Permanent Plastic Deformation Research Articles

Finite Element Cold Bending Residual Stress Evaluation on Submarine Pressure Hull Instability Assessment

During the pressure hull manufacturing, processes like cold bending and welding are often applied. These processes lead to permanent plastic deformations which are associated with residual stresses. The presence of residual stresses is equivalent to the introduction of an initial preload in the structure, which accelerates the plastification process, decreasing pressure hull resistance. To quantify this reduction, a case study that considers residual stresses due to cold bending on hull plates and frame flanges had been performed using finite element models. The study encompasses hull diameters of 6, 8, and 10 m with hull plates and frame flange thickness from 20 to 30 mm, with HY100 steel. Finite element numerical analyses were done considering material and geometric nonlinearities. First, the cold bending residual stresses were determined using finite element models. Then, these cold bending residual stresses were introduced as initial stresses in the submarine pressure hulls' finite element models. In the end, it was possible to verify that the presence of cold bending residual stress reduces the submarine hull collapse pressure up to 4.3%.

Constitutive modeling of large inelastic deformation of amorphous polymers: Free volume and shear transformation zone dynamics

Due to the lack of the long-range order in their molecular structure, amorphous polymers possess a considerable free volume content in their inter-molecular space. During finite deformation, these free volume holes serve as the potential sites for localized permanent plastic deformation inclusions which are called shear transformation zones (STZs). While the free volume content has been experimentally shown to increase during the course of plastic straining in glassy polymers, thermal analysis of stored energy due to the deformation shows that the STZ nucleation energy decreases at large plastic strains. The evolution of the free volume, and the STZs number density and nucleation energy during the finite straining are formulated in this paper in order to investigate the uniaxial post-yield softening-hardening behavior of the glassy polymers. This study shows that the reduction of the STZ nucleation energy, which is correlated with the free volume increase, brings about the post-yield primary softening of the amorphous polymers up to the steady-state strain value; and the secondary hardening is a result of the increased number density of the STZs, which is required for large plastic strains, while their nucleation energy is stabilized beyond the steady-state strain. The evolutions of the free volume content and STZ nucleation energy are also used to demonstrate the effect of the strain rate, temperature, and thermal history of the sample on its post-yield behavior. The obtained results from the model are compared with the experimental observations on poly(methyl methacrylate) which show a satisfactory consonance.

Quasi-static compression behavior of nickel oxide, nickel oxide:zirconia, nickel:zirconia and nickel foams

An effective material for use in shock mitigation should spread the deflection of the shock wave over a longer period of time and should minimize the force felt by the object under impact. Ductile or brittle cellular materials are currently gaining importance due to their unique high energy absorption characteristics. Reticulated cellular foam structures of nickel oxide (NiO) and nickel oxide:zirconia (NiO:YSZ 60:40 percentage by wt.) were fabricated by polymeric sponge replication process. These foams are reduced under hydrogen atmosphere to produce metallic nickel (Ni) and nickel:zirconia (Ni:YSZ) cermet foams, respectively. X-ray diffraction studies on the struts confirmed the corresponding phase formation. Further, the volume fraction of the solid in foam is estimated through image analysis. All the foams are subjected to uni-axial compression and the stress–strain curves were recorded. A comparative evaluation of progressive deformation behavior at room temperature was also carried out. Stress–strain curve of the nickel foam shows distinctly three regimes under compression, a deformation regime showing a linear dependence in the strain with stress. This is followed by a second region showing a plateau corresponding to the energy absorption resulting from the permanent plastic deformation while retaining the integrity and finally densification region through the wall collapse resulting in the maximum compressive strength. Stress–strain curves of all other foams such as NiO, NiO:YSZ and Ni:YSZ has demonstrated a similar fracture behavior under compression which caused not only by unstable crack propagation originating from a single crack, but also by merging of many cracks leading to the formation of the crushed zone. Compressive strength is found to be a strong function of solid fraction supporting the load and percentage porosity of NiO foams. Estimation of relative energy absorption has exhibited higher energy absorption irrespective of the material of construction at higher strain rates.

