Papers by Keyword: Crack Tip Opening Displacement (CTOD)

Abstract: According to the BS7448 fracture toughness test standard, this paper conducted a CTOD(crack tip open displacement) fatigue crack precast test with the bridge steel and analysis the tests results, thus obtained the related conclusions. With the increase of thickness and strength of the steel plate, the security of fatigue performance and toughness of the plate is becoming more important for the structure. So the evaluation of fatigue performance is a very significant work for engineer. This dissertation through the text study the CTOD Specimens Fatigue Crack Prefabricated process of bridge steel and its characteristics, and its results will provide the important experimental basis and computation basis.

Abstract: Based on the CTOD test, metallographic test is done. The effect of microstructure on CTOD of welded joints for offshore high-strength steel has been analyzed by SEM. The microstructure of different samples in a welding procedure and two different welding procedures are compared. From the microscopic point of view, the reasons why the toughness of the second group welded joints is better than the first group’s and why the toughness of the same group welded joints is different have been determined. And the main microstructure found in EQ70 steel welded joints is the ferrite, tempered sorbite and a small amount of pearlite. Besides some carbide particles are on the matrix of tempered sorbite. The material toughness with a small amount of acicular ferrite is better. Inclusions and pores also seriously affect the toughness of welded joints. Reducing the welding heat input appropriately can refine the grain of CGHAZ and improve toughness. But hardened microstructure may occur owing to too low welding heat input.

Abstract: The CGHAZ Microstructure has great effect on the toughness of welding joints and CTOD. This paper discussed the relationship between the CGHAZ and CTOD .The samples, carried out on high-strength steel EQ56, were observed by scanning electron microscopy. The Microstructural analysis result shows CGHAZ microstructure of EQ56-5, presenting high CTOD with 0.272mm, is low-carbon lath martensite and little M-A constituent, which contribute to the better toughness of EQ56-5. CGHAZ microstructure of EQ56-4, presenting the low CTOD with 0.082 mm, contains M-A constituent. The M-A constituent makes the CGHAZ brittle. The crack would grow easily along the M-A grain boundaries.

Abstract: Several tests including CTOD and Charpy impact have been performed on the weld joints of high strength low alloy steel S335G10+N to identify the toughness. The fracture surface and microstructure of samples extracted from the weld joints are characterized using scanning electron microscopy. Based on all data obtained some relationships between toughness and microstructure has been detected. (1) The tendency of Charpy impact energy distribution with the position of notch is similar to the CTOD values in WP and FL but different in FL+2.CTOD values of FL+2 are greater than those of FL, but the Charpy impact energy has an opposite tendency. (2) The appearance of Pearlite and upper bainite found in WP sample S3554 is regarded as one of the reasons for the lowest CTOD values. The cavity and quasi-cleavage in S35510 fracture surface indicate an inferior toughness compared with some dimples and ripple morphology of slip separation in S3552. (3)Grains close to the fracture surface in the 35511 (Stable crack growth) of FL +2 were elongated owing to plastic deformation caused by crack growth. (4)Hardness of the HAZ is equal to fusion zone and lower than base metal or weld position in S35511. Hardness of fusion zone increases with the depth. (5)A big difference of Average Grain Size is found in microstructure close to the fracture surface among S3557, S3556 and S3559.The greater the Average Grain Size is, the lower the CTOD value will appear.

