High-strength Steel Specimens Research Articles

Predicting failure initiation at V-notch tips in an AISI 4340 alloy by a continuum damage model

Predicting the fracture in quasi-brittle structures composed of steel alloys near V-notch tips presents a significant challenge. We investigate to which extent the continuum damage approach predicts the failure load FcExp when applied to experiments performed on AISI 4340 alloy specimens tempered at two different temperatures and containing three different V-notch opening angles loaded to fracture under a four-point bending configuration.A series of experiments was performed for the characterization of the material parameters and fracture surface (including triaxiality) for the continuum damage model coupled with the J2 theory of plasticity. Subsequently, this model was implemented within a finite element framework to compute the predicted failure load, termed FcFE. Sensitivity analyses were performed to identify the influence of notch radii and the representation of the supports on FcFE.A strong correlation was observed between FcFE and FcExp for all configurations for both tempering temperatures, with predicted loads falling within the range of experimental observations for all V-notch angles. Additionally, a linear relationship was identified between the increase of FcFE and V-notch radii. These non-linear analyses have also revealed that for these high-strength steel specimens, it is crucial to model supports as linear elastic, as rigid supports led to underestimated FcFE.The presented methodology has shown to be superior in predicting failure loads in these quasi-brittle steel structures compared to other more simplified linear approaches.

In-situ neutron imaging of delayed crack propagation of high strength martensitic steel under hydrogen embrittlement conditions

This paper presents an in-situ observation, using neutron imaging, of delayed crack propagation in a high-strength martensitic steel specimen. Delayed cracking is believed to be caused by hydrogen embrittlement occurring due to the slow diffusion and accumulation of hydrogen ahead of a crack front, causing decreased ductility and eventual cracking under constant load. The experiment involved mechanical loading of a single-edge-notch bend specimen while submerged in an electrolyte solution (H2O + 3.5% NaCl) under cathodic polarization to facilitate hydrogen ingress. Intermittent crack propagation was observed for 12 h after the environment had been removed. The stress state at each crack configuration was extracted from a three-dimensional elastic–plastic finite element simulation, which was tailored to match the quantitative information acquired from the neutron radiographs of the fracture process. To gain insight into the evolution of hydrogen concentration with crack propagation, a modeling scheme for stress-assisted hydrogen diffusion was also employed.

Fracture prediction and damage evolution of Q690 HSS under various stress states

The high strength and light weight of the high strength steel (HSS) make it extensively used. However, the reduction in ductility often leads to prominent fracture issues. The Q690 steel, with a yield strength greater than 690 MPa, is one type of Chinese HSS, and its fracture behavior, especially under complex stress states, is a growing concern. Therefore, this study tests a series of monotonic tensile specimens of Q690 HSS with various stress states to obtain the fracture results. Subsequently, a new Lode-dependent enhanced Lemaitre (NLEL) model is proposed. This model integrates the damage effect of the Lode parameter into the original Lemaitre model, enhancing the accuracy of fracture prediction specifically under shear-dominated stress scenarios. Comparative analysis among the Lemaitre model, NLEL model, and another Lode-dependent enhanced Lemaitre (LEL) model reveals that the proposed NLEL model delivers more comprehensive fracture prediction across all tested specimens, including smooth round bars, notched round bar, flat grooved plates, pure-shear plates as well as tensile-shear plates. Furthermore, this study delves into the analysis of damage evolution under representative stress states and initiates discussions regarding the coupled damage model.

I+II mixed mode fatigue crack propagation under combination of tensile loading and corrosion in a HSS specimen

Both fatigue and corrosion are two phenomena that can be very often found in metallic components. Thus, it is necessary to study their mutual effect. In this paper, a high strength steel specimen was modelled via finite element method in order to investigate the interaction of a fatigue crack and a corrosion pit. Particularly, a rectangular specimen with a corrosion pit and a nearby angled crack under remote tensile loading was modelled and the crack deflection angle was investigated via MTS and SED criteria for various crack lengths and various crack inclination angles. The results obtained clearly show how the crack behavior is affected by the presence of the corrosion pit.

Corrosion vs. fatigue in a high-strength steel specimen investigated via FEM

The work is devoted to investigations of the stress distribution/concentration in a corroded specimen loaded via remote tensile loading. Existence of corrosion pits of various size and various mutual distance affects stress distribution in a specimen which can have influence on its lifetime. Thus, numerical simulations via finite element method were performed in order to assess the stress field near corrosion pits in specimens made of high-strength steel. The geometry of the numerical model was suggested based on the dimensions of the real specimens produced for fatigue experiments. The results obtained are discussed and mutual comparison with experimental data is intended in oncoming months.

