Axial Loading Direction Research Articles

Revealing the failure mechanism of 2D triaxially braided composites under off-axial tension through mesoscale simulations

The external load angle is known to have a significant influence on the mechanical behavior of two-dimensional triaxially braided composites (2DTBCs). However, the experimental data for 2DTBCs under off-axial loading provide limited information for understanding the failure mechanisms. In this study, a comprehensive mesoscale finite element (FE) model for simulating 2DTBC specimens was established to evaluate the mechanical responses and damage characteristics when off-axial tensile loads are applied. The FE model effectively captured the mechanical response at five distinct angles (0°, 30°, 45°, 60°, and 90°) and revealed the evolving patterns of failure behavior, damage morphology, and out-of-plane deformation mechanisms corresponding to the different loading angles. The findings indicate that, when the external load aligns with the axial fiber bundle direction, the primary failure mechanism involves the fracture of load-bearing fiber bundles. In contrast, deviations from the axial loading direction resulted in failure that was primarily due to the undulation of the bias fiber bundles, resulting in a loading angle–dependent warping at the edge of the specimen due to local shear stress concentration. The findings of this study provide valuable insights that can inform the design of structures with improved application.

Hyperbolic load–displacement analysis of helical and expanded piles: database approach

Recent years have witnessed a focus on improving geotechnical systems by implementing and constructing new deep foundations such as helical and expanded piles. In this study, the effects of parameters such as embedment depth, pile geometry and axial loading direction on the load–displacement behaviour of these piles were examined. To this end, a database was compiled consisting of 80 axial loading test records for different piles. The embedment depth of the piles was in the range 2.4–36.8 m and the diameter of the helices (DH) or expanded parts (DEP) was in the range 254–1500 mm. The ultimate load of the piles was determined using the 2.5% and 5% displacement ratio criteria and the Brinch Hansen 80% method. Hyperbolic functions were fitted to the load–displacement curves, allowing for consistent estimation of the limit load and the initial tangent modulus. Analysis of the results from the database revealed that the dominant factors influencing the ultimate load, limit load, maximum measured load, initial stiffness and load–displacement behaviour were the ratio of DH or DEP to the shaft diameter, the shaft area and the toe area, and the load direction. Correlations derived from the database were validated using measurements from eight full-scale helical and expanded piles.

Spalling Criterion of Hard Brittle Rock Mass Based on Dilatation Lateral Strain.

Spalling failure is a typical failure phenomenon after excavation and unloading of a deep, hard brittle rock mass, which seriously threatens the safe construction of deep roadways (tunnels) and other projects. From the engineering viewpoint, it is essential to accurately evaluate the range and depth of surrounding rock spalling failure. From the perspective of the laboratory and engineering site, the strength and formation mechanism of hard rock spalling failure were statistically summarized and analyzed. Under uniaxial and low confining pressure conditions, when the load reached the rock damage stress, cracks in the rock penetrated to form a failure plane approximately parallel to the axial loading direction, and the strength of rock mass spalling was much smaller than that of intact rock spalling. A triaxial compression test was conducted to analyze the dilatation axial strain and dilatation lateral strain characteristics of gneiss. The results showed that dilatation axial strain gradually increased with the increase of confining pressure, whereas dilatation lateral strain was almost unchanged. Therefore, a safety factor (FS) based on dilatation lateral strain was developed. Through comparison with other strain-based spalling criteria, the establishment and physical meaning of the method were described in detail. In addition, FS was applied to analyze the deep roadway of the Hongtoushan Copper Mine in China and the Rm415 test tunnel in Canada. The results showed that the spalling criterion could accurately indicate the range and depth of the surrounding rock spalling failure, which verified the rationality and applicability of the new spalling criterion. Thus, FS can be utilized as a new theory and analysis tool for the assessment and prevention of spalling failure in deep hard rock roadways.

