An experimental study of square tube crushing under impact loading using a modified large scale SHPB

This paper presents an experimental study on square tubes made from a rate insensitive material under static and impact loading. Rate insensitivity of the base material (Cu–Zn alloy) is confirmed by static and dynamic tests on small samples cut from the tubes. A direct impact large scale Hopkinson bar (80 mm diameter, 10 m length) system is used to perform tube crushing tests. A two-point measurement method is applied to extend measuring duration of the pressure bar, which is usually limited by its length. The proposed method permits to monitor the whole tube crushing process.Static and impact tests (7–15 m/s) on these square tubes reveal that there is a significant increase under impact loading of both initial and successive peak loads with respect to quasi-static loading. Such a study is useful for the understanding of strength enhancement under impact loading observed for cellular materials such as honeycombs.     

Pressure drop of slush nitrogen flow in converging–diverging pipes and corrugated pipes

Cryogenic slush fluids such as slush hydrogen and slush nitrogen are solid–liquid, two-phase fluids. As a functional thermal fluid, there are high expectations for use of slush fluids in various applications such as fuels for spacecraft engines, clean-energy fuels to improve the efficiency of transportation and storage, and as refrigerants for high-temperature superconducting equipment. Experimental flow tests were performed using slush nitrogen to elucidate pressure-drop characteristics of converging–diverging (C–D) pipes and corrugated pipes. In experimental results regarding pressure drop in two different types of C–D Pipes, i.e., a long-throated pipe and a short-throated pipe, each having an inner diameter of 15 mm, pressure drop for slush nitrogen in the long-throated pipe at a flow velocity of over 1.3 m/s increased by a maximum of 50–60% as compared to that for liquid nitrogen, while the increase was about 4 times as compared to slush nitrogen in the short-throated pipe. At a flow velocity of over 1.5 m/s in the short-throated pipe, pressure drop reduction became apparent, and it was confirmed that the decrease in pressure drop compared to liquid nitrogen was a maximum of 40–50%. In the case of two different types of corrugated pipes with an inner diameter of either 12 mm or 15 mm, a pressure-drop reduction was confirmed at a flow velocity of over 2 m/s, and reached a maximum value of 37% at 30 wt.% compared to liquid nitrogen. The greater the solid fractions, the smaller the pipe friction factor became, and the pipe friction factor at the same solid fraction showed a constant value regardless of the Reynolds number. From the observation of the solid particles’ behavior using a high-speed video camera and the PIV method, the pressure-drop reduction mechanisms for both C–D and corrugated pipes were demonstrated.     

Tubular channel angular pressing (TCAP) as a novel severe plastic deformation method for cylindrical tubes

A severe plastic deformation (SPD) technique based on tubular channel angular pressing (TCAP) is proposed suitable for deforming cylindrical tubes to extremely large strains without changing their dimensions. The tube constrained by inner and outer dies is pressed by a hollow cylindrical punch into a tubular angular channel with three shear zones. This technique was applied to a commercial AZ91 magnesium alloy and a significant grain refinement was achieved even after single cycle TCAP. Microhardness of the tube increased to 78 Hv from an initial value of 51 Hv. This new SPD process is promising for future industrial applications.     

Ultrasonic spot welding of Al/Mg/Al tri-layered clad sheets

Solid-state ultrasonic spot welding (USW) was used to join Al/Mg/Al tri-layered clad sheets, aiming at exploring weldability and identifying failure mode in relation to the welding energy. It was observed that the application of a low welding energy of 100 J was able to achieve the optimal welding condition during USW at a very short welding time of 0.1 s for the tri-layered clad sheets. The optimal lap shear failure load obtained was equivalent to that of the as-received Al/Mg/Al tri-layered clad sheets. With increasing welding energy, the lap shear failure load initially increased and then decreased after reaching a maximum value. At a welding energy of 25 J, failure occurred in the mode of interfacial failure along the center Al/Al weld interface due to insufficient bonding. At a welding energy of 50 J, 75 J and 100 J, failure was also characterized by the interfacial failure mode, but it occurred along the Al/Mg clad interface rather than the center Al/Al weld interface, suggesting stronger bonding of the Al/Al weld interface than that of the Al/Mg clad interface. The overall weld strength of the Al/Mg/Al tri-layered clad sheets was thus governed by the Al/Mg clad interface strength. At a welding energy of 125 J and 150 J, thinning of weld nugget and extensive deformation at the edge of welding tip caused failure at the edge of nugget region, leading to a lower lap shear failure load.     

