Application Of Cyclic Loading Research Articles

Use of acoustic techniques to analyse interlayer shear-torque fatigue test in asphalt mixtures

Shear-torque tests were used for the first time to assess the fatigue behaviour of double-layered asphalt specimens. During the test, an acoustic emission (AE) technique was used to evaluate the damage process leading to failure. Results showed that the application of cyclic loading led to the decrease of the complex shear modulus, until the physical separation of the two layers. AE analysis showed that damage mostly occurred in the interlayer zone. The comparison between mechanical and AE results suggested that the damage evolution phase occurred when approaching the end of the test when the stiffness decrease was about 70%.

Response of pavement foundations incorporating both geocells and expanded polystyrene (EPS) geofoam

The suitability of geocell reinforcement in reducing rut depth, surface settlements and/or pavement cracks during service life of the pavements supported on expanded polystyrene (EPS) geofoam blocks is studied using a series of large-scale cyclic plate load tests plus a number of simplified numerical simulations. It was found that the improvement due to provision of geocell constantly increases as the load cycles increase. The rut depths at the pavement surface significantly decrease due to the increased lateral resistance provided by the geocell in the overlying soil layer, and this compensates the lower competency of the underlying EPS geofoam blocks. The efficiency of geocell reinforcement depends on the amplitude of applied pressure: increasing the amplitude of cyclic pressure increasingly exploits the benefits of the geocell reinforcement. During cyclic loading application, geocells can reduce settlement of the pavement surface by up to 41% compared to an unreinforced case – with even greater reduction as the load cycles increase. Employment of geocell reinforcement substantially decreases the rate of increase in the surface settlement during load repetitions. When very low density EPS geofoam (EPS 10) is used, even though accompanied with overlying reinforced soil of 600 mm thickness, the pavement is incapable of tolerating large cyclic pressures (e.g. 550 kPa). In comparison with the unreinforced case, the resilient modulus is increased by geocell reinforcement by 25%, 34% and 53% for overlying soil thicknesses of 600, 500 and 400 mm, respectively. The improvement due to geocell reinforcement was most pronounced when thinner soil layer was used. The verified three-dimensional numerical modelings assisted in further insight regarding the mechanisms involved. The improvement factors obtained in this study allow a designer to choose appropriate values for a geocell reinforced pavement foundation on EPS geofoam.

Analysis of shear fatigue properties of iron single crystals regarding possible effects of the resolved normal stresses on the slip planes

Crystal orientation effects on the shear fatigue limits of iron single-crystals were investigated by considering the influence of normal stress of the slip plane on the critical resolved shear stress (CRSS). The shear fatigue limit with the (123) plane in the shear loading position was experimentally confirmed to exceed that with shear loading on the (110) plane. On the contrary, the shear yield stress of the (123) plane, evaluated by monotonic shear tests, was lower than that of the (110) plane. To solve this seeming contradiction, it was postulated that body-centred cubic crystals possessed 24 slip systems and that the CRSS was sensitive to the resolved normal stress (RNS) on the slip plane. That is, it was assumed that the CRSS decreased under the tensile RNS according to its magnitude, but increased under the compressive RNS. The mathematical model of the CRSS dependence on the RNS was simply introduced to the work-hardening law proposed by Peirce et al. allowing formulation of the algebraic equations yielding the crystalline slip rates. This crystal plasticity model was applied to analyse monotonic shear and shear fatigue tests. By considering the effect of the RNS sensitivity, the shear yield stress with the (123) plane in the shear loading position was calculated to be lower than that with the (110) plane, showing agreement with the experimental results of the monotonic shear tests. Furthermore, in the application of cyclic shear loads corresponding to the fatigue limits, it was confirmed that the plastic strain range of the primary slip plane converged to similar values despite the differences in the crystal orientations of the shear plane and the normal stresses to it. These results imply that the present method allows the prediction of fatigue limits under different crystal orientations and loading conditions.

