Reversed Bending Fatigue Research Articles

Fatigue Failure of a Distortion Screen during Testing

Abstract A distress was observed in a distortion screen during testing. This screen is positioned in front of an aero engine during testing to check the engine performance with a distorted air intake. To ensure a distorted air intake, the screen contains dense meshes in some areas whereas in other areas they are less dense. Non-uniformity in the mesh density results in distorted air flow into the engine thereby facilitating evaluation of the engine performance with disturbed aerodynamic inlet. The component, i.e. the distortion screen was under development and was subjected to aerodynamic tests on the test bench. It was found to have fractured from a rib connecting two meshes. Loose end of the mesh had detached and resulted in permanent deformation of the adjoining ribs and meshes. The fractured surface exhibited glossy crystalline appearance. Scanning electron microscope revealed striations at higher magnifications (>500X) as was observed in the case of fatigue failure. Two crack fronts were adjacent and near to one corner of the rib, and the third crack front had originated from the other corner placed diagonally opposite to the other two crack fronts. This observation indicated failure of the screen under reverse bending fatigue condition. Design and material condition were reviewed. After modification it was found that the newly modified distortion screen did not show any sign of deformation after testing.

Dissipated thermal energy and damage evolution of Glass/Epoxy using infrared thermography and acoustic emission

In this paper, we present an experimental approach for characterizing energy dissipation and degradation evolution in a woven Glass/Epoxy (G10/FR4) laminate subjected to fully reversed bending fatigue test. During cyclic loading, a fraction of the input mechanical energy is converted to thermal energy, which results in an increase in the temperature of the specimen. By analyzing the surface temperature and its drop rate after halting the cyclic operation, the dissipated thermal energy (DTE) is estimated. Infrared thermography is used to assess the temperature evolution and to various damage states. Acoustic emission is also utilized to corroborate the thermography results in characterizing the degradation progression. The results of these two non-intrusive techniques show similar evolutionary response revealing the existence of degradation stages. Using calculated DTE, a damage growth model is developed that appropriately characterizes the three damage phases during fatigue process of Glass/Epoxy.

Effect of Hardness upon Temperature Rise of Steel Specimens during Bending Fatigue by Using Infrared Thermography

In this study, the effects of hardness on temperature increase of ST 37 steel during fatigue loading were investigated. Steel specimens are made of ST 37 steel and subjected to heat treatment to obtain different hardness. The specimens were subjected to reverse bending fatigue loading and the specimens were observed by using a infrared (IR) camera during the test. The obtained thermal images were recorded by FLIR E45 IR camera and then transferred to the image processing program developed by using MATLAB. Thus after image processing, thermal values used to detect the temperature rise of the surface of the steel specimen under fatigue loading were obtained. During the fatigue, the material is subjected to strain energy input which result in plastic or/and elastic deformation. This event results in an increase of temperature within material. The energy conservation requires that the generated heat shows itself as heat transfer by conduction, convection and radiation and internal energy increase. Besides, if the material has undergone plastic deformation, an additional term which accounts this effect should be included within energy conservation equation. In order to observe the effect of plastic deformation upon temperature increase of material, the ability of plastic deformation has been changed through the change of hardness and the thermal variations during fatigue has been investigated.

Loading-Frequency Effects on Fatigue Crack Growth Behavior of a Low Carbon Steel JIS S10C in Hydrogen Gas Environment

In order to clarify the loading-frequency effect on the fatigue crack growth behavior of a low carbon steel JIS S10C in a hydrogen gas environment, fully reversed bending fatigue tests were carried out. The main results are as follows. The loading-frequency effect on the FCG revealed a complex behavior; that is, not only acceleration, but also deceleration even in the same low loading-frequency range. The slight acceleration appears in the low growth rate range in which the ductile fracture mode is predominant. The deceleration appears due to the transition behavior from a quasi-cleavage fracture mode with a higher FCGR to a ductile one with a lower FCGR. This shows that lowering the load frequency does not necessarily lead to an unpredictable fatigue crack growth.

Testing and Modeling of a New Moment Connection of Concrete-Filled FRP Tubes to Footings under Monotonic and Cyclic Loadings

This study explores a new moment connection of concrete-filled fiber-reinforced polymer (FRP) tube (CFFT) to concrete footing. The tube is tightly fitted and adhesively bonded to a short reinforced concrete stub protruding from the footing, which facilitates concrete filling of the tube without the need for shoring. To establish the critical stub length (Xcr), specimens with heavily steel-reinforced stubs varying in length from 0.5D to 2.0D, where D = diameter of the CFFT, were fabricated and tested in flexure by using a cantilever setup. The Xcr required to achieve flexural failure of the CFFT was 1.05D. Additional specimens with a sufficient stub length of 1.5D were then fabricated to examine the effect on strength and ductility of the steel reinforcement ratio (ρs) in the stub. The optimal ρs of the stub required for the CFFT to reach flexural failure was 3.2%. Finally, the effect of low-cycle reversed bending fatigue was studied with and without an axial compression load. Remarkable ductility associat...

