Cyclic Mechanical Loading Research Articles

Twinning contributions to strain localizations in magnesium alloys

The effect that twin activity in Magnesium alloys has in both local and global strain is investigated in this article. The need to investigate this effect is related to difficulties in associating the spatially varying twinning with both plasticity and failure of this particular class of alloys. To address this need, mechanical testing both inside and outside the Scanning Electron Microscope was conducted and it was coupled with Digital Image Correlation (DIC) deformation and Electron Backscattered Diffraction (EBSD) texture measurements. In addition volume recordings of Acoustic Emission (AE) and surface measurements of microstructural changes were used to further complement the in situ tracking of twinning activity throughout monotonic and cyclic mechanical loading experiments. The definition of appropriate monitoring regions allowed the direct correlation of evolving twin activity with associated deformation in Magnesium alloys, for the first time to the authors’ best knowledge, from which twinning contributions to strain localizations were computed.

Effect of Different Computer-aided Design/Computer-aided Manufacturing (CAD/CAM) Materials and Thicknesses on the Fracture Resistance of Occlusal Veneers.

The aim was to evaluate, in vitro, the influence of different computer-aided design/computer-aided manufacturing (CAD/CAM) materials (IPS e.max CAD, Vita Enamic, and Lava Ultimate) and thicknesses (0.6 mm and 1.5 mm) on the fracture resistance of occlusal veneers. Sixty human third molars were prepared to simulate advanced erosion of the occlusal surface, and the teeth were randomly divided into six experimental groups (n=10) according to the material and thickness used to build the veneers. Ten sound teeth formed the control group. The veneers were adhesively luted and submitted to mechanical cyclic loading (1 million cycles at 200-N load). The fracture resistance test was performed in a universal testing machine. The failures were classified as "reparable" and "irreparable." According to two-way analysis of variance and the Tukey test, the interaction (material × thickness) was significant ( p=0.013). The highest fracture resistance was obtained for IPS e.max CAD at a 1.5-mm thickness (4995 N) and was significantly higher compared to the other experimental groups ( p<0.05). The lowest fracture resistance was obtained for Vita Enamic at 0.6 mm (2973 N), although this resistance was not significantly different from those for IPS e.max CAD at 0.6 mm (3067 N), Lava Ultimate at 0.6 mm (3384 N), Vita Enamic at 1.5 mm (3540 N), and Lava Ultimate at 1.5 mm (3584 N) ( p>0.05). The experimental groups did not differ significantly from the sound teeth (3991 N) ( p>0.05). The failures were predominantly repairable. The occlusal veneers of IPS e.max CAD, Vita Enamic, and Lava Ultimate, with thicknesses of 0.6 mm and 1.5 mm, obtained fracture resistances similar to those associated with sound teeth.

Elastic-plastic-creep response of multilayered systems under cyclic thermo-mechanical loadings

The modified Ohno and Abdel-Karim nonlinear kinematic hardening model considering cyclic hardening and creep effect are developed to investigate the deformation and residual stress of multilayered systems subjected to cyclic thermal and mechanical loads. Results reveal that, if the creep behavior is considered, significant stress relaxation takes place during about the first several cycles, and the accumulated strain of multilayered systems under deformation-controlled loads finally remains constant, which leads to shakedown. Remarkable changes of the residual stress, location of neutral axis and curvature of Si-Cu beams are observed during about the first 100 cycles. Therefore, the proposed model can be used for the engineering design of multilayered systems under complex loads.

Electrochemical characterization of automotive aluminum alloys regarding their corrosion fatigue behavior

Abstract„Downsizing“ is a well‐established practice in a wide range of industrial application. The aim is to reduce energy and raw material consumption through the enhancement of the efficiency of materials in service. In automotive and supply industry this is related to an increasing demand on performance and comfort whilst decreasing fuel consumption and pollutant emission. This is resulting in a higher and more complex material stress. Hence new testing methods have to be developed and implemented in order to meet the upcoming specifications. Aluminium alloys in chassis components are subjected to corrosion fatigue load and therefore have to be qualified for their use. The behavior of the aluminium alloys EN AW‐6082 T6 and EN AW‐7075 T73 are investigated in this study under simultaneous cyclic mechanical and corrosion loading. Load synchronized electrochemical as well as electrochemical noise measurements are performed to investigate the activation‐ and repassivation processes during corrosion fatigue testing. By monitoring the current these effects are assessed. The data monitored is conditioned by Fast‐Fourier‐Transformation in order to investigate the signal of the current density at different frequencies. Electrochemical noise measurements are performed without polarization and therefore a current transient describes local corrosion at very early stages.

