Kerbformoptimierung ohne FEM: Ein einfacher Weg,um Kerbspannungen abzubauen

Shape optimization without the use of FEM: An easy approach for the reduction of stress concentrations An analytical procedure for the shape optimization of engineering components is introduced which works without use of FEM. Significant notch stress reduction can be obtained with minimum effort.     

Kerbspannungen sind Biegespannungen – was sind gute und böse Kerben?

Notch stresses are bending stresses – what are good and bad notches? The origin of a stress concentration in an engineering component can be explained by the imagination of superimposed bending stress. The curved surface of a notch can also be seen as the concave side of a heavily curved beam under bending. The nonlinear stress distribution of the beam under bending superimposes with the nominal stress and causes the stress concentration. In order to avoid such stress concentrations the increase of stress along the notch contour has to compensate with the decrease of the nominal stress. A multi‐linearized model demonstrates the interaction between the flow of stresses and the increase of the cross‐section of the component.     

Measurement of the in situ ply fracture toughness associated with mode I fibre tensile failure in FRP. Part I: Data reduction

The fracture toughness associated with fibre tensile failure was measured for a T300/920 laminated carbon/epoxy material system using the compact tension specimen configuration. Six methods of data reduction were investigated for calculation of the toughness with the aim of finding the best technique, in terms of reproducibility of results and simplicity. The calculated fracture toughnesses were found to correlate well, though varying amounts of scatter were produced by each method. An optimum method was proposed that does not rely on the use of an optically measured crack length.     

Velocity of shear waves in an initially stressed incompressible anisotropic medium

The paper aims at the study of propagation of shear waves in an initially stressed anisotropic medium. The velocity equation is obtained and it is seen that the velocity of propagation depends upon the direction of propagation, the anisotropic factor and also on the initial stresses. Numerical computation indicates that the initial compressive stress diminishes the velocity of a shear wave which propagates along the direction 0° to 45° but increases it along the direction 45° to 90°. The reverse effect is obtained when initial stress is tensile. The anisotropy increases the velocity and even in the absence of initial stresses the velocity of the wave depends on the direction of propagation.     

Simulation‐based optimization of the local material state in the field of cyclically highly stressed case hardened construction details with notch effect*

Steel components very often show construction details, such as cross holes and rounded shaft shoulders which lead to local stress concentrations of the multiaxial stress state in case of mechanical loads (notch effect). Under cyclic loading these stress concentrations (hot spots) can cause crack initiation, crack propagation and finally failure of structural components. The fatigue strength of cyclically loaded components can be considerably increased by the heat treatment case hardening. The shape of the construction detail has a significant influence for the sub‐processes of the case hardening. This can be related to the carbon diffusion process during carburizing and the local heat transfer during quenching. As a result, the local material state following a case hardening process is often not optimal with respect to phase composition and residual stress field. In order to optimize the hardening process a heat treatment simulation based on the Finite Element Method was coupled with procedures for sensitivity analysis and optimization. Taking into account the operational loading conditions for the component, it was possible to adapt technological parameters of the case hardening process for the specific shape of the construction detail, leading to a substantially increased fatigue strength and therewith improvement of the efficiency of the case hardening process itself.     

On the determination of interfacial stresses in a composite material

The conventional finite element method has been modified to allow the elastic stresses along the fibre matrix interfaces of a composite to be determined with improved accuracy. The results obtained by this 'modified' method are compared with both a photoelastic and some traditional finite element solutions.     