Lateral Variations of Interplate Coupling along the Mexican Subduction Interface: Relationships with Long-Term Morphology and Fault Zone Mechanical Properties

Although patterns of interseismic strain accumulation above subduction zones are now routinely characterised using geodetic measurements, their physical origin, persistency through time, and relationships to seismic hazard and long-term deformation are still debated. Here, we use GPS and morphological observations from southern Mexico to explore potential mechanical links between variations in inter-SSE (in between slow slip events) coupling along the Mexico subduction zone and the long-term topography of the coastal regions from Guerrero to Oaxaca. Inter-SSE coupling solutions for two different geometries of the subduction interface are derived from an inversion of continuous GPS time series corrected from slow slip events. They reveal strong along-strike variations in the shallow coupling (i.e. at depths down to 25 km), with high-coupling zones (coupling >0.7) alternating with low-coupling zones (coupling 0.7) and transitions to uncoupled, steady slip at a relatively uniform \(\sim \)175-km inland from the trench. Along-strike variations in the coast-to-trench distances are strongly correlated with the GPS-derived forearc coupling variations. To explore a mechanical explanation for this correlation, we apply Coulomb wedge theory, constrained by local topographic, bathymetric, and subducting-slab slopes. Critical state areas, i.e. areas where the inner subduction wedge deforms, are spatially correlated with transitions at shallow depth between uncoupled and coupled areas of the subduction interface. Two end-member models are considered to explain the correlation between coast-to-trench distances and along-strike variations in the inter-SSE coupling. The first postulates that the inter-SSE elastic strain is partitioned between slip along the subduction interface and homogeneous plastic permanent deformation of the upper plate. In the second, permanent plastic deformation is postulated to depend on frictional transitions along the subduction plate interface. Based on the location and friction values of the critical state areas identified by our Coulomb wedge analysis, we parameterise frictional transitions in plastic-static models of deformation over several seismic cycles. This predicts strong shear dissipation above frictional transitions on the subduction interface. The comparison of modelled surface displacements over a critical zone at a frictional transition and over a stable area with no internal wedge deformation shows differences of long-term uplift consistent with the observed along-strike variations in the coast-to-trench distances. Our work favours a model in which frictional asperities partly control short-term inter-SSE coupling as measured by geodesy and in which those asperities persist through time.

On the dynamic plastic response of steel membranes subjected to localised blast loading

Permanent plastic deformation is expected when close-in blasts due to e.g. detonation of Improvised Explosive Devices (IED's) hit thin metallic targets. A circular thin steel plate i.e. a membrane is studied in the present work subject to a general form of a localised blast loading. The spatial shape of the pulse is fixed and different temporal shapes are investigated. Dynamic analyses are conducted and the permanent transverse displacements are found for each case.For high amplitude pulse loads of short duration, it was found that the permanent transverse displacement can be found by replacing the load by an impulse without the loss of accuracy. Excellent correlation with numerical simulations obtained from ABAQUS/Explicit is achieved. The predicted final displacements for different pulse shapes are also found to be similar, thus where membrane action is dominant, the response is insensitive to pulse shape.

Biomechanical properties of five different currently used implants for open-wedge high tibial osteotomy.

BackgroundAs several new tibial osteotomy plates recently appeared on the market, the aim of the present study was to compare mechanical static and fatigue strength of three newly designed plates with gold standard plates for the treatment of medial knee joint osteoarthritis.MethodsSixteen fourth-generation tibial bone composites underwent a medial open-wedge high tibial osteotomy (HTO) according to standard techniques, using five TomoFix standard plates, five PEEKPower plates and six iBalance implants. Static compression load to failure and load-controlled cyclic fatigue failure tests were performed. Forces, horizontal and vertical displacements were measured; rotational permanent plastic deformations, maximal displacement ranges in the hysteresis loops of the cyclic loading responses and dynamic stiffness were determined.ResultsStatic compression load to failure tests revealed that all plates showed sufficient stability up to 2400 N without any signs of opposite cortex fracture, which occurred above this load in all constructs at different load levels. During the fatigue failure tests, screw breakage in the iBalance group and opposite cortex fractures in all constructs occurred only under physiological loading conditions (<2400 N). The highest fatigue strength in terms of maximal load and number of cycles performed prior to failure was observed for the ContourLock group followed by the iBalance implants, the TomoFix standard (std) and small stature (sm) plates. The PEEKPower group showed the lowest fatigue strength.ConclusionsAll plates showed sufficient stability under static loading. Compared to the TomoFix and the PEEKPower plates, the ContourLock plate and iBalance implant showed a higher mechanical fatigue strength during cyclic fatigue testing. These data suggest that both mechanical static and fatigue strength increase with a wider proximal T-shaped plate design together with diverging proximal screws as used in the ContourLock plate or a closed-wedge construction as in the iBalance design. Mechanical strength of the bone-implant constructs decreases with a narrow T-shaped proximal end design and converging proximal screws (TomoFix) or a short vertical plate design (PEEKPower Plate). Whenever high mechanical strength is required, a ContourLock or iBalance plate should be selected.

