Cyclic Compressive Loading Research Articles

Study of creep test and creep model of hydraulic concrete subjected to cyclic loading

Although the structure of hydraulic concrete depends on the history of cyclic loading and unloading, most studies focus on a single loading and unloading creep test, rendering dubious the assumptions of elastic creep theory. To clarify this situation, we designed uniaxial tensile and compressive cyclic loading and unloading creep tests and implemented them under uniform conditions. The tests were done with two-grade concrete with a water/binder ratio of 0.5, which is typical of a concrete dam project. The results elucidate how creep in sealed dam concrete depends on tensile and compressive cyclic loading and unloading. The Boltzmann superposition model was then used with the Grey Wolf Optimization algorithm to determine the parameters of the eight-parameter tensile and compressive creep model. The results show that the tensile specific creep is about 1.5–2.5 times greater than the compressive specific creep at the same loading age. When the compressive specific creep is superimposed during unloading for either tensile or compressive specific creep, the goodness of fit between the specific creep curve calculated by Boltzmann superposition and the measured specific creep curve exceeds 0.85. The results of this analysis indicate that when the compressive specific creep is superimposed during unloading, the specific creep calculated based on Boltzmann superposition is consistent with the measured specific creep.

Biodegradation-affected fatigue behavior of extrusion-based additively manufactured porous iron–manganese scaffolds

Additively manufactured (AM) biodegradable porous iron-manganese (FeMn) alloys have recently been developed as promising bone-substituting biomaterials. However, their corrosion fatigue behavior has not yet been studied. Here, we present the first study on the corrosion fatigue behavior of an extrusion-based AM porous Fe35Mn alloy under cyclic loading in air and in the revised simulated body fluid (r-SBF), including the fatigue crack morphology and distribution in the porous structure. We hypothesized that the fatigue behavior of the architected AM Fe35Mn alloy would be strongly affected by the simultaneous biodegradation process. We defined the endurance limit as the maximum stress at which the scaffolds could undergo 3 million loading cycles without failure. The endurance limit of the scaffolds was determined to be 90 % of their yield strength in air, but only 60 % in r-SBF. No notable crack formation in the specimens tested in air was observed even after loading up to 90 % of their yield strength. As for the specimens tested in r-SBF, however, cracks formed in the specimens subjected to loads exceeding 60 % of their yield strength appeared to initiate on the periphery and propagate toward the internal struts. Altogether, the results show that the extrusion-based AM porous Fe35Mn alloy is capable of tolerating up to 60 % of its yield strength for up to 3 million cycles, which corresponds to 1.5 years of use of load-bearing implants subjected to repetitive gait cycles. The fatigue performance of the alloy thus further enhances its potential for trabecular bone substitution subjected to cyclic compressive loading. Statement of significanceFatigue behavior of extrusion-based AM porous Fe35Mn alloy scaffolds in air and revised simulated body fluid was studied. The Fe35Mn alloy scaffolds endured 90 % of their yield strength for up to 3 × 106 loading cycles in air. Moreover, the scaffolds tolerated 3 × 106 loading cycles at 60 % of their yield strength in revised simulated body fluid. The Fe35Mn alloy scaffolds exhibited a capacity of withstanding 1.5-year physiological loading when used as bone implants.

Seismic experiments and shear resistance prediction of multi‐celled corrugated‐plate CFST walls

AbstractMulti‐celled corrugated‐plate CFST walls (MC‐CFST walls) are innovative composite members for load bearing and energy dissipation. The corrugated steel plates effectively enhance the confinement capacity of the infilled concrete and reduce steel consumption. In this study, the seismic behavior of MC‐CFST walls was investigated through experimental, numerical, and theoretical analyses. Eight specimens were designed considering different key parameters, including the width and depth of the corrugated cell, the amplitude of the corrugated steel plate, and the type of boundary columns. These specimens were tested under axial compression and horizontal cyclic loading. The test results indicated that MC‐CFST walls exhibited excellent energy dissipation capacity and ductility. Specimens with boundary columns exhibited shear failure modes, while those without boundary columns exhibited flexural failure modes. Increasing the width of the corrugated cell or reducing the effective depth of the composite wall had a significant negative impact on its seismic performance. Subsequently, the finite element (FE) model was established and verified against experimental results. Finally, based on the full‐sectional plasticity assumption and the superposition principle, theoretical formulas for predicting the compression‐bending and shear capacities of MC‐CFST walls were proposed and validated against experimental data. The results showed that both theoretical formulas could effectively and accurately predict the shear resistance of MC‐CFST walls, providing valuable references for practical designs.

