Cyclic Strain Range Research Articles

Influenced of different austenite finish temperature (Af) to the cyclic properties of pseudo elastic NiTi and NiTi hybrid with steel for self-recovery of seismic reinforcing bar

This research highlighting the novel properties of pseudo-elastic Ni-Ti bar owing to their ability to reverse macroscopically inelastic deformation during earthquake known as recentering capability and large elastic strain capacity which originated from the reversible austenite to martensite phase transformation. The homogeneity of Ni and Ti are extremely sensitive to the compositional in austenite finish temperature (Af) which led to phase change transformation behavior and the effect of different size of NiTi bar in design have not comprehensively considered in the previous work. Hence, this paper is presented and evaluates the cyclic properties of three different sizes and austenite finish temperature (Af) of pseudoelastic NiTi and reused NiTi to assess their prospective use as reinforcing bar to be exploited as seismic-resistant design and mitigation. The correlation of hysteretic behavior of Ni-Ti alloy in terms of cyclic loading number and history, mechanical properties at ambient temperature, equivalent damping, energy dissipation, and stress recovery were evaluated. The tensile cyclic test obtained demonstrated a rounded loading curve based on a 0.2% offset and increased with the increase of loading rate corresponding with the values of the stress plateau. The as-received bar and reused NiTi bar exhibited superior pseudo-elastic behavior and recentering through repeated cycling without significant degradation or permanent deformation but low energy damping due to narrow hysteresis while the steel rebar shows vice versa. Experimental results show potential for the use of SMAs in seismic applications and provide areas for continued research. It was concluded that the as-received and the reused pseudo elastic Ni-Ti reinforcing bar are suitable to use for seismic mitigation despite their ability to undergo cyclical strains is 6% to a maximum 8% with minimal residual strain, 0.004% to maximum less than 1%. Due to narrow hysteresis and phase transformation behavior, the 12.7 mm NiTi exhibit the perfect superelasticity with the residual strain (εR) was 0.1076% corresponding with strain recovery capacity of 98.21% for the range of cyclic strains up to 6%. The reused NiTi 12.7 mm exhibits the highest energy dissipation residual strains up to 99.95% for their wide hysteresis and successive to cyclic loading at an 8% strain cycle. In comparison, the strain recovery capacity (%) increased with a decrease in bar size. The energy dissipation and damping increasing with the increasing of the bar size, but decreasing with the increasing of the bar size for reused NiTi Af −6.3, hence correlate in a good agreement corresponding with 8 mm and 12 mm diameter of the bar. However, different relationship demonstrated for NiTi 12.7 mm due to different composition ratio that exhibits different phase transformation behavior.

Evaluating the effectiveness of combined hardening models to determine the behavior of a plate with a hole under combined loadings

Geometrical discontinuities in a material such as holes and notches on machine elements are called as critical regions due to the stress concentrations. They are the potential failure initiation locations Therefore, researchers put significant effort on the prediction of the material response in these discontinuities under repetitive loadings. 
 Cyclic plasticity is concerned with the nonlinear material response under cyclic loadings. In this study, numerical cyclic stress – strain response of a plate with a hole was evaluated under the combined loadings which are cyclic bending and tensile loadings. Oxygen Free High Thermal Conductivity (OFHC) Copper alloy was considered as material, and finite element simulations were performed in Marc software. A user defined material subroutine known as Hypela2 was utilized in order to define the material response. The plasticity model used in the present study comprises J2 plasticity along with combined isotropic – kinematic hardening model. Evolution of the backstress was introduced by Armstrong – Frederic type kinematic hardening model. The results were compared with the literature study, and it was seen that presented hardening model provides accurate results in small cyclic strain range.

