Measurements of the temperature rise ahead of a fatigue crack

Summary The temperature rise ahead of a fatigue crack has been measured using a Thermovision camera. Observations were made on three kinds of polymer and an austenitic stainless steel. The maximum temperature difference in macro-scale between the zone ahead of the crack and the bulk of the material was found to be, for the polymers, about 30°C at 725 N/min and amplitude o.1–0.8 kp/mm² and, for the steel, 14°C at 6000 N/min and amplitude 3.0–24.0 kp/mm².     

AN ANALYSIS OF LOW CYCLE FATIGUE BASED ON HYSTERESIS ENERGY

Abstract— Cyclic deformation behaviour of medium carbon steel, armco iron and copper has been investigated. A model based on the energy absorbed for the fracture process at the crack tip, given in the form (Δw*_p)^y where Δw*_p is the hysteresis energy at the crack front and the exponent y decides the energy spent for the fracture process, is proposed for the crack growth in low cycle fatigue. The value of the exponent y has been found to be around 0·26 to 0·28 for medium carbon steel and armco iron and around 0·52 for copper. These values of y give good prediction of the relations between the total fracture energy and the stress amplitude and the total fracture energy versus the fatigue life. The energy based approach has been extended to predict the cumulative damage in a single step stress variation. Good correlation has been obtained between the experimental data and the theoretical prediction.     

Nondestructive detection of fatigue damage in austenitic stainless steel by positron annihilation

Austenitic stainless steel specimens have been examined by positron-lifetime measurements at various stages until failure during fatigue tests at constant stress or plastic strain amplitudes. A positron-source-detector assembly has been mounted on the servohydraulic testing machines that allowed truly non-destructive positron annihilation studies without removing the specimens from the load train. Positrons were generated by a ⁷²Se/⁷² As source with a maximum activity of 0.9 MBq (25 μCi). The average positron lifetime has been determined by a β⁺ − γ-coincidence applying a simplified data evaluation procedure. Under constant stress or plastic strain amplitudes early stages of fatigue damage could be detected. The strong increase of the average positron lifetime already during the first 10% of fatigue life could be related to the fatigue life of the specimens. Issues of lifetime prediction by positron annihilation measurements are discussed.     

Fatigue life prediction: A Continuum Damage Mechanics and Fracture Mechanics approach

Continuum Damage Mechanics (CDM) approach is used to predict crack initiation life and Fracture Mechanics approach predicts crack growth life. Strain controlled fatigue life of a ferrous alloy, EN 19 steel, has been determined using CDM and Fracture Mechanics approach. By combining these two approaches, life could be predicted with damage value in the material. All inputs required for the models have been determined by conducting monotonic, cyclic and fracture tests. Predicted life is also compared by conducting strain controlled fatigue tests. Predicted life in the strain amplitude range of 0.3–0.7% (fatigue life range of 10²–10⁵), compares well with the experimental results. All tests have been conducted at specimen level, stress ratio of −1 and at room temperature. The variation of crack initiation and crack propagation life with strain amplitude shows that maximum life is consumed by crack growth process at higher strain amplitude and at lower strain amplitudes, maximum life is spent for crack initiation process.     

The cyclic stress behaviour of a 355 stainless steel 2024-T8 aluminium alloy

The push-pull fatigue behaviour of a 355 stainless steel 2024-T8 aluminium alloy composite, has been studied at constant stress. The S-N curve shows a fatigue strength of 20 kg/mm². Microhardness measurements reveal that little fatigue hardening takes place within the matrix; also, hardness numbers are similar in fatigued specimens, irrespective of the applied stress amplitude.The increase in damping capacity for increasing stress amplitudes, is attributed to increased delamination at the fibre-matrix interface, in the early fatigue stages. This result is also confirmed by optical microscopy. It is inferred that the sequence of failure weakness in the composite is: fibre-matrix interface, matrix and, finally, fibres.A fatigue strength/tensile strength ratio of 0.16 for this material is noticeably low, but fatigue properties of the composite can be improved by enhancing the fibre-matrix bonding.     