Seismic behavior of SMA–FRP reinforced concrete frames under sequential seismic hazard

Accumulation of plastic deformation under excessive loads, is one of the most critical drawbacks in steel reinforced concrete structures. Permanent plastic deformation of steel rebars is among the main reasons for the disruption of the functionality of RC structures after major seismic events. It can also pose life threatening risks in case of strong aftershock occurrence. In an attempt to address the problem of excessive permanent deformations and their impact on the post-earthquake functionality of concrete moment resisting frame (MRF) structures, this paper studies analytically a new type of reinforcing bars made of fiber reinforced polymer (FRP) with embedded superelastic shape memory alloy (SMA) fibers. SMA–FRP reinforcement is characterized with both ductility and pseudo-elasticity which are two important characteristics that are sough in this study to enhance the ability of RC moment frames to withstand strong sequential ground motions (i.e. main shock followed by one or more aftershocks). In this study, experimentally validated SMA–FRP material models are used in structural level models to assess the performance of RC frame structures under seismic loading. Three-story, one-bay prototype RC MRFs, reinforced with steel and SMA–FRP composite reinforcements are first designed using performance based criteria and then subjected to incremental dynamic analysis under sequential ground motions. Comparison is drawn between steel and SMA–FRP reinforced frames based on accumulation of damage and residual drifts. Numerical results show superior performance of SMA–FRP composite reinforced MRF in terms of dissipation of energy and accumulation of lower residual drifts. Increased demands from the effects of aftershock causes accumulation of residual drifts in steel reinforced frames which is mitigated in SMA–FRP reinforced frame through re-centering capability.

Nano-scale wear characterization of CoCrMo biomedical alloys

Low amplitude motions at the micro and the nano-scale at the femoral stem–cement interface under physiological loads can result in fretting and nano-wear on the stem surface. These are important wear processes in cemented total hip replacements as the release of metal debris and ions can trigger adverse local tissue reactions within the body, bone resorption and subsequent aseptic loosening of the femoral component resulting in the implant failure. However, the influence of the microstructure and manufacturing processes on the nano-wear behaviour of different cobalt chromium molybdenum (CoCrMo) alloys has not been studied extensively. Four CoCrMo alloys were tested under reciprocating wear conditions at the nano-scale level. Tangential friction forces, coefficient of friction and plastic deformation values were recorded. A new white-light-interferometer system was validated against atomic force microscopy and Nano Vantage Test System measurements to analyse the permanent plastic deformation caused in each of the samples. Significant differences were found in the total plastic deformation achieved by the as cast alloy compared to the forged, as cast single thermal treated and as cast double thermal treated samples. In addition thermal treated samples presented a tendency to produce a higher quantity of wear debris around the nano-wear scars. These findings indicate a possible relation between the wear resistance at the nano-scale and the manufacturing and thermal processes applied on the CoCrMo biomedical alloys.

영구동토 급속안정화를 위한 롤타입강재매트의 모델링과 해석(II) - 변수해석 -

Using the finite element analysis model presented in accompanying paper, parametric study was performed in this paper. Various parameters were considered such as the width of wheel loads-induced permanent plastic deformation, backfill, equivalent thickness and orthogonal characteristic of steel mats. The effects of these parameters were analyzed for vertical and rotational displacements, maximum moment and tensile stress. From the parametric studies, it is found that great vertical deflection and tensile stress above allowable flexural tensile strength are developed in steel mats by the wheel loads-induced permanent plastic deformation. Backfill or increasing the thickness of steel mats is a feasible solution on this problem.