Analysis of the whole implementation process and optimization of a Nitinol superelastic stent

Nitinol superelastic stents have been widely used to treat the vascular stenosis due to its excellent mechanical behavior and biocompatibility. However, there exist conflicts between the functional properties and mechanical properties of the stent. An optimization method has been employed to deal with the conflictions with the consideration of the whole implementation process of the stent in this paper. A straight vascular with tumor inside is considered. A commonly used NiTinol superleastic stent with diamond shape strut is employed. The vascular wall tissue and stenotic plaque are also treated as hyperelastic materials. Softwares Isight, ABAQUS and Solidworks are utilized to perform the optimization job. It can be seen that the stresses are high at the areas around the fillets of the stent due to the stress concentration from a primary analysis. Therefore, the two fillets radius, thickness and radius of the stent are chosen as four optimization variables. The optimization object is to decrease the maximum stress of stent and increase the displacement of the plaque. After the optimization, the maximum stress can be decreased by 8.2 %, which implies that the stent's work life can be increased. The stenosis of the blood vessel can be decreased from 56 % to 40.0 %.     

Identifying process parameters influencing gear runout

The hardening distortions with respect to base body, clutch teeth and helical gears are investigated for a serial‐produced main shaft gear of a 20NiCrMoS6‐4 steel. The influences of casting geometry, annealing heat treatment and stress relief annealing of blanks, as well as vertical and horizontal loading arrangements during case hardening, are studied. The concentricity, roundness and runout of clutch teeth and helical gears are measured in the soft machined, hardened and hard‐machined conditions. The Brinell hardness is measured on blanks obtained from different manufacturing routes showing differences in hardness and scatter. Stress relief annealing lowers the hardness and the scatter for all groups, but has no significant effect on distortions. The case depth, core hardness and surface hardness are measured after hardening. The study shows that the surface hardness correlates with the oil flow measured in the quench tank. The effect of casting geometry is stronger for the clutch teeth compared to the helical gears. For the clutch‐teeth roundness and runout, significantly lower values are found for square geometry compared to rectangular. It is also seen that the major part of the runout comes from roundness errors which are mainly induced by the hardening. Horizontal loading reduces roundness errors and runout but produces conical base‐bodies with worse backplane flatness.     

Shear deformation effect in second-order analysis of frames subjected to variable axial loading

In this paper a boundary element method is developed for the second-order analysis of frames consisting of beams of arbitrary simply or multiply connected constant cross section, taking into account shear deformation effect. Each beam is subjected to an arbitrarily concentrated or distributed variable axial loading, while the shear loading is applied at the shear center of the cross section, avoiding in this way the induction of a twisting moment. To account for shear deformations, the concept of shear deformation coefficients is used. Three boundary value problems are formulated with respect to the beam deflection, the axial displacement and to a stress function and solved employing a BEM approach. The evaluation of the shear deformation coefficients is accomplished from the aforementioned stress function using only boundary integration. Numerical examples with great practical interest are worked out to illustrate the efficiency, the accuracy and the range of applications of the developed method. The influence of both the shear deformation effect and the variableness of the axial loading are remarkable.     

Die Werkstoffermüdung bei Wälzbeanspruchung – Eine Hypothese zum Mechanismus

Materials fatigue by rolling contact stressing – A hypothesis of the mechanism Research in Rolling contact fatigue produced plenty of experimental observations and theoretical knowledge. By considering all these detailed informations a hypotheses for/describing the mechanism of the fatigue process was developed and will be presented. The fatigue process may be devided up in three phases. In phase 1 and 2 the structure will be deformed microplastically. The strength localy becomes smaller. Residual compressive stresses grow. Added to the load stresses, the stressing continual change. In phase 2 the deformations nearly stop. There is a critical point by material behaviour and stressing when phase 2 attains phase 3. The microplastic deformation changes to a macroplastic deformation. The structure “flows” in the direction of the relative tension stress, more and more (in direction) towards the surface. At both sides of the track vaults arise. In the middle of the track a vault also tends to arise, but it will be smoothed by over rolling. So normal to the raceway residual tensile stresses are created. They become biger and biger. Finally they start the subsurface cracks. By the hypotheses nearly all phenomena detected in rolling contact fatigue research can be explained. The thesis reveals chances, rolling contact systems to optimize.     

