Statistical Modelling of the Fatigue Bending Strength of Norway Spruce Wood

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.

Numerical analysis on fatigue behavior of ultrahigh-performance concrete-Orthotropic steel composite bridge deck

Ultrahigh-performance concrete-orthotropic steel composite bridge deck is composed of the orthotropic steel deck and a thin ultrahigh-performance concrete (UHPC) overlayer. In the previous fatigue tests, two typical fatigue failure modes were found and identified. As a supplementary test after fatigue tests, air penetration method is capable of providing a reference to the quantitative and non-destructive damage detection of fatigue damage of UHPC. To further the previous study, a detailed numerical investigation is accomplished through complimentary finite element (FE) analysis. Compared with the solid element model, the refined shell-solid element model can better reflect the mechanical behavior. It is illustrated that the vertical stress can be adopted in assessing the fatigue strength of rib-to-diaphragm welded connection in the field test by means of nominal stress method. The combination of various factors would lead to fatigue shear failure of the short headed-studs. The fatigue strength of rib-to-diaphragm welded connection predicted by the hot spot stress method and the consistent nominal stress (CNS) method can basically meet the requirements of FAT90. The consistent nominal stress method can be used as the optimization method of nominal stress of fatigue detail. It is demonstrated that the fatigue life of UHPC can be estimated by S-N curves of ordinary concrete conservatively. The allowable equivalent maximum stress level can be taken as 0.55 for two million cycles of fatigue loading, and 0.52 for five million cycles of fatigue loading.

Finite Element Calculation of the Polymer Stamp Material Redistribution under Restrictions on Fatigue Life

The numerical method of stamp topological optimization taking into account fatigue strength is presented in the work. It is proposed to take into account the restrictions on the stress state in accordance with the curve of the dependence of the maximum stresses on the number of loading cycles in the ESO topological optimization method. An approach to the selection of the evolutionary coefficient with a step-by-step increase in the rejection coefficient is proposed when constructing an iterative scheme for the rejection of elements by the method of topological optimization. The calculation of the stamp optimal topology with a decrease in volume due to the removal and redistribution of material was carried out in the study. The new geometric model of the optimal topology stamp is based on the predicted distribution of elements with a minimum stress level. The verification calculation of the stress state of the stamp of optimal topology with an assessment of fatigue strength was carried out in the work. The numerical calculation was carried out using the finite element method in the Ansys software package. The minimized stamp volume decreased by 35% according to the calculation results. The results of the study can be further applied in the development of topological optimization methods and in the design of stamping tools of optimal topology.

Improving the Thermal Fatigue Strength of Hot-Working Tools by Laser Treatment

A method for increasing the thermal fatigue strength of a tool used in hot forming of bearing rings by applying circular laser tracks to the working surfaces is considered. Laser treatment is carried out with a power of 2.0...2.5 kW by applying on the end face working surface of the tool in the direction from the center to the periphery of the circular tracks with a common center coinciding with the center of the circumference of the end face; the tool is rotated at a constant angular rate, the spot diameter ds for each track is selected according to the dependence ds,i+1/ds,i = 0.85...0.90, and the laser radiation spots of adjacent tracks have a common point of contact. The results of pilot testing are presented, which confirmed the high technical and economic efficiency of the use of laser quenching for ejectors and punches. A tool made by machining, for example, an ejector of an AMP-70 automatic press, is subjected to volume quenching and tempering. The ejector material was steel 3Kh3M3F, quenching temperature in oil – 1030...1050 °С, tempering temperature – 580...610 °С. After volume quenching, additional machining is carried out, usually grinding, in order to remove the decarbonized layer of material formed during heat treatment and to give the working surface the required roughness class. The final stage in the tool manufacturing is the quenching of its working surface by laser treatment. Pilot testing showed that the use of laser treatment made it possible to increase the durability of ejectors of various types by 2 ... 3 times, of deforming punches – by 2.2 times.