Abstract:Abstract:It is well known that the mean stress or stress ratio of fatigue loadings has a strong effect on the shape of S-N curves. An understanding of the relationships among S-N curves corresponding to different mean stresses or stress ratios would be very useful in engineering applications. In this study, based on continuum damage mechanics, a mathematical expression of an S-N curve is deduced from a new damage evolution law. This mathematical expression can well represent the whole S-N curve, not only the l… Show more
“…Gerber curve provides a good fitting for the axial average stress of ductile materials, but it cannot correctly predict the harmful effects of compressive average stress. However, Goodman curve is suitable for low‐toughness materials . Therefore, Goodman curve shown in Figure is selected to perform the necessary fatigue life estimation.…”
Section: The Fatigue Cumulative Damage Rule For Calculating the Fatigmentioning
confidence: 99%
“…The conclusion drawn from MEMS fatigue life research is that damage in MEMS devices is accumulative and suggests that failure occurs after progressive accumulation of damage during loading cycles …”
Fatigue affects the durability of radio frequency microelectromechanical systems (RF MEMS) devices. It is common to rely on actuation voltage cycles or resonant frequencies shift to estimate the fatigue life of MEMS devices. Actuation of moving mechanical parts within the device causes transient stresses because of transient vibration. Such stresses and stress cycles are difficult to account for when fatigue life is estimated by observing actuation voltage or resonant frequencies shift only. This work investigates the effect of transient stresses on the durability of RF MEMS switch, using a benchmarked finite element‐based structural dynamics model of the switch to generate the stress‐time history of the device. Principal normal stresses are calculated from raw stress data and ordered such that (σ1 ≥ σ2 ≥ σ3). Maximum principal stress amplitudes at pull‐in and pull‐out are calculated, and their corresponding number of cycles are determined by rainflow counting method. A Cumulative damage index (Dc) is determined using Miner's rule. This study revealed that, when transient stresses are taken into consideration, the cumulative damage caused by a single on‐off cycle, of the switch studied, is 1.031 × 10−6, and its life‐to‐failure is 9.7 × 105 on‐off cycles. Whereas the life‐to‐failure of the switch, without considering transient stresses, is approximately 10.3 × 108 cycles. Results are compared with published data for similar switches, and they are in good agreement.
“…Gerber curve provides a good fitting for the axial average stress of ductile materials, but it cannot correctly predict the harmful effects of compressive average stress. However, Goodman curve is suitable for low‐toughness materials . Therefore, Goodman curve shown in Figure is selected to perform the necessary fatigue life estimation.…”
Section: The Fatigue Cumulative Damage Rule For Calculating the Fatigmentioning
confidence: 99%
“…The conclusion drawn from MEMS fatigue life research is that damage in MEMS devices is accumulative and suggests that failure occurs after progressive accumulation of damage during loading cycles …”
Fatigue affects the durability of radio frequency microelectromechanical systems (RF MEMS) devices. It is common to rely on actuation voltage cycles or resonant frequencies shift to estimate the fatigue life of MEMS devices. Actuation of moving mechanical parts within the device causes transient stresses because of transient vibration. Such stresses and stress cycles are difficult to account for when fatigue life is estimated by observing actuation voltage or resonant frequencies shift only. This work investigates the effect of transient stresses on the durability of RF MEMS switch, using a benchmarked finite element‐based structural dynamics model of the switch to generate the stress‐time history of the device. Principal normal stresses are calculated from raw stress data and ordered such that (σ1 ≥ σ2 ≥ σ3). Maximum principal stress amplitudes at pull‐in and pull‐out are calculated, and their corresponding number of cycles are determined by rainflow counting method. A Cumulative damage index (Dc) is determined using Miner's rule. This study revealed that, when transient stresses are taken into consideration, the cumulative damage caused by a single on‐off cycle, of the switch studied, is 1.031 × 10−6, and its life‐to‐failure is 9.7 × 105 on‐off cycles. Whereas the life‐to‐failure of the switch, without considering transient stresses, is approximately 10.3 × 108 cycles. Results are compared with published data for similar switches, and they are in good agreement.
“…So a unified S-N curve model of different stress ratios is particularly important. Hou and Xu (2015) deduced a mathematical expression of the whole S-N curve, which takes into account of not only the linear part, but also the transitional part near the fatigue limit where σfm is the fatigue limit; σ-1,C-1,β, and γ are proportional coefficients or material constants which are related to fatigue tests ( r= −1); η and α are originated from fatigue tests (r≠-1), which are only associated with those tests whose amplitude is near to fatigue limit; κ' is a material constant which can be obtained from high amplitude tests.…”
Section: Modified S-n Model Of Different Stress Ratiosmentioning
This article aims at modeling unified S-N curves based on multifailure modes and stress ratios. Heterogeneous mixture distributions are used to describe life distribution of multifailure modes. Bayes clustering is applied to cluster failure data to avoid convergence to global optima. Derived from regression analysis, failure data are compensated by nonrandom error. The relationship between distribution parameters and stress is established, and the ultimately unified probabilistic S-N curve is modeled. Two assumptions are presented to modify an original S-N curve model with varied stress ratios. The first one is to take into account the effects of low-amplitude load “exercising,” which we call results in the “pseudo-fatigue damage.” The other one is to set up probability density function of fatigue limit considering uncertainty. Combined with material parameters modification, thus a new modified and unified S-N curves model is available for not only researchers, but engineers as well.
“…Further, maybe the greatest advantage of the proposed model with respect to the Basquin law is the possibility to continuously diminish the slope up to zero when approaching the fatigue limit, as it usually happens in real fatigue testing, where most probably the fatigue limit is not reached as a knee, but gradually. Many clear examples of this trend first theorized by Weibull in his book are available in literature, eg, Hou and Xu . Therefore, empirical models for fatigue life prediction, despite not being motivated by physical concepts, are extremely widely used both in academia and (even more frequently) in industry.…”
Starting from the classical definition of stress‐life Wöhler curve in the form of the Basquin law, an analytical procedure for the calibration of the four parameters' Wöhler curve (the Weibull law) for a plain specimen is proposed. The obtained parameters are then adjusted by means of an additional slope factor preserving the inflection point of the curve while changing its slope in order to model the experimental observations in which an increase of the scatter in life prediction is observed when reducing the stress amplitude. The same approach has then been adopted to calibrate the Weibull law parameters for a notched specimen, and the fitting slope factor has been found to be a value that changes with the material but remains constant with the stress concentration factor. The findings have been validated with existing experimental data on 2024‐T3 aluminum alloy and normalized SAE 4130 steel.
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