Effect of compaction and moisture content on the deformation characteristics of unsaturated sandy soils varying fines content

ABSTRACT This research explores how the dynamic behaviour under cyclic compression loading affects compaction characteristics. Three types of sand, varying in fines content, are compacted under different saturation ( S r ) and compaction ( D c ) levels. Suction-controlled drained cyclic loading tests are conducted to predict axial strain accumulation, with initial suction measured for each compacted state. The study seeks to understand the correlation between axial strain accumulation and initial suction across soil types. Findings indicate that initial soil suction, influenced by saturation, significantly impacts axial strain under cyclic loading. Lower saturation enhances particle bonding, increasing initial suction and reducing strain accumulation. Moreover, an increase in fines content results in greater strain accumulation. Despite that, sands with elevated fines (18.8%) can achieve similar soil performance through effective compaction with precise saturation control. This underscores the role of saturation in moisture regulation during compaction, particularly for sands with higher non-plastic fines content, in achieving effective compaction.

Study on Seismic Performance of Concrete Columns and Beams Containing Recycled Brick Aggregate

In order to study the reinforced concrete beam and column components composed of natural concrete (NC), recycled aggregate concrete (RAC), recycled brick grain concrete (RBGC) and recycled brick aggregate concrete (RBC), the reinforced columns under the axial tension and compression cyclic load and the horizontal cyclic load and the simply supported beam under concentrated mid-span cyclic load are simulated respectively. The transient time history analysis is conducted to analyze their failure characteristics, hysteretic performance and energy dissipation performance. The results show that the damage of the column under horizontal cyclic loading is more obvious than that under vertical cyclic loading, and the energy dissipation is better, while the energy dissipation capacity of the beam is not as good as that of the column due to local concentration. The seismic performance of reinforced concrete columns and beams composed of RBGC and RBC materials is lower than that of NC and RAC materials. The materials of RBGC and RBC are more suitable for beam members.

Structural features of biobased composite foams revealed by X-ray tomography.

Polymer foams can have heterogeneous and complex internal structures, especially when material blends or particles have been integrated to create composites. It becomes even more challenging to probe and understand foam structure/properties when using non-uniform particles, such as biobased fillers. Optical or SEM imaging can only provide limited information as these are two-dimensional (2D) surface techniques. In this study, 3D X-ray tomography was applied to comprehensively analyze the structural features of biobased polyurethane foams containing porous rice hull fillers. The in-depth characterization at a wide range of length scale enabled us to quantify and obtain statistics of the unique trends in foam pore size and pore orientation corresponding to rice hull particle fraction and particle size. Rice hull particles were found to induce smaller cell formation. In addition, these biobased particles influenced cell expansion and caused cells to have less consistent orientation. Furthermore, after foam samples were subjected to cyclic compressive loading, X-ray tomography showed fractures in large (>100 μm) particles. This helps reveal the premature failing mechanism of composite foams with highly porous and coarse particles. The study elucidates novel microstructural evolution and deformation mechanisms using 3D X-ray tomography. The results offer new insights on internal structures for biobased composites and foams that are not previously possible through the conventional characterization tools.

