Combined mode I-mode III fracture of a high strength low-alloy steel

Special formulations of theJ integral were used to quantify the combined mode I-mode III fracture behavior of ASTM A710 Grade A steel, a tough material which displays elastic-plastic behavior at room temperature. Experimental fracture initiation loci for two heat-treated conditions were linear inJ _i-J_iii space, agreeing with analytical predictions based on linear elasticity. As commonly observed in mode I fracture behavior, large tearing modulus values accompanied largeJ values for mode I and mode III components, individually as well as for totalJ values. There was a pronounced tendency toward antiplane shear flow in the modified compact tension specimens tested. This may result from the concentration of mode III shear strains in the plane of the propagating crack as predicted by slip line theory. Shear fractures with the crack propagating at inclined angles to both load line and plate free surfaces are favored energetically over pure mode I cracks even though the total surface area formed by the latter is much smaller. formerly Graduate Research Assistant, The Ohio State University     

Combined mode I-mode III fracture toughness of a spherodized 1090 steel

The aim of this investigation was to determine the fracture behavior of a spherodized 1090 steel under combined mode I-mode III loading conditions. Suitably defined formulations of the J integral denoted J_ic and J_iiic were used to characterize the elastic-plastic fracture of this steel. As the mode III component in the system is increased, the resolved mode I J integral at initiation decreases, its mode III counterpart increases and the total J value remains nearly a constant. This implies a constant energy requirement for fracture initiation under mixed mode loading. As the crack plane becomes less inclined to the load line, the slopes of the mode I and total J resistance curves increase from their pure mode I values until a crack inclination angle of about 65° is reached. Somewhere in the region of 65-55°, a maximum in these values is reached and they fall off rapidly for larger mode III components. This drop is accompanied by the breakup of the crack front into mode I and mode III steps, which is shown to be an energetically more favorable process for this steel.     

Stress corrosion cracking in high strength steel under mode III loading

Stress corrosion cracking (SCC) of high-strength steel in aqueous environment and hydrogen induced cracking (HIC) during dynamic charging under Mode III loading were investigated. The threshold stress intensities for SCC and HIC under Modes III and I were measured and compared. It was found that both SCC and HIC under Mode III loading initiated and propagated on the planes inclined at 45 deg to the notch plane, differing from that under Mode I loading. The fracture surfaces, however, revealed intergranular facets, similar to that under Mode I loading. The addition of thiourea decreased the threshold value for SCC under Mode III and Mode I loading, which was still higher than that for dynamic charging. The threshold values of both SCC and HIC under Mode III were larger than that under Mode I,i.e., K_IIIH> K_IH, K_IIISCC > K_ISCC. Based upon the fracture mechanics analysis, this difference is attributed to the different equilibrium hydrogen concentration between Modes III and I loading. These results give strong evidence that the SCC mechanism in high strength steel under Mode III loading is also related to hydrogen induced cracking. Formerly Student at Beijing University of Iron and Steel     

Effects of microstructure on fracture toughness of a high-strength low-alloy steel

Fracture toughness and ductile-brittle transition behavior were measured for a copper-bearing HSLA steel. The value ofK _lc for cleavage failure was independent of heat treatment, whileJ _lc for ductile failure decreased monotonically with increasing strength level. With both failure modes, fracture appears to be controlled by cracking of sulfide inclusions. The decrease in ductile-failureJ _lc is caused by decreased work-hardening rates that suppress cleavage and facilitate void coalescence. Both higher austenitizing temperature and quenching rate after austenitization influence the ductile/brittle transition temperature, either through grain-size and precipitate refinement or through an increase in the resistance of the steel to shear failure. Formerly Graduate Student, The Ohio State University     

Mixed mode I/III fracture toughness of an experimental rotor steel

Mixed mode I/III toughness tests were performed on a Ni-Cr-Mo-V rotor steel that exhibited very high mode I toughness. The minimum toughness occurred under mixed mode loading conditions. The influence of mode III loading on total toughness is analyzed in terms of incompatibility effects at particles ahead of the crack tip.     

