Variable Strain Amplitude Research Articles

ステンレス鋼SUS316Lの低サイクル疲労強度に及ぼす変動振幅ひずみ条件の影響

Low cycle fatigue tests were performed for austenitic stainless steel, JIS SUS316L, used for pressure vessel and piping of engineering plants. This study investigated the effect of variable amplitude strain conditions on the fatigue strength. Fatigue tests were conducted under two-step and repeated two-step variable amplitude strain conditions, in which the influence of straining history on the hardening and softening behavior of the material had an important role on the fatigue strength. The experimental results were compared with predictions of the fatigue lives based on the fracture mechanics approach in conjunction with the hardening and softening behavior. These comparisons were performed for a fatigue usage factor, UF, calculated by the linear damage accumulation law using the fatigue strength diagram obtained under constant amplitude straining tests. The UF value at fracture, UFf, reduced under the variable amplitude straining conditions depending on the straining history. Variations of stress under the strain controlled tests were measured in detail and used for the fatigue life predictions. The results indicate that the minimum value of 0.7 for the UFf gives conservative evaluation of the low-cycle fatigue life.

A non-linear damage accumulation fatigue model for predicting strain life at variable amplitude loadings based on constant amplitude fatigue data

A new phenomenological technique for using constant amplitude loading data to predict fatigue life from a variable amplitude strain history is presented. A critical feature of this reversal-by-reversal model is that the damage accumulation is inherently non-linear. The damage for a reversal in the variable amplitude loading history is predicted by approximating that the accumulated damage comes from a constant amplitude loading that has the strain range of the particular variable amplitude reversal. A key feature of this approach is that overloads at the beginning of the strain history have a more substantial impact on the total lifetime than overloads applied toward the end of the cycle life. This technique effectively incorporates the strain history in the damage prediction and has the advantage over other methods in that there are no fitting parameters that require substantial experimental data. The model presented here is validated using experimental variable amplitude fatigue data for three different metals.

Nitinol Fatigue Life for Variable Strain Amplitude Fatigue

Nitinol fatigue testing results are presented for variable strain amplitude cycling. The results indicate that cycles smaller than the constant amplitude fatigue limit may contribute to significant fatigue damage when they occur in a repeating sequence of large and small amplitude cycles. The testing utilized two specimen types: stent-like diamond specimens and Z-shaped wire specimens. The diamond specimens were made from nitinol tubing with stent-like manufacturing processes and the Z-shaped wire specimens were made from heat set nitinol wire. The study explored the hypothesis that duty cycling can have an effect on nitinol fatigue life. Stent-like structures were subjected to different in vivo loadings in order to create more complex strain amplitudes. The main focus in this study was to determine whether a combination of small and large amplitudes causes additional damage that alters the fatigue life of a component.

Cyclic strain amplitude dictates the growth response of vascular smooth muscle cells in vitro: role in in-stent restenosis and inhibition with a sirolimus drug-eluting stent

The putative effects of changes in mean strain and cyclic strain amplitude on vascular smooth muscle cell (vSMC) growth (proliferation and apoptosis) were examined. Subsequently, a quantitative measure of vSMC growth was obtained to determine the prolonged effect of changes in mechanical burden following bare-metal stent (BMS) and sirolimus drug-eluting stent (DES) deployment in vitro. Bovine aortic vSMCs were exposed to prolonged cyclic strain using a Flexercell(TM) Tension system and a novel Sylgard(TM) phantom vessel following stent implantation before the level of vSMC proliferation and apoptosis was assessed by FACS analysis, cell counting, and immunocytochemistry. Physiological cyclic strain (5%) decreased vSMC proliferation and increased apoptosis in a temporal manner. There was no significant difference in cell growth following exposure to varying mean strains with similar amplitude. In contrast, exposure to varying strain amplitudes with similar mean strains resulted in significant differences in cell proliferation and apoptosis. In parallel studies, the level of vSMC proliferation and cell survival was significantly increased within low amplitude, high mean strain regions of a phantom vessel following BMS implantation when compared to regions of higher strain amplitude upstream and downstream of the stent, respectively. Moreover, the level of vSMC growth within the stented region was significantly attenuated following implantation of a sirolimus-coated DES independent of significant changes in cell survival. Cyclic strain amplitude is an important regulator of vSMC growth capacity within a stent and is a target for inhibition using a sirolimus-coated DES.

