Cyclic Mechanical Loading Research Articles

Performance of organic coatings upon cyclic mechanical load

A number of engineering structures fail due to the fatigue damage resulting from cyclic mechanical stress. However, as far as organic coatings are concerned this degradation factor remains underestimated. In the paper the authors propose a methodology combining global electrochemical impedance spectroscopy and local atomic force microscopy measurements for evaluation of coating resistance to an impact of repetitive mechanical stress. Typical epoxy coating designated for ship hull protection has been tested in as-received state as well as after pre-exposure to elevated temperature. Application of atomic force microscopy-based approach helped to reveal and spatially localize the spots of organic coating degradation onset. The mechanism and process of defect formation and propagation has been shown on a microscopic scale.

Investigation of exhaust valve failure in a marine diesel engine

The present investigation is dealing with an exhaust valve failure of a main diesel engine in a vessel for serving marine petroleum sites. The failure of the valve occurred after 6490 h in service at 380 °C ambient temperature. Fracture is located at a cross-section of the spindle near to the connection between spindle length and neck of the valve dish, while the spindle was subjected to bending. It was found that the upper part of the spindle was subjected to partial dissociation of the martensite structure coexisting with carbides embedded between the martensite laths. The lower part of the spindle, near the fracture surface, was containing coagulated carbide formations (clusters). The precipitates mainly contain Cr-carbides in addition to some few Mo-carbides due to the effect of inadequate Mo-alloying. The carbides clusters represent a suitable location for micro-crack initiation during valve service. Running time and mechanical cyclic loads in addition to hot temperature environment was leading to exaggeration of micro-cracks to become an easy pass for fracture.The spindle fracture surface showed facets of ferrite grains embedded with deep clusters of carbides due to creep rupture, which is leading to strength collapse due to thermal yielding. It is recommended to use steel-alloy containing higher C- and Mo-contents to raise martensite start temperature and impedes coagulation of carbides during servicing respectively.

Effectiveness of Sealing Gel on Vertical Misfit at the Implant-Abutment Interface and Preload Maintenance of Screw-Retained Implant-Supported Prostheses.

This in vitro study evaluated the effectiveness of a sealing agent on vertical misfit of the implant-abutment interface in an external hexagon implant system before and after mechanical cyclic loading. Effects on preload maintenance of retaining screws after mechanical cycling were also assessed. External hexagon implant systems were divided into two groups (n = 12) according to the presence of an anaerobic sealing gel (control group-no sealing gel; experimental group-sealing gel applied). A prefabricated UCLA abutment was attached to the external hexagon implant with a digital torque wrench in accordance with the recommendations of the manufacturer. Specimens were tested through mechanical cyclic loading (1 × 106 cycles, 2 Hz, and 130 N). Vertical misfit of the implant-abutment interface was analyzed using a stereomicroscope, and reverse torque values were obtained using a digital wrench. Misfit and reverse torque data were measured and analyzed via analysis of variance (ANOVA) and the Tukey least significant difference test (α = .05). The sealing gel promoted lower vertical misfit values in the test group after mechanical cycling compared with before cycling (P = .009). The control group showed lower reverse torque values for retaining screws compared with the initial torque after mechanical cyclic loading (P < .0001). However, sealing gel application promoted higher reverse torque values in the test group postcycling (P = .0003). Anaerobic sealing agent application improved vertical misfit of the implant-abutment interface and preload maintenance of screw-retained implant-supported prostheses post-mechanical cycling.

Acoustic Emission of Frozen Soils under Quasi-Static Mechanical and Cyclic Thermal Loading

The effects of acoustic emission (AE) in soils during freezing and thawing under cyclic thermal and quasi-static mechanical loading have been studied. The composition and characteristics of the developed instrumentation set are presented. The informative AE parameters and primary data processing approaches were justified. The acoustic emission patterns of soil material behavior under the variable thermobaric conditions were obtained. The comparative tests were performed using static probing and ultrasonic scanning.