On the effect of subgrade strength on the performance of geogrid-reinforced railway ballast

This paper presents the results of a series of ballast box tests aimed at investigating the effectiveness of geogrid reinforcement in reducing track settlement in a 300mm-thick layer of railroad ballast supported by three different artificial subgrades. In each experiment, the ballast layer supports a model tie subjected to cyclic compressive loading applied at a frequency of 0.8Hz with stress extrema at the tie-ballast interface of 57kPa and 400kPa for a total of 40,000 cycles. The three artificial subgrades considered in this study have CBR readings of 25, 13, and 5. For each subgrade, four tests are performed whereby one corresponds to an unreinforced condition (i.e., no geogrid) and three are reinforced with a single geogrid placed at either 150mm, 200mm, and 250mm below the bottom of the tie. The results indicate that geogrids exhibit a superior ability to minimize the tie’s settlement when the ballast layer is supported by a weak subgrade. The experiments further allude to the fact that the influence of the geogrid’s placement depth is exacerbated by the subgrade’s strength. In ballast layers supported by competent subgrades, the geogrid placement depth wields a marginal influence on the resulting tie settlement. However, the geogrid’s location becomes a key factor in ballast beds underlain by soft subgrades, with geogrids placed closer to the bottom of the tie being the most effective at minimizing the tie settlement.

In situ forming, mechanically resilient hydrogels prepared from 4a-[PEG-b-PTMC-Ac] and thiolated chondroitin sulfate for nucleus pulposus cell delivery.

Intervertebral disk degeneration (IVDD) is a common condition that causes severe back pain and affects patients' mobility and life quality considerably. IVDD originates within the central region of the disk called the nucleus pulposus (NP). Removing the damaged tissue and replacing it with NP cells (NPCs) delivered within an in situ forming hydrogel is a promising treatment approach. Herein we describe a hydrogel formulation based on 4-arm [poly(ethylene glycol)-b-poly(trimethylene carbonate)-acrylate] (4a[PEG-b-PTMC-Ac]) crosslinked with thiolated chondroitin sulfate via Michael-type reaction for this purpose. A library of hydrogels based on 15 kDa 4a-[PEG] with PTMC blocks of varying molecular weight were prepared and characterized. The instantaneous moduli of the hydrogels were adjustable from 24 to 150 kPa depending on the length of the PTMC block and the polymer volume fraction. The influence of each of these parameters was effectively explained using both scaling or mean field theories of polyelectrolyte hydrogels. The hydrogels were resistant to cyclic compressive loading and degraded gradually over 70 days in vitro. A hydrogel formulation with an instantaneous modulus at the high end of the range of values reported for human NP tissue was chosen to assess the ability of these hydrogels for delivering NPCs. The prepolymer solution was injectable and formed a hydrogel within 30 minutes at 37 °C. Bovine NPCs were encapsulated within this hydrogel with high viability and proliferated throughout a 28 day, hypoxic culture period. The encapsulated NPCs formed clusters and deposited collagen type II but no collagen type I within the hydrogels. Despite an initial gradual decrease, a steady-state modulus was reached at the end of the 28 day culture period that was within the range reported for healthy human NP tissue. This in situ forming hydrogel formulation is a promising approach and with further development could be a viable clinical treatment for IVDD.

Strength characteristics of foamed concrete under coupling effect of constant compressive loading and freeze-thaw cycles

In cold regions, foamed concrete used for the tunnel reserved deformation layer is usually subjected to environmental factors, such as the groundwater erosion, peristaltic pressure of surrounding rock, and freeze-thaw cycles. In this research, experiments considering single-factor (i.e., water immersion, constant compressive loading, and freeze-thaw cycles) and coupled multi-factor effects (the coupling of water saturated - constant loading and constant loading - freeze thaw cycles) were conducted on foamed concretes with different densities to examine the strength variation under different conditions. The results show that under the effect of single-factor environment, the average compressive strength of saturated foamed concrete is about 10% lower than the dry foamed concrete. The compressive strength of foamed concrete significantly decreases at the beginning of constant compressive loading but gradually increases with the loading time. Furthermore, the strength loss of foamed concrete after 30 freeze-thaw cycles can exceed 10%. Under the multi-factor coupled environment, the hydraulic effect accelerates the degradation of foamed concrete, particularly in terms of water saturation - constant loading coupling. Furthermore, for loading - freeze thaw cycles coupling, the compressive loading of 0.3 fc (fc is the unconfined compressive strength at 28 days) can improves the frost resistance of foamed concrete. In addition, the results of orthogonal analysis prove that the density has the most important influence, followed by the freeze-thaw cycle and the slightest effect of constant compressive loading. Additionally, a prediction equation of compressive strength as a function of density, freeze-thaw cycles, and the constant pressure value was established to describe the strength variation of foamed concretes under coupled-loading effects.

