Fatigue damage of low amplitude cycles in low carbon steel

The influences of low load cycles on fatigue damage in 0.15% C steel (C15E, No. 1.1141) are investigated in the very high cycle fatigue regime using ultrasonic fatigue testing equipment. Constant amplitude (CA) endurance limits at limiting lifetime of 10⁹ cycles are determined in cyclic tension–compression and cyclic torsion tests. Non-propagating fatigue cracks are found in specimens subjected to cyclic torsion loading at the endurance limit. The endurance limit is considered as maximum stress amplitude where possibly initiated fatigue cracks do not propagate to failure. Two-step variable amplitude (VA) tension–compression endurance tests are performed with repeat sequences consisting of high stress amplitudes above the endurance limit and far greater number of cycles below. The measured lifetimes are compared with linear damage accumulation calculations (Miner calculations). If the high stress amplitude is more than approximately 13% above the CA endurance limit, detrimental influences of low load cycles and failures at low damage sums are found. If the high stress is less than 13% above the CA endurance limit, numerous low load cycles cause prolonged fatigue lifetimes and specimens can sustain large damage sums without failure. Two-step VA fatigue crack growth investigations show that load cycles below the threshold stress intensity accelerate crack growth, if the high stress intensity is 18% or more above the CA threshold stress intensity. In repeat sequences with high stress intensities 14% above threshold stress intensity, low load cycles decelerated and stopped fatigue crack growth. Low load cycles can reduce or prolong fatigue lifetimes of low carbon steel and one reason is the accelerated or retarded fatigue crack growth due to numerous low amplitudes, and the maximum load amplitude of a VA load sequence determines whether detrimental or beneficial effects prevail.     

Very high cycle fatigue of VDSiCr spring steel under torsional and axial loading

Very high cycle fatigue (VHCF) properties of VDSiCr spring steel are investigated with ultrasonic equipment under fully reversed cyclic torsion loading and under cyclic axial loading at load ratios R = –1, R = 0.1 and R = 0.5. Shot‐peened specimens with surface finish similar to valve springs in combustion engines are tested until limiting lifetimes of 10¹⁰ cycles. Under cyclic torsion loading, specimens either fail below 10⁶ cycles with crack initiation at the surface or they do not fail. Under cyclic axial loading, failures above 10⁹ cycles were found for all load ratios with crack initiation at the surface or at internal inclusions. Ratio of mean endurance limit (50% failure probability at 10¹⁰ cycles) under fully reversed cyclic torsion and cyclic tension‐compression loading is 0.86. Cyclic torsion loading slightly below the endurance limit leads to cyclic softening first followed by cyclic hardening whereas cyclic stability is found for tension‐compression loading. Cyclic torsion reduces surface compression stresses whereas they are hardly affected by cyclic tension‐compression loading. Mean endurance limit at 10¹⁰ cycles for R = 0.1 is 61% of the endurance stress amplitude at load ratio R = –1, and for R = 0.5 it is 44% of the tension‐compression endurance limit. Endurance limits for cyclic torsion and cyclic tension‐compression loading are comparable, if effective stress amplitude is used that considers cyclic normal stresses and residual compression stresses at the surface.     

Short‐crack thresholds and propagation in an AISI 4340 steel under the effect of SP residual stresses

The effect of residual stresses induced by shot‐peening in a high‐strength AISI 4340 steel has been studied with the purpose of deriving a consistent fatigue model incorporating the results of fatigue crack growth experiments in the threshold region for a broad range of load ratio (R‐ratio ranging from ?2.5 to 0.7), and the effect of short cracks by means of a modified El‐Haddad model. The proposed model, taking into account the effect of crack closure and being capable to assess the conditions for fatigue propagation of short cracks partially embedded in the shot‐peened surface layer, was validated against constant amplitude fatigue experiments conducted in the endurance strength region, ie, for fatigue lives up to 10⁷ cycles, with micronotched specimens in the presence of shot‐peening residual stresses. The proposed model was also validated by comparing the results of fatigue crack propagation simulations with fatigue crack growth experiments under variable amplitude loading, experimentally reproducing the combined effect of service fatigue loads and shot‐peening residual stresses.     

