• Journal of Advanced Marine Engineering and Technology
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• v.33 no.4
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• pp.533-538
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• 2009

The magnitude of constraint effect $A_2$ values on the non-linear elastic plastic fracture toughness was experimentally estimated by using displacement at various measuring positions near crack tip. Constraint effect $A_2$ value was dependent on specimen configuration and on the measured displacement near crack front. The crack tip opening displacement in the vicinity of the crack tip front should be estimated within plastic region when appropriately constraint effect was calculated. It was found that the magnitude of constrain effect |$A_2$| is below 8.0 at the crack tip. But an appropriate location to measure the effective constraint effects $A_2$ at the critical value of J that crack initiation is characterizable by is r = 2mm and ${\theta}=90^{\circ}$ away from original crack tip, and the constraint effect |$A_2$| estimated is 5.3.

• Journal of Advanced Marine Engineering and Technology
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• v.33 no.4
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• pp.539-544
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• 2009

The constraint effect $A_2$ has to be evaluated within plastic region near crack tip front using opening displacement. Plastic boundary and stress or strain conditions in the vicinity of the crack tip using recrystallization heat treatment was represented. It was found that the plastic deformation boundary by recrystallization heat treatment method was the true strain of ${\epsilon}t$ = 0.05mm/mm. With the estimation of constraint effects $A_2$, the region of proper displacement measurement point near crack tip was between 0mm and 1mm distance toward direction of crack propagation, and was between 1mm to 3mm distance toward direction of load line.

• Korean Journal of Metals and Materials
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• v.46 no.6
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• pp.345-350
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• 2008

The magnitude of constraint effect $A_2$ value was experimentally estimated by using crack tip opening displacement(CTOD) between elastic and plastic regions near crack tip front for CT specimen with $25.4t{\ss}{\AE}$ SS400 steel. The constraint effect, $A_2$ was dependent on specimen configuration and on the measured positions of CTOD near crack front. $A_2$ should be estimated using the opening displacement calculated within crack front plastic region. If not, it's not reliable to evaluate of constraint effect at crack growth initiation in this paper.

• Transactions of the Korean Society of Mechanical Engineers A
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• v.27 no.9
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• pp.1531-1538
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• 2003

This paper compiles solutions of plastic $\eta$ factors and crack tip stress triaxialites for standard and nonstandard fracture toughness testing specimens, via detailed three-dimensional (3-D) finite element (FE) analyses. Fracture toughness testing specimens include a middle cracked tension (M(T)) specimen, SE(B), single-edge cracked bar in tension (SE(T)) and C(T) specimen. The ligament-to-thickness ratio of the specimen is systematically varied. It is found that the use of the CMOD overall provides more robust experimental J estimation than that of the LLD, for all cases considered in the present work. Moreover, the J estimation based on the load-CMOD record is shown to be insensitive to the specimen thickness, and thus can be used for testing specimen with any thickness. The effects of in-plane and out-of-plane constraint on the crack tip stress triaxiality are also quantified, so that when experimental J value is estimated according to the procedure recommended in this paper, the corresponding crack tip stress triaxiality can be estimated. Moreover, it is found that the out-of-plane constraint effect is related to the in-plane constraint effect.

• Transactions of the Korean Society of Pressure Vessels and Piping
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• v.16 no.2
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• pp.47-57
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• 2020

A new elastic-plastic-creep constraint parameter is proposed to quantify the effect of constraint on creep crack initiation times. It represents the difference between the transient elastic-plastic-creep crack-tip opening stress and the Riedel-Rice opening stress field in plane strain, which can be determined analytically. Application of the proposed parameter to a large set of creep crack growth test data using C(T) and SEN(B) specimens of Type 316H stainless steel at 550℃ shows that creep crack initiation times can be more accurately characterized by the C⁎-integral together with the proposed parameter.

• Transactions of the Korean Society of Mechanical Engineers A
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• v.27 no.7
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• pp.1159-1169
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• 2003

This paper investigates the effects of T-stress and plastic hardening mismatch on the interfacial crack-tip stress field via finite element analyses. Plane strain elastic-plastic crack-tip fields are modeled with both MBL formulation and a full SEC specimen under pure bending. Modified Prandtl slip line fields illustrate the effects of T-stress on crack-tip constraint in homogeneous material. Compressive T-stress substantially reduces the interfacial crack-tip constraint, but increases the J-contribution by lower hardening material, J$\_$L/. For bimaterials with two elastic-plastic materials, increasing plastic hardening mismatch increases both crack-tip stress constraint in the lower hardening material and J$\_$L/. The fracture toughness for bimaterial joints would consequently be much lower than that of lower hardening homogeneous material. The implication of unbalanced J-integral in bimaterials is also discussed.

• 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.

• Transactions of the Korean Society of Mechanical Engineers A
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• v.38 no.11
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• pp.1207-1214
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• 2014

Under excessive plasticity, the fracture toughness of a material depends on its size and geometry. Under fully yielded conditions, the stresses in a material near its crack tip are not unique but rather depend on the geometry. Therefore, the single-parameter J-approach is limited to a high-constraint crack geometry. The JQ theory has been proposed for establishing the crack geometry constraints. This approach assumes that the crack-tip fields have two degrees of freedom. In this study, the crack-tip stress field of a fully circumferential surface-cracked pipe under combined loads is investigated on the basis of the JQ theory by using finite element analysis. The combined loads are a tensile axial force and the thermal gradient in the radial direction. Q-stresses of the crack geometry and its loading state are used to determine the constraint effects. The constraint effects of secondary loading are found to be greater than those of primary loading. Therefore, thermal shock is believed to be the most severe loading condition of constraint effects.

• Proceedings of the Korean Society of Marine Engineers Conference
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• 2006.06a
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• pp.91-92
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• 2006

For the CT specimen of 25.4mm thickness SS400 steel, the fracture toughness and the magnitude of constraint effect, $A_2$ on the non-linear elastoplastic fracture behaviors were experimentally estimated by crack tip opening displacement. In order to estimate constraint effect, displacement measurement position near crack front should be the existed within plastic region. But it is found that the displacement measurement positions by the ${\delta}_5$ method are in elastic region at crack growth initiation. Hence the estimate of constraint effect, $A_2$ by the ${\delta}_5$ method was not reliable.

• Transactions of the Korean Society of Mechanical Engineers A
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• v.38 no.12
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• pp.1441-1446
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• 2014

In the present paper, the effects of the thickness and width of a curved wide-plate, the crack length, and the strain hardening exponent on the crack-tip constraint of the curved wide-plate were investigated. To accomplish this, detailed three-dimensional elastic-plastic finite element (FE) analyses were performed considering various geometric and material variables. The material was characterized by the Ramberg-Osgood relationship, and the Q-stress was employed as a crack-tip constraint parameter. Based on the present FE results, the variations in the Q-stress of the curved wide-plate with the geometric variables and material properties were evaluated. This revealed that the effect of out-of-plane constraint conditions on the crack-tip constraint was closely related to the in-plane constraint conditions, and out-of-plane constraint conditions affected the crack-tip constraint more than in-plane constraint conditions.