سید حجت هاشمی

Determination of fracture energy is one of the most important aims in drop weight tear testing (DWTT) for assessment of material properties. The purpose of this study is to investigate a new experimental method for measuring fracture energy of tested steel. In doing so, the wavelet transformation was applied for API steel for the first time to analyze acceleration signals from impact tests. The acceleration signal from DWTT on API X65 steel was captured and studied using wavelet transformation. The DWT tests were conducted on three specimens with chevron notch according to API 5L standard. After impact test, noisy acceleration signal from each specimen was smoothed using discrete wavelet transformation. Then, force-displacement response was derived for each specimen from which fracture energy was calculated using the area under each plot. The obtained average fracture energy was 6326 J and the peak load was 219 kN. It was found that about 39% of the total fracture energy was used in crack initiation while 61% used in crack propagation. The comparison of the obtained results with previous data from similar research showed the effectiveness of wavelet transformation for accurate measurement of fracture energy using DWTT.

Thermomechanical steels are widely used in oil and gas pipelines due to their high toughness and high resitance against crak growth. A large part of the steel pipelines used in the oil and gas industry in Iran is made of API X65 steel. The fluctuations of internal gas pressure in steel pipes can cause fatigue failure and lead to gas leakage and explosion. So, the control of damage initiation and structural integrity of gas pipelines is of great importance. In this study, the S-N curve and the fatigue strength of the base metal of the API X65 steel were estimated by performing fatigue tests. For this purpose, 24 and 25 test specimens along the seam weld in the coil transverse direction, and perpendicular to the seam weld along the coil rolling direction were prepared according to ISO 1143 standard, respectively. All test samples were cut from an spirally welded pipe with 1219mm outside diameter and 14.3mm wall thickness and were tested on a completely reverse rotating-bending fatigue machine. Statistical analysis of the results was performed by considering the normal logarithmic distribution. The mean curve, characteristic curve, and confidence interval of the results were obtained both in the finite fatigue life range and in the fatigue resistance. The mean endurance limit of the base metal perpendicular to and parallel to the seam seam were 305 and 291 MPa, respectively which were in the range of 0.4 to 0.6 of material tensile strength and above the seam weld endurance limit (258 MPa).

The purpose of this research, in addition to determining the characteristic forces, including the yield force and maximum force, was to calculate the correction factors to predict the onset of failure in the energy transmission pipelines with high toughness under dynamic (impact) loading. To achieve this goal, an instrumented Charpy impact machine, which plots the force-displacement diagram during the impact test, was used. Then, by dividing the area under the force-displacement diagram into two parts, both the initiation energy and crack growth energy in API X65 steel were calculated. The results showed that the total energy provided by the machine dial was in agreement with the total energy calculated from the area under the force-displacement curve. Characteristic forces were also determined from the force-displacement curve as described in the BS 14556 standard. After that, power law expressions with high accuracy were extracted to describe the behavior of the tested steel against variations of the Charpy sample thickness for crack initiation energy, crack propagation energy, and characteristic forces. Additionally, the average correction factor, which is used in prediction models of energy transmission steel pipelines, was found to be 1.26, which is in good agreement with the available results for the current steel in the literature. It was shown that by increasing the thickness, due to the transition from plain stress to plain strain condition, the correction factor changed from 1.26 to 1.3 in 8 to 10 mm thicknesses, while it did not change so much from 4 to 8 mm thicknesses. By examining the characteristic forces and plotting the ratio of the yield force to the maximum force versus thickness variation, it was also found that increasing the thickness leads to decreasing the work hardening of the steel.

The critical Crack Tip Opening Angle ( ) is considered as a convenient parameter to characterize the crack arrest toughness of natural gas pipeline. Load-displacement curve is a comprehensive reflection of geometry, mechanical property and fracture behavior of a loaded specimen, so it would be highly advantageous to deduce   from load-displacement curve directly. From force-displacement curves, maximum force of 209kN and 207kN were obtained for experimental and numerical data, respectively. In this article, a combination of load-displacement and deformation of the drop weight tear test specimen was used to calculate the critical crack tip opening angle. For this purpose, the simplified single-specimen test method was used. The   is dependent on the slope of the steady-state crack growth region and the plastic rotation factor. Firstly, based on the fact that load decreases linearly with the increment of displacement during steady-state crack growth, the slope of the experimental load-displacement curve of API X65 steel in the steady-state crack growth region was obtained as 21.583. Secondly, by modeling the drop weight tear test in Abaqus software and using two methods of mises stress and neutral axis, the plastic rotation factor was obtained as 0.5688 and 0.5651, respectively. Finally, these parameters were used and the critical crack tip opening angle was determined as 12.00 and 12.08 degrees.

