Two-step Loading Research Articles

Finite Element Analysis of Tibial Bone-Implant Load Transfer and Bone Strains with a Modern Fixed-bearing Total Ankle Replacement

Category: Ankle Arthritis Introduction/Purpose: Loosening of the tibial component is the primary failure mode in total ankle arthroplasty (TAA). The mechanics of the tibial component loosening has not been fully elucidated. Clinically observed radiolucency and cyst formation in the periprosthetic bone may be associated with unfavorable load sharing at and adjacent to the tibial bone-implant interface contributory to implant loosening. However, no study has fully investigated the load transfer from the tibial component to the bone under multiaxial loads in the ankle. The objective of this study was to utilize subject-specific finite element (FE) models to investigate the load transfer through tibial bone-implant interface, as well as periprosthetic bone strains under simulated multiaxial loads. Methods: Bone-implant FE models were developed from CT datasets of three cadaveric specimens that underwent TAA using a modern fixed-bearing tibial implant (a cobalt-chrome tray with a polyethylene bearing, Salto Talaris, Integra LifeSciences). Implant placement was estimated from the post-operative CT scans. Bone was modeled as isotropic elastic material with inhomogeneous Young’s modulus (determined from CT Hounsfield units) and a uniform Poisson’s ratio of 0.3. The tibial tray (Young’s modulus: 200,000 MPa, Poisson’s ratio: 0.3) and the polyethylene bearing (Young’s modulus: 600 MPa, Poisson’s ratio: 0.4) were modeled as isotropic elastic. A 100-N compressive force, a 300-N anterior force, and a 3-Nm moment were applied to two literature based loading regions on the surface of the polyethylene bearing. The proximal tibia was fixed in all directions. The bone-implant contact was modeled as frictional with a coefficient of 0.7, whereas the polyethylene bearing was bonded to the tray. Results: Along the long axis of the tibia, load was transferred to the bone primarily through the flat bone-contacting base of the tibial tray and the cylindrical top of the keel, little amount of load was transferred to the bone between those two features (Fig. 1A). Low strain was observed in bone regions medial and lateral to the keel of the tibial tray, where bone cysts were often observed clinically (Fig. 1A). On average, approximated 70% of load was transferred through the anterior aspect of the tibial tray at the flat bone-contacting base, which corresponded to the relatively high bone strain adjacent to the implant edge in the anterior bone-implant interface (Fig. 1B). Conclusion: Our results demonstrated a two-step load transfer pattern along the long axis of the tibia, revealing regions with low bone strain peripheral to the keel indicative to stress shielding. Those regions were consistent with the locations of bone cysts observed clinically, which may be explained by the stress shielding associated remodeling of bone. These findings could also describe the mechanism of implant loosening and failure. Future studies may use our model to simulate more loading scenarios, as well as different implant placement and design, to identify means to optimize load transfer to the bone and prevent stress shielding.

Two-step load balancing scheme for fairness improvement in HetNets

The problem of load imbalance in HetNets among wireless access technologies is addressed in this article. A two-step strategy algorithm was adopted in this work, which considers both load and bandwidth. The first step is a randomized algorithm based on the Monte Carlo scheduling strategy while the second step was a load leveling algorithm that used the brute force method to classify the load on access technologies as average, below average and above average. The obtained results when compared to those of Least Connected Algorithm (LCA) performed better. The results achieved a global load balancing fairness of 0.9119 which was 12.37% better than LCA. The significance of the achieved result translates to better resource utilization among the wireless access technologies and better Quality of Service (QoS) for users. Keywords : two-step strategy, load balancing, least connected algorithm, quality of service, Monte Carlo algorithm

Finite element analysis of pre-stretch effects on ballistic impact performance of woven fabrics

