN stainless steel over a wide range of strain rates between 0.001 and 8300 s −1 at temperatures from 77 to 1000 K. The concept of thermal activation analysis as well as the dislocation interaction mechanism is used in developing the plastic flow model for both the isothermal and adiabatic plastic deformation. In addition, the experimental observations of AL-6XN conducted by Nemat-Nasser et al. [Nemat-Nasser, S., Guo, W., Kihl, D., 2001. Thermomechanical response of AL-6XN stainless steel over a wide range of strain rates and temperatures, J. Mech. Phys. Solids 49, 1823–1846] are utilized in understanding the underlying deformation mechanisms. The plastic flow is considered in the range of temperatures and strain rates where diffusion and creep are not dominant, i.e., the plastic deformation is attributed to the motion of dislocations only. The modeling of the true stress–true strain curves for AL-6XN stainless steel is achieved using the classical secant modulus for the case of unidirectional deformation. The model parameters are obtained using the experimental results of three strain rates (0.001, 0.1, and 3500 s −1). Good agreement is obtained between the experimental results and the model predictions. Moreover, the independency of the present model to the experiments used in the modeling is verified by comparing the theoretical results to an independent set of experimental data at the strain rate of 8300 s −1 and various initial temperatures. Good correlation is observed between the model predictions and the experimental observations.

Thermomechanical response of HSLA-65 steel plates: experiments and modeling

To understand and model the thermomechanical response of high-strength low-alloy steel (HSLA-65), uniaxial compression tests are performed on cylindrical samples, using an Instron servohydraulic testing machine and UCSD’s enhanced Hopkinson technique. True strains exceeding 60% are achieved in these tests, over the range of strain rates from 10 −3/s to about 8500/s, and at initial temperatures from 77 to 1000 K. The microstructure of the undeformed and deformed samples is examined through optical microscopy. The experimental results show: (1) HSLA-65 steel displays good ductility and plasticity (strain > 60%) even at low temperatures (even at 77 K) and high strain rates; (2) at relatively high temperatures and low strain rates (especially below about 0.1/s), its strength is temperature-insensitive, indicating that the material has good high-temperature weldability; (3) slight dynamic strain aging (DSA) occurs at temperatures over 400 K and in the range of strain rates from 0.001/s to 3000/s, the maximum values of the stress shifting to higher temperatures with increasing strain rates; and (4) the microstructure of the material is not affected much by the changes in the strain rate and temperature. Finally, based on the mechanism of dislocation motion, and using our experimental data, the parameters of a physically-based model developed earlier for AL-6XN stainless steel [J. Mech. Phys. Solids 49 (2001) 1823] are estimated and the model predictions are compared with various experimental results, excluding the dynamic strain aging effects. Good agreement between the theoretical predictions and experimental results is obtained. In order to further verify the model independently of the experiments used for the evaluation of the model parameters, additional compression tests at a strain rate of 8500/s and various initial temperatures are performed, and the results are compared with the model predictions. Good correlation is observed. As an alternative to this model, the experimental data are also used to estimate the parameters in the Johnson–Cook model [Proceedings of the Seventh International Symposium on Ballistics, The Hague, The Netherlands, p. 541] and the resulting model predictions are compared with the experimental data, again excluding the dynamic strain-aging effects. These and related results suggest that the physically-based model has a better prediction capability over a broader strain rate and temperature range.

Microstructural banding and failure of a stainless steel

The presence of microstructural bands in AL-6XN stainless steel plate has been examined. The bands, which consist of a high density of second-phase particles, are located near the midthickness of the plate, range in thickness up to 300 µm, and are continuous over lengths up to 50 mm. Chemical analyses of the microstructural bands indicate elevated levels of chromium and molybdenum, while orientation imaging microscopy identified primarily sigma-phase particles within the bands; a small volume fraction of chi phase was also found. Tensile specimens oriented in the short transverse direction of the plate show low ductility and exhibit a large variation in failure strains, depending on the continuity of the bands as well as the presence of large precipitate particles within the bands. When oriented in either the longitudinal or the long transverse direction of the plate, circumferentially notched tensile specimens exhibit comparatively high ductility, although at high stress triaxialities, the material was susceptible to specimen splitting parallel to the tensile axis due to cracking along microstructural bands.

Thermomechanical response of DH-36 structural steel over a wide range of strain rates and temperatures

Abstract To understand and model the thermomechanical response of DH-36 Naval structural steel, uniaxial compression tests are performed on cylindrical samples, using an Instron servohydraulic testing machine and UCSD’s enhanced Hopkinson technique. True strains exceeding 60% are achieved in these tests, over the range of strain rates from 0.001/s to about 8000/s, and at initial temperatures from 77 to 1000 K. The microstructure of the undeformed and deformed samples is examined through optical microscopy. The experimental results show: (1) DH-36 steel displays good ductility and plasticity (strain > 60%) at low temperatures (even at 77 K) and high strain rates; (2) at relatively high temperatures and low strain rates (especially below about 0.1/s), its strength is not temperature sensitive, indicating that the material has good weldability; (3) dynamic strain aging (DSA) occurs at temperatures between 500 and 1000 K and in the range of strain rates from 0.001/s to 3000/s, the peak value of the stress shifting to higher temperatures with increasing strain rates (it is about 600 K at 0.001/s, about 650 K at 0.1/s, and about 800 K at 3000/s); (4) adiabatic shearbands develop when the strain exceeds about 30% at 77 K, and at higher strains for higher temperatures; and (5) the microstructural evolution of the material is not very sensitive to changes in strain rates and temperatures. Finally, based on the mechanism of dislocation motion, and using our experimental data, the parameters of a physically-based model developed earlier for AL-6XN stainless steel [J. Mech. Phys. Solids 49 (2001) 1823] are estimated and the model predictions are compared with various experimental results, excluding the dynamic strain aging effects. Good agreement between the theoretical predictions and experimental results is obtained. In order to further verify the model independently of the experiments used in the modeling, additional compression tests at a strain rate of 8000/s and various initial temperatures are performed, and the results are compared with the model predictions. Good correlation is observed. As an alternative to this model, the experimental data are also used to estimate the parameters in the Johnson–Cook model [G.R. Johnson, W.H. Cook, 1983, in: Proceedings of the Seventh International Symposium on Ballistic, The Hague, The Netherlands, p. 541] and the resulting model predictions are compared with the experimental data, again excluding the dynamic strain-aging effects. These and related results suggest that the physically based model has a better prediction capability over a broader strain rate and temperature range.

Adhesion of Stainless Steel to Fiber Reinforced Vinyl Ester Composite

Joints between stainless steel and composites have many applications. For example, marine vessels may be composed of a steel hull joined to a composite bow, stern, and/or topside structure. Joining these components adhesively is a major structural challenge, and overcoming this challenge necessitates the development of strong adhesion between steel and composites. The present study investigates AL-6XN stainless steel and glass fiber reinforced vinyl ester. Several specimens were manufactured with different surface preparations, adhesives, and/or additives. Short-term transverse tensile strength in dry conditions was used as the measure by which the different strategies were compared. All specimens were manufactured using a vacuum infusion technique, with the steel embedded within the fiber reinforcements before the molding. No secondary bonding was done.