The paper presents a numerical approach to compute the total amount of dissipated energy under arbitrary rate independent uniaxial thermomechanical fatigue (TMF) loading. Such an approach is based on the stress controlled Prandtl type operator enabling continuous energy dissipation calculation. It focuses on non-isothermal stress-strain conditions of a spring-slider segment, and relates slider movements to irreversible energy dissipation. The correct amount of dissipated energy is computed at any moment (online), without the need of "waiting" the cycle to finish. The operator approach guaranties monotonically increasing progress of energy dissipation, which is within the framework of thermodynamics. It is also shown that the operator approach results in the same dissipated energy after closed isothermal cycle as the traditional integration approach.
COBISS.SI-ID: 12269595
This paper presents how the piston shape of an air-spring can influence both its load-deflection characteristic and the fatigue life. Two piston shapes are considered in this study for which load-deflection characteristics and fatigue lives are compared. A method for the estimation of air-spring fatigue life is upgraded by adding the influence of the mean stress level and afterwards used together with finite element analysis to predict the fatigue life and, ultimately, the timing and global location of failure within the air-spring. These predictions are then compared with measured results and show good agreement thus proving the validity of the method used here for calculating fatigue life. Both experimental and predicted results show that the highest fatigue life can be expected if a noncylindrical, back tapered piston is used. This is only the case if the air-spring is mounted at its optimal design height as the study also shows that moving away from optimal design height does have a detrimental effect on the fatigue life of back tapered air-springs. This is due to the appearance of higher stress amplitudes in the flex member during operation. Such stress amplitudes and consequently fatigue damage can be reduced by avoiding sharp transitions in the piston design that cause additional bending of the flex member in a direction opposite to the deflection in the flex member fold.
COBISS.SI-ID: 15773211
This article describes how a selected material's fatigue-life-curve model influences the calculated reliability of a structure subjected to a dynamic loading. A uni-axially loaded structural beam with a fully-reversal constant loading amplitude was considered. The reliability for a certain number of cycles-to-failure was calculated as a cross-section of the probability distributions representing the load-amplitude scatter and the scatter of the material's fatigue-life curve. The probability density function (PDF) of the loading amplitude was modelled by a uniform and a Gaussian PDF. The scattered fatigue-life curve was modelled by a conditional two-parametric Weibull's PDF. Its parameters were estimated using two procedures: (i) a two-phase procedure and (ii) a direct procedure. Following the two-phase procedure a conditional PDF of the number of cycles-to-failure was estimated first and then converted into a corresponding conditional PDF of the stress amplitudes. In the direct procedure the conditional PDF of the stress amplitudes was modelled directly from the fatigue-life data. The two procedures were tested on 12 sets of simulated fatigue-life data and a set of experimental fatigue-life data. The two fatigue-life-curve models for the experimental data set were applied for calculating the reliability for the selected structural beam.
COBISS.SI-ID: 14035739
Church bells are exposed to severe loading conditions during ringing, which results in different damage modes due to material wear, fatigue loading, material deficiencies, different clapper-to-bell layouts, etc. As part of the activities of an EU-funded project called Maintenance and Protection of Bells (PROBELL), experimental investigations and finite-element simulations of the local contact between the clapper and the bell were carried out to study the wear-related damage to bells. First a simplified model was built to assess under the laboratory-controlled conditions the consequences of the repetitive impacts between a spherical body made from steel and a flat block made from bronze. After the results of the finiteelement simulations for a simplified model were in reasonable agreement with the measured data a full-scale finite-element model for simulating the repetitive clapper-to-bell strokes was built. The simulations with the full-scale model were performed for variations of the parameters that influence the structural behaviour of the bell and the clapper: the clapper material, the clapper mass, the relative impact velocity of the clapper, the shape of the clapper, the clappers pin support, the clappers impact angle, the clappers guide accuracy, the bells sound-burp thickness and the coefficient of friction between the clapper and the bell. The agreement between the simulated and the measured results and the relation between the local stressestrain state and the damage to the bell in the contact area are discussed.
