Fracture behaviour and numerical study of resistance spot welded joints in high-strength steel sheet

Cross tension tests of resistance spot welded joints with varying nugget diameter were carried out using 980 MPa high strength steel sheet of 1.6 mm thickness. In proportion, as nugget diameter increased from 3√t to 5√t (where t is thickness), cross tension strength (CTS) increased while fracture morphology simultaneously transferred from interface fracture to full plug fracture. In cases of interface fracture, circumferential crack initiation due to separation of the corona bond arose at an early stage of loading. The crack opening process without propagation was recognized until just before fracture and then the crack propagated to the nugget immediately in a brittle manner around CTS. In full plug fracture, main ductile crack initiation from the notch-like part at the end of sheet separation occurred with the sub-crack initiated at an early stage. The ductile crack propagated toward the HAZ and base material to form full plug fracture. The mode I stress intensity factor was considered as a suitable fracture parameter because the circumferential crack behaved pre-crack for brittle fracture in the nugget region at the final stage. Based on the FE analysis, the mode I stress intensity factor was calculated as 116 MPa √m at CTS as fracture toughness for the nugget. With respect to full plug fracture, ductile crack initiation behaviour from the notch-like part was expressed by concentration of equivalent plastic strain. On the assumption that the ductile crack arose in critical value of equivalent plastic strain, the value was calculated as 0.34 by FE analysis. Reasonable interpretation for interface fracture and full plug fracture in the resistance spot welded joint was proposed due to first crack initiation by stress concentration, brittle fracture by using mode I stress intensity factor, and ductile crack initiation by using equivalent plastic strain.     

A model of heating a flux-cored wire in arc metallizing and analysis of the structure of the coating

Summary

This paper describes HAZ‐notched CTOD tests of multipass welds in SMYS = 420–460 MPa class high‐strength steels for offshore structural applications. The weld metal strength overmatch causes different fracture behaviour depending on the actual CGHAZ toughness. When the CGHAZ is completely embrittled, the weld metal strength overmatch leads to the lower bound critical CTOD value. This is due to elevation of the local stress in the CGHAZ caused by the restraint effect of the overmatched weld metal. The fracture surface is generally flat, and brittle fracture originates from the CGHAZ sampled by the fatigue crack front. A larger fraction of the CGHAZ along the crack front gives a smaller critical CTOD value. When the CGHAZ has moderate toughness, however, the weld metal strength overmatch may produce a higher critical CTOD value at brittle fracture initiation. This is due to crack growth path deviation towards the base metal. Plastic deformation preferentially accumulates to a greater extent on the softer base metal side before the critical stress conditions for brittle fracture initiation occur in the CGHAZ. This asymmetrical plastic deformation promotes deviation of ductile crack growth from the crack tip CGHAZ. In this case, the critical CTOD value does not always reflect the CGHAZ toughness itself.

A notch location nearer the weld metal sometimes causes fracture initiation in the weld metal if the fatigue crack tip samples the CGHAZ. Such experimental data do not reflect the real CGHAZ toughness.

The significance of the critical CTOD value obtained in the tests must be determined in the fracture toughness evaluation of the weld CGHAZ. This paper presents a procedure for evaluation of CTOD test results obtained for HAZ‐notched welds that considers the strength mismatch effect.     

EFFECT OF CREEP FRACTURE TOUGHNESS ON CRACK INITIATION AND GROWTH

本文研究了蠕变断裂韧性对二种低合金耐热钢蠕变裂纹开裂和扩展的影响。试验表明:随着蠕变断裂韧性提高,抗蠕变裂纹开裂和扩展能力增加。材料呈韧性或脆性状态时,蠕变裂纹萌生和扩展过程不同。韧性状态时,裂纹为穿晶和晶界二种混合形式:穿晶裂纹可在晶内碳化物处发生,或在晶界上形核后向晶内扩展,晶界裂纹仍是由晶界上空洞形成和相互连接而成,裂纹可沿晶界和晶内扩展,但不连续。脆性状态时,裂纹沿晶界发生,它是由晶界形成空洞和相互连接而成,扩展仅沿晶界发生。     

