Multi-objective optimization of loading path design in multi-stage tube forming using MOGA

Pre-bending is a critical process required prior to hydroforming. The bending has an effect on the tube thickness and strain which will use up a portion of the formability of the as-received tube. To compensate for this loss of formability, a multi-objective optimization method was applied to improve the hydroforming process after pre-bending. A multi-objective genetic algorithm (MOGA) and Kriging surrogate model were used to optimize the loading path. The Kriging model was used to replace the finite element simulation in constraint handling. The optimal loading parameters in the hydroforming process were obtained for a tube that was previously bent 90°, and showed an improvement in reducing the corner radii of the part at the extrados and intrados of the bend (8.73 mm and 11.24 mm for the extrados and intrados of the bend, respectively). The corresponding corner fill expansion (CFE) was improved by 16.7% (or 1.79 mm) compared to the maximum expansion of 10.73 mm obtained experimentally.     

Investigation into the Cracking of SAILMA 350 HI During Forming

Vanadium microalloyed hot rolled steel of 13.8 mm thickness, when hot formed at 900 °C into axle housings were rejected because of a prominent crack running along the length of the component. A detailed metallurgical investigation revealed that the cracking during forming was the result of the opening up of pre-existing fine crack along the length of the hot rolled coils. These cracks were barely visible on the hot rolled coils that were formed into the axle housings. Metallographic and hardness tests indicated that these cracks might have been generated as a result of decarburization of the slab during soaking that led to a softer surface layer of the slab. During subsequent hot rolling mechanical defect might have been caused on the decarburized surface that is much softer than a material with a usual carbon content. It was concluded that soaking in the reheating furnace with low carbon potential might have led to extensive decarburization of the slabs. Soaking in a furnace atmosphere with adequate carbon potential was recommended as a remedial measure to prevent decarburization of the surface layers of the slabs.     

Quantitative Evaluation of Corrosion in a Thin Small-Bore Piping System Using Bobbin-Type Magnetic Camera

Nondestructive testing (NDT) methods have been developed to ensure the integrity of heat exchanger pipes. NDT systems can be used to not only locate cracks on the pipes but also evaluate the size and position of the cracks. A bobbin-type magnetic camera (BMC), an electromagnetic NDT system, was developed to inspect both the inner and outer diameters (ID and OD, respectively) for stress corrosion cracking on a small-bore piping system with a high spatial resolution at high speeds. In this paper, an algorithm that discriminates the ID cracks from the OD cracks and estimates the crack volume is proposed. Artificial ID and OD hole-type cracks which have diameter of 1–4.5 mm and depth of 0.3–27 mm were used to imitate the corrosions on a 1.27 mm thickness and 16.56 mm inner diameter copper alloy pipe to verify the proposed algorithm. The inspection results of the BMC with respect to excited frequencies from 1–9 kHz are presented. 100 % of the ID and OD cracks were discriminated, and their volumes were estimated with a standard deviation of 1.132 mm³ for volume from 1.00–9.01 mm³.     

Unexpected High-Energy Failure of Type 304L Stainless Steel Piping in the Tail Gas System in Nitric Acid Plant Due to Intergranular Stress Corrosion Cracking (IGSCC)

An incident occurred at a Nitric Acid Plant when a failed 18-in. (457 mm)-diameter 304L stainless steel pipe connecting the Converter to the Tail Gas Catalytic Reactor ruptured. The rupture resulted in thrown fragments and some damage to equipment in the vicinity of the pipe. Normally, pipe operating conditions are about 80 psig (0.55 MPa) and 850–900 °F (454–482 °C), and the nominal thickness of the pipe was 0.180-in. (0.46 mm). This article summarizes the discovery of a very unusual condition of intergranular stress corrosion cracking due to sensitization of the low-carbon stainless steel used in manufacture of the pipe. The lessons learned from this failure investigation are vital to those who use stainless steel in Tail Gas systems.     