Evaluation of distribution of fatigue lives of the extruded magnesium alloy AZ61

In this study, fatigue tests were carried out to study the distribution of fatigue lives of the extruded Mg alloy AZ61 under constant stress amplitudes. The scattering in the distribution of fatigue lives becomes larger at lower stress amplitudes compared to higher stress amplitudes. During the fatigue process of the alloy, it was observed that cracks initiated from inclusions existing on the specimen surface, then propagated, and finally led to the final failure of the specimen. Statistical distributions of both densities and areas (sizes) of the inclusions were experimentally investigated in detail and were approximated by Weibull distributions. In addition, the distribution of ${{A}_{\rm max}^{1/2}}$ , the square root of the maximum inclusion areas within a division of 0.75 mm², was also investigated. The experimental data were approximated well by the extreme-value distribution. The distributions of the fatigue lives of the extruded Mg alloy were evaluated using both Monte-Carlo simulation and surface fatigue crack propagation behaviour. In the simulation, the random numbers corresponding to the above statistical distributions of the inclusions, i.e., densities, areas, and square root of the maximum inclusion areas were utilized. The evaluated distribution of the fatigue lives corresponded well with the experimental one. It was concluded that for the evaluation of the distribution of fatigue lives, employing the extreme-value distribution of the inclusions is recommended because of convenience and accuracy of the method.     

The influence of particle content on the equi-biaxial fatigue behaviour of magnetorheological elastomers

The equi-biaxial fatigue behaviour of silicone based magnetorheological elastomers (MREs) with various volume fractions of carbonyl iron particles ranging between 15% and 35% was studied. Wöhler curves for each material were derived by cycling test samples to failure over a range of stress amplitudes. Changes in complex modulus (E¹) and dynamic stored energy during the fatigue process were observed. As for other elastic solids, fatigue resistance of MREs with different particle contents was shown to be dependent on the stress amplitudes applied. MREs with low particle content showed the highest fatigue life at high stress amplitudes while MREs with high particle content exhibited the highest fatigue resistance at low stress amplitudes. E¹ fell with the accumulation of cycles for each material, but the change was dependent on the particle content and stress amplitude applied. However, each material failed in a range suggesting a limiting value of E¹ for the material between 1.22 MPa and 1.38 MPa regardless of the particle content and the magnitude of the stress amplitude. In keeping with results from previous testing, it was shown that dynamic stored energy can be used to predict the fatigue life of MREs having a wide variation in particle content.     

Effect of substructure on crack formation and total failure of industrial alloys of the aluminium-copper-magnesium system under low-cycle fatigue conditions

On the example of pressed semifinished products made of D16 and AK4-1 type alloys of different chemical composition in the Al-Cu-Mg system in the T and T1 conditions a study has been made of the effect of a number of structural elements on the fatigue life of plane specimens with a concentrator (stress concentration factor 2.6) under low-cycle fatigue conditions. The duration of the stage to development of a fatigue crack N₀ with an area of about O.05 mm² and the overall life of specimens to total failure N_f have been determined. Tests have been carried out for low-cycle fatigue under repeated tension conditions with a cycle asymmetry factor R = O.01, frequency 5 Hz, and maximum stress in the cycle of 160 MPa. It has been established that a reduction in iron and silicon content in alloys D16, D16ch, and 1163 decreases the content in the structure of coarse particles of AlFeMnCuSi phase of variable composition and it increases the life of specimens in the naturally aged condition as a result of increasing the N₀ stage. A reduction in copper content in alloy 1163T from 4.3 to 3.9% and introduction of zirconium in an amount of 0.10%, which weakly affects N₀, increases the duration of the period of crack growth N_f-N_o by 50 kcycles. All of the alloys of the D16ch and 1163 type in the artificially aged condition have similar values of parameters N₀ and N_f with a much lower level of them in the T condition, which is explained by a change in the mechanism for crack development and its growth rate.Translated from Problemy Prochnosti, No. 1, pp. 43–49, January, 1990.     

Nonlinear constant fatigue life diagrams for carbon/epoxy laminates at room temperature

Exploration of a full shape of constant fatigue life (CFL) diagram and development of an efficient CFL diagram-based fatigue life prediction method are attempted for multidirectional CFRP laminates. On three kinds of CFRP laminates of [45/90/−45/0]_2s, [0/60/−60]_2s and [0/90]_3s lay-ups, tension–tension, tension–compression and compression–compression fatigue tests are performed at room temperature for two different stress ratios each. Experimental results clearly show that a stress ratio has a significant influence on the fatigue behavior of those CFRP laminates, and the CFL diagrams delineated using alternating stress and mean stress become asymmetric about the alternating stress axis. The alternating stress component of fatigue load for a given constant value of fatigue life turns maximum in the case of fatigue loading at a critical stress ratio that is nearly equal to the ratio of compressive strength to the tensile one. The shape of CFL diagrams progressively changes from a straight line to a nonlinear curve as a given constant value of fatigue life increases. A new and efficient method for accurately predicting an asymmetric nonlinear CFL diagram is then developed which is based on the static strengths in tension and compression and the reference S–N relationship fitted to the fatigue data for the critical stress ratio. The theoretical CFL diagram constructed following the proposed procedure agrees well with the experimental CFL diagram, regardless of the type of CFRP laminate. It is also demonstrated that the S–N relationships predicted using the proposed CFL diagram-based fatigue life prediction method adequately coincide with the experimental results for fatigue loading with a variety of different stress ratios in the range of fatigue life up to 10⁶ cycles.     