Fatigue strength of small-notched specimens under variable amplitude loading within the fatigue limit diagram

Although the fatigue limit diagram is defined in principle for constant stress amplitude, it is often considered that fatigue failure would not occur, even in varying loading, if applied stresses were kept within the fatigue limit diagram. However, it was shown in the case of small‐notched specimens that fatigue failure occurred in some special cases of variable amplitude loading, even when all stress amplitudes were kept within the fatigue limit diagram. The cause of this phenomenon was examined using two‐step stress and repeated two‐step stress patterns in which the first step stress was chosen to be equal to the fatigue limit with zero mean stress and a mean stress was superposed on the second step stress. A non‐propagating crack was formed by the first step stress. This crack functioned as a pre‐crack for the second step stress with high mean stress. Consequently, fatigue failure occurred even when all stress amplitudes were kept within the fatigue limit diagram. It was an unexpected fracture caused by the interference effect of a non‐propagating crack and a mean stress change.     

Numerical fatigue life analysis of a high frequency mechanical impact treated industrial component based on damage mechanics models

After post weld treatment with high frequency mechanical impact (HFMI) treatment of welds, a significant increase of fatigue life (up to a factor of 10) can be achieved. During the last years numerous experimental tests of welded joints with simple geometry under constant amplitude loading have been performed to quantify the positive effect of high frequency mechanical impact treatment. Due to the lack of methods for the prediction of the high frequency mechanical impact benefits, a widespread use of this process is not the case yet. Furthermore, it is still not clear if the results of these fatigue tests can be transferred to complex geometries and complex loading conditions such as in industrial applications. Therefore, an approach to assess the fatigue life of complex welded structures under variable amplitude loading was developed. For this purpose, high frequency mechanical impact treatment and fatigue load of simple welded specimen made of S690QL steel were simulated with finite element analysis (FEA) firstly. Then, the needed damage parameters for the fatigue life correlation were evaluated from the finite element post‐processing. The calculated life time to crack initiation was in good agreement with the experimental fatigue test results. In the next step, this procedure was implemented on a welded arm of an evacuator of type EW180B of the company Volvo Construction Equipment made of S700MC. The variable amplitude load measured under real service condition was transferred to single constant amplitude load cycles using a rainflow‐counting algorithm. By simulation and damage mechanics evaluation of each load cycle the total damage sum could be calculated and compared with the experimental results from Volvo Construction Equipment.     

Fretting fatigue under variable loading below fretting fatigue limit

The fatigue limit diagram provides the critical condition of non‐failure against fatigue under constant amplitude loading. The fatigue limit diagram is usually considered to give the allowable stress if every stress component is kept within the fatigue limit diagram. In the case of variable amplitude fretting fatigue, however, this study showed that fatigue failure could occur even when all stresses were within the fatigue limit diagram. An example of such a condition is a repeated two‐step loading such as when the first step stress is R=?1 and the second step stress has a high mean value. The reason why such a phenomenon occurs was investigated. A non‐propagating crack was formed by the first step stress even when well below the fatigue limit. The resultant non‐propagating crack functioned as a pre‐crack for the second step stress with a high mean value. Consequently, fatigue failure occurred even when every stress was within the fatigue limit diagram of constant amplitude loading. The fatigue limit diagram obtained in constant amplitude fatigue test does not necessarily guarantee safety in the case of variable amplitude loading in fretting fatigue.     

Cumulative fatigue damage of stress below the fatigue limit in weldment steel under block loading

To investigate the cumulative fatigue damage below the fatigue limit of multipass weldment martensitic stainless steel, and to clarify the effect of cycle ratios and high‐stress level in the statement, fatigue tests were conducted under constant and combined high‐ and low‐stress amplitude relative to stress above and below the fatigue limit. The outcomes indicate that neither modified Miner's nor Haibach's approach provided accurate evaluation under repeated two‐step amplitude loading. Moreover, effect of cycle ratios has been determined. Additionally, the cumulative fatigue damage saturated model is established and validated. Cumulative fatigue damage contributed by low‐stress below the fatigue limit in high stress of 700 MPa is higher than that with 650 MPa at identical conditions (fatigue limit 575 MPa). Thus, high stress affects fatigue damage behaviour below the fatigue limit. A new predicted approach has been proposed based on Corten‐Dolan law, whose accuracy and applicability have been proven.     