Cracking of 5Cr steel tee-pipe during start-up operation in heavy oil upgrade refinery

During the start-up operation of an HOU refinery plant, leaking through a crack at the top of a T-pipe was found in a hydrocracking process. Various kinds of metallurgical inspections of the failed T-pipe were performed in order to find the cause of cracking e.g., metallography, fractography, hardness measurement, EDS analysis and hydrogen concentration measurement. Since thermal loading in the T-pipe was usually higher during the start-up operation than under steady operations, a system load analyses of the pipe was conducted. Detailed finite element analysis of the T-pipe was also conducted to identify high stress locations and to assess the stress intensity factor of the crack partly penetrated the T-pipe.The 5Cr T-pipe steel satisfied the chemical composition requirements for ASTM A234 but the tensile strength and hardness were significantly higher than recommended in the standard, which made the pipe steel vulnerable to a hydrogen environment formed by the feed inside of the pipe. The cracking started from the inclusions near the inner surface of the pipe and grew due to the hydrogen effect. Inspection results supporting this argument are explained. And recommendations for preventing similar cracking failures were made.     

CREEP CRACK GROWTH MICROMECHANISMS AT 405°C IN COLD BENT TUBES OF A C-Mn-Mo STEEL

Abstract— Creep Crack Growth (CCG) tests were performed at 405°C on specimens cut out of the cold bent extrados of five tubes of a C-Mn-Mo steel. Intergranular fracture and grain boundary cavitation was less in the C-Mn-Mo than in the C-Mn steels, in accordance with better CCG resistance of the former material. The dimensions and hardness variation across the crack tip process zone were measured by microhardness profiles performed on metallographic sections of the broken samples. TEM analysis of the dislocation patterns close to the fracture surface confirmed the presence of temperature- and stress-induced plasticity phenomena. A significant enrichment of N at grain boundaries (GB) inside the process zone was detected by Auger spectroscopy; N not only inhibits dislocation motion and stress field relaxation at the crack tip but also causes a decrease in GB cohesion ahead of the crack tip. These results help in understanding the micromechanisms which reduce the creep ductility of C-Mn-Mo and C-Mn cold bent tubes and the role of chemical composition in improving CCG resistance.     

On Thermally Induced Multiple Cracking of a Surface: An Experimental Study

An experimental study of the thermally induced surface multiple cracking, on the example of steam superheater collector was carried out. The structure’s perforated area has 3 rows of holes, to which the connection sleeves are welded. On the first stage, the cracks initiate on the inner surface of the perforated area of the collector. These cracks are due to the thermal fatigue, caused by temperature changes at start-stops and by temperature fluctuations during operation. The histograms and cumulative distribution functions of crack lengths and the inclination angles with respect to the axes of fastening pipes holes were obtained. The length of most cracks was less than 2 mm. Also, an important localized damage on the collector inner surface in the form of a partly circumferential crack with a length of 149 mm and with the largest depth of 37.8 mm was observed.     

动态流量控制法挤出镁合金三维弯曲管件

为了开发新的弯管件加工工艺,提出了一种动态调整传统分流挤压模具中分流孔内金属流量(dynamic flow control extrusion,DFCE)的挤压变形方法.在带有辅助调控挤压杆的630 T卧式挤压机上挤出镁合金弯管件,采用OM、SEM、TEM、拉伸试验等方法,研究了DFCE制造的镁合金弯管件的晶粒细化方式、微观组织结构和性能.结果表明:在变形温度450℃、直管挤压速度3 mm/s、弯管挤压速度1.5 mm/s、辅助调控挤压杆速度30 mm/s时,成功挤出变形均匀的弯管件;挤压后的直管部分和弯管部分的晶粒尺寸分别为7.9和12.8μm,且合金晶粒大小均匀;弯管部分室温拉伸强度和屈服强度分别由217和124 MPa提高到296和179 MPa,延伸率由12.9%提高到26.2%.DFCE挤压变形可以显著细化AZ91镁合金晶粒,其挤压过程中晶粒细化机制为位错驱动和动态再结晶,机械性能较铸态大幅度提高,坯料和挤出合金的拉伸断口分别呈现为准解理断裂和韧窝断裂的特征.     