Comparison of dynamic mechanical properties of dentin between deciduous and permanent teeth

ABSTRACT Purpose: Even though differences between deciduous and permanent dentin have been widely studied, their dynamic mechanical behavior has never been compared. The objective of the present study was to quantify the differences between deciduous and permanent dentin under cyclic mechanical loading, which is similar to masticatory stress. Materials and Methods: Deciduous and permanent teeth, respectively from children (9 ~ 12 years old) and young people (18 ~ 25 years old), were wet-sectioned perpendicular to the longitudinal axis and the central specimens of coronal dentin were evaluated by nanoscopic dynamic mechanical analysis (nanoDMA). Results: The average storage, loss, and complex moduli, as well as the hardness of deciduous dentin were significantly (p < 0.05) lower than those of permanent dentin. Moreover, the tan δ value of permanent dentin was significantly (p < 0.05) lower than that of deciduous dentin across the loading frequency range, indicating that viscoelastic behavior and loss of elastic energy were significantly reduced in the stiffer permanent dentin. All the nanoDMA responses showed a significant influence of the dynamic loading frequency (p < 0.05): Both deciduous and permanent dentin showed reduced viscoelasticty with increased loading frequencies. Conclusions: Compared with deciduous dentin, permanent dentin exhibits higher stiffness with reduced energy loss during deformation, and therefore superior mechanical characteristics for the mastication process.

The influence of storage temperature on fracture behavior of cryopreserved teeth-An in vitro study.

ObjectivesCryopreservation is discussed as a viable method of preserving teeth for determined autogenous tooth transplantation. Unchanged physical properties of hard tooth tissues are crucial for functional healing. Due to different thermal expansion coefficients of enamel and dentin or the crystallization process, the freezing process may lead to crack formation, which could adversely impact the long‐term prognosis of the teeth.Material and methodsTwenty third molars (n = 20) were frozen slowly using a conservative cryopreservation protocol and stored at −80°C (group 1) and −196°C (group 2). After a storage time of 2 weeks, the samples were thawed to a temperature of +36°C and embedded in polymethyl methacrylate blocks. Cyclic loading was carried out using a spherical steel test specimen with 50,000 mechanical load cycles, followed by load to failure testing for determination of critical load.ResultsNo significant difference in the first load drop could be detected during the load to failure test under different storage conditions. The values until fracture correlated very closely in contralateral tooth pairs, which emphasizes the importance of crown geometry in load to failure tests.ConclusionsConclusions: Cryopreservation, specifically the storage temperature, does not appear to have a significant effect on the physical properties of tooth transplants.

Comparison of Fracture Load and Surface Wear of Microhybrid Composite and Ceramic Occlusal Veneers.

To compare in vitro fracture load, surface wear, and roughness after thermal cycling and cyclic mechanical fatigue loading among cemented microhybrid resin-based composite and ceramic occlusal veneers fabricated at two thicknesses (1.5 and 2.5 mm). Sixty-four extracted premolars without root canal treatment were prepared and restored with occlusal veneers of two thicknesses (1.5 and 2.5 mm), using four different materials: microhybrid composite (MC), fiber-reinforced microhybrid composite (FMC), heat-pressed lithium disilicate ceramic (HPC), and computer-aided design/computer-aided manufactured lithium disilicate ceramic (CCC). The specimens underwent thermal cycling and cyclic mechanical fatigue loading, and were then subjected to fracture testing, with loads at failure recorded as fracture load. Wear and surface roughness were recorded before and after fatigue loading. Results were analyzed using one-way ANOVA, two-way ANOVA (α= 0.05). All specimens survived thermal cycling and cyclic mechanical fatigue loading. At 1.5-mm thickness, the mean fracture load of FMC was highest (3926.48± 556.54 N), while that of CCC was highest (3066.45± 559.94 N) at 2.5 mm. Regardless of thickness, the fracture load of CCC was higher than that of HPC (p= 0.004 and p= 0.023). The wear of MC and FMC was significantly higher than those of HPC and CCC (p≤ 0.001), but was similar in terms of the wear rate of tooth enamel. HPC exhibited the lowest surface roughness after fatigue loading (p≤ 0.001). All tested occlusal veneers exhibited a fracture load considerably exceeding the maximum occlusal force in the posterior dentition. When the attainable space for restoration varies, different occlusal veneer materials should be considered. The surface wear and roughness also need to be considered when selecting materials.

