THE USE OF MARKOV CHAINS THEORY FOR APPROXIMATION OF ATIGUE CURVE OF COMPOSITE MATERIAL
Some simple mathematical model for fatigue life of composite m aterial consisting mainly of parallel rigidd components (strands) is offered.. The model,, whi chh is foundedd onn the use of Markov chainn theory,, allows too get S-type curve of internal stress growthh and, finally,, too s ee the connectionn betweenn static strengthh distributionn parameters andd S-N fatigue curve (Wholer curve).. The model is tooo simple andd does not provide numerica l coincidence with experimental fatigue test data but by the use of some additional parameters we cann fit the model too actual fatigue datasets,, we can use it for nonlinear regressionn model of S-N fatigue curveandd predict consequence of static strengthh parameter changes.
Estimating the parameters of fatigue curve of a composite material
By using the method of maximum likelihood, the parameters of two versions of a mathematical model for fatigue damage accumulation in a laminate are estimated. The models, which are founded on the Markov chain theory, are very simple: they do not take into account the specific structural features of a composite and therefore cannot provide numerical coincidence with experimental fatigue test data, but they can be used for a nonlinear regression analysis of fatigue curves. A simple method is offered for approximately estimating model parameters, some of which characterize the distribution of the local static strength. By using such models, we can predict the relative changes in fatigue curves from known relative variations in the parameters of static strength and also predict the distribution function of fatigue life in program fatigue tests.
Neutron Residual Stress Measurements on Rail Sections for Different Production Conditions
Rail sectioning with subsequent neutron diffraction experiments has been used to assess residual stresses in the rails. In this study we present the results of neutron stress measurements performed at the NIST Center for Neutron Research (NCNR) on rail sections from rails that were produced under various conditions. Specifically, these are air-cooled, air-cooled and roller-straightened, head-hardened and head-hardened and roller-straightened. More significantly, a head-hardened and roller-straightened rail was also studied after service to elucidate evolution of the service-induced residual stresses. In the latter case both a transverse-cut slice and the central region of a 0.53 m long piece were studied. Measurements on this piece are the first in which triaxial stresses have been determined for an intact rail. Neutron strain measurements with 3×3×3 mm3 spatial resolution were successfully employed for transversally cut slices to verify the difference in the stress state depending on the production process. Although examination of slices allows determination of only two-dimensional stresses in the plane of the slice, additional measurements on obliquely-cut slices, which were also carried out, and utilization of FEM gives the possibility of reconstructing the full triaxial stress distribution. Together, these approaches provide a better understanding of rail fabrication and the possibility of improving the durability and safety of rails in the future.Copyright © 2004 by ASME