Redesigning of the diesel engine turbocharger compressor wheel to improve the low-cycle fatigue life

ABSTRACTThe compressor wheel of a turbocharger is one of the highly stressed components in automotive engines. The compressor wheel undergoes variations in rotational speed, depending on vehicle operating conditions. These variations cause cyclic mechanical load in the wheel which results in fatigue stress. The continuous operation with fatigue stress results in fatigue fracture and failure. The fatigue is classified as low-cycle fatigue and high-ycle fatigue based on the number of load cycles to failure and type of fracture growth. The present work is focused on the analysis of the compressor wheel that includes measurement of load–time histories in terms of turbocharger speed for different vehicle duty cycles, finite element analysis simulation of stress distribution in the compressor wheel for given load conditions, prediction of fatigue life of the compressor wheel using cumulative damage accumulation theory and the various possibilities for improving the compressor wheel fatigue life.

Foil Strain Gauges Using Piezoresistive Carbon Nanotube Yarn: Fabrication and Calibration.

Carbon nanotube yarns are micron-scale fibers comprised by tens of thousands of carbon nanotubes in their cross section and exhibiting piezoresistive characteristics that can be tapped to sense strain. This paper presents the details of novel foil strain gauge sensor configurations comprising carbon nanotube yarn as the piezoresistive sensing element. The foil strain gauge sensors are designed using the results of parametric studies that maximize the sensitivity of the sensors to mechanical loading. The fabrication details of the strain gauge sensors that exhibit the highest sensitivity, based on the modeling results, are described including the materials and procedures used in the first prototypes. Details of the calibration of the foil strain gauge sensors are also provided and discussed in the context of their electromechanical characterization when bonded to metallic specimens. This characterization included studying their response under monotonic and cyclic mechanical loading. It was shown that these foil strain gauge sensors comprising carbon nanotube yarn are sensitive enough to capture strain and can replicate the loading and unloading cycles. It was also observed that the loading rate affects their piezoresistive response and that the gauge factors were all above one order of magnitude higher than those of typical metallic foil strain gauges. Based on these calibration results on the initial sensor configurations, new foil strain gauge configurations will be designed and fabricated, to increase the strain gauge factors even more.

Augmentation with a reinforced acellular fascia lata strip graft limits cyclic gapping of supraspinatus repairs in a human cadaveric model

A reinforced biologic strip graft was designed to mechanically augment the repair of rotator cuff tears that are fully reparable by arthroscopic techniques yet have a likelihood of failure. This study assessed the extent to which augmentation of human supraspinatus repairs with a reinforced fascia strip can reduce gap formation during in vitro cyclic loading. The supraspinatus tendon was sharply released from the proximal humerus and repaired back to its insertion with anchors in 9 matched pairs of human cadaveric shoulders. One repair from each pair was also augmented with a reinforced fascia strip. All repairs were subjected to cyclic mechanical loading of 5 to 180 N for 1000 cycles. All augmented and nonaugmented repair constructs completed 1000 cycles of loading. Augmentation with a reinforced fascia strip graft significantly decreased the amount of gap formation compared with nonaugmented repairs. The average gap formation of augmented repairs was 1.5 ± 0.7 mm after the first cycle vs. 3.0 ± 1.2 mm for nonaugmented repairs (P = .003) and 5.0 ± 1.5 mm after 1000 cycles of loading, which averaged 24% ± 21% less than the gap formation of nonaugmented repairs (7.0 ± 2.8 mm, P = .014). Cadaveric human supraspinatus repairs augmented with a reinforced fascia strip have significantly less initial stroke elongation and gap formation than repairs without augmentation. Augmentation limited gap formation to the greatest extent early in the testing protocol. Human studies are necessary to confirm the appropriate indications and effectiveness of augmentation scaffolds for rotator cuff repair healing in the clinical setting.

