Subsequent Cyclic Loading Research Articles

An investigation about interference fit effect on improving fatigue life of a holed single plate in joints

In this article, the effect of interference fit on fatigue life of holed plate of mechanical joints was investigated both experimentally and numerically. In the experimental part, fatigue tests were carried out on the holed specimens of Al 7075-T6 alloy in which oversized steel pins were force fitted to them. These fatigue tests were conducted on open hole specimen and specimens with 1, 1.5, 2 and 4% nominal interference fit sizes at different cyclic longitudinal loads . From these tests the stress-life ( S – N ) data for different interference fit sizes were obtained. The results show that interference fit increases fatigue life compared to open hole specimens. In the numerical part of the investigation, 3D finite element simulations have been performed to obtain stress (or strain) histories and distributions around the hole due to interference fit and subsequent cyclic longitudinal loading using FEM package. The stress history from finite element (FE) simulation was used to explain the reason for fatigue life improvement in the interference fitted specimens.

Axial crush simulation of braided carbon tubes using MAT58 in LS-DYNA

This paper examines a composite damage constitutive model, MAT58, in LS-DYNA and its application for use in braided composite tube axial crush simulations. The constitutive response of MAT58 was investigated using single element simulations. It was found that MAT58 reproduced the softening behavior of the braided composite under monotonic compressive loading, but failed in subsequent unloading and tensile loading cycles. A deficiency in the damage law in MAT58 was identified. Unloading and reloading a volume of material that had suffered some degree of damage was a part of the process with the progressing of crush zone during the axial crush of composite tubes. Consequently, this deficiency hinders the success of MAT58 in such applications. In tri-axial braided composite tube axial crush simulations, although the predicted initial peak forces were within 20% of the experimental values, the predictions for the specific energy absorption (SEA) values were consistently low, particularly for tubes without a plug as crush initiator. These discrepancies are attributable to the deficiency in the damage law in MAT58.

In Vitro Testing of Femoral Impaction Grafting With Porous Titanium Particles: A Pilot Study

The disadvantages of allografts to restore femoral bone defects during revision hip surgery have led to the search for alternative materials. We investigated the feasibility of using porous titanium particles and posed the following questions: (1) Is it possible to create a high-quality femoral graft of porous titanium particles in terms of graft thickness, cement thickness, and cement penetration? (2) Does this titanium particle graft layer provide initial stability when a femoral cemented stem is implanted in it? (3) What sizes of particles are released from the porous titanium particles during impaction and subsequent cyclic loading of the reconstruction? We simulated cemented revision reconstructions with titanium particles in seven composite femurs loaded for 300,000 cycles and measured stem subsidence. Particle release from the titanium particle grafts was analyzed during impaction and loading. Impacted titanium particles formed a highly interlocked graft layer. We observed limited cement penetration into the titanium particle graft. A total mean subsidence of 1.04 mm was observed after 300,000 cycles. Most particles released during impaction were in the phagocytable range (< 10 μm). There was no detectable particle release during loading. Based on the data, we believe titanium particles are a promising alternative for allografts. However, animal testing is warranted to investigate the biologic effect of small-particle release.

Proximal humerus fractures: a comparative biomechanical analysis of intra and extramedullary implants