Rock fractures developed in the Class II post-peak unloading

In this study, post-peak tests were conducted on sandstone specimens subjected to uniaxial compression under two different loading conditions: axial strain control and lateral strain control. The fractures developed in both loading regimes were investigated. Under axial strain control, Class I post-peak stress–strain curves (with negative slope) were obtained until the complete failure of the specimens. The observed macroscopic fractures have mixed orientations: sub-parallel and inclined to the axial loading direction. Under lateral strain control, the specimens were tested until different residual post-peak stresses; the tests yielded consistent Class II post-peak stress–strain curves (with positive slope). Interestingly, the tested specimens were found to be relatively intact, with only non-penetrating fractures observed on the lateral surfaces. X-ray CT scanning revealed that the subvertical macroscopic fractures were developed close to the lateral surfaces of the specimens in the post-peak stages, while the internal central parts remained visually undamaged. Analysis of longitudinal and shear wave velocities measured in the axial direction of sandstone specimens before and after post-peak tests under lateral strain control provided evidence of internal fracture (crack) development parallel to the loading axis in the internal central parts of the tested specimens. Concentration of these small fractures (cracks) is high, but it only marginally increases as the loading proceeds in post-peak region from 90% to 70% of the peak stress. This suggests that the Class II post-peak regime obtained under lateral strain control cannot be explained by pure unloading; also, the Class II post-peak unloading is mainly produced by the development of the large subvertical tensile fractures identified in the CT scans, rather than by the growth of these small fractures (cracks).

Clinical challenges of biomechanical performance of narrow-diameter implants in maxillary posterior teeth in aging patients: A finite element analysis.

This study evaluated the biomechanical performance of narrow-diameter implant (NDI) treatment in atrophic maxillary posterior teeth in aging patients by finite element analysis. The upper left posterior bone segment with first and second premolar teeth missing obtained from a patient's cone beam computed tomography data was simulated with cortical bone thicknesses of 0.5 and 1.0 mm. Three model groups were analyzed. The Regimen group had NDIs of 3.3 × 10 mm in length with non-splinted crowns. Experimental-1 group had NDIs of 3.0 × 10 mm in length with non-splinted crowns and Experimental-2 group had NDIs of 3.0 × 10 mm in length with splinted crowns. The applied load was 56.9 N in three directions: axial (along the implant axis), oblique at 30° (30° to the bucco-palatal plane compared to the vertical axis of the tooth), and lateral load at 90° (90° in the bucco-palatal plane compared to the vertical axis of the tooth). The results of the von Mises stress on the implant fixture, the elastic strain, and principal value of stress on the crestal marginal bone were analyzed. The axial load direction was comparable in the von Mises stress values in all groups, which indicated it was not necessary to use splinted crowns. The elastic strain values in the axial and oblique directions were within the limits of Frost's mechanostat theory. The principal value of stress in all groups were under the threshold of the compressive stress and tensile strength of cortical bone. In the oblique and lateral directions, the splinted crown showed better results for both the von Mises stress, elastic strain, and principal value of stress than the non-splinted crown. In conclusion, category 2 NDIs can be used in the upper premolar region of aging patients in the case of insufficient bone for category 3 NDI restorations.

Crashworthiness of Foam-Filled and Reinforced Honeycomb Crash Absorbers in Transverse Direction

AbstractHoneycomb crash absorbers have been widely studied as energy absorption devices for use in automotive industries. However, none of these investigations have studied the side impact of empty and foam-filled honeycomb absorbers and adding stiffeners between the different layers of the corrugated sheets which are composing the honeycomb structure to analyse the structure under transverse (L-direction) impacts. In this paper, the foam-filled and reinforced honeycomb crash absorbers are investigated under axial (T) and transverse (L) loading directions. Experimental results for both empty and foam-filled specimens under quasi-static and impact loads were implemented to validate the developed finite element model. Finite element analysis (FEA) was performed to find out the crashworthiness behaviour of the structure under axial and transverse impacts according to road conditions. Finally, a new design of stiffened honeycomb crash absorber was developed and investigated to reduce the level of acceleration experienced by the passengers during the crash event. In this regard, it is concluded that all the requirements related to the energy absorption capabilities and generated deceleration under impact loading can be met by introducing an advanced method to reinforce honeycomb absorbers using stiffeners. It is also proven that the thickness of these stiffeners will not significantly influence the force levels. Due to increase of wall thickness from 1 to 3 mm, the mean crushing force increased from 129 kN to 148 kN. This growth is not sufficient as the goal is to obtain a mean crushing force of 300 kN. Thickening the stiffeners would lead to a loss of efficiency of the structure, as the small increase in mean force would not make up for the gain in mass. Thus, increasing the corrugated sheet’ thickness becomes necessary.