On factors affecting CFRP-steel bonded joints

Failure of structural steel members strengthened with Carbon Fibre Reinforced Polymers (CFRP) may occur at the joints CFRP-steel and this study examines variables that alter or explain the corresponding reduction of load capacity for a specific CFRP laminate, adhesive and steel. Factors and parameters likely to be influential like surface treatment prior to bonding, the bonded length, the glass transition temperature (Tg) of the adhesive, the exposure to aggressive environment, the temperature at service and different types of loading were examined. The experimental program selected double strap CFRP-steel bonded joints under shear for the analysis. The steel surfaces to be bonded were subjected to sand blasting (6.3 bar) or abrasive grinding (6.9 bar) corresponding to thorough blast cleaning Sa2; surfaces rusted after exposure to salt fog at 35 °C were also considered. Differences detected in responses of specimens treated by sand or steel spheres blasting were relatively minor. Tests made at increasing ambient temperatures confirmed that service temperature near and above adhesive Tg caused rapid deterioration of ultimate capacity and change of failure modes. Salt fog cycles (SF) originated the most significant losses of joint capacity. Application of cyclic static loading above the critical loading threshold obtained for unaged joints did not reduce the capacity of joints previously aged by freeze-thaw. The same cyclic loading after salt fog cycles, reduced bond capacity and increase the ultimate slip, suggesting larger effective length. Despite the losses of capacity, microscopic changes of structural nature could not be identified.

A numerical investigation on multiaxial fatigue assessment of Nitinol peripheral endovascular devices with emphasis on load non-proportionality effects

This work first numerically investigates on the effect of the application of cyclic remote loads on the local stress/strain response of a peripheral stent. For this purpose, three stents of identical geometry, but different material properties (linear elastic, elastic-plastic and Nitinol material), were modelled and subjected to uniaxial and multiaxial proportional cyclic loads. The local response under uniaxial loads is non-proportional only for the case of Nitinol stent, while it happens for all the materials when multiaxial loads are applied. This paper also aims to evaluate how the use of different multiaxial fatigue criteria may affect the fatigue assessment for Nitinol peripheral stents. The same stent model was subjected to different proportional and non-proportional multiaxial cyclic loads and the results were interpreted using the standard von Mises and three approaches based on the concept of critical plane. The results showed that the von Mises is the most conservative criterion.

Mechanical Modeling and First Case Study on ITER TF CICC Loading Cases With Upgraded Finite Element Code Simulations

The strands of the Cable-In-Conduit Conductors (CICCs) for the ITER magnet system TF coils are made of Nb3Sn, which is a brittle and strain-sensitive superconducting material. The strain distribution in the filaments is driven by the cool-down process and the electromagnetic forces acting on the strands in operation. The impact of electromagnetic and thermal warm-up-cool-down cycles on the conductor performance degradation has been investigated experimentally both on short samples in the SULTAN facility and in the solenoidal configuration of the TF Insert tests in the CSMC facility. A deeper understanding on how cyclic loading of the conductor affects its electrical performance could allow one to identify the best working conditions so as to limit the degradation. In this paper, a mechanical model of one last sub-cable (petal) of the TF CICC is presented. The strain distribution in the strands is simulated via a new version of the MULTIFIL finite element code. The effects of the alternate application of thermal and electromagnetics cyclic loading are tentatively analyzed, similarly to experiments performed in the SULTAN facility in the tests of the TFIO1 conductor sample. An upgrade of the model is implemented, concerning the thermal cycle simulation.

Biomechanical evaluation of traditional posterior versus anterior spondylolisthesis reduction in a cadaveric grade I slip model.