The biomechanical stability of a newly developed humerus nail (Sirustrade mark) for the treatment of fractures of the proximal humerus was analyzed in comparison to established systems. In total, three randomized groups were formed (n = 4 pairs) from 12 matched pairs of human cadaver humeri. All intact bones were mechanically characterized by five subsequent load cycles under bending and torsional loading. The bending moment at the osteotomy was 7.5 N m the torsional moment was 8.3 N m over the hole specimen length. Loading was consistently initiated at the distal epiphysis and the deformation at the distal epiphysis was continuously recorded. Prior to implant reinforcement, a defect of 5 mm was created to simulate an unstable subcapital humerus fracture. For paired comparison, one humerus of each pair was stabilized with the Sirus proximal humerus nail while the counterpart was stabilized by a reference implant. In detail, the following groups were created: Sirus versus Proximal humerus nail (PHN) with spiral blade (group I); Sirus versus PHILOS plate (group II); Sirus versus 4.5 mm AO T-plate (group III). The Sirus nail demonstrated significantly higher stiffness values compared to the reference implants for both bending and torsional loading. The following distal epiphyseal displacements were recorded for a bending moment of 7.5 N m at the osteotomy: Sirus I: 8.8 mm, II: 8.4 mm, III: 7.7 mm (range 6.9-10.9), PHN 21.1 mm (range 15.7-25.2) (P = 0.005), PHILOS plate 27.5 mm (range 21.6-35.8) (P < 0.001), 4.5 AO T-plate 26.3 mm (range 24.3-33.9) (P = 0.01). The rotations corresponding to 8.3 N m torsional moment were: Sirus I: 9.1 degrees , II: 9.3 degrees , III: 10.6 degrees (range 7.5-12.2), PHN 13.5 degrees (range 10.3-15.6) (P = 0.158), PHILOS plate 15.6 degrees (range 13.7-20.8) (P = 0.007), 4.5 AO T-Platte 14.1 degrees (range 11.5-19.7) (P = 0.158). The intramedullary load carriers were biomechanically superior when compared to the plating systems in the fracture model presented here. Supplementary, the Sirus Nail showed higher stiffness values than the PHN. However, the latter are gaining in importance due to the possibility of minimal invasive implantation. Whether this will be associated with functional advantages requires further clinical investigation.

Static and kinetic friction of rapeseed

he present paper examines the static and kinetic coefficient of friction of rapeseed. The project utilized two methods of determination of coefficient of friction of rapeseed: according Eurocode 1 (kinetic) in direct shear test and (static) in model silo. Samples of rapeseed in a range of moisture content from 6 to 15% w.b. were used and the tests were performed for galvanized steel, stainless steel and concrete B 30. Coefficient of friction for both steel types approached stable value for all levels of moisture content w.b. in a range from 0.11 to 0.18, for concrete B 30 it was found in a range from 0.25 to 0.43. The coefficient of static friction found in model silo decreased with an increase in vertical pressure from 0.3 to 0.2 for first loading, while in subsequent loading cycles decreased from 0.2 to 0.1.

Energy-based equivalence between damage and fracture in concrete under fatigue

Damage in concrete members, occur in a distributed manner due to the formation and coalescence of micro-cracks, and this can easily be described through a local damage approach. During subsequent loading cycles, this distributed zone of micro-cracks get transformed into a major crack, introducing a discrete discontinuity in the member. At this stage, concepts of fracture mechanics could be used to describe the behavior of the structural member. In this work, an approach is developed to correlate fracture and damage mechanics through energy equivalence concepts and to predict the damage scenario in concrete under fatigue loading. The objective is to smoothly move from fracture mechanics theory to damage mechanics theory or vice versa in order to characterize damage. The analytical methods developed here have been exemplified with some already available data in the literature. The strength and stiffness reduction due to progressive cracking or increase in damage distribution, has been characterized using the available indices such as the strength reduction and stiffness reduction factors. It is seen through numerical examples, that the strength and stiffness drop indices using fracture and damage mechanics theory agree well with each other. Hence, it is concluded, that through the energy approach a discrete crack may be modeled as an equivalent damage zone, wherein both correspond to the same energy loss. Finally, it is shown that by knowing the critical damage zone dimension, the critical fracture property such as the fracture energy can be obtained.