Variable amplitude fatigue of autofrettaged diesel injection parts

Experimental and analytical investigations of constant and variable amplitude fatigue life of not autofrettaged and autofrettaged components have been performed. In variable amplitude loading the new standardised CO mmon‐ RA il‐ L oad sequence CORAL has been used as well as two‐level‐tests with small cycles at high mean stresses interrupted by large cycles for the evaluation of load sequence effects. The results of the two level tests show that small cycles with amplitudes far below the fatigue limit cause fatigue damage. Life calculations have been performed according to the nominal stress approach with S‐N‐curves and improved Miner’s Rule, linear‐elastic fracture mechanics with 3D‐weight functions, elastic‐plastic fracture mechanics applying an extended strip yield‐model, and explicit 3D‐FE‐simulation of fatigue crack growth with predefined crack fronts. All approaches are appropriate for predicting realistic variable amplitude lives. From a practical point of view the explicit 3D‐FE‐simulation of fatigue crack growth is too time‐consuming. However, such simulations show that the approaches based on linear‐elastic fracture mechanics and elastic‐plastic fracture mechanics with extended strip yield‐model capture the essential physics of fatigue crack growth in a realistic way.     

(Al2O3)f/Al复合材料在强界面结合下的疲劳损伤模式

在拉-拉载荷下测定了(Al₂O₃)_f/Al复合材料的疲劳寿命(S-N)曲线。通过夭折试验以及SEM疲劳断口和纵截面组织结构分析,研究了复合材料的疲劳损伤模式。研究结果表明,(Al₂O₃)_f/Al复合材料的疲劳极限为750MPa,远高于SCS-6碳化硅纤维增强钛基复合材料。该复合材料兼有钛基和树脂基纤维复合材料疲劳损伤的特点,高应力下由单个裂纹的起源和生长导致复合材料的失效;低应力下,疲劳损伤模式包括纤维劈裂、众多基体裂纹和单个基体裂纹的横向扩展。其中纤维劈裂是主控机制。其更高的疲劳极限可归因于低应力下纤维的纵向劈裂。     

Effect of submillimeter size holes on the fatigue limit of a high strength tool steel

The very high cycle fatigue and small fatigue crack growth behaviour of a generic tool steel material for diesel fuel injector application are described. The small crack growth tests for the tool steel material with and without the hardening heat treatment revealed the mechanisms of crack propagation and threshold behaviour. Based on the small fatigue crack propagation threshold value, an elastic plastic fracture mechanics methodology for the prediction of the endurance limit of specimens with submillimeter holes is proposed. The advantages of the new methodology are discussed in relation to existing methodologies for endurance limit prediction of specimens with small holes.     

Effect of small notch and absorbed hydrogen on the fatigue fracture in two-step stress test within fatigue limit diagram

It is usually regarded as a common understanding that fatigue failure would not occur if all stresses were kept within fatigue limit diagram. However, it was shown that fatigue failure occurred in some special cases of variable amplitude loading condition even when all stresses were kept within fatigue limit diagram in the case of small-notched specimen. The cause of such a phenomenon was examined using two-step stress pattern for low alloy steel SCM440H. In the case of constant stress amplitude loading, non-propagating crack was formed only at low mean stress and not formed at high mean stress. However, in the case of two-step stress pattern in which the first step stress was chosen as  R  =−1 and the second step stress was with high mean stress, a non-propagating crack was formed by the first step stress. This crack functioned as a pre-crack for the second step stress with high mean stress. Consequently, fatigue failure occurred by the stresses within fatigue limit diagram. In this study, the effect of notch size and shape were examined. The effect of absorbed hydrogen was also investigated. Absorption of 0.3 ppm hydrogen caused more reduction of fatigue limit.     

Crack propagation of pipeline steel exposed to a near-neutral pH environment under variable pressure fluctuations

This investigation attempts to understand the corrosion fatigue crack propagation behavior of pipeline steels exposed to near-neutral pH environments. The fatigue loading was designed to simulate the underload-type variable amplitude pressure fluctuations found during pipeline operation. The effects of amplitudes (R ratios) of underload and minor cycles were investigated. It has been found from this investigation that the crack growth rate is enhanced significantly through load interaction of the variable amplitude fatigue. The acceleration factor is found to be up to 2.7 and 5.3 for tests in air and in the near-neutral pH solution, respectively. The crack growth rate decreases with R ratios of underload and minor cycles for tests both in air and in near-neutral pH environments. The latter could enhance crack propagation by a factor of up to 11, as compared with the crack growth rate in air. The critical R ratio of minor cycles at which the minor cycles do not contribute to crack propagation through load interaction was determined to be as high as 0.982, which is much lower than the threshold determined by constant amplitude fatigue. This critical R ratio could be utilized to demarcate stress corrosion cracking and corrosion fatigue, and should be incorporated as one of the design principles for components/structures subjected to variable amplitude cyclic loading.     