The crack evolution on the atomistic scale during the pyrolysis of carbon fibre reinforced plastics to carbon/carbon composites

The development of the crack patterns during the pyrolysis of carbon fibre reinforced plastics (CFRP) to carbon/carbon composites as the second manufacturing step in the liquid silicon infiltration (LSI) process was investigated. In the basic examination reported previously, it was discovered that a substantial amount of cracking occurs beyond 650 °C, when the mesoscopic crack pattern has already fully developed. This additional cracking could not be visualized by using standard microscopy. Thus additional investigations were conducted by using conventional and high-resolution transmission electron microscopy to obtain information on the atomistic scale on the assumed cracking activity.It was found that the crack development starts at pores that develop as a compensation for the rough fibre surface. Crack propagation takes place by evolution of new nanoscopic cracks caused by fibre–matrix-debonding in the tensile stress field in front of the crack tip and subsequent connection with the main crack. Thus the interconnection mechanism – cracking as the connection of cracks on a subordinated scale to form a new crack – is the second main cracking mechanism beside transversal cracking (leading to a regular mesoscopic crack pattern) active during the carbonization of CFRP components.     

JCOE成型的X80输油钢管开裂分析

在JCOE成型过程中一根X80输油钢管母材发生纵向开裂,裂纹长度达到了700 mm。通过化学成分分析、力学性能测试、断口分析及金相检验等方法分析了钢管的开裂原因。结果表明:大尺寸外来非金属夹杂物的存在导致了钢板在连铸或轧制过程中产生裂纹。而较低的断裂韧度为裂纹进一步扩展创造了条件,在成型拉应力的作用下微裂纹扩展并延伸。     

Limit loads and approximate J estimates for axial through-wall cracked pipe bends

This paper presents plastic limit loads and approximate J estimates for axial through-wall cracked pipe bends under internal pressure and in-plane bending. These loads and estimates are based on small strain finite element limit analyses using elastic-perfectly plastic materials. Geometric variables associated with the crack and pipe bend are systematically varied, and three possible crack locations (intrados, crown and extrados) are considered. Effects of the bend and crack geometries on plastic limit loads are quantified, and closed-form limit load solutions are given. Based on the proposed limit load solutions, a reference stress based the J estimation scheme for axial through-wall cracked pipe bends under internal pressure and in-plane bending is proposed.     

Modeling Solids Friction for Dense-Phase Pneumatic Conveying of Powders

This article results from an ongoing investigation aimed at developing a new validated test-design procedure for the accurate prediction of pressure drop for dense-phase pneumatic conveying of powders. Models for combined pressure drop coefficient (“K”) for solids-gas mixture were derived using the concept of “suspension density” by using the steady-state “straight pipe” pressure drop data between two different tapping locations of the same pipe and also for two different diameter pipes. It was observed that the derived models were different depending on the location of tapping points (for the same pipe) and selected pipe diameters. The derived models were then evaluated by predicting the pressure drop for pipelines with various diameters or lengths (69 mm I.D. × 168 m, 105 mm I.D. × 168 m, 69 mm I.D. × 554 m) for the conveying of power station fly ash. A comparison between the predicted pneumatic conveying characteristics (PCC) and the experimental plots showed that the models resulted in significant over-predictions. In the second part of the article, the “system” approach of scaleup was evaluated. “Total” pipeline pressure drop characteristics for test-rig pipelines were scaled up to predict the PCC for larger/longer pipes. It was found that the “system” approach generally resulted in grossly inaccurate predictions. It was concluded that further studies are needed for a better understanding of the solids-gas flow mechanism under dense-phase conditions.     