Development of novel high damping rubber damper under axial compression dynamic load for energy-dissipation application

High Damping Rubber (HDR) has been a reliable and durable material used in bearings as seismic isolation devices for several decades. However, there has been no development of HDR as dampers to mitigate structural response caused by earthquake excitations, specifically by dissipating the induced energy applied to structures under direct axial load. This paper aims to address this gap by developing a novel HDR damper utilizing Hyper Elastic Composite Material (HECM) and conducting experimental investigations to determine its damping ratio, compressibility, and elasticity behaviour under axial dynamic compression load for effective dissipation of dynamic loads. The research methodology involved performing response spectrum analysis (RSA) on a model of actual building structures, following the guidelines of National Annex to MS EN 1998–2017. Subsequently, the dampers were incorporated into the model to achieve a seismic base shear value without seismic actions, as determined by the RSA. Full-scale dampers were then developed using three different HECM variants (NR- A, B and C), selected based on their achievement of a damping ratio of more than 10% as bearing isolators using the shear method according to EN 15129:2009 cl. 8.2.4.1.5. The developed dampers underwent testing for damping and wind load in accordance with EN 15129 cl. 7.4.2.7 and cl. 7.4.2.8, respectively, incorporating the testing parameters derived from the model's RSA. Finite element models (FEM) were developed and accurately simulating the hysteresis curves and damping properties of the HDR damper. The properties of HECM in the FEM were derived from the simulation results and can be utilized for future damper design, ensuring compliance with industry requirements. Additionally, an empirical formula will be developed to enable structural designers to estimate the mechanical properties of the tested damper. The testing shows that NR-C HECM damper is able to achieve 10.46% under 1.8 Hz requirement and able to exceed more than10% damping ratio up to frequency 2.3 Hz, thus providing an alternative energy dissipation damper in the building industry.

Effects of predeformation on torsional fatigue in R260 rail steel

Rolling contact loading induces severe plastic deformations and initiates cracks near the rail surface. Prevention of such rolling contact fatigue cracks requires a better understanding about the mechanical behavior of the deformed material in this region. Even so, current rail standards do not consider the plasticity-induced changes to mechanical behavior. They only evaluate the mechanical performance of virgin rail steels under uniaxial loading conditions, which is not representative for the material’s performance after years of service. This study proposes a new method for fatigue life evaluation of deformed material under loading conditions similar to rolling contact loading. Both virgin and predeformed test bars with a circumferential notch were subjected to strain-controlled torsional fatigue loading to evaluate the influence of axial loading, predeformation, and torsional loading direction. Superimposed compressive axial loads increase the fatigue life without affecting the torque response. The predeformed test bars exhibited longer lives, an effect we attribute to the combination of different torque responses, hardening, and anisotropy.

Elasticity and Characteristic Stress Thresholds of Shale under Deep In Situ Geological Conditions.

The mechanical properties of shale are generally influenced by in situ geological conditions. However, the understanding of the effects of in situ geological conditions on the mechanical properties of shale is still immature. To address this problem, this paper provides insight into the elasticity and characteristic stress thresholds (i.e., the crack closure stress σcc, crack initiation stress σci, and crack damage stress σcd) of shales with differently oriented bedding planes under deep in situ geological conditions. To accurately determine the elastic parameters and crack closure and initiation thresholds, a new method-i.e., the bidirectional iterative approximation (BIA) method-which iteratively approaches the upper and lower limit stresses of the linear elastic stress-strain regime, was proposed. Several triaxial compression experiments were performed on Longmaxi shale samples under coupled in situ stress and temperature conditions reflecting depths of 2000 and 4000 m in the study area. The results showed that the peak deviatoric stress (σp) of shale samples with the same bedding plane orientation increases as depth increases from 2000 m to 4000 m. In addition, the elastic modulus of the shale studied is more influenced by bedding plane orientation than by burial depth. However, the Poisson's ratios of the studied shale samples are very similar, indicating that for the studied depth conditions, the Poisson's ratio is not influenced by the geological conditions and bedding plane orientation. For the shale samples with the two typical bedding plane orientations tested (i.e., perpendicular and parallel to the axial loading direction) under 2000 and 4000 m geological conditions, the ratio of crack closure stress to peak deviatoric stress (σcc/σp) ranges from 24.83% to 25.16%, and the ratio of crack initiation stress to peak deviatoric stress (σci/σp) ranges from 34.78% to 38.23%, indicating that the σcc/σp and σci/σp ratios do not change much, and are less affected by the bedding plane orientation and depth conditions studied. Furthermore, as the in situ depth increases from 2000 m to 4000 m, the increase in σcd is significantly greater than that of σcc and σci, indicating that σcd is more sensitive to changes in depth, and that the increase in depth has an obvious inhibitory effect on crack extension. The expected experimental results will provide the background for further constitutive modeling and numerical analysis of the shale gas reservoirs.