Improving Low-Cycle Fatigue Performance of High-Performance Buckling-Restrained Braces by Toe-Finished Method

The purpose of this research is to improve the low-cycle fatigue life of the buckling-restrained brace (BRB) for the development of the high-performance buckling-restrained brace (HPBRB) used in bridge engineering. The HPBRB is expected to suffer three strong earthquakes without severe damage during the lifecycle of a bridge. This article employs a low-cycle fatigue experiment, including two series of BRB specimens, all of which had been tested to failure. In the second batch, the weld toes of the rib had been ground to improve the weld's low-cycle fatigue life. According to test results under constant and variable strain amplitudes, the specimens have a great low-cycle fatigue performance. The weld of the rib clearly affects the performance of the as-welded specimens tested with the relatively small strain amplitude. The toe-finished method effectively improves the BRB's low-cycle fatigue performance. Besides, the in-plane gap width of the BRB slightly affects the low-cycle fatigue life. Finally, the low-cycle fatigue curves are compared between the BRB and the materials and the low-cycle fatigue damage evaluation formulae of the as-welded and toe-finished BRBs are recommended for the strain-based evaluation of the low-cycle fatigue damage.

A simple approach for the modeling of an ODS steel mechanical behavior in pilgering conditions

The optimization of the forming of ODS tubes is linked to the choice of an appropriated constitutive model for modeling the metal forming process. In the framework of a unified plastic constitutive theory, the strain-controlled cyclic characteristics of a ferritic ODS steel were analyzed and modeled with two different tests. The first test is a classical tension–compression test, and leads to cyclic softening at low to intermediate strain amplitudes. The second test consists in alternated uniaxial compressions along two perpendicular axes, and is selected based on the similarities with the loading path induced by the Fe–14Cr–1W–Ti ODS cladding tube pilgering process. This second test exhibits cyclic hardening at all tested strain amplitudes. Since variable strain amplitudes prevail in pilgering conditions, the parameters of the considered constitutive law were identified based on a loading sequence including strain amplitude changes. A proposed semi automated inverse analysis methodology is shown to efficiently provide optimal sets of parameters for the considered loading sequences. When compared to classical approaches, the model involves a reduced number of parameters, while keeping a good ability to capture stress changes induced by strain amplitude changes. Furthermore, the methodology only requires one test, which is an advantage when the amount of available material is limited. As two distinct sets of parameters were identified for the two considered tests, it is recommended to consider the loading path when modeling cold forming of the ODS steel.

Developing high‐performance aluminum alloy buckling‐restrained braces based on series of low‐cycle fatigue tests

SUMMARYIn order to enhance the durability of high‐performance buckling‐restrained braces (BRBs) used in bridge engineering, which are expected to withstand severe earthquakes three times without being replaced, aluminum alloys were employed to manufacture BRBs. A series of low‐cycle fatigue tests, including 18 specimens, were conducted to address the low‐cycle fatigue strength of the aluminum alloy BRB. Test results of all specimens show that stable hysteretic curves were obtained without overall buckling occurrence. Failure mode of the welded aluminum alloy BRB is obviously affected by the ribs' welding under the variable or constant strain amplitude condition. Therefore, another type of aluminum alloy BRB, the bolt‐assembled BRB with or without spot‐welded stoppers, is proposed and tested. Results showed that the low‐cycle fatigue performance of bolt‐assembled BRBs with stoppers improved four to five times compared with welded BRBs. However, the stoppers' spot welding has an adverse effect on the failure mode because the crack, which induced the specimen's failure, initiated from the spot weld toes of the stoppers. Both bolt‐assembled BRBs with and without stoppers can meet the cumulative inelastic deformation requirement proposed for high‐performance BRBs under the constant strain amplitude, not larger than 2%. In addition, under the variable strain amplitude condition, only the bolt‐assembled BRB without stoppers has an excellent cumulative inelastic deformation capacity and sustains two cycles of 2.5% strain amplitude. Finally, recommended Manson–Coffin equations and preliminary cumulative damage formulae for welded and bolt‐assembled BRBs are given as the references of the strain‐based damage evaluation. Copyright © 2011 John Wiley & Sons, Ltd.