Mechanical properties and microstructure characteristics of wire arc additively manufactured high-strength steels

Additive manufacturing is a revolutionary emerging technology in the manufacturing industry. Wire arc additive manufacturing (WAAM) is a metal additive manufacturing technology suitable for the production of medium to large steel elements of structure. The aim of this study is to investigate the microstructural and mechanical behavior of high-strength steels produced by WAAM through experimental program. The test specimens were additively manufactured from three types of feedstock wires (ER100S-G, ER110S-G, ER120S-G). The microstructure characteristics of WAAM high-strength steels were identified. The tensile coupon specimens were extracted from flat regions of the WAAM runway-shaped steel tubes at five orientations θ = 0˚, 30˚, 45˚, 60˚, 90˚ with respect to the print direction to investigate the anisotropy of mechanical properties. Both the milled and as-built specimens were included to study the impact of geometric irregularity. The dimensions of the as-built specimens were measured using 3D laser scanning and the geometric characteristics were discussed. A total of 60 tensile tests were conducted with the assistance of digital image correlation (DIC) system in order to investigate the stress–strain responses of WAAM high-strength steels. Details of test program, including the specimen design, test setup and key test results, are presented. The anisotropy of WAAM high-strength steel specimens in terms of both the geometries and mechanical properties are discussed.

A lode-dependent plasticity model for high-strength structural steel

In this study, a new plasticity model was developed to characterize the mechanical behavior of high-strength structural steel under multiaxial stress states. This model utilizes a Lode-dependent yield criterion in conjunction with a non-associated flow rule and isotropic hardening law to determine the evolution of the plastic flow stress. Further, a detailed numerical implementation algorithm for this plasticity model is presented, including the integration algorithm of the constitutive equations using an implicit elastic predictor-return mapping method and formulation of the consistent tangent modulus. To validate the proposed plasticity model, six types of Q690 high-strength steel specimens were tested under monotonic loading. These specimens can be used to obtain the elastoplastic response of the material under uniaxial tension, compression, pure shear, tensile shear, and plane strain state, respectively. Corresponding numerical simulations were performed for each specimen using ABAQUS/Standard with the proposed model being introduced into the analysis via the UMAT user subroutine. Then, the prediction performance of the proposed plasticity model was investigated by comparing the load-displacement curves of each test specimen obtained from both experiments and numerical simulations. It is shown that the proposed plasticity model can accurately predict the mechanical response of the high-strength steel under different stress states and enables to visualize the key stress state parameters and material strength variation at the failure region of each specimen, which confirms the promising prospect of this new model in practical applications.

High-cycle fatigue performance of corroded Q690E high strength steel and Q690qENH high-strength weathering steel

High-strength steel and high-strength weathering steel have been increasingly used in engineering constructions. Corrosion leads to a decrease in the fatigue strength of steel, posing a threat to the safety and reliability of the engineering project. The fatigue strength reduction caused by corrosion usually depends on the type of steel and the corrosion time. In this study, indoor acetic salt spray accelerated corrosion test (ASS) and high-cycle fatigue test were carried out on Q690E high-strength steel and Q690qENH high-strength weathering steel, obtaining the time-varying laws of corrosion damage, the composition of corrosion products, and the fatigue life under different corrosion times. The results show that in the corrosion test process, the uniform corrosion depth C and the pitting depth Sz of both types of steel conform to a power function model. In contrast, other pitting parameters conform to a linear model. On the surface of the Q690qENH high-strength weathering steel specimen, α-FeOOH was generated first, which hinders the progress of corrosion. During the fatigue test, under the same stress range, the fatigue life decreases with the increase in corrosion time. For the corroded specimens, the corrosion parameters (C,Sz) were negatively correlated with the fatigue life linearly. This study can provide a reference for the design and use of high-strength and high-strength weathering steel.

