In real engineering applications, machine parts are rarely completely homogeneous; in most cases, there are at least some minor notch effects or even more extensive inhomogeneities, which cause ...critical local stress concentrations from which fatigue fractures develop. In the present research, a shift of the Coffin-Manson
-
material curve in a structure with random porosity subjected to dynamic LCF loads was studied. This allows the rest of the fatigue life prediction process to remain the same as if it were a homogeneous material. Apart from the cyclic
-
curve, which is relatively easy to obtain experimentally, the
-
curve is the second most important curve to describe the correlation between the fatigue life
and the strain level
. Therefore, the correct shift of the
-
curve of the homogeneous material to a position corresponding to the porous state of the material is crucial. We have found that the curve shift can be efficiently performed on the basis of numerical simulations of a combination of five porosity-specific geometric influences and the associated regression analysis. To model the modified synthetic
-
curve, five geometric influences of porosity by X-ray or
-CT analysis are quantified, and then the porosity-adjusted coefficients of the Coffin-Manson equation are calculated. The proposed approach has been successfully applied to standard specimens with different porosity topography.
When wood is used as a structural material, the fact that it is a highly inhomogeneous material, which significantly affects its static and fatigue properties, presents a major challenge to ...engineers. In this paper, a novel approach to modelling the fatigue-life properties of wood is presented. In the model, the common inverse-power-law relationship between the structural amplitude loads and the corresponding number of load cycles to failure is augmented with the influence of the wood's mass density, the loading direction and the processing lot. The model is based on the two-parametric conditional Weibull's probability density function with a constant shape parameter and a scale parameter that is a function of the previously mentioned parameters. The proposed approach was validated using the example of experimental static and fatigue-strength data from spruce beams. It turned out that the newly presented model is capable of adequately replicating the spruce's S-N curves with a scatter, despite the relatively scarce amount of experimental data, which came from different production lots that were loaded in different directions and had a significant variation in density. Based on the experimental data, the statistical model predicts that the lower density wood has better fatigue strength.
Rubber fatigue crack growth tests are one method of characterising fatigue behaviour. Due to the possibility of using one specimen to measure fatigue at multiple load levels, it is often considered a ...faster and less expensive method compared to the time-consuming fatigue crack initiation tests. In this work, the possibility of a new specimen grip geometry for crack growth tests was investigated, enabling the use of specimens cut from compression moulded plates. Several variations of a crack growth predictor parameter for tests with different mean strain levels were investigated. In addition, the optimum variation of the crack growth angle with respect to the loading direction was evaluated using test measurements. The results show that the use of the simplified crack growth predictor can provide the same results as the commonly used tearing energy. Using the Euclidean length of the crack growth increments gives better results than their horizontal projection, with the possibility of using angle deviations up to 90° without major drawbacks.
•A novel specimen grip geometry for planar tension specimens is presented.•Different variations of the tearing energy parameter have been investigated.•A modified tearing energy parameter has advantageous computational simplifications.•The effect of the crack growth angle on the scatter of the data was investigated.
In this article, a response of the complex-phase high-strength steel SZBS800 was modelled by considering the strain-rate influence. The material's response was first measured with a series of ...standard tensile tests at lower strain rates. Higher strain rates were achieved using the unconventional test of shooting the ball into flat specimens. A viscoplastic formulation of the Cowper-Symonds material model was applied to consider the strain-rate effects. The parameters
,
, and
of the material model were estimated using a step-wise procedure. First, rough estimates of the three parameters were obtained from the tensile tests using the grid search method. Then, the parameters
and
were fine-tuned using the reverse engineering approach. With the help of explicit dynamic simulations and all the experimental data, a multi-criteria cost function was defined and applied to obtain a smooth response function for the parameters
and
. Its optimum was determined by a real-valued genetic algorithm. The optimal values of the estimated parameters model the material response well, although a domain of optimum candidates spans two orders of magnitude for the parameter
and a few orders of magnitude for the parameter
.
The problem of characterizing the structural residual life is one of the most challenging issues of the damage tolerance concept currently applied in modern aviation. Considering the complexity of ...the internal architecture of composite structures widely applied for aircraft components nowadays, as well as the additional complexity related to the appearance of barely visible impact damage, prediction of the structural residual life is a demanding task. In this paper, the authors proposed a method based on detection of structural damage after low-velocity impact loading and its classification with respect to types of acting stress on constituents of composite structures using the developed processing algorithm based on segmentation of 3D X-ray computed tomograms using the
package, real-oriented dual-tree wavelet transform and supporting image processing procedures. The presented algorithm allowed for accurate distinguishing of defined types of damage from X-ray computed tomograms with strong robustness to noise and measurement artifacts. The processing was performed on experimental data obtained from X-ray computed tomography of a composite structure with barely visible impact damage, which allowed better understanding of fracture mechanisms in such conditions. The gained knowledge will allow for a more accurate simulation of structural damage in composite structures, which will provide higher accuracy in predicting structural residual life.
