Circumferential Notch Research Articles

Application of ANN Concepts for Prediction of Crack Growth and Remaining Life of Circumferentially Cracked Piping Components under Different Loading Scenarios

This paper presents the details of Artificial Neural Network (ANN) based models to predict crack depth and remaining life of circumferentially part-through cracked piping components used in the nuclear industry. Crack growth data from experimental studies on (i) straight pipes made up of SA 312 Type 304LN stainless steel having part-through circumferential notch under combined bending and torsion and (ii) dissimilar metal pipe weld joints having circumferential through-wall crack in the weld were used. The dissimilar metal pipe weld joint is made up of SA312 Type 304LN austenitic stainless steel and SA508 Gr. 3 Cl. 1 low alloy carbon steel and joined by Nickel-rich Inconel alloy weld. About 75% of the mixed experimental data has been used for development of model and the remaining data for validation. For the development of ANN model, (i) back propagation technique (ii) sigmoidal transfer functions and (iii) Levenberg-Marquardt algorithm are used. The maximum percentage difference observed between the predicted and the experimental results for crack depth and remaining life was 19.59% and 15.78% respectively. The efficiency of the developed model was verified using several statistical parameters. The developed models are useful for the structural integrity assessment of piping components under different loading scenarios.

Microstructural Engineering of Mn-Alloyed Austenitic Steel for Hydrogen Storage and Delivery

Austenitic stainless steels are commonly used for hydrogen storage and transportation. These alloys have a high nickel (Ni) content, which increases alloy cost. In this study, high manganese (Mn) austenitic alloys were evaluated as potential lower cost alternatives. Two heats of high Mn alloys with different stacking fault energies (SFE) of ~29 mJ·m-2 and 49 mJ·m-2 were acquired. Additionally, a new vanadium (V)-microalloyed high Mn alloy was designed to achieve a SFE of 47 mJ·m-2 to minimize planar slip deformation mechanisms. Post-processing via cold working in conjunction with aging was also performed on the V-microalloyed high Mn steel. Hydrogen embrittlement sensitivity was investigated using circumferential notch tensile specimens cathodically charged with hydrogen in a 0.05M NaOH electrolytic solution. The alloys were compared to a cold-worked 316L stainless steel, which exhibited no strength loss due to hydrogen. The high Mn alloys with SFE of ~29 mJ·m2 and 49 mJ·m-2 had notch strength losses of 11 and 6 pct, respectively. The V-microalloyed high Mn steel in the as-hot-rolled condition had a notch strength loss of 17 pct. The V-microalloyed high Mn steel in the cold worked and aged condition indicated no notch strength loss in hydrogen, which was comparable to the performance of the 316L stainless steel.

Effects of predeformation on torsional fatigue in R260 rail steel

Rolling contact loading induces severe plastic deformations and initiates cracks near the rail surface. Prevention of such rolling contact fatigue cracks requires a better understanding about the mechanical behavior of the deformed material in this region. Even so, current rail standards do not consider the plasticity-induced changes to mechanical behavior. They only evaluate the mechanical performance of virgin rail steels under uniaxial loading conditions, which is not representative for the material’s performance after years of service. This study proposes a new method for fatigue life evaluation of deformed material under loading conditions similar to rolling contact loading. Both virgin and predeformed test bars with a circumferential notch were subjected to strain-controlled torsional fatigue loading to evaluate the influence of axial loading, predeformation, and torsional loading direction. Superimposed compressive axial loads increase the fatigue life without affecting the torque response. The predeformed test bars exhibited longer lives, an effect we attribute to the combination of different torque responses, hardening, and anisotropy.

Analysis of Phenomenon of Plasticity Loss of Steel Core Made by Selective Laser Melting Method in Zone of Pressure Mould Conformal Cooling Channel.

