Matjaž Šraml

A computational model for determination of service life of gears

Abstract A computational model for determination of service life of gears in regard to bending fatigue in a gear tooth root is presented. The fatigue process leading to tooth breakage is divided into crack initiation and crack propagation period. The strain-life method in the framework of the FEM-method has been used to determine the number of stress cycles N i required for the fatigue crack initiation, where it is assumed that the crack is initiated at the point of the largest stresses in a gear tooth root. The simple Paris equation is then used for the further simulation of the fatigue crack growth. The functional relationship between the stress intensity factor and crack length K = f ( a ), which is needed for determination of the required number of loading cycles N p for a crack propagation from the initial to the critical length, is obtained using displacement correlation method in the framework of the FEM-method. The total number of stress cycles N for the final failure to occur is then a sum of N i and N p . The model is used for determination of service life of real spur gear made from through-hardened steel 42CrMo4, where required material parameters have been determined previously by the appropriate test specimens.

Numerical procedure for predicting the rolling contact fatigue crack initiation

Abstract A computational numerical model for contact fatigue damage analysis of mechanical elements is presented in this paper. The computational approach is based on continuum mechanics, where a homogenous and elastic material model is assumed in the framework of the finite element method analysis. Cyclic contact loading conditions are simulated with moving Hertzian contact pressure. The time-depending loading cycles are defined for each observed material point on and under the contact area. Furthermore, the influence of friction upon rolling–sliding contact loading cycles is analysed in detail, using Coulomb’s friction law. The model for prediction of the number of loading cycles, required for initial fatigue damages to appear, is based on Coffin–Manson relations between deformations and loading cycles, and includes characteristic material fatigue parameters. As a general example, the model is used to analyse a fundamental contact problem of a cylinder and flat surface, which is usually a substitutional model for analysing real mechanical problems. However, the results concerning the identification of critical material points and the number of loading cycles, required for initial fatigue damages to appear at those points, are the main purpose of the presented study.

Travel time models for automated warehouses with aisle transferring storage and retrieval machine

This paper presents analytical travel time models for the computation of travel time for automated warehouses with the aisle transferring S/R machine (in continuation multi-aisle AS/RS). These models consider the operating characteristics of the storage and retrieval machine such as acceleration and deceleration and the maximum velocity. Assuming uniform distributed storage rack locations and pick aisles and using the probability theory, the expressions of the cumulative distribution functions with which the mean travel time is calculated, have been determined. The computational models enable the calculation of the mean travel time for the single and dual command cycles, from which the performance of multi-aisle AS/RS can be evaluated. A simulation model of multi-aisle AS/RS has been developed to compare the performances of the proposed analytical travel time models. The analyses show that regarding all examined types of multi-aisle AS/RS, the results of proposed analytical travel time models correlate with the results of simulation models of multi-aisle AS/RS.

Numerical calculation of bending fatigue life of thin-rim spur gears

Abstract Mechanical elements subjected to cyclic loading have to be designed against fatigue. The aim of this paper is to examine the bending fatigue life of thin-rim spur gears of truck gearboxes. The gear service life is divided into the initiation phase of the damage accumulation and the crack growth, respectively. The analysis of thin-rim gear fatigue life has been performed using the finite element method and the boundary element method. The continuum mechanics based approach is used for the prediction of the fatigue process initiation phase, where the basic fatigue parameters of the materials are taken into account. The remaining life of gear with an initial crack is evaluated using the linear-elastic fracture mechanics.

Calibration of microsimulation traffic model using neural network approach

This paper presents the results of research on the applicability of neural networks in the process of computer calibration of a microsimulation traffic model. VISSIM microsimulation model is used for calibration done at the example of roundabouts in an urban area. The calibration method is based on the prediction of a neural network for one traffic indicator, i.e. for the traveling time between measuring points. Besides the traveling time, the calibration process further/also involves a comparison between the modeled and measured queue parameters at the entrance to the intersection. The process of validation includes an analysis of traveling time and queue parameters on new sets of data gathered both at the modeled and at a new roundabout. A comparison of the traffic indicators measured in the field and those simulated with the calibrated and uncalibrated microsimulation traffic model provides an insight into the performance of the calibration procedure.

