Applied Mechanics and Materials Vol. 831

Abstract: The AQUASONIC project is aimed to develop a sounding rocket including a hybrid propulsion system based on the propellant combination nitrous oxide and polyethylene. It takes place in the frame of the STERN (Student Experimental Rockets) programme founded by the German Space Agency (DLR) in order to promote students in the area of launch vehicles. Main element of the project is the AQUASONIC rocket, which shall reach a flight altitude of 5-6 km and a velocity of MACH 1. All major activities like design, manufacturing, verification and, finally, the launch campaign will be performed by students. The rocket shall be launched at Esrange Space Centre (Sweden) in 2016. Thus, students are able to apply their skills and knowledge to a real project like it is conducted by the space industry or research organisations.

Abstract: Previous studies on one-sided lapping allow to state that not only parameters of lapping elements (i.e. properties of workpiece, abrasive grains and lapping plate) impact on a lapping efficiency. Influential are also kinematics and dynamics of the process. It is crucial to control an average velocity of lapping (v), a distribution of tangential acceleration (at), a nominal pressure (p), a lapping time (t) and disposal of workpiece in separator.Based on kinematics equations and the tribological models, the dimensionless distribution of the material removal volume and the trajectories of abrasive grains cutting on the lap were numerically simulated. Obtained information about excessive wear on lapping plate was the starting point to work out non-standard, single-plate lapping systems.In this paper ideas of unconventional lapping systems were presented. After numerous simulations and careful analysis it was observed, that the most desirable system is radial lapping system. It was point out that other systems are not very different from the standard kinematic system. Generated trajectories of an abrasive grain were almost identical. What is more, effects of kinematical parameters on the trajectories, velocities and accelerations observed in radial lapping system are shown.

Abstract: New tools for flat grinding are the subject of the paper. Electroplated diamond tools with different grains - D64 and D107 - were used in a modified single-disc lapping machine configuration. The results from flat grinding, such as the material removal rate (MMR), surface roughness and plane-parallelism are presented in the paper. Apart from ceramic samples, the additional experiments were carried out on cemented carbide workpieces (H10S) with the use of a diamond electroplated tool (D64). SEM microscopic images of unworn and worn active tool surface are presented with abrasive grains worn by attrition and cavities in the nickel bond after the grains pull out.

Abstract: The paper investigates the possibility of applying a heuristic algorithm to solve an inverse heat conduction problem encountered in Temperature Oscillation IR Thermography method. The idea of TOIRT [24] is to infer about convection coefficient in given point h(x,y) from phase delay φ(x,y) of the temperature signal measured by means of thermography on the opposite side of heat conducting wall in steady-periodic conditions. Since the exact solution for inverse 3D heat conduction problem defined by TOIRT method is not available, there is a need to employ some kind of optimisation scheme to find the distribution h(x,y). In [22] it has been done by iterative solution of associated direct problem with finite difference method and updating the distribution h(x,y) in every iteration according to relative error between measured and computed values of φ(x,y) on the surface of the conducting wall. This paper presents an alternative approach based on symbolic regression with Genetic Algorithm to find an explicit formula relating amplitude as well as phase shift angle of temperature signal and the value of convection coefficient. Proposed procedure involves an offline training of the model with data previously generated with Finite Element Method which assures that time-consuming part of the calculation is carried out once and since then obtained explicit formula may be used in fast and straightforward manner. The results show that proposed approach gives solution with at least comparable accuracy to that obtained in [22], but requires careful selection of training data mainly in terms of their diversity and values of gradient of h(x,y).

Abstract: Metal structures and components made of fiber reinforced plastics are often tied using bolts, rivets or adhesive bonding methods. To avoid the disadvantages of these techniques, hybrid transition structures can be used. Two different concepts are investigated and a numerical approach to calculate the failure properties of such transition structures is proposed. This is done using cohesive zone models for a wire concept and probability functions in an extended failure tree analysis for a foil concept. Numerical results based on the finite element method are presented.