Fatigue failure is the most common type of failure in structures under oscillatory loading. Fatigue damage in steel gas pipelines is very important due to internal pressure fluctuation. A large part of pipelines in oil and gas industry of Iran are made of thermomechanical steel of grade API X65, made by spiral submerged arc welding. In this study, the stress-life curve and fatigue limit of the spiral weld seam of this steel are determined by fatigue tests. For this purpose, 20 test specimens (12 specimens used in the limited fatigue life zone and 8 specimens used to estimate fatigue strength) according to ISO 1143 standard. All test samples were cut from an actual spirally welded pipe with 1219mm outside diameter and 14.3 mm wall thickness and were tested on a completely reverse rotating-bending fatigue machine. Statistical analysis of the results was performed by considering the normal logarithmic distribution. Mean curve, confidence interval, and characteristic curve of the results were obtained in the finite fatigue life range using Basquin fatigue model according to ISO 12107 and ASTM E-739 standards. In the fatigue resistance range ISO 12107 standard was used. The mean endurance limit of the seam weld of the tested steel was 258.5 MPa which is located in the conventional range of 0.4 to 0.6 of the ultimate tensile strength of this steel.

In this paper, dynamic behavior of API X65 steel under impact loading was studied using an instrumented Charpy machine of 450J capacity. The fracture energy can be measured using this testing machine in two different ways. In the first method, the difference in potential energy before and after impact test is reported via machine dial. In the second method, the output voltage from load-cell (mounted on the machine tup) gives load-displacement data from which energy is calculated via curve integration. Using instrumented data, the initiation energy, propagation energy and total fracture energy for eight series of specimens made from tested steel were measured and found to have exponential behavior versus initial notch depth. Furthermore, the characteristic forces used for the design of dynamically loaded structures (namely general yielding, Fgy, and maximum force, Fm) were determined from the force-displacement diagram. The important innovation of this research is presentation of eight new correction factors for different notch depth of specimens. These new correction factors and their mean value were compared with a similar data from a previous research work for conventional fracture models of energy transportation steel pipelines which showed good agreement.

The most important objective in the drop weight tear test (DWTT) is obtaining the fracture energy. DWTT use in the military, steel and pipeline industries. By installing the proper equipment, the dynamic impact load can be measured in terms of the hammer displacement of the DWTT machine and it can calculate many parameters such as initiation energy, propagation and total fracture energy. For this purpose, numerical analysis by ABAQUS software was used to determine the proper location of the strain gauge on the hammer. Then, the strain gauge circuit (load-cell) was installed and calibrated using analytical and experimental methods. In order to use the circuit, the voltage must be converted to the load values. The analytical and experimental methods were used to obtain the desired coefficient. The two coefficient had an acceptable difference. In addition, in order to increase the accuracy and repeatability, other equipment such as guide and micro-swich were used in the DWTT machine. Experiments with the DWTT machine showed the repeatability of the results.

The Charpy impact test is an experimental method for determination of materials dynamic properties at different temperatures to investigate the ductile to brittle transition behavior of tested materials. The percentages of ductile and brittle fractures can be evaluated based on fracture area of Charpy specimen (according to API E23 standard) by visual techniques which do not provide exact percentages of these fractures. In this study, a method is proposed to calculate the exact percentage of ductile fractures using image processing, which makes it possible to quantitatively examine different parts of the fracture surface with high accuracy. All steps of image processing are described for eleven Charpy standard specimens of API X70 steel, tested at temperatures between +20 to -80 °C with a temperature increment of 10 °C. In this research, converting a qualitative image of fracture surface to a quantitative matrix is described for the first time. Prediction of the shape of ductile and brittle parts of the fracture surface at temperatures between +20 and -80 °C is one of the results of this study. The percentages of ductile fractures using image processing for temperatures of +20, 0, -20, -40, -40, -60 and -80 °C were obtained as 100, 100, 86, 53, 36 and 0, respectively. The transition temperature was -45 °C for this steel, corresponding of 50% ductile fracture.