Abstract: The design and fabrication of offshore structure depend on material science and solid mechanics, such as steel processing technology and welding and materials testing and evaluation and so on. In this paper, CTOD (Crack Tip Opening Displacement) toughness test for weld joint of heavy thickness S355G10+N steel has been carried out to evaluate the fracture resistance of this weld joint under Submerged Arc Welding(SAW). The results show that (1) the weld joint of heavy thickness S355G10+N steel tested possesses excellent toughness. That is to say the materials in the weld joint possess enough capacity resisting the crack and propagation. Therefore the welding procedure specification evaluated can be applied to fabricate offshore structure. What is more, being at the case of as-weld condition for the weld joint the CTOD test was carried out herein, hence the offshore structure fabricated with the welding procedure specification evaluated can avoid the post welding heat treatment. (2) Different positions in weld joint have their special toughness. The position which is 2mm away from the fusion line (located in base metal) (FL+2) possesses best toughness (average value is 0.825mm). The fusion line position (FL) possesses slightly worse toughness (average value is 0.672mm). The weld metal position possesses worst toughness (average value is 0.553mm). This distribution character is different from other references reported and comparision has been made in this study. (3) Being a lower fracture toughness in weld metal, as a design suggestion high stresses should be avoided in this position.

Abstract: This paper presents a simplified strain-based fracture mechanics approach to study the effect of pressure induced hoop stress on bi-axially loaded through walled cracked (TWC) pipes subjected to an external bending load in combination with internal pressure. Elastic-plastic finite element analyses are conducted to establish the relation between global strain and Crack tip opening displacement (CTOD). In the finite element model X65 pipeline steel is considered using power-law idealization of stress-strain, and the inelastic deformations, including ductile tearing effects, are accounted for by use of the Gurson–Tvergaard–Needleman model. Several parameters are taken into account, such as crack length, internal pressure and material hardening. Strain based crack driving force equation is used and maximum load criterion is adopted to determine the critical strain from ductile tearing in the cracked pipeline. The results suggest that presence of pressure-induced hoop stresses increases the fracture response in high-hardening materials and their effects are significant due to large plastic-zone size.

Abstract: This paper presents the implementation of fatigue crack growth power law equations based on ΔK, ΔJ-integral and ΔCTOD fracture mechanics parameters determined in an FE analysis, to plates with multiple site damage (MSD). Results of fatigue tests with constant amplitude tensile loading carried out on mild steel plate specimens damaged with a single central crack and with three collinear cracks are presented. A relatively larger plastic zone occurred in the crack tip region at higher fatigue crack growth rate (FCGR), from 10^-7 to 10^-6 m/cycle. The crack growth models based on the elastic-plastic fracture mechanics (EPFM) parameters describe better fatigue crack growth in this range as compared to the liner elastic models.

Abstract: According to BS7448 fracture toughness test standard, crack tip opening displacement (CTOD) for ASTM A333 Gr.6 low temperature steel which is used in offshore platform were carried out in low temperature (0°C, -29°C). Three point bending tests were conducted on the specimens, the dimension of which is B×2B (B is the thickness) and notch direction is NP. The final value of CTOD was calculated by P-V curves of both the welds and HAZ (heat affected zone). And the experimental results were analyzed and discussed in the paper. The low temperature steel welded joints show good low temperature toughness, the selected welding procedure is applied to construction of pipelines.The CTOD value of HAZ is larger than weld metal and weld is the weakest position in the whole structure. The experiment results provide an important basis for the choice of welding method and welding process parameters, ECA assessment and construction of offshore platform.

Abstract: According to the BS7448 fracture toughness test standard and DNV-OS-C401,this dissertation conducted a CTOD(crack tip opening displacement) fatigue crack precast test with the used steel Q370qE in bridge.Respectively tested different thickness mather materials’condition of fatigue crack,and analysis the test’s results,thus obtained the related conclusions.

Abstract: Three point bending tests were carried out on 14mm-thick Q460C the high-strength structural steel at low temperature, and scanning electronic microscope of the fracture appearance was analyzed. The results showed that the obvious feature of brittle mechanism was shown on the three point bending specimen fracture whose testing took place at -40°C. And the crack tip opening displacement value of Q460C steel, which was less than that of Q235 steel, Q345 steel and Q390 steel at low temperature, tended to decrease with respect to the temperature reduction. Moreover, a Boltzmann function fitting analysis was applied to the experimental data, and the ductile-brittle transition temperature and the changing regularity were obtained.