High Amplitude Direct Compressive Strain Enhances Mechanical Properties of Scaffold-Free Tissue-Engineered Cartilage

Adult cartilage has a limited healing capacity. Damages resulting from disease or injury increase over time and cause severe pain. One approach to reinstate the cartilage function is tissue engineering (TE). However, the generation of TE cartilage is time consuming and expensive and its properties are so far suboptimal. As in vivo cartilage is subject to loading, it is assumed that mechanical stimulation may enhance the quality of TE cartilage. In this study the short-term influence of variable compressive strain amplitudes on mechanical and biochemical properties of scaffold-free TE cartilage was investigated. Primary porcine chondrocytes were isolated, proliferated, redifferentiated, and transferred onto hydroxyapatite carriers, resulting in scaffold-free cartilage-carrier constructs. These constructs were placed in a custom-made bioreactor. Compression amplitudes of 5%, 10%, and 20% were applied. In each experiment four constructs were loaded with dynamic compression (3000 cycles/day, 1 Hz) for 14 days and four constructs served as unloaded control. The cartilage was evaluated biochemically, histological, and mechanically. No difference in glycosaminoglycan or collagen content between the loaded and the control groups was found. However, a positive correlation between compression amplitude and normalized Young's modulus was detected (R(2)=0.59, p<0.001). The highest compression amplitude of 20% had the strongest positive effect on the mechanical properties of the TE cartilage (Young's modulus increase of 241±28% compared to unloaded control). The data presented suggest that preconditioning with higher load amplitudes might be an attractive way of generating stiffer tissue and may help accelerating the cultivation of mechanically competent TE cartilage.

Durability Analysis of an Automobile Coil Spring Using Hybrid I-Kaz

A durability related analysis has been performed using a new statistical-based method, called the Hybrid Integrated Kurtosis-based Algorithm for Z-notch filter (Hybrid I-kaz) Technique. This method provides a two dimensional graphical representation of the signal amplitude and the Hybrid I-kaz coefficient that were used to measure the degree of data scattering. For validation purposes, road tests have been conducted in order to measure the strain and vibration signals on a coil spring. Three road surfaces were used for the road test and data collection purposes. This study used test signals which excited based on three different road surfaces. The 5-mm strain gauge and an accelerometer were fixed on the outer surface of the coil spring for measuring the variable amplitude strain loadings together with the vibration response. The time domain strain and vibration signals were then analysed using global signal statistics and Hybrid I-kaz coefficients. Finally, it was found that the respective coefficients for each signals showed an increment with the total value of strain range and vibration amplitudes.

S031012 鉄鋼材料の低サイクル疲労における表面および内部の疲労損傷([S03101]低サイクル疲労とその応用(1))

In this study, low cycle fatigue tests were conducted for low ally steel, SFVQ1A, austenitic stainless steel, SUS316NG, and carbon steel, STS410, in order to investigate internal fatigue crack growth characteristics. The tests were conducted under repeated two-step variable amplitude strain condition to appear beach marks of cracks on the fracture surface. In addition to these tests, tests with surface removal were also conducted to suppress surface fatigue cracks by polishing the specimen surface during the fatigue tests. Cracks initiated from internal inclusion and those fatigue life were slightly larger under surface removal test for SFVQ1A and SUS316NG compared to those under no surface removal tests. STS410 specimen under surface removal test was not broken until its specimen diameter became too small. There was an internal crack observed on the section of the specimen cut longitudinally. Internal crack growth rate calculated from beach marks for SFVQ1A and SUS316NG was similar to that of surface cracks.

Fatigue Life Calculation Concepts for Structures with Locally Modified Properties

The influence of forming on the cyclic behaviour of various thin sheet metals with austenitic phases is looked into, with the main focus lying on the variation of cyclic parameters and the progression of work hardening over the materials’ fatigue life. There is a marked interaction of hardening due to increased dislocation density resulting from forming and hardening due to cyclic straining under varying strain amplitudes of strain controlled LCF testing. This is strongly dependant on the degree of forming and the amplitude of the following cyclic loading. Also, the temperature influence during forming of addtional forming elements in a temperature controlled die on the fatigue performance of austenitic and complex three phase steels was analysed, showing a marked influence of the forming temperature on the fatigue performance, enabling positive effects to be achieved by choosing suitable material specific forming temperatures.