A MICROCRACK GROWTH LAW FOR MULTIAXIAL FATIGUE

Within the past decade, critical plane approaches have gained increasing support based on correlation of experimentally observed fatigue lives and microcrack orientations under predominately low cycle fatigue (LCF) conditions for various stress states. In this paper, we further develop an engineering model for microcrack propagation consistent with critical plane concepts for correlation of both LCF and high cycle fatigue (HCF) behavior, including multiple regimes of small crack growth. The critical plane microcrack propagation approach of McDowell and Berard serves as a starting point to incorporate multiple regimes of crack nucleation, shear growth under the influence of microstructural barriers, and transition to linear crack length-dependent growth related to elastic-plastic fracture mechanics (EPFM) concepts. Microcrack iso-length data from uniaxial and torsional fatigue tests of 1045 steel and IN 718 are examined and correlated by introducing a transition crack length which governs the shift from nonlinear to linear crack length dependence of da/dN. This transition is related to the shift from strong microstructural influence to weak influence on the propagation of microcracks. Simple forms are introduced for both the transition crack length and the crack length-dependence of crack growth rate within the microcrack propagation framework (introduced previously by McDowell and Berard) and are employed to fit the 1045 steel and IN 718 microcrack iso-length data, assuming preexisting sub-grain size cracks. The nonlinear evolution of crack length with normalized cycles is then predicted over a range of stress amplitudes in uniaxial and torsional fatigue. The microcrack growth law is shown to have potential to correlate microcrack propagation behavior as well as damage accumulation for HCF-LCF loading sequences and sequences of applied stress states.     

On the application of the stochastic approach in predicting fatigue reliability using Miner's damage rule

The present paper investigates the application of the stochastic approach when the commonly adopted Miner's linear damage rule is implemented, both in its traditional and modified forms to include the presence of a random stress threshold (random fatigue limit), below which the rate of damage accumulation is reduced. Main steps are provided to obtain the simulated distribution of the accumulated damage under variable amplitude loading. When the stochastic approach is applied in the presence of a random fatigue limit, an additional correlation structure, which takes into account the fatigue limit value, must be introduced in the analysis. If the number of cycles to failure under constant amplitude loading is Weibull (Log‐Normal) distributed, then the corresponding accumulated damage is Fréchet (Log‐Normal) distributed. The effects of the correlation structure on reliability prediction under variable amplitude loading are also investigated. To this aim, several experimental datasets are taken from the literature, covering various metallic materials and variable amplitude block sequences. The results show that the choice of the damage accumulation model is a key factor to value the improvement in the accuracy of reliability predictions introduced by the stochastic approach. Comparison of the predicted number of cycles to failure with experimental data shows that larger errors are non‐conservative, regardless of the adopted correlation structure. When the analysis is limited to reliability levels above 80%, for these large non‐conservative errors, it is the quantile approach to be closer to actual experimental data, thus limiting the overestimation of component's life. For the experimental datasets considered in the paper, adoption of a stochastic approach would improve the accuracy of Miner's predictions in 10% of cases.     

The effect of stress transients on the HCF endurance limit

Constant amplitude fatigue of a material at a fixed stress ratio, R, and at some limiting stress level, may produce high cycle fatigue (HCF) lives in excess of some large number, typically 10⁷ or higher, which can be treated as an endurance limit. Under vibratory loading, stress transients can exceed this endurance limit amplitude and cause damage that accumulates with repeated transient loading. These HCF transients normally occur at lower stress amplitudes than those needed to cause low cycle fatigue (LCF) where lives, N, are typically in the range N < 10⁴–10⁵. Therefore, the HCF transient stresses produce cycles to failure beyond the normal LCF regime but correspond to amplitudes that are above the fatigue limit stress. In this investigation, a titanium alloy, Ti-6Al-4V, is subjected to HCF stress transients while being cycled under constant amplitude HCF. The HCF transients correspond to blocks of loading above the fatigue limit stress applied for a specified fraction of their expected life. A step-loading procedure is used to determine the fatigue limit stress at a frequency of 420 Hz. Stress transients applied at stresses up to 40% above the endurance limit for cycle counts up to 25% of expected life are found to have little or no effect on the fatigue limit stress. Simple calculations of the propagation life in a test specimen show that most of the life at these transient stress levels is spent in the nucleation phase. Fractography, aided by heat tinting, was unable to detect any prior cracks due to the HCF stress transients on the fractured specimens.     

