Reversed Bending Fatigue Research Articles

Application of inverse heat transfer to fatigue fracture

Cyclic actuation tends to cause self-heating in the material as the structure experiences fatigue, where the movement and coalescence of defects lead to crack formation, propagation, and eventual fracture. This study explores the modeling aspects of the self-heating phenomenon using a one-dimensional inverse heat problem to analyze heat generation and dissipation to quantify the plastic work rates needed for predicting fatigue life. The approach is based on analyzing the surface thermography obtained using an infrared camera and numerically solving an inverse Fourier heat conduction equation. Formulation of the inverse problem via constrained optimization and method of solution for analyzing the fatigue behavior of CS 1018 flat dog bone specimens subjected to fully reversed bending fatigue are presented. The proposed model demonstrates superior accuracy in predicting the location of maximum heat generation and identifying potential fracture zones compared to traditional methods.

Effect of surface roughness and polishing orientation on fatigue life and fracture fatigue entropy

AbstractThe effect of surface roughness and polishing orientation on fatigue life is investigated. Fully reversed bending fatigue tests are conducted on ASTM A1008 specimens having surface roughness asperities ranging from mirror finish (Ra = 0.05 μm) to coarse (Ra = 1.5 μm). Two polishing directions (i.e., orientations) are examined. For the range of roughness values tested, the results suggest that polishing orientation has a strong influence on fatigue life. Specifically, within the range of surface roughness values tested, fatigue life was found to be relatively insensitive to surface roughness asperities when the roughness orientation was parallel to the direction of surface stress but sensitive to surface roughness asperities when the orientation crossed the direction of surface stress. Results are generalized using the concept of fracture fatigue entropy (FFE). It is shown that, unlike fatigue life, FFE is independent of polishing orientation, a noteworthy result. To explain the behavior of FFE in relation to surface roughness asperities, a fatigue thermodynamics theory is presented that hypothesizes that all specimens of the same material fail at equivalent entropy levels.

Flexural Bending and Fatigue Analysis of Functionally Graded Viscoelastic Materials: Experimental and Numerical Approaches

This work synthesized a thermoplastic polymer with varying densities along one direction using additive manufacturing technology to study the dynamic and static characteristics of functionally graded viscoelastic materials (FGVMs). To describe the mechanical properties of FGVMs, an analytical formulation based on the sigmoid-law formulation was proposed. The experimental program is conducted on 3D-printed samples, and various tests are conducted to examine the performance of such materials. Furthermore, the finite element method was used to evaluate the structural system's flexural properties. The influences of FG parameters and geometrical properties on flexural and reverse bending fatigue life are analyzed. The results show that increasing porosity from 10% to 30% at a power-law index (k = 2) reduces bending strength by 31.25 percent and deflection by around 11.2 percent for VE samples. Changing the power-law exponent from 0.5 to 10 increases fatigue strength by 35 %.

Toward superior fatigue and corrosion fatigue crack initiation resistance of Sanicro 28 pipe super austenitic stainless steel

The present work deals with the fatigue and corrosion-fatigue crack initiation in cold-pilgered Sanicro 28 super austenitic stainless steel. Toward this end, the stress-controlled symmetric reverse bending fatigue tests were conducted in air and mixed acid aqueous solution under the stress amplitude of 200–570 MPa with the stress ratio of −1 and frequencies of 1 and 15 Hz at ambient temperature. The fatigue study was supplemented with electrochemical experiments and advanced microstructural analyses to explore the damage characteristics in detail. The planar character of dislocation substructure owing to the low stacking fault energy of the material, and formation of parallel slip bands led to a more homogenous distribution of the imposed strain which retarded the progress of damage accumulation. According to the X-ray photoelectron spectrometer results, the formation of the insoluble and stable MoO3 oxide increased the rate of repassivation. This sticky oxide resisted against breakdown rendered by the attack of aggressive chloride ions. The corrosion fatigue behavior of the experimented alloy under the various test frequencies was discussed relying on the synergistic effects of mechanical and environmental damages.