Is implant choice associated with fixation strength for displaced radial neck fracture: a network meta-analysis of biomechanical studies

The multitude of fixation options for radial neck fractures, such as pins, screws, biodegradable pins and screws, locking plates, and blade plates, has led to a lack of consensus on the optimal implant choice and associated biomechanical properties. This study aims to evaluate the biomechanical strength of various fixation constructs in axial, sagittal, and torsional loading directions. We included biomechanical studies comparing different interventions, such as cross/parallel screws, nonlocking plates with or without augmented screws, fixed angle devices (T or anatomic locking plates or blade plates), and cross pins. A systematic search of MEDLINE (Ovid), Embase, Scopus, and CINAHL EBSCO databases was conducted on September 26th, 2022. Data extraction was carried out by one author and verified by another. A network meta-analysis (NMA) was conducted in accordance with the Preferred Reporting Items for Systematic Reviews and Meta-analyses guidelines. Primary outcomes encompassed axial, bending, and torsional stiffness, while the secondary outcome was bending load to failure. Effect sizes were calculated for continuous outcomes, and relative treatment ranking was measured using the surface under the cumulative ranking curve (SUCRA). Our analysis encompassed eight studies, incorporating 172 specimens. The findings indicated that fixed angle constructs, specifically the anatomic locking plate, demonstrated superior axial stiffness (mean difference [MD]: 23.59 N/mm; 95% CI 8.12–39.06) in comparison to the cross screw. Additionally, the blade plate construct excelled in bending stiffness (MD: 32.37 N/mm; 95% CI − 47.37 to 112.11) relative to the cross screw construct, while the cross-screw construct proved to be the most robust in terms of bending load failure. The parallel screw construct performed optimally in torsional stiffness (MD: 139.39 Nm/degree; 95% CI 0.79–277.98) when compared to the cross screw construct. Lastly, the nonlocking plate, locking T plate, and cross-pin constructs were found to be inferior in most respects to alternative interventions. The NMA indicated that fixed angle devices (blade plate and anatomic locking plate) and screw fixations may exhibit enhanced biomechanical strength in axial and bending directions, whereas cross screws demonstrated reduced torsional stability in comparison to parallel screws. It is imperative for clinicians to consider the application of these findings in constraining forces across various directions during early range of motion exercises, taking into account the distinct biomechanical properties of the respective implants.

PERFORMANCE OF CORRODED THIN-WALLED STEEL TUBULAR COLUMNS FILLED WITH CONCRETE UNDER DIRECT MONOTONIC LOADING

The current experimental work investigates the structural behavior of steel tubular columns filled with concrete (STCFC) subjected to corrosion conditions as well as direct axial and uniaxial loading. To find out the ultimate capacity for bearing corroded columns, eight steel tubular square columns filled with normal strength concrete (NSC) having a cross-section of (100×100mm), a thickness of (1.5mm) and a length of (1000-1200mm) were tested under concentric and eccentric loads. The main variables that were adopted in this experiment are the column type (slender or short), the type of the applied loads (concentric or eccentric), and the thickness of the steel columns (before and after corrosion). The experimental results indicated that, after the steel columns were exposed to (5%) dilute sulfuric acid for (8 days), it was found that the thickness of the steel columns had decreased by about (55%), which led to a decrease in the bearing capacity of the tested corroded steel columns by about (48% and 43%) for slender columns subjected to concentric and eccentric loads respectively, and by about (47% and 44%) for short columns subjected to concentric and eccentric loads respectively.