In addition to non-destructive testing, the weld joints in large structures must also be confirmed by mechanical destructive testing (such as tensile and impact tests). Drop weight tear test (DWTT) is a reliable test to evaluate ductile fracture and crack arrestability of pipelines. Previous research has focused on measuring and comparing the base metal fracture energy from DWTT using finite element and experimental methods. However, in the present research, the spirally welded seam of API X65 steel specimens were machined from an actual pipe and tested using DWTT. The load-displacement curves obtained from both numerical and experimental methods have been evaluated and compared regarding correction factors. Using the obtained correction factors (from important characteristics of the load-displacement curve such as total absorbed energy, crack initiation and propagation energy are used), the load-displacement curve of simulation can be relied upon with acceptable accuracy. Real curves can be obtained with a relatively large amount of time and money. For the steel used in the present study, the correction factor for base and weld metal is 1.8. The use of this value will also be acceptable for steels of the same grade. The weld metal correction factor using the simulation (based on Gurson-Tvergaard-needleman method) was 1.7, which was slightly different from the experimental value.

To use higher capacities in Iran's energy transmission systems, API standardized pipes made of API X65 steel have been utilized (made of thermo-mechanically controlled rolling process, TMCR steels). The TMCR inherently increases the anisotropic properties of steel coils and plates used for pipe manufacturing. In addition, the production of helical welded pipe involves steps that can lead to different mechanical properties in different directions. The aim of the present study is to measure the orientation dependence of the Charpy fracture energy. Therefore, the effect of changing the angle of specimens relative to the rolling direction and also the effect of changing the notch orientation (three notch A, B and C in total) on the fracture energy in API X65 steel has been experimentally determined. The maximum change in the average Charpy fracture energy at different angles relative to the rolling direction is a maximum of 13% (in notch B), but the largest change in the average Charpy fracture energy between different notches is a maximum of 12.2% (at an angle of 0 °). As a result, the effect of changing the angle of the specimen relative to the rolling direction is greater than the effect of changing the notch orientation on the Charpy fracture energy. Also, at an angle of 67.5 degrees to the direction of rolling (equivalent to the diagonal direction (D-D)), the most fracture energy for all notches was obtained. To quantitatively compare the fracture energy changes in different notches, an index called anisotropy index has been presented.

In the present study Charpy impact tests on a steel plate of API X65 (made from a pipe with adiameter of 1219 and a thickness of 14.3 mm) with full size (10 × 10 × 55 mm) with differentnotch depth (in the range of 1.25 to 3 mm) were conducted and the fracture energy wasmeasured. For this purpose, 24 specimens were made in triple series with 8 different notchdepths, and then the failure energy was obtained by Charpy impact tests. Also, computersimulation of the experiment with a three-dimensional model was performed based onGurson's modified damage law in Abaqus software. The experimental results showed that therelationship between the fracture energy (E) and the depth of the Charpy samples (a) for thetested steel is E = 499.65e-0.3501a. Using this relationship, the Charpy energy can bedetermined for this steel for any notch depth.

The purpose of this research is to investigate low velocity impact effect on fracture energy of DWTT specimen made of API X65 steel. As in some cases due to sudden impact (pre Loading) and permanent deformation, fracture energy of steel may be changed, the investigation of this study is important. Test specimen is API X65 steel of single edge notch type with Chevron notch with a depth of 5.1 mm which is cut from a spiral seam welded steel pipe with an outside diameter of 1219mm and wall thickness of 14.3mm. This steel is widely used in oil and gas industery in the world. The experiments were performed on 21 specimens. In the first stage, the drop height of the hammer was low so that the specimens did not fail and only the area adjacent to the crack tip entered the plastic zone. Subsequently, the samples were fractured by applying a second impact from a standard height of 2 meters. By analyzing experimental data, the fracture energy for each sample was calculated and compared with each other. By plotting the fracture energy in terms of the initial impact energy, it was observed that by increasing the initial impact energy in the sample, its fracture energy decreased. Finally, a linear relationship with acceptable accuracy for this energy drop was proposed, which showed a direct relationship between the fracture energy and the initial impact energy.