This paper presents the results of testing the mechanical properties of maraging steel 1.2709 that were obtained by the SLM method under uniaxial and triaxial states of stress. The triaxial state of stress was realised by making circumferential notches in the samples with different radii of rounding. The specimens were subjected to two types of heat treatment, which consisted of ageing at 490 °C and 540 °C for 8 h. The results of the tests that were conducted on the samples were considered as references and compared with the results of the strength tests that were conducted directly on the SLM-made core model. Differences were found between the results of these tests. Based on the experimental results, the relationship between the equivalent strain of the specimen in the bottom of notch εeq and triaxiality factor η was determined. Function εeq = f(η) was proposed as a criterion for the decrease in the plasticity of the material in the area of the pressure mould cooling channel. Using the FEM method, equivalent strain field εeq and triaxiality factor η were determined in the conformal channel cooled core model. Based on the proposed criterion of plasticity loss and the results of numerical calculations, it was shown that the values of equivalent strain εeq and triaxiality factor η in the core that was aged at 490 °C did not meet this criterion. On the other hand, the values of strain εeq and triaxiality factor η did not exceed the safety limit when ageing was carried out at 540 °C. The plasticity loss method presented in this paper assumes that the value of the triaxiality factor in the vicinity of the channel is influenced by the shape, cross-sectional dimensions and trajectory of the channel axis. Using the methodology proposed in this paper, it is possible to determine the value of allowable deformations in the cooling channel zone and to determine whether the heat treatment applied to the SLM steel does not cause an excessive reduction in the plastic properties.

Effect of Reference Reflector Orientation for Ultrasonic Inspection of Piping Welds

In this paper, the sensitivity responses from both axial and circumferential notches on piping calibration blocks are compared and their uses discussed when performing ultrasonic inspection of piping welds.

Comparison of fatigue performance of polyethylene pipe grades in the form of extruded and compression molded specimens

Due to the very long operational lifetimes (up to 100 years) of polyethylene pipes, there is a strong demand for accelerated testing of pipe materials to experimentally prove the ability of a certain material to achieve such lifetime. Among the accelerated tests of performance, the cracked round bar (CRB) test is the most convenient. The CRB test takes advantage of cyclic loading that is applied on cylindrical specimens with a circumferential notch in the middle. The specimens are usually manufactured from plates produced by compression molding from the investigated material. However, for polyethylene pipe producers, the possibility of making the specimens by extrusion would save time and lower the costs. This study is dealing with the comparison of CRB test, and a new test carried out on notched pipe samples produced by extrusion. The goal is to investigate the possibility of measuring the performance directly on extruded pipe specimens. The comparison is illustrated on experimental results of several polyethylene pipe grades of PE80, PE100 and PE100RC often used for pipe manufacture.

Analysis of ultimate fracture bearing capacity of CHS XK-joints welds

The current study presents advanced finite element (FE) models that combine the stress weighted damage model and the crack propagation using explicit dynamic approach of commercial FE software ABAQUS. The applicability of the subroutine for ductile fracture prediction and crack propagation modeling of structural steel is confirmed by comparing the results of fracture tests performed on circumferential notch specimens. FE analysis is performed on 17 circular hollow section (CHS) XK-joint models with various sized partial penetration welds to simulate the fracture process of the joint models, and the obtained results are used to analyze the fracture ultimate bearing capacity of the joint weld. The results indicate that the crack first appears on the inner side of the weld at the crown point of the intersecting line of the tensile web member. The findings also demonstrate that the joint weld does not lose the bearing capacity completely after the initial cracking. Instead, the joint weld's bearing capacity increased with the displacement at the early stages of fracture propagation to reach the maximum value prior to gradual decrease in bearing capacity. A design formula of weld bearing capacity suitable for partial penetration weld has been proposed herein to incorporate the effects of uneven distribution of joint weld stress in the considered XK-joints.

Effect of retrogression and reaging (RRA) on pitting and stress corrosion cracking (SCC) resistance of stir zone of high strength AA7075-T651 alloy joined by friction stir welding

The main purpose of this study is to investigate the influence of retrogression and re-aging post weld heat treatment (RRA-PWHT) on potentiodynamic corrosion (PC) and stress corrosion cracking resistance (SCC) of stir zone (SZ) of AA7075-T651 alloy butt joints developed using friction stir welding (FSW). FSW was employed to overcome the problems in fusion welding of AA7075-T651 alloy such as solidification cracking, heat affected zone (HAZ) softening, grain coarsening, dissolution of strengthening precipitates and inferior mechanical performance. The SZ of joints were subjected to potentiodynamic corrosion test in 3.56 wt% NaCl solution. The circumferential notch tensile (CNT) specimens were subjected to SCC test at various conditions of axial stress level in 3.56 wt% NaCl solution. The microstructural features of butt joints SZ were characterized using optical (OM), scanning electron (SEM) and transmission electron microscope (TEM). The results disclosed that RRA-PWHT joints exhibited greater PC and SCC resistance compared to as-welded joints. The threshold stress level for the failure of CNT specimens of parent metal, as-welded joints and RRA-PWHT joints was 242 MPa, 175 MPa and 198 MPa, respectively. The threshold stress level of RRA-PWHT joints and as-welded joints is 81.81% and 72.31% of parent metal threshold stress level. It is correlated to increase in vol% and coarsening of grain boundary precipitates with enrichment of copper at the grain boundaries.