Posterior reduction with pedicle screws is often used for stabilization of unstable spondylolisthesis to directly reduce misalignment or protect against micromotion while fusion of the affected level occurs. Optimal treatment of spondylolisthesis combines consistent reduction with a reduced risk of construct failure. The authors compared the reduction achieved with a novel anterior integrated spacer with a built-in reduction mechanism (ISR) to the reduction achieved with pedicle screws alone, or in combination with an anterior lumbar interbody fusion (ALIF) spacer, in a cadaveric grade I spondylolisthesis model. Grade I slip was modeled in 6 cadaveric L5-S1 segments by creation of a partial nucleotomy and facetectomy and application of dynamic cyclic loading. Following the creation of spondylolisthesis, reduction was performed under increasing axial loads, simulating muscle trunk forces between 50 and 157.5 lbs, in the following order: bilateral pedicle screws (BPS), BPS with an anterior spacer (BPS+S), and ISR. Percent reduction and reduction failure load-the axial load at which successful reduction (≥ 50% correction) was not achieved-were recorded along with the failure mechanism. Corrections were evaluated using lateral fluoroscopic images. The average loads at which BPS and BPS+S failed were 92.5 ± 6.1 and 94.2 ± 13.9 lbs, respectively. The ISR construct failed at a statistically higher load of 140.0 ± 27.1 lbs. Reduction at the largest axial load (157.5 lbs) by the ISR device was tested in 67% (4 of 6) of the specimens, was successful in 33% (2 of 6), and achieved 68.3 ± 37.4% of the available reduction. For the BPS and BPS+S constructs, the largest axial load was 105.0 lbs, with average reductions of 21.3 ± 0.0% (1 of 6) and 32.4 ± 5.7% (3 of 6) respectively. While both posterior and anterior reduction devices maintained reduction under gravimetric loading, the reduction capacity of the novel anterior ISR device was more effective at greater loads than traditional pedicle screw techniques. Full correction was achieved with pedicle screws, with or without ALIF, but under significantly lower axial loads. The anterior ISR may prove useful when higher reduction forces are required; however, additional clinical studies will be needed to evaluate the effectiveness of anterior devices with built-in reduction mechanisms.

Investigation of factors affecting the formability of metallic sheets in dieless incremental hole-flanging

Incremental sheet forming (ISF) is recognized to have the capability to enhance the formability of metallic sheets in comparison to press working (PW), but there is insufficient knowledge regarding the physical mechanisms underlying this phenomenon. This paper examines the forming limits of Al 6061 and AISI 304 sheets in dieless incremental hole-flanging (IHF), which is an important supplement to conventional ISF. The effect of discontinuous deformation and strain-induced martensitic transformation in 304 steel on deformation is especially considered. It was found that the limiting forming ratios (LFRs) of all the blanks in IHF were improved compared to those in PW, and the LFRs of the 6061 blanks were improved more. In the uniaxial tensile tests under repeated loading–unloading, the elongation of the 6061 specimens slightly improved, while that of 304 steel obviously decreased with the application of cyclic loading. The results indicate that cyclic straining can contribute to the ductility of 6061 blanks in IHF. For the 304 blanks, the strain-induced martensitic transformation was found in all the processes, and the martensite content increases with the deformation amount and the frequency of cyclic loading. The TRIP (transformation-induced plasticity) effect of 304 sheets does not dominate in the tensile tests and IHF. Moreover, a special failure mode of the 304 blanks, the petaloid neck, which is prone to appear before fracturing in IHF, was addressed.

