Numerical simulation and experimental verification of fatigue crack propagation in high-strength bolts based on fracture mechanics

To investigate the fatigue crack propagation behavior of high-strength bolts for high-speed train brake discs, the fatigue crack propagation of high-strength bolts with initial defects under various load ratios was numerically simulated and experimentally verified based on fracture mechanics in this paper. Firstly, the fracture mechanics model of a three-dimensional hexahedral mesh with initial root defects was established using ABAQUS-FRANC3D interactive technology. Then the stress intensity factor (SIF) of the crack front was calculated by the stress superposition of the crack surface to simulate the coupling effect of preload and axial cyclic load. Based on it, fatigue crack propagation was simulated. Finally, the corresponding fatigue experiments on prefabricated crack bolts were carried out. The results show that mode I cracks dominate in the process of crack propagation. The stable crack propagation zones of the fractured high-strength bolts all show a semi-elliptical cross-section. The SIF of the crack front decreases with the increase of the load ratio, thus making the crack propagation life increase with the increase of the load ratio. The experimental outcomes are in great agreement with the simulation results, which verify that the numerical simulation method can effectively and accurately evaluate the fatigue life of high-strength bolts with initial defects and provides an effective means for predicting the fatigue crack propagation life of the same type high-strength bolts in engineering applications.

Influence of Electroless Nickel-DLC (Diamond-like Carbon) Multilayer Coating on the Mechanical Performance of the Heat-Treated AlSi10Mg Alloy Produced by Powder Bed Fusion-Laser Beam

This study characterizes the mechanical performance of the AlSi10Mg alloy produced by powder bed fusion-laser beam (PBF-LB) subjected to two combined cycles consisting of multilayer coating deposition (electroless nickel (Ni-P) + diamond-like carbon (DLC)) and heat treatment. In particular, the DLC deposition phase replaces the artificial aging step in the T5 and T6 heat treatments, obtaining the following post-production cycles: (i) Ni-P + DLC deposition and (ii) rapid solution (SHTR) (10 min at 510 °C) before Ni-P + DLC deposition. Microstructural characterization shows no appreciable modifications in the morphology and dimensions of the hard Si-rich phase of the eutectic network and secondary spheroidal Si phase. However, overaging phenomena induced by DLC coating deposition and differences in elastic-plastic properties between the multilayer coating and the PBF-LB AlSi10Mg substrate lead to a reduction in tensile strength by up to 31% and a significant decrease in ductility by up to 58%. In contrast, higher resistance to crack opening thanks to improved surface hardness and residual compressive stresses of the coating and reduced defect sensitivity of the substrate increase the fatigue resistance by 54% in T5-coated alloy and 24% in T6R-coated alloy. Moreover, the coating remains well adherent to the substrate during fatigue testing, not becoming a source of fatigue cracks.

Experimental and Theoretical Study on the Fatigue Crack Propagation in Stud Shear Connectors

Steel-concrete composite girder bridges are subjected to reciprocal cyclic loading from vehicles, and the stud shear connectors are the key components for transmitting shear forces. Thus, it is necessary to study the fatigue performance of the stud shear connectors. At present, there are few studies on the fatigue crack propagation process of studs, and the variation curve of the crack depth of studs with the number of fatigue loading cycles is not clear. In this study, the degradation law of fatigue properties and the fatigue crack propagation law of stud shear connectors in steel-concrete composite structures are examined under fatigue loading. The fatigue properties, i.e., failure mode, the dynamic slip-fatigue number curve, cross-sectional characteristics, and the residual bearing capacity of the stud specimens, are first systematically studied through ten standard push-out specimen tests. The test results show that the relative value of the fatigue crack extension area increases, while the relative value of the residual bearing capacity of the studs decreases approximately linearly. Then, the expression of the relationship between the fatigue crack depth and the residual load-bearing capacity of the stud is proposed, based on the fatigue crack theory of fracture mechanics. Finally, combined with the ABAQUS and FRANC3D software, a fatigue crack propagation finite element analysis (FEA) model of the stud is established. The FEA results showed that the trends in the number of cyclic loads and the fatigue crack depth of studs are basically the same for the simulation curve, test curve and theoretical calculation curve.

A Novel Ultrasonic Fatigue Test and Application in Bending Fatigue of TC4 Titanium Alloy

The very high cycle fatigue (VHCF) problems of thin-plate structures are usually caused by high-frequency vibrations. This paper proposes an accelerated fatigue test method based on ultrasonic loading technology in order to develop a feasible bending testing method and explore the bending fatigue characteristics of thin-plate structures in the VHCF regime. A new bending fatigue specimen with an intrinsic frequency of 20 kHz was designed based on cantilever bending through finite element simulation. It was verified by the axial load test with R = -1. The results showed that the method could effectively transfer the dangerous cross-section at the first-order cantilever bending restraint to the internal part of the specimen, thereby making the fracture location independent of the complex stresses. The linear relationship between the vibration amplitude and the maximum stress was also verified using strain measurements. Furthermore, the S-N curves and fracture morphology for different loading types were consistent with conventional studies to a certain extent, which indicated that the design of the bending test model was reasonable.

