Constraint loss correction for assessment of CTOD fracture toughness under welding residual stress. Part I: Methodology using the equivalent CTOD ratio

This part I of a two-part paper presents a method of assessing the effects of welding residual stress and constraint loss on the cleavage fracture of a wide plate subjected to membrane stress based on the Weibull stress criterion. It has been found that the Weibull stress criterion is efficient for evaluating the fracture instability of wide plates with and without a welding residual stress field. The concept of an equivalent crack-tip opening displacement (CTOD) ratio β_r under a welding residual stress field is introduced for assessment of constraint loss effects on CTOD fracture toughness of wide plates. The equivalent CTOD ratio β_r is defined as the ratio of the CTOD in the standard fracture toughness specimen to the CTOD in a wide plate with a welding residual stress at the same level of the Weibull stress. Fracture assessment procedures using β_r for wide plates are shown within the framework of the failure assessment diagram. It has been found that the excessive conservatism observed in the conventional procedure can be reduced reasonably by applying the proposed method. The companion part II of the paper presents applications of the CTOD toughness correction method using β_r to the fracture test data of welded joints and verifies the proposed method in the ductile-brittle transition temperature region.     

Fracture toughness of ISO 3183 X80M (API 5L X80) steel friction stir welds

Friction stir welding (FSW) is a solid-state joining process with numerous advantages such as good dimensional stability and repeatability, which is widely used Al alloys and with a great potential for critical joining applications involving high melting temperature alloys. Twelve millimeter thick plates of ISO 3183 X80M (API 5L X80) steel was friction stir welded using two passes on both sides of the plate using ceramic tools. Different heat inputs were obtained using a fix travel (welding) speed in combination with several spindle speeds. The fracture toughness of the two-pass joints was evaluated at 25 °C using the critical crack tip opening displacement (CTOD_m), revealing that joints produced with lower spindle speeds presented higher toughness at the heat-affected zone (HAZ) and stir zone (SZ), which are comparable with the base metal (BM) toughness. On the other hand, joints produced using higher spindle speeds presented low fracture toughness at the SZ and elevated CTOD_m toughness at the HAZ. The joints produced with low spindle speeds showed CTOD_m-values above the offshore standard (DNV-OS-F101) requirements.     

行程受限COD规在裂纹尖端张开位移试验中的应用

分析了COD规在裂纹尖端张开位移(CTOD)试验中由于行程受到限制而随载荷的变化所产生的各种可能结果，提出了用EXCEL数据处理软件对采集到的COD规的张开位移进行处理的理想方法，得到了令人满意的P-V曲线，从而可较准确地从该曲线上求得停机点的载荷Ps和COD规张开位移的塑性分量Vo，为CTOD试验的顺利完成提供了保证。     

Analysis of vacuum fatigue crack growth results and its implications

Conventionally, the reduction of ΔK_th with load ratio R has been interpreted in terms of crack closure arising from plasticity, oxide or crack surface roughness. Since, plane-strain conditions exist near-threshold, plasticity-induced closure is absent. Therefore, to account for R-ratio effects near-threshold, the oxide and roughness closure mechanisms have been proposed. Further analysis has shown that these other two closure effects also are small, when the results taken in vacuum were included. The present analysis shows that there is a unique relation of the oxide thickness with a threshold K_max, rather than with a threshold ΔK. This threshold K_max (K*_max,th) depends on environment. When the applied K_max<K*_max,th, the crack is stationary in the presence of the oxide formation and grows only when the applied K_max>K*_max,th. Thus, the oxide thickness—K_max relation seems to have a bearing on the criterion for crack growth in the presence of the environment. Oxide formation passivates the crack surfaces and retards the environmental damage ahead of the crack-tip. Examples from CrMo and NiCrMo steels support this viewpoint and suggest that there is a pressing need for quantifying the crack-tip environmental effects and fatigue thresholds.     

Welding of aluminum alloy door units for large SUVs using a compound laser method

A study was made on the relation between CTOD value and M–A constituent for the single-heat-cycled weld heat-affected zone (HAZ) of YS 320–360 N/mm² class thermomechanical control process steels with increased heat input. It was found that the M–A constituent disappeared and lost its deterioration effect on the HAZ CTOD toughness when the heat input exceeded about 15 kJ/mm, although this boundary heat input depended on the steel chemistry. On the other hand, the austenite grain size increased monotonically with increasing the heat input. But the austenite grain size could not be the controlling factor of the HAZ toughness, and its effect deferred between base metals. However, the HAZ toughness was related to the fracture facet size for the large heat input conditions. This fracture facet size, which represents the fracture toughness, is considered to be a measure of the uniformity of the transformed microstructure from austenite in the HAZ.     

A crack propagation criterion based on ΔCTOD measured with 2D‐digital image correlation technique

The fatigue cracks growth rate of a forged HSLA steel (AISI 4130) was investigated using thin single edge notch tensile specimen to simulate the crack development on a diesel train crankshafts. The effect of load ratio, R, was investigated at room temperature. Fatigue fracture surfaces were examined by scanning electron microscopy. An approach based on the crack tip opening displacement range (ΔCTOD) was proposed as fatigue crack propagation criterion. ΔCTOD measurements were carried out using 2D‐digital image correlation techniques. J‐integral values were estimated using ΔCTOD. Under test conditions investigated, it was found that the use of ΔCTOD as a fatigue crack growth driving force parameter is relevant and could describe the crack propagation behaviour, under different load ratio R.     

桥梁钢Q420qE低温CTOD断裂韧性试验研究

依据英国标准BS7448断裂韧度试验标准和DNV-OS-C401,对桥梁常用钢Q420qE进行了三点弯曲CTOD(裂纹尖端张开位移)断裂韧度试验,测试了在低温(-20℃)母材、热影响区、焊缝的CTOD特征值,得出相应的结论。     

Characterization of delayed cracking in deep‐drawn Swift cups of metastable austenitic stainless steels

Fracture behaviour of metastable austenitic stainless steels (Cr‐Ni grade 301 and low‐Ni high‐Mn grades 201 and 204Cu) that suffer of delayed cracking after deep drawing was examined. The phenomenon of delayed cracking is known to be related to the coexistence of internal hydrogen, strain‐induced α′‐martensite and tensile residual stresses. The objective of this study was to maximize the information attainable from the common Swift cup test, which is used to evaluate the formability and delayed cracking susceptibility of sheet metals. Fractures in deep‐drawn Swift cups were analysed macroscopically, by fracture surface study and electron backscatter diffraction analysis. Residual stresses in the cups were measured with X‐ray diffraction and α′‐martensite content using a Ferritescope. Estimation of fracture toughness of the materials was made using two different approaches: measuring the width of shear lips and relating it to the plastic zone size, and measuring the crack‐tip opening angle CTOA. On the basis of the results, grade 301 presents the highest fracture toughness, and 204Cu the lowest. Toughness of the materials seems to correlate with their nickel content. The order between the studied materials is similar when considering the susceptibility to delayed cracking.