Biosorption of heavy metals by activated sludge and their desorption characteristics

The biosorption of different metals (Cu 2+, Cd 2+, Zn 2+, Ni 2+ and Pb 2+) was investigated using activated sludge. The optimum pH was 4 for Cd, Cu and Pb sorption and 5 for Ni and Zn. Biomass metal uptake clearly competed with protons present in the aqueous medium, making pH an important variable in the process. Protons consumed by biomass in control tests versus protons exchange in biosorption tests confirmed a maximum exchange between metal cations and protons at pH 2. The study of the influence of biomass concentration revealed that the amount of protons released from biomass increased with biomass concentration. This would confirm the hypothesis of ion exchange between both types of ions. The application of the Langmuir and Freundlich models showed a better fitting of experimental data to the first model. The maximum sorption uptake of the studied metals by the activated sludge showed the following decreasing order: Pb>Cu>Cd⩾Zn>Ni. Desorption experiments showed that HCl was a good eluent for the five metals tested, particularly at low pH values (1 and 2). At pH 3 or 4 the desorption yield was significantly lower. However, its use did not allow the reuse of biomass in subsequent loading and unloading cycles. EDTA was also a good desorption agent, achieving the total recovery for the five metals tested at a concentration of 1 mM, with the advantage that biomass could be reused for three sorption–desorption cycles.

A study of crack growth retardation due to artificially induced crack surface contact

The contact of the cracked surfaces during a part of a loading cycle generally results in a reduced crack growth rate. A critical experiment was designed to evaluate the influence of the crack surface contact on crack growth. A round compact specimen made of 1070 steel with a round hole at the wake of the fatigue crack was designed. Two mating wedges were inserted into the hole of the specimen while the external load was kept at its maximum in a loading cycle. In this way, the wedges and the hole in the specimen were in firm contact during the entire loading cycle in the subsequent loading. Experiments showed that the addition of the wedges resulted in a reduction of crack growth rate in the subsequent constant amplitude loading. However, crack growth did not arrest. With the increase in the subsequent loading cycles, crack growth rate increased. The traditional crack closure concept cannot explain the experimental phenomenon because the effective stress intensity factor range was zero after the insertion of the wedges. The detailed stress–strain responses of the material near the crack tip were analyzed by using the finite element method with the implementation of a robust cyclic plasticity theory. A multiaxial fatigue criterion was used to determine the fatigue damage based upon the detailed stresses and strains. The crack growth was simulated and the predicted results were in good agreement with the experimental observations. It was confirmed that the stresses and strains near the crack tip governed cracking behavior. Crack surface contact reduced the crack tip cyclic plasticity and the result was the observed retardation in crack growth.

Experimental study of the deformation behaviour of compacted metal powders used in a new rapid tooling process

To examine the feasibility of a newly proposed rapid tooling process based on the backing of a metal shell with metal powders, the compressive deformation behaviour of selected metal powders and metal shells was investigated. It was found that the mechanical properties of the powder material and the mixing ratio of powders of different sizes had a significant effect on the compressive deformation behaviour of the compacted powders. Softer metal powders with the same compressive stress tended to produce greater plastic deformation and formed a block under a stress of 138 MPa. Mixing two or more powders of different sizes could significantly increase the deformation resistance of the compacted powders. Minimum plastic deformation was achieved when the fraction of the coarse powder in a binary powder mixture reached 0.77. Initial loading at a higher stress could minimize or eliminate the plastic deformation in subsequent loading cycles. This feature could be used advantageously in powder packing for the proposed new rapid tooling process. Some elastic properties of the compacted metal powders were also determined from the compression experimental data.

Computer simulation on fatigue behavior of cemented hip prostheses: a physiological model.

This paper is concerned with the investigation on the fatigue failure of implant fixation by numerical approaches. A computer algorithm based on finite element analysis and continuum damage mechanics was proposed to quantify the fatigue damage rate of cement mantle under physiological conditions. In examining the interfacial debonding effect, the interface elements were introduced at cement-stem interfaces and calibrated with the increase of loading cycles. Current results reveal that the major sites for failure initiation are in the proximal anterior-medial regions and at the distal prosthesis tip, which clearly demonstrate the same failure scenario as observed in clinical studies. Such fatigue failures not only result in the corruption of cement-stem interfaces, but also greatly affect the cement stress distribution and the damage rate in subsequent loading cycles. Another significant result is that the predicted damage rate increases steadily with gait cycles. This trend in damage development is consistent with the findings obtained from fatigue tests available in literature. It is anticipated that presented methodology can serve as a pre-clinical validation of cemented hip prostheses.