Pre-stretch effects on ballistic impact performance of woven fabrics that made of Kevlar KM2 is investigated with finite element simulation. Individual yarns of the fabrics are modeled with truss elements incorporated with equivalent material properties of real fiber yarns. Before the projectile is issued, a fabric panel that has a rectangular configuration is pre-stretched by directly applying displacement boundary conditions. This two-step loading condition is realized through results transferring capability offered by the commercial software ABAQUS. Simulation results reveal that pre-stretch can significantly influence the fabric’s ballistic response such as ballistic limit, energy absorption and wave propagation. The fabric with higher pre-stretch absorbs more energy, however fails earlier than the fabric with lower pre-stretch. Parametric studies show that with the increase of pre-stretch, the deformation contour evolves from a pyramid shape that is conventionally observed in non-pre-stretched fabrics into a conical shape.

Experiments and constitutive modeling of deformation behavior of a magnesium sheet during two-step loading

Two-step tension tests, namely pre-tension in transverse direction followed by re-tension along different directions, were performed on an AZ31 magnesium alloy sheet. Compared with monotonic tension, early yielding phenomenon (strain path changes induced softening) was observed in the second loading of two-step tension. Moreover, the degree of softening increases with increasing loading angle from transverse direction to rolling direction. To model the deformation behavior, especially the early yielding phenomenon of AZ31 magnesium alloy sheet in two-step tension, more fluctuating components were introduced in the HAH model. The early yielding phenomenon of AZ31 magnesium alloy sheet in two-step tension was qualitatively well described by the extended HAH model.

Zeolite Supported Bimetallic Catalyst System: The Effect of Metal Loading for Catalytic Pyrolysis of Jatropha Residue

Two transition metals were loaded on H-ZSM-5 zeolite to produce bimetallic zeolite supported catalysts for catalytic pyrolysis reaction. Ni and Co metal were loaded on H-ZSM-5 via wet impregnation method. The loading sequence was applied using one-step and two-step loading method. The different loading sequence affect surface properties of catalyst and catalytic activity in pyrolysis reaction. The bimetallic catalysts were prepared at Ni+Co metal loading content of 10+10 wt% (Ni:Co=1:1) to 10+20 wt% (Ni:Co=1:2 or 2:1). All bimetallic catalysts supported on H-ZSM-5 were calcined and characterized by X-ray Diffraction (XRD), Surface area analysis (BET) and Temperature Programmed Desorption of ammonia (NH3-TPD). The XRD patterns of bimetallic zeolite supported catalysts revealed that loading of two metals at high content affect crystalline structural of ZSM-5 support. All XRD patterns illustrated peaks characteristic of ZSM-5, cobalt oxide and nickel oxide. The NH3-TPD results showed number of acid sites of the catalyst which revealed that the acid sites of ZSM-5 support was weakened with transition metal added. The two-step loading of 10+20 wt% metals on ZSM-5 reduced the peak intensities of NH3 desorption due to the metal particles aggregate on acid sites of ZSM-5. The two-step 10+20 wt% bimetallic catalysts has the lowest surface acidity, followed by the one-step 10+20 wt%, the two-step 10+10 wt% and the one-step 10+10 wt% bimetallic catalysts, respectively. Jatropha residue was used for catalytic pyrolysis study. Jatropha residue and bimetallic catalyst was pyrolyzed at 500 °C in a pyrolysis-gas chromatography/mass spectrometry (Py-GC/MS). The product vapor was analyzed by GC/MS for the different groups of organic products such as fatty acid, aldehydes, ketones, aliphatic hydrocarbons, aromatic hydrocarbons and nitrogen compounds. The product from catalytic pyrolysis of jatropha residue with bimetallic zeolite supported catalysts enhance deoxygenation reaction that resulted in high aliphatic and aromatic hydrocarbons product. The one-step loading at ratio Ni:Co = 1:1 (10+10 wt%) gave the highest hydrocarbons product yield at 57.81%.

