Advances in Fracture and Strength

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作者: Xu Chang, Junjie Liu, Chun An Tang, Yong Bin Zhang, Juan Xia Zhang
摘要: Equally spaced opening-mode fractures always evolve in top layer attached to underlying layer. With a newly developed Material Failure Process Analysis code (MFPA2D), we have firstly investigated the stress distribution between two adjacent fractures as a function of the fracture-spacing-to-layer-thickness ratio using a two-layer model with a fractured top layer. The numerical results indicate the horizontal stress perpendicular to the fractures near the top surface changes from tensile to compressive when the fracture-spacing-to-layer-thickness ratio changes from greater than to less than a critical value. Then, the process from fracture initiation to fracture saturation is numerically modeled. The modeling of fracture process shows that the fractures initiate at the top surface and propagate to the interface between the two layers in the first stage. In the following stage, new fractures can infill between the earlier formed fractures and they always initiate at the interface and propagate to the top surface. Numerical simulation clearly demonstrates that the stress state transition precludes further infilling of fractures and the fracture spacing reaches a constant state, i.e. the so-called fracture saturation.
作者: Gyu Baek An, Mitsuru Ohata, Masahito Mochizuki, Han Sur Bang, Masao Toyoda
摘要: It has been well known that ductile fractures of steels are accelerated by triaxial stresses. The characteristics of ductile crack initiation in steels are evaluated quantitatively using two-parameter criterion based on equivalent plastic strain and stress triaxiality. It has been demonstrated by authors using round-bar specimens with circumferential notch in single tension that the critical strain to initiate ductile crack from specimen center depends considerably on stress triaxiality, but surface cracking of notch root is in accordance with constant strain condition. This study fundamentally clarifies the effect of strength mismatch, which can elevate plastic constraint due to heterogeneous plastic straining under static loading, on critical conditions for ductile cracking from the pre-notch root. In order to evaluate the stress/strain state in the pre-notch root of specimens, a thermal elastic-plastic finite element (FE) analysis has been carried out.
作者: Byeong Choon Goo, Sung Yong Yang, Jung Won Seo
作者: J. Sun, L.H. Han, L. Li
摘要: The ductile to brittle transition (DBT) phenomena in pure metal was reviewed briefly with the emphasis on effects of stress triaxiality and confined dislocation slip. Experimental and numerical methods were used to investigate the DBT of inter pure Al sheet sandwiched by 2024 Al alloy through explosive cladding. The experimental results clearly showed that this transition has taken place with its decreasing thickness, h , characterized by decreasing fracture toughness and local cleavage patterns where the {100} facets are dominant. Finite element calculations show that it is the constraint of elastic fraction of substrates on the yield zone in inter Al layer that results in an increasingly high triaxial tensile stress, which makes the material become brittle. The banded-grain boundaries have a similar role to strong interface in preventing dislocations slip.
作者: Jong Sung Kim, Tae Eun Jin
摘要: In the paper, the validity of the modified mesh-insensitive SS (structural stress) procedure to apply to the welded joints with local thickness variation is identified via the comparison of SCFs (stress concentration factors) calculated for various FE (finite element) models. FCI (fatigue crac kinitiation) cycles are determined by using the SS/EPFM (elasto-plastic fracture mechanics) approach and the various fatigue crack growth models. Fatigue test is performed to identify the validity of the fatigue analysis results. Finally, as a result of comparison among test and various analysis results, it is found that the SS/FM (fracture mechanics) approach agrees well with the fatigue test results over all cycle regions and the SS/EPFM approach is more reliable than the SS/LEFM (linear elastic fracture mechanics) approach.
作者: Jeong Woo Han, Seung Ho Han, Byung Chun Shin, Jae Hoon Kim
摘要: The fatigue life of welded joints is associated with crack initiation and propagation life. Theses cannot be easily separated, since the definition of crack initiation is vague due to the initiation of multiple cracks that are distributed randomly along the weld toes. In this paper a method involving a notch strain and fracture mechanical approach, which considers the characteristics of welded joints, e.g. welding residual stress and statistical characteristics of multiple cracks, is proposed, in an attempt to reasonably estimate these fatigue lives. The fatigue crack initiation life was evaluated statistically, e.g. the probability of occurrence in 2.3, 50 and 97.7%, in which the cyclic response of the local stress/strain in the vicinity of the weld toes and notch factors derived by the irregular shape of the weld bead are taken into account. The fatigue crack propagation life was simulated in consideration of the Mk-factor and the mechanical behavior of mutual interaction/coalescence between two adjacent cracks. The estimated total fatigue life as a sum of crack initiation and propagation life was found to be in good agreement with the experimental results.
