Dynamic Fatigue Resistance Research Articles

Surface integrity and high-cycle fatigue life of direct laser metal deposited AISI 431 alloys modified by plasticity ball burnishing

Laser metal deposition (LMD) as an additive manufacturing (AM) is widely used to repair and extend wear and fatigue life of the critical components. This paper has investigated the application of ball burnishing (BB) to improve surface integrity and high-cycle fatigue resistance of LMDed AISI 431 alloys. Results showed that the BB treated samples exhibited significant surface finish improvement by lowering roughness by 91 %. Microhardness increased from 490 to 530 HV0.1, an increase by 10 % with a modified depth of 400 µm from the top surface. XRD results showed a peak shift and increase in FWHM by up to 17 %. This had been corroborated by EBSD exhibiting a 20 % increase in dislocation density and 24 % increase in localised misorientation within microstructure. As a result, the overall high cycle fatigue strength of the burnished sample increased by 50 %, and the cracks initiated from sub-surface level defects at a depth of 350 μm below the top surface, delaying the crack propagation and fracture failure. The findings clearly highlight that the burnishing treatment can be a plausible approach in improving the dynamic fatigue resistance and the overall service life of LMDed AISI 431 steel alloys components in engineering applications.

Zirconia specimens printed by vat photopolymerization: Mechanical properties, fatigue properties, and fractography analysis.

The mechanical and fatigue properties of zirconia specimens printed by vat photopolymerization (VPP) were evaluated and compared with those of zirconia specimens milled by computer numerical control (CNC). Bar-shaped specimens were printed by stereolithography (SL) and digital light processing (DLP). CNC-milled specimens were used as control samples. The fracture toughness, hardness, and flexural strength properties of the zirconia specimens were evaluated via single edge V-notch beam tests, Vickers hardness tests, and 3-point bending tests. Dynamic fatigue tests were carried out in distilled water using a step-stress test. After static bending and dynamic step-stress testing, fractography analysis was performed. Statistical analysis was carried out to compare the fracture toughness, hardness, flexural strength, and fatigue cycle results of each group (α = 0.05). The fracture toughness values did not significantly differ among the groups (p>0.05). The flexural strength was 894.10MPa for SL, 831.46MPa for DLP, and 1140.39MPa for CNC. The flexural strength of CNC was greater than that of SL and DLP (p<0.01). The mean fatigue cycles were 23498.07 for SL, 19858.60 for DLP, and 31566.80 for CNC. The mean fatigue failure strength was 643.13MPa for SL, 530.63MPa for DLP, and 903.75MPa for CNC. The fatigue failure strength of CNC was greater than that of SL and DLP (p<0.05). Fractography analysis revealed material defects at the fracture origin for each group. A partially fused structure of the incompletely debonded resin could be observed in SL, and a porous region of incompletely sintered zirconia grains could be observed in CNC. The fracture toughness and hardness of zirconia printed by VPP are comparable to those of zirconia milled by CNC. However, zirconia milled by CNC has superior static flexural strength and dynamic fatigue resistance. Further studies are needed to explore the clinical applications of VPP-printed zirconia.

REACTION OF O-PHENYLENEDIAMINE WITH EPICHLOROHYDRIN AND EVALUATION OF THE PRODUCT AS A RUBBER ANTIAGING AGENT

The epichlorohydrin - o-phenylenediamine reaction product has have obtained. With reverse titration the fact has been proven that the epoxy groups were completely used up to produce the adduct. The Shimadzu FTIR-8400S Fourier spectrometer was used to register the IR spectra. Through IR spectroscopy it was demonstrated that the resulting product is a secondary amine. Thus, epichlorohydrin epoxide groups react with the o-phenylenediamine amino groups. The product has been isolated in the form of a solid crystalline substance having the melting temperature range of 104-106 °С. The rubber mixes based on SKS-30ARKM-15 butadiene-styrene rubber containing sulfur, 2-mercaptobenzthiazole, diphenylguanidine, zinc white, industrial stearin, N-550 carbon black and rubber anti-aging agents at 6.6 mmol/100 g were also studied. The epichlorohydrin - o-phenylenediamine reaction product, N-isopropyl-N'-phenyl-p-phenylenediamine, and 2,2-methylene-bis(4-ethyl-6-tert-butylphenol) were used as the anti-aging agents. A rubber mix containing no anti-aging additives was studied. The rubber mixes have been prepared in LB 320 160/160 rollers, at 45-55 °C. The vulcanization properties of the rubber compounds were tested using Prescott Instruments Rheo-Line MDR rheometer at 160 °C. It was established that the studied adduct has virtually no effect on the vulcanization rate. High values of the difference between the maximum and minimum values of the torque of rubber with the adduct under study indicated a high degree of cross-linking of macromolecules. The mixture has been vulcanized in a 600-600 2E hydraulic vulcanization press at 160 ºC for 15 min. Studies of the properties of vulcanized rubber were carried out in accordance with the standards accepted for the rubber industry. High tensile stress values at given elongations for the rubber containing the product studied have also indicated a high degree of vulcanization. The use of the epichlorohydrin - o-phenylenediamine reaction product in the SKS-30ARKM-15 rubber based compounds has ensured high thermal aging resistance as well as dynamic fatigue resistance.

