Load Hold Times Research Articles

The Influence of Loading Rate and Hold Time on the Nano-mechanical Properties of γ-TiAl and Plasma Mo-Si-Ti Coating

The Influence of Loading Rate and Hold Time on the Nano-mechanical Properties of γ-TiAl and Plasma Mo-Si-Ti Coating

Fatigue crack acceleration and retardation induced by high temperature load hold in Inconel 718

The effect of high temperature tension hold on fatigue crack growth in Inconel 718 is investigated. A single tension hold at maximum load was applied during cyclic loading at 650 °C, and the effect of load level and hold time are examined. In all experiments, a rapid acceleration of fatigue crack growth is observed immediately after tension hold. A retardation is observed after the initial acceleration at lower stress intensity factors, while only an acceleration occurs at higher intensity factors. These results are discussed by considering two competed mechanisms: grain boundary damage related to oxygen in front of the crack tip, and stress relaxation due to creep deformation.

Effect of creep-fatigue loading condition on crack tip fields of grade 91 steel at crack initiation and growth

In this paper, the effect of creep-fatigue loading condition on crack tip deformation and stress fields of Grade 91 steel at 600 °C at crack initiation and growth is studied. By using finite element debond analysis method, creep-fatigue crack growth tests using C(T) specimens under various load ratios and hold times are simulated, and the effect of creep-fatigue load ratio and hold time on the crack opening profile, equivalent stress, crack-opening stress and triaxiality at crack initiation and growth is presented. It is found that the crack tip deformation and stress fields under tension–compression creep-fatigue loading are overall different from those under tension-tension creep-fatigue and pure creep loading, due to re-sharpening of the crack tip.

Development of a cycle counting algorithm with temporal parameters

Load cycle counting is an important tool for estimating the fatigue life of structures. Rainflow counting methods have been developed to count load reversals and extract load cycles from load time histories. Rainflow counting methods were originally developed to assess fatigue damage associated with mechanical cycling where time-dependent factors such as dwell time and loading rate were not considered significant contributors to fatigue failure. However, loading rate and hold times can be significant factors in some fatigue failures, such as solder interconnects under temperature cycling. In these cases, fatigue life models may require the load transition time and load hold times to account for time-dependent material changes, such as creep. This paper develops a new multi-parameter cycle counting algorithm that provides a range-based hold time estimation for use with time-dependent fatigue damage models. To show the applicability of the new method, the life of solder joints under a complex thermal cycling regime was estimated using the data extracted by the new multi-parameter cycle counting algorithm. This life estimate was then compared with test results and found to be within the estimated uncertainty bounds.

Fatigue and creep-fatigue crack growth in alloy 709 at elevated temperatures

ABSTRACTFatigue crack growth (FCG) and creep-fatigue crack growth (CFCG) in austenitic stainless-steel Fe-25Ni-20Cr (Alloy 709) were measured experimentally and simulated using the finite element method. Temperature conditions investigated were 550°C, 600°C and 700°C, with load hold times of 0 s, 60 s and 600 s. Fracture surface was investigated using scanning electron microscopy and optical imaging. Experiments indicated that crack growth exhibits minimal sensitivity to the various loading conditions evaluated. At 600°C, crack growth rates were independent of hold time or loading frequency. At 700°C, there was a small increase in crack growth rate as a function of hold time, with a 600s hold time causing a factor of 2 increase in crack growth rate over FCG. Finite element simulations were performed to compute plasticity-induced crack closure in the presence of creep deformations at the crack tip. The simulations produced FCG and CFCG rates similar to the experimental results.

Investigation of Sectional-Stage Loading Strategies on a Two-Stage Turbocharged Heavy-Duty Diesel Engine under Transient Operation with EGR

The influence of loading strategies on combustion and emissions parameters is experimentally and numerically studied under typical 5 s transient conditions of constant speed and increasing torque. The experiment is conducted on a two-stage turbocharged heavy-duty diesel engine with a constant opening valve high-pressure exhaust gas recirculation (EGR) system. The test results show that: compared with the full-stage loading (FSL) strategy (constant loading rate during the entire transient process), the sectional-stage loading (SSL) strategies (holding a certain time at 50% load) can significantly reduce soot emissions (by 41.3%); the greater the first-stage loading rate, the better the torque response performance, which maximally increases by 56.7%. Besides, longer loading holding time can effectively restrain the overshoot of EGR rate and advance the combustion phase (CA10, CA50) at medium and large loads. However, the larger second-stage loading rate slightly deteriorates the combustion and emission performance. This deterioration situation can be markedly suppressed by adopting a suitable loading hold time.