Metal Plasticity and Fatigue at High Temperature

The situation in which a component or structure is maintained at high temperature under the action of cyclic thermal and/or mechanical loadings represents, perhaps, one of the most demanding engineering applications—if not, in fact, the most demanding one [...]

Effects of microstructural heterogeneity on fatigue properties of cast aluminum alloys

Cast Al alloys are widely employed for engine components, structural parts, gear box, chassis, etc. and subjected to mechanical cyclic load during operation. The accurate fatigue life prediction of these alloys is essential for normal operation as fatigue cracks initiated during operation induce the lubrication oil leak and serious safety hazard. Microstructural heterogeneity, including shrinkage/gaspores and secondary phase particles, is the most detrimental factor that affects fatigue life of cast Al alloys. The approximate fatigue life cycles could be estimated based on the size distribution and locations of shrinkage pores/defects. The relationship between crack population and stress was reported by statistical distributions and the cumulative probability for cast Al alloys fail at a certain stress could be predicted by combination of Paris law and pore size distribution. Pore depth was found to dominate the stress field around the pore on the surface and the maximum stress increases sharply when the pore intercepted with the surface at its top. The microstructure of cast Al alloys usually is composed of primary Al dendrites, eutectic silicon, Fe-rich particles and other intermetallic particles are dependent upon alloy composition and heat treatment. The coalescence of microcracks initiated from the fractured secondary phases was clearly found and can accelerate the initiation and propagation of the fatigue cracks. A link between defect features and the fatigue strength needs to be established through a good understanding of the fatigue damage mechanisms associated with the microstructural features under specific loading conditions. This paper reviews the influences of shrinkage/gaspores and secondary phase particles, formed during casting process, on the fatigue life of Al-Si-Mg cast Al alloys.

Experimental Investigation of the Inelastic Response of Pig and Rat Skin Under Uniaxial Cyclic Mechanical Loading

Skin is a highly non-linear, anisotropic, rate dependent inelastic, and nearly incompressible material which exhibits substantial hysteresis even under very slow (quasistatic) loading conditions. In this paper, a series of uniaxial cyclic loading tests of porcine and rat skin at different strain rates and with samples oriented in different directions (with respect to the spine) were conducted to study the effect of strain rate and samples orientations with respect to spine on mullins effect and skin inelastic response. A noteworthy feature of skin is that, similar to certain filled rubbers, its mechanical response shifts after the first extension and exhibits softening and hysteresis when loaded under cyclic tension and mullins effect is observed. The results of these strain-controlled cyclic loading tests also indicated that the extent of softening is different for different strain rates and orientations. Also a substantial hysteresis persists even at very low strain rates indicating inelastic behavior beyond the rate sensitive viscoelastic response. Through this series of experiments, by investigating the effect of strain rate on pig skin and rat skin, we conclude that the skin response is rate dependent but inelastic and shows irreversible changes in fiber orientation which are observed in histology results. Also, skin shows persistent deformation that is only partially recovered even after a long period of unloading.

Interface crack growth rate and fatigue life of multilayer-coated conductor tapes

Rare-earth barium copper oxide (REBCO) coated conductor (CC) tapes in high-field magnet applications usually experience fatigue due to a variety of cyclic mechanical and thermal loads. Therefore, estimation of the fatigue life of a particular REBCO CC is vital for the successful development and applications of high-field magnets devices, which are crucial components for aerospace, automotive, and maglev train transportation. For these needs, the authors evaluated the interfaces fatigue crack growth rate, and fatigue life of multilayered yttrium barium copper oxide (YBCO) coated conductor (CC) tape at room temperature via a displacement-energy model (DEM) approach. The DEM approach can measure the damage initiation and propagation rate in specific interfaces of a multi-layer film structure simultaneously. It assumed transverse intrinsic tensile/compressive cyclic loading and adopted the linear-elastic damage law with the maximum normal stress failure criterion to characterize the degradation in the tape. With an initial Eigenfunction (W0) (a parameter equivalent to the stress loading amplitude range) set as 2.30 × 10-12 m, the computed results show a strain (ε) range from 0.35 to 0.46 % and fatigue ratio range from 0.002 to 0.07 in the interfaces of the tape. Under the given parameters, it was found out that the tape has a critical interface fatigue crack growth rate of 2.54 pm/cycle corresponding to fatigue life of 3.9372 × 105 cycles. According to our previous studies, the results on the DEM approach has good agreement with those on the Finite Element Method (FEM) counterpart model, closed-form solution (CFS) model, and X-ray diffraction method (XRD). This technique can help to avoid enormous resources usually spent experimentally on the calculation of fatigue life of REBCO CC tapes.