Plain fatigue resistance of shot peened high strength aluminium alloys: Effect of loading ratio

The effect of different shot-peening treatments on the reverse and pulsating bending fatigue behaviour of Al 7075 T651 was studied. The fatigue improvements with respect to the unpeened condition and the influence of the peening intensity on fatigue were discussed accounting for the effects of surface modifications and residual stresses. In particular, the extent of the residual stress redistribution during loading was investigated by means of X-ray diffraction (XRD) measurements. No significant residual stress relaxation was observed in samples tested to a load level corresponding to the fatigue endurance at 5⋅106 cycles. Residual stress relaxation was observed only when the material plastic flow stress was achieved during the compressive part of the fatigue load cycle. Accordingly, shot peened samples with deep sub-superficial compressive residual stress peak showed a reversed fatigue endurance level corresponding to the condition of incipient plastic flow. This phenomenon was also accompanied by subsuperficial fatigue crack initiation. On the contrary, samples tested at shorter fatigue lives or under pulsating loading conditions showed crack initiation close to the surface. The initial and the stabilised residual stress profiles were considered for discussing the improvement in the fatigue behaviour due to peening. For this purpose, a multiaxial fatigue criterion was adopted to account for the biaxial residual stress field. The fatigue life was quite accurately predicted as long as fatigue initiation occurs on the surface.

Repair Welding Effects on the Bending Fatigue Strength of AISI 4130 Aeronautical Steel Used in a Critical to the Flight-Safety Structure

The aim of this study was to analyze the effect of successive TIG (tungsten inert gas) welding repairs on the reverse bending fatigue strength of AISI 4130 steel, which is widely used in components critical to the flight-safety. In order to simulate the abrupt maneuvers, wind bursts, motor vibration and helixes efforts, which generate cyclic bending loadings at the welded joints of a specific aircraft component called "motor cradle", experimental reverse bending fatigue tests were carried out on specimens made from hot-rolled steel plate, 1.10 mm (0.043 in) thick, by mean of a SCHENK PWS equipment, with load ratio R = -1, under constant amplitude, at 30 Hz frequency and room temperature. It was observed that the bending fatigue strength decreases after the TIG (Tungsten Inert Gas) welding process application on AISI 4130 steel, with subsequent decrease due to re-welding sequence as well. Microstructural analyses and microhardness measurements on the base material, heat-affected zone (HAZ) and weld metal, as well as the effects of the weld bead geometry on the obtained results, have complemented this study.

Considerations on corrosion and weld repair effects on the fatigue strength of a steel structure critical to the flight-safety

The aim of this study is to analyze the effects of corrosion and successive tungsten inert gas (TIG) welding repairs on the reverse bending fatigue strength of AISI 4130 steel used in components critical to the flight-safety. The tests were performed on hot-rolled steel plate specimens, 1.10 mm and 1.60 mm thick, by means of a SCHENK PWS equipment, with load ratio R = −1, constant amplitude, 30 Hz frequency and room temperature. It was observed that the reverse bending fatigue strength of AISI 4130 steel decreases due to the corrosion and the TIG welding and re-welding processes.

Numerical Simulation of Residual Stress Relaxation in Shot Peened High-Strength Aluminum Alloys Under Reverse Bending Fatigue

The mechanism of the residual stress relaxation during the fatigue life of shot peened high-strength aluminum alloys was investigated. Experiments were conducted on specimens subjected to three different shot peening treatments and tested under reverse bending fatigue. x-ray diffraction (XRD) measurements were carried out to determine the initial and stabilized residual stress fields. The residual stress field created by the surface treatments has been introduced into a finite element (FE) model by means of a fictitious temperature distribution. The elastic-plastic response of the superficial layers affected by the shot peening treatments has been derived through reverse strain axial testing combined with microhardness tests and implemented in the FE model. The proposed numerical/experimental approach is able to satisfactorily predict the residual stress field evolution. Notably, relaxation has been correctly simulated in the low-cycle fatigue regime and imputed to plastic flow in compression when the superposition of compressive residual and bending stresses exceeds the local cyclic yield strength of the material. Conversely, the residual stress field remains stable at load levels corresponding to the 5×106 cycles fatigue endurance.

