FEM prediction of welding residual stresses in a SUS304 girth-welded pipe with emphasis on stress distribution near weld start/end location

A finite element approach based on Quick Welder software is developed to simulate welding temperature field and welding residual stress distribution in a 3D multi-pass girth-welded pipe model. The characteristics of welding residual stress distributions in a SUS304 stainless steel pipe induced by heating with a tungsten inert gas arc welding torch are investigated numerically. Meanwhile, an emphasis is focused on examining the welding residual stress distributions in and near the weld start/end location. Moreover, the residual stresses predicted by the present computational approach are compared with the measured data; and the comparison suggests that the numerical simulation method has basically captured the feature of welding residual stress distribution near the weld start/end region. The numerical simulation results show that both the hoop and the axial residual stresses near the weld start/end region have sharp gradients and are significantly different from those in the steady range.     

The influence of hygrothermal conditions on the damage processes in quasi-isotropic carbon/epoxy laminates

The effects of hygrothermal conditions on damage development in quasi-isotropic carbon-fiber/epoxy laminates are described. First, monotonic and loading/unloading tensile tests were conducted on dry and wet specimens at ambient and high temperatures to compare the stress/strain response and damage development. The changes in the Young's modulus and Poisson's ratio were obtained experimentally from the monotonic tensile tests. The critical stresses for transverse cracking and delamination for the above three conditions are compared. The delamination area is measured by using scanning acoustic microscopy (SAM) at various loads to discuss the effects of delamination on the nonlinear stress/strain behavior. Next, the stress distributions under tensile load including hygrothermal residual stresses are computed by a finite-element code and their effects on damage initiation are discussed. Finally, a simple model for the prediction of the Young's modulus of a delaminated specimen is proposed. It is found that moisture increases the critical stresses for transverse cracking and delamination by reducing the residual stresses while high temperature decreases the critical stresses in spite of relaxation of the residual stresses. The results of the finite-element analysis provide some explanations for the onset of transverse cracking and delamination. The Young's modulus predicted by the present model agrees with experimental results better than that predicted by conventional models.     

Finite Element Analysis of Modeling Residual Stress Distribution in All-position Duplex Stainless Steel Welded Pipe

On the basis of the thermal-elastic-plastic theory, a three-dimensional finite element numerical simulation is performed on the girth welded residual stresses of the duplex stainless steel pipe with ANSYS nonlinear finite element program for the first time. Three-dimensional FEM using mobile heat source for analysis transient temperature field and welding stress field in circumferential joint of pipes is founded. Distributions of axial and hoop residual stresses of the joint are investigated. The axial and the hoop residual stresses at the weld and weld vicinity on inner surface of pipes are tensile, and they are gradually transferred into compressive with the increase of the departure from the weld. The axial residual stresses at the weld and weld vicinity on outer surface of pipes is compressive while the hoop one is tensile. The distributions of residual stresses compared positive-circle with negative-circle show distinct symmetry. These results provide theoretical knowledge for the optimization of p     

Nickel-alumina interfacial fracture toughness: experiments and analysis of residual stress effects

The purpose of the present work is to account for the influence of residual stresses on the measured fracture toughness of a representative metal-ceramic system and, in conjuction with a maximum hoop stress criterion, to explain the observed increase in toughness with increasing mixity of loading. For the sandwich specimen geometry adopted in the current study, a simple argument yields a critical layer thickness below which residual stress effects are expected to be minimized. The measured fracture toughness is found to be independent of thickness for thicknesses below this threshold. For such specimens a general result is demonstrated: compared to the same loading without residual stresses present, the effect of residual stresses is to decrease the magnitude of the phase angle of the stresses which develop along the interface. It is argued that when small-scale yielding conditions hold, both the mixity and the critical hoop stress corresponding to fracture should be reported at a length which falls within the -dominant region in the sample. In this manner, good quantitative agreement between theory and experiment is demonstrated.     

