Low Fatigue Strength Research Articles

Microstructure, Mechanical, and Fatigue Strength of Ti-54M Processed by Rotary Swaging

TIMETAL 54M is a newly developed (α + β) titanium alloy with nominal composition Ti-5Al-4V-0.6Mo-0.4Fe. The alloy can provide a cost benefit over Ti-6Al-4V due to improved machinability and formability. In the present work, evolution of mechanical properties in terms of tensile and hardness values is investigated as a function of deformation degrees imposed via rotary swaging (RS). Microstructure, mechanical properties, and fatigue performance of Ti-54M are investigated after severe plastic deformation by RS conducted at 850 °C and after being subjected to two different post-swaging annealing conditions. Optical microscopy and scanning electron microscopy using electron back scatter diffraction were utilized to document the evolution of the microstructure. Tensile tests were conducted to characterize mechanical properties. RS, to a true strain of 3.0, is found to lead to a marked ultrafine-grained structure of about 1 μm grain size with low content of high angle grain boundaries (HAGBs). Post-swaging heat treatment at 800 °C followed by air cooling did not change the grain size but exhibited high content of HAGBs. Post-swaging heat treatment at 940 °C followed by furnace cooling resulted in a grain size of about 5 μm and enhanced work-hardening capability and ductility, which resulted in less fatigue notch sensitivity, but at the same time lower fatigue strength at 107 cycles.

Effect of Interlayer Thickness on Fatigue Behavior in A5052 Aluminum Alloy with Diamond-Like Carbon/Anodic-Oxide Hybrid Coating

Anodic oxide layer was formed on A5052 aluminum alloy, and subsequently diamond-like carbon (DLC) was deposited to fabricate DLC/anodic-oxide hybrid coating. Plane bending fatigue testes have been performed using the hybrid-coated specimens with different thicknesses of anodic-oxide interlayer. The interlayer thicknesses were 10 and 50 µm, where the thickness of DLC film was fixed as 3 µm. The specimens without coating, with DLC single layer and with anodic-oxide single layer were also used for comparison. The fatigue strengths of the specimens with anodic-oxide single layer were lower than those of the substrate without coating because the corner edge cracking or pin-hole defects could be fatigue crack initiation sites. However, when DLC was deposited on the anodic-oxide layer, fatigue strengths were improved. The hybrid-coated specimens with the interlayer thickness of 10 µm had higher fatigue limit than the substrate and specimens with the interlayer thickness of 50 µm. Thin DLC layer with the thickness of 3 µm could have suppressed the cracking in interlayer, and led to the improvement of fatigue strengths than the substrate. When the interlayer became relatively thicker compared with the DLC film, such as 50 µm, DLC film could not suppress the cracking from the corner edge defects, resulting in the lower fatigue strengths in spite of the presence of hybrid coating.

J0450303 炭素織維強化プラスチックの先進レーザー加工による熱損傷評価

Cross-ply carbon fiber reinforced plastics (CFRP), which is expected to reduce the weight of transportations, was cut by a CW (continuous wave) fiber laser (λ=1.06μm, 2.0kW or 3.3kW) and machining with milling. Although the high-power laser cutting processes make it possible to improve cutting rate of CFRP, the laser-cut specimens clearly showed a thermal damage with a heat-affected zone (HAZ) identified by a micro X-ray CT analysis and infrared thermography. These specimens were lower tensile strength and fatigue strength than milling-cut specimens. In laser-cut specimens, specimens cut by the 2.0 kW fiber laser exhibited the lowest strength of all processes because of large HAZ extension. While, the 3.3 kW CW fiber laser (6 m/min), which is the highest cutting speed, was minimized the HAZ extension. It could be resulted of preventing the degradation and reduction of strength.

Computational parametric study of low-velocity impact response of composite cylinder for automotive components

Composite materials have found applications in a wide range of industries, such as automotive and aerospace, because of numerous advantages including low weight, high monotonic and fatigue strengths and the possibility of manufacturing large integral shell structures. In this study, a material model was applied in order to simulate low-velocity impact analysis on filament winding composite cylinders made of epoxy resin reinforced by carbon fibres. The damage mechanisms were evaluated by a material model developed by Ribeiro et al. (2012), which was implemented as a FORTRAN subroutine (VUMAT - user material subroutine for explicit integration analyses) and linked to the ABAQUSTM software. In this investigation, several cases, such as mesh density, element type, contact algorithm and damping effects, are investigated in order to verify the influence of theses parameters. In each case, the numerical results were compared with experimental analysis. The best procedure is proposed which can serve as benchmark method in low-velocity impact modelling of composite structures for future investigations.