Analysis of material degradation and life assessment of 25Cr–38Ni–Mo–Ti wrought alloy steel (HPM) for cracking tubes in an ethylene plant

Radiant tubes made of wrought 25Cr–38Ni–Mo–Ti alloy steel (HPM) have been in-service for 76,500 h as cracking tubes in an ethylene plant and they are expected to provide reliable service for 100,000 h (11.4 years) or more. During service, the tube inner surfaces were operated at temperature in the range of 820–835 °C within which thermal cracking process occurred. These aged tubes were assessed to ensure continued safe operation. The assessment of material degradation was carried out using optical microscopy, scanning electron microscopy (SEM) in combination with energy dispersive X-ray (EDX) analysis, X-ray powder diffraction (XRD) analysis, Vickers microhardness measurement and stress rupture test to obtain stress–Larson–Miller parameter (LMP) curves for remaining life prediction. Results showed that microstructural degradation was observed at the inner surface of the radiant tubes marked by the damage of protective oxide film containing Cr₂O₃, Fe₂O₃ and SiO₂. Once this film was removed, carburization occurred and free C atoms involved during cracking of ethylene easily penetrated along austenitic grain boundaries. In addition, carbon diffusion into the tube metal seemed to promote precipitation of Cr₂₃C₆ at grain boundaries and within the grains resulting in a sharp increase in hardness. The outer surface of the radiant tubes, on the other hand, was exposed to higher temperature, typically 1040–1100 °C during operation and creep damage seemed to be the main cause of material degradation. Based on stress rupture test, the remaining life of the radiant tubes is expected to be 21,107 h (2.4 years) consistent with the design life. In the present investigation, factors affecting creep are discussed.     

Joint properties and their improvement of AISI 310S austenitic stainless steel thin walled circular pipe friction welded joint

This paper describes the joint properties and their improvement in thin walled circular pipe friction welded joint for an AISI 310S austenitic stainless steel. Pipes were welded with the combination of the same thickness and outer diameter by a continuous drive friction welding machine that has an electromagnetic clutch. Then, when the clutch was released, the relative speed between both specimens instantly decreased to zero. When the joint with a pipe thickness of 1.50 mm was made at a friction pressure of 120 MPa, the joining could be successfully achieved and that had 100% efficiency with the base metal fracture. However, the joining became difficult with decreasing pipe thickness, and it was not successful at a pipe thickness of 0.50 mm. On the other hand, when the joint with a pipe thickness of 0.50 mm was made at a friction pressure of 30 MPa, the joining could be successfully achieved, although that did not have 100% efficiency. Then, when the joint was made under a friction time of 0.6 s, i.e. the friction torque reached just after the initial peak, and a forge pressure of 60 MPa, it had 100% efficiency with the base metal fracture. However, when that was made with high forge pressure such as 120 MPa, the joining could not be achieved because the adjacent region of the weld interface had heavy buckling. To obtain the successful joining and 100% joint efficiency with the base metal fracture for the thin walled circular pipe, the joint should be made with opportune friction welding condition as follows: low friction pressure, a friction time of just after the initial peak of the friction torque, and a forge pressure of double value of a friction pressure.     