Wechselverformung und Gefügeänderungen von Ck 15 und Ck 45

Microstructural Changes and Cyclic Deformation The crack initiation starts due to weakening and strengthening process during rotating bending. However a smaller plastic deformation amplitude is noticed at the same nominal stress compared to tension-compression stressed specimens. This results in a higher fatigue life. The different cyclic deformation behaviour was proofed by SEM (rotating bending specimens showed a lower slip line density compared to tension-compression specimens at the same nominal stress) and TEM investigations (the rotating bending specimens showed a smaller dislocation density at the same nominal stress). Furthermore it is showed, a correlation of cyclic stress strain data σ(ε_pls) between tension-compression and rotating bending specimens exists. This is also valid for the Manson-Coffin-relationship. the relation between lg ε_pls and lg N_B depends on the material (Ck 15, Ck 45) but not on the state of stress.     

Assessment of fatigue response of thermally aged reduced activation ferritic-martensitic steel based on finite element analysis

Abstract

In the present investigation, effect of thermal ageing on low cycle fatigue (LCF) behaviour of Reduced Activation Ferritic Martensitic steel has been assessed by finite element analysis. The steel was thermally aged at 873 K for 3000 hour. Low cycle fatigue tests were carried out on both the as-received and thermally aged material at strain rate of 3×10^?3 s^?1 at 823 K, over strain amplitudes in the range of ± 0.25 to ± 0.8%. Continuous cyclic softening till final failure, except for initial few cycles especially at relatively lower strain amplitudes, was observed in both the material conditions. Thermal ageing resulted in marginally higher cyclic stress response accompanied by lower fatigue life. The differences in fatigue responses have been attributed to the coarsening of precipitates on thermal ageing. Finite element analysis has been carried out considering combined isotropic and kinematic hardening as material model to estimate the effect of thermal ageing on the response of material under LCF loading. Thermal ageing was found to decrease both the isotropic and kinematic hardening with appreciable effect on isotropic hardening. The predicted cyclic stress response and hysteresis loops were found to be in good agreement with the experimental data. The LCF life of the steel has been estimated based on the hysteresis energy approach.     

Effects of shot peening on bending fatigue strength of spring steel SS 2090

Abstract

The influence of prior surface condition and of a shot peening treatment on the bending fatigue strength of a standard Si–Cr spring steel (SS 2090) has been investigated. This steel was initially hardened and tempered to a hardness of 52–54 HRC. After shot peening, compressive residual stresses had been introduced into a surface layer of depth ~0·3 mm, with the maximum value of ~1000 MN m^?2 being found close to the surface. The effect of this treatment was to increase the fatigue limit by ~40% to 890 MN m^?2. Coincident with this increase was a change in the site of fatigue initiation from a surface to a subsurface location beneath the compressive residual stress layer. The initiating inclusions, which were 20–40 μm in size, were analysed and found to be Al₂O₃. At stress amplitudes greater than the fatigue limit, initiation was invariably found to occur at the surface and was not always due to inclusions. Inclusion initiated failure has been modelled using the size and spatial distribution of inclusions in the test bars in addition to the variation of applied and residual stresses through the section. A crack propagation criterion based on linear elastic fracture mechanics is used, assuming that propagation is controlled by stress intensity threshold value. It is assumed that small cracks exist at oxide inclusions from the beginning of the fatigue life and that failure is associated with the propagation of one of these cracks.