The fracture load analysis of different material thickness in adhesively bonded joints subjected to fully reversed bending fatigue load

The adhesive joints used in airframe and wing sections of aircrafts are generally subject to dynamic (fatigue) loading rather than static loading due to environmental conditions. The dynamic loading formed on wing sections of aircraft is generally in the form of fully reversed fatigue. This study experimentally and numerically investigated the lifetimes of single-lap joints (SLJs) of five different adherend thicknesses (2, 3, 4, 5 and 6 mm) – obtained using AA2024-T3 aluminum alloy and Araldite-2015 structural adhesive – under fully reversed bending fatigue loads. Accordingly, before performing fatigue tests, static bending strength tests were carried out on the SLJs to determine the maximum loads to be used for bending fatigue tests and also to evaluate their static performances. Fatigue tests were performed at a loading ratio (R) of −1 and a frequency of 4 Hz. It was determined that the static bending strength of the joints increased by approximately between 43% and 74% with increasing adherend thickness. However, this increase was not the same as the increase in material thickness. In addition, the increased adherend material thickness changed (decreased or increased) the maximum load applied to the joints over infinite cycles (taken to be 2 × 105 cycles). This was because the change in material thickness changed the bending moment formed in the joint and the flexural rigidity of the material.

Failure Analysis of Bolt of Rear Mounting Trunion of an Aero-Engine: A Case of Bending Induced Chevron Pattern as well as Fatigue Failure on the Same Fracture Surface

Present work describes the failure investigation of failed bolt of starboard rear mounting trunion of an aero-engine. Multiple fracture initiation points are noticed. This is a classic case of a single bending type of load initiating reversed bending fatigue as well as chevron pattern on the same fracture surface. Visually observed bending phenomenon supports the each type of failure mode. More interestingly, point of initiation of fast as well as the reversed bending fatigue failure is the same, although those two events have been found to be separate phenomena. It has been established that two different fatigue crack fronts, typical of reversed bending fatigue phenomenon propagated towards each other to make half of the cross-section fractured, while the another half failed by chevron patterned fast fracture. In this, the fast fracture of one half has preceded the reversed bending fatigue fracture of the other half, although the former is not responsible for happening of the later. Modes of fracture and factors influencing have been established in this article with emphasis on circumstantial evidence involving background information and visual examination, supported often by the open literature. Presence of cadmium (Cd) and its possible source, residence time and relative presence on differently fractured surfaces have offered important clues on establishing the sequence and relative inter-dependence of the said two fracture types. Presence of cadmium on the fracture surfaces, multiple crack initiation sites and numerous well-revealed secondary cracks on Branson cleaned fracture surface indicate that the cracks pre-existed on the material even before the cadmium plating and manifestation of chevron pattern is its extreme revelation. This pre-existing chevron pattern primarily aggravated the present failure through bending fatigue phenomenon in the later stages. Low alloy steel (ASTM grade 16) with presently used hardness (340 HV) level does not seem to suit the present application, as it is clear from its extreme brittleness as manifested by pre-existing cracks.

Application of thermoelectricity in fatigue of metals

AbstractAn online approach is proposed to predict the fatigue life of metallic specimens under cyclic loading using the concept of thermoelectricity. The working principle of the methodology is described, and experimental results are performed with a fully reversed bending fatigue testing apparatus to evaluate the efficacy of the approach. Results are presented that correlate the measured thermoelastic potential for low‐carbon steel 1018 specimens measured via an infrared camera. It is shown that the application of thermoelectricity provides a practical methodology for early prediction of fatigue.

Failure analysis of fretting fracture in frame rails of heavy duty trucks – Case study

This paper analyzed fatigue fracture of a frame rail during accelerated endurance test of a heavy duty truck. This fracture was noted in an area of high shear stresses where contact and fretting fatigue was suspected. A cross section through the fracture initiation site showed areas of surface contact wear in the microstructure. Fretting wear resulted in crack initiation and propagated to form a through crack under reverse bending fatigue crack growth mechanism. A combination of crack growth simulation tool (FRANC3D) and finite element analysis solver (MSC NASTRAN) was used and automated to handle multiple crack growth iterations. The simulation results obtained showed good correlation to failure crack path and cycles to failure. These simulation results were also used to understand the root cause of failure, identify critical parameters causing failure and recommend design modifications to prevent fretting fatigue in frame of heavy duty trucks. A modified design using the proposed simulation methodology was identified and was shown to provide improved frame durability.