The drop weight tear test has been used for nore than seven decades in oil and gas industries to measure fracture energy and to study the features of ductile and brittle fracture of energy trsnportaton pipelines. In this study, the fracture energy of API X65 steel was measured using the acceleration signal of the drop weight tear test equipped with accelerometer. The test sample was cut from the natural gas transmission pipe with outside diameter of 1219 mm and wall thickness of 14.3 mm. The test was carried out in accordance with API 5L standard on drop weight tear test apparatus with a 700 kg hammer equipped with accelerometer having 3 m drop. Using the obtained natural frequency and the low-pass Batterworth filter, the unwanted oscillations were removed from the high-velocity test data. The fracture energy (area under load-displacement record) obtained as 6791 J, which was around 4 percent less than the energy value of the drop weight tear test equipped with strain gauge in previous studies. Due to this small difference, the accuracy of the drop weight tear test equipped with accelerometer was confirmed.

The purpose of this paper is to experimentally investigate the natural frequencies of notched homogeneous and inhomogeneous specimen made from API X65 steel. The specimens cut from spiral welded pipe, tested on an equipped low blow drop weight tester with accelerometer. The tests were performed according to the API 5L standard. The homogeneous specimen was seamless and included only the base metal, while the inhomogeneous specimen included the weld seam and three zones of base metal, heat affected zone and weld. In the present study the specimens were subjected to hammer low blow in the middle without plastic deformation. The laboratory data (voltage-time) were transferred from time to frequency domain using Fourier transformation and the imposed oscillations were removed from the frequency signal by the Butterworth low pass filter. As the hammer drop height increased, the natural frequency in the specimens was almost constant. The natural frequency in the inhomogeneous specimen was less than the homogeneous specimen. Having information about the natural frequency, it is possible to prevent the destructive phenomenon of resonance in the main test (complete fracture of the specimen). Also, using the results of equipped low blow drop weight test and knowing the natural frequency, the dynamic stress intensity factor of the test specimen can be determined.

In this study for the first time, changes in the thickness of the fracture cross-section of the inhomogeneous sample (with horizontal weld seam) of the API X65 steel, using drop weight tear test specimen have been investigated experimentally. The fracture surface of the test specimen consisted of three zones of base metal, heat affected zone and weld metal with different microstructure and mechanical properties. The most thickness reduction was in the cleavage fracture area of the notch root. In the base metal zone, thickness changes were constant which indicated the stable crack growth in this area. In both heat affected zones before and after the weld zone, the thickness changed with a constant slope. Due to the high hardness and low fracture energy of the weld zone, the lowest percentage of thickness changes was in this zone. Thickness in the weld zone increased with a constant slope due to the stretching of the weld zone to the end of the crack growth path by the force caused by the change of fracture mode from tensile to shear. Also in the reverse fracture zone, due to the increased in compressive strain caused by impact of the hammer on the sample, the thickness increases with a significant slope and reached the maximum value.

Because of the inherent structure of welded pipelines, the seam weld can be a potential source for initiation and propagation of crack that can eventually lead to failure of the structure. Due to the critical conditions in the welding region, the investigation of failure energy in gas transportation pipeline is very important for engineers and line designers. In this paper, the three-point bending test (according to the standard specimen of drop-weight tear test) was performed quasi-statically on the seam weld pipe and base metal of spiral seam weld pipe of API X65 steel from which force diagrams were extracted. The presence of sudden load drops in the force-displacement diagram of the specimen in the weld indicated the inhomogeneous structure of the weld. The diagrams of force-displacement, yield and ultimate force, amount of steady crack growth and fracture energy of the metal and seam weld specimens including initiation and propagation energy of crack were investigated and compared. Also, the ratio of the force drop to the ultimate force at the same displacement rate was investigated. The results showed that in seam weld compared to the base metal specimen, the yield force was higher and the ultimate force, the amount of steady crack, initiation and propagation energy of crack were lower. In addition, the lower ratio of force to ultimate force (at the same displacement) in the base metal also indicated a high resistance of the base to the crack propagation.