COBISS.SI-ID: 12166171
A commercial woodcutting circular sawblade was analysed in this work. The lateral stiffness on the periphery was measured, and the natural frequencies were determined by modal analyses. The sawblade was modelled by the finite element method, where the influence of the internal stresses caused by roll-tensioning of the sawblade was considered. The roll-tensioning force was determined based on the measurement of the sawblade rolling profile, where it was established that the sawblade had been rolled with a force of 7800 N. The analysis showed that at the aforementioned force, the lateral stiffness was a maximum; here, the calculated and measured stiffnesses were 81 and 60 N/mm, respectively. The calculated natural frequencies agree well with the measured ones, where in the most important vibrational modes there is only a 7% difference. The maximum rotational speed for the sawblade was determined to be 85% of the critical speed. Because the sawblade was clamped with a ratio of clamping of only 0.25, the maximum rotational speed was amounted to 6630 rpm. Increasing the rolling force would increase the critical speed but greatly reduce the lateral stiffness.
COBISS.SI-ID: 2783625
A damage operator approach for random non-isothermal loading is demonstrated in this paper. The approach is then used to predict the damage of a thermomechanically loaded exhaust muffler. Material properties of the basic material and the weld have been treated separately. The results from thermal and structural analyses using FEA have been applied to the exhaust muffler in LMS Virtual.Lab and both fatigue and creep damages predicted. Tested exhaust mufflers were then subjected to the same loading conditions as in the calculation, and load cycles were repeated up to the point of failure or 2 million cycles. Simulated and test results are comparable.
COBISS.SI-ID: 13429275
Experimental and numerical analysis of a compression test carried out on samples of as-received and thermally-treated beech (Fagus sylvatica L.) wood is presented. In a normal climate, samples with the dimensions of 20 x 20 x 20 mm were exposed to static compressive loads parallel and transverse to the grain. Afterwards, the test was modelled using the finite element method. It was confirmed that, after thermal modification, the wood’s density decreased and the stiffness in both tested directions increased. After the thermal treatment, the strength of beech wood increased in the direction parallel to the grain and decreased in the direction transverse to the grain. Based on the comparison of experimental and numerical results, it is possible to use the hyperelastic constitutive law to reasonably model the force and displacement obtained in the compression test samples.
COBISS.SI-ID: 2468745
We present an alternative approach to the rapid estimation of S-N curves and their scatter. A simultaneous estimation of the SN curve and its scatter is achieved by applying a two-parametric Weibull distribution to describe the scatter of a number of load cycles to failure at an arbitrary amplitude stress level. The shape of the S-N curve is generally modelled as a linear dependence between the logarithmic value of the number of load cycles to failure and the logarithmic value of the amplitude stress level. This dependence is described by two parameters: a constant term and a scale coefficient of the S-N curve in a loglog scale. Therefore, the same formulation was applied to model the dependence between a scale parameter of the Weibull distribution and the logarithmic value of the amplitude stress level. In this manner the S-N curve and its scatter are described by three parameters: the constant term, the scale coefficient and the shape parameter of the Weibull distribution. The three parameters are estimated with a differential ant-stigmergy algorithm from the experimental data. In the article a mathematical background of the approach is presented and applied to three cases of experimentally obtained durability data. The results are analysed and discussed.
COBISS.SI-ID: 12242203
We present an approach for estimating E-N curves and their scatter. The scatter of a number of load cycles to failure at an arbitrary amplitude-strain level is modelled using a two-parametric Weibull distribution with the constant shape parameter and the scale parameter dependent on the strain amplitude by the Coffin-Manson equation. In this way the E-N curve and its scatter can be described using five parameters: the four parameters of the Coffin-Manson equation for the scale parameter of the Weibull distribution and the shape parameter of the Weibull distribution. The objective was to estimate these five parameters, which are generally unknown (since the data from the literature are manly known only for the median E-N curves), on the basis of the known fatigue-life data to obtain not only the trend of the E-N curve, but also its scatter. In order to estimate these parameters on the basis of the fatigue-life data, two evolutionary algorithms were applied: a real-valued genetic algorithm (GA) and the differential ant-stigmergy algorithm (DASA). In the article a mathematical background of the approach is presented and applied to 27 test cases of simulated fatigue-life data and one real case of experimentally obtained fatigue-life data. The results are analysed and discussed.
COBISS.SI-ID: 13042971
A method is presented that enables predictions of long-term creep-rupture strength based on a small sample of short-term experimental results. Smoothed bootstrapping in combination with the most commonly used time-temperature parameters is used for the determination of the optimal values of coefficients. For ten metals, creep%rupture strengths are evaluated for both full-size and sub-size data sets. Predictions in the case of small data sets always lie on the conservative side, whereas the confidence interval of predicted strengths decreases with an increasing number of experimental results. However it is shown here that for the evaluated materials, usable interim predictions of creep-rupture strengths can be achieved.
COBISS.SI-ID: 13803803