Untersuchungen zur transkristallinen und interkristallinen Spannungsrißkorrosion austenitischer Chrom-Nickel-Stähle in heißen Magnesiumchlorid-Lösungen

Investigations Into transgranular and intergranular stress corrosion cracking of austenitic stainless steels In hot magnesium chloride solutions The stress corrosion cracking (SCC) of austenitic stainless steels in hot magnesium chloride solutions is known to be transgranular. Therefore the slip-step-dissolution model is most favourable when explaining the failure mechanism. Constant load and constant extension rate tests (CERT) show that both methodes are almost equivalent. Moreover constant extension rate tests in more concentrated magnesium chloride solutions at 135°C reveal a small potential range of intergranular stress corrosion cracking more negative than the range of transgranular SCC. Observations of crack nucleation and crack propagation make plain that crack nucleation is a localized corrosion process. Pitting produces crack nucleis in the elastic range whereas cracks start along slip lines after plastic deformation. Fractography of specimens which failed by intergranular and transgranular SCC show macroscopically brittle fracture surfaces. Therefore a model is proposed which explains crack propagation by hydrogen-induced intermitted cracking at high-stressed sites at the crack tip.     

EFFECT OF CREEP FRACTURE TOUGHNESS ON CRACK INITIATION AND PROPAGATION UNDER CREEP FATIGUE INTERACTION

本文研究了蠕变断裂韧性对三种低合金Cr-Mo-V钢在蠕变一疲劳交互作用下裂纹开裂和扩展的影响.试验表明,材料韧性对裂纹开裂和扩展起重要作用,脆性状态时,裂纹开裂时间比蠕变时短,二者间的裂纹扩展速度无明显差别;韧性状态时,裂纹开裂时间不仅比蠕变时短,而且其裂纹扩展速度比蠕变时大得多.此外,低合金Cr-Mo-V钢经蠕变一疲劳交互作用后有脆化倾向,其脆性程度取决于钢的原始韧性值.韧性状态时,蠕变-疲劳交互作用显著促使三种Cr-Mo-V钢由韧性向脆性转变;而在脆性状态时,这种脆性转变不明显。     

Localised cyclic plastic deformation on translamellar fracture surfaces in a P/M γ-TiAl-based alloy

Large numbers of fine parallel steps are generated on translamellar fracture surfaces during slow crack growth (da/dN≤5.0×10⁻⁶ mm/cycle) under cyclic loading. These are seen only rarely during fast crack growth (da/dN≥1.0×10⁻⁴ mm/cycle) and are not seen during catastrophic fracture. These steps occur due to intense plastic deformation as a result of cleavage on {111} planes along twin boundaries and dislocation bands. Such action may promote an inherent resistance to crack growth owing to higher energy dissipation. TEM examinations show that the intense deformation structure consists of both microtwins and dislocation bands if the lamellae are orientated to allow easy glide. Conversely, microtwin activity dominates when easy dislocation glide is prevented. Crack initiation and growth resistance are sensitive to such microscopic features within the deformation zone. Lamellar volume fraction, the proportion and distribution of γ grains inside lamellae and lamellar interfacial strength may all influence the local plastic deformation through their interaction with underlying slip and twinning processes.     

