Issue 41

F. Chebat et alii, Frattura ed Integrità Strutturale, 41 (2017) 447-455; DOI: 10.3221/IGF-ESIS.41.56 453 F ATIGUE STRENGTH IN TERMS OF STRAIN ENERGY DENSITY AVERAGED IN A FINITE SIZE VOLUME y using the first model with a regular and very fine mesh the SED has been evaluated circumferentially all around the roller in the zone surrounding the weld root. The maximum SED value occurs outside the line of the application of the load. The angle of rotation is strongly dependent on the geometry of the bearing housing. In the case of the roller PSV 133 315 the maximum SED occurs at about 30 degrees from the line of load application. In that point all the modes of failure are contemporary present as will be discussed in the following. For this specific model an analysis of sensitivity of SED as a function of the length of the lack of penetration c has been carried out [27]. From a micrographic analysis conducted on a large amount of welded rollers c has been found to vary in the range between 0.6 and 1.0 mm. A typical image of the weld root is shown in Fig. 1b. The sensitivity analysis has been made varying the length of the lack of penetration and evaluating the SED in a control volume of radius R C = 0.28 mm. The variation of the SED is very limited in the range of c considered. The SED varies from 0.31 MJ/m 3 to 0.35 MJ/m 3 for a value of c corresponding to 0.6 and 1.0 mm, respectively. Considering the low variation of the SED as a function of the initial lack of penetration, the length c = 1 mm has been set in all FE analyses. This choice is in the safe direction because the worst configuration has been considered. Some fatigue tests have been conducted on the two rollers shown in Fig. 2 [27]. A test system has been created for reproducing the service conditions on the roller. The load has been applied by means of an external counter-roll which press with a constant pressure the tested roller which rotates with a regular speed. Altogether 22 new tests have been carried out considering the two investigated geometries. The new results reconverted in terms of the local SED have been compared with the scatterband proposed for structural welded steels [36]. That band is shown in Fig. 3 together with the new data. It is evident that the previous scatter band can be satisfactorily applied also to the new data from failure at the weld root of rollers tested at different load levels. Figure 3: Synthesis of new data in terms of local SED and comparison with the scatterband by Lazzarin and co-authors. C ONCLUSIONS he present paper deals with a local energy based approach employed for the fatigue assessment of rollers with failure occurring at the weld root. The rollers considered in the present investigation are particularly suited to conveyors that operate in very difficult conditions, where working loads are high, and large lump size material is conveyed; and yet, despite these characteristics, they require minimal maintenance. The bearing housings are welded to the tube body using autocentralising automatic welding machines utilizing a continuous wire feed. From the point of view of the fatigue behavior under loading, the weakest point of the entire structure is the lack of penetration of the weld root. Therefore, if the roller is loaded well above its declared nominal admitted load [26], it would B T