1713 二段多重変動ひずみ振幅下の低サイクル疲労強度と内部き裂進展(OS17-4 低サイクル疲労とその応用-き裂進展評価・モニタリング-)

1713 二段多重変動ひずみ振幅下の低サイクル疲労強度と内部き裂進展(OS17-4 低サイクル疲労とその応用-き裂進展評価・モニタリング-)

Amplitude‐frequency dependencies of wave attenuation in single‐crystal quartz: Experimental study

The experiments have been conducted to investigate the effect of strain amplitude and frequency on the compressional and shear wave attenuation in quartz samples of three types: the intact quartz, fractured quartz, and smoky quartz. The measurements were performed using the reflection method on a pulse frequency of 1 MHz with changing strain in the range 0.3 ≤ ɛ ≤ 2.0 μstrain under a confining pressure of 10 MPa and at ambient temperature. The essential difference in amplitude‐frequency characteristics of wave attenuation in three quartz types has been detected. The intact quartz shows the more “simple” behavior in comparison with the fractured and smoky quartz. The attenuation (the inverse quality factor Q) depends on strain amplitude as Q−1(ɛ) ∼ ɛ−n, where n ≅ 0.005–0.085, with the greatest decrease in the smoky and fractured quartz reaching of about 15%. Relaxation spectra of attenuation are presented in the frequency range from 0.4 to 1.4 MHz. The dependence Qp−1(f) ∼ f−1.2 characterizes the intact and fractured quartz, whereas the smoky quartz has the relaxation peak. The dependence Qs−1(f) ∼ f−0.84 presents S wave relaxation spectrum in the intact quartz; in the fractured and smoky quartz, the attenuation peaks take place. The strain amplitude variation exerts influence on the relaxation strength, the peak frequency, and the width of the relaxation peak. Such behavior of attenuation can be explained by a joint action of viscoelastic and microplastic mechanisms. These results can be considered as a contribution for providing the experimental background to the theory of attenuation in rocks. They can also be used in solving applied problems in material science, seismic prospecting, etc.

Cyclic behaviour simulation of X38CRMOV5-47HRC (AISI H11)-tempered martensitic hot-work tool steel

The cyclic behaviour of X38CrMoV5 (AISI H11) tool steel with a nominal hardness of 47HRC has been predicted. Basically, thermo-elastoplastic and thermo-elastoviscoplastic constitutive laws are investigated. First, various uniaxial isothermal conditions (LCF) with different strain rate, strain amplitude and temperature level are investigated. Then the constitutive laws are examined under various TMF loading conditions. The simulated results by both the approaches are compared with experimental results in terms of stress?strain behaviour and cyclic softening. Some applications of the model for simulation of thermal fatigue sample are shown. Taking into consideration the results of this work, the goal is to further characterise the limitations of these constitutive laws under complex and severe loading conditions, i.e., under variable temperature, variable strain amplitude and thermal fatigue structural specimen.

Correlation between stresses and microstructure in concentrated suspensions of non-Brownian spheres subject to unsteady shear flows

Stresses within unsteady simple shear flows of suspensions of non-Brownian spheres constrained to move in the velocity-gradient plane are calculated using Stokesian dynamics simulations. The unsteady flows considered include shear reversal and oscillatory flows of varying strain amplitude. The evolution of the stresses in time are reported along with the corresponding microstructural development for all flow conditions. For shear reversal, the shear stress rapidly decreases to a minimum before gradually returning to the steady state value reached in the previous direction, whereas the normal stress briefly changes sign upon reversal of shear before returning to the steady state value. For oscillatory shear flow, the shear stress increases with total strain before attaining a steady state that depends upon the applied strain amplitude, indicating irreversible behavior even at small strain amplitudes. The shear stresses show a nonmonotonic dependence on the applied strain amplitude that agrees with experimental results [Bricker and Butler, J. Rheol. 50, 711–728 (2006)]. The steady state normal stresses also depend on the strain amplitude and may change signs at low strain amplitudes.

Evaluation of Low-Cycle Fatigue in Simulated PWR Environment

For developing fatigue design curve of cast stainless steel that is used in piping material of nuclear power plants, a low-cycle fatigue test rig was built. It is capable of performing tests in pressurized high temperature water environment of PWR. Cylindrical solid fatigue specimens of CF8M were used for the strain-controlled environmental fatigue tests. Fatigue life was measured in terms of the number of cycles with the variation of strain amplitude at 0.04%/s strain rates. The disparity between target length and measured length of specimens was corrected by using finite element method. The corrected test results showed similar fatigue life trend with other previous results.