Ductilisation of tungsten (W) through cold-rolling: R-curve behaviour

Here we show that cold-rolling of tungsten (W) decreases the stable crack growth onset temperature. Furthermore, we show that stable crack growth is accompanied by crack bridging, which in turn is triggered by dislocation activity. The entire stable crack growth regime shows ductile intergranular fracture.Our ductilisation approach is the modification of microstructure through cold-rolling. In this work, we assess two different microstructures obtained from (i) cold-rolled and (ii) severely cold-rolled tungsten plates. From these plates, single-edge cracked-plate tension (SECT) specimens were cut and tested in the L-T direction. Crack growth resistance (R) curves were obtained using the direct-current-potential-drop method (DCPM). The experiments show the following results: cold-rolled plates are brittle at room temperature (RT), but show stable crack growth at 250 °C (523 K) and a fracture toughness, K_IQ, of about 100 MPa(m)^1/2 at a crack extension, Δa, of 0.6 mm. Severely cold-rolled tungsten plates show stable crack growth at RT and a fracture toughness, K_IQ, of 100 MPa(m)^1/2 at a crack extension, Δa, of 0.3 mm. Scanning electron microscopy (SEM) analyses of the stable crack growth region show intergranular fracture with microductile character.The question of why cold-rolling causes the stable crack growth onset temperature to decrease (or in other words, why cold-rolling causes the brittle-to-ductile transition (BDT) temperature to decrease) is discussed against the background of (i) intrinsic and extrinsic size effects, (ii) crystallographic texture, (iii) impurities and (iv) the role of dislocations. Our results suggest that the spacing between the dislocation nucleation sites (high angle grain boundaries (HAGBs) act as dislocation source) is the most important parameter responsible for the decrease of the stable crack growth onset temperature.     

TiAl裂纹形核和塑性变形关系的研究

对层状结构ＴｉＡｌ在扫描电镜中原位拉伸,发现裂纹前端首先出现滑移带,只有当局部塑性变形发展和临界状态,位错塞积应力等于原子键合务时才会使微米尺寸卑贱 裂纹不连续形核,裂纹可沿滑移带形核,可沿其它是面形核,方扩展,新的不连续裂纺的阻力不为民增大     

CERT-Untersuchungen an Leitungsrohrstählen über eine Korrosionsgefährdung durch wasserstoffhaltige Gase bei hohen Drücken

CERT tests on line pipe steels for corrosion risks by hydrogen containing gases under high pressure Notched and unnotched round tensile specimens of line pipe steels were subjected to constant extension rate testing in autoclaves. The testing included pulling the specimens at a slow strain rate to fracture as well as slow strain rate cyclic loading. Besides base material, welded joints with varying hardness values ranging up to 400 HV were tested. Testing environments were inert gases (N₂, Ar, air, natural gas), 70 bar H₂ and various mixtures of H₂ and natural gas. In all gases, fracture occurred in the region of highest plastic deformation. When H₂ was present in the environment secondary cracking and fractions of brittle fracture were found on the fracture surface, their extent being reduced by the presence of O₂ and CO. No crack growth occurs in H₂ environment under cyclic loading when there is no plastic deformation. For the range investigated, there is no influence of hardness on the resistance of the materials to cracking, but surface finish and notch acuity exercise a strong influence.     

非调质钢YG1401的冲击韧性和断口形貌探讨

通过采用示波冲击试验机对贝氏体非调质钢YG1401进行系列温度冲击试验,并采用体式显微镜和扫描电镜对断口宏观形貌及微观扩展形貌进行观察,结合示波冲击试验所测的裂纹起裂功及裂纹扩展功研究该材料从24～-80 ℃的冲击断口形貌变化规律.结果 表明:该材料的DBTT为-12℃.试验材料在24～-10℃区间冲击时,材料代表高韧...     

Influence of Cr element on impact fracture process of ductile Ni-resistant alloyed iron at low temperature

In this study, in order to investigate the influence of Cr element on the impact fracture process of ductile Ni-resistant alloyed iron at low temperature, different contents of Cr element were added to ductile Ni-resistant(DNR) austenitic alloyed iron. The experimental results show that Cr addition can increase the hardness of the DNR alloyed iron, but it has an destructive effect on low-temperature impact properties. Through the analysis of the dynamic load and absorbed energy of samples with different Cr contents in the impact fracture process, and the comparison of the impact fracture process at room and low temperatures, it reveals that Cr addition into the DNR alloyed iron can facilitate the formation of the carbide mixture in Mn23C6 and Cr23C6 with homogeneous and discontinuous distribution. Meanwhile, Cr addition also can improve the the maximum dynamic load and crack initiation energy at low temperature, but has no obvious effect on the yield behavior of the DNR alloyed iron in the impact fracture process. Compared with the impact crack propagation process at room temperature, the metastable propagation energy at low temperature declines significantly with an increase in Cr content. This is because the micro-cracks that caused by the carbides weaken the matrix, resulting in the decline of impact crack propagation resistance. The fracture analysis results also show that the impact fracture mechanism gradually transforms from ductile to brittle with an increase in Cr content at low temperature. It explains that too much Cr addition can lead to brittle fracture even though the austenitic matrix has a good toughness at low temperature.     