Experimental Study on Variable-Amplitude Fatigue Performance of M60 High-Strength Bolts of Grid Structures with Bolted Spherical Joints

The high-strength bolts of grid structures with bolted spherical joints under the action of suspension cranes are at risk of severe fatigue failure. Thus, this paper studies the variable-amplitude fatigue performance of M60 high-strength bolts. The test results for eight specimens in four loading modes are obtained using an Amsler fatigue testing machine. The fatigue life is also estimated based on Miner and Corten-Dolan's theories, and the applicability of Corten-Dolan's theory is verified. The fracture induced by the variable-amplitude fatigue is microscopically analyzed using scanning electron microscopy (SEM), revealing the mechanism of the variable-amplitude fatigue failure. Our findings provide valuable experimental data supporting the fatigue life estimation of grid structures with bolted spherical joints in service.

Post-Fatigue Fracture Resistance of Lithium Disilicate and Polymer-Infiltrated Ceramic Network Indirect Restorations over Endodontically-Treated Molars with Different Preparation Designs: An In-Vitro Study

The aim of the present study was to evaluate the fatigue to cyclic and static resistance of indirect restorations with different preparation designs made either of lithium disilicate (LS) or polymer-infiltrated ceramic network (PICN). Eighty-four (n = 84) molars were chosen, endodontically treated, and prepared with standardized MOD cavities. The molars were randomly divided into 6 study groups (n = 14) taking into account the "preparation design'' (occlusal veneer with 1.2 mm occlusal thickness; overlay with 1.6 mm occlusal thickness; adhesive crown with 2 mm occlusal thickness) and the "CAD/CAM material'' (E-max CAD, Ivoclar vivadent; Vita Enamic, Vita). A fatigue test was conducted with a chewing simulator set at 50 N for 1,500,000 cycles. Fracture resistance was assessed using a universal testing machine with a 6 mm diameter steel sphere applied to the specimens at a constant speed of 1 mm/min. A SEM analysis before the fracture test was performed to visually analyze the tooth-restoration margins. A statistical analysis was performed with a two-way ANOVA and a post-hoc pairwise comparison was performed using the Tukey test. The two-way ANOVA test showed that both the preparation design factor (p = 0.0429) and the CAD/CAM material factor (p = 0.0002) had a significant influence on the fracture resistance of the adhesive indirect restorations. The interaction between the two variables did not show any significance (p = 0.8218). The occlusal veneer had a lower fracture resistance than the adhesive crown (p = 0.042) but not lower than the overlay preparation (p = 0.095). LS was more resistant than PICN (p = 0.002). In conclusion, in the case of endodontically treated teeth, overlay preparation seems to be a valid alternative to the traditional full crown preparation, while occlusal veneers should be avoided in restoring non-vital molars with a high loss of residual tooth structure. LS material is more resistant compared to PICN.

Study on Static and Fatigue Behaviors of Steel-UHPFRC Composite Deck Structure

Ultra-high-performance fiber-reinforced cementitious composite (UHPFRC) is used in orthotropic steel deck (OSD) to form a lightweight composite deck structure (LWCD), which is expected to solve the problems of fatigue cracking of traditional steel deck and pavement damage. This paper aims to study the influence of key design parameters on longitudinal bending and transverse fatigue performance, as well as the ultimate bearing capacity calculation theory of the LWCD. A local finite-element (FE) model was built to evaluate the vehicle-induced stress ranges of six typical fatigue-prone details. In total, eight negative bending tests on steel-UHPFRC composite beams and one fatigue test on a steel-UHPFRC composite plate were conducted to investigate the longitudinal bending performance and the transverse flexural fatigue behavior of the LWCD, respectively. The results show that adding a 60-mm UHPFRC layer can significantly reduce the stress amplitude of six typical fatigue details by 44.8% to 90%. The failure mode of the longitudinal bending tests is the U-rib buckle and all UHPFRC layers exhibit multiple cracking behaviors when the specimens failed. The longitudinal cracking stresses of the specimens are between 20.0 MPa to 27.3 MPa. The reinforcement ratio and cover thickness have a great influence on the cracking stress. While the ultimate bearing capacity of specimens with different parameters has little difference. The calculation method of the ultimate bearing capacity of a steel-UHPFRC composite structure is proposed. When the strain at the bottom of the u-rib is taken as 1.2 times the design yield strain, the calculated results are in good agreement with the experimental results. No fatigue failure was observed after 66.12 million fatigue cycles under the design load, highlighting the favorable fatigue resistance of the proposed LWCD.

New anterior controllable antedisplacement and fusion surgery for cervical ossification of the posterior longitudinal ligament: a biomechanical study

OBJECTIVE

The traditional anterior approach for multilevel severe cervical ossification of the posterior longitudinal ligament (OPLL) is demanding and risky. Recently, a novel surgical procedure-anterior controllable antedisplacement and fusion (ACAF)-was introduced by the authors to deal with these problems and achieve better clinical outcomes. However, to the authors' knowledge, the immediate and long-term biomechanical stability obtained after this procedure has never been evaluated. Therefore, the authors compared the postoperative biomechanical stability of ACAF with those of more traditional approaches: anterior cervical discectomy and fusion (ACDF) and anterior cervical corpectomy and fusion (ACCF).