Evaluation of Load Parameters and Hold Time of a Vickers Ultra-Micro Hardness Tester for Measure of Hardness and Modulus of Elasticity of Dental Composites

The aim of this study was to standardize the parameters for testing the hardness and elastic modulus for dental composites. It used the equipment Dynamic Ultra Micro Hardness Tester DUH-211 / DUH-211S (Shimadzu) varying the load and the hold time. two composites were used with various inorganic particles that were embedded and sanded for the assay. one Vickers indenter was used being held nine rows with ten penetrations. For each composite loads were used (20, 50 and 100gf) and times (zero, five and ten seconds). The results showed that the effect of the load and the hold time had influence on the modulus and hardness values of the studied composites. The authors concluded that loads and hold larger team showed greater stability of hardness values and modulus of elasticity for the studied composites and these values for both hybrid composites as for nanoparticulate were dependent on load and time.

The influence of the cumulated deformation energy in the measurement by the DSI method on the selected mechanical properties of bone tissues.

The goal of the study was to determine the influence of DSI test conditions, i.e., loading/unloading rates, hold time, and the value of the maximum loading force on selected mechanical properties of trabecular bone tissue. The test samples were resected from a femoral head of a patient qualified for a hip replacement surgery. During the DSI tests hardness (HV, HM, HIT) and elastic modulus (EIT) of trabecular bone tissue were measured using the Micro Hardness Tester (MHT, CSEM). The analysis of the results of measurements and the calculations of total energy, i.e., elastic and inelastic (Wtotal, Welastic, Winelastic) and those of hardness and elasticity made it possible to assess the impact of the process parameters (loading velocity, force and hold time) on mechanical properties of bone structures at a microscopic level. The coefficient k dependent on the EIT/HIT ratio and on the stored energy (ΔW = Wtotal - Welastic) is a measure of the material reaction to the loading and the deformation of tissue.

Experimental study of elastic properties of different constituents of partially saturated argillite using nano-indentation tests

Callovo-Oxfordian argillite, obtained from the ANDRA underground research laboratory in Meuse/Haute-Marne (France), is characterized as a multiphase material. The argillite is composed of carbonate inclusions (10–50μm) embedded in an argillaceous matrix (representing expanding clay minerals, such as smectites). The matrix itself is also multiphase, composed of clay aggregates of 1μm-size carbonate and quartz inclusions. The high sensitivity of the mechanical behaviour of argillite to saturation is an important characteristic of this material as it has been previously demonstrated in macro-scale mechanical experiments performed under varying degrees of humidity.The study presented here consists of grids of nano-indentation tests performed under controlled saturation conditions. The influence of the load hold time before unloading has been studied. Series of indentations were performed without or with load hold, which led to consider instantaneous or deferred unloading moduli respectively. The experimental procedure employed allows the micro-mechanical properties of the different phases (matrix and inclusions) to be determined under controlled hydration and therefore under partially saturated conditions. Several series of measurements were performed at relative humidity levels of 50%, 85%, 90% and 95%, and at a constant temperature of 20°C. A statistical analysis enabled to discriminate the deferred unloading modulus of the different phases. At 50% humidity, we measured a mean deferred unloading modulus of 16GPa for the clay matrix (a mean instantaneous unloading modulus of 13GPa was observed for the clay matrix and moduli higher than 70GPa were measured for the carbonate macro-inclusions). The mean deferred unloading modulus of the matrix appears to decrease with increasing saturation; at 95% humidity (near-saturation) it is less than 5GPa. However, it was impossible to verify the instantaneous unloading modulus of the carbonate macro-inclusions at this high-level of saturation.

Analysis of creep–fatigue life prediction models for nickel-based super alloys

Creep fatigue is the main damage form of engine parts operating under both cyclic mechanical and thermal loading conditions. In the present work time-dependent creep and cycle-dependent fatigue of nickel-based super alloys is discussed. Three different kinds of creep–fatigue life models are investigated for the nickel-based super alloys. Effects of temperature, loading ratio and loading hold time to material failure are considered. Comparison with experiments reveals that the linear accumulation concept based on Forman model provides reasonable results on the creep–fatigue interaction if the life is dominated by mechanical fatigue damage. For higher loading ratios, however, deviations become significant. In the lower frequency region the modified FCE models and Saxena models give more reasonable predictions, but show large discrepancy to tests with hold time. The modified Saxena model suggested in the present work contains a separate term for hold time effects and agrees with the experimental observations more accurately.

