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Load History (load + history)
Selected AbstractsFrequency-based fatigue analysis of non-stationary switching random loadsFATIGUE & FRACTURE OF ENGINEERING MATERIALS AND STRUCTURES, Issue 11 2007D. BENASCIUTTI ABSTRACT The service loadings in real systems are not only random, but also non-stationary. The spectral methods based on a frequency-domain characterization of random loads, which have been used in alternative to classical time-domain approaches, cannot be applied to non-stationary loads, because the conventional spectral density spectrum is not able to capture the evolutionary frequency characteristics of non-stationary loads. This clearly restricts the applicability of the existing frequency-based methods only to loads which are stationary. At the same time, it is also very difficult to propose general models valid for all types of load non-stationarity encountered in practice. Therefore, a practical approach is to restrict the analysis to a specific class of non-stationary loads; in this work, we consider particular non-stationary loads (i.e. switching loads), which are piecewise stationary in their variance. A frequency-domain analysis of such loads is proposed, which is based on a combination of the frequency-based analysis of each adjacent stationary segment, which can be either Gaussian or non-Gaussian. Numerically simulated load histories, as well as loads measured on mountain bikes in special tracks, are analysed to validate the proposed methodology. The presented results also show the correlation between load non-stationarity and non-Gaussianity. [source] Prediction of crack opening stress levels for 1045 quenched and tempered steel under service loading spectraFATIGUE & FRACTURE OF ENGINEERING MATERIALS AND STRUCTURES, Issue 1 2006M. KHALIL ABSTRACT The opening stresses of a crack emanating from an edge notch in a 1045 quenched and tempered steel specimen were measured under two different Society of Automotive Engineers (SAE) standard service load histories having different average mean stress levels. The two spectra are the Grapple Skidder history (GSH), which has a positive average mean stress, and the Log Skidder history (LSH), which has a zero average mean stress. To capture the behaviour of the crack opening stress in the material, the crack opening stress levels were measured at 900X using an optical video microscope, at frequent intervals for each set of histories scaled to two different maximum stress ranges. A crack growth analysis based on a fracture mechanics approach was used to model the fatigue behaviour of the steel specimens for the given load spectra and stress ranges. Crack growth analysis was based on an effective strain-based intensity factor, a crack growth rate curve obtained during closure-free loading cycles and a local notch strain calculation based on Neuber's rule. The crack opening stress (Sop) was modelled and the model was implemented in a fatigue notch model, and the fatigue lives of the specimens under the two different spectra scaled to several maximum stress levels were estimated. The average measured crack opening stresses were between 6 and 12% of the average calculated crack opening stresses. In the interest of simplifying the use of Sop in design, the average Sop was correlated with the frequency of occurrence of the cycle reducing the Sop to the average crack opening stress level. The use of an Sop level corresponding to the cycle causing a reduction in Sop to a level reached once per 10 cycles gave a conservative estimate of average crack opening stress for all the histories. [source] Biaxial testing and analysis of bicycle-welded components for the definition of a safety standardFATIGUE & FRACTURE OF ENGINEERING MATERIALS AND STRUCTURES, Issue 6 2003N. PETRONE ABSTRACT This paper presents the experimental evaluation of the fatigue behaviour of welded components under non-proportional variable amplitude biaxial loads. The study was undertaken on welded mountain bike handlebar stems, which were different in terms of geometry and technology and tested with load histories that were reconstructed and accelerated from recorded field data. Loads measured in the field were decomposed into bending and torsional components; a synchronous Peak-Valley counting, a spectrum inflation technique, a spline interpolation and a final amplification were applied to the measured signals in order to obtain test drive signals with the correct content of biaxial non-proportional loadings. After evaluation of the bending and torsion load-life curves of components under constant amplitude fatigue, the resulting data from biaxial variable amplitude fatigue tests were analysed in order to evaluate the damage contribution as a result of the two load components and an equivalent simplified two-stage constant amplitude fatigue test was proposed to the working group ISO/SC1/TC149/WG4. [source] Finite element analysis and evaluation of design limits for structural materials in a cyclic state of creepINTERNATIONAL JOURNAL FOR NUMERICAL METHODS IN ENGINEERING, Issue 14 2003M. Boulbibane Abstract In this paper a direct non-time stepping method derived from the minimum theorems given by the authors (European Journal of Mechanics , A/Solids 2002; 21:915,925) is outlined. This method can be used in the prediction of the deformation and life assessment of structures subjected to cyclic mechanical and thermal loadings. It produces accurate predictions of failure modes based on material behaviour incorporated into