A novel defect-related cyclic damage model driven by strain energy density for LPBF titanium alloy

The unavoidable defects in additive manufacturing (AM) material have an impact on the deformation behavior and fatigue lifetime. In this study, a series of experiments on Laser powder bed fusion (LPBF) Ti-6Al-4 V under symmetrical tension–compression cyclic loading were conducted, and the corresponding damage mechanism was then investigated. It is revealed that due to the nucleation, growth, and coalescence of micro-voids, LPBF Ti-6Al-4 V exhibits accelerated softening behavior after reaching cyclic stability. Based on the physical mechanism, a cyclic damage model with a critical initiation criterion was developed using fracture mechanics principles, in which plastic strain energy as the driving parameter is introduced to describe the defect evolution, and the void’s closure effect during the tension–compression cycle is considered. Experimental validation has demonstrated that the damage model can accurately predict the cyclic stress–strain curves for LPBF Ti-6Al-4 V vertical and horizontal specimens, with errors in predicting crack initiation and growth lifetime falling within the ± 1.6 and ± 2.5 scatter bands.

Experimental investigation on seismic performance of an enhanced embedded base for CFST columns

To reduce the embedded depth and improve the seismic performance of the embedded column base for concrete-filled steel tubes, an enhanced embedded column base is proposed. The column base is strengthened by a pair of strengthening steel beams and anchor bolts. Four embedded column bases were tested under axial compression load and cyclic lateral load, and the test parameters included the type of column base, height of strengthening beam, and axial load ratio. The hysteresis curves, skeleton curves, stiffness degradations, energy dissipation capacities, and strain developments were analyzed based on the test results. The test results indicated that the enhanced embedded column base exhibited high strength and a high energy dissipation capacity compared with the traditional embedded column base. An embedded depth ratio of 0.7 enabled the enhanced embedded column base to achieve the ultimate strength of the CFST column. Finally, a calculation method for the ultimate strength of the enhanced embedded column base was proposed. This method considers the resistance resulting from the interaction between the steel tube and concrete, the interaction between the strengthening steel beam and concrete, and the deformation of anchor bolts.

Autogenous self-healing induced by compressive fatigue in carbonated hydraulic lime mortars

This study investigates the autogenous self-healing mechanisms in carbonated natural hydraulic lime mortar exposed to cyclic compressive loads. We sought to explain the observed 40% surge in residual compressive strength compared to the quasi-static compressive strength following fatigue testing at frequencies of 5 Hz, 0.5 Hz, and 0.05 Hz [Theoretical and Applied Fracture Mechanics 118 (2022) 103201]. Central to our investigation is the hypothesis that cyclic loads mobilize the embedded water in the mortar, catalyzing chemical reactions that foster autogenous self-healing. To substantiate this, intact and post-compressive fatigue specimens were analyzed using differential thermal analysis (DTA), thermogravimetric assessments (TGA), mercury intrusion porosimetry (MIP), and scanning electron microscopy with energy dispersive X-ray analysis (SEM-EDX). The TGA-DTA highlighted the evolution of new hydration pathways, characterized by C–S–H gel and portlandite formation. The MIP results indicated a decline in pore volume, particularly in fine pores, affirming the synthesis of novel hydration compounds. The SEM-EDX analysis further authenticated the composition of these emergent microstructures. This work, therefore, presents a comprehensive understanding of the factors influencing autogenous self-healing in lime mortar systems.