Effects of cyclic tensile strain and microgravity on the distribution of actin fiber and Fat1 cadherin in murine articular chondrocytes

Chondrocytes as mechano-sensitive cells can sense and respond to mechanical stress throughout life. In chondrocytes, changes of structure and morphology in the cytoskeleton have been potentially involved in various mechano-transductions such as stretch-activated ion channels, integrins, and intracellular organelles. However, the mechanism of cytoskeleton rearrangement in response to mechanical loading and unloading remains unclear. In this study, we exposed chondrocytes to a physiological range of cyclic tensile strain as mechanical loading or to simulated microgravity by 3D-clinostat that produces an unloading environment. Based on microarray profiling, we focused on Fat1 that implicated in the formation and rearrangement of actin fibers. Next, we examined the relationship between the distribution of Fat1 proteins and actin fibers after cyclic tensile strain and microgravity. As a result, Fat1 proteins did not colocalize with actin stress fibers after cyclic tensile strain, but accumulated near the cell membrane and colocalized with cortical actin fibers after microgravity. Our findings indicate that Fat1 may mediate the rearrangement of cortical actin fibers induced by mechanical unloading.

Analysis of Fatigue Indicator Parameters for Ti-6Al-4V microstructures using extreme value statistics in the transition fatigue regime

Extreme value (EV) statistics have been previously utilized to model the tails of distributions of computed Fatigue Indicator Parameters (FIPs) to provide a method of ranking microstructures in terms of resistance to fatigue crack formation under identical loading conditions. FIPs that capture the two primary transgranular mechanisms for fatigue crack formation and early stages of growth in bimodal Ti-6Al-4V are computed for cyclic strain-controlled loading conditions corresponding to transition fatigue where there is perceptible macroscopic plastic cyclic strain, but the polycrystal cyclic elastic strain range dominates the cyclic plastic strain range. A statistical volume element (SVE) ensemble approach is employed, in which a large number of microstructure samples are constructed for simulation. The peaks-over-threshold method is used to identify extreme values within the FIP distributions which are then fit to the Generalized Pareto Distributions, enabling rank-ordering of various microstructures with regard to resistance to fatigue crack formation. Microstructure attributes including the grain size, orientation, and nearest neighbor arrangements are quantified in the neighborhood of FIP ‘hot spots’.

Modeling of cyclic hardening and evaluation of plastic strain range in the presence of pre-loading and ratcheting

An extended evolution equation of cyclic hardening is proposed to include the effect of maximum plastic strain due to pre-loading and ratcheting. A set of extended evolution equations is also proposed for a plastic strain surface to properly evaluate the cyclic plastic strain range in the presence of pre-loading and ratcheting. These extensions are verified by performing uniaxial cyclic experiments of 316 stainless steel at 600°C. The experiments are accurately simulated using a constitutive model with the extensions in addition to the resetting scheme of the plastic strain surface recently proposed by the authors. The extension for the plastic strain surface allows fairly good simulation of the experiments without recourse to the resetting scheme if the strain range does not markedly decrease with the number of cycles. The extended equations are further used to discuss the effect of maximum plastic strain considering previous experiments.

Multiscale analysis of thermomechanical stresses in double wall transpiration cooling systems for gas turbine blades

Double wall transpiration cooling (DWTC) is a new technology that allows the gas turbine inlet temperatures to be increased beyond current levels to promote higher engine efficiency. DWTC systems consist of outer hot and inner cooler walls, connected by pedestals, which contain film cooling and impingement holes, respectively. In order to employ these new systems, an evaluation of the stresses that drive fatigue and ratchetting at critical stress raisers is essential. We present a modelling framework which combines Computational Fluid Dynamics (CFD)-heat transfer solutions for the temperature field in DWTC systems, with theoretical and Finite Element (FE) elastic solutions for the thermal (T) stress and centrifugal (CF) stress fields. We demonstrate that uniaxial tensile CF loading causes much higher stress concentration factors (SCF) at cooling holes and wall-connecting pedestals than the thermally induced biaxial stresses. A theoretical framework is developed, supported by FE studies, that captures the dependence of the SCF on important geometric parameters, such as wall thicknesses, pedestal height and hole size, spacing and inclination angle, which provides important information for the optimisation of these systems. A key observation of relevance to both conventional and non-conventional turbine blade designs, is that the superposition of tensile CF stresses to compressive T stresses is beneficial for the performance at the critical film hole features; for double wall blades, however, the superposition degrades the performance at impingement holes and pedestals, as in these locations the T stresses are also tensile. These stresses can be balanced by using an optimal wall thickness ratio. Our elastic solutions can be readily used in analyses for predicting structural ratchet boundaries based on shakedown theory and the local cyclic strain range that drives thermomechanical fatigue in DWTC systems.