TiC_p/W复合材料的断裂行为

利用扫描电镜观察了含３０ ％ ＴｉＣ 颗粒（ 体积分数） 的钨基复合材料在室温和高温的微观断裂过程, 讨论了裂纹萌生、扩展条件及其影响因素。室温下的断裂过程受控于裂纹萌生阶段, 相应的应力－ 挠度曲线表现为线性, ＴｉＣ 颗粒和Ｗ 基体在微观上都呈现脆性断裂。高温下的断裂则存在一个亚稳态的初始裂纹长大和合并过程, 使应力－ 挠度曲线呈现出非线性, 在微观上ＴｉＣ 颗粒呈现脆性断裂,Ｗ 基体呈现韧性撕裂。同时也指出了复合材料在室温和高温下的强化机制     

An experimental investigation of constraint effects on mixed mode fracture initiation in a ductile aluminium alloy

The effect of constraint on ductile fracture initiation from a notch tip under mode I and mixed mode (involving modes I and II) loading is investigated. To this end, mixed mode fracture experiments are performed with Compact Tension Shear (or CTS) specimen of a ductile 2014-O aluminium alloy. The constraint effects are investigated by considering specimens with two crack length to width ratios. The effect of crack tip constraint on the relationship between the critical value of the J-integral at fracture initiation (J_c) and M_p is examined. Further, the micromechanics of mixed mode ductile fracture initiation is investigated by performing fractographic studies and metallographic examination of the mid-plane region of the specimen near the notch tip.     

Study on ductile crack initiation limit from notch root of pipeline girth welded joints with strength mismatch

This study investigated ductile crack initiation limit of pipeline girth welded joints with different strength mismatches. The ductile crack initiation limit for the girth welded joints was evaluated by conducting three-point bending fracture toughness tests and wide plate tensile tests with a surface notch. In addition, effect of heat input on the ductile crack initiation limit of weld metal (WM) was evaluated on the assumption that a welding condition would be varied in the field in the actual pipeline construction. As the results, the equivalent plastic strain at the notch tip for the ductile crack initiation of the three-point bending tests was consistent with those of the wide plate tests, and the heat input hardly affected the ductile crack initiation limit within the range of this study. This meant that the ductile crack initiation limit of the pipeline girth welded joints with strength mis-match was able to be estimated using the equivalent plastic strain obtained from the three-point bending tests. Based on these results, we proposed a procedure to determine the rational fracture toughness requirements which took into account the difference in the plastic constraint between standard fracture toughness test and pipeline girth welded joints. This procedure was also possible to determine the required strength matching level for a strain-based design for girth welded joint containing surface notch in the centre of the WM in terms of preventing the ductile crack initiation.     

柔度法确定延性金属动态断裂起裂点及阻力曲线的建立

运用柔度法校正了动态冲击的起裂能。借助试验及理论分析,得到J-△a曲线。预制裂纹在起裂之前处于以塑性变形为主导的钝化状态,起裂之后则处于微孔形成与长大机制控制下的延性断裂过程,而起裂点实际上是这两个过程之间的过渡。     

The nature of delayed fracture of quenched steel

Conclusions Delayed fracture occurs at stresses exceeding the stress at the beginning of microdeformation. Nucleation of microcracks under load occurs due to the development of microtears in areas of peak residual stresses that occur in the process of the martensitic transformation. The most probale centers of crack nucleation in quenched steel under load are the boundaries of the original austenite grains (where the largest martensite crystals appear).Steady growth up to the critical size is uneven and jumplike, which is due to elastic energy during delayed fracture tests. Fracture in the boundaries of the original austenite grains becomes possible due to the high elastic energy of the boundaries resulting from the martensitic transformation. Slow crack propagation is the result of periodic slowdowns of brittle fracture in the grain boundaries of areas where plastic deformation occurs (ductile fracture). The process determining the low rate of steady crack growth is evidently thermally activated plastic deformation of microsections of ductile fracture.I. P. Bardin Central Scientific-Research Institute of Ferrous Metallurgy. Translated from Metallovedenie i Termicheskaya Obrabotka Metallov, No. 12, pp. 36–41, December, 1976.     

