Cyclic behavior of 9–12% Cr steel under different control modes in low cycle regime: A comparative study

Cyclic behavior of 9–12% Cr steel under both stress and strain control modes was investigated at 873 K. Significant asymmetric deformation and cyclic softening were observed for both modes. Under the strain-controlled fatigue, a strain level-independent softening factor (SF) was observed. The SF was dependent on applied stress under the stress-controlled fatigue. For the strain-controlled fatigue, the magnitude of cyclic asymmetry decreases with the increase of strain amplitude, while under the stress-controlled fatigue, the asymmetry increases with the increase of stress amplitude. The anomalous ratcheting strain resulted from the asymmetry under the stress-controlled mode has a detrimental effect on fatigue life.     

Investigating environmental effects on small fatigue crack growth in Ti–6242S using combined ultrasonic fatigue and scanning electron microscopy

In situ ultrasonic fatigue with a cyclic frequency of 20 kHz was employed in an environmental scanning electron microscope (ESEM) to characterize fatigue crack formation and growth in the near alpha titanium alloy Ti–6242S. The role of environment on small fatigue crack initiation and growth was investigated in vacuum and in variable pressures of saturated water vapor, as well as in laboratory air. Small crack growth behavior from cracks initiated at FIB-produced micro-notches indicated a significant environmental dependence, with fatigue crack growth rates increasing with increasing partial pressures of water vapor. Environment also influenced crack initiation lifetime in that cracks initiated earlier in laboratory air than in vacuum or saturated water vapor environments. Transgranular, crystallographic crack growth was observed in each environment, with the crack path in primary α grains producing facets parallel to basal planes when crack size was small. Small crack growth resistance had a marked sensitivity to microstructural features, such as α/α grain boundaries with high misorientation and α/α + β boundaries. These initial investigations demonstrate the usefulness of in situ ultrasonic fatigue instrumentation (UF-SEM) as a new tool for the characterization of environmental and microstructural influences on very high cycle fatigue (VHCF) behavior.     

Mechanics and mechanisms of fatigue in a WC–Ni hardmetal and a comparative study with respect to WC–Co hardmetals

There is a major interest in replacing cobalt binder in hardmetals (cemented carbides) aiming for materials with similar or even improved properties at a lower price. Nickel is one of the materials most commonly used as a binder alternative to cobalt in these metal-ceramic composites. However, knowledge on mechanical properties and particularly on fatigue behavior of Ni-base cemented carbides is relatively scarce. In this study, the fatigue mechanics and mechanisms of a fine grained WC–Ni grade is assessed. In doing so, fatigue crack growth (FCG) behavior and fatigue limit are determined, and the attained results are compared to corresponding fracture toughness and flexural strength. An analysis of the results within a fatigue mechanics framework permits to validate FCG threshold as the effective fracture toughness under cyclic loading. Experimentally determined data are then used to analyze the fatigue susceptibility of the studied material. It is found that the fatigue sensitivity of the WC–Ni hardmetal investigated is close to that previously reported for Co-base cemented carbides with alike binder mean free path. Additionally, fracture modes under stable and unstable crack growth conditions are inspected. It is evidenced that stable crack growth under cyclic loading within the nickel binder exhibit faceted, crystallographic features. This microscopic failure mode is rationalized on the basis of the comparable sizes of the cyclic plastic zone ahead of the crack tip and the characteristic microstructure length scale where fatigue degradation phenomena take place in hardmetals, i.e. the binder mean free path.     

A physically based fatigue damage model for fibre-reinforced plastics under plane loading

The subject of the present article is a new layer-based fatigue damage model (FDM) for laminated multidirectional laminates exposed to general states of plane stress, which allows for simulating the stiffness and the strength degradation by means of a FEM analysis. The essential of the new model is its use of an energy approach which makes the fatigue life prediction of composites more physical and therewith its characterisation much less extensive. Since the failure analysis bases on an interacting failure criterion, the material degradation depends also on the failure mode and is layer-based. (Typical fatigue phenomena as stress redistributions and sequence effects can be analysed with the new model and due to its efficiency it is also applicable to larger structures.)The paper presents a partial validation of the model based on experimental results from the literature and different application examples. These are a shell with a hole and a rotor blade of a wind energy converter demonstrating the analysis of stress redistribution, sequence effects and the applicability of the model to large structures respectively.     