Estimation of the endurance fatigue limit for structural steel in load increasing tests at low temperature

The endurance fatigue strength of structural steel S355 was investigated in fatigue tests according to the method of increasing stress amplitude. The so‐called ‘load increasing test’ is based on the direct correlation between the fatigue limit and the temperature changes caused by local plastic deformation ahead of the tip of a micro crack, which was initiated as a result of cyclic loading. In the present work the fatigue limit for testing temperatures 40°C and ?20°C was estimated not only from the temperature measurements but also from the electrical potential measurements. Further, the obtained results were validated in standard fatigue tests with constant stress amplitude and a very good agreement was found.     

INTERACTION OF OVERLOADS ON FATIGUE CRACK GROWTH FOR 40CrNi STEEL

This work was designed to improve the general understanding of loading sequence effects on fatigue crack growth and lead to the development of improved methods for predicting crack propagation behaviour. Two loading histories were selected (1) a baseline amplitude with periodic overloads or underloads, and (2) several overloads without interactive effects. The specimen used was of a Wedge Opening Loading type and the material was a low alloy high strength steel, i.e. 4OCrNi. It was found that the Linear Summation of Damage (LSD) assumption could be applied in predicting fatigue crack growth rate (FCGR) under periodic overloads or underloads within the scatterband of the constant amplitude data obtained using an Alternating Current Potential Drop technique for measuring crack length for multiple specimens with several load amplitudes. A discrepancy existed between the FCGR predicted from LSD and the one actually measured during several hundreds of loading cycles immediately following every non-interactive overload of the latter loading history although the overload ratio was the same as that of periodic overloading. The causes of this phenomenon are discussed.     

Variable amplitude cyclic deformation and fatigue behaviour of stainless steel 304L including step,periodic, and random loadings

This paper discusses cyclic deformation and fatigue behaviours of stainless steel 304L and aluminium 7075‐T6 under variable amplitude loading using strain‐controlled as well as load‐controlled tests. Load sequence effects were investigated in step tests with high‐low and low‐high sequences. For stainless steel 304L, strong hardening induced by the first step of the H‐L sequence significantly affects the fatigue behaviour, depending on the test control mode used. For periodic overload tests of stainless steel 304L, hardening due to the overloads was progressive throughout life and more significant than in H‐L step tests. For aluminium 7075‐T6, no effect on deformation behaviour was observed due to periodic overloads. However, the direction of the overloads was found to affect fatigue life, as tensile overloads led to longer lives, while compressive overloads led to shorter lives. Deformation and fatigue behaviours under random loading were also studied for the two materials. To correlate a broad range of fatigue life data for a material with strong deformation history effect, such as stainless steel, it is shown that a damage parameter with both stress and strain is required. The Smith‐Watson‐Topper parameter as such a parameter is shown to correlate the data reasonably well under different control modes and loading conditions.     

A computationally efficient cohesive zone model for fatigue

A cohesive zone model has been developed for the simulation of both high and low cycle fatigue crack growth. The developed model provides an alternative approach that reflects the computational efficiency of the well‐established envelop‐load damage model yet can deliver the accuracy of the equally well‐established loading‐unloading hysteresis damage model. A feature included in the new cohesive zone model is a damage mechanism that accumulates as a result of cyclic plastic separation and material deterioration to capture a finite fatigue life. The accumulation of damage is reflected in the loading‐unloading hysteresis curve, but additionally, the model incorporates a fast‐track feature. This is achieved by “freezing in” a particular damage state for one loading cycle over a predefined number of cycles. The new model is used to simulate mode I fatigue crack growth in austenitic stainless steel 304 at significant reduction in the computational cost.