Characterization of High-Cycle Bending Fatigue Behaviors for Piston Aluminum Matrix SiO2 Nano-composites in Comparison with Aluminum–Silicon Alloys

In automotive industries, one failure mechanism in engine pistons is due to the fatigue phenomenon. Therefore, to enhance fatigue properties of piston aluminum alloys is a major concern for designers. One reinforcement method could be the addition of nano-particles in the aluminum matrix. In this article, high-cycle fatigue properties of the aluminum matrix nano-composite were characterized under bending loadings and then compared to those of the aluminum–silicon alloy. For this objective, fully reversed bending fatigue tests were performed on standard specimens, based on the ISO-1143:2010 standard. Before testing, nano-composite samples were stir-casted by the addition of 1 wt% SiO2 nano-particles, and aluminum specimens were gravity-casted in a cast-iron mold. The microstructure of materials and the distribution of nano-particles in the aluminum matrix were evaluated by the optical microscopy and the field emission scanning electron microscopy. Experimental data indicated that nano-particles had a significant effect on the high-cycle fatigue lifetime. The reason for this improvement in high-cycle fatigue properties could be finer grains, higher hardness, the proper distribution of nano-particles in the aluminum matrix and stronger bonding strength at the Al/Si interface. However, based on fracture surfaces, all samples had the brittle behavior due to cleavage and quasi-cleavage marks.

Resistance to mechanically small fatigue crack growth in ultrafine grained interstitial-free steel fabricated by accumulative roll-bonding

Effects of ultrafine grain refinement on fatigue crack growth were investigated using an interstitial-free (IF) steel with a grain size of 590 nm produced by accumulative roll bonding. The fatigue properties and associated microstructures were characterized by fully reversed bending fatigue tests, replica method coupled with optical microscopy, and electron backscattering diffraction measurements near the fracture surfaces. Compared with a coarse-grained IF steel tested at the same stress amplitude, the fatigue strength of ultrafine-grained steel was higher, which was attributed to an increase in hardness. Compared with the coarse-grained steel at the same ratio of stress amplitude to hardness, the crack growth rates in the ultrafine grained steel were higher in the short-crack regime because of the smaller crack roughness and perhaps the difference in the strain gradient at the crack tip.

Design And Fabrication Of Fatigue Test Rig And Preliminary Investigation On Flax Composite Beam

Reinforced polymer laminates are used increasingly as structural parts in automobile, aircraft and navel industries due to their excellent mechanical characteristics for relatively low density when compared to the conventional materials and therefore much information on mechanical properties are necessary. Composite materials behaviour subjected to fatigue load is very complex due to its non-homogeneous and anisotropic properties. So it is very important to understand the fatigue behaviour of FRP laminates to extend its usage for various applications in the current industries. Main objective of the project is to understand the fatigue behaviour of the composite laminates when it is subjected to fully reversed bending fatigue cycle. In this case it is very important to develop a specific test setup and approach to perform fatigue test avoid the utilization of expensive test methods available in the market. So a fatigue test rig was designed and developed for this project. Fatigue behaviour of composite laminates was studied in terms of stiffness degradation by performing experimental static deflection test on the test specimen before and after subjecting it to the fatigue load.