Investigation of microstructure and flexural behavior of steel-fiber reinforced concrete

This paper presents microstructure and flexural behavior of steel-fiber reinforced concrete produced with different steel fibers volume fraction and aspect ratio. Prismatic concrete specimens of 100 × 100 × 350 mm were prepared with and without steel fiber. Two different steel fiber types (both is hooked-end) were used by ratio of 0% (control), 0.2, 0.4, 0.6 and 0.8% by volume. Specimens were de-molded after 24 h and cured in water until 7, 28, 56, 180 and 360 days. On the prisms, flexural strength has been defined for every age. The crack widths have also been measured after maximum bearing loads. Microstructure of SFRC was studied by scanning electron microscopy and optical microscopy for 180 aged specimens. The results showed that the polarized microcopy images may be used for observing the bond characteristic of SFRC as alternatively to SEM. A good bond was observed between steel fiber and concrete matrix interface zone by using polarizing microscopy, too. Flexural strength of SFRC increased with the concrete age and fiber volume fraction. Besides, the first crack development significantly decreased by increasing of fiber volume fraction in the all concrete ages.     

Microstructural evolution and solidification cracking susceptibility of Fe–18Mn–0.6C–xAl steel welds

In this work, the solidification behavior and solidification cracking of Fe–18Mn–0.6C–xAl (x = 1.49, 2.37, 4.79, 6.04 wt%) alloys were investigated. A longitudinal Varestraint test was applied to evaluate the solidification cracking tendency of Al-added high-Mn steel welds. In terms of total crack length and maximum crack length at 4 % applied strain, the solidification cracking susceptibility of high-Mn steel decreased with increasing Al content. Addition of Al suppressed the formation of low melting point eutectics (γ + (Fe,Mn)₃C) along the grain boundaries during the final stage of solidification, which resulted in the decrease of solidification cracking tendency. The Al segregated extensively to the dendrite core opposite to Mn and C during solidification, which promoted the formation of δ ferrite. Further, the transition of the solidification sequence from the primary austenitic to primary ferritic mode provided a noticeable improvement in solidification cracking resistance in high-Mn steel welds similar to austenitic stainless steel welds.     

MECHANICALLY SHORT CRACK GROWTH FROM NOTCHES IN A MILD STEEL

Abstract— Two L-notched specimens made of mild steel (average grain size =30 μm) and having root radii of 0.1 mm and 3 mm, and also a smooth surface specimen were cyclically loaded at different stress levels at R =−1 and at R = 0. A technique based on miniature strain gauges was successfully used to monitor the depth and the opening level of mechanically short cracks of depths from 0.015 mm to 0.5 mm. Three dimensional FEM computations were made to obtain appropriate calibration curves for varying crack aspect ratios and gauge eccentricities as well as notch plastic strain distributions. The fracture of L-notched specimens having a root radius of 0.1 mm was characterized by an early and multiple crack initiation phase (defined by a crack depth of 30 μm), and the short crack growth rates showed a mechanical behaviour different from that of long cracks (large discrepancies at the same Δ K -value, crack deceleration at R =−1 even beyond the notch plastic zone). For smooth surface specimens both the initiation and the propagation of a single short crack represented important fractions of the total life; the short crack growth rates were high and continuously increasing. The notch influence was highly reduced when the stress singularity is truncated by a 3 mm radius. The cracking behaviour was, in several aspects, close to that at smooth surfaces. The evolutions of crack closure were analyzed in each condition (transient decrease and stabilized value of the closure ratio U =Δ K _eff/Δ K ) and were shown to have a strong influence on short crack growth. Most of the short crack growth rates obtained in the various geometry/loading conditions are well consolidated with LEFM long crack growth rates using the Δ K _eff parameter.     

Failure Investigation of Cold-Rolled ZnTiCu Alloy Fracture During Bending Under Ambient Temperature Conditions

Bending ductility is an important quality parameter of the ZnTiCu rolled sheet for the fabrication of specially formed members in construction industry. Fabrication of such components is usually realized under severe loading conditions (i.e., high strain rate forming using manual tools or press brake machinery) and even sometimes under low temperature environment in the construction field, which is very often close to or less than 0 °C. Bending orientation is also an important parameter, since texture-sensitive Zn alloy cold-worked strip affects ductility performance in direction transverse to the rolling direction. A failure analysis process was implemented on a ZnTiCu bent sheet possessing cracks on outer bent areas. Light optical metallography, and scanning electron microscopy coupled with energy dispersive spectrometry, along with mechanical testing were the principal investigation techniques employed to evaluate the failure. The presence of pronounced elongated grain structure combined with coarse and continuous Ti-rich intermetallic phase, likely reduces the fracture resistance against the transverse stress field imposed by bending, stimulating, thus crack initiation and propagation.     

