Elastic Fatigue Research Articles

Electric-field-induced stress relaxation in α-phase poly(vinylidene fluoride) films

The relationship between elastic fatigue and electrical cyclic loading in α-phase poly(vinylidene fluoride) films has been investigated. Our experimental studies have shown that the electric-field-induced fatigue behavior can be described by a stress relaxation, which belongs to the Kohlrausch function group, and the corresponding exponent is a modified two-parameter Weibull distribution function.

The effect of elastic fatigue on the output transmission of a Mach–Zehnder interferometer comb filter

The elastic characteristics of linear birefringence and its effect upon the output of an optical fiber dual-pass MZI comb filter are theoretically and experimentally analyzed. The result shows that the linear birefringence is induced by the mutual pressure applied on fiber due to the applied stress. As a result of induced birefringence, the peak wavelength shifts continuously with the continuous change in stress up to a certain value of the applied stress. But for discrete change in stress the peak wavelength shifting is not continuous. Theoretically, the gradients of peak wavelength position shifting 0.16nm/N and 0.36nm/N are calculated for the maximum and minimum effective length respectively. Experimental observations of the gradients acquire values of 0.14nm/N and 0.40nm/N. These results might be useful for pressure sensing technique.

Research on the Effect of Load on the Fatigue Tearing of the Elastic Element

According to engineering implement, the elastic element of the rubber isolator is easy prone to fatigue tearing, the model of the pump group floating raft vibration isolation system is established by ADAMS and the finite element ANSYS software, the force distribution of the system is calculated in the different swing angles in ADAMS, and the main reason of the elastic element fatigue tearing is found. Then, the strain change rule of the elastic element is researched in ANSYS, the elastic element which is the most vulnerable to fatigue tearing and its crack location are found, which plays an important role in solving the fatigue tearing of the rubber elastic element in the engineering practice.

Fatigue Life Prediction of Girth Gear-Pinion Assembly Used in Kilns by Finite Element Analysis

This study is focused on predicting the fatigue life expectancy of Girth gear-pinion assembly used in cement industries. Gear design and modeling was carried out using a CAD package and analysis was done using finite element analysis software, ANSYS. AISI 4135-low alloy steel material properties are considered and linear elastic finite element analysis and fatigue life analysis were carried out. The variable amplitude load is applied to simulate the real time loading of the gear-pinion assembly. Rainflow cycle counting algorithm and Minars linear damage rule is employed to predict the fatigue life. The critical stress and the corresponding deformation are discussed in the results. Finally the life expectancy of the girth gear and pinion assembly is estimated which would be useful for the periodical maintenance of the gear assembly.

Study on the Inwall Crack Extension and Fatigue Life Evaluation for Deep-Sea Steel Catenary Riser under Wave and Current Action

Aimed at deep-sea steel catenary risers(SCRs) transporting supercritical or dense CO2, this paper uses the macroscopic pipeline-transporting finite element model and the microscopic pipewall crack growth model to numerically simulate corrosive-inwall-defect-induced crack extension along radial direction for the deep-sea riser which is subjected to wave and current loads. Meanwhile, based on linear elastic fracture mechanics and fatigue crack extension theory, the effect of wave height on the fatigue life of the pipewall at touchdown zone is discussed. The above-mentioned study in this paper is expected to give a good reference to the similar study for oil&gas SCRs.

Combining Elastic and Plastic Fatigue Damage in Steel Pipelines, Risers, and Coiled Tubing

Summary Coiled tubing is a continuous pipe that, having been coiled around a reel for storage, can be deployed and used as a pipeline or riser. During deployment as a riser, the coiled tubing is unspooled from the reel, run into the water, and connected to the wellhead. This process plastically strains the pipe, causing plastic (or low-cycle) fatigue damage. When the coiled tubing is connected to the wellhead, the environmental loading causes elastic-stress cycles, resulting in elastic (or high-cycle) fatigue damage. Numerous methods are available to determine fatigue life from either plastic or elastic cycling; however, few data are available within the industry on how the fatigue damages from elastic and plastic cycles combine. This paper presents the experimental work conducted to show the combined fatigue life of notched samples of flat steel used to manufacture coiled tubing that has been plastically and elastically cycled. The data show that the combined fatigue life can be lower than the total of the plastic and elastic fatigue damages by use of Miner's rule. Existing theory suggests that the combined fatigue life could be as low as 10% of the Miner's-rule fatigue damages; however, the experimental data indicate that a more appropriate value is closer to 75%.