METHODS

To determine and assess pre- and postsurgical range of motion (ROM) (2 Nm torque) in flexion-extension, lateral bending, and axial rotation in the cervical spine, the authors collected cervical areas (C1-T1) from 18 cadaveric spines. The cyclic fatigue loading test was set up with a 3-Nm cycled load (2 Hz, 3000 cycles). All samples used in this study were randomly divided into three groups according to surgical procedures: ACDF, ACAF, and ACCF. The spines were tested under the following conditions: 1) intact state flexibility test; 2) postoperative model (ACDF, ACAF, ACCF) flexibility test; 3) cyclic loading (n = 3000); and 4) fatigue model flexibility test.

RESULTS

After operations were performed on the cadaveric spines, the segmental and total postoperative ROM values in all directions showed significant reductions for all groups. Then, the ROMs tended to increase during the fatigue test. No significant crossover effect was detected between evaluation time and operation method. Therefore, segmental and total ROM change trends were parallel among the three groups. However, the postoperative and fatigue ROMs in the ACCF group tended to be larger in all directions. No significant differences between these ROMs were detected in the ACDF and ACAF groups.

CONCLUSIONS

This in vitro biomechanical study demonstrated that the biomechanical stability levels for ACAF and ACDF were similar and were both significantly greater than that of ACCF. The clinical superiority of ACAF combined with our current results showed that this procedure is likely to be an acceptable alternative method for multilevel cervical OPLL treatment.

Mechanical Study of Various Pedicle Screw Systems including Percutaneous Pedicle Screw in Trauma Treatment

Background and Objectives: Spine surgery using a percutaneous pedicle screw placement (PPSP) is widely implemented for spinal trauma. However, percutaneous systems have been reported to have weak screw-rod connections. In this study, conventional open and percutaneous systems were biomechanically evaluated and compared. Material and Methods: The experiments were performed in two stages: the first stage was a break test, whereas the second stage was a fatigue test. Four systems were used for the experiments. System 1 was intended for conventional open surgery (titanium rod with a 6.0 mm diameter, using a clamp connecting mechanism). System 2 was a percutaneous pedicle screw (PPS) system for trauma (titanium alloy rod with a 6.0 mm diameter, using ball ring connections). System 3 was a PPS system for trauma (cobalt-chromium alloy rod with a 6.0 mm diameter, using sagittal adjusting screw connections). System 4 was a general-purpose PPS system (titanium alloy rod with a 5.5 mm diameter, using a mechanism where the adapter in the head holds down the screw). Results: Stiffness values of 54.8 N/mm, 43.1 N/mm, 90.9 N/mm, and 39.3 N/mm were reported for systems 1, 2, 3, and 4, respectively. The average number of load cycles in the fatigue test was 134,393, 40,980, 1,550,389, and 147,724 for systems 1 to 4, respectively. At the end of the test, the displacements were 0.2 mm, 16.9 mm, 1.2 mm, and 8.6 mm, respectively. System 1, with a locking mechanism, showed the least displacement at the end of the test. Conclusion: A few PPS systems showed better results in terms on stiffness and life than the open system. The experiments showed that mechanical strength varies depending on the spinal implant. The experiments conducted are essential and significant to provide the mechanical strength required for surgical reconstruction.

Device for Torsional Fatigue Strength Assessment Adapted for Pulsating Testing Machines

The torsional fatigue test determines the fatigue limit for a certain asymmetry coefficient of the cycle. The assessment of fatigue tests is performed on specialized machines. There are two types of torsion testing machines: universal machines that have the torsion component and specialized machines only for torsion testing. Nevertheless, no matter which proposed option we choose, the purchase prices for these testing machines or the values spent for self-management are quite high. This paper presented a device used for torsion fatigue testing, adaptable to a universal pulsating testing machine, designed to determine the torsion fatigue limit for different materials. The built device is simple and reliable, and therefore inexpensive. By using this device, we can determine the limit of the torsional fatigue after any stress cycle and we can use the parameters obtained from the universal machine to which it was attached. The torque and twisting angle of the test specimen during the test can be determined by calculation. The paper also presented an experimental method for determining shear strains based on calibration experiment, using a specimen on which strain gauges were mounted. The values taken from this calibration experiment were compared with those obtained from the theoretical calculation.

The Hardness Evolution of Cast and the High-Cycle Fatigue Life Change of Wrought Ni-Base Superalloys after Additional Heat Treatment

Concerning the use of modern technologies and manufacturing systems in the production of high-stress components from Ni-base superalloys and the optimization of the production process, knowledge of the microstructure–mechanical properties relationship is very important. The microstructure of Ni-base superalloys is very closely related to the chemical composition. With the high number of alloying elements, various phases are presented in the structure of Ni-base superalloys, which have a predominantly positive effect on the mechanical properties, but also phases that reduce, in particular, the heat resistance of these materials. The aim of the presented paper is the quantification of structural parameters of two types of cast alloys, ZhS6K and IN738, where the effect of dwell at 10 and 15 h at 800 °C on the change in morphology and volume fraction of the γ′-phase precipitate was studied. The detected changes were verified by the Vickers hardness test. The IN718 superalloy was chosen as a representative of the wrought superalloy. This alloy was also annealed for 72 h at a temperature of 800 °C, and the quantification of structural parameters was performed by EDS mapping and TEM analysis. Another partial goal was to assess the effect of changes in the volume fraction of the γ′-phase and δ-phase on the change in the high-cycle fatigue life of superalloy IN 718. This superalloy was tested by dynamic cyclic loading with cycle asymmetry parameter R = −1 at an ambient temperature of 22 ± 5 °C and at a temperature of 700 ± 5 °C and with cycle asymmetry parameter R < 1 (three-point bending load) after annealing at 700 °C/72 h. The results of the quantitative analyses and fatigue tests will be further used in optimizing the design of Ni-base superalloy components by modern technologies such as additive technologies for the production of turbine blades and implemented within the philosophy of Industry 4.0.