A comprehensive ESPI based system for combined measurement of shape and deformation of electronic components

This paper describes a very simple and reliable procedure for measuring shape and deformation of electronic components with a single experimental set-up. The procedure is based on two electronic speckle pattern interferometry (ESPI) techniques referred to as conventional ESPI and phase shifting ESPI (PS-ESPI). The present research is motivated by the fact that mismatch in thermal expansion coefficients of the different materials included in electronic packaging (EP) may cause mechanical failures since thermal stresses will change sharply through subsequent loading cycles. ESPI is particularly suitable for measurements on EP since it allows us to perform non–contact testing of non-planar heterogeneous surfaces. It is apparent that gathering detailed topographic information will certainly help us to measure accurately surface deformations of EP along with modeling correctly numerical analysis. As is known, the accuracy of results obtained with ESPI may be significantly improved by phase shifting techniques (PST). Therefore, this paper compares the relative merits of different phase-stepping strategies in order to find which strategy will perform the best for the optical set-up utilized in the experiments. Preliminary investigations on a standard specimen under three-point-bending served to choose properly the optical set-up and phase-stepping procedure which yield the best fringe visibility. Four-phases achieved the best fringe visibility and the minimum number of invalid pixels. These information have been utilized in the experimental campaign on standard and surface mounted technology (SMT) electronic components. ESPI and PS-ESPI have been used for analyzing the transient state and the steady state of devices, respectively. From the experimental results obtained here, it appears possible to measure strains induced by thermal loading cycles. The experimental set-up, based on the Lendeertz's interferometer, proved itself also able to contour specimen surface at a good level of detail. Remarkably, by using the same set-up for deformation and shape measurements we can preserve the pixel by pixel relationship between displacements and surface depth which will hold true if correlation between different exposures is not destroyed. The results obtained in this research justify using PS-ESPI in order to understand better failure mechanisms of electronic components. This fact along with the exact knowledge of object shape may be particularly useful in the different design stages (including FEM modeling and analysis) of electronics for special applications.

A Study of Inhomogeneous Plastic Deformation of 1045 Steel

An experimental study was conducted on the inhomogeneous plastic deformation of 1045 steel subjected to monotonic tension and cyclic tension-compression. Small strain gages were used to characterize the local deformation and transmission electron microscopy (TEM) was used to study the microstructures associated with the observed deformation phenomenon. Under monotonic tension, the strain of the Lu¨ders front was lower than the full Lu¨ders strain. The local strains were inhomogeneous even at the work-hardening stage. Under stress-controlled cyclic loading with stress amplitudes below the lower yield stress, cyclic deformation consisted of an incubation stage, a significantly inhomogeneous cyclic softening stage, and a saturation stage. Inhomogeneous cyclic plasticity persisted at the saturation stage. A monotonic tension preload can eliminate the incubation stage in the subsequent cyclic loading. Under asymmetric cyclic loading with a mean stress, the cyclic plastic deformation was accompanied by noticeable inhomogeneous ratcheting deformation. TEM was used to analyze the distribution and evolution of the dislocation structures at different locations of the specimen. With increasing loading cycles, the dislocation substructures evolved in a sequence of loose tangles, thick veins, long walls, elongated cells, and equiaxial cells. Based upon the mechanical experiments and observations of dislocation arrangements, the mechanisms governing inhomogeneous cyclic plastic deformation were discussed.