G040012 水素マイクロプリント法を用いた7075アルミニウム合金の疲労変形における水素放出の可視化

To improve tank safety and promote the application of aluminum alloys to high-compression hydrogen gas tank liners, it is important to clarify the fatigue properties of aluminum alloys as affected by atmospheric hydrogen. In this study, the effect of fatigue frequency and maximum applied stress on hydrogen evolution in 7075 aluminum alloy was studied by means of a hydrogen microprint technique (HMT). In fatigue deformation, accumulations of hydrogen were observed in the interface between a second phase inclusion (such as an Al_7Cu_2Fe phase) and the matrix. Hydrogen accumulations were also observed within the Al_7Cu_2Fe phase. The amount of hydrogen accumulation increased with decreasing fatigue test frequency. This shows that under elastic fatigue deformation, hydrogen diffusion is influenced by deformation rate. Hydrogen accumulation increased with increasing maximum stress, which indicates that hydrogen evolution during elastic fatigue deformation is controlled by stress-induced diffusion. Depending on the applied stress condition, stress-induced diffusion acted to shift the hydrogen diffusion route from the Al_7Cu_2Fe phase to the interface between that phase and the matrix. This result indicated that the deformation took place over a time sufficiently long for hydrogen to diffuse through that phase.

PS10 7075アルミニウム合金の疲労変形における水素拡散経路の可視化

To improve tank safety for fuel cell vehicles and promote the application of aluminum alloys to high-compression hydrogen gas tank liners, it is important to clarify the fatigue properties of aluminum alloys as affected by atmospheric hydrogen. In this study, the effect of elastic fatigue deformation on hydrogen diffusion for 7075 aluminum alloy was investigated by means of hydrogen microprint technique. The fatigue test was carried out under the various parameters for frequency (10 Hz, 1 Hz, or 0.1 Hz) and applied maximum stress (80%, 60%, or 40% of proof stress (525 MPa for 7075-T6 alloy)). In all fatigue tests, accumulated hydrogen was mainly observed within second phase inclusion such as Al_7Cu_2Fe. This shows that Al_7Cu_2Fe acts as hydrogen diffusion path during elastic fatigue deformation. The amount of hydrogen accumulation increased with decreasing fatigue test frequency, which shows that the hydrogen diffusion is influenced by deformation time. Hydrogen accumulation also increased with increasing maximum stress, which indicates that hydrogen evolution during elastic fatigue deformation is controlled by stress-induced diffusion.

Numerical Modeling and Simulation of Delamination Crack Growth in CF/Epoxy Composite Laminates under Cyclic Loading Using Cohesive Zone Model

This paper presents the mathematical modelling of fatigue damage able to carry out simulation of evolution of delamination in the laminated composite structures under cyclic loadings. A new elastic fatigue damage evolution law is proposed here. A classical interface damage evolution law, which is commonly used to predict static debonding process, is modified further to incorporate fatigue delamination effects due to high cycle loadings. The proposed fatigue damage model is identified using Fracture Mechanics tests like DCB, ENF and MMB. Simulations of delamination under fatigue loading are performed and results are successfully compared with reported experimental data on HTA/6376C unidirectional material. Delamination crack growth with variable fatigue amplitude is also performed and simulation results show that the proposed fatigue damage law can also accommodate this variable amplitude phenomenon. A study of crack tip behaviour using damage variable evolution is also carried out in this paper. Finally the effect of mesh density on crack growth is also discussed.

A high-cyclic elastic fatigue damage model for carbon fibre epoxy matrix laminates with different mode mixtures

This paper presents the development of a fatigue damage model able to carry out simulation of evolution of delamination in the laminated composite structures under cyclic loadings. A classical interface damage evolution law, which is commonly used to predict static debonding process, is modified further to incorporate fatigue delamination effects due to high cycle loadings. The proposed fatigue damage model is identified using fracture mechanics tests DCB, ENF and MMB. Then a non-monotonic behaviour is used to predict the fatigue damage parameters able to carry out delamination simulations for different mode-mixtures. Linear Paris plot behaviour of the above mentioned fracture mechanics tests are successfully compared with available experimental data on HTA/6376C and AS4/PEEK unidirectional materials.

Calculation of strict error bounds for finite element approximations of non‐linear pointwise quantities of interest

AbstractThis paper deals with the verification of simulations performed using the finite element method. More specifically, it addresses the calculation of strict bounds on the discretization errors affecting pointwise outputs of interest which may be non‐linear with respect to the displacement field. The method is based on classical tools, such as the constitutive relation error and extraction techniques associated with the solution of an adjoint problem. However, it uses two specific and innovative techniques: the enrichment of the adjoint solution using a partition of unity method, which enables one to consider truly pointwise quantities of interest, and the decomposition of the non‐linear quantities of interest by means of projection properties in order to take into account higher‐order terms in establishing the bounds. Thus, no linearization is performed and the property that the local error bounds are guaranteed is preserved. The effectiveness of the approach and the quality of the bounds are illustrated with two‐dimensional applications in the context of elastic fatigue problems. Copyright © 2010 John Wiley & Sons, Ltd.