[Research Method of Fatigue Performance of Surgically Implanted Heart Valve Stent]

The aim of this study was to evaluate the fatigue property in vitro of a domestic surgically implanted heart valve stent (SIHVS). The domestic SIHVS was used as a testing sample, and a protocol was developed according to the standards GB 12279-2008 and ISO 5840-2:2015. A dynamic fatigue testing machine was used to apply for a 0.365 mm cyclic radial displacement at the end of the SIHVS. The SIHVS were observed after 400 million cycles by a scanning electron microscope. The observed results show that the structure of SIHVS is intact. There are no fracture, no obvious wear, and no loosening of the wire and casing. In the tested stents, no structural distortion, no cracks and obvious pits were observed. This study demonstrated that SIHVS structure meets the expected fatigue performance requirements.

The Experimental Verification of Direct-Write Silver Conductive Grid and ARIMA Time Series Analysis for Crack Propagation

The paper presents experimental verification of customized resistive crack propagation sensors as an alternative method for other common structural health monitoring (SHM) techniques. Most of these are sensitive to changes in the sensor network configuration and a baseline dataset must be collected for the analysis of the structure condition. Sensors investigated within the paper are manufactured by the direct-write process with electrically conductive, silver-microparticle-filled paint to prepare a tailored measuring grid on an epoxy or polyurethane coating as a driving/insulating layer. This method is designed to enhance the functionality and usability compared to commercially available crack gauges. By using paint with conductive metal particles, the shape of the sensor measuring grid can be more easily adapted to the structure, while, in the previous approach, only a few grid-fixed sensors are available. A fatigue test on the compact tension (CT) specimen is presented and discussed to evaluate the ability of the developed sensors to detect and monitor fatigue cracks. Additionally, the ARIMA time series algorithm is developed both for monitoring and predicting crack growth, based on the acquired data. The proposed sensors' verification reveal their good performance to detect and monitor fatigue fractures with a relatively low measurement error and ARIMA estimated crack length compared with the crack opening displacement (COD) gauge.

Manufacturing a First Upper Molar Dental Forceps Using Continuous Fiber Reinforcement (CFR) Additive Manufacturing Technology with Carbon-Reinforced Polyamide

3D printing is an emerging and disruptive technology, supporting the field of medicine over the past decades. In the recent years, the use of additive manufacturing (AM) has had a strong impact on everyday dental applications. Despite remarkable previous results from interdisciplinary research teams, there is no evidence or recommendation about the proper fabrication of handheld medical devices using desktop 3D printers. The aim of this study was to critically examine and compare the mechanical behavior of materials printed with FFF (fused filament fabrication) and CFR (continuous fiber reinforcement) additive manufacturing technologies, and to create and evaluate a massive and practically usable right upper molar forceps. Flexural and torsion fatigue tests, as well as Shore D measurements, were performed. The tensile strength was also measured in the case of the composite material. The flexural tests revealed the measured force values to have a linear correlation with the bending between the 10 mm (17.06 N at 5000th cycle) and 30 mm (37.99 N at 5000th cycle) deflection range. The findings were supported by scanning electron microscopy (SEM) images. Based on the results of the mechanical and structural tests, a dental forceps was designed, 3D printed using CFR technology, and validated by five dentists using a Likert scale. In addition, the vertical force of extraction was measured using a unique molar tooth model, where the reference test was carried out using a standard metal right upper molar forceps. Surprisingly, the tests revealed there to be no significant differences between the standard (84.80 N ± 16.96 N) and 3D-printed devices (70.30 N ± 4.41 N) in terms of extraction force in the tested range. The results also highlighted that desktop CFR technology is potentially suitable for the production of handheld medical devices that have to withstand high forces and perform load-bearing functions.

Fatigue Resistance of the Advanta V12/iCast and Viabahn Balloon-Expandable Stent-Graft as Bridging Stents in Experimental Fenestrated Endografting

PURPOSE

Bridging stents undergo millions of cycles during respiratory movements of the kidneys throughout the patient's life. Thus, understanding the response of fabric and endoskeleton of the stent to cyclic loading over the time is crucial. In this study, we compare the fatigue resistance of the Viabahn Balloon-Expandable stent-graft (VBX) with the widely used Advanta V12/iCast under prolonged stress induction.

MATERIALS AND METHODS

A polyester test sheet with 10 fenestrations was used simulating a fenestrated endograft. Five 6×59 mm VBX stent-grafts and five 6×58 mm Advanta stent-grafts were implanted into 6×6 mm fenestrations. The stents were flared with a 10×20 mm PTA (percutaneous transluminal angioplasty) catheter and connected with a fatigue stress machine. All stent-grafts were evaluated by microscopy and radiography at baseline and after regular intervals until 50,000,000 cycles were applied, simulating a life span of approximately 75 months. Freedom from fracture (FF), freedom from initial polytertafluoroethylene (PTFE) changes (FIC), and from PTFE breakpoint (FBP, all-layer defect) were calculated.