Ultrashort echo time T2∗ values decrease in tendons with application of static tensile loads

In early stages of tendon disease, mechanical properties may become altered prior to changes in morphological anatomy. Ultrashort echo time (UTE) magnetic resonance imaging (MRI) can be used to directly detect signal from tissues with very short T2 values, including unique viscoelastic tissues such as tendons. The purpose of this study was to use UTE sequences to measure T2∗, T1 and magnetization transfer ratio (MTR) variations of tendon samples under static tensile loads. Six human peroneal tendons were imaged before and under static loading using UTE sequences on a clinical 3T MRI scanner. Tendons were divided into two static tensile loading groups: group A that underwent one-step loading (15N) and group B that underwent two-step loading (15 and 30N). The T2∗, T1 and MTR variations were investigated in two selected section regions of interest (ROIs), including whole and core sections. Mean T2∗ values for the first step of loading (groups A and B) in both whole section and core section ROIs were significantly decreased by 13±7% (P=0.028) and 16±5% (P=0.017), respectively. For the second loading step (group B), there was a consistent, but non-significant reduction in T2∗ value by 9±2% (P=0.059) and 7±5% (P=0.121) for whole and core sections, respectively. Mean T1 did not show any consistent changes for either loading steps (P>0.05). Mean MTR increased slightly, but not significantly for both loading steps (P>0.05). Significant differences were found only in T2∗ values of tendons by static tensile load application. Therefore, T2∗ monitoring during loading is suggested for quantitative investigation of the tendons biomechanics.

Structural behavior of a lightweight, textile-reinforced concrete barrel vault shell

Textile-reinforced concrete (TRC) as a novel composite material offers a wide range of capabilities and flexibility in the manufacturing of thin-walled, lightweight structures. The application of textile reinforcement in fine aggregate high-performance concrete has enabled the dimensioning of structural concrete in very small thicknesses. This possibility allows for the fabrication of thin-walled TRC shell structures with complex geometries. On the other hand, structural planning and construction require new modeling approaches to comprehend the structural behavior of such forms. In this paper, we present the fabrication procedure of a large-scale TRC vault shell, together with the performed experimental study. The shell structure was tested under a two-step loading scenario to study the load-bearing behavior. The particular focus of the paper is on the analysis of the structural behavior by means of an anisotropic strain-hardening material model specifically developed for the simulation of TRC shells. The prediction obtained using the nonlinear finite element simulation has been compared with the test results to validate the modeling approach. The performed studies are used to evaluate and discuss the structural redundancy included in the applied linear ultimate limit state assessment procedure.

Variable amplitude fatigue behavior of neat and short glass fiber reinforced thermoplastics

Variable amplitude fatigue behavior of injection molded thermoplastics was investigated. A neat thermoplastic (polypropylene impact co-polymer) and a thermoplastic composite made of polybutylene terephthalate (PBT) with 30wt% short glass fibers were used for the experimental study. Fatigue tests were conducted on both un-notched and notched specimens using plate-type specimens. For the notched specimens, a circular hole was drilled in the center of the specimens. Two-step loading (high-low and low-high) tests at two cycle ratios of 0.2 and 0.5 and stress ratios of R=0.1 and −1 were conducted to investigate load sequence and mean stress effects, as well as the accuracy of linear damage rule. Periodic overload and service load history tests were also conducted to investigate the effect of overloads, cycling frequency, small stress cycles, and maximum stress level. A strengthening effect of increased frequency was observed for both materials and the effect of small stress cycles was found to be negligible. A general fatigue damage model along with the rainflow cycle counting method and linear damage rule were used for life predictions of variable amplitude service load history tests. More than 89% of the life predictions based on the modeling procedure used were within a factor of 3 of the experimental lives for both materials.

