Papers by Keyword: CT Specimen

Abstract: In this work, the effect of the prefabricated crack offset angle on the fracture toughness of the CT specimen was studied by means of the numerical simulation of tensile fracture. It is found that offset angle has a great influence on the fracture toughness of the CT specimen with a straight crack, and the effect is obvious when the bias angle is greater than 3 degrees. For the CT specimen with an oblique crack tip, with the increase of the bias angle of the crack tip, the J-integral value and the fracture toughness are not significantly changed. At the same time, as the angle increases, the stress applied load has more obvious influence on the crack tip stress field.

Abstract: Three-dimensional (3D) elastic-plastic finite element model (FEM) is adopted to research the effect of side groove on the crack-front J-integral for different size of Compact Tension (CT) specimens. Although the side-grooved CT specimen is widely used in the existing test method, such as ASTM E1820-13, the test data of fracture toughness is varying with the various geometric parameters. Before FE calculation, the material properties of Q345 steel were obtained by uniaxial tensile test, especially for the true stress-strain relationship. In this paper, it focuses on the numerical study of geometric parameter effects on the fracture toughness. Toward this end, the commercial FE software of ABAQUS is adopted to calculate the J-integral. Since the side groove of CT specimen is so important to make the fracture test success, the various parameters of side groove is intensively analyzed for obtaining the accurate J-integral along the crack front, including the effects of the angle, depth and root radius. In fact, the side groove effect is so significant around the crack front that cannot be ignored in the J-integral calculation. Through rigorous FE investigation, the influence of the side groove on the fracture toughness testing is fully disclosed, and the appropriate side groove configuration is recommended accordingly.

Abstract: At the moment a problem of fracture and lifetime estimation for piezoelectric materials is not completely solved. The paper considers fundamentals of linear fracture electromechanics, fracture parameters and fracture criteria. The main difference from linear mechanics is crack face boundary conditions taking into account relative permeability of media inside the crack gap and coulomb traction. Different types of crack face boundary conditions and their numerical implementation are described. The paper presents results of finite element modeling of fracture toughness experiments on the compact tension specimens under combined electromechanical loading. Different types of crack face boundary conditions were tested and comparison of fracture parameters and fracture criteria was carried out.

Abstract: The simultaneous effect of crack length and crack front shape on plasticity-induced crack closure (PICC) for a 304L austenitic stainless steel is simulated through 3D numerical modelling using finite element software Abaqus for through-thickness cracks with different curved crack fronts in CT specimens in comparison with bidimensional through crack with a straight front. The influence of possible loading history effect is avoided by applying constant K amplitude. The local stress intensity factor range for crack opening K_op is evaluated from the simulation of the loss of the last local contact between the crack lips near the crack tip. The pertinence of the different crack front shapes is discussed in term of the effective stress intensity factor range K_eff and in comparison with the experimental crack front observations.

Abstract: Stress intensity factor (K) is the measure of severity of stress at the crack tip. When K exceeds the critical limit (i.e., the material fracture toughness), the crack grows. K is valid in brittle materials (LEFM) and to some extent in ductile materials also provided there is small scale yielding (SSY) at the crack tip. The paper reviews the numerical methodology to obtain KI of ductile, Mode I cracked, CT and SENB test specimens in LEFM and SSY regimes with the help of J integral method. The numerical values are successfully compared with the theoretical values.

Abstract: In the context of a R&D project concerning the new Alcácer do Sal composite railway bridge, a study of the fatigue crack growth on samples of its base material and weldments was performed. For this purpose, tests were carried out on CT specimens designed according to ASTM E647 standard, using the approximate thickness (B) of a structural detail of interest, B=32mm. The choice of B led to a relatively large specimen and was justified by the desire to better simulate service conditions, which would not be possible with smaller specimens, particularly in the case of weldments. The test matrix used included three values of R ratio (maximum/minimum load), 0.1, 0.4 and 0.7, and three material conditions, namely base material (BM), heat affected zone (HAZ) and weld metal (WM). When the nominal range of the stress intensity factor (DK) is used, the measured data displays a strong effect of the weldments on the FCG rates, with the base material presenting higher da/dN values. An evaluation of opening load behaviour was carried out, and it showed extensive closure caused by residual stresses in the HAZ and WM specimens. The investigation included the full field measurement of the residual stress perpendicular to the crack plane, using the contour technique. When the opening load effect was taken into consideration it was found that the da/dN vs. ∆K of the BM, HAZ and WM specimens is approximately identical. Furthermore if loading effects are considered, no significant difference is found for the three R values used, even if, as expected, higher R corresponds to higher da/dN values.

