Fatigue Strength Degradation Research Articles

High cycle fatigue behavior prediction of shot-peened parts

This paper presents an approach to predict high cycle fatigue behavior of shot-peened parts based on the multi-axial high cycle fatigue criteria of Crossland and Dang Van. The induced shot peening modifications on the surface layers, which are: (i) in-depth profile of compressive residual stresses, (ii) superficial work-hardening, (iii) surface finish imperfections, (iv) superficial defects, have been taken into account. This approach has been applied to a case of a shot-peened ductile steel SAE 3415 using two conditions: (i) a standard condition of 100% coverage (ii) a severe condition of 1000% coverage (high exposure time). This approach allows to evaluate all the effects of the induced shot peening modifications at the surface and in deeper layers. Good agreement has been observed between the predicted results and those obtained from experimental investigations. A computational procedure has been proposed to predict either high cycle fatigue strength enhancement or degradation of shot-peened parts.

Effects of ballistic impact damage on fatigue crack initiation in Ti–6Al–4V simulated engine blades

The ingestion of debris into jet engines creates nicks and dents on the leading edges of blades and vanes. This is commonly known as foreign object damage (FOD). Such damage, which can often result in premature failure, was simulated in the laboratory using diamond cross-section axial fatigue samples that were impacted with 1 mm diameter glass beads at 305 m s−1 at either 0 or 30° angle of incidence. The samples had either a thin leading edge (LE) with a radius of 0.127 mm or a thick LE with a radius of 0.381 mm. Fatigue strength of impacted specimens showed degradation of 10–50% due to LE damage, regardless of the depth of the damage zone. FOD related impact notch depth, loss of material (LOM), shear, folds, embedded shattered glass, and microstructural damage were characterized by SEM. Fatigue strength degradation was found to be higher for the 30° impacts than for the 0° impacts. No clear correlation between notch depth or LE thickness and fatigue strength was found.

Study of steam turbine blade degradation Part 2 – Damage mechanical model of blade materials during long term service

A damage mechanical model has been proposed from the theory of continuum damage mechanics that describes the service damage evolution of steam turbine blade materials during long term service. In the model, the damage variable is selected according to the variation in fatigue limit of the blade materials after service. Furthermore, a model of the degradation of fatigue strength during service has been created. The experimental results are in good agreement with the model. Compared with the low cycle fatigue damage model, the strength design of the blade is conservative early in its service life. If the degradation in fatigue strength is not taken into account, the life prediction of the blade is inadequate for long term service lives.

Evaluation of thermal degradation of unidirectional CFRP rings

We have studied the thermal degradation of unidirectional carbon-FRP (CFRP) rings. First, we developed a method based on the ring tensile test for evaluating hoop directional strength. And we calculated the distribution of hoop stress by a finite-element method considering friction. Then, we measured the thermal reduction of fatigue strength of unidirectional CFRP rings. To evaluate the degradation of fatigue strength, we applied the Arrhenius equation to confirm the relation between temperature and time. Using this evaluation method, we predicted the thermal reduction of fatigue strength under high temperature and long exposure time.

６０６３アルミニウム合金のフレッティング疲労特性に及ぼす陽極酸化処理の影響

Wear, scratch, plain fatigue and fretting fatigue tests were carried out using aluminum alloy JIS 6063–T5 specimens with anodic film and lubricating anodic film. Though the surface hardness of lubricating anodic film was lower than that of anodic film, lubricating anodic film had excellent wear resistance and crack resistance compared to anodic film. Plain fatigue strength of the specimen with anodic film was lower than that of the specimen with lubricating anodic film, which almost coincided with that of the specimen without film. Fretting fatigue strength of the specimen with lubricating anodic film was higher than that of the specimen with anodic film, which was higher than that of the specimen without film. The lubricating anodic film had the lowest coefficient of friction and a higher cracking resistance, which contributed to the late crack initiation and the low crack growth rates. The anodic film also had low coefficient of friction compared to the specimen without film and prevented metal-contact, which may result in higher fretting fatigue strength compared to the specimen without film. The lubricating anodic film is of significant benefit to fretting fatigue strength without degradation of plain fatigue strength.