高強度鋼のギガサイクル疲労強度予測式に関する考察

Predictions of gigacycle fatigue strength in high-strength steel were discussed by using reported fatigue test results. The discussed prediction models were Tanaka-Akiniwa model, Murakami’s equation and a new model. The prediction based on the Tanaka-Akiniwa model overestimated the effects of inclusion sizes, resulting in much lower fatigue strength for a large inclusion than actual. The prediction based on the Murakami’s equation provided us good estimations for the effects of the inclusion sizes. However, estimated ODA sizes, which were necessary in the calculation, were imaginary since the estimated fatigue strength did not agree with the fatigue test results. The new model, proposed in this research, was a modification of the Tanaka-Akiniwa model. The new model used a new fatigue crack growth law instead of conventional Paris law. In this new model, coefficient of determination, R2 value, in the fitting of the fatigue test results was maximized to determine a new constant. The prediction based on the new model provided us much better estimations than that based on the Tanaka-Akiniwa model, and the new fatigue crack growth law showed good agreements with the measured crack growth rates for internal small cracks. Validity of the new model was thus confirmed in this research.

Mechanical Characterization of a Nickel-based Superalloy Repaired using MicroPlasma and ESD Technology

In this work, the possibility using two different technologies to repair industrial flat components made of Waspaloy superalloy was investigated. Specimens present a V notch in the central zone of the gage length, which has been refilled by MicroPlasma and Electro Spark Deposition in order to recover the original thickness of the material. These specimens have been used for a complete mechanical characterization, carrying out static, low-cycle fatigue and creep tests. Finally, results have been compared to base material. Static and low-cycle fatigue test have been carried out at Room Temperature and 538°C, while creep test considered the temperature of 704°C.Results of uniaxial tensile tests showed that the two repair process have a different effect on the mechanical properties. While MicroPlasma produced a reduced yield stress and tensile strength but a good performance whit respect to the elongation to failure, Electro Spark Deposition assured a better mechanical strength but a reduced elongation to failure.Low-cycle fatigue properties have been determined carrying out tests at different temperature (Room Temperature and 538°C). Repaired material showed lower fatigue strength and an increase of the data scatter, especially ESD at 538°C.Finally, creep test carried out on a limited number of specimens allowed establishing some changes about the creep rate and time to failure. MP behaviour was more similar to base material, while ESD show the presence of a marked tertiary creep.

Modification of creep and low cycle fatigue behaviour induced by welding

In this work, the mechanical properties of Waspaloy superalloy have been evaluated in case of welded repaired material and compared to base material. Test program considered flat specimens on base and TIG welded material subjected to static, low-cycle fatigue and creep test at different temperatures. Results of uniaxial tensile tests showed that the presence of welded material in the gage length specimen does not have a relevant influence on yield strength and UTS. However, elongation at failure of TIG material was reduced with respect to the base material. Moreover, low-cycle fatigue properties have been determined carrying out tests at different temperature (room temperature RT and 538°C) in both base and TIG welded material. Welded material showed an increase of the data scatter and lower fatigue strength, which was anyway not excessive in comparison with base material. During test, all the hysteresis cycles were recorded in order to evaluate the trend of elastic modulus and hysteresis area against the number of cycles. A clear correlation between hysteresis and fatigue life was found. Finally, creep test carried out on a limited number of specimens allowed establishing some changes about the creep rate and time to failure of base and welded material. TIG welded specimen showed a lower time to reach a fixed strain or failure when a low stress level is applied. In all cases, creep behaviour of welded material is characterized by the absence of the tertiary creep.

Low cycle fatigue assessments of corner welded joints based on local strain approach

Low cycle fatigue strengths of corner welded joints with single bevel groove welding were studied. Low cycle fatigue tests were performed with three types of specimens which have different sizes of weld root face. The test results indicated that the low cycle fatigue strength of the corner welded joints strongly depends on the weld root size. Then, the fatigue strength of the corner joints was evaluated based on the effective notch strain at the weld root tip which was calculated by elasto-plastic finite element analyses. A unique relationship between the effective notch strain and the fatigue life of the joint was observed regardless of the weld root size. Therefore, it was revealed that the effective notch strain approach is applicable to evaluate the low cycle fatigue strength of the corner welded joints.

High vitamin E content, impact resistant UHMWPE blend without loss of wear resistance.