Abstract: In the paper,numerical analysis of dynamics of a variable mass manipulator is presented. A revolute joints composed manipulator is considered. Payload of the gripper is considered as the only element characterized by unknown value of its mass (variable between subsequent operations). As in other cases of the revolute joints composed manipulators, its behaviour dependents significantly on the pose of the manipulator. When the manipulator is driven over a larger operating range, nonlinear terms can be observed in the equations, and linear controller does not ensure satisfactory its performance. Thus, nonlinear control technique is employed. To ensure such non-linear control, nonlinear models of the plant are introduced in the controller. Initially, there is an open-loop feedforward model-based controller used to enforce the manipulator to follow the required paths. Work of the proposed configuration is tested numerically. To deal with it, a numerical model of the manipulator is prepared and rules of multibody modelling are used for it. Two identical models (equations) are implemented in two separate blocks of the model. In the controller block, primary set of parameters of the robot structure is introduced. As the actual gaol is to observe effects of the incorrect identification of the payload masses, the plant (the controlled object) model may not be described by identical values of their masses in the performed tests. As a result, significant path errors are observed during simulations. To eliminate these errors, the controller model is enlarged. A feedback loop is accepted and a proportional-derivative controller is considered in this feedback part. Subsequently, collaboration between these feedforward and feedback blocks is tested. This collaboration runs well (error of the realised path is reduced). However, intensive work of the feedback controller is non-required in the considered application. Possibility of mass identification is tested. Performed tests proofed that signal of the proportional-derivative controller is a useful data base for the required identification process. A short run of a trial path is performed at beginning of the motion. PD signal obtained during such test motion are collected. They allow us to identify the mass of the payload and to improve data in the model-based controller. Thanks of it, the feedback control signal is reduced and realisation of the required path is acceptable.

Abstract: The influence of post-heat treatment on the mechanical properties of complexly deformed low carbon steel is studied in this work to improve a forming process to ensure and enhance the overall product quality. Since there is no constant deformation degree, there are big differences in the mechanical properties within the part: material seen a high deformation degree shows increased strength but decreased ductility, which carries the risk of formation of cracks. A post heat treatment reduces that risk by reducing inner stresses and work/strain hardening. This study involves two steel component varying in thickness and forming process: stretching plus bending and stretching only. Post heat treatment after cold forming is necessary to provide the safety margin needed in the application. The forming process which consists of stretching only delivers a more uniform deformation over the length of the nozzle. However, adding bending to stretching keeps the deformation degree lower and leads to a more homogenous property distribution after annealing. Material only exposed to stretching shows strong softening by deforming above critical deformation degree with resulting values below initial properties. Even during annealing of around 40 min, recrystallization took part: more globular grains can be seen. Post heat treatment also homogenizes hardness over the cross-section of the work piece. Increased hardness towards the tension and compression surface side as a result of friction between tool and work piece should be avoided to provide the safety margin also on the more local level. Annealing retrieves the yield point phenomenon and strongly increases ductility up to 35 % and reduces the ratio of tensile yield strength to ultimate tensile strength to more moderate levels of around 0.6.

Abstract: Lubricated bearings are usually used to support and guide the shafts in gas turbines, exhaust gas turbochargers and other turbomachines. The lubricant is also used for cooling the hot parts such as the turbine side of the rotor. The high temperature condition effects a contamination of the compressed air by evaporating the lubricant. In special applications an oil-free operation has to be guaranteed. In other applications an oil lubricated bearing system prevents an immediate engine stop. The use of air foil bearings eliminates the disadvantages of the oil lubricated bearings. An air foil bearing works self-adjusting the necessary gap which depends on relative speed of the rotor and the stator surfaces and on the viscosity of the lubricating fluid. A low rotor speed requires a small bearing gap, whereas the gap must be enlarged for high speed at constant viscosity of the lubricant. An air foil bearing works contact-free in a wide speed range even at small rotation speed depending on design. On an experimental basis an air foil bearing system was developed and tested for a standard automotive exhaust gas turbocharger. The developed bearings need no infrastructure for supplying with lubricant and its cooling and filtering. The air foil bearings was calculated and designed based on the estimated bearing loads. For the manufacturing of the radial wave springs a procedure was developed. A test facility was set up for the experiments. The modified rotor works at the standard rotation speed of 80000 rpm. The bearings support and guide the shaft contact-free (without wear) in a range of a few thousands of revolutions per minute up to the standard rotation speed.

Abstract: The three-dimensional numerical study of natural convection in a cubical enclosure, discretely heated, was carried out in this study. Two heating square sections, similar to the integrated electronic components, are placed on the vertical wall of the enclosure. The imposed heating fluxes vary sinusoidally with time, in phase and in opposition of phase. The temperature of the opposite vertical wall is maintained at a cold uniform temperature and the other walls are adiabatic. The governing equations are solved using Control volume method by SIMPLEC algorithm. The sections dimension ε = D / H and the Rayleigh number Ra were fixed respectively at 0,35 and 10⁶. The average heat transfer and the maximum temperature on the active portions will be examined for a given set of the governing parameters, namely the amplitude of the variable temperatures a and their period τ_p. The obtained results show significant changes in terms of heat transfer, by proper choice of the heating mode and the governing parameters.