A continuum fatigue damage model for the cyclic thermal shocked ceramic-matrix composites

In the present work, the tension-compression fatigue behavior of the 2-D woven oxide/oxide-ceramic-matrix composites is investigated. Experimental investigations reveal different fatigue failure mechanism for the original and the cyclic thermal shocked ox/ox-CMCs, and both materials undergo cyclic softening in the whole loading history. Based on the decrease of the elastic modulus, a new fatigue damage model is proposed to characterize the fatigue damage evolution and predict the fatigue life of both the original and the cyclic thermal shocked ox/ox-CMCs. The total damage is separated into the thermal stress-related damage from thermal shocks and the fatigue damage from applied mechanical loads. The good agreement between the proposed model and experimental data indicates that the damage model has the potential to describe complex thermomechanical damage of the ox/ox-CMCs under coupled cyclic thermal shocks and mechanical loading.

Deterioration Behavior Analysis and LSTM-Based Failure Prediction of GIB Electrical Contact Inside Various Insulation Gases

Plug-in connector of gas insulated bus (GIB) could be subject to the wear and electrical contact resistance (ECR) deterioration processes under cyclic mechanical and current loads. A specific test platform is designed to simulate the current-carrying wear and ECR degradation process of Ag-plated GIB electrical contact spot inside SF6, N2 and Air insulation mediums under current loads. Worn surface morphology and chemical composition are observed and analyzed. Influence of insulation medium and current load on deterioration mechanism of GIB electrical contact are discussed. Raw ECR curve obtained from current-carrying experiment is smoothed by the exponentially weighted moving average (EWMA) method. A long short-term memory (LSTM) neural network is then proposed to realize both single-step and multi-step ECR and failure predictions of GIB electrical contact. Results show that adhesive wear on contact surface happened due to materials transfer. Deterioration of contact spot inside air medium is much severer than those in SF6 and N2 due to oxidation effect. Deterioration of contact spot inside SF6 medium under high current load is severer than those under low current load due to the change of wear behavior and chemical reactions. Compared with other methods, LSTM could achieve better performance in both single-step and multi-step failure prediction of GIB electrical contact.

ОБ ИСПОЛЬЗОВАНИИ АКУСТИКОЭМИССИОННОГО ЭФФЕКТА ПАМЯТИ ДЛЯ ОЦЕНКИ СТРУКТУРНОЙ УСТОЙЧИВОСТИ МЕРЗЛЫХ ГРУНТОВ ПРИ ИХ ЦИКЛИЧЕСКОМ ОТОГРЕВЕ И МЕХАНИЧЕСКОМ НАГРУЖЕНИИ

ОБ ИСПОЛЬЗОВАНИИ АКУСТИКОЭМИССИОННОГО ЭФФЕКТА ПАМЯТИ ДЛЯ ОЦЕНКИ СТРУКТУРНОЙ УСТОЙЧИВОСТИ МЕРЗЛЫХ ГРУНТОВ ПРИ ИХ ЦИКЛИЧЕСКОМ ОТОГРЕВЕ И МЕХАНИЧЕСКОМ НАГРУЖЕНИИ

Cyclic mechanical loading of unsaturated silty clay soils

Cyclic mechanical loading of soils due to natural causes such as earthquakes or due to loads coming from structures causes the accumulation of strains and excess pore-water pressures endangering the long term serviceability and stability of the structures built on them. In this study, un-drained cyclic loading tests at 0.1 Hz frequency are carried out on a normally consolidated unsaturated silty clay soil at different deviatoric stresses for up to around 1400 cycles. The results of the tests, which were performed using a recently developed cyclic electromechanical tri-axial device, show a significant dependence of the cyclic mechanical behavior of the silty clay soil, such as in the form of cyclic axial and accumulated plastic strains, on the number of cycles and applied deviatoric stresses.