Antioxidant stabilization of radiation cross-linked ultrahigh molecular weight polyethylene (UHMWPE) has been introduced to improve the oxidative stability of total joint implant bearing surfaces. Blending of antioxidants (most commonly vitamin E) with UHMWPE resin powder followed by consolidation and uniform radiation cross-linking is currently available for use in both total hips and total knees. It was previously shown that the fatigue resistance of vitamin E-blended and irradiated UHMWPEs could be further improved by spatially manipulating the vitamin E concentration throughout the implant and limiting cross-linking to the surface of the implant where it is necessary for wear resistance. This was possible by designing a low concentration of vitamin E on the surface and higher concentration in the bulk of the implant because cross-linking is hindered in UHMWPE as a function of increasing vitamin E concentration. In this study, we hypothesized that such a surface cross-linked UHMWPE with low wear rate and high fatigue strength could be obtained by limiting the penetration of radiation into UHMWPE with uniform vitamin E concentration. Our hypothesis tested positive; we were able to obtain control of the surface cross-linked region by manipulating the energy of the irradiation, resulting in extremely low wear, and high impact strength. In addition, we discussed alternatives of improving the oxidation resistance of such a material by using additional vitamin E reservoirs. These results are significant because this material may allow increased use of antioxidant-stabilized, cross-linked UHMWPEs in high stress applications and in more active patients.

Degradation in the fatigue strength of dentin by cutting, etching and adhesive bonding

The processes involved in placing resin composite restorations may degrade the fatigue strength of dentin and increase the likelihood of fractures in restored teeth. ObjectiveThe objective of this study was to evaluate the relative changes in strength and fatigue behavior of dentin caused by bur preparation, etching and resin bonding procedures using a 3-step system. MethodsSpecimens of dentin were prepared from the crowns of unrestored 3rd molars and subjected to either quasi-static or cyclic flexural loading to failure. Four treated groups were prepared including dentin beams subjected to a bur treatment only with a conventional straight-sided bur, or etching treatment only. An additional treated group received both bur and etching treatments, and the last was treated by bur treatment and etching, followed by application of a commercial resin adhesive. The control group consisted of “as sectioned” dentin specimens. ResultsUnder quasi-static loading to failure there was no significant difference between the strength of the control group and treated groups. Dentin beams receiving only etching or bur cutting treatments exhibited fatigue strengths that were significantly lower (p≤0.0001) than the control; there was no significant difference in the fatigue resistance of these two groups. Similarly, the dentin receiving bur and etching treatments exhibited significantly lower (p≤0.0001) fatigue strength than that of the control, regardless of whether an adhesive was applied. SignificanceThe individual steps involved in the placement of bonded resin composite restorations significantly decrease the fatigue strength of dentin, and application of a bonding agent does not increase the fatigue strength of dentin.

Reduction in Fatigue Strength of Arc Welded Aluminium 5083-H111 on Immersion in NaCl

The design of welded aluminium structures subjected to fatigue loading is usually carried out on the basis of reference fatigue curves published in design codes. The reference curves are usually relevant to dynamic loading in ambient air and make provision for the presence of a corrosive environment by downgrading the detail category for a particular joint type. This investigation studied the corrosion-fatigue behaviour of Al 5083-H111 welded using ER5356 wire. Comparison between experimentally determined fatigue curves and reference fatigue design curves published in Eurocode 9 confirms that 5083-H111 welds display significantly lower reference fatigue strengths than unwelded base material. Immersion in a NaCl solution during testing reduced the reference fatigue strengths even further. The reduction in detail category number recommended in Eurocode 9 for aluminium butt welds on immersion in sea water appears suitable (or even marginally conservative) for the 5XXX series Al-Mg-Mn welds examined in this investigation.

The Effect of Pitting Corrosion on the Fatigue Strength of 304 and 316 Stainless Steel Alloys

Pitting corrosion behavior of 304 and 316 stainless steel alloys was investigated in 3.5% NaCl solution containing 100 ppm thiosulfate ion as pitting corrosion initiator. Experiments were performed using potentiodynamic polarization and potentiostatic techniques at room temperature. The shape, size, and pit depth of the formed corrosion pits were also determined using the metallurgical light microscope. The fatigue strength of the pitted samples (either single or multipitted) was determined using plain-bending fatigue test machine. The fracture surface was also investigated using scanning electron microscope (SEM). The results show that SS 304 is more susceptible to pitting corrosion and has lower fatigue strength than SS 316 for the unpitted alloys samples. For both alloys, the single pitted samples shows that a deterioration percentage in fatigue is of about 15% while the multi pitted samples shows a deterioration percentage in fatigue of about 33% compared to the unpitted samples. The crack through the specimen surface was noticed to be initiated from the pit and propagates in two perpendicular sides to each other.

