• Journal of the Korean Institute of Gas
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• v.25 no.6
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• pp.14-21
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• 2021

Inner shell material of LNG storage tanks that store ultra-low temperature LNG at -162℃ requires structural integrity assessment of a crack-like defect. From the viewpoint of conventional fracture mechanics, the assessment has mainly performed by single parameter using stress intensity factor K, J-integral and CTOD. However, the stresses in a material of crack tip are not unique caused by constraint loss due to size and geometry of the structure. Various attempts have been made to complement a single parameter fracture mechanics, typically with Q-stress. In this paper, we have performed a two-parameter approach by deriving the Q-stress coupling with J-integral suitable for the evaluation of the crack tip stress field in the non-linear elastic region. A quantitative evaluation of the constraint effect has performed by using the J-Q approach. It was evaluated that the SENB type specimen had a crack ratio of 0.1 to 0.7 and the wide type specimen had a crack ratio of 0.2 to 0.6.

• Structural Engineering and Mechanics
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• v.44 no.5
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• pp.601-609
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• 2012

Effect of different crack sizes on fracture criterion of some engineering materials was investigated in this work. Using finite element (FE) method coupled with a newly developed algorithm, J-integral values for different crack sizes were obtained for single-edge notched bend (SENB) and compact type (CT) specimen. Specimens with initial a/W ratio from 0.25 to 0.75 varying in crack size in steps of 0.125 were investigated. Several different materials, like 20MnMoNi55, 42CrMo4 and 50CrMo4, usually used in engineering structure, were investigated. For one of mentioned materials, numerical results were compared with experimental and their compatibility is visible.

• Korean Journal of Materials Research
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• v.9 no.2
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• pp.132-138
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• 1999

In order to investigate the toughening characteristic by microcrack formation in ceramic composites, $Al_2$O$_3$/(0~20)vol% YAG composites containing equiaxed second grains were fabricated using$ Al_2$$O_3$ during hot-pressing. AE(acoustic emission) measurements have been coupled with fracture toughness experiments of SENB method, to evaluate the microcrack formation and the improvement in fracture toughness of ceramic composites. Formation of microcrack was detected by Ae. The generation of AE events increased with increasing of load when load was applied at specimen. The AE events are generated mainly around at maximum load. Specially, the detected AE evetns of composites are many as compared with monolithic $Al_2$$O_3$. Fracture toughness of composites was improved than that of monolithic alumina. $Al_2$O$_3$/YAG composites exhibit main toughening effects by microcracking, results from mutual coalesence of microcracks being generated under applied load. However, there are few toughening mechanism like microcracking in monolithic alumina.

• Computers and Concrete
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• v.33 no.2
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• pp.147-162
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• 2024

This study presents a random-aggregate mesoscale model integrating the random distribution of the coarse aggerates and the damage mechanics of the mortar and interfacial transition zone (ITZ). This mesoscale model can generate the random distribution of the coarse aggregates according to the prescribed particle size distribution which enables the automation of the current methodology with different coarse aggregates' distribution. The main innovation of this work is to propose the "correction factor" to eliminate the dimensionally dependent mesh sensitivity of the concrete damaged plasticity (CDP) model. After implementing the correction factor through the user-defined subroutine in the randomly meshed mesoscale model, the predicted fracture resistance is in good agreement with the average experimental results of a series of geometrically similar single-edge-notched beams (SENB) concrete specimens. The simulated cracking pattern is also more realistic than the conventional concrete material models. The proposed random-aggregate mesoscale model hence demonstrates its validity in the application of concrete fracture failure and statistical size effect analysis.

• Journal of Advanced Marine Engineering and Technology
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• v.30 no.4
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• pp.494-504
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• 2006

It is well known that the two-parameter $J-A_2$ solution can well characterize the crack-tip fields and quantify the crack-tip constraint for different flawed geometries in variety of loading conditions. However, this solution fails to do so for bending dominated specimens or geometries at large deformation because of the influence of significant global bending stress on the crack-tip field. To solve this issue, a modified $J-A_2$ solution is developed in this paper by introducing an additional term to address the global bending influence. Using the $J_2$ flow theory of plasticity and within the small-strain framework detailed finite element analyses are carried out for the single edge notched bend (SENB) specimen with a deep crack in A533B steel at different deformation levels ranging from small-scale Yielding to large-scale Yielding conditions. The numerical results of the crack-tip stress field are then compared with those determined from the $J-A_2$ solution and from the modified $J-A_2$ solution at the same level of applied loading Results indicate that the modified $J-A_2$ solution largely improves the $J-A_2$ solution, and match very well with the numerical results in the region of interest at all deformation levels. Therefore, the proposed solution can effectively describe the crack-tip field and the constraint for bending dominated specimens or geometries.