Proper Generalized Decomposition (PGD) for the numerical simulation of polycrystalline aggregates under cyclic loading

The numerical modelling of the behaviour of materials at the microstructural scale has been greatly developed over the last two decades. Unfortunately, conventional resolution methods cannot simulate polycrystalline aggregates beyond tens of loading cycles, and they do not remain quantitative due to the plasticity behaviour. This work presents the development of a numerical solver for the resolution of the Finite Element modelling of polycrystalline aggregates subjected to cyclic mechanical loading. The method is based on two concepts. The first one consists in maintaining a constant stiffness matrix. The second uses a time/space model reduction method. In order to analyse the applicability and the performance of the use of a space–time separated representation, the simulations are carried out on a three-dimensional polycrystalline aggregate under cyclic loading. Different numbers of elements per grain and two time increments per cycle are investigated. The results show a significant CPU time saving while maintaining good precision. Moreover, increasing the number of elements and the number of time increments per cycle, the model reduction method is faster than the standard solver.

Mechanically enhanced grain boundary structural phase transformation in Cu

It has been previously proposed that grain boundary can be treated as a two-dimensional phase in materials with possible transformations between its multiple metastable states under external stimuli such as high temperature. In this work, the influences of another type of common external stimuli, i.e. mechanical load, on grain boundary structural phase transformation (GBSPT) are studied by molecular dynamics simulations. Two types of high angle symmetrical grain boundaries in Cu are used as model systems to investigate their structural transformation under both constant and cyclic mechanical loading. While in general GBSPT can be enhanced or weakened under a constant tensile or compressive stress as compared to that caused by temperature only, the influences of tension and compression are not the same. For this reason, cyclic loading consisting of symmetric tensile/compressive half-cycles can significantly influence the overall GBSPT. Furthermore, it is found that the macroscopic strain in the material caused by GBSPT under compression/tension can be well described by Coble creep, which implies that GBSPT could be a grain boundary diffusional process during the creep of polycrystalline materials that has been overlooked before. Additionally, it is shown that together with shear coupled grain boundary motion and sources of free volume such as voids or cracks, cyclic shear loading can also cause GBSPT between its metastable structures of different atomic density, which can be possibly utilized to heal damage or microcracks in engineering materials.

Effect of mechanical preconditioning on the electrical properties of knitted conductive textiles during cyclic loading

This paper presents, for the first time, the electrical response of knitted conductive fabrics to a considerable number of cycles of deformation in view of their use as wearable sensors. The changes in the electrical properties of four knitted conductive textiles, made of 20% stainless steel and 80% polyester fibers, were studied during unidirectional elongation in an Instron machine. Two tests sessions of 250 stretch–recovery cycles were conducted for each sample at two elongation rates (9.6 and 12 mm/s) and at three constant currents (1, 3 and 6 mA). The first session assessed the effects of an extended cyclic mechanical loading (preconditioning) on the electrical properties, especially on the electrical stabilization. The second session, which followed after a 5 minute interval under identical conditions, investigated whether the stabilization and repeatability of the electrical features were maintained after rest. The influence of current and elongation rate on the resistance measurements was also analyzed. In particular, the presence of a semiconducting behavior of the stainless steel fibers was proved by means of different test currents. Lastly, the article shows the time-dependence of the fabrics by means of hysteresis graphs and their non-linear behavior thanks to a time–frequency analysis. All knit patterns exhibited interesting changes in electrical properties as a result of mechanical preconditioning and extended use. For instance, the gauge factor, which indicates the sensitivity of the fabric sensor, varied considerably with the number of cycles, being up to 20 times smaller than that measured using low cycle number protocols.