In this study investigated the effects of momentum variations on fracture energy in Charpy impact testing of API X65 steel by experimental and numerical methods. Experimental analysis was conducted in the various speed of impact about 3.50 to 5.72 m/s and impact energy varied about 450 to 1200 J. The experimental results showed that increase of about 63% in impact speed increased the fracture energy about 15%, because of material properties dependence on loading rate. Numeral studies were performed in two categories with ABAQUS software. First mass variation in constant velocity assumed standard quantity about 5.5 m/s in which impact energy varied about 300 to 1200 J and the second, velocity variation with constant mass assumed 50 kg that impact energy varied about 625 to 1600 J. The simulation results showed the variations in mass had not any effect in fracture energy and in all analyses, it was about 265 J. However, increasing the velocity variations with constant mass, caused a slight reduction of about 5% in the fracture energy. The reason for the difference between experimental and numerical results is the lack of consideration of the effect of strain rate on mechanical properties of tested steel in numerical analysis.

Drop weight tear test (DWTT) is a common experiment to determine the dynamic properties and evaluate the fracture energy of steel specimens. The purpose of present research is presenting experimental, numerical and analytical methods for determination energy transferred to the specimen during impact. Test specimen was cut from an actual spiral seam welded steel pipe with an outside diameter of 1219mm and wall thickness of 14.3mm and then machined to standard size. Then chevron notch was placed in the middle of specimen and test sample was fractured under dynamic loading with initial impact velocity of 6.3m/s. Experiment was impelemented on three specimens by dropping the impactor from 2m height. By drawing and analyzing the energy-displacement and energy-velocity diagrams a linear relation between fracture energy and hammer velocity was. Using this relation by having the impactor velocity in each time, one could evaluate the transferred energy to the specimen and finally the fracture energy of tested specimen. Also, using kinetic energy and potential energy formula by knowing the hammer location and velocity in each time and by neglecting energy losses one could calculate the absorbed energy in the specimen

One of the most important purposes of the drop weight tear test (DWTT) is to achieve the value of fracture energy for better evaluation of tested steel properties. In the present research, experimental and numerical measurement of fracture energy in drop weight tear test specimen with chevron notch on API X65 steel has been carried out. The purpose of the determination of this energy is to estimate the strength of material due to fracture. The test specimen was cut from an actual spiral seam welded steel pipe of API X65 grade with an outside diameter of 1219mm and wall thickness of 14.3mm and then it has been machined to standard size. Then chevron notch with a length of 5.1 was placed in the middle of the specimen and the specimen was fractured under dynamic loading with an initial impact velocity of 6.3m/s. The maximum force of 229kN and 225kN were achieved for experimental and numerical data, respectively by drawing force-displacement and energy-displacement curves. The fracture energy of the test sample for experimental and numerical data was obtained as 7085J and 6800J, respectively by evaluation of the area under the force-displacement curve. Based on the results of experimental curves, about %59 of fracture energy was used for crack propagation and the remaining was used for crack initiation and plastic deformation of test sample near anvils and striker regions. In the end, drawing a linear curve for fracture energy of specimen based on the hammer velocity showed that the slope of this curve could be a good criterion for estimating the energy loss and fracture behavior of the test specimen.

Different factors such as initial notch angle, notch depth, notch root radius, notch, specimen size, hammer geometry and striker velocity, and initial notch creation method (i.e. broaching and machining) influence the experimental determination of Charpy impact energy. Thus, the study of the influence of these parameters on Charpy fracture energy is important. In the present research, Charpy impact experiments are conducted on test specimens made from API X65 steel (used for gas transportation pipelines). The tests samples have standard size with different notch radius (within the range of 0.13 - 0.41 mm). By measurement of fracture energy, Charpy impact energy is determined as a function of variations of initial crack radius. Through having geometrical dimensions of the initial notch and considering the stated standard tolerance the aforementioned function allows the determination of measurement error of API X65 steel Charpy impact energy. Then, the test samples are modeled as 3D in ABAQUS using Gurson damage theory. The comparison of test and simulation results for different notch radius are presented in details and compared.