Feasibility Evaluation of Novel High-Damping Rubbers as Energy-Dissipation Material under Axial Dynamic Load for Damper Devices

High-damping rubber (HDR) material has been widely used in bearings for seismic-isolation devices in structures. Nevertheless, HDR has not yet been developed in dampers to reduce the response of structures to earthquake excitations by dissipating the energy applied to the structures under direct axial load. The purpose of this paper was to evaluate the feasibility of using novel hyperelastic composite material (HECM), which is an HDR material, in experimental investigations to determine its damping ratio, compressibility, and elasticity behavior under axial dynamic load for the development of novel dampers in the future. First, a series of tests on HECM was conducted using the double-shear method to determine the most suitable sample for a purely dynamic compression test. Subsequently, the HECM was used in a device working as a scaled-down damper under both direct tension and compression dynamic load conditions, and pure direct compression dynamic load conditions were tested. Various thicknesses of the HECM (6, 8, and 10 mm) used in the testing damper were examined under a constant force with various frequencies of 0.01, 0.1, 0.25, and 0.5 Hz. The results show that the 10 mm thick HECM can provide a high damping ratio of 10% to 13% under axial conditions. Hence, this study is important for evaluating HECM, which has the potential for use in developing a full-scaled rubber damper system to resist axial force in the future. The damper is a novel rubber damper with high damping capability to dissipate energy under axial load. Furthermore, the damper can serve as an alternative choice that is more durable and overcomes the current weaknesses of passive dampers.

A multiscale computational framework for wear prediction in knee replacement implants

Wear damage represents a long-term material failure that affects the service life of joint implants. Prediction of wear in bearing inserts under arbitrary joint kinematics and kinetics holds an important key to improving the performance of entire joint implant device through new material development and customized geometry design. In this work, a hierarchical multiscale computational framework that predicts the wear rate of ultra-high molecular weight polyethylene (UHMWPE) insert by resolving the microstructure details and microscale deformation mechanisms is developed. A fully dynamic finite element simulation is carried out at the macroscale level to reproduce the interactions between femoral component and UHMWPE insert according to the knee simulator input profiles. The stress evolution history from the initial wear site is extracted and applied back to the representative volume element (RVE) to further study the microscale deformation mechanisms that are related to wear damage. Activation of different slip systems in the crystalline phase as well as the interplay between the crystalline phase and amorphous phase are captured through a semi-crystal plasticity model. Results shown that improving wear resistance requires minimizing the plastic deformation upon the same external loading while maintaining a load-sharing balance between the crystalline phase and the amorphous phase. A combination of high crystallinity, well-dispersed crystallite with small size and texture alignment along the axial load direction can promote activation of chain pull slip that leads to improved wear resistance.

The effect of grain size and porosity on the nature of compaction localisation in high-porosity sandstone

Compaction bands are a type of deformation band with negligible shear in porous rocks such as sandstones and are important because they can impede subsurface fluid flow. However, the microstructural properties which favour their growth are not fully understood. We experimentally investigate the effect of porosity and grain size on compaction localisation using synthetic sandstones that permit control of the starting porosity (27%, 32%, 37%) and mean grain size (ranging from 314 to 987 μm). Each sample is subjected to the same triaxial test conditions and shortened by 5% axial strain at an effective confining pressure of 85% of their grain crushing pressure (P*). Discrete compaction bands oriented normal to the axial loading direction are found only in the sample with the lowest starting porosity (27%) and smallest grain size (314 μm), while diffuse bands are observed for the same porosity at a larger grain size of 411 μm. No compaction bands are observed for any grain size in either the 32% or 37% starting porosity samples. Porosity analysis indicates grain size reduction does not necessarily correspond to porosity reduction indicating that compaction by grain rearrangement is as effective as localisation through comminution for these high-porosity synthetic sandstones.