Influence of different back laminate layers on ballistic performance of ceramic composite armor

This paper mainly reported a new type of ceramic composite armor with a back laminate of Ti6Al4V/UHMWPE/Ti6Al4V against the 12.7 mm armor piercing projectile at a velocity of 818 m/s. The mechanism of the whole ceramic composite armor against the projectile, and the function of each layer in the back laminates were systematically investigated around the experiments and numerical simulations. The results indicated that the outermost Ti6Al4V layer provided support for the UHMWPE layer, leading to the UHMWPE layer displaying the extremely high buffer performance during the impact process. Meanwhile, the middle UHMWPE layer also had an energy balance function between the first and outermost Ti6Al4V layers to cause small damage in the back laminate layers. Thus, this configuration contributed to absorb or dissipate the more energy of the impact projectile, successfully preventing the perforation of the projectile.     

Design and prototyping of a large capacity high frequency pulse tube

This document describes the design and the prototyping performed at CEA/SBT in partnership with AIR LIQUIDE of a high frequency large cooling power pulse tube. Driven at 58 Hz by a 7.5 kW flexure bearing pressure wave generator, this system provides a net heat lift of 210 W at 65 K. The phase shift is obtained by an inertance and a buffer volume. This type of cryogenic cooler can be used for on site gas liquefaction or drilling site and for high temperature superconductivity power device cooling (transmission lines, large generators, fault current limiters).In this paper, we focus on two essential points, the regenerator and the flow straightener. The regenerator is a key component for good performance of the pulse tube cooler. It must have a large thermal inertia, a low dead volume, a good heat transfer gas/matrix and at the same time, small pressure drop. In the present case and unlike typical moderate cooling power pulse tubes, the regenerator is very compact. However, the resulting conductive losses remain negligible compared to the cooling power targeted. The goal of the flow straightener is to avoid as much as possible any jet stream effect and to guarantee the uniformity of the velocity field at both ends of the pulse tube. Indeed multi-dimensional flow effects can significantly impact the performances of the machine.     

Fatigue in laser welded titanium tubes intended for use in aircraft pneumatic systems

The pneumatic system conducts the pressurized hot air from the engine to the environmental systems of the aircrafts. In-service failures of arc-welded pneumatic parts have driven further developments of laser beam welding as an alternative method. Here, a fiber laser with 2 kW power had been employed to weld commercial purity titanium tubes with 0.5 mm wall thickness and 50 mm diameter. For comparison purposes, semiautomatic TIG welding was realized. The chosen parameters speed and laser power for laser welding were 200 W–2 m/min and 250 W–3 m/min. The laser welded tubes presented 1 mm wide weld beads composed by partially twinned α-Ti grains. The TIG welded tubes showed 5 mm wide beads composed by acicular α-grains. These observed differences had been associated with the cooling rates, which are ten times higher in the laser case. Both laser and TIG welded tubes were cycled 44,000 times in a pneumatic bench at 350 °C without failures or cracks that could release the internal pressure. After the pressurization tests, the tubes were tested for tensile and fatigue resistance. The yield stresses, tensile strengths and total elongation did not change comparing base material, TIG welded and laser welded cases. The condition 200 W–2 m/min presented superior fatigue resistance values compared to other welding conditions, and could be considered similar to the tubes in the unwelded condition. The microstructural and mechanical results had shown that the current laser technology can replace, with advantages, the arc welding for the joining of the titanium tubes.     

Effects of winding angle on the behaviour of glass/epoxy pipes under multiaxial cyclic loading

The effects of winding angle on the behaviour of glass/epoxy composite tubes under multiaxial cyclic loading were investigated. The performance of such composite tubes was studied using an indigenous automated test procedure that is compatible with the internal qualification requirements of the composite pipe manufacturers. Glass fibre reinforced epoxy (GRE) composite pipes with three winding angles, namely, [± 45°]₄, [± 55°]₄, and [± 63°]₄, were tested. A novel automated test rig was fabricated to accommodate five stress ratios, ranging from pure axial to pure hoop loadings. The cyclic pressure test was conducted until droplets of water were seen on the outer surface of the pipe. Failure envelopes were then constructed based on the first ply failure (FPF) points determined from the axial stress to hoop strain response at five stress ratios. Three functional failure modes, namely, tensile axial, weepage, and local leakage failures, were observed during the tests. The results indicate that each winding angle dominates a different optimum pressure loading condition, namely, [± 55°]₄ for pure hydrostatic loading, [± 45°]₄ for hoop to axial loading, and [± 63°]₄ for quad hoop to axial loading. The envelopes show a strong dependence on the stress ratio and winding angle.     

