| 1. |
- Holmberg, L. Joakim, 1971-
(författare)
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Musculoskeletal Biomechanics in Cross-country Skiing
- 2012
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Doktorsavhandling (övrigt vetenskapligt/konstnärligt)abstract
- Why copy the best athletes? When you finally learn their technique, they may have already moved on. Using muscluloskeletal biomechanics you might be able to add the "know-why" so that you can lead, instead of being left in the swells.This dissertation presents the theoretical framework of musculoskeletal modeling using inverse dynamics with static optimization. It explores some of the possibilities and limitations of musculoskeletal biomechanics in cross-country skiing, especially double-poling. The basic path of the implementation is shown and discussed, e.g. the issue of muscle model choice. From that discussion it is concluded that muscle contraction dynamics is needed to estimate individual muscle function in double-poling. Several computer simulation models, using The Anybody Modeling System™, have been created to study different cross-country skiing applications. One of the applied studies showed that the musculoskeletal system is not a collection of discrete uncoupled parts because kinematic differences in the lower leg region caused kinetic differences in the other end of the body. An implication of the results is that the kinematics and kinetics of the whole body probably are important when studying skill and performance in sports. Another one of the applied studies showed how leg utilisation may affect skiing efficiency and performance in double-poling ergometry. Skiing efficiency was defined as skiing work divided by metabolic muscle work, performance was defined as forward impulse. A higher utilization of the lower-body increased the performance, but decreased the skiing efficiency. The results display the potential of musculoskeletal biomechanics for skiing efficiency estimations. The subject of muscle decomposition is also studied. It is shown both analytically and with numerical simulations that muscle force estimates may be affected by muscle decomposition depending on the muscle recruitment criteria. Moreover, it is shown that proper choices of force normalization factors may overcome this issue. Such factors are presented for two types of muscle recruitment criteria.To sum up, there are still much to do regarding both the theoretical aspects as well as the practical implementations before predictions on one individual skier can be made with any certainty. But hopefully, this disseration somewhat furthers the fundamental mechanistic understanding of cross-country skiing, and shows that musculoskeletal biomechanics will be a useful complement to existing experimental methods in sports biomechanics.
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| 2. |
- Pettersson, Robert, 1981-
(författare)
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Simulation of Human Movements through Optimization
- 2012
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Doktorsavhandling (övrigt vetenskapligt/konstnärligt)abstract
- Optimization has been used to simulate human neural control and resulting movement patterns. The short term aim was to develop the methodology required for solving the movement optimization problem often arising when modelling human movements. A long term aim is the contribution to increased knowledge about various human movements, wherein postures is one specific case. Simulation tools can give valuable information to improve orthopeadic treatments and technique for training and performance in sports. In one study a static 3D model with 30 muscle groups was used to analyse postures. The activation levels of these muscles are minimized in order to represent the individual’s choice of posture. Subject specific data in terms of anthropometry, strength and orthopedic aids serve as input. The specific aim of this part was to study effects from orthopedic treatment and altered abilities of the subject. Initial validation shows qualitative agreement of posture strategies but further details about passive stiffness and anthropometry are needed, especially to predict pelvis orientation. Four studies dealt with movement optimization. The main methodological advance was to introduce contact constraints to the movement optimization. A freetime multiple phase formulation was derived to be able to analyse movements where different constraints and degrees of freedom are present in subsequent phases of the movements. The athletic long jump, a two foot high jump, a backward somersault and rowing were used as applications with their different need of formulation. Maximum performance as well as least effort cost functions have been explored. Even though it has been a secondary aim in this work the results show reasonable agreement to expected movements in reality. Case specific subject properties and inclusion of muscle dynamics are required to draw conclusions about improvements in the sport activity, respectively.