Effect of interface geometric parameters on the mechanical properties and damage evolution of layered cemented gangue backfill: Experiments and models

• Mechanical properties of LCGB as a function of interface geometry parameters. • Effects of interface geometric parameters on damage evolution of LCGB. • Failure mode of LCGB is affected by interface geometric parameters. • Dynamic crack evolution law of LCGB during loading. Subsequent backfilling of widespread goaf generally necessitates multiple fillings to enrich the goaf, resulting in the layered structure of cemented gangue backfill. Therefore, it is of great significance to study the mechanical properties of layered cemented gangue backfill (LCGB) with different interface geometric parameters to ensure the stability of underground engineering. In this paper, the effects of interface roughness and interface angle on the mechanical properties and damage evolution of LCGB were comprehensively explored using uniaxial compression test, acoustic emission (AE) and digital image correlation (DIC). A numerical model of LCGB was built using simulation software (PFC2D) to demonstrate the dynamic crack evolution of LCGB during loading. The results demonstrate that LCGB's compressive strength and elastic modulus have positive linear and quadratic relationships with interface undulating angle and interface angle respectively, and that the rupture angle, which represents the worst mechanical properties of LCGB, is between 45° and 60°. Increasing the interface undulating angle can enhance the strain hardening characteristics of LCGB during the post-peak deformation stage and release more active AE signals. As the interface angle approaches the rupture angle, the peak stress of LCGB continues to deteriorate, and its AE signal appears more concentrated in the post-peak deformation stage. The surface principal strain of LCGB rises as the interface undulating angle increases, while it reduces first and then increases with the interface angle increases. The simulation process reveals that shear cracks mainly extend along the interface, whereas tension cracks propagation and coalescence the adjacent interface along the axial stress loading direction, and they converge to generate macroscopic defects that cause LCGB unstable failure.

Influence of shear stress state on dynamic fracture of a glass ceramic Macor

The effect of shear stress on the dynamic failure of ceramic materials is not sufficiently investigated in the published literature. With the use of a bespoke split Hopkinson pressure bar, this paper presents an effort to investigate the dynamic shear compressive response of Macor, a model ceramic material with zero porosity and light weight characteristics. A cone specimen and cylindrical specimens with varying inclined angles are used to introduce the shear stress to the Macor ceramic. The dynamic failure initiation and crack propagation are monitored by the high speed photography and Digital Image Correlation techniques. It is found that the equivalent stress of Macor at the initiation of failure decreases nonlinearly with the increase of shear stress. The high speed images show that the crack originates from the minimum cross-section of the cone specimen and the obtuse angle corner of the inclined cylindrical specimens. The cracks propagate parallel to the inclined plane instead of the axial loading direction. The fractographic analysis shows the compacted zone in the shear fracture surfaces of the cone specimen and the inclined cylindrical specimens. This indicates a significant role of shear loading in the dynamic failure process of Macor.

The influence of infiltration casting technique on properties of metal syntactic foams and their foam-filled tube structures

This research provides a comparative study between metal syntactic foams (MSFs) manufactured using two different infiltration casting techniques. Counter-gravity infiltration (CGI) and low-pressure infiltration (LPI) casting methods are employed to manufacture aluminium alloy matrix MSFs with embedded lightweight expanded clay aggregate particles. The cast MSFs from both methods are then used as the core material in the empty aluminium tubes with or without adhesive to produce foam-filled tube (FFT) samples. The MSFs manufactured using the LPI technique show higher density compared to their CGI cast counterparts. The quasi-static compression of the MSFs in axial and lateral loading directions showed the shearing of the MSFs during their deformation. The deformation of the FFTs in axial compression was influenced by the type of adhesive material, while in their lateral compression, the effect of adhesive type was insignificant. The mechanical properties of the MSFs and FFTs using the LPI technique showed higher values. For both casting techniques, the mechanical properties of the MSFs and FFTs in axial compression outperformed their structural values obtained from lateral compression.