作者: Han Ki Yoon, Dong Hyun Kim, Won Jo Park, Akira Kohyama
摘要: In order to apply a reduced activation ferritic (JLF-1) steel to the blanket/first-wall structure of a fusion reactor, its fracture toughness is very important for the strict estimation of material life. Fracture toughness testing of irradiated materials requires the use of miniaturised specimens and evaluation of TIG welding (tungsten inert gas arc welding) weldment properties is an important issue because necessary for production of nuclear fusion reactors. In this study, the fracture toughness tests were carried out according to the ASTM E1820-99. It was performed on various sizes (ligament and thickness) and various side-grooves of specimens and the TIG welding joint of JLF-1. The test results showed the standard specimen with side-groove of 40% represented valid fracture toughness. Fracture resistance curve (R-curve) increased with increasing specimen ligament and decreased with increasing specimen thickness. However, the R-curve of half size specimen was similar to that of the standard (1inch thickness) specimen. The fracture toughness test results of the TIG welded specimen showed a slight increase in the TIG welded specimen compared with JLF-1 base metal specimen.
作者: Byeong Soo Lim, Bum Joon Kim, D. Kim, J.W. Kim, Dong Bok Lee
摘要: In this study, the creep-fatigue crack growth behavior was investigated at 600°C under the maximum load with various dwell times. Test material was P92 steel (9%Cr-2%W) weldment. The creep-fatigue crack growth behavior of the HAZ(heat affected zone) and base metal was compared. The relationship between the crack growth behavior and dwell time was studied and it was characterized using ΔK and (Ct)avg parameters. The area fraction of micro-voids/cavities at the crack tip and fracture modes were examined and the relationship between crack growth rate and dwell time was investigated. The cycle dependant crack growth rate increased with the increase in dwell time. From the results of creep-fatigue test, the crack growth rate of the HAZ was found to be faster than that of base metal. As the dwell period increased, the intergranular fracture was observed along the crack growth path with the effect of creep during the dwell period.
作者: Ja Myeong Koo, Young Ho Lee, Sun Kyu Kim, Myung Yong Jeong, Seung Boo Jung
摘要: The mechanical and electrical properties of the Sn-3.5Ag solder/Cu BGA packages were investigated as a function of number of reflows. A continuous scallop-shaped Cu6Sn5 intermetallic compound (IMC) layer was formed at the solder/Cu interface upon 1 reflow process. After 3 reflows, very thin layer of Cu3Sn IMC was observed at the Cu6Sn5/Cu interface. As the number of reflows increased, the thickness of these IMCs increased with a cube root of reflow time. Shear force slightly increased up to 4 reflows and then gradually decreased with increasing the number of reflows. The maximum shear force was 11.7N when the thickness of the IMC layers was 3.7㎛. The fracture surfaces of all the specimens showed ductile failure characteristics up to 4 reflows, and then the fraction of the brittle IMC fracture increased with the number of reflows. The electrical resistance of the BGA packages increased with the number of reflows.
作者: Feng Jin, Kikuo Kishimoto, Hirotsugu Inoue, Takashi Tateno
摘要: The linear ultrasonic technique has been extensively used as a powerful, non-destructive test tool for reliability testing and failure analysis of electronic packaging. This is used most often in the inspection of defects such as delaminations, voids, or cracks through use of a SAM (Scanning Acoustic Microscope). Then, as the reliability level that is required of electronic packaging becomes higher and the thickness of package becomes thinner, the possible defect which needs to be detected becomes smaller. In the conventional SAM, however it is very difficult to detect small defects less than m µ 1 . 0 , such as micro-delaminations. In order to solve such a problem, this paper proposes a nonlinear ultrasonic method, where the nonlinearity caused by the effect of crack-face interactions is considered. The basic concept of this method involves harmonic frequencies that are generated in the transmitted ultrasonic wave due to the partial contact at the interface of micro-delamination. As an evaluation index, the nonlinear parameter dependent on the amplitude of the second order harmonic frequency component is obtained by spectral analysis of the transmitted signal. Experimental results show that the nonlinear parameter has good correlation with the micro-gap and the proposed method can detect the micro-delamination even less than nm 1 .