Numerical simulations of response of tubular steel beams to close-range explosions

A numerical study of hollow and concrete-filled square tubular steel columns subjected to near-field detonations has been undertaken and validated through the experimental results. The experiments used concrete-filled and hollow square tubular columns (100×5mm SHS Grade C350) made out of cold-formed structural steel hollow sections (SHS) that were simply supported at both ends. High explosives TNT charges were placed above the top surface of the column at two different scaled standoff distances of 0.12m/kg1/3 and 0.15m/kg1/3. Failure patterns and permanent mid-span deformations were recorded and compared with the numerical analysis results. Arbitrary Lagrangian–Eulerian (ALE) formulations coupled with fluid–structure interaction (FSI) algorithms that are available in the advanced finite element code LS-DYNA were used in the numerical study. A detailed description of the numerical technique adopted in the study is presented. The models were validated with the experimental results and were used to obtain the failure pattern, permanent plastic deformation, pressure and impulse time histories, stress distribution, and energy absorption of the different configurations of the columns. The performance of hollow and concrete-filled SHS tubes for blast load mitigation was assessed and discussed.

Effect of contact stresses on shape recovery of NiTiCu thin films

Permanent plastic deformation induced by mechanical contacts affects the shape recovery of shape memory alloys. To understand the shape recovery of NiTiCu thin films subjected to local contact stresses, systematic investigations are carried out by inducing varying levels of contact stresses using nanoindentation. The resulting indents are located precisely for imaging using a predetermined array consisting of different sized indents. Morphology and topography of these indents before and after shape recovery are characterized using Scanning Electron Microscope and Atomic Force Microscope quantitatively. Shape recovery is found to be dependent on the contact stresses at the low loads while the recovery ratio remains constant at 0.13 for higher loads. Shape recovery is found to occur mainly in depth direction of the indent, while far field residual stresses play very little role in the recovery.

Effect of Internal Pressure and Dent Depth on Strain Distribution of Pressurized Pipe Subjected to Indentation

A dent in a pipeline is a permanent plastic deformation of the circular cross section of the pipe. This paper discusses numerical results obtained from finite element (FE) simulation of pressurized pipe subjected to radial denting by a rigid indenter. Dent produced by rectangular shape indenter is assessed and the strain distribution of the pipe is investigated. The effect of internal pressure and dent depth on the distribution of strain is also studied. The results show that the circumferential and longitudinal strains increase with increasing the internal pressure and the depth of the dent. Numerical results are compared with an empirical theoretical model in order to demonstrate the accuracy of the analysis.

Deformation twinning and detwinning in the lamella copper observed by in-situ TEM

Deformation twinning in metal is normally a permanent plastic deformation mechanism. In this study, deformation twining and detwinning were observed at a crack tip by in-situ transmission electron microscope and the post fracture surface characterization for the lamella copper. At deformation twinning process, the larger twins grew at expense of the smaller twins adjacent to them via the partial dislocations. The detwinning process is accomplished via the collective glide of multiple twinning dislocations that form an incoherent twin boundaries under the action of shear stress. This discovery of reversible twinning is the evidence that small scale deformation twinning is unstable.

A Transforming Metal Nanocomposite with Large Elastic Strain, Low Modulus, and High Strength

Freestanding nanowires have ultrahigh elastic strain limits (4 to 7%) and yield strengths, but exploiting their intrinsic mechanical properties in bulk composites has proven to be difficult. We exploited the intrinsic mechanical properties of nanowires in a phase-transforming matrix based on the concept of elastic and transformation strain matching. By engineering the microstructure and residual stress to couple the true elasticity of Nb nanowires with the pseudoelasticity of a NiTi shape-memory alloy, we developed an in situ composite that possesses a large quasi-linear elastic strain of over 6%, a low Young's modulus of ~28 gigapascals, and a high yield strength of ~1.65 gigapascals. Our elastic strain-matching approach allows the exceptional mechanical properties of nanowires to be exploited in bulk materials.