• Nuclear Engineering and Technology
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• v.52 no.7
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• pp.1579-1586
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• 2020

Dissimilar metal joints (DMJs) are more common in the application of piping system of many industries. A 2- D and 3-D finite element analysis (FEA) is carried out on dissimilar metal Single Edged Notch Bending (DMSENB) specimens fabricated from ferritic steel, austenitic steel and Inconel - 182 alloy to study the behavior of DMJs with constraints by using linear elastic fracture mechanics (LEFM) principles. Studies on DMSENB specimens are conducted with respect to (i) dissimilar metal joint width (DMJW) (geometrical constraints) (5 mm, 10 mm, 20 mm, 30 mm and 50 mm) (ii) strength mismatch (material constraints) and (iii) crack lengths (16 mm, 20 mm and 24 mm) to study the DMJ behavior. From the FEA investigation, it is observed that (i) SIF increases with increase of crack length and DMJWs (ii) significant constraint effect (geometry, crack tip and strength mismatch) is observed for DMJWs of 5 mm and 10 mm (iii) stress distribution at the interfaces of DMSENB specimen exhibits clear indication of strength mismatch (iv) 3-D FEA yields realistic behavior (v) constraint effect is found to be significant if DMJW is less than 20 mm and the ratio of specimen length to the DMJW is greater than 7.4.

• Journal of the Korean Ceramic Society
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• v.34 no.11
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• pp.1129-1138
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• 1997

Al2O3/20 vol%YAG composite containing equiaxed grains and Al2O3/20 vol%LaAl11O18 composite containing elongated grains were fabricated using Al2O3-Y2O3 composition and Al2O3-La2O3 composition, respectively, by hot-pressing. In order to investigate the influence of microstructural control of second phase on toughening effect of toughened ceramic composites, AE (acoustic emission) measurements have been coupled with fracture toughness experiments(SENB and SEPB method). A separation of the fracture toughness and analysis of toughening mechanism was possible using the AE technique. The fracture toughness of hot-pressed materials was estimated to be 3.2 MPam0.5 for monolithic alumina, 4.7 MPam0.5 for Al2O3/20 vol%YAG composite and 6.2 MPam0.5 for Al2O3/20 vol%LaAl11O18 composite. In monolithic Al2O3, toughening does not occur as a result of either microcracking or grain bridging, whereas, composites exhibit toughening effects by both microcracking in the frontal zone and gain bridging in the wake zone, resulting in an improvement of fracture toughness as compared with monolithic Al2O3. The fracture toughness of Al2O3/20 vol%LaAl11O18 composite is higher than that of Al2O3/20 vol%YAG composite. It may be attributed to the elongated microstructure of Al2O3/20 vol%LaAl11O18 composite, resulting relatively greater bridging effect.

• Proceedings of the Korean Society For Composite Materials Conference
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• 2002.10a
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• pp.60-63
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• 2002

Metal/intermetallic laminated composites have been manufactured by SHS reactions between Ni and Al elemental metal foils. Microstructure showed that the intermetallic volume fraction was 55%, 45%, 35% in the 1:1, 2:1, 4:1 thickness ratio(Ni:Al) specimen and the main phases of the intermetallic were transformed from $Ni_2Al_3$ to NiAl when the thickness ratio was increased. Tensile strength and elongation were increased when the volume fraction of Ni metallic phase was increased. Under assumptions of isostrain condition, the tensile strength of metal/intermetallic laminated composites didn't obey the ROM due to the thermal residual stress and this was confirmed by X-ray residual stress analysis. Fracture toughness results by the SENB test showed R-curves with upward curvature based on LSB condition. Bridging stress based on LSB condition was determined by the curve fitting analysis, In-situ observed microstructure during fracture test showed that the various bridging mechanism such as crack bridging, crack branching and ductile failure of metallic layer were occurred

• Journal of Mechanical Science and Technology
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• v.18 no.12
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• pp.2158-2173
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• 2004

In this paper, the higher order terms in the crack tip stress fields are investigated macroscopically for more realistic assessment of structural material behaviors. For reactor pressure vessel material of A533B ferritic steel, effects of crack size and temperature have been evaluated using 3-point SENB specimens through a series of finite element analyses, tensile tests and fracture toughness tests. The T-stress, Q-parameter and q-parameter as well as the K and J-integral are calculated and mutual relationships are investigated also. Based on the evaluation, it has proven that the effect of crack size from standard length (a/W=0.53) to shallow length (a/W=0.11) is remarkable whilst the effect of temperature from -20$^{\circ}C$ to -60$^{\circ}C$ is negligible. Finally, the cleavage fracture toughness loci as a function of the promising Q-parameter or q-parameter are developed using specific test results as well as finite element analysis results, which can be applicable for structural integrity evaluation considering constraint effects.

• Carbon letters
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• v.7 no.1
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• pp.19-26
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• 2006

Three point bending tests of single edge notched beam (SENB) specimens were carried out to evaluate the fracture behavior of the fine-grain isotropic nuclear grade graphite, IG-11. To measure the crack initiation point and the subsequent crack growth, the direct current potential drop (DCPD) method and a traveling microscope were used. The effects of test variables like initial crack length, specimen thickness, notch type and loading rate on the measured fracture toughness, $K_Q$, were investigated. Based on the test results, the ranges of the test variables to measure the reliable fracture toughness value were proposed. During the crack growth, the rising R-curve behavior was observed in IG-11 graphite when the superficial crack length measured on the specimen surface was used. The increase of crack growth resistance was discussed in terms of crack bridging, crack meandering, crack branching, microcracking and crack deflection, which increase the surface energy and friction force.