MST/1392     

ELEVATED TEMPERATURE FATIGUE IN Ni3Al-BASED ALLOYS

Both high-cycle and low-cycle fatigue properties of hot-extruded powders of a Ni₃Al-based alloy, IC218, have been evaluated. High cycle fatigue measurements were performed under stress controlled conditions at temperatures ranging from 25°C to 850°C. Tests were made in both laboratory air and vacuum environments. Low cycle fatigue tests were conducted under total strain control in a laboratory air environment at 650°C. In high cycle fatigue, high ratios of the fatigue limit (Δσ at 10⁶ cycles) to monotonic yield strength (σ_ys), of approximately Δσ/σ_ys～1, were obtained in the powder extruded IC218 alloy for temperatures ranging from 25°C to 650°C. In low cycle fatigue, a substantial decrease in fatigue life occurred at 650°C, compared to results obtained previously at 25°C. High cycle fatigue performance at low stress/strain amplitudes is better than expected when compared to precipitation strengthened superalloys. The improved performance is explained in terms of the cyclic hardening behavior of the alloy.     

Role of machining defects and residual stress on the AISI 304 fatigue crack nucleation

Machining defects as rebuilt material and dislodgement were often induced by cutting of difficult to machining AISI 304 stainless steel. Their density increases with a decreasing of cutting speed. The effect of these defects on surface topography and residual stress was evaluated by roughness and X‐ray diffraction measurements coupled with numerical simulation. The role of the rebuilt material on the distribution of fatigue crack nucleation sites was investigated by scanning electron microscope examination of post fatigue samples loaded at different imposed strain amplitudes. The association of machining defects and fatigue crack nucleation sites was attributed to the contribution of additional tensile residual stresses induced by rebuilt material rather than local stress concentration. Moreover, the fatigue crack coalescence is promoted by increasing the rebuilt material density. When the machining defect density increases from 5 to 60 particles/mm², the fatigue life decreases from ?22% to ?65% with respect to the electropolished surface.     

Multiaxial fatigue behaviour of Sintered Steels under combined in and out of phase bending and torsion

This paper discusses the fatigue behaviour of sintered steels under multiaxial loading. These steels are the Fe-1.5% Cu and the Fe-2.0% Cu-2.5% Ni, sintered at low and high temperatures, in the densities 7.1 and 7.4 g/cm³, which are used in the production of several ready to assemble automotive parts. Fully reversed or pulsating combined loading with constant frequency and amplitudes acting in and out of phase, was applied to round notched specimens (K_tb = 1.49, K_tt = 1.24) in the finite fatigue life region (10⁴ ≤ N_f ≤ 2 · 10⁶). The mechanics of crack initiation and propagation as well as rupture were studied using fractography and microfractography. These analyses led to a mechanical model based on local normal stresses for the fatigue life evaluation. The fatigue life evaluation on the base of the local bending stress obtained under uniaxial loading describes the test results for in phase combined bending and torsion satisfactorily. But the increase of fatigue strength and life by out of phase loading is overestimated in the case of fully reversed loading. However for design purposes the out of phase loading can be neglected because of its beneficial effect in increasing fatigue life for this type of material. If the dependence of the different stress concentrations under combined in and out of phase loading on the supportable local bending stress obtained under uniaxial loadings is considered, then the calculation procedure covers all test results.     

Rißfortschritt in ultrafesten Stählen und deren Schweißverbindungen bei dynamischer Beanspruchung

Crack propagation in ultra-high-strength steels and their welded joints under dynamic loading . Reported are results of investigation into the propagation of cracks in the base metal and weld metal of an ultra-high-strength steel. The material used in the investigations was a Ni? Co? Mo? alloy maraging steel with a yield point of 170 kp/mm². The steel was arc welded and TIG welded. The joints exhibited a drop of static strength in the range of 5 to 8 percent related to the base metal. Under zero-to-tension stress cycles the fatigue strength corresponded that of other high-strength steels, under tension-compression stress cycles the steel exhibited a higher fatigue strength. It was possible to show striations with the aid of scanning microscopy. Comparing the track propagation calculated in the microscopic range with the results obtained from the crack growth curves produced approximate agreement.     

Fatigue behaviour of SiCp-reinforced aluminium composites in the very high cycle regime using ultrasonic fatigue

The fatigue behaviour of a 2009/SiC/15p‐T4 DRA composite has been examined in the very high cycle fatigue (VHCF) regime where 10⁷≤N_f≤ 10⁹ cycles. Ultrasonic fatigue was used to achieve the very high cycle counts. Careful processing yielded a composite with a very homogeneous particle distribution with minimal clustering. Fatigue crack initiation was observed almost exclusively at AlCuFe inclusions with no crack initiation observed at SiC particle clusters. Fatigue lives at a given stress level exhibited minimal scatter and subsurface crack initiation was observed in all cases. This behaviour is consistent with the presence of a low number density of critical inclusions that are responsible for crack initiation very early in fatigue life.     