Fatigue property of cerium oxide modified vulcanised natural rubber under uniaxial loading

Cerium oxide (CeO2) modified natural rubber (NR) exhibits good mechanical properties, such as high abrasion resistance, high tear strength and good thermal aging and creep resistance. Rubber components are often subjected to long-term cyclic loading in service. Therefore, fatigue performance needs to be considered for their application. In the investigations, tensile properties and fatigue properties of CeO2 modified vulcanised NR under uniaxial loading were investigated. The fatigue tests were carried out under constant cyclic strain range at room temperature. Results showed that the CeO2 modified NR demonstrated higher elastic modulus and larger elongation than that of unmodified rubber, which were related to the enhanced interaction and bonding force of rubber chains by CeO2 additive. Moreover, a remarkably enhanced fatigue life could be achieved in the CeO2 modified NR. The internal friction and impact in the rubber matrix was relieved by CeO2 additive, which can reduce the hysteresis loss during cyclic loading, resulting in improved fatigue resistance in CeO2 modified vulcanised NR.

Creep-plasticity-fatigue calculations in the design of porous double layers for new transpiration cooling systems

New porous double layer (PDL) transpiration cooling technologies can allow gas turbine entry temperatures to be increased beyond current limits towards higher engine efficiency. However, PDL systems require inclined film holes with stress concentration factors in excess of 3.8. Combination of thermoelastic Finite Element (FE) analysis with Neuber type local strain approaches gives similar cyclic strain range predictions with cyclic plasticity-creep FE analysis. Fatigue crack initiation at film holes occurs with a low number of cycles due to excessive plasticity. Our study establishes links between elastic-inelastic analyses and between material phenomena-PDL geometry and indicates pathways of improving life.

Fatigue crack initiation and energy-based life analysis for Q345qD bridge steel at low temperatures

The lack of in-depth understanding on the low-temperature fatigue cracking performance has limited the extensive application of the Q345qD bridge steel in cold and severely cold regions. This study investigates the resistance to the fatigue crack initiation of the Q345qD steel through a series of strain-controlled fatigue tests of smooth coupons at room and low temperatures. Experimental results present the cyclic stress-strain relationship, non-Masing properties and Coffin-Manson-type fatigue crack initiation resistance. Thereafter, this study analyzes the effect of low temperatures on the stain-controlled fatigue life of the studied Q345qD steel using the different energy approaches. Results indicate that the low temperatures within -60 °C increase the resistance to the fatigue crack initiation and the cyclic strain-hardening coefficients. A comparison with other structural steels demonstrates the excellent fatigue performance of the Q345qD steel at room and low temperatures. Since the corresponding stress amplitudes of the Q345qD steel under each cyclic strain range exhibit some strain-range-dependent cyclic softening/hardening behavior, the plastic strain energy density and the concept of fatigue toughness are more suitable and effective to predict the fatigue life of the Q345qD steel at room and low temperatures. Finally, a simple and accurate method for calculating the plastic strain energy density has been proposed.

Features of the kinetics of cyclic elastoplastic deformation diagrams at dwells in cycles and superimposition of variable stresses on them