Fatigue Stress-Life Model of RC Beams Based on an Accelerated Fatigue Method

Several standard fatigue testing methods are used to determine the fatigue stress-life prediction model (S-N curve) and the endurance limit of Reinforced Concrete (RC) beams, including the application of constant cyclic tension-tension loads at different stress or strain ranges. The standard fatigue testing methods are time-consuming and expensive to perform, as a large number of specimens is needed to obtain valid results. The purpose of this paper is to examine a fatigue stress-life predication model of RC beams that are developed with an accelerated fatigue approach. This approach is based on the hypothesis of linear accumulative damage of the Palmgren–Miner rule, whereby the applied cyclic load range is linearly increased with respect to the number of cycles until the specimen fails. A three-dimensional RC beam was modeled and validated using ANSYS software. Numerical simulations were performed for the RC beam under linearly increased cyclic loading with different initial loading conditions. A fatigue stress-life model was developed that was based on the analyzed data of three specimens. The accelerated fatigue approach has a higher rate of damage accumulations than the standard testing approach. All of the analyzed specimens failed due to an unstable cracking of concrete. The developed fatigue stress-life model fits the upper 95% prediction band of RC beams that were tested under constant amplitude cyclic loading.

The influence of a cyclic loading history on soil-geogrid interaction under pullout condition

The knowledge of soil-geosynthetic interface behaviour is a key point in the design of geosynthetic-reinforced soil structures. The pullout ultimate limit state can be reproduced conveniently by means of pullout tests performed with large-size laboratory apparatuses, which allow studying the interaction mechanisms that develop in the anchorage zone. During the service life of geosynthetic-reinforced soil structures, reinforcements may be subjected to long-term cyclic vehicular loads or short-term seismic loads in addition to dead loadings, such as the structure's self-weight and other sustained loads. In order to study the influence of a cyclic loading history (a sinusoidal function with fixed amplitude A, number of cycles N and frequency f) on the post-cyclic peak pullout resistance, the writers carried out a series of multi-stage pullout tests on a high density polyethylene extruded uniaxial geogrid embedded in a compacted granular soil for different vertical effective stress σ′v values. Moreover, the stability of the soil-geosynthetic interface from a point of view linked to the cyclic loading application has also been investigated. Test results showed that the design pullout resistance parameters are affected by the applied cyclic loading history for specific combined conditions (A, N and σ′v) and it should be taken into account for designing geosynthetic reinforced soil structures.

Degradation of piezoelectric device as an energy harvester under equivalent traffic stress condition

Piezoelectric materials have widely been used for applications in microelectromechanical systems, electromechanical devices, civil infrastructure, in transportation and smart devices. The performance of these devices is largely affected by their loading and unloading history when used in electromechanical applications. Lead Zirconium Titanate (PZT), a specific material of piezoelectric ceramics, which upon repetitive cyclic compressive load undergoes both electrical and mechanical degradation over a period of time and number of cycles. The degradation of piezoelectric properties with the application of cyclic compressive loading and the mechanical fatigue behavior were investigated on PZT-5H.

Evolution of internal damage during actuation fatigue in shape memory alloys

Shape Memory Alloys (SMAs) are subject to multiple types of fatigue including structural fatigue due to cyclic load application as well as functional fatigue due to cyclic phase transformation. Due to the thermomechanical nature of the phase transformation, the area of functional fatigue can be further subdivided into pseudoelastic fatigue, which is fatigue due to stress induced phase transformation, or actuation fatigue which is fatigue due to repeated thermally induced phase transformation. In the area of actuation fatigue, many works have attempted to predict the actuation fatigue lifetime, however the evolution of the microstructure during the actuation fatigue lifetime has yet to be examined. This work utilizes X-ray computed microtomography to examine the evolution of internal damage during actuation fatigue and has found that internal damage evolves in a non-linear manner unique to SMAs. Specifically, internal damage is shown to nucleate quickly during initial actuation cycling, followed by a steady state growth and leading to rapid coalescence at the end of the actuation fatigue lifetime. This experimentally observed non-linear internal damage growth has been introduced into a SMA constitutive model and the model is shown to give good lifetime predictions both for uniaxial as well as multiaxial loading conditions.