Analysis of nanoindentation curves in the case of bulk amorphous polymers

Abstract Load versus displacement curves obtained from nanoindentation experiments on polymers are particularly difficult to analyze due to their complex mechanical and especially time-dependent behavior. The indentation curves obtained on two amorphous polymers (polycarbonate (PC) and polymethyl methacrylate (PMMA)) show an increase in the indentation depth during load hold time. A non-linear time-dependent model shows that this phenomenon could be decomposed in a first part due to a viscoelastic response and a second part assigned both to a viscoelastic and a viscoplastic response. If the first part is not achieved, the values of the elastic modulus computed using the Oliver and Pharr method are abnormally high. If it is achieved, the beginning of the unloading curve presents an abnormality that is linked to the increase in indentation depth during hold load time. Nevertheless, if the load hold time is high enough and if the contact stiffness is calculated from the initial slope of the power fit of the unloading curve, the values of the elastic modulus computed using the Oliver and Pharr method are close to those obtained by tensile tests.

Modeling of Seed Cotton Viscoelastic Properties

Seed cotton is commonly compressed into modules following harvest for storage and transport, yet mathematical models for the prediction of compressed density are unavailable. The ability to predict final seed cotton density based on the applied compressive force, deformation, and time is needed to provide a basis for engineering systems that can maintain seed cotton quality during extended storage. The impact of factors such as loading density, harvest method, hold time, and loading method was evaluated, and predictive models were developed. A Burgers model with an inelastic strain element originally used with forages was used to model the compression and creep relationships of seed cotton. Compressed density was adequately described (R2 ~0.98), but residual errors indicated that some aspect of the compression behavior remained unexplained. The creep (time-dependent strain) was also accurately modeled (R2 ~0.99). The pattern of the residual errors indicated that additional terms may improve the model. Improved distribution of seed cotton within the module builder is necessary to produce a desirably shaped module. The improved understanding of seed cotton compressive properties will aid in the development of systems to improve the module builder and maintain seed cotton quality during storage.

Nanoindentation-derived elastic modulus of an amorphous polymer and its sensitivity to load-hold period and unloading strain rate

An amorphous polymer was contacted by a Berkovich indenter using the same loading history but with four different unloading rates following a wide range of load-hold time periods. The strain-rate sensitivity index of the creeping solid was determined at each load-hold period based on two readily determinable parameters, which are the effective contact stiffness and strain rate at the onset of unloading. The measured strain-rate sensitivity index was found to increase with decreasing load-hold period, suggesting that the elastic moduli of the amorphous polymers determined by nanoindentation (together with the true contact area) depends significantly on the selection of the load-hold period. The rheological condition of the creeping solid under constant load changes substantially with time to affect the subsequent unloading recovery process. It is therefore advisable to control not only the unloading strain rate but also the load-hold period when testing time-dependent materials.

High Temperature Crack Growth Behavior of 304 Stainless Steel under Creep and Fatigue Loading

The crack growth behavior in a 304 stainless steel has been investigated at 538°C in air environment. Compact tension specimens were subjected to fatigue, creep and creep-fatigue loading. The combined effects on crack growth rates of load level and hold time have been examined. Stress intensity factors are found to correlate crack growth rates reasonably well for fatigue crack growth. Creep crack growth rates are found to correlate with stress intensity factor and C*(t). Crack growth rates under hold time cycles are successfully correlated with C*(t)avg under various load levels and hold times. Crack growth under creep-fatigue loading has been simulated by elastic-plastic-steady state creep finite element analyses. The results of analysis show that fatigue loading interrupts stress relaxation around the crack tip during hold time and causes stress reinstatement, thereby giving rise to accelerated crack growth compared with crack growth under static loading. Analysis of hold time crack growth based on the cyclic stress-strain response yields crack closure during unloading, and creep deformation during hold time tends to lower the closure load.

Nanoindentation and nanoscratch characteristics of Si and GaAs

The nanomechanical characteristics of Si and GaAs were investigated using nanoindentation and nanoscratch techniques. The hardness, Young’s modulus and plastic energy were calculated from the loading–unloading curve under different load conditions, hold times and indentation cycles. The scratching volume removal rate and wear properties were also obtained. The results indicate that Young’s modulus and the hardness of Si and GaAs decreased as the applied load, hold time and indentation cycle were increased. In addition, the loading curve for GaAs clearly shows a pop-in phenomenon. During the nanoscratch test, the wear depth for Si and GaAs decreased as the feed was increased and the wear depth of GaAs apparently increased as the applied load was increased. The volume of the removed material was used to evaluate the hardness of the material using the scratch technique.