Evaluation of plasticity and radiopacity of elastic separators by means of traction tests and radiography

INTRODUCTION: Elastic separators are widely used in the orthodontic clinic, in different sizes and thicknesses. Their function is to promote the separation between the teeth that will receive orthodontic bands, however, when subgingivally lost, they may cause periodontal problems and, ultimately, cause tooth extraction. OBJECTIVE: Concerned not only about their function but also about their properties, this experimental work was performed in order to evaluate, with a clinical perspective, the plasticity and radiopacity of the elastic separators, usually used in orthodontic clinic, through physical traction tests and radiographs. METHODS: Six different brands were analyzed and divided in two groups: extruded separators (American Orthodontics, GH, Morelli Blue and Morelli Green) and modular separators (Abzil, Dentaurum, GAC, Blue Modular Morelli and Green Modular Morelli). The plasticity was evaluated after physical traction tests, stretching the separators to 3 times their initial diameter. While the radiopacity was assessed using x-rays. RESULTS: When stretched to three times their inner diameter (ID), the extruded separators American Orthodontics, GH, Morelli Blue and Morelli Green and the modular ones Dentaurum, Morelli Blue Modular and Morelli Green Modular, presented elastic deformation, recovering their original inner diameter. The modular separators from Abzil and GAC showed permanent plastic deformation. With respect to radiopacity, the Abzil, American Orthodontics, Dentaurum, GAC and GH separators were radiopaque. CONCLUSION: Correlating the results concerning the plasticity and radiopacity, the separators which showed the most satisfactory results on both tests were, among the extruded separators, American Orthodontics and GH, and among the modular separators, GAC and Dentaurum.

Revisiting ASME Strain-Based Dent Evaluation Criterion

Oil and gas transmission pipelines can be subjected to concentrated lateral loads and as a result, a dent can form. A dent is a localized defect in the pipe wall in the form of a permanent inward plastic deformation. This kind of defect is a matter of serious concern for the pipeline operator since a rupture or a leak may occur. Dent may not pose an immediate threat to the structural integrity of a pipeline. However, it can possibly hinder the operational and inline inspection activities. In the long run, it can cause a leak or rupture in the pipeline under sustained or cyclic pressure load. Hence, AMSE B31.8 recommends a strain-based criterion for the assessment of dents. This strain-based criterion was developed based on several assumptions. This study was undertaken using full-scale laboratory tests and finite element analyses to review and revisit the ASME strain-based dent evaluation criterion and its assumptions. It was found that some of these assumptions are incorrect, and hence, this dent evaluation criterion can lead to inaccurate estimations of critical (effective) strain values in dents, which in turn can lead to inaccurate assessments of the dents.

Expansion-Cone Material: Heat-Treatment Effects

This article, written by Senior Technology Editor Dennis Denney, contains highlights of paper IPTC 14784, ’Effect of Heat Treatment on Mechanical Properties of Expansion-Cone Material,’ by Sayyad Zahid Qamar and Tasneem Pervez, Sultan Qaboos University, prepared for the 2011 International Petroleum Technology Conference, Bangkok, Thailand, rescheduled to 7-9 February 2012. The paper has not been peer reviewed. To withstand repeated high-level thermomechanical stresses, expansion-cone material must have specific properties. This work covered heat treatment and mechanical testing of tensile and impact specimens of American Iron and Steel Institute (AISI) D6 (DIN 1.2436) tool steel. The heat-treatment process comprises annealing, hardening/austenitizing, air or oil quenching, and single or double tempering. Mechanical testing includes hardness, tensile properties (elastic modulus, yield strength, ultimate strength, and ductility), and impact strength. An optimal heat-treatment sequence was developed for the AISI D6-steel expansion mandrel. Introduction Solid-expandable-tubular (SET) technology is applied for completion and production purposes. SET systems can optimize fracturing parameters by maintaining larger diameters, and they can provide effective sealing for isolating multizone environments. Expandable tubulars can help in first-time fracturing, refracturing, and multizone fracturing, and in refurbishing older wells, thus providing enhanced production and fracturing applications. Maintenance of many oil and gas wells requires workover operations ranging from simple interventions to costly remediations. These operations include correcting production or injection profiles, converting wells from producers to injectors (or vice versa), mitigating cementation problems, repairing corroded casings, fracturing or refracturing of zones, and perforating or reperforating new or existing zones. SET systems enable such strategies. Also, SET technology can assist techniques such as smart completions, multifracturing strategies, steam-assisted gravity-drainage applications, and unconventional enhanced-oil-recovery methods. SET technology uses cold expansion of a tubular by forcing a conical mandrel made from special tool steel through it. The tubular is cold worked beyond its elastic limit, causing permanent plastic deformation, as shown in Fig. 1. An SET test rig was designed, fabricated, and commissioned for full-scale expansion tests of SETs. Expansion cones, shown in Fig. 2, used at this facility are made from cold-worked AISI D6 tool steel. AISI D6 tool steel is a high-carbon high-chromium steel with 2.0–2.25% car-bon, 0.2–0.6% manganese, 0.2–0.4% silicon, 11–13% chromium, 0.15–0.3% vanadium, and 0.8–1.25% tungsten. This steel is known for its high wear resistance, high compressive strength, high surface hardness after hardening, good through-hardening properties, and good stability during hardening. Published work on AISI D6 steel focused on surface-hardening methods and wear-related issues. However, little published literature was available about an optimum heat-treatment strategy, especially for AISI D6 steel used as a mandrel for expanding petroleum tubulars. The main objectives of this work were to investigate mechanical properties of AISI D6 steel after various heat-treatment operations and to develop an optimum heat-treatment strategy.