Fatigue crack growth behaviour of wire arc additively manufactured steels

Combining welding technology with robotics, wire arc additive manufacturing (WAAM) is emerging as a viable method of construction. To facilitate its wider application, in particular with structural integrity in mind, an improved understanding of the fatigue properties of WAAM materials is needed. Hence, an experimental investigation into the fatigue crack growth (FCG) behaviour of WAAM plates made of normal- and high-strength steels has been conducted and is reported herein. Following material characterisation, FCG tests on 13 machined compact tension specimens were undertaken, extracted in different directions from WAAM plates built using a parallel deposition strategy. Fractography of the tested specimens was also performed to analyse the mechanisms of crack growth. The material testing demonstrated an approximately proportional relationship between the hardness and ultimate strength. From the FCG tests, Paris’ law constants were derived for the WAAM normal- and high-strength steel specimens, revealing consistently lower material constants m for the latter. Similar FCG behaviour to that of equivalent, conventionally-produced steels, with no significant anisotropy, was found. The observed response was shown to be generally well captured by the FCG laws given in BS 7910, with the recommended curve for unwelded steels providing a close match to the experimental data and the recommended curve for welded steels providing conservative predictions. Additional FCG data on WAAM steels printed using the parallel and oscillatory deposition strategies were collected and analysed. It was shown that the two deposition strategies led to similar crack growth rates for WAAM normal-strength steels, but distinct FCG behavioural trends for WAAM high-strength steels. Finally, the fractographic analysis showed that the WAAM specimens exhibited principally brittle behaviour during FCG but somewhat ductile behaviour before fracture.

Elastic-Wave Characteristics from Crack Initiation and Propagation of High-Strength Steel Immersed in Acetic-Acid Solution

In this study, elastic waves were detected when different bending stresses were applied to cracked specimens of high-strength steel (SKD11: HV550) immersed in a 0.057 M solution of acetic acid (CH3COOH), and frequency characteristics were analyzed using time-frequency analysis. The dominant frequency obtained using the tensile test was approximately 103 kHz, and those in the acetic-acid solution without stress were approximately 32 and 101 kHz. The dominant frequencies of the crack specimens in which cracks propagated were approximately 30–40 (F1), 60–85 (F2), and 100–110 (F3) kHz. An elastic wave was obtained by corrosion, pitting, crack initiation, and propagation but not during the hydrogen aggregation time. The dominant frequencies of the crack specimens without crack propagation were approximately 28–33 (F1) and 94–109 (F3) kHz. These were the same as the dominant frequency in the acetic-acid solution under nonstress conditions. The fractured surface showed many traces of pitting and corrosion regardless of the applied stress, resulting in microcracks in the Cr carbide. -----------------------------------------------------------------------------------------------------------

Microstructure and mechanical properties of wire-fed arc-based directed energy deposition of high-strength steel after ultrasonic impact oxidation treatment

The ultrasonic impact oxidation treatment (UIOT) was applied to the local region of a high-strength steel specimen fabricated via a wire-arc directed energy deposition (DED) process. The microstructure and shear-punch strength of the specimen were investigated. The effects of temperature on the UIOT were also studied. Results show that UIOT produces an oxide layer on the treated surface and the elements in the pin are transferred to form the oxides. UIOT at room temperature results in the highest element transfer amount from the pin. The UIOT-induced oxides become tiny oxide inclusions dispersed in the DEDed specimen. Contents of Al, Si, Ti, Mn, and O of the inclusions are significantly higher than those in the deposited metal and the Mn-deplete zone is generated near the inclusion, which has the favorite conditions for the nucleation of acicular ferrite. The UIOT results in finer grains in the treated region than those in the untreated region, and it decreases the strength of the treated material while increasing the equivalent yielding-to-tensile ratio.

Extension of methods for estimating the fatigue strength of components made of ultra-high strength steels

Various high and ultra-high strength steels are investigated experimentally by tensile and fatigue testing. The results are used to extend existing methods for the estimation of cyclic material behaviour for cases in which a limited amount of experimental data is available. Furthermore, fatigue tests of component-like high strength steel specimens are performed, including the determination of residual stresses and fractographic investigations of failure-relevant inclusions. The results serve to adapt and apply a proof of structural durability based on the local strain approach for components made of ultra-high strength steels.