Experimental method for determining through thickness residual hoop stresses in thin walled pipes and tubes without inside access

The determination of through thickness residual stresses in pipes and pressure vessels is of growing interest because of emphasis placed on life prediction, design, and failure analysis of piping systems. Most of the through thickness residual stress measurement techniques require the placement of gauges on the outside and inside of the pipe. These methods are severely hampered when gauges cannot be placed on the inside of the pipe. This constraint could arise for small diameter pipes, long pipes or for pipes that have been used in a service condition causing corrosion or fouling of the inner surface.
This paper focuses on the first step of a three step procedure for determining residual hoop stresses in thin walled pipes and tubes. The method described is designed for cases where it is impossible to place gauges on the inside of the pipe. The method yses biaxial strain gauges on the outside of the pipe and involves a through thickness axial cut of the pipe. Based on the change in strain on the outside of the pipe, changes in the hoop residual stress distribution due to the axial cut are obtained with the method presented here. The method provides a means to evaluate changes in stresses on both the outside surface and the inside surface of the pipe as well as an evaluation of the change in through thickness hoop stress distribution at any location in the pipe cross section. This paper further demonstrates that the problem of shortening long pipes to enable placement of gauges on the inside of the pipe can result in the loss of significant residual stress information.     

Predicting the mechanical behaviour of Kevlar/epoxy and carbon/epoxy filament-wound tubes

The axial, hoop and shear moduli and failure conditions of carbon/epoxy and Kevlar/epoxy filament-wound tubes have been determined through respective applications of internal pressure, tension and torsion. The introduction in the laminated plate theory of a gradual reduction in individual moduli makes it possible to overcome the limitations of the theory and enables accurate predictions to be made of the linear and non-linear stress/strain curves of 90°/ ±0/90° tubes. The existence of a dominant layer in the failure of the multilayered tubes has been shown experimentally. When associated with a failure criterion applied to the dominant layer, the new model permits the prediction of tube failure. Agreement between calculated and experimental data is better than 5%.     

A note on fracture criteria for interface fracture

Several criteria for interface fracture are examined and compared to test results obtained from glass/epoxy specimens. These include two energy release rate criteria, a critical hoop stress criterion and a critical shear stress criterion. In addition, approximate plastic zone size and shape within the epoxy are determined for these tests.     

The rate-dependent behaviour of ± 55 ° filament-wound glass-fibre/epoxy tubes under biaxial loading

This paper presents the results of an experimental investigation into the behaviour of glass-fibre-reinforced epoxy tubes subjected to monotonic biaxial loading. Commercially available tubes with a filament winding pattern of ± 55 ° were tested in a biaxial testing machine with various ratios of axial stress to hoop stress. In addition, the tubes were tested at three rates of monotonic loading. The resulting stress/strain curves were analyzed and biaxial failure envelopes in terms of stress and strain were constructed demonstrating the complexity of the behaviour of the tubes. It is shown that the rate and ratio of biaxial loading affect the monotonic failure strength, damage accumulation and the mode of failure. In addition, these results are discussed based on macro and micro observations of damage and failure modes.     

Analysis of the fatigue failure of a mountain bike front shock

We present an analysis of a mountain bike front shock failure. The failure of the 1-year-old shock occurred catastrophically as the bike was ridden off of a 1-m drop. The failure was the result of fast fracture through both shock tubes at the location where the tubes were press fit into the shock upper crown. Examination of the fracture surfaces of the tubes revealed regions of fatigue crack growth that nearly penetrated the entire thickness of both tubes. An estimate of the forces during use, coupled with stress analysis, revealed three stresses near the fracture site—axial compression, bending, and hoop stresses. During operation, the axial compressive stress is negligible while the hoop and bending stresses are significant. Based on fracture mechanics, and an estimate of the bending stress from a 1-m drop, it is confirmed that the fatigue cracks present on the fracture surface were large enough to induce fast fracture. Prior to the existence of the fatigue cracks, the stresses were magnified locally near the fracture site by a significant stress concentration caused by the sharp transition from the shock tube to the crown. The fatigue cracks initiated at a circumferential location in the tube commensurate with high tensile bending stress and the stiffest region of the crown (highest stress concentration). Based on the evidence, the most probable cause of the bike shock fatigue failure was the shock design, which facilitated high local stresses during use.     