Dynamic Viscoplastic Response of a Circular Plate Subjected to Underwater Shock

A structure subjected to underwater shock exhibits surprising dynamic behavior, different from the permanent plastic deformation of a structure subjected to air blast, due to the presence of complicated fluid-structure interaction (FSI) effect. Previous studies of a circular plate subjected to underwater shock indicate that there exist large discrepancies between theoretical and experimental results of plastic deformations. Herein we thus propose a new double-scale and double-phase (DSDP) FSI model for correct prediction of the dynamic plastic behavior of a circular plate subjected to underwater shock. Results obtained from this DSDP model are compared with several experimental tests, with excellent agreement observed. This model is believed useful for further implementation in those software programs that handle underwater explosion and its effects on marine structures.

On the chaotic dynamic response of deterministic nonlinear single and multi-degree-of-freedom systems

This paper presents a comprehensive study on counter-intuitive or chaotic dynamic response of prototype discrete parameter models of single or multiple degrees of freedom subject to blast or impact loading. Non-linear dynamic behaviour of a typical single degree-of-freedom (SDOF) and multi-degree-of-freedom (MDOF) systems are studied by taking into account cubic and quintic non-linearities, elastic perfect-plastic, and elastic-plastic-hardening and softening behaviours. The first part is founded on Duffing’s equation and Ueda’s work on strange attractors which indicates the presence of chaos in deterministic systems by using chaos detection techniques such as Poincare’s mapping and Lyapunov’s exponents and in some cases by fractal dimensions. In these deterministic problems, the system hesitates to settle between two different possible settling regimes despite the fact that the input parameters to the system are deterministic. In a SDOF model with elastic perfectly-plastic or elastic-plastic-hardening resistance function the sign of the permanent plastic deformation may or may not coincide with that of loading direction hence chaotic behaviour can be observed. For an elastic-plastic-softening system, subjected to blast loading, the problem is sensitive to the ratio of post yield stiffness to initial stiffness and for certain ranges of this parameter a small change can replicate into an abrupt change in response. Examples are included of finite elements models with many degrees of freedom of beams and plates. As most intricate engineering structures are composed of these structural elements the existence of component chaos can imply global counter-intuitive behaviour.

Effects of the Loading Direction on High Strain Rate Behavior of Woven Graphite/Epoxy Composites

Effects of the loading direction on high strain rate behavior of cylindrical woven graphite/epoxy composites are presented. Compressive split Hopkinson pressure bar (SHPB) was used for high strain rate experiments. Cylindrical specimens were loaded diametrically and transversely at the impact energies of 67 J, 163 J, and 263 J. Micro Laser Raman spectroscopy and scanning electron microscopy (SEM) were used for surface characterization. It is observed that diametrically loaded specimens show permanent plastic deformation with high ductility resulting in a catastrophic failure while transversely loaded specimens exhibit viscoplastic deformation with some recoverable damage. As a result of this, Raman peak shifted to higher values for the diametrically loaded fibers whereas almost no change was observed in the Raman shift of transversely loaded fibers.