Mechanical Property Model of Q620 High-Strength Steel with Corrosion Effects

High-strength steel (HSS) is widely used in engineering structures, due to its superior material performance, but corrosion tends to occur in steel structures with time. The corrosion effects on mechanical performance of Q620 HSS were investigated experimentally. The electrochemical accelerated corrosion test was conducted to generate corroded Q620 HSS specimens (ρ = 0~60%). With increases in the corrosion degree, corrosion performance became more and more non-uniform. The tensile coupon test was conducted to clarify mechanical properties of corroded Q620 HSS specimens. With increases in corrosion degree, fy, fu, εu and E decreased, correspondingly. The effect of corrosion on εy could be ignored. With the deepening of corrosion, the necking of Q620 HSS specimens was weakened, which reduced their ductility. The simplified constitutive model consisting of nominal yield point (εy, fy) and ultimate point (εu, fu) was proposed to quantify the mechanical properties of Q620 HSS with different corrosion degrees. After the numerical fitting, relationships between the corrosion degree and mechanical properties were clarified. Based on the results of numerical fitting, mechanical properties of corroded Q620 HSS specimens were worse than those of specimens with idealized uniform corrosion. The adverse effect of corrosion on εuc was more obvious than that on strength properties. Comparison among different mild steels and HSSs was performed. Different indexes were chosen to clarify corrosion effects on the ductility of corroded Q620 HSS specimens. This study considers and discusses the research on corrosion rates, the relationships between service time, service environment, corrosion form and strength properties of Q620 HSS.

Investigation of block shear strength of high strength steel bolted connections

This study presents an experimental and numerical investigation of block shear strength of high strength steel (HSS) bolted connections under double shear. A total of 16 HSS (grades Q690 and Q960) and eight mild steel (MS) (grade Q345) bolted connection specimens with various geometric configurations were tested and failed in block shear failure mode. The test and finite element (FE) analysis results confirmed that the block shear failure is a combination of fracture of the net tension plane and yielding of the effective shear plane, which is located between the net and gross shear planes. The effects of material properties (such as ductility and tensile-to-yield strengths ratio (fu/fy)) of steel materials on the block shear strength were comprehensively investigated based on the validated FE models. It was found that the tensile stress across the net tension plane could be fully developed irrespective of the steel grade due to effective stress redistribution along the net tension plane and the beneficial effect of the biaxial tensile stress state. However, the shear stress magnitude along the shear plane is influenced by the ductility and fu/fy ratio of steel materials and the length of the shear plane. The low ductility and fu/fy ratio of HSS materials did not allow for effective stress redistribution along the long shear plane and the shear yielding could only partially develop along the shear plane at the ultimate state. For the HSS specimens with short shear planes, the shear stress could develop up to the shear yield strength in general. Finally, the accuracy of the existing design equations in various design codes (i.e., prEN 1993–1–8: 2021, ANSI/AISC 360–16, and CSA S16–19) and relevant literature for predicting the block shear strength of bolted connections was evaluated and design recommendations were provided.

Local and Global Interactive Buckling Capacity of Thin-Walled Box-Section Members of BS700 High-Strength Steel under Axial Compression

The local and global buckling capacity of the thin-walled box-section members formed by cold bending of ten BS700 high-strength steel (HSS) specimens are experimentally determined through an axial compression test. The mechanical properties of the materials are evaluated through material performance test, and a material model suitable for finite element analysis is proposed. The initial geometric imperfection of the members is measured, and it is found that the maximum initial deflection can be 1/1000 of the length of the members, as specified in the Chinese code. Based on the test, the ultimate bearing capacity and failure modes of the component with local and global interactive buckling are obtained. Further, a finite element model is established, and the corresponding results are compared with the test results. Furthermore, the test results are compared with those obtained using the existing specifications. The results show that the failure modes of the specimens are primarily local and global buckling failure. The influence of residual stress and initial geometric imperfection is considered in the proposed finite element model. Comparing the ultimate bearing capacity and load-displacement curves with the corresponding test results, it is found that the finite element model can effectively reproduce the test results. By comparing the test results with those obtained based on the steel structure design codes of China, the United States, and Europe, it is found that the test results are all higher than the existing code results, and the Chinese and European codes are relatively conservative with a difference of more than 20%, while the difference between the test results and the American code results is nearly 10%. Therefore, it is necessary to further improve the calculation methods of local and global buckling capacity of thin-walled box-section members of BS700 HSS under axial compression.

Ultra-low cycle fatigue performance of Q690 high-strength steel after exposure to elevated temperatures

High-strength steel (HSS) can reduce steel consumption compared to ordinary steel and has extensive applications in the building industry. HSS members in building structures are inevitably subjected to fire. The elevated temperature caused by fire leads to significant changes in the service properties of HSS. Therefore, research on the service properties of HSS after a fire is of great significance when evaluating whether the HSS structure can continue to serve. A series of studies on the ultra-low cycle fatigue (ULCF) performance of Q690 HSS after fire was carried out to evaluate the ability of Q690 HSS to resist strong earthquakes after fire. Q690 HSS specimens were subjected to heat treatment at 600–900 °C, and ULCF tests with different strain amplitudes were carried out on the cooled specimens. The hysteretic curves, fatigue life, and total energy dissipation density of the Q690 HSS after the fire were obtained. On this basis, the stress and strain evolution laws under the ULCF load of Q690 HSS, with different fatigue strain amplitudes and exposed temperatures, were analysed. Subsequently, the metallographic structure and ULCF fracture behaviour of Q690 HSS after fire were discussed. Based on the test results, two sets of independent prediction equations were proposed to evaluate the ULCF performance of Q690 HSS after a fire, which was in good agreement with the test results.