Performance of fibre-reinforced landfill mined soil like fraction as an environment friendly fill material

Landfill mining is emerging as an effective strategy towards mitigation of the problems associated with legacy landfills. About 40–80% of the landfill mined waste is soil-like fraction (landfill mined soil like fraction, LMSF) that is obtained after removing the combustibles and recyclables. However, due to its lower calorific value and presence of organic content, LMSF has not found many applications except landfill cover material and filling of low-lying areas. In this context, the present study explores the strength properties of LMSF with and without fibres (coir and polypropylene fibres) through unconfined compression, direct shear, UU triaxial, cyclic UU triaxial, indirect tension and bending strength tests. Overall, the shear strength properties of LMSF are similar to sand with some cohesion. Further, the performance of LMSF with optimum fibre proportion (2% for coir fibres and 1.5% for polypropylene fibres) is quite superior under monotonic and cyclic compressive loading conditions, as well as indirect tension and flexural loading conditions. An assessment of 1 m replacement of soft soil with LMSF with/without fibre reinforcement infers substantial increase in the allowable bearing pressure of LMSF. Overall, though LMSF may need assessment/pre-treatment (as required) due to presence of organic content and any possible heavy metal concentration, utilization of LMSF would encourage the adoption of landfill mining process. The current research insights contribute in the direction of achieving sustainable infrastructure development by reduction in the landfill area that would reduce the depletion of conventionally utilized natural resources for fill applications such as sand and gravel, and contribute positively towards environment.

Role of confinement in coseismic pulverization: Testing the rock record of rupture directivity on the San Jacinto fault, Southern California

The San Jacinto fault juxtaposes Pleistocene Bautista Formation sediments against Cretaceous tonalite near Anza, California. While the tonalites show typical features of pulverized fault zone rocks on the NE side of the fault, the weakly consolidated Bautista Formation shows evidence of pulverization at a depth of 120 m, yet only background level deformation at 70 m, consistent with the expectation that a minimum confining pressure is required to initiate pulverization in poorly consolidated sediments. We test this hypothesis by simulating pulverization using a modified Split Hopkinson Pressure Bar apparatus, in which dry and wet sediments are subjected to up to three repeated dynamic triaxial compressive load cycles. We suggest unconsolidated sediments require a minimum confining pressure under axial compression to initiate pulverization and multiple seismic cycles to reach the observed fracture densities. Companion experiments conducted on Ottawa sand suggest sub-grain scale inelastic deformation in the bulk (i.e., not localized in deformation bands) occurs preferentially at high strain rates. Furthermore, the compressive origin of sediment pulverization, along with evidence of tensile fragmentation in cross-fault tonalite, supports a preferred northwest rupture direction along the southern section of the San Jacinto fault.

Evaluating quasi-static and fatigue performance of IN718 gyroid lattice structures fabricated via LPBF: Exploring relative densities

Additive manufacturing (AM) technology has unlocked the possibilities of fabricating complex structures of components such as lattice structures. Based on the literature review, the significance of the thickness effect on lattice structures has not been completely investigated yet. Therefore, this research aims to study the wall thickness effect on the mechanical performance of sheet-based lattice structures under quasi-static compression and fatigue loading conditions. For this aim, four geometries of lattice structures with designed relative densities of 12.5%, 25.0%, 37.5%, and 50.0% were fabricated via Laser Powder Bed Fusion (LPBF) process using Inconel alloy 718. The specimens were heat-treated to eliminate residual stresses and then tested under monotonic and cyclic compressive loading conditions to obtain the mechanical responses for each respective relative density. The experimental results from the quasi-static and fatigue compression tests on different geometries were compared and analyzed with the use of Scanning Electron Microscopy (SEM) to observe the fatigue failure locations and their dependency on the surface morphology of those failure locations. In addition, the actual relative densities were also calculated to evaluate the geometrical deviations and correct the experimental results. Overall, the experimental data revealed that there are strong associations between the lattices with various wall thicknesses and corresponding quasi-static and fatigue performance.