High velocity impact response of Kevlar-29/epoxy and 6061-T6 aluminum laminated panels

The high velocity impact response of composite laminated plates has been experimentally investigated using a nitrogen gas gun. Tests were undertaken on sandwich structures based on Kevlar-29 fiber/epoxy resin with different stacking sequence of 6061-T6 Al plates. Impact testing was conducted using cylindrical shape of 7.62 mm diameter steel projectile at a range of velocities (180–400 m/s) were investigated to achieve complete perforation of the target. The numerical parametric study of ballistic impact caused by same conditions in experimental work is undertaken to predict the ballistic limit velocity, energy absorbed by the target and comparison between simulation by using ANSYS Autodyn 3D v.12 software and experimental work and study the effects of shape of the projectile with different (4, 8 and 12 mm) thicknesses on ballistic limit velocity. The sequence of Al plate position (front, middle and back) inside laminate plates of composite specimen was also studied. The Al back stacking sequence plate for overall results obtained was the optimum structure to resist the impact loading.The results obtained hereby are in good agreement with the experimental (maximum error of 3.64%) data where it has been shown that these novel sandwich structures exhibit excellent energy absorbing characteristics under high velocity impact loading conditions. Hence it is considered suitable for applications of armor system.     

Zn-Al钎料固相率及组分对Cu/Al管磁脉冲-半固态复合辅助钎焊接头质量的影响初探

由于节能环保以及轻量化的要求,Cu/Al异种金属复合管件被广泛应用于各工业领域,为此探索一种Cu/Al管件间高效可靠的连接技术具有较大应用价值和研究意义。为了探究Cu/Al管半固态钎焊连接机理并优化相关工艺参数,基于新提出的磁脉冲-半固态复合辅助钎焊方法制备了Cu/Al异种金属管件钎焊接头,利用LS-DYNA分析了半固态钎料不同表观粘度参数下外管内壁的受力情况,采用材料力学拉伸试验机,能谱仪和电子探针显微分析仪研究了半固态Zn-Al系钎料固相率及组分对焊接接头质量的影响。结果表明:外管内壁所受压力幅值随着表观粘度的增大而减小,剪切应力随着表观粘度的增大而增大;半固态钎料固相率为0.8时钎料与母材间没有良好冶金结合,固相率为0.4时会导致钎料填充不足的缺陷。在二次放电电压7 kV,半固态Zn-Al钎料固相率为0.6的条件下,磁脉冲-半固态复合辅助钎焊工艺能够实现Cu/Al管的无钎剂有效连接;与Zn-3Al和Zn-22Al相比,Zn-15Al钎料制得的接头综合性能更好,是较为合适的Cu/Al管磁脉冲辅助半固态钎焊用Zn-Al钎料。     

Fabrication of Mg alloy tubes for biodegradable stent application

Though Mg alloys are promising candidates for biodegradable stents, it is very difficult to fabricate stent tubes with high dimensional accuracy using Mg alloys because of their low deformability. This study aimed to develop thin-walled, high-quality Mg alloy tubes with good performance in stent applications. Cold drawing with a fixed mandrel was carried out for extruded Mg-0.8%Ca and AZ61 alloy tubes using optimized drawing parameters and lubrication, and stent tubes with 1.5–1.8 mm outer diameter and 150 μm thickness were fabricated. A dimensional evaluation showed that the tube dimensional errors were within 0.02–2.5%. Also, an immersion test of pure Mg with different crystal orientations showed that the crystal orientation affected the corrosion properties, results that are the same with other Mg alloys. The crystal orientation of the stent tube could be controlled by changing the deformation amount and direction in the drawing, showing that it is possible to further improve the biodegradability of stents by approaching their fabrication from a processing aspect.     