Critical plane modelling of fatigue initiation under rolling and sliding contact

Contact fatigue is a phenomenon of important practical significance for engineering applications involving localized contacts, such as gears, rail-wheel system and rolling bearings. The service lifetime of such components is related to damage, which results from the contact fatigue. The process in the material structure that causes this kind of failure is quite complicated. The aim of the present paper is to describe a contact fatigue initiation criterion, based on the critical plane approach for the general contact problem. On the basis of contact stress analysis with modified Hertzian boundary conditions, the loading cycle of characteristic material points in the contact area is determined. The Dang Van damage initiation criterion is based on the critical plane approach, which combines the largest allowable shearing and hydrostatic stresses (tensile and compressive), with an assumed elastic shakedown behaviour and it is used in this work. The material point of initial fatigue damage is then determined at the transition of the loading cycle stresses over the critical plane. The model assumed a homogeneous and elastic material model, without any imperfections or residual stresses, and elastic shakedown is considered. A proper determination of loading cycles and their characteristic values is of significance for contact fatigue initiation analysis. Finally, determination of the most critical material point on or under the contact surface and related number of loading cycles required for fatigue damage initiation is calculated with the strain-life (ε—N) method.

An examination of the construct and predictive validity of the self-reported speeding behavior model

The present study deals with the problem of speeding behavior on rural roads. The purpose of the paper is to examine the construct validity and the internal consistency and reliability of a questionnaire that measures the determinants of speeding behavior. In addition, it aimed to test the predictive validity of a modified theoretical framework of a theory of planned behavior (TPB) in relation to speeding behavior. A total of 546 car drivers from five local communities in the Republic of Srpska successfully completed the questionnaire after reading the scenario. The principal component analysis revealed seven components interpreted as: personal norm, perceived behavioral control, affective attitude toward speeding, subjective norm, habit, descriptive norm, and cognitive attitude toward speeding. A speeding behavior model was developed by structural equation modeling. Personal norm, subjective norm, and affective attitudes were shown to be important variables within the modified TPB in understanding speeding behavior. Overall, the present findings provide significant support for the concept of the modified theoretical framework of TPB in relation to speeding behavior on rural roads. Implications for a speeding behavior model and interventions are discussed.

Designing Unit Load Automated Storage and Retrieval Systems

The successful performance of the unit load automated storage and retrieval systems is dependent upon the appropriate design and optimization process. In this chapter a model of designing unit load automated storage and retrieval system for the single- and multi-aisle systems is presented. Because of the required conditions that the unit load automated storage and retrieval systems should be technically highly efficient and that it should be designed on reasonable expenses, the objective function represents minimum total cost. The objective function combines elements of layout, time-dependant part, the initial investment and the operational costs. Due to the nonlinear, multi-variable and discrete shape of the objective function, the method of genetic algorithms has been used for the optimization process of decision variables. The presented model proves to be a useful and flexible tool for choosing a particular type of the single- or multi-aisle system in designing unit load automated storage and retrieval systems. Computational analysis of the design model indicates the model suitability for addressing industry-size problems.

Modelling of Rolling Contact Fatigue of Rails

Rail dark spot defect, also termed as squat failure or shelling, is a rolling contact fatigue failure which occurs frequently on the high speed traffic railway rails. The main goal of this study is to develop a computational model for simulation of the squat phenomena on rails in rail-wheel contact. The proposed computational model consists of two parts: (i) Contact Fatigue Crack Initiation (CFCI) and (ii) Contact Fatigue Crack Propagation (CFCP). The results of proposed unified model enable a computational prediction of a probable number of loading cycles that a wheel-rail system can sustain before development of the initial crack in the rail, as well as the number of loading cycles required for a crack to propagate from initial to critical length, when the final fatigue failure (squat) can be expected to occur.

Fatigue crack initiation and propagation in lotus-type porous material

The investigation of fatigue strength of lotus-type structure with nodular cast iron as a base material using computational model is analysed in present study. The irregular pores distribution in transversal and longitudinal direction, regarding the external loading, is considered in the computational models. The complete fatigue process of analyzed porous structure is then divided into the crack initiation (Ni) and crack propagation (Np) period where the total fatigue life (N) is defined as: N = Ni + Np. The crack initiation period is determined using strain life approach where elastic-plastic numerical analysis is performed to obtain the total strain amplitude in the critical stress fields around the pores. The simplified universal slope method is then used to determine the number of stress cycles, Ni, required for formation of initial cracks. The number of stress cycles, Np, required for crack propagation from initial to the critical crack length is also numerically determined using finite element (FE) models, in the frame of Abaqus computation FEM code. The maximum tensile stress (MTS) criterion is considered when analyzing the crack path inside the porous structure. The performed computational analyses show that stress concentrations around individual pores are higher when external loading is acting in transversal direction in respect to the pore distribution. Therefore, further computational analyses regarding crack initiation and crack propagation period have been done only for pores distribution in transversal direction.