Explosive scarf welding of aluminum to copper plates and their interface properties

Explosive welding was used to produce scarf joint between aluminum and copper plates. This process is known as explosive scarf welding (ESW). In a scarf joint, the final bond interface is oblique. In this study, chamfered end of aluminum and copper plates were joined explosively and named scarf joint, employing changes in chamfered angle at different stand‐off distance and explosive loading. The geometry of scarf joint enables consideration of both flyer and base plate thickness and explosive loading and the effects on mechanical properties of interface such as bond shear strength and micro‐hardness can be investigated. Mathematical models developed on the interface properties of scarf joint to make relationship between the bond shear strength and explosive loading ratio. To check the adequacy of developed models, mechanical properties of interface, such as bond shear strength was predicted and compared with actual values in explosive cladding process. The results show reasonable agreement with theoretical predictions. Consequently, mathematical model which is based on scarf joints, can predict bond shear strength of cladding metals under desired explosive loading and flyer plate thickness.     

Berechnung der Dauerschwingfestigkeit bei biaxialen Mittelspannungen

Calculation of the fatigue endurance limit under biaxial mean stresses Investigated is a relative simple plane stress state with one alternating normal stress and two static biaxial normal stresses. This state of stress can be realized for example with hollow cylinders, which are loaded by internal or external pulsating pressure and by a force in axial direction. This state of stress is of special practical importance if the effect of biaxial residual stresses has to be estimated. It is shown how the shear stress intensity hypothesis SIH is describing the effect of biaxial mean stresses. There are some differences to other hypotheses, so f.e. high negative mean stresses are reducing the allowable stress amplitude. A comparison between calculation and test results for unalloyed and low alloyed steel in the range of R_p0,2 = 294 ‐ 940 MPa from the literature shows a good agreement.     

Measurement of the in situ ply fracture toughness associated with mode I fibre tensile failure in FRP. Part II: Size and lay-up effects

The in-plane size, thickness and lay-up effects on the measured fracture toughness associated with fibre tensile failure were investigated for a T300/920 laminated carbon/epoxy material system. Compact tension specimens were tested with scaled in-plane size, increased thickness, and having various proportions of plies orientated at 0° and 90° to the loading direction. No in-plane size effects were discovered; however, testing revealed a thickness dependence. It was found that the ply toughness is significantly dependent on the thickness of the 0° layers. Propagation values of toughness were measured to be 132 kJ/m² for specimens made up of [90/0₂] sub-laminates and between 57 and 69 kJ/m² for all other configurations. Investigation of the fracture surfaces using SEM revealed that the increase in measured toughness for specimens with thicker 0° plies was due to an increase in the amount of pulled-out 0° fibres.     

Materials and machining trends in terms of the existing axioms of the machining theory

The paper provides new insight into existing axioms of theory of machining. The existing theory of metalworking was based on the development of cutting materials during the 20^th century. In particular, the improvement of cutting materials, the new special materials and progressive technologies and the trends to increase cutting speed call for a review of existing definitions and relations. Appling previously defined relations between cutting conditions and results of machining, the inaccuracies occur and this may lead to an incorrect choice of cutting conditions in the specific conditions of machining. The paper analyses existing well‐known equations used in practice and on the basis of extensive experimental analysis it modifies them. Furthermore, the paper concerns the evaluation of physical and technological parameters, such as chips compression, machined surface roughness, cutting forces, cutting tool‐wear and tool‐life.     