RESULTS

Digital radiographic images did not show any stent fracture in both groups after 50,000,000 cycles. The VBX stent-graft was free from any all-layer defects at the conclusion of 50,000,00 cycles resulting in a significant higher FBP compared with Advanta V12 (50,000,000 vs 33,400,000; p<0.01). All-layer defects were observed only in the Advanta group. Two of 5 Advanta stents showed early penetration of the nitinol ring causing a defect of PTFE. Regarding FIC, there was no significant difference between the stents (3,400,000 in VBX vs 3,200,000 in Advanta).

CONCLUSIONS

In fatigue tests simulating respiration movements, VBX and Advanta V12 performed equally well in terms of fracture resistance and freedom from initial PTFE changes. VBX maintained freedom from PTFE breakpoint throughout the full 50,000,000 cycles. All-layers defects were detected only in Advanta and were mainly caused by penetration of the nitinol ring through the PTFE.

Multi-Stage Cold Forging Process for Manufacturing a High-Strength One-Body Input Shaft

A multi-stage cold forging process was developed and complemented with finite element analysis (FEA) to manufacture a high-strength one-body input shaft with a long length body and no separate parts. FEA showed that the one-body input shaft was manufactured without any defects or fractures. Experiments, such as tensile, hardness, torsion, and fatigue tests, and microstructural characterization, were performed to compare the properties of the input shaft produced by the proposed method with those produced using the machining process. The ultimate tensile strength showed a 50% increase and the torque showed a 100 Nm increase, confirming that the input shaft manufactured using the proposed process is superior to that processed using the machining process. Thus, this study provides a proof-of-concept for the design and development of a multi-stage cold forging process to manufacture a one-body input shaft with improved mechanical properties and material recovery rate.

Influence of Connection Type and Platform Diameter on Titanium Dental Implants Fatigue: Non-Axial Loading Cyclic Test Analysis

Two-pieces dental implants must provide stability of the implant-abutment-interface. The connection type and platform diameter could influence the biomechanical resistance and stress distribution. This study aims to evaluate the fatigue for different types of connections, external and internal, and different platform diameters. Three implant designs with the same length were used: (a) external hexagon/narrow platform; (b) internal double hexagon/narrow platform; (c) internal octagon/regular platform. A fatigue test was developed to establish the number of cycles needed before fracture. A 30º oblique load with a sinusoidal function of fatigue at a frequency of 15 Hz and 10% stress variation was applied to each system. The fatigue load limit (FLL) for design (a) was 190 N, being the nominal-curvature-moment (NCM) = 1.045; FLL = 150 N, with a NCM = 0.825 for (b), and FLL = 325 N, with a NCM = 1.788 for (c). The platform diameter affects the FLL, obtaining lower FLL on a narrow platform. The connection type interferes with the implant walls’ width, especially in narrow implants, making internal connections more unstable at this level. Long-term clinical studies to assess the restoration’s success rate and survival are mandatory.

Study on statics and fatigue analysis of dental implants in the descending process of alveolar bone level

Alveolar bone atrophy can directly cause a decrease in bone level. The effect of this process on the service life of dental implants is unknown. The aim of this study was to determine the failure forms of the two-piece dental implants in the descending process of alveolar bone level, and the specific states of the components during the failure process. The CAD software SolidWorks was used to establish the model of alveolar bone and dental implants in this article. The finite element analysis was used to analyze the statics of the dental implants in the host oral model. The finite element analysis results showed that the stress concentration point of the implant and abutment in the implant system has changed greatly during the descending process of alveolar bone level, and indirectly increased the fatigue life of the same fatigue risk point. At the same time, the dental implants were tested in vitro in the descending process of alveolar bone level. Then, the fracture of the implant system was scanned by scanning electron microscope. The fatigue test results proved the finite element analysis hypothesis the central screw first fractured under fatigue and then caused an overload break of the implant and abutment.

Experimental Study on a Novel Shear Connection System for FRP-Concrete Hybrid Bridge Girder

The study presents experimental results of an investigation on a novel shear connection system for hybrid bridge girders composed of laminated composite beams and concrete slabs. The special connector comprised of a steel plate and welded bolts is attached to beam’s top flange by adhesive bonding and with a preset torque of nuts. The study’s purpose is to check ductility, safety, reliability and robustness of the shear connection before its implementation in the first Polish composite bridge. Three static push-out tests and fatigue test were performed to evaluate the shear connection behavior under static and cyclic loading. The load–slip curves, shear capacity, fatigue strength and failure mechanisms of the novel shear connectors are discussed. The high-slip modulus indicates that the connectors can very efficiently promote the composite action. The ultimate resistance and the fatigue strength obtained from the test was about 12% and 66% higher than the characteristic resistance and the fatigue strength of common headed studs, according to Eurocode 4, respectively. An estimated global safety factor of 3.67 showed the high safety, reliability and robustness of the novel connection system. The study discusses the structural performance of the proposed connection system, demonstrating its technical suitability.