In this study, the fractograghy of the inhomogeneous specimen (including seam weld) of API X65 steel was performed, for the first time by the drop weight tear test. The fracture surface of the test specimen consisted of three zones of base metal, heat affected zone and weld metal. The test was performed according to API 5L standard. The crack initiated with a cleavage flat fracture from notch root and continued up to 14.7 mm from the fracture path. The cleavage fracture immediately changed to a ductile shear fracture with a 45 degree angle to the surface plane, which continued to the inverse fracture zone. SEM images of all three zone of the base metal, heat affected zone and weld metal showed signs of ductile fracture, including dimples of varying size, orientation and shape. Also, faint chevron marks with high density were observed in the root of crack in the base metal. Furthermore, in the hammer impact zone, an inverse cleavage fracture was observed, which was enclosed on both sides by shear surfaces with an angle of 45 degrees. The calculated ductile fracture surface was within the standard range and indicated the confirmed toughness of these steels for the use in gas transportation pipe lines.

Welding residual stresses decrease designing stress in natural gas transmission pipes with large diameter under high internal pressure. The outside diameter and wall thickness of API X70 steel in this research are 1423 and 19.8 millimeter. Hole drilling is the most common technique in order to measure residual stresses. Because of large diameter of this pipe, its transportation to conduct hole drilling test is a big problem so cutting a finite sample is desired. In this study standard dimension of this sample plate is analyzed and simulation of welding process is done from which and residual stresses in different directions are obtained. Residual stresses in the thickness direction is presented for the first time. The results showed separating a finite sample with the size of 320×440 millimeter is appropriate to do hole drilling test. The location and amount of the maximum residual stress is evaluated and compared for both simulation and experimental samples. Variation in hoop and longitudinal residual stresses on both internal and external surfaces of pipe samples are investigated. Also validation of simulation results with the experimental results of the same pipe is perfomed. Maximum residual stress (460MPa) is measured on inner surface of the pipe (96 percent of yield stress) which is reduced to 200MPa (42 percent of yield stress) after hydrostatic test. Because residual stress after hydrostatic test is lower than half of yield stress, hole drilling technique is validated after hydrostatic test.

The welded zone of API X70 steel pipe, due to the inherent defects of welding, is a potential area for initiation and propagation of cracks which can eventually lead to damage of the structure. In this research, mechanical behavior of spiral seam weld was evaluated in API X70 pipe steel in three zones (base metal, HAZ and weld metal) using uniaxial tensile and three point bend (3PB) experiments. Three tensile specimens were used in each zone for measurement of mechanical properties. For studying the mechanical behaviour of the pipe, one specimen with 3PB geometry was tested for each zone. Specific values including yield, peak and final load for 3PB specimen were determined from load-displacement plots. The associated energy for each load plus initiation and propagation energies were calculated and the results were analyzed in relation to microstructure in each zone. With an analytical equation based on slip-line field analysis, yield strength was determined with 3PB specimen in each zone and compared with the result of uniaxial tensile experiment. From uniaxial tensile experiment, yield strength levels of 560, 514 and 507 MPa were found for base metal, HAZ and weld metal respectively and from 3PB experiment 604, 582, 575 MPa respectively. Keywords: Pipe line steel; API X70; Three point bending test; Spiral seam weld; Static fracture energy.

The API X65 steel (with a minimum yield strength of 65ksi equivalent to 448MPa) is one of the most common types of pipe steels in the transportation of natural gas in Iran. By studying the ductile and brittle fracture areas at the fracture surface of this steel, we can show the quality of this type of steel. In the present study, macroscopic fracture surface characteristics in three-point bending test specimen are studied (based on the geometry and standard notch of drop-weight tear test specimen). Test specimens were machined from an actual steel pipe of API X65 grade with an external diameter of 1219 mm (48 inches) and wall thickness of 14.3 mm. Due to the quasi-static test conditions and speed of the machine's jaw (0.1 mm/s), the test was carried out on base metal specimens with machine chevron notch of 15, 10, and 5.1 mm depth, respectively, that was controlled with changing location. By applying the test load, cracking initiated from the notch root in each specimen and continued without crack specimen (ligament). At the end of the test, test specimens were cooled by liquid nitrogen and were broken in a brittle manner. In this paper, after investigation of the failure mode and the crack expansion in the standard specimen, investigation of macroscopic fracture surface characteristics was conducted by optical microscopy. By observing the fracture surface, different features such as thickness variation, shear regions (ductile fracture), cleavage fracture, shear lips, inverse fracture, and brittle fracture were studied. Having above 85% shear area, the ductile fracture of specimen was confirmed.