Abstract: In this work, a nondimensional load separation conforming to similarity principle was proposed. Based on this new principle, similarity simulation of the physical phenomenon between the prototype sample and the dimension-changed model sample can be implemented. Then, a modified separation parameter S_pb method was developed, which can absolutely eliminate the effect of referencing blunt cracked specimen on the instantaneous crack length estimation of the sharp cracked specimen, and the forced blunt-corrected initial crack length and the final crack length are recommended as the calibration points for J-resistance curve estimation. Finally, the modified S_pb method is successfully applied in the J-resistance curve estimation of two steels with CT and SEB specimens, respectively.

Abstract: Three sets of fatigue crack growth data tested under different constant-amplitude loads for CT specimens made of 2024 T-351 aluminum alloy are released, and the analyzed results presented in this study are specially emphasized on the correlation between statistics of these scattered fatigue data and their applied loads. Investigating the scatters of initiation cycle and specimen life, it was found that both the mean and standard deviation of initiation cycle, as well as the mean and standard deviation of specimen life, decrease as applied stress amplitude increases. Moreover, the negatively linear correlation between the median values of initiation cycle and applied stress amplitudes presented in linear scale, and between the median values of specimen life and applied stress amplitudes presented in logarithmic scale were found, where the initiation cycle and specimen life are all best depicted by normal distributions for all three data sets. Finally, the mean of intercepts and mean of exponents of Paris-Erdogan law for each data set were studied, and it was found that the mean of intercepts decreases greatly as applied stress amplitude increases, while the mean of exponents decreases slightly.

Abstract: Based on the finite element analysis software ABAQUS, the CT specimens of bimetallic material and the metallurgical composite bimetallic pressure tube with axial crack were simulated on the fracture mechanical properties, after that the crack growth residual strength and the plastic failure stress of the metallurgical composite bimetallic pressure tube model were obtained. The results indicate that the crack growth residual strength generates near the crack tip and the crack growth residual strength of the pressure tube model is smaller than the CT specimens. Meanwhile, the plastic failure stress values obtained from the CT specimens and the pressure tube model are basically consistent with the theoretical calculative values. With the crack depth a increasing, the plastic failure stress values will be reduced. For the same crack depth, the plastic failure stress values of the pressure tube model are slightly lower than the CT specimens and the theoretical calculative values.

Abstract: It is known that fracture characteristics are changed due to the geometric configuration. Also, it is known that toughness data obtained from the standard specimen test are conservative to predict fracture behavior of the real piping. Thus fracture behavior by tests of pipes would to be applied to the integrity evaluation for the piping system. However, fracture test with real pipe is not only difficult to perform but also very expensive, and requires lots of experience. So an estimation method of pipe’s fracture behavior is necessary to solve this problem. The objective of this thesis is to propose a method to estimate the fracture behavior of a pipe from the result of the standard specimen fracture test. For this, fracture tests for standard specimens and pipes are conducted. The resultant load - load-line displacement record of the standard specimen was transformed to that of a pipe by load separation method. To begin with, the load versus load-line displacement curve of a standard specimen extracted from a pipe is normalized by a geometry function of the CT specimen. Then this normalized curve was converted to pipe’s load versus displacement curve by a geometry function of pipe. To verify the constraint factor and the geometric function of pipe, finite element analyses were performed. To demonstrate the proposed method, experimental results of pipes are compared with predicted results. Calculated results from CT specimens are similar to experimental results of pipes. Therefore the transformability from a CT specimen to a pipe by load separation method is proved. Consequently the applicability of the proposed method was proved.