Influence of foreign-object damage on crack initiation and early crack growth during high-cycle fatigue of Ti–6Al–4V

The objective of this work is to provide a rationale approach to define the limiting conditions for high-cycle fatigue (HCF) in the presence of foreign-object damage (FOD). This study focused on the role of simulated FOD in affecting the initiation and early growth of small surface fatigue cracks in a Ti–6Al–4V alloy, processed for typical turbine blade applications. Using high-velocity (200–300 m/s) impacts of 3.2 mm diameter steel spheres on the flat surface of fatigue test specimens to simulate FOD, it is found that the resistance to HCF is markedly reduced due to earlier crack initiation. Premature crack initiation and subsequent near-threshold crack growth is primarily affected by the stress concentration associated with the FOD indentation and the presence of small microcracks in the damaged zone (seen only at the higher impact velocities). Furthermore, the effect of residual stresses and microstructural damage from FOD-induced plastic deformation at the indent sites are assessed in terms of fatigue strength degradation. It is shown that FOD-initiated cracks, that are of a size comparable with microstructural dimensions, can propagate at applied stress-intensity ranges on the order of ΔK∼1 MPa √ m , i.e., a factor of roughly two less than the “worst-case” threshold stress-intensity range in Ti–6Al–4V for a crack of a size large compared to microstructural dimensions (a “continuum-sized” crack). Correspondingly, for FOD-initiated failures, where the critical condition for HCF must be defined in the presence of microstructurally small cracks, the Kitagawa–Takahashi diagram, with the limiting conditions of the stress-concentration corrected 10 7-cycle fatigue limit and the “worst-case” ΔK TH fatigue threshold, is proposed as a basis for design against FOD-induced HCF failures.

Modeling of Load Frequency Effect on Fatigue Life of Thermoplastic Composites

This paper presents a simple prediction scheme correlating fatigue life to the thermal degradation of fatigue strength. Shifting factors similar to the time-temperature shifting in viscoelastic media were employed to account for the effect of temperature on fatigue strength and an iso-strength plot was introduced for fatigue life prediction under non-isothermal conditions. The scheme presented in this paper can predict the load frequency effect associated with hysteretic heating from limiting basic material information. The load frequency effect on the fatigue life of an AS4/PEEK ±45 thermoplastic composite laminate was investigated at 1 Hz, 5 Hz and 10 Hz. Fatigue life prediction for 5 Hz and 10 Hz based on S-N data at 1 Hz was also demonstrated. The predictions agreed reasonably with the experimental data.

Electromechanical Fatigue of Lead Zirconate Titanate Ceramics

Mechanical fatigue of lead zirconate titanate ceramics when both an ac electric field and mechanical stress were applied to the specimen was investigated. Fatigue strengths under an ac electric field degraded compared with the fatigue strengths under short-circuit conditions while the slope of the S-N curve was the same as that under the short-circuit condition. The reason for the degradation of fatigue strength under an ac electric field is considered to be the microscopic internal stress caused by the electric-field-induced strain. Degradation of fatigue strengths under an ac electric field became larger when the electric field became bipolar even though the amplitude of the field was the same. The reason for the dependence of fatigue strength under an ac electric field on the applied field polarity is considered that the internal stress became large when the ac electric field became bipolar.

快削ステンレス鋼ＳＵＳ３０３同種摩擦圧接継手の疲労特性

Fatigue tests were conducted on the free cutting stainless steel SUS303/SUS303 friction welded joints under pulsating zero-tension load to study the effect of microstructure produced by friction welding process. The specimens with circumferential notch which was machined near the welding interface were used in the fatigue tests. Fatigue crack initiation and propagation were examined by using DC potential drop method.The fatigue limit of the friction welded joints was almost the same as that of the base metal. But the difference in fatigue strength between the base metal and the joints increased with increasing stress level. The crack initiation strength of the joints was lower than that of the base metal and the non-propagating cracks were found in the specimens of the joints which did not fracture above 107 stress cycles. The fatigue crack growth rate of the joints decreased and reached the minimum value near crack length of 0.2mm. The crack growth rate of the joints was smaller than that of the base metal in the low stress intensity factor range, while it increased and gained upon that of the base metal. The degradation of fatigue strength of the joints, especially for the fatigue crack initiation strength, was mainly caused by the shape, size, direction and distribution of non-metallic inclusions which differed from those of the base metal.