Torsional, Static, and Dynamic Cyclic Fatigue Resistance of Reciprocating and Continuous Rotating Nickel-Titanium Instruments

IntroductionThe aim of this study was to evaluate torsional, dynamic, and static cyclic fatigue resistance of the reciprocating One RECI (OR; Micromega, Besançon, France), WaveOne Gold (WOG; Dentsply Maillefer, Ballaigues, Switzerland), rotary One Curve (OC, MicroMega), and ProTaper Next X2 (PTN X2; Dentsply Sirona, Charlotte, NC) instruments. MethodsA total of 120 OR (n = 30), WOG (n = 30), OC (n = 30), and PTN X2 (n = 30) nickel-titanium instruments were used. Torque and rotation angle until failure under static torsion loading were measured according to ISO 3630-1. Static and dynamic fatigue resistance was measured as the time to fracture in an artificial stainless steel canal with a 60° angle and 5-mm radius of curvature at intracanal temperature. The results were analyzed with 1-way analysis of variance and the post hoc Tukey test. The alpha-type error was set at 5%. Fracture instruments from torsion and fatigue tests were examined with a scanning electron microscope. ResultsOR showed higher static fatigue resistance and rotation angle at fracture than WOG, OC, and PTN X2 (P < .05). WOG exhibited higher torsional resistance than the others (P < .05). The cyclic fatigue tests in dynamic mode had higher TTF than static for the PTN X2 and WOG groups (P < .05). In the dynamic tests, OR and WOG showed higher TTF than OC and PTN X2 (P < .05). ConclusionsUnder these experimental conditions, One RECI exhibited suitable mechanical properties with the highest cyclic fatigue resistance and angle of rotation among the tested instruments.

Dynamic fatigue resistance and marginal leakage of endocrowns fabricated with two different CAD/CAM ceramic materials.

Dynamic fatigue resistance and marginal leakage of endocrowns fabricated with two different CAD/CAM ceramic materials.

Mechanical Behavior of Single and Bundled Defect-Free Carbon Nanotubes

ConspectusSuperstrong materials can be utilized in many fields such as bulletproof vests, airframes, suspension bridges, flywheel energy storage, etc. As one of the strongest materials, carbon nanotubes (CNTs) can potentially be used to fabricate superstrong fibers. However, the tensile strength of CNTs is impaired a lot by defects, and the CNT fibers prepared so far have strengths much lower than that of a single CNT, showing a “size effect”. The conventional study of solid mechanics is usually based on the assumption that materials contain defects. Defect-free ultralong CNTs can hopefully help us avoid the “size effect” of nanomaterials and produce superstrong CNT fibers. They would also provide a system for the study of the mechanical behavior of ideal solids with a nonlocalized “quantum stress singularity”.In this Account, we discuss our recent studies on the mechanical behavior of defect-free single CNTs and CNT bundles. First, we introduce the defect-free structure of ultralong CNTs, which is one type of ideal solid. Second, we review the investigation of the static tensile properties and dynamic fatigue resistance and their temperature-dependence of single centimeters long defect-free CNTs. The results showed that defect-free CNTs have superior comprehensive mechanical properties, including superstrength, -toughness, and -durability. Different from traditional materials, the fatigue lifetime and fracture of CNTs are dominated by the first single-bond-sized defect (quantum stress singularity), showing “superbrittleness”. Third, by using a gas flow focusing in situ synthesis method as well as a synchronous tightening and relaxing strengthening strategy, we successfully fabricated CNT bundles with tensile strengths approaching that of single CNTs and showed that the “size effect” can be avoided. In addition, the advantages and promising future of using CNTs in flywheel energy storage are discussed. Finally, we provide our perspectives on the challenges and future directions in this field.