Effect of water on the damage and energy dissipation feature of coal under uniaxial cyclic loading–unloading condition

AbstractTo gain insight into the reduction mechanism of water on the dynamic destabilization failure character of coal under mining disturbance conditions, the effect of water on the damage and energy dissipation feature of coal was investigated via cyclic uniaxial compressive loading–unloading test, acoustic emission (AE) monitoring technology, and theoretical analysis. It manifests that the softening effect of water on the mechanical properties of coal is still remarkable, lower uniaxial compressive strength and elastic modulus, while greater axial strain and higher Poisson's ratio are observed under the cyclic loading–unloading condition. Water‐saturated specimens dissipate more energy during the cyclic loading–unloading process, which reduces the proportion of input energy accumulated in the coal mass, thus lowering the burst proneness of coal. A greater amount of damage generates in air‐dried specimens than that in water‐saturated specimens at the first cyclic loading–unloading cycle, while lower damage increments are observed in the following cycles. This may be the reason for the minimal energy dissipation of air‐dried specimens in the following cycles. The elastic modulus behaves logarithmically versus the increasing cyclic loading–unloading cycle counts. The energy dissipated due to the damage evolution and plastic deformation of coal specimens positively correlates with the energy released by AE activities, the AE energy in air‐dried specimens has a greater concentration around the peak axial stress and a stronger correlation with the dissipated energy. The cumulative AE energy and energy dissipation density are linearly correlated during the cyclic loading–unloading process, which indicates a possibility in estimating the damage evolution and the dynamic failure risk of coal via cumulative AE energy, under the cyclic loading–unloading condition.

Implant-Supported Fixed Partial Dentures with Posterior Cantilevers: In Vitro Study of Mechanical Behavior.

Rehabilitation with dental implants is not always possible due to the lack of bone quality or quantity, in many cases due to bone atrophy or the morbidity of regenerative treatments. We find ourselves in situations of performing dental prostheses with cantilevers in order to rehabilitate our patients, thus simplifying the treatment. The aim of this study was to analyze the mechanical behavior of four types of fixed partial dentures with posterior cantilevers on two dental implants (convergent collar and transmucosal internal connection) through an in vitro study (compressive loading and cyclic loading). This study comprised four groups (n = 76): in Group 1, the prosthesis was screwed directly to the implant platform (DS; n = 19); in Group 2, the prosthesis was screwed to the telescopic interface on the implant head (INS; n = 19); in Group 3, the prosthesis was cemented to the telescopic abutment (INC; n = 19); and in Group 4, the prosthesis was cemented to the abutment (DC; n = 19). The sets were subjected to a cyclic loading test (80 N load for 240,000 cycles) and compressive loading test (100 KN load at a displacement rate of 0.5 mm/min), applying the load until failure occurred to any of the components at the abutment-prosthesis-implant interface. Subsequently, an optical microscopy analysis was performed to obtain more data on what had occurred in each group. Results: Group 1 (direct screw-retained prosthesis, DS) obtained the highest mean strength value of 663.5 ± 196.0 N. The other three groups were very homogeneous: 428.4 ± 63.1 N for Group 2 (INS), 486.7 ± 67.8 N for Group 3 (INC), and 458.9 ± 38.9 N for Group 4 (DC). The mean strength was significantly dependent on the type of connection (p < 0.001), and this difference was similar for all of the test conditions (cyclic and compressive loading) (p = 0.689). Implant-borne prostheses with convergent collars and transmucosal internal connections with posterior cantilevers screwed directly to the implant connection are a good solution in cases where implant placement cannot avoid extensions.

Marginal Discrepancy and Internal Fit of Bi-Layered and Monolithic Zirconia Fixed Dental Prostheses: An In Vitro Study