Effects of dislocation dynamics and microstructure on crack growth mechanisms in two-phase titanium aluminide alloys

The fracture behaviour of two-phase titanium aluminide alloys was characterized by fracture toughness tests performed in a wide temperature range on chevronnotched three point bending bars. Temperature and rate dependent deformation processes were characterized by temperature and strain rate cycling tests. The alloy investigated had compositions and microstructures which are currently being considered for engineering applications. The paper considers the effects of microstructure and crack tip plasticity on the crack growth resistance. The temperature dependence of the fracture toughness was rationalized in terms of micro-processes which determine the glide resistance of the dislocations in the plastic zone of crack tips. The implications of such observations for the engineering application of the materials are addressed briefly.     

Effect of metal layer thickness on the decohesion of high purity copper–sapphire interfaces

The decohesion of high purity Cu–sapphire interfaces has been studied by using double cleavage drilled compressive (DCDC) specimens, accompanied by in situ optical observations of the crack extension mechanisms. The Cu layer thickness was varied between 10 and 100 μm. Decohesion occurred along the interface subject to a resistance, Γ_R(Δa), that rises with crack extension. The magnitude of the resistance was determined to be much larger (by a factor 4) for specimens with the thicker (100 μm) Cu layer. The observations revealed that the rupture occurred by a ductile mechanism involving void formation at the interface, followed by plastic void growth in the Cu, and subsequent coalescence by the separation of the interface between voids. Aspects of the failure sequence change with the Cu layer thickness. These changes, as well as the difference in the toughness, are attributable to a transition in constraint between the thin and thick layers. Namely, the thin layer develops high constraint (relative to the thicker layer) that elevates the mean stress ahead of the crack and causes the failure process to progress to a location further from the crack front. The measured trends in Γ_R(Δa) for the two different layer thickness are shown consistent with models of the constraint transition for interface failure by ductile mechanisms.     

Influence of Martensite Volume Fraction on Impact Properties of Triple Phase (TP) Steels

Ferrite-bainite-martensite triple phase (TP) microstructures with different volume fractions of martensite were obtained by changing heat treatment time during austempering at 300 °C. Room temperature impact properties of TP steels with different martensite volume fractions (V _M) were determined by means of Charpy impact testing. The effects of test temperature on impact properties were also investigated for two selected microstructures containing 0 (the DP steel) and 8.5 vol.% martensite. Test results showed reduction in toughness with increasing V _M in TP steels. Fracture toughness values for the DP and TP steels with 8.5 vol.% martensite were obtained from correlation between fracture toughness and the Charpy impact energy. Fractography of Charpy specimens confirmed decrease in TP steels’ toughness with increasing V _M by considering and comparing radial marks and crack initiation regions at the fracture surfaces of the studied steels.     

Effect of carbide distribution on the fracture toughness in the transition temperature region of an SA 508 steel

An investigation was conducted into the effect of carbide distribution on fracture toughness in the ductile–brittle transition temperature region of an SA 508 steel used for nuclear reactor pressure vessels. Tensile properties and elastic–plastic cleavage fracture toughness were measured in the transition temperature region, and the fracture toughness data were interpreted by using a simple fracture model containing carbide size distribution. This modeling study indicated that the critical nearest-neighbor distance between coarse carbides was an important microstructural factor affecting elastic–plastic fracture toughness, since it satisfied a linear relationship with the critical distance between a crack tip to a cleavage initiation site. These findings suggested that reducing the total number of carbides, particularly the number of M₃C carbides larger than the critical size, and homogeneously distributing fine M₂C carbides, were useful ways to improve fracture toughness in the transition temperature region.