喷丸起落架外筒焊接热影响区的残余应力测定

本文用X射线法测定了喷丸起落架外筒焊缝影响区的残余应力,并从残余应力角度考察了起落架外筒现行工艺路线的优劣。测定结果表明,在相同构件上虽然所测方向和部位不同,但残余应力的分布、大小及符号基本相同。与前起外筒相比,喷丸处理在主起外筒上形成的残余压应力值较低。分析认为消除焊接残余拉应力的合理工序,是两种外筒均应采用焊后热处理,但应防止表面脱碳。     

A study on the mechanical stress relieving and safety assessment without post-weld heat treatment

For full welded body valve, the temperature of grommet cannot exceed 150 °C in order to prevent it from damaging and assure the tightness and the service life of valve. Therefore, post-weld heat treatment (PWHT) cannot be used to relieve the residual stresses. In this study, the effect of the mechanical stress relieving (MSR) treatment on the residual stresses was studied by the finite element method and experimental work. A pressure and time diagram of MSR treatment was established. A two-dimensional axisymmetric finite element model was used to simulate the residual stresses field. Before and after MSR treatment, the residual stresses on the outer surface were measured by the blind hole drilling method. Finally, the fracture toughness behaviors of weld zone (WZ) and heat affected zone (HAZ) were investigated in terms of crack tip opening displacement (CTOD) according to BS7448 and DNV-OS-C401 fracture toughness tests standards. The safety of the valve in active service was assessed without PWHT. Through comparison and analysis, the axial residual stresses and the hoop residual stresses on the outer surface of valve are mainly tensile. The peak value of tensile stress occurs nearer to the outer surface of the valve. MSR treatment can decrease the peak value of axial residual stresses and hoop residual stresses on the outer surface obviously and make the residual stresses distribution more uniform. The safety of the valve in active service is reliable without PWHT.     

The adhesive behaviour of poly(p-phenylene benzobisthiazole) (PBT)/epoxy composites

A modified scarf joint specimen was developed for characterizing the adhesive behaviour of poly (p-phenylene benzobisthiazole) (PBT) film/epoxy composites. This method subjected samples to varying amounts of normal stress (tensile or compressive) and shear stress. This resulted in the determination of two adhesive strengths; one in the absence of shear stress and one in the absence of normal stress. As a result, the dependence of the adhesive strength on the degree of normal stress was determined. The adhesive behaviour of PBT/epoxy composites was investigated at cure temperatures of 55, 85, 115 and 215°C. Adhesive strengths of 3.5 and 8.2 MPa were measured in the absence of shear and normal stress, respectively, for samples cured at 55° C. A decrease in adhesive strength with increasing cure temperature was attributed to residual cure and thermal stresses. The fracture of these composites was predominantly adhesive, resulting in a clean delamination of the PBT film from the epoxy surface. A modified Tsai-Wu failure criterion is suggested for these composites.     

Effect of shape factor, operating stresses, and electricalconductors on the energy density of rotating machines

The ability to store as much energy as possible for the least weight, defined by the rotor's energy density (U/m~J/g), is of paramount importance to the pulsed power supply of electromagnetic guns. This paper extends the work presented in a complementary paper in which the shape factors, F_s, for a representative number of rotating machines were derived and compared. The effects of shape factor, maximum allowable operational stress σ_&thetas;max, and effective rotor density ρ_eff (determined by the addition of electrical conductors to the rotor for power extraction) on rotor energy density and tip speed are presented in this paper. A comparison between two candidate composite designs for electromagnetic pulsed power generation-a laminated disk (LD) and a rim rotor (RR)-is performed. Our analyses have determined that the true discriminator between the LD and the RR designs is the allowable stress each can operate at since they both have the same shape factor and about the same composite density. The LD design is limited by the interlaminar shear strength of the epoxy bond to hoop stresses in the range of 150 ksi(1.0·10⁹ N/m²), while the RR design is able to take full advantage of the strength limit of the composite fiber and operate at hoop stresses as high as 350 ksi (3.1·10⁹ N/m²)     

Residual stresses in CSM/vinyl ester resin laminates due to post-cure shrinkage

In the design of glass reinforced plastic vessels for the chemical industry it should be recognised that the matrix can undergo significant chemical shrinkage during elevated temperature post-curing. If this shrinkage is restrained from occurring freely then residual stresses can be set up. These can quite easily exceed the levels of stress or strain allowed for by accepted design codes and are sufficient to promote environmentally assisted cracking in certain operating environments. Furthermore, in many cases the residual stresses will be significantly larger than the stresses associated with mechanical and thermal loading. This work provides some experimental results for this important type of stress. Post-cure shrinkage strain results are presented for castings of two vinyl ester resin types, flat CSM laminates and for thick walled tubes. The results for the tubes are compared to values predicted by commercial finite element software and by the use of the Rayleigh-Ritz method. Correlation between the predicted and measured values is shown to be inconsistent and possible reasons for this are explored.     