A comparative study of ultrasonic impact cladding of steel surface using titanium alloy pin

The ultrasonic impact cladding (UIC) of steel surface using titanium alloy pins is comparatively studied. The cross-sectional appearance, roughness, hardness, and corrosion properties of EQ70 high-strength low-alloy steel specimens after UIC were compared to those after ultrasonic impact treatment (UIT) with steel pins. And the effects of substrate strength and pin diameter on the UIC layer were investigated. Results show that the specimens treated by the UIC have lower roughness and better corrosion resistance than the UIT-treated specimen. The hardness of the pin, as well as the hardness and strength of the treated material, has a decisive effect on the thickness and element content of the cladding layer. The prerequisite for generating a cladding layer on the steel surface with the UIC is that fragments should be produced from the pin under the impact.

Model description of hydrogen-induced damage evolution in high-strength steel weld metal based on results of a novel test method

In this paper, the development of a novel method for testing welded tensile specimens under two-stage external tensile loading is presented. The test method is intended to investigate the susceptibility of submerged arc welded high-strength steel specimens of type S690QL-S3Ni2.5CrMo to hydrogen-induced damage. Multiple cracking events within one specimen prove that the locally prevailing boundary conditions decisively determine the hydrogen-induced damage sequences.By means of SEM and EBSD analyses on specimens after test completion, interrelationships about the microstructural damage behavior are being examined. The crack tip environments are of particular importance here, since complex stress and strain conditions tend to cause hydrogen accumulation and further propagation of inter- and transgranular cracks here. In the vicinity of crack tips, retained austenite is of special importance. This metastable phase might undergo a deformation-induced transformation into crack-sensitive martensite and simultaneously release high local hydrogen contents as a former hydrogen trap. Finally, a model description including various sections of the stress- and hydrogen-induced damage evolution is derived from the empirical-analytical results.

Effects of exposure temperature on low–cycle fatigue properties of Q690 high–strength steel

Fire brings lasting effects on mechanical performance of structural steel. For the high strength steel (HSS) structures experiencing fire while do not collapse, it is necessary to evaluate the service performance after fire, including the ability to resist earthquake. During earthquake, the low–cycle fatigue load is applied on the structural steels. In this paper, the low–cycle fatigue properties of Q690 HSS exposed to different elevated temperatures are investigated experimentally. Totally, five exposure temperature levels (25, 600, 700, 800 and 900 °C) and four fatigue strain amplitudes (1%, 1.5%, 2% and 4%) are considered. After the heating test, the appearance characteristics and microstructures of Q690 HSS specimens are discussed. Later, the fatigue test is conducted and results show that the hysteretic curve tends to be fuller due to the increase in plastic strain when the exposure temperature ranges from 700 to 900 °C. Based on the test results, effects of exposure temperature and strain amplitude on cycles to failure and total dissipated energy density are analyzed. Besides, stress and strain coefficients are selected to analyze the stress and strain properties of post–fire Q690 HSS under low–cycle fatigue loading. Finally, a fatigue model is proposed to quantify the relation between total strain amplitude and cycles to failure, where the coefficient β and exponent a of Q690 HSS exposed to different elevated temperatures are determined by test results, after which the predictive equations are suggested to the quantify effects of exposure temperature on β and a.

Interaction of a fatigue crack and a corrosion dimple in a high-strength steel specimen

Both corrosion and fatigue crack propagation are the most common phenomena degrading metal/steel structures. Therefore, an extensive study is performed on this topic. Namely, results of a numerical analysis made on 2D model of a rectangular specimen containing a corrosion pit near a short edge crack that is arbitrarily oriented towards the cyclic tensile loading. Thus, I+II mixed mode conditions are created and the behavior of the crack is investigated by means of the concept of the equivalent stress intensity factor range. Discussion on effect of various parameters (such as crack length, corrosion pit size, crack inclination angle) on fatigue crack propagation is presented and important conclusions, that can help in further numerical/experimental analyses, are summarized.