Эластокалорический эффект в быстрозакаленном сплаве Ti-=SUB=-2-=/SUB=-NiCu при периодическом воздействии растягивающей силы с частотой до 50 Hz

Abstract The elastocaloric effect (ECE) is studied in samples of rapidly quenched ribbons of a Ti_2NiCu alloy at a periodic action by a mechanical stress to 300 MPa at a frequency to 50 Hz. ECE is maximal near the temperatures of a first-order thermoelastic martensitic phase transition. The ECE maximum is observed in a point corresponding to the completion of the reverse martensitic transition ( T = 67.5°C) and is 21 and 6 K at cyclic mechanical loads of 300 and 100 MPa, respectively. The ECE value is shown to be independent of the frequency of the external loads in the range from 0 to 50 Hz. The specific power of a rapidly quenched ribbon as a thermal energy transformer is estimated at the external mechanical stress of 100 MPa; its value is 150 W/g at a frequency of 50 Hz and ECE is 6 K.

Combined effects of cyclic load and temperature fluctuation on the mechanical behavior of porous sandstones

Rocks in cold regions tend to experience exacerbated degradation under the combined effects of environmental and anthropogenic factors, which may arise from, for example, temperature fluctuation, mechanical excavation, and blasting. Activities related to rock support or open-pit slope optimization in cold regions require a complete understanding of the failure mechanisms of rock under the complex conditions. This paper quantitatively documents the impact of combined cyclic mechanical load and freeze-thaw cycles (i.e., the effect of stress “history”) on the microstructural evolution and mechanical degradation of three porous sandstones with distinct porosity values (from 3.9 to 14.1%). The three sandstone samples were collected from different geological regions in China. The microstructural evolution of the tested samples was quantitatively analyzed using the low-field Nuclear Magnetic Resonance (NMR) technique. To investigate sample degradation arising from the impact of the stress “history”, the cyclic-loaded and freeze-thaw cycled samples were eventually compressed to failure, during which an acoustic emission system was used to monitor microseismic activities. The results of the study show that the porosity of all tested sandstone samples was increased after cyclic load, with a much more rapid and further increase in porosity observed for samples being subsequently treated under the freeze-thaw cycles. More interestingly, the Chuxiong sandstone with relatively small porosity values were much more sensitive to the impact of cyclic load compared with the Linyi sandstone, exhibiting a somewhat larger increase rate in porosity. However, the Linyi sandstone with larger initial porosity values exhibited a relatively large increase rate in porosity under the multiple freeze-thaw treatments. The multiple freeze-thaw treatments mainly resulted in the development of relatively large pores. The results of the uniaxial compression tests show that the strength reduction of the samples being solely treated by freeze-thaw cycles was within the range of 5–10%, whereas it was within the range of 20–40% for those samples subjected to the combined cyclic load and freeze-thaw cycles.

The behavior of a zirconia or metal abutment on the implant-abutment interface during cyclic loading