Ratchetting of viscoplastic material with cyclic softening, part 1: experiments on modified 9Cr–1Mo steel

Uniaxial and multiaxial ratchetting tests were conducted at temperatures between 200 and 600 °C on modified 9Cr–1Mo steel, which exhibits both viscoplastic and cyclic softening behavior. Anomalous behavior was observed in the stress-controlled uniaxial ratchetting tests; the material exhibited outstanding ratchetting in the tensile direction under zero mean stress. Under the uniaxial conditions, the ratchetting deformation significantly depended on the loading rate and hold time in addition to parameters such as the maximum stress and stress ratio. The uniaxial ratchetting was also accelerated to a great extent when cyclic deformation was given before the ratchetting tests. Under the multiaxial conditions, the ratchetting depended on the steady stress, cyclic strain range and strain rate. The ratchetting progressed faster as the steady stress or strain range became larger, or the strain rate became smaller, as expected. Monotonic compression tests were carried out to investigate the reason for the rachetting under no mean stress. Strain range change tests were also conducted to investigate the effect of strain range on the cyclic softening behavior of the material in detail.

Fatigue crack growth mechanisms in Ti6242 lamellar microstructures: Influence of loading frequency and temperature

Fatigue crack growth experiments were carried out on Ti6242 alloy with large colony size. The alloy was heat treated to provide three different lamella size; fine, coarse, and extra coarse. Tests were conducted at two temperatures, 520 °C and 595 °C, using two loading frequencies, 10 and 0.05 Hz. The latter frequency was examined with and without a 300-second hold time. All tests were performed in air environment and at a stress ratio of 0.1. This study shows that at 520 °C, the Fatigue crack growth rate (FCGR) is not significantly influenced by changes in the microstructure. For 0.05 Hz/low ΔK, however, the FCGR is higher in the fine lamellar microstructure and is accompanied by- the appearance of a plateau, which disappears in the extra large lamella microstructure. Furthermore, the addition of a 300-second hold time does not alter the crack growth rate. At 595 °C, while the general level of the FCGR is higher than that at 520 °C, the effects of loading frequency and hold time remain similar to those reported at the lower temperature. Unlike the results at 520 °C, however, the FCGR at low δK is not influenced by variations in lamellar microstructure. Under all test conditions, the fatigue process is predominantly controlled by one single mechanism associated with transcolony fracture and formation of quasi-cleavage facets. The fatigue crack growth results and the associated fracture behavior as obtained in this study are correlated to the crack-tip shear activity and transmission at the α/β interfaces. A general hypothesis accounting for the role of loading frequency, temperature, and microstructure on the observed cracking mechanisms is presented.

Creep-Fatigue Crack Growth at CrMo Steel Weld Interface

Creep-fatigue crack growth behavior was experimentally measured particularly when a crack was located in the heat affected region of lCr-5Mo steel. Load hold times of the tests for trapezoidal fatigue waveshapes were varied among 0, 30, 300 and 3,600 seconds. Time-dependent crack growth rates were characterized by the parameter. It was found that the crack growth rates were the highest when the crack path was located along the fine-grained heat affected zone(FGHAZ). Cracks located in other heat affected regions had a tendency to change the crack path eventually to FGHAZ. Creep-fatigue crack growth law of the studied case is suggested in terms of (da/dt) vs. () for residual life assessment.

Optimization of microencapsulating and compacting conditions for hydrogen storage alloy

Thermophysical properties and cycling strength of a compact of copper-microencapsulated magnesium-nickel alloy have been studied for different fabrication conditions. In preparing the compact, the conditions of copper content (film thickness), loading pressure, loading hold time and pressing method were changed to assess their effect on thermal conductivity/diffusivity and strength against hydrogen absorption-desorption cycling. Thermal conductivity of the Cu-coated compact was shown to increase significantly compared with the Cu-free one; however, very significantly, thermal conductivity did not depend on copper content of the compact at all. Heavy loading pressure for compacting also increased thermal diffusivity due to an increase of contacted area among particles within the compact. However, the compact showed viscoelastic behavior; namely, it took as long as 1.8 ks until thermal diffusivity reached a constant value under a fixed loading pressure. Upon hydrogen absorption-desorption cycling the Cu-coated compacts did not crack, even after 200 cycles, while the Cu-free compacts broke easily within 100 cycles. These results show the possibility for the practical use of hydrogen storage alloy compacts with less copper microencapsulation.

Crack Growth Rate Behavior of a Titanium-Aluminide Alloy During Isothermal and Nonisothermal Conditions

Observations of isothermal fatigue, isothermal fatigue with superimposed load hold times, and thermomechanical fatigue (TMF) crack growth rate behavior of Ti-24Al-11Nb are presented and compared with results from previous studies on titanium and nickel-base superalloys. Elevated-temperature crack growth mechanisms in this alloy, which involve fatigue, oxidation and creep, and the influence of frequency, temperature, and hold-time are discussed. These mechanisms are used to interpret the observations of TMF crack growth. The limitations of current crack growth rate models based on the linear-elastic fracture mechanics parameter, K, are addressed.