This in vitro study evaluated the influence of restoration design (bi-layered vs. monolithic) and manufacturing technique on the marginal discrepancy and internal fit of 3-unit zirconia fixed dental prostheses (FDPs). Mandibular second premolars and second molars were prepared as abutments in a 3-unit zirconia bridge to develop four groups (n = 10 FDPs): MZ: Monolithic zirconia FDPs, ZL: zirconia framework veneered by the hand-layering technique, ZP: zirconia framework veneered by the heat-pressed technique, and CAD-on: zirconia framework veneered by CAD/CAM lithium-disilicate glass–ceramic. All the zirconia FDPs were cemented to their corresponding die replicas using dual-cure resin cement and were subjected to compressive cyclic loading at a load range for half a million cycles using a universal testing machine. FDPs were sectioned mesiodistally to measure the marginal gap and internal fit using scanning electron microscopy. The measurements were taken at pre-assigned points of each abutment. Data were statistically analyzed via a Kruskal–Wallis test (α = 0.05). No significant differences were found between the monolithic and bi-layered zirconia groups in terms of the marginal discrepancy. However, there was a significant difference in the marginal gap between the zirconia groups. The marginal gap between monolithic and bi-layered zirconia FDPs was within the clinically acceptable range (&lt;100 μm). Comparable mean values of the marginal gaps of 3-unit monolithic and veneered zirconia FDPs were found. Therefore, the FDP design and veneering methods did not affect the marginal discrepancy. However, the mean internal gap varied among the experimental groups. As the current in vitro investigation demonstrated equivalent mean values of marginal gaps of both 3-unit monolithic and bi-layered zirconia FPDs, the use of monolithic 3-unit zirconia FPDs would be a viable alternative fabrication technique.

Degradation mechanisms of polyurethane grouting materials under quasi-static and cyclic compression loading: Density and size effects

Foamed polyurethane (PU) grouting materials are widely used for road trenchless rehabilitation. However, the density and size effects on their mechanical properties and the microscopic mechanisms under quasi-static and cyclic compression are still unclear. In this paper, the degradation mechanisms of PU grouting materials with different densities and specimen sizes were studied by quasi-static compression, cyclic compression, and macro–micro morphological characterization. As the specimen diameter increased, the compressive strength decreased and the elastic modulus increased. As the diameter-to-height ratio increased, the compressive strength increased and the elastic modulus decreased. The compressive strength/elastic modulus increased as a power function with density. When the stress levels were above the fatigue threshold, cyclic failure occurred. When the stress level reached the fatigue threshold, the loading rate, elastic strain, and loss factor were concentrated at 0.25 s−1, 0.013, and 0.023, respectively. The initial dynamic stiffness increased by 55.94 % compared to the quasi-static stiffness. The fatigue threshold tended to decrease with the increasing specimen diameter and diameter-to-height ratio. For specimens with D20H20, the density effect on the fatigue threshold was not significant. For specimens with D40H80 and D40H40, the fatigue threshold decreased with increasing density. Density and size effects were due to the combined effects of relative skin thickness, cell structure, and matrix content. Moreover, the S-N curve was established.

THE EFFECT OF CEMENT AND SCREW ON THE RESISTANCE OF AN IMPLANT CROWN TELESCOPICALLY ENGAGED TO A HEXAGONAL-SHAPED ABUTMENT UNDER COMPRESSIVE DYNAMIC CYCLIC FATIGUE LOADING.

To investigate the resistance of an implant crown telescopically engaged to a geometrically defined hexagonal abutment with and without cement under compressive dynamic cyclic fatigue loading. 40 crowns, 9 mm in height, were cast from prefabricated plastic copings, each telescopically engaged to a 3-mm-high machined hexagonal abutment. 20 crowns were retained with zinc phosphate cement, and 20 crowns were retained with a screw. A vertical load of 15 kg was applied to all samples under oscilla)on. A Periotest was used to measure the mobility of the implant crown in Periotest values (PTV). Test endpoints were defined by the following: fatigue cycles ≥ 20 million; crown PTV > 10; or if samples became visibly loose or component fracture. Cement-retained crowns failed on average at 2.60 x 106 cycles ± 2.27, while screw-retained crown samples failed at 2.17 x 106 cycles ± 1.27 with no significant difference (P > .05). Implant and abutment screw fractures were the most prevalent mode of failure in the cement-retained group, while in the screw-retained group, failures were caused by the loosening of one or both screw joints. The rate of increase in PTV was higher in the screw-retained group than in the cement-retained group. Under the experimental conditions, an implant crown telescopically engaged vertically to a 3-mm-tall hexagonal abutment, under compressive dynamic cyclic fatigue loading with or without cement, demonstrated no differences in resistance or failure outcomes. A cement-retained implant crown with telescopic engagement to the abutment is more rigid, resulting in more implant and abutment screw fractures than loosening.