Durability of recycled aggregate concrete under coupling mechanical loading and freeze-thaw cycle in salt-solution

In this study, a novel coupling testing protocol with separated repetitive loading and freezing-thaw cycles in salt-solution is designed to simulate coupling mechanical loading and complex environmental effects on durability and deterioration of recycled aggregate concrete (RAC). The Micromechanical properties and porosity of RAC were also characterized by scanning electron microscopy (SEM) and microhardness. The results show that the number and width of cracks of RAC and NAC under freeze-thaw cycles obviously increased with the increase of alternating times of repetitive load and the compressive stress level. The compressive strength losses for both RAC and NAC increase with the increase of compressive stress level and alternative times of repetitive load. However, the compressive strength of natural aggregate concrete (NAC) became lower than that of RAC after freeze-thaw cycles. It was found that the freeze-thaw resistance of RAC seems even better than that of NAC under the same freeze-thaw attacks and cyclic mechanical loading. It indicates that after freeze-thaw cycles in salt-solution, the durability of RAC is better than that of NAC. On the other hand, the microhardness and SEM characterization results indicate that the interface transition zone (ITZ) was a weak part in both RAC and NAC, and the ITZ in NAC obviously deteriorated faster than that of RAC.

Simulation of Fatigue Fracture of FeMn-based Shape Memory Alloys at Cyclic Mechanical Tests

In this microstructural simulation of the mechanical behavior of FeMn-based shape memory alloy samples at mechanical cycling the threefold symmetry of the close-packed planes {111} of the austenitic fcc phase and basic planes {0001} of the martensitic hcp structure and the multi-variance of the reverse martensitic transformation are taken into account. Damage accumulation and resulting fatigue fracture are described in the terms of the internal variables associated with a damage variable and the densities of the oriented and scattered deformation defects. A deformation-and-stress criterion of fracture is proposed. It takes into consideration the effect of hydrostatic pressure, deformation defects and material damage. It is shown that the approach is suitable for describing the fatigue fracture of iron-based shape memory alloys at cyclic mechanical loading.

Fatigue behavior of laminated glass fiber reinforced polyamide

The fatigue performance of laminated glass fiber reinforced polyamide composites was investigated in this study. Fatigue tests were conducted on both unidirectional and cross-ply specimens at room temperature. The S-N diagram of [0]8 followed a bilinear curve, however, the [90]8 laminates presented a linear S-N diagram. Compared to the same lamination of glass/epoxy, [0]8 and [90]8 glass/polyamide laminates presented a lower fatigue resistance, while [02/902]s laminates exhibited a superior fatigue resistance. In addition, stiffness degradation was investigated for the laminates and was compared with that of glass/epoxy with the same lamination. Glass/polyamide composites had lower stiffness reduction compared to glass/epoxy.During fatigue tests, an infrared (IR) camera was also used to monitor the temperature rise in [02/902]s and [04/904]s laminates resulting from mechanical cyclic loading, and to capture the temperature profile associated with the failure area in the specimens. The maximum temperature in [04/904]s laminates was comparable to that of [02/902]s laminates and the final temperature in some of the cross-ply specimens reached 50 °C, which was near the glass transition temperature of the polyamide matrix.

Thermomechanical fatigue – Mechanism-based considerations on the challenge of life assessment

The combination of cyclic mechanical and cyclic thermal loading leads to thermomechanical fatigue (TMF) which is considered to be the primary life-limiting factor for engineering components in many high-temperature applications. Extensive low-cycle fatigue (LCF) data, which is traditionally used for design purposes, has been generated isothermally on various high-temperature materials, and thus, it is tempting to try to predict TMF life based mainly on isothermal LCF data. In this contribution, studies on different metallic structural high-temperature materials, which have mainly been carried out in the author's laboratory, are reviewed addressing the question, in which way and to which extent a reliable, unerring and robust TMF life assessment is possible on the basis of isothermally obtained fatigue life data. It is shown by means of examples that a sound TMF life prediction first of all requires a detailed mechanistic understanding of the isothermal cyclic stress-strain response and the relevant damage mechanisms. Furthermore, the TMF-specific peculiarities in both the non-isothermal cyclic stress-strain behaviour and the non-isothermal damage evolution process must be known. If all these requirements are fulfilled and reflected in the TMF life assessment methodology applied, a reasonable predictive accuracy can be attained.