The goal of the study is determination of the regularities of changes in cyclic strains and related deformation diagrams attributed to the existence of time dwells in the loading modes and imposition of additional variable stresses on them. Analysis of the obtained experimental data on the kinetics of cyclic elastoplastic deformation diagrams and their parameters revealed that in contrast to regular cyclic loading (equal in stresses), additional deformations of static and dynamic creep are developed. The results of the studys are especially relevant for assessing the cyclic strength of unique extremely loaded objects of technology, including nuclear power equipment, units of aviation and space systems, etc. The experiments were carried out on the samples of austenitic stainless steel under low-cycle loading and high temperatures of testing. Static and dynamic creep deformations arising under those loading conditions promote an increase in the range of cyclic plastic strain in each loading cycle and also stimulate an increase in the range of elastoplastic strain due to active cyclic deformation. At the same time the existence of dwells on extrema of stresses in cycles without imposition of additional variable stresses on them most strongly affects the growth of plastic strain ranges in cycles. Imposition of additional variable stresses on dwells also results in the development of creep strains, but their growth turns out to be somewhat less than in the presence of dwells without stresses imposed. The diagrams of cyclic deformation obtained in the experiments are approximated by power dependences, their kinetics being described in terms of the number of loading cycles using corresponding temperature-time functions. At the same time, it is shown that increase in the cyclic plastic deformation for cycles with dwells and imposition of additional variable stresses on them decreases low cycle fatigue life compared to regular loading without dwells at the same stress amplitudes, moreover, the higher the values of static and dynamic creep, the greater decrease in low-cycle fatigue life. This conclusion results from experimental data and analysis of conditions of damage accumulation for the considered forms of the loading cycle using the deformation criterion of reaching the limit state leading to fracture.

Nonlinear damping properties of recycled aggregate concrete short columns under cyclic uniaxial compression

Although recycled aggregate concrete (RAC) has been studied intensively over past two decades, its damping property is still not analyzed sufficiently. In this paper, a nonlinear damping model for RAC prisms subjected to cyclic uniaxial compression are presented. Then with the combination of the compressive strength and damping property of RAC, an optimum recycled concrete aggregates (RCA) content including particle size and replacement percentage are recommended. The cyclic uniaxial compression test was conducted to investigate the dynamic modulus and nonlinear damping property of RAC, meanwhile tests for the RAC’s static compressive strength and modulus of elasticity were also carried out. Then based on the nonlinear damping model and compressive strength, relationships among the compressive strength, damping property, load indicators and RCA content are analyzed to obtain the optimum RCA content. Test results indicate that RAC obtains a higher loss factor than NAC. The loss factor of RAC is an increasing power function of RCA particle size and a decreasing power function of RCA replacement percentage. However, the compressive strength and dynamic modulus of RAC decreases compare with that of natural aggregate concrete. The different choice of cyclic uniaxial compression load indicators (cyclic stress range, stress level or strain range) has significance impact on RAC damping property and optimum RCA content.

Characteristics of the complex modulus of recycled cold mix with foamed bitumen and recycled concrete aggregate

This article presents research on recycled cold mix with foamed bitumen (MCAS) containing recycled concrete aggregate. The primary concept driving this research was to determine if recycled concrete aggregate (RC) could be used as a substitute for reclaimed asphalt pavement (RAP). Recycled concrete aggregate was used in the MCAS mix in amounts ranging from 20%, 60% and 80%. The reference mix was the MCAS mix containing 50% reclaimed asphalt pavement (RAP) and virgin aggregate. Identical 0/31.5-mm continuously graded dolomite virgin aggregate was used in all mixes. 2.5% foamed bitumen (FB) and 2.0% CEM I 42.5R Portland cement (PC) were used to increase the cohesion of the mineral mix. Foamed bitumen was produced from 50/70 penetration paving bitumen. The behaviour of the recycled base course was tested in the range of cyclic sinusoidal strain with amplitude εo = 25–50 με. The tests were carried out in the (-7°C, 5°C, 13°C, 25°C, 40°C) temperature and (0.1 Hz, 0.3 Hz, 1 Hz, 3 Hz, 10 Hz, 20 Hz) loading time range. The complex modulus was tested in a direct tension-compression test on cylindrical samples (DTC-CY) in accordance with EN 12697-26. The results of the tests were used to assess the complex modulus (E*), phase angle (φ) and complex modulus components (E1) and (E2).Tests of the mixes indicate that recycled concrete aggregate can be used in recycled cold mixes in amounts of up to 80%. Increasing the amount of recycled concrete aggregate does not cause excessive stiffness of the recycled mix in comparison with the reference mix. The tests did not demonstrate a significant difference in terms of the phase angle (φ), which indicates a similar content of the viscous part and elastic part in the obtained complex modulus for the reference mix (RAP + MCAS) and the mix containing recycled concrete aggregate (RC + MCAS).