Mullins Effect and Crack Growth in Natural Rubber Vulcanizates during Heat Aging and Cyclic Loading

The effect of thermal aging and cyclic loading on mechanical properties and development of cracks in natural rubber vulcanizates was studied. After aging at 70oC and 110oC vulcanizates were subjected to cyclic loading. At a certain number of loading cycles, the samples were conducted in a tension test. At the aging condition of 70oC, the static tensile properties of material stay almost unchanged even after 88 aged hours and 8000 loading cycles. On the contrary, the dynamic fatigue resistance of vulcanizates decreases with increasing aging time. These results are attributed to the post-curing and the development of microcracks that might be caused by Mullins effect: in the case of static loading, the strain-induced crystallization may prevent cracks growth, but in the case of cyclic loading the strain-induced crystallization does not occur, so cracks develop without hindrance. However, at 110oC both static properties and dynamic fatigue resistance of material reduced dramatically because at high temperature the heat degradation exceeds both post-curing and strain-induced crystallization. Crack formation and propagation were examined by a digital optical microscope in the progress of cyclic loading. Results showed that natural rubber vulcanizate filled with carbon black has the best crack growth resistance (CGR) while the addition of modified and unmodified silica reduces CGR of materials. Moreover, the vulcanizate with unmodified silica has the lowest CGR.

Modification of Surface of Carbon Fiber Materials by Plasma Treatment (Review)

Modern processes for plasma modification of carbon fiber materials with the aim of improving the degree of adhesive interaction at the boundary between the polymeric matrix and the carbon reinforcing filler are reviewed. The strength of adhesion of carbon fibers to the polymeric matrix at the interface has a deciding effect on the strength of the carbon plastics in shear and compression and determines to a significant degree the crack resistance of the carbon plastic materials, the specific impact viscosity, and the dynamic fatigue resistance, thereby determining the durability of constructions made from them.

Is a Zirconia Dental Implant Safe When It Is Available on the Market?

The market share of zirconia (ZrO2) dental implants is steadily increasing. This material comprises a polymorphous character with three temperature-dependent crystalline structures, namely monoclinic (m), tetragonal (t) and cubic (c) phases. Special attention is given to the tetragonal phase when maintained in a metastable state at room temperature. Metastable tetragonal grains allow for the beneficial phenomenon of Phase Transformation Toughening (PTT), resulting in a high fracture resistance, but may lead to an undesired surface transformation to the monoclinic phase in a humid environment (low-temperature degradation, LTD, often referred to as ‘ageing’). Today, the clinical safety of zirconia dental implants by means of long-term stability is being addressed by two international ISO standards. These standards impose different experimental setups concerning the dynamic fatigue resistance of the final product (ISO 14801) or the ageing behavior of a standardized sample (ISO 13356) separately. However, when evaluating zirconia dental implants pre-clinically, oral environmental conditions should be simulated to the extent possible by combining a hydrothermal treatment and dynamic fatigue. For failure analysis, phase transformation might be quantified by non-destructive techniques, such as X-Ray Diffraction (XRD) or Raman spectroscopy, whereas Scanning Electron Microscopy (SEM) of cross-sections or Focused Ion Beam (FIB) sections might be used for visualization of the monoclinic layer growth in depth. Finally, a minimum load should be defined for static loading to fracture. The purpose of this communication is to contribute to the current discussion on how to optimize the aforementioned standards in order to guarantee clinical safety for the patients.

Cast polyurethane elastomers with improved dynamic fatigue resistance

AbstractCast polyurethane (PU) elastomers have been widely used in dynamic applications, such as rollers and wheels, where a low heat buildup, high‐loading resistance, and good cut‐growth resistance are desired. Because of their different molecular structure, cast PU elastomers do not perform in the same way in highly demanding dynamic applications. Small variations in the viscoelastic properties can result in significant differences in the longevity of the wheels and rollers that are subjected to a large number of cyclic compressive deformations. Therefore, it is of great interest to understand dynamic performance of urethane elastomers based on various backbones. Dynamic mechanical analysis (DMA) is commonly used to differentiate the dynamic performance of elastomers, but it only provides characterization of the dynamic behavior of a given elastomer at small deformation in the linear viscoelastic regime, where information such as the heat buildup and load‐bearing capability of elastomers cannot be obtained. As such, we developed a stress‐controlled flexometer on the basis of the ISO4666/4 method that enabled measurement of the heat buildup and load‐bearing capabilities of urethane elastomers in a large dynamic deformation environment. Moreover, a dynamometer was constructed to evaluate wheels based on different urethane elastomers under load at various speeds; this allowed close simulation of the urethane elastomers in a real application environment. In this article, the viscoelastic properties of cast urethane elastomers based on different backbones were studied by DMA. The heat buildup and load‐bearing capabilities of the elastomers were investigated via the stress‐controlled flexometer, and the performance of the wheels based on various prepolymer systems were evaluated with the dynamometer. Correlations between the material viscoelastic behavior and the heat buildup were established. © 2011 Wiley Periodicals, Inc. J Appl Polym Sci, 2012

Dynamic fatigue resistance of implant-abutment junction in an internally notched morse-taper oral implant: influence of abutment design.