Fatigue behavior and modeling of short fiber reinforced polymer composites: A literature review

Applications of short fiber reinforced polymer composites (SFRPCs) have been rapidly increasing and most of the components made of these materials are subjected to cyclic loading. Therefore, their fatigue behavior and modeling have been of much interest in recent years. This literature review presents a broad review of the many factors influencing cyclic deformation, fatigue behavior, and damage development in SFRPCs. These include microstructural related effects as well as effects related to loading condition and their service environment. Microstructural related effects include those related to fiber length, content and orientation, surface treatment, and failure mechanisms. Cyclic deformation and softening, viscous characteristics, and dissipative response used to characterize and model their fatigue damage behavior and accumulation are discussed. The effects of stress concentrations and their gradient on fatigue behavior are also discussed, due to their significant influence. The effects related to the loading condition include mean stress effects which may be accompanied by cyclic creep, variable amplitude loading, and multiaxial stress effects. Since fatigue behavior is substantially influenced by the testing frequency with self-heating as the primary consequence of increased frequency, this effect is also investigated. Environmental effects considered include the effects of moisture content and temperature, as well as thermo-mechanical fatigue behavior. The effect of welded joints in manufactured components made of SFRPCs and fatigue analysis and life estimation techniques used for such components are also included.     

Fatigue strength assessment of laser stake-welded T-joints using local approaches

This study investigates the applicability of local stress- and energy-based approaches to the fatigue strength assessment of laser stake-welded T-joints. The T-joint has two crack-like notches with infinitesimal tip rounding on each side of the weld. The local approaches used are fictitious tip rounding of 1 and 0.05 mm and the approaches that assume zero rounding within this study, namely the stress intensity factor, the average strain energy density and the J-integral.It is shown that the slopes of the fatigue resistance curves vary between 4 and 8 under different loading conditions imposed on the joint. The slope value exhibits a linear relation with the dimensionless gradient of the maximum principal stress evaluated at the critical notch tip. The same linear relation between the slope and the gradient is valid for all approaches. Because of the slopes, which differ from the usual 3, the agreement of the T-joints with other steel joints is obtained at five million load cycles. The exception is the fictitious rounding concept of 0.05 mm, in which case the recommended design S–N curve with a slope of 3 appears overly conservative in a high-cycle regime.     

Thermal aging effect on the ratcheting-fatigue behavior of Z2CND18.12N stainless steel

The uniaxial tensile and ratcheting-fatigue behaviors of the Z2CND18.12N austenitic stainless steel at room temperature were studied with different thermal aging periods (from 1 h to 500 h) at different thermal aging temperatures (500 °C and 700 °C). The thermal aging process resulted in apparent changes in the ratcheting behavior and the ratcheting-fatigue life. The precipitates under different thermal aging conditions were identified by SEM observation. Considering the deterioration of the material induced by thermal aging process, aging damage factor was introduced to predict the ratcheting-fatigue life, which resulted in good prediction for all the thermal aging conditions.     

Fatigue of swollen elastomers

The compatibility of the properties of elastomer with conventional diesel fuel has made it favourable in many engineering applications. However, due to global energy insecurity issues, there is an urgent need to find alternative renewable sources of energy as replacements to conventional diesel. In this respect, biodiesel appears to be a promising candidate. Hence, research into the compatibility and fatigue characteristics of elastomers exposed to biodiesel becomes essential. The present paper introduces the first attempt to investigate the effect of different solvents on the fatigue of swollen elastomers. The filled nitrile rubbers are immersed in the palm biodiesel and conventional diesel to obtain the same degree of swelling prior to the application of uniaxial fatigue loading. Field Emission Scanning Electron Microscopy (FESEM) analysis is carried out to observe the fracture surfaces. Stretch-N curves are plotted to illustrate the fatigue life duration. These curves showed that the fatigue lifetime of rubber is the longest for dry rubber and the least for rubber swollen in biodiesel. FESEM micrographs reveal that the loading conditions have no effect on the crack initiation and propagation patterns regardless of the swelling state.     

Calculating fatigue limits of notched components of arbitrary size and shape with cracks growing in mode I

This paper describes the implementation of an alternating technique that uses a short crack growth model in combination with the Finite Element Method (FEM) to calculate fatigue limits. Components of any size and shape can, in principle, be analyzed, but the technique is specially suitable for “small” notched components, i.e., components that are obviously larger than the crack itself but not so large as to allow the adoption or use of infinite-medium solutions and where the “back” surfaces or boundaries of the component (other than the notch itself) may influence the propagation of the fatigue crack. This work represents a first application of the technique and is limited to plane problems where fatigue cracks, for reasons of symmetry, for example, grow in mode I alone. The tool is validated by applying it to several problems of specimens with notches of different forms and sizes. Comparisons with experimental results and with prediction obtained by other methods are presented.     

Random multiaxial fatigue: A comparative analysis among selected frequency and time domain fatigue evaluation methods

Fatigue analysis of mechanical components subjected to random loads has been recently upgraded through several developments of calculation procedures, with the scope to support the designer within the loading condition numerical simulation. Under such scenario, the frequency domain approach is characterized by interesting features, which support its adoption in alternative or in conjunction with the classic time-domain approach, especially when the frequency domain is applied for the individuation of the component critical locations. The major goal of this paper consists of an overview about the strength and weaknesses of frequency approach with respect to the time domain one by comparing the reference time domain methods with their frequency domain translation. A significant test case development will be shown, representing a classic automotive one (chassis validation). Promising results of the frequency method application will be presented, encouraging its adoption on large scale.