Failure analysis of 316L stainless steel tubing of the high-pressure still condenser

Sherritt International Corporation experienced corrosion failures with the 316L stainless steel tubing in a high-pressure still condenser employed for ammonia recovery. A detailed failure analysis was conducted on the condenser tubing to determine the mode and the root cause of the failure. The analysis included both optical and scanning electron microscopy (SEM) of the inner and outer surfaces of the tube as well as characterization of the corrosion products using energy-dispersive X-ray spectroscopy (EDX). Results revealed that the corrosion attack was confined to the first ～100 mm of the tubing at the inlet where the tube was connected to the top tubesheet. The tube suffered both external stress-corrosion cracking (SCC) and crevice corrosion from the shell side (water side), and wall thinning of the inner surface (the tube side) due to erosion corrosion. It was evident that failure of one of the tubes occurred due to SCC that penetrated the whole wall thickness and resulted in a leak failure. Some prevention measures are proposed to avoid this type of corrosion attack in the future.     

A new hybrid identification method for determining the material parameters of thin-walled tube under compressive stress state

Accurate and fast determination of material parameters of thin-walled tube under compressive stress state is essential for analyzing the compressive-type tube forming process. For the thin-walled tube with hollow structure, it is difficult to determine the material parameters directly from the experiment since buckling occurs easily when the tube suffers axial compressive loading. To accurately and rapidly identify the material parameters of thin-walled tube under compressive stress state, a hybrid inverse identification method is proposed based on tube lateral compression test with combining finite element simulation, regression analysis and genetic algorithm. By employing the proposed method, the Swift law hardening parameters of thin-walled tubes with different materials and specifications under compressive stress state are identified. Furthermore, the efficiency and accuracy of the proposed method are discussed in comparison with the previous researches. The results show that: (1) for 6061-T4 and 1Cr18Ni9Ti tubes, the maximum relative predicting errors of forces in tube lateral compression using the identified material parameters are less than 9%; (2) for aluminum tube ∅100 × 2 (diameter × thickness, mm), the maximum discrepancies between the simulated and experimental circumferential strains are less than 0.0274 for 30–70% reductions, and the simulated tube profiles deviate from the experiment less than 10% at reductions of 0–78%; and (3) the proposed method almost saves 80% computational time compared with the previous stepwise optimization method.     

Effect of anisotropy on fatigue properties of 2198 Al–Li plates joined by friction stir welding

Al–Li alloys are characterized by a strong anisotropy in mechanical and microstructural properties with respect to the rolling direction. In the present paper, 4 mm sheets of 2198 Al–Li alloy were joined via friction stir welding (FSW) by employing a rotating speed of 1000 mm/min and a welding speed of 80 mm/min in parallel and orthogonal direction with respect to the rolling one. The joints mechanical properties were evaluated by means of tensile tests at room temperature. In addition, fatigue tests were performed by using a resonant electro-mechanical testing machine under constant amplitude control up to 250 Hz sinusoidal loading. The fatigue tests were conducted in axial control mode with R = σ_min/σ_max = 0.33, for all the welding and rotating speeds used in the present study.     

Film condensation heat transfer characteristics of R134a on horizontal stainless steel integral-fin tubes at low heat transfer rate

Condensation heat transfer characteristics of R134a on the integral-fin tubes are experimentally investigated. The test tubes are made of stainless steel, and the root diameter of the tubes is 13.27 mm. The height of fin is 1.19 mm, and the densities of the integral fin are 19 fpi and 26 fpi. The present tests were conducted at the saturation temperatures of 20 °C and 30 °C. The condensation heat transfer coefficients of the tubes having 19 fpi and 26 fpi at the saturation temperature of 20 °C are higher than that of the plain tube by 4.4 and 3.1 times, respectively. When the temperature difference across the condensate film is less than 0.7 °C, the enhancement of the tube of 19 fpi is much larger than that of the tube of 26 fpi. The Honda and Nozu model shows the smallest mean deviation between the estimated values and experimental results among the existing models.     