constitutive equations. It also can be used to define limit loads related to certain design criteria. Generally, for complex geometries and load histories, the identification of load histories that correspond to predefined design conditions, in the form of time or number of cycles to failure, can only be achieved by extensive and repeated calculations. For the Linear Matching Method, however, the representation of materially non-linear stress and strain fields by linear behaviour with spatially varying moduli, indicates the possibility that direct evaluation of loads and temperature ranges that correspond to a design restriction may be evaluated directly through the construction of the exact cyclic state and via sequence of approximations. The technique employs the finite element method combined with the cyclic state solution. The description of the material behaviour is given by a non-linear viscous model (Norton's law). It can also apply to any class of material behaviour that includes internal state variables. This technique has been applied successfully to a set of characteristics problems (Bree problem and plate containing a circular hole and subjected to radial temperature gradient). Copyright © 2003 John Wiley & Sons, Ltd. [source] Damage evolution in low velocity impacted unreinforced vinyl ester 411-350 and 411-C50 resin systemsPOLYMER COMPOSITES, Issue 6 2000M. Motuku Assistant Professor Damage evolution in plaques made of vinyl ester resin systems was investigated as a function of specimen thickness, impact energy level and matrix material. Dow DERAKANE vinyl ester 411-350 and 411-C50 resin systems, which have low viscosity and are ideally suited for low-cost liquid processing techniques like vacuum assisted resin transfer molding (VARTM), were considered for the low velocity instrumented impact testing. Characterization of damage evolution was undertaken using optical microscopy and analysis of impact load histories recorded during the impact event. Radial cracking, perforations at the point of impact (in the form of a truncated cone), and damage resulting from the support constraints were identified as the dominant failure characteristics in both resin systems. Radial cracking, which originated from the bottom surface, was operative in all failed specimens and was attributed to the catastrophic failure due to extensive flexural tensile strength losses. For specimens that could deflect significantly, radial cracking and support-constraint-induced damage were the operative failure mechanisms. Radial cracking and through-thickness shearing led to failure in stiffer plaques. The DERAKANE 411-350-vinyl ester resin system was found more damage resistant than the 411-C50 system. [source] A reliability-based data treatment system for actual load historyFATIGUE & FRACTURE OF ENGINEERING MATERIALS AND STRUCTURES, Issue 10 2005J. J. XIONG ABSTRACT This paper seeks to establish an integrated and practical data treatment system for actual load history reliability analysis. A convergence,divergence counting procedure is presented to extract all load cycles from a load history of divergence,convergence waves. The lowest number of load history sampling is established on the basis of the damage-based prediction criterion. A parameter estimation formula is proposed for hypothesis testing of the load distribution. The examples of its application for the data treatment of actual load history are given. The proposed data treatment system has been shown to have valid and practical characteristics in analysing reliability results. [source] Fatigue as a process of cyclic brittle microfractureFATIGUE & FRACTURE OF ENGINEERING MATERIALS AND STRUCTURES, Issue 3 2005R. SUNDER ABSTRACT While fatigue crack growth in vacuum may occur by slip alone, environmental fatigue including crack growth in air is strongly influenced by crack-tip surface chemistry that adversely affects ductility. Cumulative diffusion, combined with adsorption and chemisorption in the loading half-cycle may promote instantaneous crack extension by brittle microfracture (BMF). Unlike slip, the BMF component will be sensitive to parameters that affect near-tip stresses, such as load history and constraint. BMF dominates near-threshold environmental fatigue. Being a surface phenomenon, it loses its significance with increasing growth rate, as slip-driven crack extension gains momentum and growth becomes less sensitive to environment. The BMF model provides for the first time, a scientific rationale for the residual stress effect as well as the related connection between stress,strain hysteresis and load-sequence sensitivity of metal fatigue including notch response. Experimental evidence obtained on a variety of materials under different loading conditions in air and vacuum appears to support the proposed model and its implications. [source] Surface deformation induced by present-day ice melting in SvalbardGEOPHYSICAL JOURNAL INTERNATIONAL, Issue 1 2009H. P. Kierulf SUMMARY The vertical movement of the Earth's surface is the result of a number of internal processes in the solid Earth, tidal forces and mass redistribution in the atmosphere, oceans, terrestrial hydrosphere and cryosphere. Close to ice sheets and glaciers, the changes in the ice loads can induce large vertical motions at intraseasonal to secular timescales. The Global Positioning System (GPS) and Very Long Baseline Interferometry (VLBI) antennas in Ny-Ålesund, Svalbard that started observations in 1991 and 1995, respectively, observe vertical uplift rates on the order of 8 ± 2 mm yr,1, which are considerably larger than those predicted by postglacial rebound (PGR) models (order 2 mm yr,1). The observations also indicate increased uplift rates starting some time in 2000. A local GPS campaign network that has been reoccupied annually since 1998, reveals a tilting away from the neighbouring glaciers. The Svalbard glaciers have been undergoing melting and retreat during the last century, with increased melting since about 2000. We compared the observed vertical motion to the motion predicted by loading models using a detailed ice model with annual time resolution as forcing. The model predictions correlate well with the observations both with respect to the interannual variations and the spatial pattern of long-term trends. The regression coefficients for predicted and observed interannual variations in height is 1.08 ± 0.38, whereas the regression coefficient for the predicted and observed spatial pattern turns out to be 1.26 ± 0.42. Estimates of the predicted secular trend in height due to PGR and present-day melting are on the order of 4.8 ± 0.3 mm yr,1 and thus smaller than the observed secular trend in height. This discrepancy between predictions and observations is likely caused by the sum of errors in the secular rates determined from observations (due to technique-dependent large-scale offsets) and incomplete or erroneous models (unaccounted tectonic vertical motion, errors in the ice load history, scale errors in the viscoelastic PGR models and the elastic models for present-day melting). [source] Habitual use of the primate forelimb is reflected in the material properties of subchondral bone in the distal radiusJOURNAL OF ANATOMY, Issue 6 2006Kristian J. Carlson Abstract Bone mineral density is directly proportional to compressive strength, which affords an opportunity to estimate in vivo joint load history from the subchondral cortical plate of articular surfaces in isolated skeletal elements. Subchondral bone experiencing greater compressive loads should be of relatively greater density than subchondral bone experiencing less compressive loading. Distribution of the densest areas, either concentrated or diffuse, also may be influenced by the extent of habitual compressive loading. We evaluated subchondral bone in the distal radius of several primates whose locomotion could be characterized in one of three general ways (quadrupedal, suspensory or bipedal), each exemplifying a different manner of habitual forelimb loading (i.e. compression, tension or non-weight-bearing, respectively). We employed computed tomography osteoabsorptiometry (CT-OAM) to acquire optical densities from which false-colour maps were constructed. The false-colour maps were used to evaluate patterns in subchondral density (i.e. apparent density). Suspensory apes and bipedal humans had both smaller percentage areas and less well-defined concentrations of regions of high apparent density relative to quadrupedal primates. Quadrupedal primates exhibited a positive allometric effect of articular surface size on high-density area, whereas suspensory primates exhibited an isometric effect and bipedal humans exhibited no significant relationship between the two. A significant difference between groups characterized by predominantly compressive forelimb loading regimes vs. tensile or non-weight-bearing regimes indicates that subchondral apparent density in the distal radial articular surface distinguishes modes of habitually supporting of body mass. [source] Langjährige Messung von Kraft- und Verformungsgrößen in Neotopf-Gleitlagern einer Spannbetonstraßenbrücke.BETON- UND STAHLBETONBAU, Issue 12 2006Brückenlager-Monitoring Zwei Lager der dreifeldrigen Spannbeton-Straßenbrücke über den Aasee in Münster wurden beim Bau in den Jahren 1987/88 als Neotopf-Gleitlager mit Meßeinrichtungen ausgestattet, die den Gleitspalt und den Kippspalt messen. Außerdem ist eine Druckmeßdose eingebaut worden, die den Druck des Silikonöls im Stahltopf unter dem Elastomer mißt. Mit diesen Meßeinrichtungen wurde die Lastgeschichte der Lager während der Bauzeit festgestellt. Im Laufe der 18jährigen Standzeit der Brücke wurden die Lager mit diesen Meßeinrichtungen überwacht. Nach der Fertigstellung der Brücke und in Abständen von einigen Jahren sind Zustandgrößen der Lager im Sinne eines Monitoring auch durch Überfahren der Brücke mit Lastkraftwagen gemessen worden. Die Lager weisen nach 18 Jahren nur geringfügige Abnutzung auf. Longterm Measuring of Forces and Displacements in Elastomer Pot Sliding Bearings of a Prestressed Concrete Road Bridge. Monitoring of Bridge Bearings Two bearings of a three span prestressed concrete road bridge over the lake Aa in the City of Münster were equipped with elastomer pot sliding bearings with measuring devices during the erection in 1987/88, that are able to measure the distance between sliding slab and covering slab and the rotation of the covering slab. Moreover there are dynamometers, which measure the compression stress of the silicone oil in the pot under the elastomer. With these measuring devices the load history of the bearings during the time of erection of the bridge was monitored. In the 18 years that the bridge is in operation, the bearings have been monitored with these measuring devices. Following the completion of the bridge and further in intervals of some years the loads and displacements of the bearings were monitored especially during lorries were crossing the bridge. The bearings show only minimal attrition after 18 years in use. [source] | |||||||||||||