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Percussive rotary drilling is recognized as the most efficient method for hard rock drilling. Despite clear advantages over conventional rotary methods, there are still some uncertainties associated with percussive drilling. For geothermal applications, drilling accounts for a large portion of the total cost. Specifically, the wear of drill bits when drilling in hard rock is a predominant cost factor and drilling parameters are often based on the experience of the field operator. Within the framework of the H2020 project GEOFIT, numerical simulations of percussive drilling are performed in order to evaluate the rock drilling process and gain insight about the trade-off between wear and Rate of Penetration (ROP). In the simulations, the rock material was represented by the Bonded Discrete Element Method (BDEM), the drill bit by the Finite Element Method (FEM), the drilling fluid by the Particle Finite Element Method (PFEM) and the abrasive wear on the surface of the drill bit was represented by Archard’s wear law. The drilling simulations were conducted for two rock materials; a sedimentary rock material corresponding to what was found when drilling at the GEOFIT pilot site in Aran Islands, Ireland, and a harder reference rock similar to granite. The results show that, at a drill bit impact force of 10 kN, the ROP in the sedimentary rock was 6.3 times faster than for granite. When increasing the impact force to 40 and 50 kN, however, the ROP for the sedimentary rock is only 1.9 and 1.6 times faster, respectively. Furthermore, the wear rate decreased with increased impact force when drilling in the granite rock. For the sedimentary rock, however, the loading resulting in the best trade-off between abrasive wear and ROP was the second highest loading of 40 kN, which suggests that an increase in impact energy may increase the rate of penetration but may not be economically motivated. Raw datais available for download here.

For the reduction of the environmental footprint of Heavy-Duty vehicles (HDV), lighter chassis components can be considered. A lighter HDV chassis gives the opportunity of lower fuel consump-tion, increased payloads, and savings of material resources. One way of achieving this, is to reduce thicknesses of components in combination of using higher strength steels. For the aim of forming UHSS into complex geometries the need to characterize thick sheet metal at elevated temperatures arises. This work aims at expanding earlier research of characterization of thinner sheet metal and create a testing methodology for tensile tests of 7 mm thick steel sheets at elevated temperatures. An experimental methodology for evaluating high strength steel under warm conditions have been de-veloped and demonstrated. A Digital Image Correlation system is used to extract strain fields for all three testing temperatures. This together with an automatized induction system pre-defined temperature cycles are applied. When the desired Hollomon-Jaffe constant is obtained the tensile test is executed. The methodology shows promising results with good repeatability of stress-strain curves. The methodology shows good stability and are promising for future development and investigations of high strength steels under warm forming conditions., This research has been funded by the Research Fund for Coal and Steel, Project: WarmLight, Grant Agreement: 800649.

The automotive industry is currently adapting to a new reality, where anthropogenic emissions need to decrease significantly. To meet present and future demands of vehicle design, press harden-ing techniques to produce complex geometries with high strength and ductility as well as good precision are of great interest. New generations of hot forming steels enable both further weight reductions by using thinner sheets as well as better crash performance due to its ability to improve the structural integrity of the body-in-white. To promote the use of these new steel grades, it is important to study their performance using well-instrumented lab scale test of full-scale compo-nents. Since these tests are often time consuming and expensive, calibrating constitutive models with tensile specimens and using finite element analysis is a more cost-effective alternative. How-ever, these calibrated models should be validated against full-scale experiments to verify their effectiveness in predicting the material behaviour in complex crash environments. In this paper, a high-speed 3D digital image correlation experiment is performed on a crash box under axial com-pression. The material is a hot forming steel grade with a specified tensile strength of 1000 MPa. The axial crash tests are modelled based on a visco-plastic model calibrated by high-speed tensile tests. The computed results in terms of force response and obtained deformations agree well with the corresponding measurements.

Numerical modelling of the fracture of heterogeneous brittle materials is of interest for several industries, such as rock excavation and comminution applications. A numerical model of brittle materials needs to be able to capture the unpredictable results, e.g. with regards to measured strength and fracture pattern, as observed experimentally. In a previous work [1], the Bonded Discrete Element Method [2] was combined with statistical methods in order to generate heterogeneous rock bodies. Grains of random sizes and shapes, consisting of multiple bonded discrete elements, were generated in the body and the micromechanical parameters of these grains were governed by the Weibull distribution [3]. In this work, this modelling approach was used to evaluate the fracture behaviour of experiments commonly found within the field of rock mechanics - the unconfined and confined axial compression test, Brazilian disc test and the three point bend test. For each test, a large set of numerical samples were generated and simulated. The fracture behaviour, e.g. initiation, propagation and coalescence of cracks, were investigated for different levels of heterogeneity and grain cement strengths. The results show that a variety of different fracture modes can be obtained with this modelling approach. Further, the results suggests that the statistical methods employed in this work improves the versatility of the Bonded Discrete Element Method for rock modelling. ISBN för värdpublikation: 978-84-123222-8-6