Nondestructive low-cycle fatigue characterization of multi-layer thin film structures

A multi-layered thin film structure (namely, electrodeposited Cu/sputtered Cr/Kapton substrate/sputtered Cr/electrodeposited Cu), utilized as a flexible component for computers, has been exposed to fatigue. Although a standardized testing method for fatigue ductility is available for a solid monolayer of electrodeposited foil, there is no method available for examining such a multi-layered thin film structure. In this study, four different methods were employed to characterize the low-cycle fatigue damage: (1) DC resistance measurement, (2) residual stress development by x-ray diffraction, (3) dislocation density calculation by using obtained x-ray diffraction line profiles, and (4) microscopic observations. Low-cycle fatigue was conducted at eight levels of applied total strain, i.e., _T =13.95%, 7.69%, 5.83%, 4.69%, 3.37%, 2.37%, 1.59%, and 1.19%. The number of fatigue cycles, when the crack was first observed on the outer Cu layer, was identical to that observed with the onset of increased resistance. This cycle number is thus designated as the number of cycles-to-fatigue crack initiation,N _c . AtN _c , the residual stresses also show a noticeable relaxation, and the dislocation density shows a remarkable increase. IfN _c is plotted against the applied total strain amplitudes, a Manson-Coffin's relationship is obtained with an exponent of 0.39. It is recommended that monitoring the continuous changes in DC resistance could provide a reliable nondestructive evaluation of low-cycle fatigue life of a multi-layered thin film structure.     

Fatigue assessment of laserbeam‐welded aluminium joints under multiaxial loading

This paper presents the results and evaluation of the multiaxial fatigue behaviour of laserbeam‐welded overlapped tubular joints made from the artificially hardened aluminium alloy AlSi1MgMn T6 (EN AW 6082 T6) under multiaxial loadings with constant and variable amplitudes. Several fatigue test series under pure axial and pure torsional loadings as well as combined axial and torsional proportional and non‐proportional loadings have been carried out in the range of 2·10⁴ to 2·10⁷ cycles. The assessment of the investigated thin‐walled joints is based on a local notch stress concept. In this concept the fatigue critical area of the weld root is substituted by a fictitious notch radius r_ref = 0.05 mm. The equivalent stresses in the notch, considering especially the fatigue life reducing influence of non‐proportional loading in comparison to proportional loading, were calculated by a recently developed hypothesis, which is called the Stress Space Curve Hypothesis (SSCH). This hypothesis is based on the time evolution of the stress state during one load cycle. In addition, the fatigue strength evaluation of multiaxial spectrum loading was carried out using a modified Gough‐Pollard algorithm.     

Mechanical fatigue of Sn-rich Pb-free solder alloys

Recent fatigue studies of Sn-rich Pb-free solder alloys are reviewed to provide an overview of the current understanding of cyclic deformation, cyclic softening, fatigue crack initiation, fatigue crack growth, and fatigue life behavior in these alloys. Because of their low melting temperatures, these alloys demonstrated extensive cyclic creep deformation at room temperature. Limited amount of data have shown that the cyclic creep rate is strongly dependent on stress amplitude, peak stress, stress ratio and cyclic frequency. At constant cyclic strain amplitudes, most Sn-rich alloys exhibit cycle-dependent and cyclic softening. The softening is more pronounced at larger strain amplitudes and higher temperatures, and in fine grain structures. Characteristic of these alloys, fatigue cracks tend to initiate at grain and phase boundaries very early in the fatigue life, involving considerable amount of grain boundary cavitation and sliding. The growth of fatigue cracks in these alloys may follow both transgranular and intergranular paths, depending on the stress ratio and frequency of the cyclic loading. At low stress ratios and high frequencies, fatigue crack growth rate correlates well with the range of stress intensities or J-integrals but the time-dependent C* integral provides a better correlation with the crack velocity at high stress ratios and low frequencies. The fatigue life of the alloys is a strong function of the strain amplitude, cyclic frequency, temperature, and microstructure. While a few sets of fatigue life data are available, these data, when analyzed in terms of the Coffin–Mason equation, showed large variations, with the fatigue ductility exponent ranging from −0.43 to −1.14 and the fatigue ductility from 0.04 to 20.9. Several approaches have been suggested to explain the differences in the fatigue life behavior, including revision of the Coffin–Mason analysis and use of alternative fatigue life models.