Analysis for tensile strength of multi-phase alloy composed of ductile matrix and brittle reinforcement with circumferential notch in As-cast AZX912 Mg alloy

Non-combustible Mg alloys have been developed, and they tend to form cast defects. A crack propagates stably from a defect before the final fracture. The stable crack propagation is divided into two steps. I: cracking of the transversely long reinforcing phase that exists ahead of the defect, and II: fracture of the matrix ligament between the cracked reinforcing phase and the defect. In this study, a tensile test of an as-cast AZX912 Mg Alloy with a circumferential notch is performed. The equivalent plastic strain in the ligament is determined by FEM analysis based on the actual crack observed during stable propagation. On the other hand, the maximum equivalent strain tolerated by the ligament is obtained from the DIC observation of tensile tests conducted on AZ31 whose fracture elongation is about 0.15. As a result of comparing the equivalent plastic strain on the ligament with the equivalent strain that the ligament can tolerate, the latter is smaller than the former. Therefore, it can be assumed that the second step occurs immediately after Step I. The authors conclude that Step I should be the focus of attention.

Consideration on stable crack propagation from cast defect in As-cast AZX912 Mg alloy

Mg alloys have the lowest density of metals commonly used as structural materials. A non-combustible Mg alloy with a high ignition point has been developed, and they tend to form cast defects. In order to determine whether a defect in the alloy can be regarded as a crack, tensile tests were conducted on specimens with circumferential notches that simulate a crack by varying the notch root radius. The results of the tensile tests showed that the strengths of the specimens were similar at all radii, and that in one specimen, the crack was initiated and grew stably only from the notch root and did not interfere with the shrinkage porosity. From the observations of longitudinal sections of the specimens subjected to interrupted tensile tests, the path of the stable crack propagation was clarified and the length scale of crack initiation was defined. Based on the comparison between the length scale and the notch depth, and the fact that stable crack propagation occurred from all notch with a 1 mm deep notch, we predict that stable crack propagation always occurs in a fracture process from notches smaller than 1 mm.

Crack Closure in Cycles with Dwell Times at High Temperature

The growth of short cracks in round bar specimens with a circumferential notch has been investigated by an extensive experimental study. This specific specimen geometry has been applied to determine notch support based on a reduced crack growth rate due to the induced stress gradient compared to situations without a notch. At the same time, this investigation of crack growth in low cycle creep-fatigue conditions reveals a significant influence of dwell times on crack closure, which is discussed in detail in this paper. Therefore, a fracture mechanical approach based on an elastic-(visco)plastic crack growth simulation is used to compute the cyclic effective fatigue and creep crack tip loading. The crack growth rates are then determined by a Paris and Nikbin–Smith–Webster model. All investigations are based on strain controlled conditions at different loading levels. The shape of the applied cycles is trapezoidal, having a dwell time of three minutes at maximum and minimum load, and is symmetric in tension and compression. It turns out that a separate consideration of crack opening and closure reveals an important difference in effective crack tip loading range and therefore has a significant impact on fatigue crack growth. Moreover, this paper shows that the effect of dwell times is linked to the influence on plasticity induced crack closure and to the stress relaxation during the dwell time in compression, yielding less crack closure.