Experimental Evaluation of the Influence of Aggregate Strength on the Flexural Cracking Behavior of Epoxy Asphalt Mixtures

The flexural cracking resistance of an asphalt concrete mixture used in a steel bridge deck pavement needs to be higher than that of one used in ordinary pavement. In this study, mechanical experimental tests were used to evaluate the influence of the aggregate strength on the flexural cracking behavior of epoxy asphalt concrete (EAC). The aggregate fracture area of beam cross sections was quantitatively analyzed by digital image processing, and crack propagation in the mixture was analyzed using fracture mechanics theory. The bending test results showed that the EAC containing high-strength aggregates exhibited the highest flexural cracking resistance among all of the aggregate mixtures under the same conditions. The use of high-strength aggregates led to a reduction in the aggregate fracture area, thereby improving the flexural cracking resistance of the mixture. The aggregate strength had a significant influence on the flexural cracking propagation behavior of the mixture. Fatigue test results at strain-controlled levels of 600–1200 με and 15 °C showed that the aggregate strength had no evident influence on the fatigue properties of the EAC. It is recommended that high-strength aggregates are used to increase the fracture resistance of aggregates and the flexural crack resistance of EACs.

High Cycle Fatigue Performance of LPBF 304L Stainless Steel at Nominal and Optimized Parameters

In additive manufacturing, the variation of the fabrication process parameters influences the mechanical properties of a material such as tensile strength, impact toughness, hardness, fatigue strength, and so forth, but fatigue testing of metals fabricated with all different sets of process parameters is a very expensive and time-consuming process. Therefore, the nominal process parameters by means of minimum energy input were first identified for a dense part and then the optimized process parameters were determined based on the tensile and impact toughness test results obtained for 304L stainless steel deposited in laser powder bed fusion (LPBF) process. Later, the high cycle fatigue performance was investigated for the material built with these two sets of parameters at horizontal, vertical, and inclined orientation. In this paper, displacement controlled fully reversed (R = -1) bending type fatigue tests at different levels of displacement amplitude were performed on Krouse type miniature specimens. The test results were compared and analyzed by applying the control signal monitoring (CSM) method. The analysis shows that specimen built-in horizontal direction for optimized parameters demonstrates the highest fatigue strength while the vertical specimen built with nominal parameters exhibits the lowest strength.

Validation and Improvement of a Bicycle Crank Arm Based in Numerical Simulation and Uncertainty Quantification

In this study, a finite element model of a bicycle crank arm are compared to experimental results. The structural integrity of the crank arm was analyzed in a universal dynamic test bench. The instrumentation used has allowed us to know the fatigue behavior of the component tested. For this, the prototype was instrumented with three rectangular strain gauge rosettes bonded in areas where failure was expected. With the measurements made by strain gauges and the forces registers from the load cell used, it has been possible to determine the state of the stresses for different loads and boundary conditions, which has subsequently been compared with a finite element model. The simulations show a good agreement with the experimental results, when the potential sources of uncertainties are considered in the validation process. This analysis allowed us to improve the original design, reducing its weight by 15%. The study allows us to identify the manufacturing process that requires the best metrological control to avoid premature crank failure. Finally, the numerical fatigue analysis carried out allows us to conclude that the new crank arm can satisfy the structural performance demanded by the international bicycle standard. Additionally, it can be suggested to the standard to include the verification that no permanent deformations have occurred in the crank arm during the fatigue test. It has been observed that, in some cases this bicycle component fulfils the minimum safety requirements, but presents areas with plastic strains, which if not taken into account can increase the risk of injury for the cyclist due to unexpected failure of the component.

Development of Real-Time Measurement Platform for Stretchable and Rollable Functions of Flexible Electronics under Multiple Dynamic Loads

Mainstream next generation electronic devices with miniaturized structures and high levels of performance are needed to meet the characteristic requirements of electronics with flexible and stretchable capabilities. Accordingly, several applied fields of innovative electronic component techniques, such as wearable devices, foldable curtain-like displays, and flexible hybrid electronic (FHE) biosensors, are considered. This study presents a novel inspection system with multifunctions of stressing tensile and bending mechanical loads to acquire the stretchable and rollable characteristics of soft specimens. The performance of the proposed measurement platform using samples of three different geometric types is evaluated in terms of its stretchability. The results show a remarkable enhancement of mechanical reliability when the sine wave geometric structure is used. A symmetrical sine wave-shaped sample is designed to measure performance under cyclic rolling. The proposed measurement platform of flexible electronics meets the testing requirements of mechanical reliability for the development of future flexible electronic components and FHE products.

High-Cycle, Low-Cycle, Extremely Low-Cycle Fatigue and Monotonic Fracture Behaviors of Low-Carbon Steel and Its Welded Joint

Low-carbon steels are commonly used in welded steel structures and are exposed to various fatigue conditions, depending on the application. We demonstrate that the various transitions in the fracture mode during fatigue testing can be distinguished by their different cyclic response curves and microstructural features after fracture. Fractography, surface damage micrographs, and microstructural evolution clearly indicated the transition of the fracture modes from high-cycle to low-cycle, extremely low-cycle fatigue, and monotonic behavior. The high-cycle fatigue mode showed initial cyclic softening, followed by cyclic stabilization, and showed inclusion-induced crack initiation at fish-eyes, while the low-cycle fatigue mode showed initial cyclic hardening followed by cyclic stabilization, where fractography images showed obvious striations. In addition, the extremely low-cycle fatigue mode showed no cyclic stabilization after initial cyclic hardening, which was characterized by quasi-cleavage fractures with a few micro-dimples and transgranular cracking, while the monotonic fracture mode predominantly showed micro-dimples.