超厚高强钢心部组织对断裂机制的影响

摘要：为了研究超厚钢板心部显微组织与微观断裂机制的关系,利用扫描电子显微镜（SEM）、透射电子显微镜（TEM）和电子背散射衍射（EBSD）等技术手段对试验钢板1/4厚度、1/2厚度（心部）显微组织及裂纹形貌进行表征和分析。结果表明,心部处低温冲击韧性较1/4厚度处有明显的降低,这是由于心部组织为回火板条贝氏体和回火粒状贝氏体,其中粒状贝氏体的比例较高,粒状贝氏体中M-A岛的存在促进了微孔或微裂纹的启裂;1/4厚度处为回火板条马氏体与回火板条贝氏体二者的混合组织,板条间的位错运动释放局部应力集中,而舒缓裂纹尖端区域所受应力,产生裂纹钝化、阻碍裂纹扩展,提高低温冲击韧性。     

Effect of center microstructure on fracture mechanism of ultra-thick high strength steel

In order to study the relationship between microstructure and fracture mechanism of ultra-thick steel plate, scanning electron microscopy (SEM), transmission electron microscopy (TEM) and electron backscatter diffraction (EBSD) were used to characterize and analyze the microstructure and crack morphology of 1/4 and 1/2 thickness (center) of the test plate. The results show that the low temperature impact toughness at the center is significantly lower than that at the 1/4 thickness, which is due to the microstructure of the center is tempered lath bainite and tempered granular bainite, and the proportion of granular bainite is higher. The presence of M-A island in granular bainite promotes the initiation of micro pores or micro cracks. The 1/4 thickness is a mixed microstructure of tempered lath martensite and tempered lath bainite. The dislocation movement between laths releases local stress concentration, and relieves the stress at the crack tip region, resulting in crack passivation, hindering crack propagation and improving low temperature impact toughness.     

Evaluation of the fracture toughness of hot-rolled low-alloy Ti-V plate steel

The effects of varying finish rolling temperature (FRT) and cooling rate on the mechanical properties of hot rolled plates of an experimental low-alloy Ti-V steel were studied. Fracture toughness was evaluated for various types of specimens at slow and high deformation rates. However, the transition temperatures determined by the various tests do not always correlate. Therefore, it is recommended that fracture toughness be evaluated by both static and dynamic testing. Following a low cooling rate, the best plate properties are obtained at the lowest FRT in the austenite-ferrite range, although occurrence of delaminations at this temperature may be detrimental for specific applications. Higher cooling rates produce higher strength but lower toughness than lower cooling rates in plates with the same FRT.     

Warm deformation and fracture behaviour of DP1000 advanced high strength steel

The effect of temperature has been investigated on the mechanical properties and fracture behaviour of DP1000 advanced high-strength steel. The variation of the mechanical properties depending on temperature has been determined by performing uniaxial tensile tests at the temperatures of 25, 100, 200, 300°C at the rolling directions of 0° (rolling), 45° (diagonal), and 90° (transverse) at the strain rate of 0.0083?s^?1. The yield strength and tensile strength have showed a slight decrease tendency between 25 and 200°C, but the highest value has been reached by showing an increase at 300°C temperature. The amount of elongation has not been affected significantly with the increase of the temperature. While hardening coefficient has increased due to the rising temperature, no effect has been observed on strength coefficient between 25 and 100°C, but an increase has occurred at higher temperature values.     

高强度无取向电工钢疲劳性能及断裂机制

测试了高强无取向电工钢的S-N曲线,并借助光学显微镜、扫描电子显微镜、透射电子显微镜分析了实验钢组织,疲劳断口形貌和位错结构.结果 表明:室温条件下,频率为20 Hz,应力比R为0.1,循环107周次时,实验钢的疲劳强度为360 MPa,疲劳裂纹萌生于实验钢的次表面,裂纹萌生点附近有沿晶开裂现象,疲劳裂纹扩展区域有解理台阶与疲劳条纹,瞬间断裂区是韧性断裂,有大量韧窝.实验钢在循环应力作用下基体中产生了大量位错,并有驻留滑移带终止在晶界位置.     

Structure, properties, and fracture of a low-alloy steel in the submicrocrystalline state

Experimental data on the structure and properties of 09G2S low-alloy steel with a submicrocrystalline (SMC) structure has been obtained; the structure was formed under various regimes of severe plastic deformation by equal-channel angular pressing. The possibility of increasing the strength characteristics and cold resistance of a ferrite-pearlite steel with a SMC structure is shown. The factors responsible for the increase in the brittle fracture resistance of an SMC material are found by studying fracture micromechanisms and fracture-surface micromorphological parameters as functions of the structure.     

先进高强度汽车用钢氢致延迟断裂研究进展

随着汽车、机械装备、航空航天、船舶等工业领域的快速发展,降低成本、提性减重、实现节能减排已成为各个行业为之努力而奋斗的目标,而高强度钢材是实现汽车工业产品轻量化和安全性的重要依托。然而,随着汽车用钢强度的不断提升,氢致延迟断裂问题越发突出,已经成为阻碍高强度汽车用钢广泛应用的一个关键因素。主要介绍先进高强度汽车用钢(热成形钢、双相(DP)钢、淬火-配分(Q&P)钢)最新的研究成果,并对其氢致延迟断裂特性、机制和测试评价技术方法等方面做了简要介绍,为相关领域的研究人员提供参考。     

Dynamic fracture behaviour of simulated weld heat affected zone of a high strength low alloy steel

Fracture toughness properties of a simulated weld coarse grain heat affected zone (CGHAZ) in a newly developed 785 MPa grade high strength low alloy (HSLA) steel were investigated at different loading rates and a series of temperatures. It is indicated that the fracture toughness K_Ic is a single-value function of the parameter ΔH=kT In (A/ε?), implying that just like the plastic deformation, the event of fracture initiation is also controlled by dislocation thermal activation movement.     