To compare dynamic fatigue resistance of one- and two-piece abutments connected to internally notched morse-taper oral implants. Sixteen synOcta ITI implants were tightened into two metal models. SynOcta abutments for cement-retained restorations and solid abutments were divided into two equal groups and were torque-tightened into the implants. Each implant received a cement-retained crown. Cyclic dynamic axial and lateral peak loads of 75+/-5 N were applied on the implants for a duty of 500,000 cycles at 0.5 Hz, and at an angle of 20 degrees. Prior to the experiments and at each 100,000 cycles of loading, periotest values (PTVs) were measured. Removal torque values (RTVs) of the abutments were also measured with a custom-made torque device at the termination of the experiment. All abutments and implants were clinically immobile and without any signs of mechanical failure. The final PTVs for both abutments were similar and the difference between groups was insignificant (P>0.05). The RTVs of solid abutments were significantly higher than synOcta abutments (P<0.05). Predictable long-term clinical results can be achieved with solid abutments and synOcta abutments for cement-retained restorations. Solid abutments possess higher removal torque resistance than synOcta abutments when connected to synOcta ITI implants.

Strength and dynamic fatigue of silicon nitride at intermediate temperatures

The flexure strength distributions and dynamic fatigue of GS44, NT551, and NT154 silicon nitrides were determined at intermediate temperatures (≤850°C) in ambient air. GS44 and NT551 were fabricated by gas pressure sintering and had a larger volume fraction of secondary phase than the hot-isostatically pressed NT154. The inert characteristic strength for the GS44 and NT551 decreased with temperature while that for the NT154 was statistically independent of temperature. The slow crack growth exponent, N, for the GS44 and NT551 exhibited a 70–80% decrease between 700 and 850°C while the strength of NT154 was comparatively independent of stressing rate at 850°C. Associated with those changes in mechanical performances, the coefficient of thermal expansion (CTE) for the GS44 and NT551 sharply increased between 700 and 850°C while that for the NT154 did not. Although the observed change in CTE (and the temperature range where it occurred) did not have any obvious correlation with a change in inert strength at these intermediate temperatures, its presence was an indicator of a threshold temperature where dynamic fatigue resistance was compromised if the temperature exceeded it. The change in material state (and the associated temperature region for which it occurs) is linked to the equilibrium state of the secondary phase in the silicon nitride.

Dynamic fatigue and degradation in solution of hydroxyapatite ceramics.

Polycrystalline hydroxyapatite was densified by hot pressing. The dissolution process in aqueous solution and the effects of environment on dynamic fatigue resistance of the resulting HAP ceramics were investigated. Pure water or Ringer's solution strongly enhances subcritical crack growth. The crack propagation exponent decreases from 22.5+/-2 in air to 10+/-4 in Ringer's solution for materials densified at 98% of the theoretical value. The residual porosity ratio is also very detrimental for the mechanical reliability. Both fatigue resistance and immediate fracture strength are decreased, with values of only 14+/-4 for the propagation exponent and 40 MPa for the tensile strength (compared to 90 MPa at 98% relative density) for materials densified at 94% tested in air. The degradation in solution appears to be governed by uniform physico-chemical dissolution of crystalline HAP phase at the surface of the material. This dissolution is accompanied by a decohesion of grains located around residual pores which leads to the growth of local surface defects.

Mechanochemical Phenomena in Polymers. II. The Effects of Initiators and Inhibitors of Chain Processes

Abstract 1. The substantial effect of inhibitors and initiators of chain radical processes on the change in the properties of rubbers in milling, as well as on the resistance of vulcanizates to fatigue, has been shown. This indicates a mechanochemical mechanism for processing of polymers and in the fatigue of elastomers. The act of mechanical scission of chemical bonds, with the formation of free radicals which initiate the secondary chain processes, rests on these processes. 2. The possibility has been shown of regulating the properties of polymers during processing, and also of increasing the dynamic fatigue resistance of vulcanizates by incorporating small quantities of additives active in regard to free radicals.

Mechanochemical Phenomena in Polymers. II. The Effects of Initiators and Inhibitors of Radical Processes

Abstract 1. The substantial effect of inhibitors and initiators of chain radical processes on the change in the properties of rubbers in milling, as well as on the resistance of vulcanizates to fatigue, has been shown. This indicates a mechanochemical mechanism of the processes occurring in processing polymers and of the fatigue of highly elastic materials (e.g., rubber vulcanizates). The act of mechanical scission of chemical bonds, with the formation of free radicals which initiate the secondary chain processes, rests on these processes. 2. The possibility has been shown of regulating the properties of polymers in the course of their industrial processing, and also of increasing the dynamic fatigue resistance of vulcanizates by incorporating small quantities of additives active in regard to free radicals.