New Damage Evolution Law for Steel-Asphalt Concrete Composite Pavement Considering Wheel Load and Temperature Variation

Epoxy asphalt (EA) concrete is widely used in constructing long-span steel bridge pavements (SBDPs). This study aims to derive a fatigue damage evolution law, conducting an experimental investigation of SBDP. First, a general theoretical form of the fatigue damage evolution law of materials is established based on the thermal motion of atoms. Then, fatigue experiments demonstrate that this evolution law well represents the known damage–life relationships of SBDP. Taking into account the experimental relationships between damage and fatigue life under symmetrical cyclic loadings with different overload amplitudes and temperature variations, a detailed damage evolution law is deduced. Finally, the role of damage accumulation is discussed on the basis of the proposed damage evolution law for the extreme situation of heavy overload and severe environments. The results show that both heavy loading and falling temperatures increase the fatigue damage of SBDP considerably. EA shows a fatigue life two to three times longer than that of modified matrix asphalt (SMA) or guss asphalt (GA). For the same thickness, EA pavement is demonstrated to be more suitable for an anti-fatigue design of large-span SBDP under high traffic flows and low temperatures.

New Dental Implant with 3D Shock Absorbers and Tooth-Like Mobility-Prototype Development, Finite Element Analysis (FEA), and Mechanical Testing

Background: Once inserted and osseointegrated, dental implants become ankylosed, which makes them immobile with respect to the alveolar bone. The present paper describes the development of a new and original implant design which replicates the 3D physiological mobility of natural teeth. The first phase of the test followed the resistance of the implant to mechanical stress as well as the behavior of the surrounding bone. Modifications to the design were made after the first set of results. In the second stage, mechanical tests in conjunction with finite element analysis were performed to test the improved implant design. Methods: In order to test the new concept, 6 titanium alloy (Ti6Al4V) implants were produced (milling). The implants were fitted into the dynamic testing device. The initial mobility was measured for each implant as well as their mobility after several test cycles. In the second stage, 10 implants with the modified design were produced. The testing protocol included mechanical testing and finite element analysis. Results: The initial testing protocol was applied almost entirely successfully. Premature fracturing of some implants and fitting blocks occurred and the testing protocol was readjusted. The issues in the initial test helped design the final testing protocol and the new implants with improved mechanical performance. Conclusion: The new prototype proved the efficiency of the concept. The initial tests pointed out the need for design improvement and the following tests validated the concept.

Structural Safety Evaluation of Precast, Prestressed Concrete Deck Slabs Cast Using 120-MPa High-Performance Concrete with a Reinforced Joint

Prestressed concrete structures are used in various fields as they can reduce the cross-sectional area of members compared with reinforced concrete structures. In addition, the use of high-performance and strength concrete can help reduce weight and achieve excellent durability. Recently, structures have increasingly been constructed using high-performance and strength concrete, and therefore, structural verification is required. Thus, this study experimentally evaluated the structural performance of a long-span bridge deck slab joint, regarded as the weak point of structures. The specimens were designed with a 4 m span for application to cable-stayed bridges. To ensure the required load resistance and serviceability, the specimens comprised of 120 MPa high-performance fiber-reinforced concrete and were prestressed. The deck slabs satisfied all static and fatigue performance as well as serviceability requirements, although they were thinner than typical concrete bridge deck slabs. The study also verified whether the deck slabs were suitable to help implement precast segmental construction methods. Finally, the results confirmed that the structural performance of the developed prestressed concrete (PSC) deck slab was sufficient for the intended bridge application as it achieved a sufficiently large safety factor in the static and fatigue performance tests, relative to the design requirement.

Motion Synchronous Composite Decoupling with Fewer Sensors on Multichannel Hydraulic Force Control for Aircraft Structural Loading Test System

The aircraft full-scale fatigue test is widely used in the modern aircraft industry for the safety of flight. Generally, the aircraft full-scale fatigue test is achieved by structural loading; multiple hydraulic actuators are used to apply load for force control. The fatigue loading test takes approximately several years. A key challenge is how to accelerate the loading frequency to shorten the total test time. Nevertheless, when pluralities of hydraulic actuator simultaneously increase the loading frequency, the mutual coupling force from the low rigidity of the aircraft structure will cause a large loading error, meaning that the test cannot be implemented. Although it is possible to reduce error by adding sensors, the force sensors need to connect several kilometers of cable. This paper proposed a novel motion synchronous composite decoupling control strategy with fewer sensors. The control method compensates the negative coupling effect of the channels by integrating the command signals and feedback signals of all channels. It can suppress coupling force and reduce errors at higher frequencies, thereby shortening the experiment time. Opposed to traditional decoupling control methods, advantages of this strategy are that it only needs force sensors and it does not need additional displacement or velocity and acceleration sensors to collect state variables for building the state space. Furthermore, it has been experimentally verified that the new motion synchronous composite decoupling control method can indeed guarantee sufficient control accuracy when the test frequency is increased. The method has great economic significance for shortening test duration.