A prediction model for extremely low cycle fatigue strength of structural steel

Miner’s rule has been accepted as a simple and effective method for predicting the low cycle fatigue life under variable strain amplitude. However, in an extremely large strain field, corresponding to the fatigue life of less than 10 cycles, the accuracy of Miner’s rule has not been verified. In this study, low cycle fatigue tests under variable strain amplitude in an extremely large strain field were conducted with a newly developed testing system. Miner’s rule always predicted a longer fatigue life than the observed life. A simple model introducing the damage mechanics concept was proposed and its validity was examined as another method for predicting low cycle fatigue life.

Small-strain dynamic mechanical behavior of magnetorheological fluids entrained in foams

An oscillatory double-lap-shear configuration was used to study the dynamic mechanical properties in shear of magnetorheological (MR) fluids entrained in open-cell polyurethane foams. The dynamic viscoelastic mechanical properties of the fluid were studied by varying strain amplitude, frequency, and magnetic field strength. We find that these samples exhibit linear viscoelastic behavior below a threshold level of strain and only in zero applied magnetic field. Above the critical strain, and in applied magnetic fields, they behave as nonlinear viscoelastic materials with significant damping capacity that is strongly dependent on the field applied perpendicular to the direction of strain. For example, the energy dissipated by the material increases by over an order of magnitude when the flux density is increased from 0 to 0.6 Tesla. In addition, the magnitude of the complex modulus increases from 170 kPa in 0 Tesla to over 2 MPa with an applied flux density of 0.6 Tesla. We also found that the overall dynamic mechanical material properties changed very little with applied mechanical frequency in the range studied, 0.1 to 10 Hz.

The Dynamic Stress Responses to Area Change in Planar Lipid Bilayer Membranes

The viscoelastic properties of planar phospholipid (dimyristoylphosphatidylcholine) bilayer membranes at 308K are studied, many of them for the first time, using the nonequilibrium molecular dynamics simulation (NEMD) method for membrane area change. First, we present a unified formulation of the intrinsic three-dimensional (3D) and apparent in-plane viscoelastic moduli associated with area change based on the constitutive relations for a uniaxial system. The NEMD simulations of oscillatory area change process are then used to obtain the frequency-domain moduli. In the 4–250GHz range, the intrinsic 3D elastic moduli of 20–27kbar and viscous moduli of 0.2–9kbar are found with anisotropy and monotonic frequency dispersion. In contrast, the apparent in-plane elastic moduli (1–9kbar) are much smaller than, and the viscous moduli (2–6kbar) comparable to, their 3D counterparts, due to the interplay between the lateral and normal relaxations. The time-domain relaxation functions, separately obtained by applying stepwise strains, can be fit by 4–6 exponential decay modes spanning subpicosecond to nanosecond timescale and are consistent with the frequency-domain results. From NEMD with varying strain amplitude, the linear constitutive model is shown to be valid up to 6 and 20% area change for the intrinsic 3D elastic and viscous responses, respectively, and up to 20% area change for the apparent in-plane viscoelasticity. Inclusion of a gramicidin A dimer (∼1mol %) yields similar response properties with possibly smaller (<10%) viscous moduli. Our results agree well with available data from ultrasonic experiments, and demonstrate that the third dimension (thickness) of the planar lipid bilayer is integral to the in-plane viscoelasticity.

A viscoplasticity theory applied to proportional and non-proportional cyclic loading at small strains

The small strain, isotropic, Viscoplasticity theory Based on Overstress (VBO) is modified so that the experimentally observed, complex cyclic hardening behavior under proportional and non-proportional loading with fixed as well as variable strain amplitude can be reproduced. This is accomplished by formulating a growth law for the isotropic, rate independent stress, a scalar valued state variable of VBO. The non-proportionality measure employed for modeling the effects of loading path, amplitude and prior history is Tanaka's fourth order tensor [Eur. J. Mech. Solids 13 (1994) 155]. Numerical experiments show the responses in step-up and down two amplitude tests and include further hardening after an increase in the strain amplitude. The differences in the responses to proportional and non-proportional loading including circular, square, one step and two steps are demonstrated. History dependence of the hardening is depicted. The cross hardening behavior, sudden increase in the stress level which is followed by softening after the path change are modeled well.