S0410202 転造加工によるSCM415焼結合金歯車の歯元曲げ疲労強度の向上([S041-02]粉末成形とその評価(2),機械材料・材料加工部門)

Sintered materials have been widely used in various industrial fields because of their higher performance properties and lower production costs. However, it has been still difficult to employ sintered material for high strength parts such as transmission gears of vehicle. Since many pores remain in the sintered body, resulted in lower fatigue strength. In this paper, rolling process is applied to sintered SCM415 gear to overcome this issue. Tooth surface on the sintered gear was densified by the rolling process. The surface area is the most critical parts of the gear, so that fatigue strength of the gear could be improved by the densification. As a result of the densification process, higher fatigue strength as same as that of wrought material was obtained.

Mean stress sensitivity of ductile iron with respect to technological and statistical size effect considering defects

Specimens of two sizes have been taken from two sampling locations within a wind turbine hub made of nodular cast iron (EN-GJS-400-18-LT) for constant amplitude fatigue testing. The sampling positions exhibit varying cooling conditions, resulting in different microstructures. Fatigue tests have been carried out at R-ratios of R = −1 and R = 0. The coarse microstructure as well as the larger specimens yielded in lower fatigue strengths. No effect of the microstructure or the specimen size on the mean stress sensitivity has been found. Fractographic analysis of the fractured specimen's surface revealed micro-shrinkages to be the source of crack initiation for all specimens. Micro-shrinkage size increases from fine to coarse microstructure and with increasing specimen size. The El-Haddad equation using the √area parameter was used to describe the fatigue limit. The results were in good agreement with the experiments.

Mechanisms of fatigue-crack initiation and their impact on fatigue life of AlSi7 die-cast components

In the course of the present study, in-situ observations of crack initiation and crack growth of naturally induced cracks in cyclically loaded specimens along with conventional fatigue tests and fracture surface analyses were performed. The specimens used were taken from different sampling positions of standard and HIPed aluminum-die-cast engine blocks, with different cooling conditions. In one sampling position within the standard engine block microporosity was able to form, acting as a source for fatigue-crack initiation. While in the absence of microporosity, as observed in specimens taken from HIPed components, crack initiation occured via slip band mechanism. If material defects such as pores were present, premature crack initiation reduced the fatigue life yielding a lower fatigue life and fatigue strength than specimens where cracks formed by slip band mechanism. For cracks formed at pores, the pore size is the determining factor for fatigue behavior. While for cracks initiated via slip band mechanism fatigue strength is a function of the local material strength.

Fatigue crack initiation and damage mechanisms during ultrasonic fatigue testing of cast aluminium alloy AlSi7Mg

During the last years, an increased interest in in situ-damage characterization during ultrasonic fatigue testing evolved. Different non-contact in situ-surveillance systems have been developed or applied to the present ultrasonic fatigue testing systems, e.g. analysis of the nonlinearity parameter and the resonant frequency [1, 2] as well as thermal measurements [3–5]. In addition, Heinz et al. [6] performed noncontact strain measurements during ultrasonic fatigue testing. The aim of the present investigations is to evaluate the fatigue crack initiation in the cast aluminium alloy AlSi7Mg (A356) in sand and die cast condition under high cycle and very high cycle fatigue loading conditions. Therefore, a combined analysis of the development of the specimen temperature, the resonant frequency fres and the nonlinearity parameter rel was performed. For such investigations it is essential to keep the ambient temperature strictly constant and the increase of temperature due to the resonant conditions as low as possible. Therefore, a spot cooler using a vortex tube was applied to the ultrasonic fatigue testing system. With this special cooling device it was possible to keep the ambient temperature constant. An infrared thermo-camera system was used for the in situ-examination of the specimen temperature and the surveillance of the ambient temperature during ultrasonic fatigue testing. The determination of the resonant frequency and the nonlinearity parameter was realized online during fatigue tests according to Mayer et al. [2]. After failure, the fracture surfaces of the specimens were investigated intensively by scanning electron microscopy (SEM). According to Krewerth et al. [5], the correlation of the results from thermographic and fractographic investigations can shed some light on the crack initiation point. Figure 1a shows the S-N curves of the fatigue tests on the aluminum specimens produced by sand casting (batch 1) and die-casting (batch 2) in hot isostatically pressed condition, respectively. Specimens of both batches failed due to casting porosity. However, the casting porosity in the sand cast material was significantly higher leading to a lower fatigue strength. Several specimens were investigated by thermographic measurements accompanied by simultaneous analysis of the resonant frequency as well as the nonlinearity parameter. Exemplarily, specimen 1 (sand casting, see Fig. 1a) is explained here in more detail. The development of the resonant frequency (fres), the nonlinearity parameter ( rel), the ambient temperature Tamb, the maximum temperature Tmax and the temperature variation T is shown in fig. 1b. This specimen failed due to an internal defect at Nf = 2.16 × 106 cycles. The dashed black