Microstructure-based fatigue life model of metallic alloys with bilinear Coffin-Manson behavior

A microstructure-based model is presented to predict the fatigue life of polycrystalline metallic alloys which present a bilinear Coffin-Manson relationship. The model is based in the determination of the maximum value of a fatigue indicator parameter obtained from the plastic energy dissipated by cycle in the microstructure. The fatigue indicator parameter was obtained by means of the computational homogenization of a representative volume element of the microstructure using a crystal-plasticity finite element model. The microstructure-based model was applied to predict the low cyclic fatigue behavior of IN718 alloy at 400 °C which exhibits a bilinear Coffin-Manson relationship under the assumption that this behavior is triggered by a transition from highly localized plasticity at low cyclic strain ranges to more homogeneous deformation at high cyclic strain ranges. The model predictions were in very good agreement with the experimental results for a wide range of cyclic strain ranges and two strain ratios (Rε=0 and −1) and corroborated the initial hypothesis. Moreover, they provided a micromechanical explanation for the influence of the strain ratio on the fatigue life at low cyclic strain ranges.

In vitro cyclic compressive loads potentiate early osteogenic events in engineered bone tissue.

Application of dynamic mechanical loads on bone and bone explants has been reported to enhance osteogenesis and mineralization. To date, published studies have incorporated a range of cyclic strains on 3D scaffolds and platforms to demonstrate the effect of mechanical loading on osteogenesis. However, most of the loading parameters used in these studies do not emulate the in vivo loading conditions. In addition, the scaffolds/platforms are not representative of the native osteoinductive environment of bone tissue and hence may not be entirely accurate to study the in vivo mechanical loading. We hypothesized that biomimicry of physiological loading will potentiate accelerated osteogenesis in bone grafts. In this study, we present a compression bioreactor system that applies cyclic compression to cellular grafts in a controlled manner. Polycaprolactone-β Tricalcium Phosphate (PCL-TCP) scaffolds seeded with Mesenchymal Stem Cells (MSC) were cyclically compressed in bioreactor for a period of 4 weeks at 1 Hz and physiological strain value of 0.22% for 4 h per day. Gene expression studies revealed increased expressions of osteogenesis-related genes (Osteonectin and COL1A1) on day 7 of cyclic loading group relative to its static controls. Cyclic compression resulted in a 3.76-fold increase in the activity of Alkaline Phosphatase (ALP) on day 14 when compared to its static group (p < 0.001). In addition, calcium deposition of cyclic loading group was found to attain saturation on day 14 (1.96 fold higher than its static scaffolds). The results suggested that cyclic, physiological compression of stem cell-seeded scaffolds generated highly mineralized bone grafts. © 2016 Wiley Periodicals, Inc. J Biomed Mater Res Part B: Appl Biomater, 105B: 2366-2375, 2017.

Numerical validation of crack closure concept using non-linear crack tip parameters

Crack closure concept has been widely used to explain different issues of fatigue crack propagation. However, some authors have questioned the relevance of crack closure and have proposed alternative concepts. The main objective here is to check the effectiveness of crack closure concept by linking the contact of crack flanks with non-linear crack tip parameters. Numerical models with and without contact of crack flanks were built for a wide range of loading parameters. The non-linear crack tip parameters studied were the range of cyclic plastic strain, the crack tip opening displacement (CTOD) and the size of reversed plastic zone. Well defined relations between non linear parameters and K were obtained without contact of crack flanks which reinforces the validity of linear elastic fracture mechanics. When the CTOD with contact is plotted versus ΔKeff the values are coincident with those obtained without contact, which shows that the crack closure concept is valid and able to explain the influence of mean stress. The analysis of overloads and high-low load blocks demonstrated the validity of crack closure concept for variable amplitude loading.