Penetration mechanisms in glass laminate/resin structures

The ballistic response of composite structures comprising differing laminated float glass/polycarbonate replacement resin (PRR) elements was studied. In order to provide materials data for future modelling work, sphere-impact tests were employed to determine the high strain-rate response of the elastomeric resin. Larger-scale armour simulants comprising glass-laminate-fronted cylinders of PRR were also investigated using lead antimony-cored 7.62 mm × 51 mm NATO Ball rounds in order to interrogate their behaviour under impact. Penetration mechanisms were studied via the use of high-speed video equipment. Projectile defeat in the resin was observed to depend on the degree of projectile disruption, with a greater degree of comminution leading to enhanced behaviour. This confirmed the importance of the elastomeric properties of the resin in behaviour under ballistic impact in these structures. The interaction between the glass disrupting layer and the backing absorber was found to be key to minimising subsequent penetration. The use of asymmetric float glass laminates incorporating a thinner disrupting outer surface was found to reduce subsequent depth of penetration by as much as 52% compared to similar areal density monolithic systems. High-speed video footage implied that the thinner outer layer acted to blunt the incident projectile, while the backing thick layer of glass exhibiting a Hertzian cone-like “plugging” failure mechanism. In addition analysis of high-speed video showed that the penetration rate in the resin was initially constant, implying penetration analogous to hydrodynamic behaviour.     

Failure analysis of superheater tube

The failure analysis of a ruptured superheater tube after 20 years service in the oil-fueled boiler, as the typical problems in power plants, was investigated. A thin-lipped rupture at failed region was observed in superheater tube. By measuring the tube’s wall thicknesses far from failed region, non-uniformity was seen. The suggested main root cause of failure was fireside corrosion of the tube during the service. Because of low grade of used fuel, sodium, sulfur, and vanadium elements were observed at the outer surface, which caused continuously scale formation and reduction of wall thickness, by metal consumption. In addition, it seems that it has been worsened by occurrence of long-term overheating. Coagulation of carbides at both outer and inner regions of tube was observed that could prove the occurrence of overheating during the service. In addition, the formation of sigma-phase particles was revealed because of being in the susceptible temperature after 20 years in service. At the end, in order to prevent or decelerate such failure, some recommended remedies were suggested.     

Corrosion failure analyses of an elbow and an elbow-to-pipe weld in a natural gas gathering pipeline

The internal corrosion of a 90° elbow was found in a natural gas gathering pipeline in Northeast China. The welded joint between the elbow and the downstream pipe was also severely corroded. The 90° elbow was forged of 16Mn steel. The downstream pipe (Φ 76 mm × 9 mm) was made of 20G steel. To determine failure causes, the elbow and the welded joint were taken as a whole and investigated systematically. The influence of the flow disturbance induced by the elbow on the damage at the welded joint was considered. The internal damage at the elbow and that at the welded joint were studied using field investigation, visual examination, scanning electron microscopy (SEM), energy-dispersive spectroscopy (EDS), X-ray powder diffraction (XRD), hardness tests, and computational fluid dynamics (CFD) techniques. The results showed that erosion corrosion from solid particles and corrosive liquid droplets entrained in the natural gas flow was the main cause of the internal damage at the elbow and the welded joint. The welded joint was attacked by the particles with the highest velocity magnitudes and the most dangerous impact angles. The flow disturbance induced by the elbow, the special location of the welded joint, and the angular misalignment due to poor welding quality jointly caused the substantially more severe damage at the welded joint.