Effect of prior cyclic damage removal on high temperature low cycle fatigue endurance

In terms of fatigue life recovery, the beneficial effect of removing a surface layer from critical locations of components subject to high frequency elastic loading is well documented. Not so well known are the consequences of surface layer removal from parts previously subject to elasto-plastic low cycle fatigue (LCF) loading. The paper describes the results of a research project conducted to evaluate the effects of surface layer removal on subsequent LCF properties using uniaxial testpieces which have been prior cyclic deformed to different life fractions. Strain-controlled LCF tests performed on a 1%CrMoV steel at 565 °C with different strain amplitudes indicate that, following: (i) prior cyclic deformation (PCD) to nominal LCF life fractions of 0.5 and 0.8, (ii) subsequent surface damage layer removal, and (iii) test resumption; the consequent overall LCF endurances are at least as long as those accumulated without intermediate surface damage removal, and in some cases are significantly extended. Unlike the possibilities associated with elastic fatigue loading, LCF lifetimes are not fully recoverable following the removal of prior cyclic surface damage.

Fatigue Life Analysis of Al 8090 Helicopter Fuselage Panels

Considering the aerospace structures, the advantages of Al-Li alloys in comparison with conventional aluminium alloys comprise relatively low densities, high elastic modulus, excellent fatigue and toughness properties, and superior fatigue crack growth resistance. Unfortunately, these alloys have some disadvantages due to highly anisotropic mechanical properties and due to a very high crack growth rate for microstructurally short cracks. This could mean relatively early cracking in high stress regions such as rivet holes in helicopter fuselage panels. Consequently a more accurate approach in fatigue life analysis is requested. Considering that the 8090 T81 aluminium alloy has been widely used in an helicopter structure, in particular in the bolted connection between the stringers and the modular joint frame in the rear of the fuselage, it is extremely important to found a reliable procedure for the fatigue life assessment of the component. Thus, using the results of experimental tests made on panel specimens, a FE general model and two submodels of the critical zone (involved in fatigue damage during the tests) have been modelled in order to investigate the complex state of stress near the rivets holes. These stress values obtained have been elaborated for a fatigue assessment.

Limitations of elastic definitions in Al–Si–Mg cast alloys with enhanced plasticity: linear elastic fracture mechanics versus elastic–plastic fracture mechanics

Linear elastic fracture mechanics describes the fracture behavior of materials and components that respond elastically under loading. This approach is valuable and accurate for the continuum analysis of crack growth in brittle and high strength materials; however it introduces increasing inaccuracies for low-strength/high-ductility alloys (particularly low-carbon steels and light metal alloys). In the case of ductile alloys, different degrees of plastic deformation precede and accompany crack initiation and propagation, and a non-linear ductile fracture mechanics approach better characterizes the fatigue and fracture behavior under elastic–plastic conditions. To delineate plasticity effects in upper Region II and Region III of crack growth an analysis comparing linear elastic stress intensity factor ranges (Δ K el) with crack tip plasticity adjusted linear elastic stress intensity factor ranges (Δ K pl) is presented. To compute plasticity corrected stress intensity factor ranges (Δ K pl), a new relationship for plastic zone size determination was developed taking into account effects of plane-strain and plane-stress conditions (“ combo plastic zone”). In addition, for the upper part of the fatigue crack growth curve, elastic–plastic (cyclic J based) stress intensity factor ranges (Δ K J ) were computed from load–displacement records and compared to plasticity corrected stress intensity factor ranges (Δ K pl). A new cyclic J analysis was designed to compute elastic–plastic stress intensity factor ranges (Δ K J ) by determining cumulative plastic damage from load–displacement records captured in load-control ( K-control) fatigue crack growth tests. The cyclic J analysis provides the true fatigue crack growth behavior of the material. A methodology to evaluate the lower and upper bound fracture toughness of the material ( J IC and J max) directly from fatigue crack growth test data ( Δ K FT ( J IC ) and Δ K FT ( J max ) ) was developed and validated using static fracture toughness test results. The value of Δ K FT ( J IC ) (and implicitly J IC) is determined by comparing the plasticity corrected elastic fatigue crack growth curve with the elastic–plastic fatigue crack growth curve. A most relevant finding is that plasticity adjusted linear elastic stress intensity factor ranges (Δ K pl) are in remarkably good agreement with cyclic J analysis results (Δ K J ), and provide accurate plasticity corrections up to a Δ K corresponding to J IC (i.e. Δ K FT ( J IC ) ). Towards the end of the fatigue crack growth test (above Δ K FT ( J IC ) ) when plasticity is accompanied by significant tearing, the cyclic J analysis provides a more accurate way to capture the true behavior of the material and determine Δ K FT ( J max ) . A procedure to decouple and partition plasticity and tearing effects on crack growth rates is given. Three cast Al–Si–Mg alloys with different levels of ductility, provided by different Si contents and heat treatments (T61 and T4) are evaluated, and the effects of crack tip plasticity on fatigue crack growth are assessed. Fatigue crack growth tests were conducted at a constant stress ratio, R = 0.1, using compact tension specimens.