700MPa冷轧低合金超高强钢的典型连续退火工艺

为了开发新一代冷轧低合金超高强钢,利用连续退火实验机对Ti-0．12％、Nb-0．076％的冷轧低合金超高强钢进行连续退火实验,设计了760~830℃四种不同退火温度,研究了退火温度对实验钢的相组成、晶粒尺寸和力学性能的影响．在800℃退火、400oC过时效的条件下,可得到铁素体和少量贝氏体的组织,铁素体晶粒尺寸约为1．4μm,屈服强度可达700MPa．同时利用扫描电镜和透射电镜观察到钢中存在大量纳米尺寸的亚晶结构、少量位错以及纳米级的Ti、Nb的析出物．这些微结构单元对强度有较大的提升作用．     

On failure mode of resistance spot welded DP980 advanced high strength steel

Abstract

This paper aims at investigating the correlation between metallurgical and mechanical characteristics of DP980 dual phase steel resistance spot welds with attention focused on the transition in failure mode from interfacial to pullout mode during the tensile shear test. It was studied whether the conventional/industrial weld size criteria can produce pullout failure modes for DP980 spot welds. Based on theoretical analysis and experimental results, there is a minimum fusion zone size which beyond that the interfacial failure mode is avoided. Results showed that by increasing welding current, pullout failure mode was promoted due to increasing the fusion zone size and encouraging martensite tempering in subcritical heat affected zone (HAZ). Fusion zone size was proved to be key factor controlling mechanical properties of DP980 welds in terms of peak load, ductility and failure energy.

Ce document vise à examiner la corrélation entre les caractéristiques métallurgiques et mécaniques de soudures par points par résistance de l’acier biphasé DP980, en mettant l’accent sur la transition du mode de défaillance de mode d’interface à mode par arrachement lors de l’essai de traction-cisaillement. On voulait savoir si les critères conventionnels ou industriels de taille de la soudure pouvaient produire des modes de défaillance par arrachement des soudures par points de DP980. L’analyse théorique et les résultats expérimentaux indiquent qu’il y a une taille minimale de la zone de fusion au-delà de laquelle on évite le mode de défaillance d’interface. Les résultats ont montré qu’en augmentant le courant de soudage, le mode de défaillance par arrachement était favorisé, à cause de l’augmentation de la taille de la zone de fusion et de l’encouragement du revenu de la martensite dans la HAZ sous critique. On a démontré que la taille de la zone de fusion constituait une composante clé contrôlant les propriétés mécaniques des soudures de DP980 en ce qui a trait à la charge de pointe, à la ductilité et à l’énergie de défaillance.     

冷轧双相超高强钢氢致延迟断裂门槛应力研究

以抗拉强度为1 000 MPa级的汽车用冷轧双相钢为对象,采用电化学充氢和恒载荷拉伸试验方法,研究了材料在不同应力和充氢条件下的延迟断裂行为以及材料延迟断裂门槛应力与氢浓度之间的关系.结果表明,在充氢条件下材料延迟断裂时间随着应力的增大而缩短,延迟断裂的门槛应力随着充氢电流密度增大而下降,延迟断裂门槛应力与临界可扩散氢浓度的对数呈线性关系.     

Tempering behaviour of a dual-phase low-alloy steel

The tempering behaviour of a dual phase steel of 0.08% C, 1.21% Mn, 1.00% Si, 0.42% Cr, and 0.41% Mo composition with two different martensite contents of 30 and 52%. (obtained by intercritical treatments at 820 and 860°C, respectively) has been studied. The ultimate tensile strength decreased and percentage elongation increased continuously with increasing tempering temperature up to 600°C for both intercritical treatments. The yield strength has, however, increased up to 300°C, beyond which it decreased for the steel with 30% martensite. In contrast it remained almost constant for 52% martensite up to 300°C, beyond which it decreased. The martensite of dual-phase steel for both the intercritical treatments has undergone microstructural changes on tempering that are akin to those of fully martensitic low carbon steels. The SEM fractographs from the as-quenched specimens indicate that the tensile specimens failed by microvoid coalescence with the martensite areas appearing facetted and featureless while those for 600°C tempered condition by the formation of equiaxed dimples.