An investigation of fretting fatigue in a circular arc dovetail assembly

The fretting fatigue of a fan blade–disk attachment in aircraft gas turbine engines was investigated, in which a sub-scaled specimen with one circular arc dovetail root at each end and its fixture were designed and tested under cyclic loading, and the stress distribution in the contact zone and its variation during loading were determined by use of the finite element method. Based on the critical plane method, an approach taking the cyclic strain range and the stress gradient into consideration was developed to predict the fatigue life of specimens, which is verified by the experiments and can be applied to real structures.

A numerical study of non-linear crack tip parameters

Crack closure concept has been widely used to explain different issues of fatigue crack propagation. However, different authors have questioned the relevance of crack closure and have proposed alternative concepts. The main objective here is to check the effectiveness of crack closure concept by linking the contact of crack flanks with non-linear crack tip parameters. Accordingly, 3D-FE numerical models with and without contact were developed for a wide range of loading scenarios and the crack tip parameters usually linked to fatigue crack growth, namely range of cyclic plastic strain, crack tip opening displacement, size of reversed plastic zone and total plastic dissipation per cycle, were investigated. It was demonstrated that: i) LEFM concepts are applicable to the problem under study; ii) the crack closure phenomenon has a great influence on crack tip parameters decreasing their values; iii) the ?Keff concept is able to explain the variations of crack tip parameters produced by the contact of crack flanks; iv) the analysis of remote compliance is the best numerical parameter to quantify the crack opening level; v) without contact there is no effect of stress ratio on crack tip parameters. Therefore it is proved that the crack closure concept is valid.

Estimation of Creep and Cyclic Damage Accumulation in Micro-Turbine Disc

The micro-turbine unit is intended to provide 100 kW of electric supply. The turbine is small size (turbine wheel diameter 150 mm), high speed (65,000 rpm), and should have big enough durability (60,000 hours). Creep in stationary regime and cyclic inelastic deformation due thermal stresses at start up and stop restricts the turbine lifetime. Numerical analysis of temperatures, the creep strains and cyclic strain range shows that points with maximum creep damage and cyclic damage are different. This fact allows estimating the structure lifetime without exact knowledge of summation rule for the damages.

Creep–fatigue crack development from short crack starters

A series of isothermal strain controlled creep–fatigue tests on fully instrumented cylindrical specimens with shallow chordal crack starters has been conducted for an advanced 9%Cr turbine rotor steel at 600 and 625°C. Cyclic/hold wave shapes involving a dwell period at peak strain in tension or compression were also performed with crack development being monitored by means of electrical potential drop instrumentation. It is found that temperature, total strain range and hold period are the most influential factors on short creep–fatigue crack propagation rates and specimen life. In order to establish a reliable relationship to represent subcritical crack development for high temperature component integrity assessment, the effectiveness of candidate correlating parameters such as cyclic strain range, cyclic J integral and strain energy density factor have been evaluated. Their application to circumstances involving short crack development due to fatigue, and interacting and non-interacting creep loading are evaluated with reference to the evidence determined from post-test metallurgical examination.

Effect of crack closure on non-linear crack tip parameters

Crack closure concept has been widely used to explain different issues of fatigue crack propagation. However, some authors have questioned the relevance of crack closure and have proposed alternative concepts. The main objective here is to check the effectiveness of crack closure concept by linking the contact of crack flanks with non-linear crack tip parameters. Accordingly, 3D-FE numerical models with and without contact were developed for a wide range of loading scenarios and the crack tip parameters usually linked to fatigue crack growth, namely range of cyclic plastic strain, crack tip opening displacement, size of reversed plastic zone and total plastic dissipation per cycle were investigated. It was demonstrated that: (i) LEFM concepts are applicable to the problem under study; (ii) the crack closure phenomenon has a great influence on crack tip parameters decreasing their values; (iii) the ΔKeff concept is able to explain the variations of crack tip parameters produced by the contact of crack flanks; and (iv) the analysis of remote compliance is the best numerical parameter to quantify the crack opening level. Therefore the crack closure concept seems to be valid. Additionally, the curves of crack tip parameters against stress intensity factor range obtained without contact may be seen as master curves.