水素ガス中における低炭素鋼S10Cの疲労き裂伝ぱ挙動に及ぼす繰返し速度の影響

In order to clarify the loading-frequency effect on the fatigue crack growth behavior of a low carbon steel JIS S10C in a hydrogen gas environment, fully reversed bending fatigue tests were carried out. The main results are as follows. The loading-frequency effect on the FCG revealed a complex behavior; that is, not only acceleration, but also deceleration even in the same low loading-frequency range. The slight acceleration appears in the low growth rate range in which the ductile fracture mode is predominant. The deceleration appears due to the transition behavior from a quasi-cleavage fracture mode with a higher FCGR to a ductile one with a lower FCGR. This shows that lowering the load frequency does not necessarily lead to an unpredictable fatigue crack growth.

Reverse bending fatigue of shot peened 7075-T651 aluminium alloy: The role of residual stress relaxation

The effect of different shot peening treatments on the reverse bending fatigue behaviour of Al-7075-T651 was studied. The fatigue improvements with respect to the unpeened condition and the influence of the peening intensity on fatigue were discussed accounting for the effects of surface modifications (roughness and strain hardening) and of residual stresses. In particular the extent of the residual stress redistribution during loading was investigated by means of X-ray diffraction (XRD) measurements. No significant residual stress relaxation was observed in samples tested to a load level corresponding to the fatigue endurance at 5 · 10 6 cycles. Residual stress relaxation was observed only when the material plastic flow stress was achieved during the compressive part of the fatigue load cycle. Accordingly, shot peened samples with deep sub-superficial compressive residual stress peak showed a fatigue endurance level corresponding to the condition of incipient plastic flow. This phenomenon was also accompanied by sub-superficial fatigue crack initiation. On the contrary, samples tested at shorter fatigue lives showed crack initiation close to the surface. The initial and the stabilized residual stress profiles were considered for discussing the improvement in the fatigue behaviour due to peening. For this purpose, a multiaxial fatigue criterion was adopted to account for the biaxial residual stress field. The fatigue life was quite accurately predicted as long as fatigue initiation occurs on the surface.

The surface treatment influence on the fatigue crack propagation of Al 7050-T7451 alloy

The objective of this research is to evaluate 100 μm thickness hard chromium electroplated coating on the fatigue crack propagation of Al 7050-T7451 alloy. The shot peening process was carried out to create residual stresses using ceramic and glass shots. Reverse bending fatigue tests were performed with base material, base material chromium electroplated and base material shot peened and chromium electroplated. In order to study the influence of residual stresses on fatigue life, the compressive residual stress field was measured by an X-ray diffraction method. Scanning electron microscopy technique was used to analyze fracture surface and identify crack origin sites. Glass shot peening results are better with respect to fatigue crack nucleation and propagation in comparison to ceramic shot peening. An overpeening may have occurred which produced damages on the ceramic shot peened surface.

Development of a Reversible Bending Fatigue Test Bed to Evaluate Bulk Properties Using Sub-Size Specimens

Abstract A non-standard fully reversible bending fatigue test bed of fixed displacement amplitude type was designed. A sub-size sample similar in design to the standard ASTM B593 sample was used to evaluate the high cycle bend fatigue behavior of 7075-T6 sheet specimens. Fatigue life was determined at four stress levels of 300, 240, 220, and 190 MPa at a stress ratio of −1.

Tension and fatigue behavior of silver-cored composite multi-strand cables used as implantable cables and electrodes

The mechanical behavior of a variety of cable architectures comprised of silver-cored wires was evaluated in uniaxial tension, and in cyclic strain-controlled fatigue with the use of a flex tester operated to provide fully reversed bending fatigue. The magnitude of cyclic strains imparted to each cable tested was controlled via the use of different diameter mandrels. Smaller diameter mandrels produced higher values of cyclic strain and lower fatigue life. Multiple samples were tested and analyzed via scanning electron microscopy. The fatigue results were analyzed via a Coffin–Manson–Basquin approach and compared to fatigue data obtained on 316LVM cables where testing was conducted in an identical manner. The effects of changes in wire diameter, cable architecture, and wire composition (i.e. silver-cored vs. 316LVM) are discussed.

Microstructural effects on tension and fatigue behavior of Cu–15Ni–8Sn sheet

The effects of systematic changes in microstructure/heat treatment on the mechanical behavior of Cu–15Ni–8Sn (Pfinodal ®) were evaluated in uniaxial tension and in cyclic strain-controlled fatigue with the use of a modified GE Flex Tester operated to provide fully reversed bending fatigue. The magnitude of cyclic strains was controlled via the use of different diameter mandrels. Smaller diameter mandrels produced higher values of cyclic strain and lower fatigue life. Multiple samples were tested and fracture surfaces were analyzed via scanning electron microscopy. The fatigue results were analyzed via a Coffin–Manson–Basquin approach and compared to fatigue data obtained from the literature, where testing was conducted on similar materials. Significant effects of cold work and heat treatment on both the tension and fatigue behavior were obtained and rationalized based on the mechanisms of failure, revealed by SEM, for this system.