Fatigue Strength of Polyethylene After Sterilization by Gamma Irradiation or Ethylene Oxide

The oxidation level of ultrahigh molecular weight polyethylene specimens sterilized by gamma irradiation in either air or Ar gas was compared with that of unsterilized and ethylene oxide sterilized ultrahigh molecular weight polyethylene. The fatigue strength of ultrahigh molecular weight polyethylene specimens sterilized by gamma irradiation in air was compared with that of unsterilized and ethylene oxide sterilized ultrahigh molecular weight polyethylene. At the specimen surface, oxidation was highest for ultrahigh molecular weight polyethylene gamma irradiated in air, lower for ultrahigh molecular weight polyethylene gamma irradiated in Ar gas, and absent in unsterilized and ethylene oxide sterilized ultrahigh molecular weight polyethylene. At a depth of 3.5 mm below the specimen surface, oxidation levels were equivalent for ultrahigh molecular weight polyethylene gamma irradiated in either air or Ar gas whereas unsterilized and ethylene oxide sterilized specimens were again unoxidized. Thus, even in an inert atmosphere, oxidative degradation of gamma irradiated ultrahigh molecular weight polyethylene occurs. The 10 million cycle fatigue strength was similar for unsterilized and ethylene oxide sterilized ultrahigh molecular weight polyethylene whereas the fatigue strength of gamma irradiated in air ultrahigh molecular weight polyethylene was lower. Results of this study show that ethylene oxide gas does not degrade ultrahigh molecular weight polyethylene whereas gamma radiation in air causes changes in the polymer that adversely affect its mechanical properties. Ethylene oxide gas is a viable alternative to gamma radiation in air that avoids oxidation and fatigue strength degradation known to accompany irradiation of ultrahigh molecular weight polyethylene polymer bearing surfaces in total joint implants.

Effect of Relative Slip Amplitude on Fretting Fatigue Behavior of Silicon Nitride

Fretting fatigue tests were carried out using HIP-sintered silicon nitride to study the effect of relative slip amplitude between the specimen and the contact pad on fretting fatigue behavior. Fretting fatigue strength decreased with increasing relative slip amplitude. Application of static contact without fretting motion had no influence on static fatigue strength. Therefore, fretting action with relative slip is essential for the degradation of fatigue strength. Fretting fatigue life prediction based on fracture mechanics analysis was also carried out, where the frictional force between the specimen and the contact pad was taken into consideration. The predicted fretting fatigue lives agreed well with the experimental results.

Degradation of Fatigue Properties of Weld-Bonded Aluminum Exposed to Moisture and Elevated Temperature

Abstract Experiments have been completed in order to characterize the fatigue behavior of weld-bonded aluminum 5754-O/bis-phenol-A epoxy adhesive joints subjected to 100% relative humidity at 38°C. It was found that the presence of water vapor at elevated temperature decreases the fatigue strength of weld-bonded joints by as much as 33% at 5 × 106 cycles. Optical microscopy, scanning electron microscopy, dynamic mechanical analysis, and tensile testing of fatigued specimens and exposed bulk adhesive revealed that fatigue strength degradation is mainly due to the plasticization and micro-cracking of adhesive by the water vapor.

Effect of Relative Slip Amplitude on Fretting Fatigue Behavior of Silicon Nitride.

Fretting fatigue tests were carried out using HIP-sintered silicon nitride to study the effect of relative slip amplitude between the specimen and the contact piece on fretting fatigue behavior. Fretting fatigue strength decreased with increasing relative slip amplitude. Application of static contact without fretting motion had no influence on static fatigue strength. Therefore, fretting action with relative slip will be essential to the degradation of fatigue strength. Fretting fatigue life prediction based on the fracture mechanics analysis was also carried out, where the frictional force between the specimen and the contact piece was taken into consideration. The predicted fretting fatigue lives agreed well with the experimental results.

A stochastic theory of cumulative fatigue damage

A Stochastic theory for the cumulative fatigue damage of structural component with random fatigue strength under random loading is proposed on the basis of the Stratonovich-Khasminskii theorem. The analytical solutions for the probability densities of the cumulative fatigue damage and fatigue life and for the reliability function are given for steel and reinforced concrete components, with constant fatigue strength subject to narrow band stationary Gaussian stress process with zero mean. The results agree very well with those of digital simulation. It is noted that the theory can be applied, in principle, to metallic and some nonmetallic materials, to both narrow band and wide band stress process, and can be adapted to a sequence of n stationary stress process or quasi-stationary stress process. The scatter and degradation of fatigue strength can also be incorporated into the theory.