The analysis of a material that is subjected to variable loads is a complex subject and generally treated separately by fatigue and fracture mechanics. We present an attempt to extend the validity of ...conventional fatigue approach (here strain-life) in the scope fracture. This was achieved by introducing a zero thickness cohesive contact element coupled with a damage parameter that was developed from material observations of strain controlled fatigue experiments. The presented simulation framework results in a predictable crack growth direction on a compact tension specimen, although further experimentation is needed to validate the proposed approach.
Due to their advantages-longer internal force delay compared to bulk materials, resistance to harsh conditions, damping of a wide frequency spectrum, insensitivity to ambient temperature, high ...reliability and low cost-granular materials are seen as an opportunity for the development of high-performance, lightweight vibration-damping elements (particle dampers). The performance of particle dampers is affected by numerous parameters, such as the base material, the size of the granules, the flowability, the initial prestress, etc. In this work, a series of experiments were performed on specimens with different combinations of influencing parameters. Energy-based design parameters were used to describe the overall vibration-damping performance. The results provided information for a deeper understanding of the dissipation mechanisms and their mutual correlation, as well as the influence of different parameters (base material, granule size and flowability) on the overall damping performance. A comparison of the performance of particle dampers with carbon steel and polyoxymethylene granules and conventional rubber dampers is given. The results show that the damping performance of particle dampers can be up to 4 times higher compared to conventional bulk material-based rubber dampers, even though rubber as a material has better vibration-damping properties than the two granular materials in particle dampers. However, when additional design features such as mass and stiffness are introduced, the results show that the overall performance of particle dampers with polyoxymethylene granules can be up to 3 times higher compared to particle dampers with carbon steel granules and conventional bulk material-based rubber dampers.
Fabricating additively manufactured thermoplastic parts with increased load-bearing capacity using material extrusion, namely fused deposition modeling, requires the incorporation of continuous ...reinforcing fibers. In this study, composites of PLA matrix and continuous flax fibers were additively fabricated using a specially designed 3D printer with multiple deposition toolheads. Unidirectional tensile and flexural specimens with a component-like shape and a fiber volume fraction of up to 18% were fabricated. The mechanical properties were determined for four different composite and matrix layers arrangements. The tensile modulus of elasticity and strength values increased by 3.6 and 2.9 times, respectively, compared to the specimens without the reinforcing fibers. In contrast, the flexural modulus of elasticity improved by 2.4 times, while the flexural strength increased by 1.4 times. It was found that it is possible to influence the mechanical properties and the type of damage by changing the volume fraction of the fibers and the arrangement of the composite layers.
A commonly used tool for estimating the parameters of a mixture model is the Expectation–Maximization (EM) algorithm, which is an iterative procedure that can serve as a maximum-likelihood estimator. ...The EM algorithm has well-documented drawbacks, such as the need for good initial values and the possibility of being trapped in local optima. Nevertheless, because of its appealing properties, EM plays an important role in estimating the parameters of mixture models. To overcome these initialization problems with EM, in this paper, we propose the Rough-Enhanced-Bayes mixture estimation (REBMIX) algorithm as a more effective initialization algorithm. Three different strategies are derived for dealing with the unknown number of components in the mixture model. These strategies are thoroughly tested on artificial datasets, density–estimation datasets and image–segmentation problems and compared with state-of-the-art initialization methods for the EM. Our proposal shows promising results in terms of clustering and density-estimation performance as well as in terms of computational efficiency. All the improvements are implemented in the rebmix R package.
Granular materials promise opportunities for the development of high-performance, lightweight vibration-damping elements that provide a high level of safety and comfort. Presented here is an ...investigation of the vibration-damping properties of prestressed granular material. The material studied is thermoplastic polyurethane (TPU) in Shore 90A and 75A hardness grades. A method for preparing and testing the vibration-damping properties of tubular specimens filled with TPU granules was developed. A new combined energy parameter was introduced to evaluate the damping performance and weight-to-stiffness ratio. Experimental results show that the material in granular form provides up to 400% better vibration-damping performance as compared to the bulk material. Such improvement is possible by combining both the effect of the pressure-frequency superposition principle at the molecular scale and the effect of the physical interactions between the granules (force-chain network) at the macro scale. The two effects complement each other, with the first effect predominating at high prestress and the second at low prestress. Conditions can be further improved by varying the material of the granules and applying a lubricant that facilitates the granules to reorganize and reconfigure the force-chain network (flowability).
PDF