Effect of Grain Orientation on Fatigue Behavior in Micro Cantilever of Magnesium Alloy AZ31

The micro scaled fatigue tests have been conducted using the micro cantilever-shaped specimens to investigate the effects of scale and grain orientation on the fatigue properties of magnesium alloy AZ31. The micro cantilevers were fabricated from a single crystal by the focused ion beam (FIB) milling. The grain orientations were analyzed by an electron backscatter diffraction (EBSD) method. The micro cantilever exhibited the higher fatigue strength than the bulk material but the scatter was large. The cantilever fabricated from a crystal whose Schmid factor in basal slip plane was high exhibited the low fatigue strength, and on the surface after the fatigue test, the slip lines were observed parallel to the basal slip plane. On the other hand, the cantilever with low Schmid factor in basal slip plane exhibited the high fatigue strength and the slip lines on the surface were bended, which indicates that the slips and twins in the plural planes were active.

Passive Damping Treatment on Aircraft's Non-Flush Patch Repair Scheme

The majority of aircrafts structure cracks due to fatigue are thin gage metallic structures and secondary support structures. Beside lower fatigue strength, they may also occur due to mechanical defects, especially at weak area and for areas exposed to high repetitive force such as vibration, friction, acoustic load and tensile force. Normally, the Original Equipment Manufacturer (OEM) recommended the aircraft operator to repair the crack by typical patch or doublers repair technique which is formally known as Non-Flush Patch RepairScheme. However, the crack tends to reappear and tend to be more severe. The natural frequency and damping ratio of a structure which is treated with Typical Non-Flush Patch Repair, with and without Passive Damping Treatment (PDT) are studied using impulse based technique. Experimental data are presented to demonstrate the difference of damping ratio (ζ) among the test specimens. The cracked structure which has been treated with PDT on Non-Flush Repair Scheme has a better damping ratio and resonates at a different natural frequency. The experiment data generated from this study can be used as a basis to further the study on aircrafts structural repair scheme.

Fatigue strength evaluation of welded structural details in corrugated steel web girders

Fatigue strength evaluation of welded structural details is of practical significance in the design of corrugated steel web girders in highway bridges and industrial structures. In this paper, the fatigue strengths of corrugated steel web girders with several welded structural details and welding methods are analytically examined by fatigue tests of corrugated web beams and small-size welded joints. The stress concentration & distribution characteristics of corrugated web beams were analysed using finite element analysis. The beam test results showed that the structure with scallops or notches in the flange has lower fatigue strength and that with butt joints is prone to suffer from shear crack on the corrugated web. Within the inside range of the scallop, the stress concentration becomes greater with the increase of scallop radius and the tension flange contributes significantly to the bending capacity of corrugated steel web beams. The tests for small-size welded joints indicated their applicability in the analysis and prediction of S-N relationship of the test corrugated steel web beams. Finally, the fatigue crack propagation lives of weld joints were compared with those of test corrugated steel web beams with respect to related design Categories of the AASHTO LRFD Bridge Design Specifications.

New Era in Denture Base Resins: A Review

AbstractIn past, the materials used for denture bases were vulcanite, celluloid & phenol formaldehyde. Acrylic resins were introduced to dentistry in 1930's. There are certain limitations of acrylics like residual monomer allergy, poor mechanical strength, low fatigue strength, brittle on impact, poor conductors of heat, low hardness, high coefficient of thermal expansion, thermal shrinkage, poor color stability of self-cured resins, porosity, crazing, warpage, poor adhesion to metal and porcelain and requirement of mechanical retention. But still they are the most widely used denture base materials till date. But recently there has been much advancement in the field of denture base resins to overcome their drawbacks and to obtain an ideal denture base material. This article reviews the various advancements in the field of denture base resins.