Stress Intensity Factors of partly circumferential surface cracks at the outer wall of a pipe

By means of the finite element method stress intensity factors were calculated for partly circumferential surface cracks at the outer wall of a pipe. The crack shape considered can be described as curved rectangular shape. The cracks considered have crack depths between 20 and 80 percent of the wall thickness of the pipe and crack lengths (defined by the angle of circumference φ) between φ = 10° and φ = 60°. The pipe is loaded by a constant axial tensile stress σ₀ (equal to 136 Nmm^?2 in the numerial calculations), and the wall thickness to inner radius ratio of the pipe was chosen to 0.1. A wall thickness of 20 mm was used for the numerical calculations.     

Investigation and Recommendations on Bottom-Dented Petroleum Pipelines

On September 21, 2015, the National Transportation Safety Board responded to a petroleum leak from a transmission pipeline in Centreville, VA. A small through crack was found leaking at a dent on the underside of the pipe, located away from any welds. The investigation found that corrosion fatigue could initiate at small dents, typically caused by impingement. While top-side dents from excavation and servicing have well-been documented and regulated, bottom-side dents, deemed acceptable per regulations, were found to be susceptible to stress corrosion and fatigue cracking. This investigation explored multiple and fundamental aspects of cracking in steel pipe dents, including nondestructive inspection, electron microscopy, finite element modeling, and long-term cyclic loading tests to characterize the cause of this pipeline accident.     

Correlating plastic shrinkage cracking potential of fiber reinforced cement composites with its early-age constitutive response in tension

It is well understood that the early-age properties of cementitious materials influences its long-term performance. Cementitious materials experience high moisture loss at early age resulting in volumetric shrinkage. When this shrinkage is restrained, tensile stresses develop resulting in cracking of the material. This issue is more pronounced at early-age when the tensile strength of cementitious materials is not fully developed. There are many methods and models available to predict the drying shrinkage of concrete. However, the relationship between early age uniaxial tensile strength and restrained shrinkage cracking characteristics at early age is not well understood. Uniaxial tests were conducted on dog-bone specimens made using various fiber types including glass, cellulose and three types of synthetic fibers. Tensile characteristics including stress versus strain plots for all the specimens along with stress at peak, post crack strength at 2 mm deflection along with elongation at peak are calculated and presented in this paper. Plastic shrinkage cracking results obtained using an innovative test developed by the authors were correlated to these tensile strength results. Results indicate that post crack residual tensile strength at early-age is inversely proportional to the total crack area resulting from restrained shrinkage. The effectiveness of various fiber types is also discussed in the paper.     

Annealing twin boundary cracking in the low-temperature brittle fracture of a high-nitrogen bearing austenitic steel

Crack propagation paths in the low temperature brittle fracture of 18Cr-18Mn-0.7N austenitic steel were investigated by means of scanning electron microscopy. Corresponding relationships of the fracture facets with microstructures were established by the simultaneous observation of the fracture surface and the microstructure of the adjacent side surface. It was shown that the annealing twin boundary cracking occurred during fracture. A great deal of twins formed during solution treatment of the steel, with steps several microns high on the twin boundaries, and a considerable amount of planar deformation structures developed on {111} planes in the fracture process. The fracture facets of the annealing twin boundary are fairly flat and smooth, with bent steps of micron-scale height, and a pattern of three sets of parallel straight-lines intersecting at 60°. But there is no river pattern on the facets. The bent steps result from partial propagation of crack along steps that are developed on annealing twin boundaries during solution treatment, while the line-pattern is the intersection traces of the planar deformation structures with the fracture facet. It is believed that the annealing twin boundary cracking is attributed to the stress concentration arising at the intersection of planar deformation structures and the annealing twin boundaries.