Fatigue Behavior of Neat and Short Glass Fiber Reinforced Polymers under Two-Step Loadings and Periodic Overloads

Fatigue Behavior of Neat and Short Glass Fiber Reinforced Polymers under Two-Step Loadings and Periodic Overloads

Deformation behavior upon two-step loading in a magnesium alloy sheet

Two-step loading tests were performed on an AZ31 rolled magnesium alloy sheet with strong basal texture in the normal direction, and the deformation behavior such as a stress–strain curve, Lankford value, and texture evolution was investigated both experimentally and numerically. When the sheet was subjected to compression in the rolling direction followed by tension in different directions, the following characteristic deformation was observed during the second loading: The sigmoidal shape of the stress–strain curve was more pronounced when the sheet was stretched in the rolling direction but less pronounced as the loading direction approached the transverse direction. The Lankford value during the second loading was much smaller than that of the virgin sheet. Observation of the microstructure showed that the detwinning activity during the second loading decreased as the loading direction approached the transverse direction. The aforementioned deformation behavior was qualitatively well predicted using a crystal-plasticity finite-element method. The crystal-plasticity analysis was then used to investigate the underlying deformation mechanism upon two-step loading, focusing especially on the effect of twinning and detwinning activities.

Energy Based Approach Calculations of Machine Components Fatigue Life

The purpose of this paper is to present a method for fatigue life calculations in energy based approach (hybrid method) and its experimental verification. Tests were performed for C45 steel. It was found, on the basis of experimental tests, that the diagram of fatigue life in energy approach is characterized by higher value of determination coefficient R2 than the diagram of fatigue life in stress based approach (both for constant amplitude and programmable loading). A comparative analysis of calculation results and experimental tests results revealed that for two-step loading the relative difference in results depends on the maximal value of stresses in the spectrum.

D-14 摩擦撹拌接合A6063合金の疲労き裂進展挙動に及ぼす二段二重変動荷重の影響

D-14 摩擦撹拌接合A6063合金の疲労き裂進展挙動に及ぼす二段二重変動荷重の影響

鋳造アルミニウム合金の疲労強度特性に及ぼす二段多重変動荷重の影響

鋳造アルミニウム合金の疲労強度特性に及ぼす二段多重変動荷重の影響

Кривые длительной прочности нелинейной модели вязкоупругопластичности типа Максвелла и правило суммирования поврежденности при ступенчатых нагружениях

Исследуется нелинейное определяющее соотношение типа Максвелла для вязкоупругопластичных материалов с двумя произвольными материальными функциями одного аргумента с целью выявления арсенала его возможностей и области применимости, в частности, возможности подключения к нему критериев для описания разрушения при ползучести с постоянным или кусочно-постоянным напряжением. При минимальных первичных ограничениях на материальные функции выведены уравнения теоретических кривых длительной прочности, порожденных соотношением типа Максвелла в сочетании с деформационными и энергетическим (диссипативным) критериями разрушения, а также с интегральными критериями, учитывающими историю деформирования. Аналитически изучены и сопоставлены общие свойства этих кривых. Показано, что они адекватно описывают типичные свойства экспериментальных кривых длительной прочности вязкоупругопластических материалов. Для найденных зависимостей времен разрушения от напряжения проверено выполнение правила линейного накопления поврежденности при ступенчатом нагружении и выведены формулы для величины (и знака) отклонений от него. В частности доказано, что для нелинейного соотношения типа Максвелла в сочетании с диссипативным критерием разрушения всегда в точности выполняется правило линейного накопления поврежденности при любом ступенчатом нагружении, а для деформационного критерия оно, наоборот, никогда не выполняется, но дает оценку сверху или снизу для времени разрушения в зависимости от знака скачка напряжения.

Granular bright fact crack propagating under very high cycle fatigue

The different crack initiation modes from different internal defects of a high carbon chromium bearing steel are investigated in the present paper. The relative granular bright facet (GBF) size is approximately proportional to . The critical applied stress intensity factor range at the periphery of inclusion ΔKinc,th, above which the GBF will no longer be formed, was obtained. For the present SUJ2 steel, ΔKinc,th = 8 MPa m1/2. The crack growth process in GBF is further investigated by the low–high two-step variable amplitude loading tests. The crack growth process in GBF can be described by . The exponent m0 is a material constant independent of the applied stress amplitude and the inclusion size and increases with increasing of HV.