Undrained Response and Liquefaction Resistance of Sand–Silt Mixtures

A series of undrained cyclic triaxial tests were performed on sand–silt mixtures, prepared after addition of non-plastic fines (in the range of 0–40%) into the fine sand. The cylindrical samples were prepared at medium dense condition (Dr = 50%) and saturated by purging CO2 and application of back and cell pressures alternatively until the pore pressure coefficient becomes about 0.98 to 1.0. The samples were consolidated at 100 kPa pressure before cyclic load application. Each consolidated sample was then subjected to cyclic loading with the sinusoidal waveform at a frequency of 1 Hz. The cyclic loads were varied with CSRs in the range of 0.127–0.178 to obtain the resistance curves and assess the liquefaction resistance of sand–silt mixtures. This paper presents the pore pressure build-up, axial strain variation with load cycles and resistance curves to study the effect of cyclic load amplitude levels, over-consolidation, cycles of preloading and non-plastic fines on liquefaction resistance of fine sands. Test results indicate that, at constant relative density, the liquefaction resistance of fine sand decreases with the addition of non-plastic fines up to 40%. It is also noted that the liquefaction resistance of sand is increased substantially with an increase in cycles of preloading and over-consolidation pressures.

Experimental Investigation on the Fracture Behaviour of Natural Stones Exposed to Monotonic and Cyclic Loading

The present paper is devoted to an experimental study on the fracture behaviour of natural stones, commonly used as elements for building cladding, under both monotonic and cyclic loading, with particular emphasis to white Carrara marble. The effect of progressive damage produced by in-service thermal fluctuations can be investigated through the application of appropriate cyclic mechanical loads. In the experimental tests conducted, some static mechanical properties of marble are characterized by means of three-point bending tests on edge-cracked prismatic specimens for the determination of Young's modulus, tensile strength and fracture energy. Moreover, cyclic three-point bending tests are conducted to determine the propagation rate of nominally Mode-I fatigue cracks. Finally, the fatigue behaviour of the marble is studied through a cohesive crack model, in which the direct tensile strength of the material is determined by a Brazilian test, and the behaviour is calibrated by means of a suitable FE model. The effect of crack path on the fracture resistance of marble is discussed.

Pre- and post-cyclic behaviour of an unsaturated clayey soil contaminated with crude oil

Besides adverse impacts on the environment, pollution with hydrocarbon compounds such as crude oil affects the geotechnical parameters like deformation modulus and shear strength. The present research involves evaluation of clayey soil properties changes in an unsaturated state due to application of cyclic loads before and after contamination with crude oil. A series of constant water content (CW) static and dynamic triaxial tests were conducted on samples that were compacted to the same dry density but different degrees of saturation and net confining pressures. The soil studied was provided from a place near an oil refinery located in Iran’s capital city and was artificially polluted with 3%, 6% and 9% crude oil. Deformation modulus reduction due to cyclic loads application was considerably more than reduction of shear strength. Also, these parameters increased due to increasing net confining pressure and initial matric suction. Several chemical tests and techniques were employed to investigate the chemical variations in soil structure and the role of different chemical components of crude oil on quantitative variations of soil characteristics.

The effect of resistance grooves on the fracture toughness of zirconia-based crowns from mono and cyclic loading.

Objective:Prosthetic molar crowns in service are subjected to chewing loads, which cause a shift or dislodgment. The objective of this study is to investigate whether the addition of resistance grooves to the proximal surfaces of the abutment teeth would enhance the fracture resistance of the zirconia crowns and to compare between the patterns of cracks development on the zirconia crowns after the application of mono loading versus cyclic loading forces.Materials and Methods:Thirty-six all-ceramic zirconia cored crowns were prepared on the same abutment. Resistance grooves were added to the mesial and distal surfaces of 16 abutments. Before testing, all specimens subjected to thermal aging. Two groups of crowns were then subjected to cyclic axial and lateral forces for 1,250,000 cycles in aqueous conditions. Two groups of samples were also tested in monoloading fashion.Results:The crack pattern between mono and cyclic loading were compared. The crown fracture resistance was compared in the two types of abutments, with and without grooves. The results confirmed that the grooves addition had no effect on critical conditions to initiate failure in the case of mono loading. In cyclic loading, grooves addition increased the critical loads in the order of two. Failure patterns and location were obtained.Conclusions:The results showed that the location of retention grooves halted the failure in the surfaces where it was located in all loading mechanisms used in this study.