The elastic moduli and fatigue properties of canine trabecular bone tissue

The elastic modulus and fatigue properties of canine and human trabecular bone tissues (single trabeculae) were experimentally determined on a microstructural level using four-point bending cyclic test, and they were compared based on microstructural characteristics and mineral density. The results showed that canine trabecular bone tissue had significantly lower modulus and lower fatigue strength than human tissue. The observed microstructural differences between the two tissues may be more responsible for the differences, although the lower mineral density in canine tissue might also have contributed to the lower modulus and fatigue strength.

Fatigue behaviour of laser welded joints in steel with a high elastic limit

Summary It is shown that lasers, steels with a high elastic limit and fatigue could, a priori, be considered incompatible elements, whereas in fact, this is not at all the case if sound precautions are taken.

A Correlation among Elastic Modulus Defect, Plastic Strain and Fatigue Life of Metals

A Correlation among Elastic Modulus Defect, Plastic Strain and Fatigue Life of Metals

Fatigue behaviour of microalloyed steels for hot-forged mechanical components

In the manufacture of automotive mechanical parts the quenching and tempering treatment following hot forging can be eliminated by using microalloyed steels. Costs are thus reduced as, too, are distortions and residual stresses, while the mechanical and microstructural properties of components are more uniform through the thickness. Continuously-cast, microalloyed steels can thus be considered interesting as candidate materials for the manufacture of all those components that must possess high mechanical and fatigue strength. This paper reports the results of investigations on the elastoplastic and elastic fatigue behaviour of a series of commercial microalloyed steels, comparing this with the behaviour of quenched and tempered steels having the same strength class and being traditionally used for automotive components. Comparison of the two different families of materials is completed by evaluations of fracture mechanics parameters. Analysis of the results as a whole shows the real possibility of replacing quenched and tempered steels by microalloyed steels of appropriate composition and microstructure.

Plastic Strain Concentration in a Cylindrical Shell Subjected to an Axial or a Radial Temperature Gradient

Plastic strain concentration factors for use in elastic fatigue analyses (like Ke in ASME Code) are usually overly conservative, but may be unsafe in certain cases. Especially for unnotched structures under thermal loading, many elastic-plastic analyses demonstrated that these plastic strain concentration factors are too restrictive. Thus, the present work derives appropriate factors for the idealized case of a cylindrical shell made of a linear kinematic hardening material and subjected to a radial or an axial temperature gradient. The results obtained are considered to be applicable to many practical problems.

Ultrasonic characteristics of graphite/epoxy composite material subjected to fatigue and impacts

Fiber-reinforced composites, because of their superior specific strengths and stiffnesses, are used in many aircraft components. However, in this application these composites are subjected not only to fatigue loading, but to occasionally high velocity impact due to the bird injection, hail, dust, and rain. Thus, it is important to evaluate the residual life and degradation due to combined fatigue and impact loadings. Unidirectional graphite epoxy composites (MA8276-Tiger) which are used in the aerospace industry were impacted by a free falling weight at energy levels of 0.567j, 1.134j, and 1.571j [impact energy toughness (j/cm3); 0.12, 0.24, 0.34], respectively. The subsequent changes/degradation in elastic moduli, strength, toughness, and fatigue properties were measured after different number of impacts. It was found that for all energy levels these properties vary linearly with the number of impacts. Furthermore, attenuation changes is not a good ultrasonic parameter for degradation estimation, since it does not incorporate the micro- and macrocracks beyond the impact point. However, these micro- and macrocracks have significant effect on the mechanical properties. In contrast to the attenuation, the stress wave factor, which indicates the efficiency of wave propagation along the specimen, correlates very well with degradation, and it can be used effectively to measure the residual strength after impact. Ultrasonic characteristic on specimens subjected to combined fatigue and impact were also studied. Based on these experiments, it is concluded that the loss in fatigue residual life due to impact loads may be predicted by measuring the effects of the impact load on attenuation and stress wave factor. It was found that the reduction in fatigue life is proportional to sudden changes in attenuation and stress wave factor. Damage accumulation models based on Coffin-Manson equation, was suggested for impact and combined fatigue and impact. It was found that residual properties and fatigue life can be estimated from these models.