Failure analysis of a 40 ton crane hook at a Hot Strip Mill

There have been accounts of repeated failure of crane hooks at the coil yard of a Hot Strip Mill which pose a serious threat to safety in the area. More than 4 hooks failed in less than 5 years. The crane hook (rated for 36000 kg) failed from the threaded shank while lifting a load of 18143 kg. The metal in the hook was revealed by chemical analysis to be killed IS: 4367 20C15 steel. The hook rod failed from a step where there was a cross sectional change and the locations were associated with machining and chatter marks. Such cross-sectional changes are the potential sites of stress concentrations leading to crack initiations. Fracture surfaces of broken pieces of hook reveal initiation of beach marks from both sides with granular rough surface at the middle of fracture zone. Beach marks initiated from both sides indicate origin of reverse bending fatigue. Distinct granular rough zone at the middle is due to final brittle fracture. Microstructure of the polished sample revealed numerous inclusions which indicate that the steel was not clean .Such a huge number of inclusions are not desirable as they can act as stress concentration sites and lead to fatigue crack initiation. Etched microstructure of failed hook reveals coarse cast structure having inhomogeneous microstructure with a mixture of ferrite and pearlite (which meant lower fatigue strength). This inhomogeneous coarser cast structure is outcome of lower reduction ratio during rolling followed by improper heat treatment process. The fracture was concluded to have occurred due to stress concentration from the step region due to inferior material (Inclusion and Improper Heat treatment process). Preventive maintenance and condition monitoring procedures should be applied to identify and minimize the risk(s) Establishment of an NDT procedure for regular basis inspection of the structural members (welded joints and hookshaped steel rods during incoming inspection and in-service).

Grain size dependence of fatigue properties of friction stir processed ultrafine-grained Al-5024 alloy

The main objective of the present study was to examine the effect of grain size on the fatigue properties of 5024 Al alloy. Two ultrafine-grained (UFG) and one fine-grain microstructures were produced via friction stir processing (FSP). Fatigue properties were investigated using a fully reversible bending fatigue testing machine. Microstructural evolution during fatigue was analyzed by carrying out interrupted fatigue testing with subsequent orientation imaging microscopy. Emphasis was given to grain size on the evolution of deformed structure, intergranular dislocation density, and overall S-N curve. A comparison with literature demonstrates the importance of the processing route to obtain the UFG microstructure. This difference is highlighted through an analysis of Basquin equation parameters. The results indicate that FSP can be utilized as an effective route to obtain stable UFG microstructure. A fractographic study was done to understand the effect of microstructure on fatigue crack initiation and propagation rates. Crack growth rates in stage-II has been compared for the three different grain sizes.

Failure Analysis of a Ti6Al4V Screw Used in a RASL Procedure

A failure analysis investigation was performed on a Ti6Al4V medical implant screw that failed after 1 month of surgical implantation. Light microscopy, scanning electron microscopy, and finite element analysis (FEA) techniques were utilized to characterize the mode(s) of failure and fracture surfaces. Complete fracture of the screw was observed near the transition from the tapered thread at the back of the screw, to the flute portion towards the front of the screw. Fractographic analysis revealed significant fretting and flattening of the threads, indicating lack of fixation to the bone, and micromotion of the screw during implant life. Off-axis, reverse bending macroscopic beach marks, as well as microscopic fatigue striations were found on opposite sides of the annular fracture surface. These features suggest intermittent cyclic bending led to crack initiation, and propagation of crack fronts originating along circumference of the tapered thread transition region. Microvoid coalescence (overload features) were also found on the fracture surface, as well as secondary cracks along the screw circumference within the thread root. FEA was used to show the development of localized stress concentrations within the thread root radius at the taper transition region, consistent with observed crack initiation areas. The fracture morphology suggests that premature failure occurred because of interacting factors including: a lack of fixation between bone and screw, micromotion of the screw threads against the bone, and reverse bending fatigue within the tapered to flute transition region. These conditions may have been exacerbated by the use of an improper screw diameter or inadequate installation torque.

Effect of Nickel and Magnesium on the Electrochemical Behavior of AA 1050 Alloys in Nitric Acid Solution

The study investigates the influence of nickel and magnesium additions to AA 1050 aluminum alloy on the electrochemical behavior of the alloy in nitric acid solution under conditions relevant to the lithographic and electronic industries. Magnesium and nickel additions are of interest, since they can improve the alloy properties for the printing process by improving reverse bending fatigue strength and thermal softening resistance, while nickel may provide uniform pitting during electrograining. Scanning electron microscopy was used to characterize the resulting surface morphologies. The addition of nickel led to an increase in the pitting and corrosion potentials; additionally, it reduced the rate of dissolution of intermetallic particles during anodic polarization and increased the rate of aluminum dissolution during cathodic polarization. In contrast, the addition of magnesium had negligible influence on the open circuit and pitting behaviors, since the magnesium is retained in solid solution and has negligible influence on the cathodic behavior of intermetallic particles, which dominate the corrosion behavior.