Effect of bi-directional operation on rolling-sliding wear of wheel steel under variable amplitude loading

The objective of this paper was to investigate the wear behaviour of railway wheel steel under the combined condition of bi-directional operation and variable amplitude loading. Rolling-sliding wear tests show that the wear rate of the wheel disc significantly drops at the moment of the bi-directional operation, and then it rises with the increase in reversed rolling cycles until a steady wear state is reached. When the test was conducted under the combined condition of the bi-directional operation and a two-step loading, the wear rate of the wheel disc after bi-directional operation decreases with the increase in the first step contact stress. These wear behaviours can be explained by the variation of the reversed plastic deformation and the movement of wave structure produced by the bi-directional operation. The wave structure moves down with the increase in reversed rolling cycles due to the reduction of the resistance to reversed plastic deformation. This reduces the resistance to the propagation of surface cracks and leads to the increase of wear rate. The resistance to reversed plastic deformation rises with the increase in the first step contact stress, which makes the wave structure move towards to the worn surface and reduces the wear rate of the wheel steel.

Effect of Pre-Compressive Strain on Work-Hardening Behavior upon Two-Step Loading in a Magnesium Alloy Sheet

In the present study, deformation behavior upon two-step loading in a rolled AZ31 Mg alloy sheet was investigated. The experimental procedure was as follows: (1) a sheet was subjected to in-plane compression, (2) small samples were cut from the compressed sheet along various directions, and (3) uniaxial tension was imparted to small samples. The angle between the first and second loading directions was set to either 0, 30, 60, or 90°. During the second loading, a strong in-plane anisotropy occurred in the stress-strain curve: a sigmoidal curve occurred during the second loading for the angles of 0 and 30°, while it did not arise for the angles of 60 and 90°. From microstructural observations, it was presumed that the aforementioned results could be explained in terms of the activity of detwinning.

Distributed load aware relay selection algorithm for cooperative relay networks

Cooperative relaying is accepted as a promising solution to achieve high data rates over large areas in the 4G wireless system. This paper presents a two-step distributed load aware relay selection algorithm LA-RS for relay-based cooperative networks. According to current user channel conditions as well as the user numbers that relay serves, LA-RS chooses the optimal relay node for each user in a distributed way. More specifically, in the first step each mobile station MS constructs relay selection functions for candidate relay node RN, and selects the appropriate RN accordingly. Furthermore, in the second step RNs also adopt a weighted channel dependent scheduling algorithm for resource allocation, so as to provide a flexible tradeoff between per-user fairness and aggregate throughput in each cell. Simulation results demonstrate that compared with other relay selection schemes, LA-RS achieves load balancing through effective relay selection method, so as to obtain a tradeoff between the system throughput and the user equity.

Influence of Stresses below the Fatigue Limit on Fatigue Life

According to the performed analysis of fatigue phenomena occurring in metals, the effects of fatigue appear in the form of lines and slip bands under loading conditions producing variable stresses with values below the fatigue limit of these metals. It is commonly accepted that variable stresses with constant amplitude of values below 0.4 of the fatigue limit do not cause plastic strain in grains (lines and slip bands), thus they do not affect the fatigue life. This study is an attempt of quantitative assessment of the influence of stresses with values below the fatigue limit on fatigue life by using tests with programed two-step loading (variable-amplitude). Tests were performed with the use of C45 steel specimens.

Experimental Investigation of Fatigue Damage Accumulation in En295 Steel

In the paper monotonic, cyclic and fatigue data for En295 steel were presented. Subsequently, the experimental results for two-step loading sequence (low-to-high (L—H) loading, and high-to-low (H—L) and three-step loading sequence (L-H-L and H-L-L) were presented. The analysis showed that all loading sequences affects the cumulative damage in En295 steel. In the analysis an energy based cumulative damage hypothesis presented by Golos and Ellyin was applied. The predictions based on the analyzed Golos-Ellyin cumulative damage hypothesis are compared with the obtained data for En295 steel and is found to be in fairly good agreement.