Влияние степени деформации в условиях низких температур на превращения и свойства метастабильных аустенитных сталей

Introduction. For reliable operation of low-temperature equipment, it is necessary to use materials capable of ensuring operability in a wide temperature range under conditions of alternating loads, exposure to corrosive media, etc. Most often, in such cases, metastable austenitic steels (MAS) of various alloying systems are used. Despite sufficient experience in the use of such materials, not enough information is collected on the behavior of such materials at low temperatures, including phase-structural transformations, the features of such transformations in different temperature zones, including when a load is applied, both static and dynamic. The subject of the study in this work is selected MAS 10Cr14NMn20 and 10Cr14Mn14Ni4Ti grades. The purpose of the study is to evaluate the performance of industrially used metastable austenitic steels for its possible use instead of steel 12Cr18Ni10Ti. Research methodology. The phase composition of the samples was studied on a DRON-3.0 X-ray diffractometer. Mechanical tests were carried out in the temperature range from +20 to -196 °C. Static uniaxial tensile tests were carried out on a R-20 tensile testing machine; cylindrical specimens with threaded heads were prepared according to GOST 11150–75, as well as samples with a circumferential notches. Dynamic bending tests were carried out on a pendulum impact tester, using samples according to GOST 9454–78. Results and Discussion. Based on the data obtained, it is found that an increase in the strain rate at low temperatures contributes to a decrease in the number of martensitic phases in the steels under study. It is found that the hardenability during elastic-plastic deformation decreases and completely disappears at the temperature of the material transition to a brittle state. It is shown that an increase in the rate of low-temperature deformation of samples prevents the development of phase martensitic transformations in steels. The results obtained can be recommended for use in the selection of materials for the manufacture of equipment operating at temperatures down to -196 °C. Conclusions. It is shown that the obtained values of the characteristics of mechanical properties make it possible to recommend the studied MAS as a substitute for steel 12Cr18Ni10Ti, down to a temperature of -196 °C.

Early Crack Growth from Notches under Creep-Fatigue Loading

Most of the components’ lifetime is consumed in the range of crack initiation and short crack growth. This paper shows the impact of notches in these early stages under creep-fatigue loading at high temperature. With the focus on round bar specimens having a circumferential notch the influence of different notch shapes, loading levels and conditions is assessed by a crack growth model. The crack tip loading is computed in form of a cyclic effective J-Integral considering the impact of (visco)plasticity induced crack closure based on finite element simulations. Based on this model, notch support factors for same local loading levels and crack depths are computed to quantify the notch support effect. The theoretical results are validated by experiments, where the short crack growth is measured. To give an idea of the applicability potential of the modelling approach, an industry relevant use case is presented.

Distinct Advantages of Circumferential Notch Tensile (CNT) Testing in the Determination of a Threshold for Stress Corrosion Cracking (KISCC).

Stress corrosion cracking (SCC) is a vexing problem for load-bearing equipment operating in a corrosive environment in various industries, such as aerospace, chemical and mineral processing, civil structures, bioimplants, energy generation etc. For safe operation, effective maintenance and life prediction of such equipment, reliable design data on SCC (such as threshold stress intensity for SCC, i.e., KISCC) are invaluable. Generating reliable KISCC data invariably requires a large number of tests. Traditional techniques can be prohibitively expensive. This article reviews the determination of KISCC using the circumferential notch tensile (CNT) technique, the validation of the technique and its application to a few industrially relevant scenarios. The CNT technique is a relatively recent and considerably inexpensive approach for the determination of KISCC when compared to traditional techniques, viz., double-cantilever beam (DCB) and compact tension (CT) that may be fraught with prohibitive complexities. As established through this article, the CNT technique circumvents some critical limitations of the traditional techniques.

Unforeseen, negative interaction-effect of adjacent circumferential notches on the fatigue limit of Ni-based superalloy 718

Investigations were conducted on the interaction-effects of adjacent, circumferential V-notches on the fatigue limit of Alloy 718, with the aid of fully-reversed, tension–compression fatigue tests. Contrary to expectation, specimen with seven adjacent circumferential notches had lower fatigue limit than that with a single circumferential notch. This phenomenon was attributed to the increased stress intensity factor (SIF) range of shear-mode fatigue cracks at notch-roots, resulting from competition between two factors: (i) stress mitigation through the mechanistic interaction of notches (decreased SIF) and (ii) the orderly, spatial alignment of crack-planes, due to the asymmetric stress-fields in the vicinity of notch-roots (increased SIF).

Potential of Metallurgical Gradients in the Design of Structural Components Made of Nodular Cast Iron.