Micro and nanoscale characterization of poly(DL-lactic-co-glycolic acid) films subjected to the L929 cells and the cyclic mechanical load

In this paper, the effect of the presence of L929 fibroblast cells and a cyclic load application on the kinetics of the degradation of amorphous PLGA films was examined. Complex micro and nano morphological, mechanical and physico-chemical studies were performed to assess the degradation of the tested material. For this purpose, molecular weight, glass transition temperature, specimen morphology (SEM, μCT) and topography (AFM) as well as the stiffness of the material were measured. The study showed that the presence of living cells along with a mechanical load accelerates the PLGA degradation in comparison to the degradation occurring in acellular media: PBS and DMEM. The drop in molecular weight observed was accompanied by a distinct increase in the tensile modulus and surface roughness, especially in the case of the film degradation in the presence of cells. The suspected cause of the rise in stiffness during the degradation of PLGA films is a reduction in the molecular mobility of the distinctive superficial layer resulting from severe structural changes caused by the surface degradation. In conclusion, all the micro and nanoscale properties of amorphous PLGA considered in the study are sensitive to the presence of L929 cells, as well as to a cyclic load applied during the degradation process.

The Influence of the Timing of Cyclic Load Application on Cardiac Cell Contraction.

Cardiac cells are subjected to mechanical load during each heart-beat. Normal heart load is essential for physiological development and cardiac function. At the same time, excessive load can induce pathologies such as cardiac hypertrophy. While the forces working on the heart as an organ are well-understood, information regarding stretch response at the cellular level is limited. Since cardiac stretch-response depends on the amplitude and pattern of the applied load as well as its timing during the beating cycle, the directionality of load application and its phase relative to action potential generation must be controlled precisely. Here, we design a new experimental setup, which enables high-resolution fluorescence imaging of cultured cardiac cells under cyclic uniaxial mechanical load and electrical stimulation. Cyclic stretch was applied in different phases relative to the electrical stimulus and the effect on cardiac cell beating was monitored. The results show a clear phase-dependent response and provide insight into cardiac response to excessive loading conditions.

Piezoelectric Tooth Aligner for Accelerated Orthodontic Tooth Movement.

In this paper, we propose a device that can induce vibration to potentially accelerate the rate of orthodontic tooth movement and thus reduce the duration of the orthodontic treatment. This is accomplished by harmonically exciting a polyvinylidene fluoride (PVDF) piezoelectric actuators to generate vibration and hence generate cyclic loading at 30 Hz. PVDF is a popular piezoelectric polymer because of its high flexibility, biocompatibility, and low cost. Generally, application of cyclic loading (vibration) reverses bone loss, stimulates bone mass, induces cranial growth, and accelerates tooth movement. This reduce the pain experience and discomfort associated with the orthodontic treatment and also enhances the patient compliance with the treatment. Vibration has the advantage of minimal side effects in comparison to medicinal treatments. The device is attached to either a positioner or teeth aligner. The main advantage of this work is that the device can be adjusted and repositioned in a different location of the teeth aligner. We propose one device that incorporates vibration, harmonic function generation, and the housing unit for the battery and processor. Current approaches require an external device, which is relatively cumbersome to the patient. Theoretical analysis based on the distributed parameter model is utilized to justify our claim. Piezoelectric actuation behavior is simulated using Finite Element Analysis.

Development of a novel testing device for fabric reinforced carbon fibre composites under cyclic biaxial load applications

In this paper, a novel testing device has been developed and adopted to a planar- biaxial servo hydraulic testing machine to examine the multiaxial fatigue behaviour of woven CFRP under superimposed interlaminar cyclic shear and static or cyclic through-thickness compression stresses.