Failure analysis of oxygen hose of the lance equipment in LD shop of Tata steel

The hose of the oxygen lancing equipment in LD shop # 2 has failed at the weld–bellows interface by reverse bending fatigue. The bending stresses were resulted due to inclined fitting of the hose at the fixed flange end. No metallurgical abnormalities or weld defects or mechanical damages were responsible for the fatigue crack initiation. It is recommended that if inclined fitment of the hose is not a process requirement, the original design be restored to realize the expected life of the component. On the other hand, if inclined fitment cannot be avoided, additional support may be provided for the hose at the fixed flange end to avoid reverse bending at the weld–bellows interface. A detailed analysis of the failure is presented in this report.

Tension and fatigue behavior of 316LVM 1 × 7 multi-strand cables used as implantable electrodes

The mechanical behavior of 316LVM 1 × 7 cables were evaluated in uniaxial tension and in cyclic strain-controlled fatigue with the use of a Flex tester operated to provide fully reversed bending fatigue. The magnitude of cyclic strains imparted to each cable tested was controlled via the use of different diameter mandrels. Smaller diameter mandrels produced higher values of cyclic strain and lower fatigue life. Multiple samples were tested and analyzed via scanning electron microscopy. The fatigue results were analyzed via a Coffin–Manson–Basquin approach and compared to fatigue data obtained from the literature where testing was conducted on similar materials, but under rotating bending fatigue conditions.

Influence of anodization on the fatigue strength of 7050‐T7451 aluminium alloy

ABSTRACTIn recent years, with higher demand for improved quality and corrosion resistance, recovered substrates have been extensively used. Consequently residual stresses originated from these coatings reduce the fatigue strength of a component. Due to this negative influence occasioned by corrosion resistance protective coatings, an effective process like shot peening must be considered to improve the fatigue strength. The shot peening treatment pushes the crack sources beneath the surface in most of medium and high cycle cases due to the compressive residual stress field (CRSF) induced. The aim of this study was to evaluate the influence on the fatigue life of anodic films grown on 7050‐T7451 aluminium alloy by sulphuric acid anodizing, chromic acid anodizing and hard anodizing. The influence on the rotating and reverse bending fatigue strength of anodic films grown on the aluminium alloy is to degrade the stress life fatigue performance of the base material.A consistent gain in fatigue life in relation to the base material was obtained through the shot peening process in coated specimens, associated to a residual stress field compressive near the surface, useful to avoid fatigue crack nucleation and delay or even stop crack propagation.

Fatigue crack growth behaviour in ferritic ductile iron with surface crack under reverse bending

In this paper, fatigue crack growth behaviour of ferritic ductile cast iron containing 3.26%C and 4.18%Si was studied. Materials were produced and reverse bending fatigue test specimens were cut from the casting with the thickness of 24 mm. Process annealing heat treatment was applied to the specimens. Semi-elliptical surface cracks were formed and reverse bending fatigue tests were carried out. Fatigue crack growth rates were determined. Stress intensity factors (Δ K) were obtained by Newman–Raju’s equations. Fatigue crack growth material constants ( n and c) were determined in Paris–Erdogan’s equation. Fractured surfaces of the specimens were examined by using SEM.

Fatigue behaviour of graphite and interpenetrating graphite–aluminium composite up to 10 9 load cycles

Fatigue behaviour of polycrystalline, isotropic graphite FU2590 (porosity 13 vol%) and of FU2590 infiltrated with the aluminium alloy AlSi7Mg (FU2590/AlSi7Mg) is investigated. The interpenetrating graphite–aluminium composite is produced by squeeze casting infiltration, where the open porosity of FU2590 (10–11 vol%) is infiltrated with AlSi7Mg. Fully reversed bending fatigue tests at cycling frequency 25 Hz and fully reversed tension–compression experiments at 20 kHz are performed. Fatigue data of FU2590 measured with both methods are comparable when presented vs. the normalized maximum stress, i.e. maximum tension stress of a cycle divided by the bending or tensile strength, respectively. The mean endurance limit of FU2590 at 10 9 cycles under tension–compression loading is at 23.0 ± 0.9 MPa (normalized maximum stress 0.65 ± 0.03), and 26.1 ± 1.5 MPa (normalized maximum stress 0.55 ± 0.03) is found for FU2590/AlSi7Mg. The increased endurance limit of FU2590/AlSi7Mg is mainly caused by the 15% higher stiffness compared with FU2590. In reversed bending experiments, at cycles to failure below 10 7, the composite shows about 30% higher cyclic strength. The beneficial influence of infiltration on static properties is more pronounced, and 50% higher bending strength and 35% higher tensile strength are determined. In the graphite, fatigue cracks predominantly initiate at porosity. In the composite, cracks initiate at pores, which are filled with AlSi7Mg. Interface failures and failures of the aluminium alloy limit the beneficial influence of infiltration on the cyclic properties.