Warping shear stresses in nonuniform torsion by BEM

 In this paper a boundary element method is developed for the nonuniform torsion of simply or multiply connected prismatic bars of arbitrary cross section. The bar is subjected to an arbitrarily distributed twisting moment, while its edges are restrained by the most general linear torsional boundary conditions. Since warping is prevented, beside the Saint–Venant torsional shear stresses, the warping normal and shear stresses are also computed. Three boundary value problems with respect to the variable along the beam angle of twist and to the primary and secondary warping functions are formulated and solved employing a BEM approach. Both the warping and the torsion constants using only boundary discretization together with the torsional shear stresses and the warping normal and shear stresses are computed. Numerical results are presented to illustrate the method and demonstrate its efficiency and accuracy. The magnitude of the warping shear stresses due to restrained warping is investigated by numerical examples with great practical interest. Received: 13 November 2001 / Accepted: 2 October 2002     

Optimization of pickling and annealing parameters in cold rolling using the Taguchi method

In this study, the optimum values of pickling and annealing parameters in cold rolling were obtained by using the Taguchi method. The materials taken from the hot rolling mill were subjected to pickling with different pickling times (3 min, 4 min, 5 min) parameters and then cold rolled. After this process, annealing was carried out with different annealing temperatures (700 °C, 750 °C, 800 °C), annealing times (5 min, 7.5 min, 10 min) in order to eliminate the cold working hardness. After these processes, sheet materials were subjected to skin-pass rolling, which is the last step of the rolling process. Optimum conditions are minimum rolling force measured from the load cell positioned on the work rolls. In this direction, S/N ratios with objective function “smaller is better” and were calculated and the optimum levels of the parameters were determined as 3 min pickling time, 750 °C annealing temperature and 10 min annealing time. In line with the findings obtained from the analysis of variance, it was determined that the most important parameter was the annealing time with a rate of 48.99 %. Confirmation experiments were carried out and it was concluded that the optimization was valid.     

Numerical aspects of finite element simulations of residual stresses in metal matrix composites

When a metal matrix composite (MMC) is cooled down from the fabrication or annealing temperature to room temperature, residual stresses are induced in the composite due to the mismatch of the thermal expansion coefficients of the matrix and reinforcement. A thermomechanical model describing these processes is presented considering that the reinforcement component has a thermo‐elastic behaviour and that the matrix material exhibits a thermo‐elastoviscoplastic behaviour. The model is implemented with a semi‐implicit forward gradient finite element method algorithm and the resulting code is used to perform numerical simulations and calculate thermally induced residual stress fields in MMCs. Several tests are performed on a continuously reinforced MMC and a short cylindrical particle MMC in order to optimize the algorithm and define its governing parameters. Good agreement was obtained with results from other authors. Copyright © 2001 John Wiley & Sons, Ltd.     

Procedure for reliable durability validation of train axles

The structural durability of wheelset axles depends on the operational loading as well as on the fatigue strength of axles. To validate an axle design the time‐varying stresses in individual critical areas of the axle, represented by their stress spectra, must be taken into account. The allowable stresses in individual axle areas depend both on the fatigue strength and on the spectrum of operational stresses. The procedure for the axle structural durability validation, presented in this paper, includes the numerical fatigue life estimation as well as the experimental durability approval. Also the influences of axle design, material properties and manufacturing technology on structural durability are discussed.     

Surface Aspects in Fatigue of Shape Memory Alloys (SMA)

The present paper briefly reviews the role of surfaces in fatigue of shape memory alloys (SMAs). When polished and defect free austenitic surfaces are cooled to lower temperatures, the martensitic transformation creates a pattern of surface upheavals; and an inverse pattern forms when polished and defect free martensite is heated. In full transformation cycles (heating followed by cooling or cooling followed by heating) high quality surfaces can only be re‐established if no fatigue occurs. After multiple cycling intrusions and extrusions can be detected which represent fatigue damage accumulation. Cracks can grow from extrusions and intrusions. We also show that fatigue cracking can start from surface defects which are for example introduced by wire drawing. We then briefly discuss surface treatments which are intended to improve fatigue resistance of shape memory alloys. These may consist in the modification of the SMA microstructure in the surface region (by deformation or diffusion treatments). They can also involve coating procedures where other elements are deposited on the SMA surface. However, SMA performance and the intensity of exploitable SM effects are usually reduced by surface treatments.