Shakedown limit of rail surfaces including material hardening and thermal stresses

ABSTRACTSliding friction between railway wheels and rails results in elevated contact temperatures and gives rise to severe thermal stresses at the wheel and rail surfaces. The thermal stresses have to be superimposed on the mechanical contact stresses. Due to the distribution of stresses, the rail surface is generally subjected to higher stresses than the wheel surface. The elastic limit is reduced and yield begins at lower mechanical loads. During the first cycles of plastic deformation, the material hardens and residual stresses build up. The residual stresses provide the structure to shake down to pure elastic behaviour in subsequent load cycles up to a shakedown limit. The kind of hardening observed for rail steel has a considerable influence on the shakedown limit. The shakedown limit is dropped to lower mechanical loads due to the thermal stresses in the rail surface as well. This might cause structural changes in the rail material and rail damage.

Effects of interfacial debonding on fatigue damage of cemented hip prostheses

Clinical studies on retrieved cement mantles have pointed out that the cemented hip prostheses failed after long‐term use due to debonding at the cement‐stem interface and local fractures in the cement mantles. These were linked to fatigue damages of cement mantles proved by fatigue experiments. In this paper, a numerical approach based on finite element analysis and continuum damage mechanics is proposed to investigate the fatigue behavior of cement mantles during gait cycles. Results reveal that the major sites for failure initiation are at the proximal medial regions and at the distal prostheses tip. Such fatigue failures not only result in the corruption of cementstem interfaces, but also greatly affect the stress distribution and damage rate of the proximal cement mantles in subsequent loading cycles. The interfacial debonding rate increases from 2.5% to 15% with gait loadings from five to twenty million cycles. Meanwhile, owing to the partial debonding of interface, the cement stresses on the remaining regions increase by 91% to 871% when compared with those generated with a fully bonded interface, which in turn accelerates the fatigue damage accumulation rate of the cement mantle from 5.99 % to 21.5%.

Thermal stresses and shakedown in wheel/rail contact

Sliding friction between railway wheels and rails results in considerable contact temperatures and gives rise to severe thermal stresses at the surfaces of the wheels and rails. An approximate analytical solution is presented for a line contact model. The increased bulk temperature of the wheel after a long period of constant operating conditions is also taken into account. The thermal stresses have to be superimposed on the mechanical contact stresses. They reduce the elastic limit of the wheel and rail, and yielding begins at lower mechanical loads. When residual stresses build up during the initial cycles of plastic deformation, the structure can carry higher loads with a purely elastic response in subsequent load cycles. This phenomenon is referred to as shakedown. Due to the distribution of temperature, the rail surface is generally subjected to higher stresses than the wheel surface. This can cause structural changes in the rail material and hence rail damage.

Defect dynamics of cement mortar under repeated loading, studied by electrical resistivity measurement

Defect dynamics, as studied by electrical resistivity measurement during repeated compression of cement mortar in the elastic regime, are characterized by defect generation, defect healing and defect aggravation. Defect generation dominates in the first compressive loading and in tensile loading in any loading cycle. Defect healing dominates in all subsequent compressive loading cycles and in tensile unloading in any cycle. Defect aggravation dominates during compressive unloading in any cycle and occurs during tensile loading in any cycle. Both the interface between sand and cement and that between silica fume and cement contribute to the defect dynamics, particularly the defect healing. The defect dynamics give similar effects to the longitudinal and transverse resistivities. Upon uniaxial compression, the resistivity decreases (except for the first cycle); upon uniaxial tension, the resistivity increases.

Stress dependence of reversible and irreversible domain switching in PZT during cyclic loading