The objective of this work is to investigate the use of metallurgical gradients (MG) in the design of structural components made of ductile cast iron (DCI). MG have been realized in this study by locally varying the pearlite fraction of the matrix. Exemplarily, the allowable cyclic load for a drive shaft as well as the allowable static displacement are calculated. The performed calculations are based on static and cyclic strength data of four different DCI with amounts of pearlite ranging from 0% to 96.8%. To show the advantage of the purposeful usage of local MG, ten different configurations are examined by numerical simulation studies of a generic drive shaft comprising a circumferential notch. Four configurations are calculated assuming homogenous material throughout the entire component. In the other six configurations the surface region of the notch root has an increased amount of pearlite. For each configuration the allowable multiaxial cyclic load by combinations of torsion and bending was calculated and subsequently the allowable static bending displacement. The results show that the targeted realization of MG results in a significant increase in the multiaxial fatigue strength of the shaft as well as in a slight improvement of the allowable static bending displacement.

Fracture Toughness Measurement of Aluminium and Steel Inserted Aluminium Using Numerical Method

The aim of the current work is to test the possibility of improvement of fracture properties of a shaft fabricated by press fitting a rod inside a tube and explore the effect of interface on the crack propagation. Measurement of Mode II fracture toughness is considered for this purpose. In this study numerical method developed by Yong Li and Viola based on Kienzler and Herrmann’s method by conducting a simple shear test on round bar specimen is adopted. Round bar specimens with a circumferential notch are subjected to shear test. Three specimens are prepared using Al6061T6 alloy tube inserted with rod of three different materials A6061T6 alloy, commercial pure Copper, and AISI1045 steel. Also a plain rod of Al6061T6 alloy is tested under shear to compare the results. Shear tests are conducted on Universal Testing Machine and fracture toughness values are calculated using modified Kienzler and Herrmann’s equations. The results indicated an improvement of 18% in the fracture toughness values in the case of A6061T6 alloy rod inserted A6061T6 alloy tube compared to the fracture toughness of plain A6061T6 alloy rod. And when steel rod is inserted inside the A6061T6 alloy tube an improvement of 33.33% in the fracture toughness.

Numerical and experimental investigation of notch-induced high-speed precise shearing of 42CrMo bar

A notch-induced high-speed precise shearing method was developed for high-strength metal bars, which prefabricated V-shape circumferential notches in batch on the bar surface to make stress concentration, and applied a high-speed load to complete separation on a new type of electric-pneumatic counter hammer. The FE simulation and experimental tests were conducted; the influences of loading speed, notch depth, and axial clearance were analyzed on the fracture behavior and blank quality; the microfracture mechanism was further investigated. The results showed that the circumferential notch inhibited the plastic distortion and obtained high precision chamfered billets, with a roundness error of 1.34%, flatness error of 0.34 mm, and incline angle of 0.87°. Besides, the surface notch effectively reduced Max. impact force and fracture energy. The fractography revealed that: for the notched bar, the cracks initiated from the thin extrusion layers at the bilateral-notch tips, and from micro extrusion and intrusion at the top-notch tip. The predominant microfracture mechanism involves microvoid coalescence and forming of quasi-parabolic dimples along with the shear stress.

Compression properties of z-pinned carbon-fibre/epoxy laminates reinforced with circumferentially notched z-pins

This paper represents results of the investigations on in-plane compression properties of unidirectional carbon-fibre/epoxy laminates with a three-dimensional z-pin reinforcement with circumferentially notched z-pins. Different notch designs - rectangular, circular, triangular and sinusoidal - were examined to identify the impact on the resulting compression strength and modulus. From the determinations it can be concluded that notching of z-pins has a significant impact on the compression strength, no matter of the notch design. Due to the changes in microstructure caused by the implementation of notches at the z-pin surface, eg partially lower in-plane fibre waviness as well as reduced swelling behaviour, the compression strength values are significantly higher compared to those of laminates reinforced with unnotched pins. With increasing notch depth and decreasing notch distances the compression strength values are rising. In contrast, the Young's modulus of the z-pinned laminate is not affected by notching of the z-pins. An influence of the notch design, depth and distance of circumferential notches on the Young's modulus was not found with the investigation carried out.

증기발생기 전열관 곡관부 덴트와 원주방향 외경노치가 혼합된 와전류신호 특성분석

증기발생기 전열관 곡관부 덴트와 원주방향 외경노치가 혼합된 와전류신호 특성분석