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| 3. |
- Rashid, Asim
(författare)
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Finite Element Modeling of Contact Problems
- 2016
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Doktorsavhandling (övrigt vetenskapligt/konstnärligt)abstract
- Contact is the principal way load is transferred to a body. The study of stresses and deformations arising due to contact interaction of solid bodies is thus of paramount importance in many engineering applications. In this work, problems involving contact interactions are investigated using finite element modeling.In the first part, a new augmented Lagrangian multiplier method is implemented for the finite element solution of contact problems. In this method, a stabilizing term is added to avoid the instability associated with overconstraining the non-penetration condition. Numerical examples are presented to show the influence of stabilization term. Furthermore, dependence of error on different parameters is investigated.In the second part, a disc brake is investigated by modeling the disc in an Eulerian framework which requires significantly lower computational time than the more common Lagrangian framework. Thermal stresses in the brake disc are simulated for a single braking operation as well as for repeated braking. The results predict the presence of residual tensile stresses in the circumferential direction which may cause initiation of radial cracks on the disc surface after a few braking cycles. It is also shown that convex bending of the pad is the major cause of the contact pressure concentration in middle of the pad which results in the appearance of a hot band on the disc surface. A multi-objective optimization study is also performed, where the mass of the back plate, the brake energy and the maximum temperature generated on the disc surface during hard braking are optimized. The results indicate that a brake pad with lowest possible stiffness will result in an optimized solution with regards to all three objectives. Finally, an overview of disc brakes and related phenomena is presented in a literature review.In the third part, a lower limb donned in a prosthetic socket is investigated. The contact problem is solved between the socket and the limb while taking friction into consideration to determine the contact pressure and resultant internal stress-strain in the soft tissues. Internal mechanical conditions and interface stresses for three different socket designs are compared. Skin, fat, fascia, muscles, large blood vessels and bones are represented separately, which is novel in this work.
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| 4. |
- Schmidt, Peter, 1965-
(författare)
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Analysis of adhesively bonded joints : an asymptotic approach
- 2001
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Licentiatavhandling (övrigt vetenskapligt/konstnärligt)abstract
- Simulations using the Finite Element Method (FEM) play an increasingly important role in the design process of joints and fasteners in the aerospace industry. In order to utilize the potential of using adhesive bonding, there is an increasing need for effective and accurate computational methods. The geometry and the nature of an adhesive joint is, however, not so simple to describe effectively using regular FEM-codes. The main reason is that the very thin and soft adhesive layer must be modelled by a large number of FEM-elements in the thickness direction to achieve sufficiently accurate calculations. To overcome this difficulty, special FEM-elements can be developed that provides a material surface treatment of the adhesive and the joined parts. In order to create a model that reflects the above features one may introduce proper scalings on the geometry and on the material properties in terms of a perturbation parameter. Within the framework of three-dimensional elasticity, together with an asymptotic expansion method, a material surface model is obtained through a systematic procedure of derivation. In such derivation no a priori assumptions on the displacements or stress fields are needed. The final result is a variational equation posed over a single reference surface, which forms the basis of a structural element for the compound joint.
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| 5. |
- Ahn, Young Ju, et al.
(författare)
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Shakedown in frictional contact of discrete elastic systems: A review
- 2022
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Ingår i: International Journal of Solids and Structures. - : PERGAMON-ELSEVIER SCIENCE LTD. - 0020-7683 .- 1879-2146. ; 241
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Forskningsöversikt (refereegranskat)abstract
- When exposed to cyclic quasi-static loading, elastic bodies in contact may develop a favourable condition where slip ceases after a few cycles, an occurrence commonly known as frictional shakedown. If the amplitude of the cyclic load is greater than a so-called shakedown limit, shakedown cannot occur. In this review paper, the validity of shakedown theorems in the context of conforming contacts with a la Coulomb friction is first discussed. Then, an optimisation method for determining the shakedown limit of elastic discrete three-dimensional systems is reviewed. Finally, an incremental Gauss-Seidel algorithm, extended to three-dimensional systems, is here illustrated in details for the first time. The algorithm allows us to describe the transient response of normal-tangential coupled systems under a given cyclic loading scenario, and to determine their possible shakedown depending on the initial conditions. An example concerning a discrete conforming contact problem, where either coupling or uncoupling conditions can be imposed, is illustrated.
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| 6. |
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| 7. |
- Andersson, Lars-Erik, 1942-, et al.
(författare)
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A Survey of Basic Mathematical Results for Frictional Contact Problems
- 2001
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Ingår i: From Convexity to Nonconvexity. - Dordrecht/Boston/London : Kluwer. - 0792371445 - 9780792371441 ; , s. -392
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Bokkapitel (övrigt vetenskapligt/konstnärligt)abstract
- Contains a collection of invited papers dedicated to the memory of two great mathematicians, Gaetano Fichera and Panagis Panagiotopoulos. The book is centered around the seminal research of G Fichera on the Signorini problem, hemivariational inequalities, nonsmooth global optimization, and regularity results for variational inequatities.