Quantitative characterization of the fracture behavior of sandstone with inclusions: Experimental and numerical investigation

To investigate the effects of inclusions on the mechanical properties of rock with holes, sandstone specimens with square openings were prepared, considering the absence and presence of inclusions as two cases. The mechanical mechanism of crack evolution during the loading of the specimens and the effect of inclusions were quantitatively studied using the digital image correlation (DIC) technique and the rock failure process analysis (RFPA). The presence of inclusions reduced the stress of the sandstone specimen with an opening and hence increased its peak strength and elastic modulus. Based on the relative movement trend of the measuring points on both sides of new cracks, the crack types were divided into direct tensile cracks, relative tensile cracks, and shear cracks, as also confirmed by the damaged element distribution obtained by the RFPA2D. The strain values of the typical monitoring points along the potential crack propagation paths were extracted, and it was found that tensile cracks were initiated at the upper and lower boundaries of the opening and in the middle of the inclusions, then propagated vertically along the axial loading direction. Comparing the strain measurements of the two types of specimens showed that inclusions reduced the horizontal strain on tensile cracks and inhibited the initiation and development of cracks. The differentiation rate of the strain field was proposed to characterize the evolution of the degree of the overall differentiation of the strain field. This indicator was found to have a substantial spike at the time of crack initiation, thereby enabling the quantitative identification of the crack initiation stress. Compared with the specimens without inclusions, the specimens with inclusions had a higher crack initiation stress, which corresponded to a smaller differentiation rate of the strain field.

Free Vibration, Buckling and Dynamical Stability of Timoshenko Micro/Nano-Beam Supported on Winkler-Pasternak Foundation Under a Follower Axial Load

Axially loaded micro/nano-beams supported on foundations are extensively applied in micro/nano-electro-mechanical systems. This paper deals with the problems of free vibration, buckling and dynamical stability of Timoshenko micro/nano-beam supported on Winkler–Pasternak foundation subjected to a follower axial periodic load. Based on Hamilton’s principle, the governing equations of the system are derived in conjunction with nonlocal strain gradient and Timoshenko theory. The transition parameter is introduced to describe the follower direction of axial load during the deformation of the micro/nano-beam. Employing the weighted residual method, the variational consistency boundary conditions (BCs) can be derived according to the governing equations. Using differential quadrature method (DQM), the governing equations are discretized and numerical solutions of the natural frequencies, critical buckling load and instability region are obtained. Numerical examples are performed to verify present solutions by those available in the literature. Significant effects of the transition parameter and variational consistency BCs are revealed on the free vibration, buckling and dynamical stability of the axially loaded micro/nano-beam. The present analysis is of significance to axially-loaded micro/nano-beam mechanical system, especially for determination of the direction of follower axial force.

Biomechanical finite element analysis of short-implant-supported, 3-unit, fixed CAD/CAM prostheses in the posterior mandible

ObjectiveTo assess the biomechanical effects of different prosthetic/implant configurations and load directions on 3-unit fixed prostheses supported by short dental implants in the posterior mandible using validated 3-D finite element (FE) models.MethodsModels represented an atrophic mandible, missing the 2nd premolar, 1st and 2nd molars, and rehabilitated with either two short implants (implant length-IL = 8 mm and 4 mm) supporting a 3-unit dental bridge or three short implants (IL = 8 mm, 6 mm and 4 mm) supporting zirconia prosthesis in splinted or single crowns design. Load simulations were performed in ABAQUS (Dassault Systèmes, France) under axial and oblique (30°) force of 100 N to assess the global stiffness and forces within the implant prosthesis. Local stresses within implant/prosthesis system and strain energy density (SED) within surrounding bone were determined and compared between configurations.ResultsThe global stiffness was around 1.5 times higher in splinted configurations vs. single crowns, whereby off-axis loading lead to a decrease of 39%. Splinted prostheses exhibited a better stress distribution than single crowns. Local stresses were larger and distributed over a larger area under oblique loads compared to axial load direction. The forces on each implant in the 2-implant-splinted configurations increased by 25% compared to splinted crowns on 3 implants. Loading of un-splinted configurations resulted in increased local SED magnitude.ConclusionSplinting of adjacent short implants in posterior mandible by the prosthetic restoration has a profound effect on the magnitude and distribution of the local stress peaks in peri-implant regions. Replacing each missing tooth with an implant is recommended, whenever bone supply and costs permit.