Stresses suffered by lead zirconate titanate (PZT) components in actuators are the origin of the gradual degradation of the microstructure and piezoelectric capability that limits their lifetime. The stress–strain behavior of a PZT ceramic has been studied in compressive uniaxial cyclic loading using a constant loading rate, in order to determine the operating stresses that cause structural damage associated to the exhaustion of twinning. Domain switching strain curves have been calculated considering that stress-induced switching of 90° domains is the mechanism responsible of non-linear stress–strain behavior. Each load–unload cycle caused a permanent strain in the PZT. Successive cycles produced incremental increases in stress-induced permanent strain, up to a maximum value or ‘saturated cyclic permanent strain’, attributed to irreversible stress-induced domain switching. The dependence of the saturated permanent strain with the maximum cyclic load showed a characteristic non-linear behavior, with a steep slope at a stress level σ I, that we called the ‘critical stress for irreversible domain switching’. Below σ I, the majority of domain switching is reversible. We have called σ R to the stress at which the increase in reversible domain switching is more pronounced. At stresses between σ R and σ I high reversible strains can be reached without resulting in permanent stress-induced depoling and thus without the exhaustion of available twinning during subsequent load cycles. At maximum cyclic stresses higher than σ I, irreversible domain switching accounts for the majority of strain and increase rapidly toward values close to the maximum 90° domain switching strain available. The number of cycles to failure also had a strong dependence with the maximum cyclic stress. Cyclic stresses above the critical stress caused a rapid accumulation of permanent strains, so the saturated value is reached after a few cycles, resulting in early catastrophic failure, because of the exhaustion of reversible domain switching.

Influence of asperity degradation on the mechanical behavior of rough rock joints under cyclic shear loading

A cyclic shear testing system was established to investigate the mechanical behavior of rough rock joints under cyclic loading conditions . Laboratory cyclic shear tests were conducted for two joint types of Hwangdeung granite and Yeosan marble: saw-cut and split tensile joints. Prior to test, the roughness of each specimen was characterized by measuring the surface topography using a laser profilometer . Several important aspects of cyclic joint behavior, such as high peak shear strength and non-linear dilation in the first loading cycle, different frictional resistance for the reversed shear loading direction, and anisotropic shear behavior and its dependence on the normal stress level were identified from the cyclic shear test results. These features and their variations in the subsequent loading cycles are mainly due to the effect of second order asperities and strength of rock material. It was also observed from experimental results that degradation of asperities under cyclic shear loading also followed the exponential degradation laws for asperity angle and the mechanism for asperity degradation would be different depending upon the shearing direction and the type of asperities. Based on the experimental results an elasto-plastic constitutive model, which can consider the degradation of second order asperities, was proposed. Numerical simulations for the monotonic and cyclic shear loading indicated agreement with the laboratory test results.

A fully packaged single crystalline resonant forcesensor

In the oil logging, automotive and spacecraft industriesthere exists a strong interest in sensing devices which canmeasure a static load with very high resolution at normal andelevated temperatures up to 500 °C. We developed,realized and tested a first- and second-order packaged forcesensor for such requirements. The system consists of a resonantstructure vibrating in its bending mode, which is hermeticallyvacuum packaged in a single-crystal silicon housing together withthe resonance detection sensor. The capacitive detectionelectronics are off-chip, giving a high degree of freedom forthe design and fabrication process. The oscillator operates witha switched lock-in amplifier. The packaged silicon load cellachieved a load sensitivity of 27 Hz N-1, a resolution of2 mN and good repeatability for subsequent load cycles. Theelectronics for the first amplifier stage are mounted togetherwith the silicon load cell in the second level package. Thetemperature limitation caused by the electronics was eliminatedby replacing it with a Fabry-Perot interferometer in thesilicon chip. The optical detection was successfully tested andshowed the resolution of the resonator deflection to be more than tentimes better than the capacitive detector.

補強土橋脚模型の鉛直圧縮特性に及ぼす繰返し・荷重保持載荷履歴の効果

To evaluate the effects of preloading and prestressing on deformation characteristics by cyclic loading for a long term performance of reinforced soil structures, a series of cyclic and creep tests were performed on small models. No distinct difference was found in the increasing rate with time of residual strain between a cyclic loading test and a constant pressure creep test at a preloading state. The amount of residual strain by cyclic loading at an unloaded state (i. e., prestressed state) decreased by applying in advance long-term constant pressure or cyclic loading at the preloaded condition. Applying cyclic loading at the preloading state was slightly more effective. When swelling was allowed to take place at a prestress state. the deformation during the subsequent cyclic loading test became larger.