Effect of residual stresses on fatigue strength of high strength steel sheets with punched holes

The effect of residual stresses on the reverse bending fatigue strength of steel sheets with punched holes was studied for steels with tensile strength grades of 540MPa and 780MPa. Tensile and compressive residual stresses were induced around the punched holes. Heat treatment of the specimens with punched holes at 873K for 1h decreased the residual stresses around the holes and improved the fatigue strength of the sheets. This result means that the tensile residual stresses induced in the sidewalls of the holes and near the hole edges by punching reduced fatigue strength. The effect of the residual stresses on the fatigue limits of the edges was estimated by the modified Goodman relation using the residual stresses after cyclic loading and the ultimate tensile strength at the fatigue crack initiation sites.

Fatigue behaviour of laser sintered Nylon 12 in rotating and reversed bending tests

This paper reports the results of a study into the fatigue behaviour of laser sintered Nylon 12 specimens in both reversed and rotating bending. Rotating bending tests are generally the most efficient in terms of test time, but reversed bending deformation explicitly promotes delamination, and so the main aim was to identify whether or not there was significant variation in fatigue behaviour between the two loading modes. The paper presents the first reversed bending fatigue test study on selective laser sintered Nylon parts and compares the results to fatigue behaviour from rotating bending. The results showed no significant variation in fatigue behaviour between the two loading modes and also showed an isotropic response in terms of fatigue behaviour in both test configurations.

Enhanced Fatigue Strength of Commercially Pure Ti Processed by Rotary Swaging

Fully reversed bending fatigue tests were performed on polished hour-glass specimens of commercially pure titanium grade 1 with three different grain sizes, that were produced by severe plastic deformation (rotary swaging) and subheat treatments, in order to examine the effect of grain size on fatigue. An improvement in fatigue strength was observed, as the polycrystal grain size was refined. The endurance limit stress was shown to depend on the inverse square root of the grain size as described empirically by a type of Hall-Petch relation. The effect of refining grain size on fatigue crack growth is to increase the number of microstructural barriers to the advancing crack and to reduce the slip length ahead of the crack tip, and thereby lower the crack growth rate. It was found that postdeformation annealing above recrystallization temperature could additionally enhance the work-hardening capability and the ductility of the swaged material, which led to a marked reduction in the fatigue notch sensitivity. At the same time, this reduction was accompanied with a pronounced loss in strength. The high cycle fatigue performance was discussed in detail based on microstructure and mechanical properties.

Laser Shock Peening on a 6056-T4 Aluminium Alloy for Airframe Applications

Laser Shock Peening (LSP) is a material enhancement process used to introduce compressive residual stresses in metallic components. This investigation explored the effects of different combinations of LSP parameters, such as irradiance (GW/cm2) and laser pulse density (spots/mm2), on 3.2 mm thick AA6056-T4 samples, for integral airframe applications. The most significant effects that are introduced by LSP without a protective coating include residual stress and surface roughness, since each laser pulse vaporizes the surface layer of the target. Each of these effects was quantified, whereby residual stress analysis was performed using X-ray diffraction with synchrotron radiation. A series of fully reversed bending fatigue tests was conducted, in order to evaluate fatigue performance enhancements with the aim of identifying LSP parameter influence. Improvement in fatigue life was demonstrated, and failure of samples at the boundary of the LSP treatment was attributed to a balancing tensile residual stress.

Observation of small fatigue crack growth behavior in the extremely low growth rate region of low carbon steel in a hydrogen gas environment

To investigate the effects of hydrogen on crack propagation in the extremely low growth rate range, fully reversed bending fatigue tests were performed on low carbon steel (JIS S10C) in hydrogen and in nitrogen gas environments at a low pressure. A crack showed almost the same non-propagation behavior in nitrogen as that in air. However, a crack in hydrogen continued to propagate even near \(10^{7}\) cycles in the same testing strain range as that in nitrogen. In hydrogen gas, a crack grew intermittently by coalescing with a new micro-crack generated by slip behavior. This implies that hydrogen could inhibit the action of any factor affecting non-propagation.