Fatigue and microgap behaviour of a three-unit implant-fixed dental prosthesis combining conventional and dynamic abutments

This is an in vitro study composed by a fatigue test followed by an optical microscopy analysis. Dynamic abutments concept, recently introduced on screw-retained implant dental prosthesis, consists on the screw channel customisation according to the individual needs of each rehabilitation. Geometry and tightening torque differences advise the assessment of their mechanical performance. Clarify whether the combination of dynamic and conventional abutments in a three-unit implant-fixed prosthesis has detrimental effects either on the mechanical performance under cyclic loading or on the implant-abutment microgap dimensions. The fatigue test was performed in agreement with the ISO standard 14801. Then on the samples that resisted 5 million cycles, the implant-abutment microgap was measured on dynamic and conventional abutments using optical microscopy. Two unloaded samples were used as control group. The samples supported a load of 1050 N. The implant-abutment microgap measurement did not show statistically significant differences (p = .086) between loaded and unloaded groups, but the loaded conventional abutments showed a significant lower implant-abutment microgap (p = .05) than the loaded dynamic abutments. The combination of conventional and dynamic abutments do not seem to produce a decrease in fatigue resistance to a level below the mastication forces or an increase in the joint dimensions.

Removal torque evaluation of three different abutment screws for single implant restorations after mechanical cyclic loading

PURPOSE

The aim of this study was to evaluate the removal torque of three different abutment screws and pull out strength of implant-abutment connection for single implant restorations after mechanical cyclic loading.

METHODS

The study was performed in accordance with ISO 14801:2007. Three implant groups (n=15) were used: group A, PW Plus® with flat head screw; group B, PW Plus® with tapered screw; and group C, Conelog® with flat head screw. All groups had the same implant-abutment connection feature: cone with mandatory index. All screws were tightened with manufacturer's recommended torque. Ten specimens in each group underwent cyclic loading (1×106 cycles, 10 Hz, and 250 N). Then, all specimens were un-tightened, measured for the removal torque, and underwent a tensile test. The force that dislodged abutment from implant fixture was recorded. The data were analysed using independent sample t-test, ANOVA and Tukey HSD test.

RESULTS

Before cyclic loading, removal torque in groups A, B and C were significantly different (B> A> C, P<.05). After cyclic loading, removal torque in all groups decreased significantly (P<.05). Group C revealed significantly less removal torque than groups A and B (P<.005). Tensile force in all groups significantly increased after cyclic loading (P<.05), group A had significantly less tensile force than groups B and C (P<.005).

CONCLUSIONS

Removal torque reduced significantly after cyclic loading. Before cyclic loading, tapered screws maintained more preload than did flat head screws. After cyclic loading, tapered and flat head screws maintained even amounts of preload. The tensile force that dislodged abutment from implant fixture increased immensely after cyclic loading.

Fatigue Damage Monitoring of a Composite Step Lap Joint Using Distributed Optical Fibre Sensors

Over the past few decades, there has been a considerable interest in the use of distributed optical fibre sensors (DOFS) for structural health monitoring of composite structures. In aerospace-related work, health monitoring of the adhesive joints of composites has become more significant, as they can suffer from cracking and delamination, which can have a significant impact on the integrity of the joint. In this paper, a swept-wavelength interferometry (SWI) based DOFS technique is used to monitor the fatigue in a flush step lap joint composite structure. The presented results will show the potential application of distributed optical fibre sensor for damage detection, as well as monitoring the fatigue crack growth along the bondline of a step lap joint composite structure. The results confirmed that a distributed optical fibre sensor is able to enhance the detection of localised damage in a structure.

Mechanical properties of resin glass fiber-reinforced abutment in comparison to titanium abutment

Purpose:

So far, definitive implant abutments have been performed with high elastic modulus materials, which prevented any type of shock absorption of the chewing loads and as a consequence, the protection of the bone-fixture interface. This is particularly the case when the esthetic restorative material chosen is ceramic rather than composite resin. The adoption of an anisotropic abutment, characterized by an elastic deformability, could allow decreasing the impulse of chewing forces transmitted to the crestal bone.

Materials and Methods:

According to research protocol, the mechanical resistance to cyclical load was evaluated in a tooth-colored fiber-reinforced abutment (TCFRA) prototype and compared to that of a titanium abutment (TA), thus eight TCFRAs and eight TAs were adhesively cemented on as many titanium implants. The swinging that the two types of abutments showed during the application of sinusoidal load was also analyzed.

Results:

In the TA group, both fracture and deformation occurred in 12.5% of samples while debonding 62.5%. In the TCFRA group, only debonding was present in 37.5% of samples. In comparison to the TAs, the TCFRAs exhibited a greater swinging during the application of sinusoidal load. In the TA group, the extrusion prevailed, whereas in the TCFRA group, the intrusion was more frequent.

Conclusion:

The greater elasticity of TCFRA to the flexural load allows absorbing part of the transversal load applied on the fixture during the chewing function, thus reducing the stress on the bone-implant interface.