Residual stresses in steel rod with collar formed by partial diameter-enlarging technique

Residual stress distribution in a carbon steel component with collar manufactured by partial diameter-enlarging (PDE) process was evaluated using the angular dispersion and time-of-flight neutron diffraction methods in the interior and X-ray diffraction method at the surface of the collar. The residual stresses in the PDE specimen were smaller compared with those of a specimen with the similar shape and dimension made by a simple compression (SC) process. The hoop residual stress varied from ?98 to 2?MPa around the circumference at the surface of the collar in the PDE specimen, whereas it was nearly constant in the SC one. The residual stresses in the central region of the both specimens were nearly in a hydrostatic compressive stress condition.     

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.     

The effects of Mandrel material and tow tension on defects and compressive strength of hoop-wound,on-line consolidated,composite rings

Even carefully controlled, on-line processing, which is superior to standard autoclave processing in a number of ways, is not immune to the development of process-induced defects. These include waviness, residual stresses and voids caused by poor process parameter selections. Several designed experiments and multivariate regression analyses were performed to formally examine the effects of mandrel material, tow tension, cooling rate and ring thickness (as well as their interactions) on the development of these defects and their effects on the compressive strength of thin and thick, hoop-wound rings. Cylinders were produced from T300/polysulfone powder-impregnated towpreg using on-line consolidation; they were wound using different mandrels (aluminum, steel), tow tensions (13.3 N, 44.5 N), cooling rates (natural and natural convection) and radius-to-thickness ratios (11, 5.5). Three or four rings per cylinder were failed using a new, compliant-ring-based, radial compression test method; the compressive hoop and radial stresses at failure were measured and compared based on either uniaxial longitudinal compressive failure (thin rings) or biaxial maximum work (or Tsai-Hill) theory (thick rings). In thin rings, independent changes in mandrel material and tow tension significantly affect the hoop and radial stresses at failure: decreasing with increasing mandrel coefficient of thermal expansion and decreasing tow tension. Void contents higher than certain threshold values (2–3% in this study) substantially degrade the rings’ compressive strengths by eroding its inter- and intralaminar shear strength. Changes in part thickness had a significant effect on the radial (but not hoop) stresses at failure. Waviness, which was isolated to the innermost six plies of any cylinder, was principally affected by mandrel material, with tow tension and thickness played lesser, moderating roles. Cooling rate had no discernable effect on waviness. Residual stresses were principally affected by tow tension and thickness, with the mandrel material's role difficult to de-convolve from its effect on waviness. Modest cooling rates did not appear to affect the residual stresses. Void contents decreased with increasing cooling rates and tow tensions in thin rings and with increasing radius-to-thickness ratios in thick rings. With the appropriate selection of process parameters, on-line processing can produce relatively ‘defect-free’ hoop-wound cylinders, which can match the compressive strengths of autoclave-consolidated, unidirectional plate laminates and be used to measure the laminates intrinsic compressive strength.     

LOW-TEMPERATURE AUTOFRETTAGE: AN IMPROVED TECHNIQUE TO ENHANCE THE FATIGUE RESISTANCE OF THICK-WALLED TUBES AGAINST PULSATING INTERNAL PRESSURE

Abstract— It is shown that autofrettage at low temperatures is superior to autofrettage at room temperature in enhancing the fatigue resistance of thick-walled tubes against pulsating internal pressure. The physical reason is based on the well-known temperature dependence of the mechanical behaviour of metals and alloys which generally exhibit an enhancement of both the yield stress and strain hardening behaviour at lower temperatures. As a consequence, significantly larger compressive residual hoop stresses can be introduced during pressurization at low temperatures than at room temperature. Experimental data obtained on thick-walled tubes of the metastable austenitic stainless steel AISI 304 L which were subjected to pulsating internal pressure at room temperature after autofrettage at temperatures between-110°C and room temperature are presented. These data demonstrate convincingly the advantages offered by low-temperature autofrettage in enhancing both the fatigue life in the finite-life region and the fatigue endurance limit in comparison with autofrettage at room temperature. In conclusion, some specific materials requirements for optimum low-temperature autofrettage performance are discussed.     

Fracture strains in biaxially loaded 2024 aluminum tubes

2024 aluminum tubes, heat treated to a T6 and T8 temper, were proportionally loaded to failure with combinations of tension and internal pressure. We measured diffuse instability and fracture strains and compared these to the ductile fracture model of Ghosh (1976). Agreement was quite good. Fracture strains measured in axial tension and pure hoop tension were equivalent even though anisotropies were observed in the plastic deformation of the tubes.