A system of analysis and prediction of the loss of forging tool material applying artificial neural networks

The article presents the use of artificial neural networks (ANN) to build a system of analysis and forecasting of the durability of forging tools and the process of acquiring the source knowledge necessary for the network learning process. In particular, the study focuses on the prediction of the geometrical loss of the tool material after different surface treatment variants.The methodology of developing neural network models and their quality parameters is also presented. The standard single-layer MLP networks were used here; their quality parameters are at a high level and the results presented with their participation give satisfactory results in line with technological practice. The data used in the learning process come from extensive comprehensive performance tests of forging tools operating under extreme operating conditions (cyclic mechanical and thermal loads). The parameterization of the factors important for the selected forging process was made and a database was developed, including 900 knowledge vectors, each of which provided information on the size of the geometrical loss of the tool material (explained variables). The value of wear was determined for the set values of explanatory variables such as: number of forgings, pressure, temperature on selected tool surfaces, friction path and the variant of the applied surface treatment. The results presented in the study, confirmed by expert technologists, have a clear applicational character, because based on the presented solutions, the optimal treatment can be chosen and the appropriate preventive measures applied, which will extend the service life.

Overexpression of Mechanical Sensitive miR-337-3p Alleviates Ectopic Ossification in Rat Tendinopathy Model Via Targeting IRS1 and Nox4 of Tendon Derived Stem Cells

Tendinopathy, which is characterized by the ectopic ossification of tendon, is a common disease occurred in population, such as athletes that suffer from repetitive tendon strains. However, the molecular mechanism underlying the pathogenesis of tendinopathy caused by overuse of tendon is still lacking. Here, we found that the mechanosensitive miRNA, miR-337-3p, reduces its expression under uniaxial cyclical mechanical loading in tendon-derived stem cells (TDSCs) and negatively controlled chondro-osteogenic differentiation of TDSCs. Importantly, down-regulation of miR-337-3p expression was also observed in both rat and human calcified tendons, and overexpressing miR-337-3p in patellar tendons of rat tendinopathy model displayed a robust therapeutic efficiency. Mechanistically, we found that the proinflammatory cytokine Interleukin-1β (IL-1β) was the upstream factor of miR-337-3p that bridges the mechanical loading with its down-regulation. Furthermore, the target genes of miR-337-3p, NADPH oxidase 4 (Nox4) and insulin receptor substrate 1 (IRS1) activated chondro-osteogenic differentiation of TDSCs through JNK and ERK signaling respectively. Thus, these findings not only provide novel insight into the molecular mechanisms underlying ectopic ossification in tendinopathy but also highlight the significance of miR-337-3p as a putative therapeutic target for clinic treatment of tendinopathy. Funding: This work was supported by grants from National Natural Science Foundation of China (No.81572123, 81401844), The Ministry of Science and Technology of China (No. 2015DFG32200), Science and Technology Commission of Shanghai Municipality (No.14431900900, 15411951100, 16430723500), Shanghai Municipal Education Commission-Gaofeng Clinical Medicine Grant Support (No.20161314). Declaration of Interest: The authors declare no conflict of interest. Ethical Approval: All experiments with animals were carried according to Experimental Safety of Chinese Academy of Sciences and the Committees of Animal Ethics. All animal treatments were guided by the NIH guidelines.

High Temperature Fatigue Crack Growth Rate Studies in Stainless Steel 316L(N) Welds Processed by A-TIG and MP-TIG Welding.

Welded stainless steel components used in power plants and chemical industries are subjected to mechanical load cycles at elevated temperatures which result in early fatigue failures. The presence of weld makes the component to be liable to failure in view of residual stresses at the weld region or in the neighboring heat affected zone apart from weld defects. Austenitic stainless steels are often welded using Tungsten Inert Gas (TIG) process. In case of single pass welding, there is a reduced weld penetration which results in a low depth-to-width ratio of weld bead). If the number of passes is increased (Multi-Pass TIG welding), it results in weld distortion and subsequent residual stress generation. The activated flux TIG welding, a variant of TIG welding developed by E.O. Paton Institute, is found to reduce the limitation of conventional TIG welding, resulting in a higher depth of penetration using a single pass, reduced weld distortion and higher welding speeds. This paper presents the fatigue crack growth rate characteristics at 823 K temperature in type 316LN stainless steel plates joined by conventional multi-pass TIG (MP-TIG) and Activated TIG (A-TIG) welding process. Fatigue tests were conducted to characterize the crack growth rates of base metal, HAZ and Weld Metal for A-TIG and MP-TIG configurations. Micro structural evaluation of 316LN base metal suggests a primary austenite phase, whereas, A-TIG weld joints show an equiaxed grain distribution along the weld center and complete penetration during welding (Fig. 1). MP-TIG microstructure shows a highly inhomogeneous microstructure, with grain orientation changing along the interface of each pass. This results in tortuous crack growth in case of MP-TIG welded specimens. Scanning electron microscopy studies have helped to better understand the fatigue crack propagation modes during high temperature testing.

