Dislocation theory based short crack model and its application for aircraft aluminum alloys

This paper presents a study on dislocation theory based short/small crack modeling, and its application for short crack growth life analysis on 2024-T351 aluminum specimens. The dislocation theory was applied to determine the crack tip opening displacement (CTOD) of a microstructurally short crack by taking into account the effects of microstructural features, such as grain size, orientation, and grain boundary. The CTOD was then used as the parameter for calculating the short crack growth rate. In this work, an existing CTOD model was modified for estimating the short crack life of 2024-T351 single edge notch tension (SENT) specimens, which were tested in an Advisory Group for Aerospace Research and Development (AGARD) program [Newman JC, Edward PR. Short-crack growth behavior in an aluminum alloy: an AGARD cooperative test programme. AGARD-R-732, NATO, Advisory Group for Aerospace Research and Development; 1988]. The analytical results matched the test results reasonably well.     

Fracture behaviour of specimens with surface notch tip in the heat affected zone (HAZ) of strength mis-matched welded joints

The generally accepted conditions for the strength overmatched welded joints of high strength steel are not clearly defined. In this paper, the fracture mechanics analysis of specimens, with surface notch tips completely embedded in the heat affected zones was conducted. The results showed that the strength of mismatching of a welded joint caused a redirection of the crack propagation towards the low strength region of the welded joint. This redirection of the crack propagation affected the values of the critical CTOD. In the cases of the overmatched welded joints containing a soft root layer it is possible to achieve a comparable fracture behaviour related to the homogeneous overmatched welded joint if the impact toughness of the soft root layer is higher than the impact toughness of the overmatched weld metal. Such a type of welded joint is therefore preferable for the welding of high strength low alloy steels, because it enables the manufacturing of a welded joint without preheating.     

热处理工艺对新型R5系泊链钢组织和断裂韧性的影响

采用裂纹尖端张开位移(CTOD)实验方法对研究了不同热处理工艺对新型R5系泊链钢断裂韧性的影响,使用偏光显微镜、光学显微镜和扫描电镜等对CTOD试样裂纹扩展区组织及断口形貌进行分析,并用高低温材料试验机测试了R5系泊链钢低温-20℃下的力学性能。结果表明,回火温度对R5系泊链钢CTOD断裂韧性有显著影响。在570~600℃范围内回火,组织保留马氏体位向,碳化物呈片状及长条状不均匀分布在原奥氏体晶界、马氏体板条界及板条内部,应力作用下容易导致局部应力集中,裂纹扩展速率大,CTOD特征值接近零。随回火温度升高,在600~690℃范围内回火,碳化物逐渐聚集、球化并在基体中弥散均匀分布,应力作用下不易引起局部应力集中,并且在裂纹扩展时可使裂纹扩展方向发生偏转,裂纹扩展路径延长,裂纹扩展受到抑制,CTOD特征值随回火温度升高不断增大。经950℃淬火+630~660℃区间回火,R5系泊链钢同时具有高强度和良好的CTOD断裂韧性。     

On the cyclic crack-tip opening displacement

This note discusses the cyclic crack tip opening displacement ( CTOD )_cyc. As is discussed, the correct expression for the ( CTOD )_cyc is based upon the effective range of the stress intensity factor.     

Crack tip opening displacement in micro-cracked concrete by an embedded optical fiber sensor

Crack tip opening displacement (CTOD) is an important parameter that is often employed in characterization of fracture in engineering materials. Due to inherent difficulties in direct determination of CTOD, the crack mouth opening displacement (CMOD) has been measured during fracture tests of concrete beams and then related to CTOD. Analytical schemes are used for the determination of CTOD from the measured crack mouth opening displacements, which leads to the determination of other fracture parameters of importance. The research presented here describes development of an experimental technique for direct determination of CTOD based on embedded fiber optic sensors. The transduction mechanism is based on the correlation between the applied strain and the light intensity variation of speckle patterns generated at the output end of the multimode optical fiber due to mode redistribution.The measured displacements, CTOD, and CMOD, are compared and the validity of analytical schemes in estimation of CTOD is evaluated.     

Measuring and calculating GTOD and the J-integral with a double clip gauge

This paper describes a technique for measuring and calculating CTOD (crack tip opening displacement) arid the J-integral with double clip gauge. CTOD (or δ) and the rotation factor (r), the relation between δ and J-integral, as well as the R-curves δ - Δa and J. Δa are discussed. From the measurements with double clip gauge, one can calculate CTOD and J througout the entire loading process. The present technique seems to be a better approach than DD19, in which r is taken as a constant (r = 1/3).     

B+级铸钢材料的研制

通过合理的优化合金成分、并配合合适的热处理工艺,成功开发了B+级铸钢,并对B+级铸钢热处理工艺、低温性能、铸造工艺性能、裂纹尖端张开位移CTOD及疲劳性能进行了研究。研制的B+级铸钢完全符合AAR-M-201标准的要求。     

Analysis of the geometry dependence of fracture toughness at cracking initiation by comparison of circumferentially cracked round bars and SENB tests on Copper

In the first part of the paper, the use of circumferentially cracked round bars (CRB geometry) for characterizing fracture toughness of a ductile material, namely copper, is assessed experimentally through a comparison with the single edge notched bend (SENB) geometry. The J _R curve method with multiple-specimens was applied, but, as unstable cracking appeared very early in the CRB specimen, an engineering definition of fracture toughness was not pertinent. Unloaded specimens were analyzed metallographically to determine the CTOD at physical cracking initiation. The fracture toughness measured using the CRB geometry was 50% larger than using the SENB geometry. The second part of the paper aims at justifying this difference of fracture toughness at cracking initiation. Finite element simulations revealed a slightly higher constraint in the SENB specimens. The main difference between the two specimen geometries lies in a 50% larger extension of the finite strain zone with respect to the CTOD in the case of the SENB specimens. Based on the observation that, in the studied material, the critical CTOD is one order of magnitude larger than the void spacing, we conclude that the geometry dependence of the fracture toughness is caused by the difference in the finite strain zone extension rather than by a stress triaxiality effect.     

A crack tip blunting analysis in the ductile-to-brittle transition of a structural steel

The J -integral versus crack growth resistance curves ( J – R curves) of an AE-460 structural steel were evaluated at different temperatures in the ductile-to-brittle transition region. This curve as well as the critical J _I value at the initiation of crack growth was found to be independent of temperature, being dependent only on the stress triaxiality state of the specimen.
The blunting expressions described in the ASTM and ESIS standards were evaluated using fractographic techniques. The ESIS blunting line fits the linear regression of the experimental results very well, while the blunting line predicted by the ASTM standard is clearly located below the experimental points.     

埋弧焊焊丝MK65HGX-Ⅲ的开发和应用

介绍了用于焊接深海天然气输送用直缝埋弧焊管的焊丝MK65HGX-Ⅲ的开发背景及研制过程。采用该焊丝和优化的H08DG焊丝与高碱度烧结焊剂SJ102G匹配焊接11.525万tX65钢级厚壁管线管,其低温韧性、断裂韧性和焊接接头的横向拉伸性能均符合标准要求,且性能稳定,富余量较大。实际生产证明:MK65HGX-Ⅲ焊丝是一种高性能的新型埋弧焊焊丝,所焊接的钢管焊缝具有高强度、高韧性和高塑性,适用于大线能量(约65kJ/cm)的焊接,可推广应用到更高钢级(X80)和抗H2S腐蚀要求的焊管焊接。