Effects of Grinding on Dynamic Fatigue Behavior and Strength Degradation of Hot Pressed Silicon Nitride Ceramics at Room Temperature

Four-point bending dynamic fatigue tests were carried out at five different crosshead speeds from 5×10-4 to 5×100mm/min in air at room temperature. Hot pressed silicon nitride ceramics was used for the tests and ground with 600, 400 and 200 grit resin bonded diamond wheels perpendicular to the direction of bending stress. Both the specimens ground with the 400 and 200 grit wheels showed a decrease in the fatigue parameter and fracture strength, while no decrease was observed in the specimen ground with the 600 grit wheel. It is suggested that grinding induces surface flaws and that the mean fracture strength of specimens ground with the 400 and 200 grit wheels tested at low crosshead speeds is lowered by slow crack growth of the surface flaws which are easily affected by stress corrosion.

The fatigue damage process of FRP laminates with U and V type notches.

This paper deals with the fatigue damage process of glassfiber reinforced polyester laminates with U and V type notches. It is shown from experimental results that the relationship between the applied stress/tensile strength ratio and fatigue life is the same on the whole, regardless of fiber content. In order to explain these fatigue behaviors, a simple fatigue and residual strength degradation model is proposed to predict the fatigue life under pulsating tensile loadings. The prediction of fatigue life by a numerical approach using the proposed fatigue damage model agrees well with the experimental data.

塩水腐食した熱間圧延鋼板の疲れ強度低下要因の検討

In order to investigate the corrosion effect on the fatigue strength of steel sheets used in automobile suspension members, plane bending fatigue tests were carried out. The materials used were 0.075%C and 0.15%C hot-rolled steel sheets. One group of specimens were intermittently dipped in 5% salt-water at room temperature and another group continuously in 20% HCl aqueous solution at 50°C. The fatigue strength of the corroded specimens was correlated with the feature of the corroded surfaces.The main factors affecting the degradation of fatigue strength due to corrosion were the reduction of the sectional area and the increase of surface roughness. The corrosion products and charged hydrogen did not decrease the fatigue strength of the corroded specimen in a dry condition.The following equation was proposed for estimation of the bending fatigue strength σwap of the specimens after the corrosion products were removed.σwap=(Zc/Z0)·(1-ηΔdmax0.4)·σw0where Z: section modulus, η: 0.45 for 0.075C steel and 0.65 for 0.15 steel, Δdmax: maximum depth of roughness due to corrosion, suffix 0: Electropolished specimens, suffix C: Corroded specimens.

Fatigue Crack Initiation in a Porous Steel

Abstract Porosity in sintered powder metals may contribute to fatigue strength degradation in two ways. First, pores will act as local stress concentrators and, second, they may act act as fatigue crack precursors. Accordingly, the effect of porosity on fatigue crack initiation was chosen as the thrust of the present study. Conventional powder metallurgical techniques were employed to generate various levels of porosity in a heat treatable steel of the AISI 4600 type. Porous steel specimens, in a modified compact tension configuration, were cyclically loaded and cycles to initiation noted. Initiation was defined as the generation of a fatigue crack 0·10 mm in length at the notch root. As expected, the greater the porosity content, the earlier the crack developed. There are two interdependent variables in porosity character for a given porosity content: these are the average interpore spacing and the average pore diameter. The region of concentrated stress around each pore is proportional to the cube of th...

An Exploratory Study Into the Fatigue of Composites Under Spectrum Loading

A fatigue and residual strength degradation model for constant amplitude cyclic loading has been modified and applied to fatigue of composites under service loading spectra. An exploratory study is made to describe the fatigue behavior of composites subject to spectrum loading using some base-line constant amplitude data. Available test data for both composite laminates and bonded joints under service loading spectra are used to correlate with the theoretical results.

Proof Test and Fatigue of Unnotched Composite Laminates

A comprehansive Fatigue and residual strength degradation model has been used to predict the effect of proof loads (or high load) on the statisti cal fatigue behavior of composite laminates. The validity of the theoretical model is confirmed by the experimental test results. The correlation be tween the test results and the theoretical distributions of the fatigue life and the residual strength for composite specimens with or without the effect of proof loads is shown to be very good.