ANALYSIS OF EFFECTS OF MECHANICAL LOADS, THERMAL FLUCTUATIONS AND CHEMICAL FACTORS ON THE BOND STRENGTH OF RESIN CEMENT TO TITANIUM AND CoCrMo ALLOYS IN IMPLANT SYSTEMS – IN VITRO STUDY

Introduction: Cements in the oral cavity are subjected to many factors affecting cement retention, the major ones being masticatory loads and thermal stress. The gold standard in cementing restorations in the contemporary implant prosthodontics are resin cements while their predisposition to the effects of oral cavity environment presents a major factor for the efficiency of dental implant treatment. Material and method: In the study, we used 40 test models made up of a combination of original components of the Nobel Biocare system (implant replica NobRplN and titanium suprastructure Easy abatement NP 0.75) and the restoration cast in CoCrMo alloy. The specimens were divided in 4 test groups with 10 specimens each. The specimens in each group were cemented with resin composite cement with or without using the metal primer. Group I – Multilink Implant, IvoclarVivadent, Liechtenstein, Group II – Multilink Implant, IvoclarVivadent, Liechtenstein + Monobond Plus, Group III G-CEM LinkAce®, Group IV - G-CEM LinkAce® + GC Metalprimer II. The specimens were stored in 100% relative humidity for 24 hours whereupon each group underwent 5 rounds of testing. The specimens were subjected to thermal and mechanical load cycling tests whose number reflected the period of simulation of the function in the oral cavity (unloaded specimens, 7 days of function, 3 months, 6 months and 12 months). The retention force was measured by the Universal testing machine. Results: The highest retention values of the resin composite cement were recorded during the initial tests, which then declined in the subsequent rounds of testing. The biggest fall was measured in the first week after the cementation, while the cross-comparison of the later rounds of testing did not show any statistically significant differences. The values of the retention force of resin composite cements 12 months after the cementation dropped by one third of the initial values. All recorded values were higher in the specimens with primer coating. Conclusion: Masticatory forces and temperature changes in the oral cavity reduced the retention values of resin cement, but its values after 12 months of function were still high and provided stability and retention of the restoration in function. The usage of metal primer had a significant effect on retention force values at all levels of testing.

Viscoelastic analysis of a spring-connected dielectric elastomer actuator undergoing large inhomogeneous deformation

A viscoelastic model is developed for a conical dielectric elastomer actuator (CDEA) connected by a spring. The CDEA is composed of a dissipative dielectric elastomer membrane, a rigid disk and a spring. The CDEA undergoes large out-of-plane inhomogeneous deformation by applying the electro-mechanical loadings. The nonlinear spring-dashpot model is employed to model the viscoelastic behavior of the membrane. A differential-algebraic system of equations is derived and the solving technique is developed. Both the creep behaviors under the constant instantaneous electro-mechanical loadings and the cyclic performance under the cyclic mechanical and electrical loadings are presented. The time-dependent behaviors of the CDEA can be tuned by changing the stiffness of the spring, the pre-stretch of the membrane as well as the loading rate and the loading type.

Welding residual stresses as needed for the prediction of fatigue crack propagation and fatigue strength

Welding residual stresses have an impact on the performance of welded structures, on their fracture resistance, their resistance against fatigue crack propagation and, most important, their fatigue strength and fatigue lifetime. The present paper provides an overview on the issue mainly from the point of view of the application of fracture mechanics to the determination of the fatigue strength as the topic of this Special issue. Besides own experimental and theoretical data a comprehensive discussion is provided in that context which includes the definition and interaction of short- and long-range (or reaction) residual stresses, the effect of cyclic mechanical loading and its treatment in fracture and fatigue analyses.