Statement of problemThe effect of different abutment materials on the misfit and stability of the implant-abutment assembly under cyclic loading is unclear. PurposeThe purpose of this in vitro study was to evaluate the initial misfit, final misfit, and wear of external hexagon zirconia and metal implant abutments upon cyclic loading. Material and methodsForty 4.1-mm regular platform external connection implants were divided into 4 groups with 10 specimens in each group. The prosthetic abutments tested included Completely cast group (CC) (antirotational, castable burnout, custom abutment completely cast in nickel-chromium-titanium), Overcast group (OC) (Co-Cr premachined, antirotational, custom burnout abutment cast in nickel-chromium-titanium), Zirkonzahn group (Z) (antirotational castable, custom Y-TZP abutment produced; Zirkonzahn Prettau), and Neodent group (N) (antirotational castable, custom Y-TZP abutment; Neoshape). Abutments were fixed to the implants with 20-Ncm torque, and all specimens were cyclically loaded at 300 N with 9 Hz for 1 million cycles. The misfit on the implant-abutment interface was evaluated before and after cyclic loading by using an optical linear measuring microscope. Then, the wear of the external hexagon was analyzed by scanning electron microscopy (SEM) at a magnification of ×120. For a statistical analysis, a 1-way ANOVA was used for wear values. The data for the gaps were analyzed with a 2-way ANOVA. Any significant differences were resolved by using the Tukey HSD test (α=.05). ResultsThe CC (37 ±8 μm) and OC (35 ±5 μm) groups showed significantly larger initial misfit than the Z (4 ±5 μm) and the N (2 ±4 μm) groups (P<.05). After the mechanical cyclic loading, a statistically significant difference was found between the CC group (58 ±9 μm) and the other 3 groups (P<.05). The OC group (41 ±4 μm) showed significantly larger final misfit than the Z (19 ±7 μm) and the N groups (20 ±4 μm) (P<.05). Despite significantly increased misfit in all groups (P<.05), the OC group showed significantly less increased gap (6 μm) than the Z (15 μm) and the N (18 μm) groups, while CC showed the most increase in misfit (21 μm). Conversely, N (1313 ±315 μm2) and Z (735 ±126 μm2) groups showed significantly higher wear rate on the hexagon of external implants. The CC (231 ±182 μm2) and OC (201 ±125 μm2) groups were not statistically different. ConclusionsDifferent materials and fabrication techniques resulted in different levels of misfit at the implant-abutment interface. The mechanical cyclic loading aggravated the misfit, regardless of material or fabrication technique. In addition, the difference in the hardness of the abutment materials may lead to different levels of wear on the external hexagon. Zirconia abutments had increased wear and misfit. Premachined, antirotational, castable custom dental implant abutments had the least amount of change in misfit, which may indicate long-term stability at the implant-abutment interface.

The Effect of Cell and Module Dimensions on Thermomechanical Stress in PV Modules

We present an evaluation of the silicon solar cell as well as the photovoltaic (PV) module size and its effect on thermomechanical stress. The evaluation is based on finite-element method (FEM) simulations. Within these simulations, we perform parameter variations of the number of solar cells within a PV module from 60–140 cells, of the cell size from 156.0–161.75 mm, and the cell format from full cells down to quarter cells. The FEM simulations cover the lamination process, mechanical load, and thermal cycling for glass–foil as well as glass–glass modules. The presented results reveal correlations between the solar cell and module size with the stress in the solar cells. We also find that the interaction of the laminate with the module frame plays a significant role in thermal cycling. Of the varitations under investigation, the increase in cell size has the largest effect on the stress. However, at a mechanical load of 2400 Pa, glass–foil modules with less than 96 solar cells have a negligible failure probability. The advantage of placing the solar cells in the neutral axis of the laminate is proven by the negligible tensile stress values for all variations of the glass–glass modules.

Thermomechanical Fatigue Testing of Dual Hardening Tool Steels

Hot‐work tool steels are exposed to complex interacting cyclic thermal and mechanical loadings. Due to the combination of strengthening via carbides and intermetallic precipitates, dual hardening steels achieve well‐balanced mechanical properties in terms of fatigue strength and fracture toughness. Therefore, dual hardening steels have a great potential for hot‐work applications. Herein, out‐of‐phase thermomechanical fatigue tests are used to simulate the loading conditions experienced in hot‐work tool steel applications on a laboratory scale. The testing is conducted on Fe–C–Cr–Mo–V and Fe–C–Cr–Mo–V–Ni–Al alloys to compare common 5% Cr and dual hardening hot‐work tool steels. The resistance to thermomechanical fatigue is therefore correlated with single or dual hardening. Both alloys experience softening during the fatigue testing. Atom probe tomography investigations reveal coarsening of the secondary hardening precipitates for both alloys. However, the number density of surface cracks is greater for the 5% Cr hot‐work tool steel. The dual hardening steel possesses higher resistance to softening and reaches a higher lifetime.