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| 8. |
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| 9. |
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| 10. |
- Andersson, Lars-Erik, 1942-, et al.
(författare)
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On the existence and uniqueness of steady state solutions in thermoelastic contact with frictional heating
- 2005
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Ingår i: Proceedings of the Royal Society of Edinburgh. Section A Mathematics. - : The Royal Society. - 0308-2105 .- 1473-7124. ; 461:2057, s. 1261-1282
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Tidskriftsartikel (refereegranskat)abstract
- It is well known that contact and friction in thermoelasticity result in mathematical problems which may lack solutions or have multiple solutions. Previously, issues related to thermal contact and issues related to frictional heating have been discussed separately. In this work, the two effects are coupled. Theorems of existence and uniqueness of solutions in two or three space dimensions are obtained - essentially extending, to frictional heating, results due to Duvaut, which were built on Barber's heat exchange conditions. Two qualitatively different existence results are given. The first one requires that the contact thermal resistance goes to zero at least as fast as the inverse of the contact pressure. The second existence theorem requires no such growth condition, but requires instead that the frictional heating, i.e. the sliding velocity times the friction coefficient, is small enough. Finally, it is shown that a solution is unique if the inverse of the contact thermal resistance is Lipschitz continuous and the Lipschitz constant, as well as the frictional heating, is small enough.
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| 11. |
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| 12. |
- Andersson, Lars-Erik, 1942-, et al.
(författare)
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Thermoelastic Contact with Frictional Heating
- 2006
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Ingår i: Nonsmooth Mechanics and Analysis,2003. - New York : Springer Science+business Media, inc.. ; , s. 61-
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Konferensbidrag (refereegranskat)
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| 13. |
- Barber, J.R., et al.
(författare)
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Shakedown in frictional contact problems
- 2007
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Ingår i: 2007 Proceedings of the ASME/STLE International Joint Tribology Conference. - 0791838110 - 0791848108 ; , s. 517-519
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Konferensbidrag (övrigt vetenskapligt/konstnärligt)abstract
- If a linear elastic system with frictional interfaces is subjected to periodic loading, any slip which occurs generally reduces the tendency to slip during subsequent cycles and in some circumstances the system ‘shakes down’ to a state without slip. It has often been conjectured that a frictional Melan’s theorem should apply to this problem — i.e. that the existence of a state of residual stress sufficient to prevent further slip is a sufficient condition for the system to shake down. Here we discuss recent proofs that this is indeed the case for ‘complete’ contact problems if there is no coupling between relative tangential displacements at the interface and the corresponding normal contact tractions. By contrast, when coupling is present, the theorem applies only for a few special two-dimensional discrete cases. Counter-examples can be generated for all other cases. These results apply both in the discrete and the continuum formulation.
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| 14. |
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| 15. |
- Bendsoe, MP, et al.
(författare)
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Joakim Petersson 1968-2002 - Obituary
- 2003
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Ingår i: Structural and multidisciplinary optimization (Print). - : Springer Science and Business Media LLC. - 1615-147X .- 1615-1488. ; 25:3
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Annan publikation (övrigt vetenskapligt/konstnärligt)
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| 16. |
- Christensen, Peter, et al.
(författare)
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An introduction to structural optimization
- 2009. - 1
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Bok (övrigt vetenskapligt/konstnärligt)abstract
- Mechanical and structural engineers have always strived to make as efficient use of material as possible, e.g. by making structures as light as possible yet able to carry the loads subjected to them. In the past, the search for more efficient structures was a trial-and-error process. However, in the last two decades computational tools based on optimization theory have been developed that make it possible to find optimal structures more or less automatically. Due to the high cost savings and performance gains that may be achieved, such tools are finding increasing industrial use.This textbook gives an introduction to all three classes of geometry optimization problems of mechanical structures: sizing, shape and topology optimization. The style is explicit and concrete, focusing on problem formulations and numerical solution methods. The treatment is detailed enough to enable readers to write their own implementations. On the book's homepage, programs may be downloaded that further facilitate the learning of the material covered.The mathematical prerequisites are kept to a bare minimum, making the book suitable for undergraduate, or beginning graduate, students of mechanical or structural engineering. Practicing engineers working with structural optimization software would also benefit from reading this book."--Publisher's website.
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| 17. |
- Christensen, P. W., et al.
(författare)
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Formulation and comparison of algorithms for frictional contact problems
- 1998
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Ingår i: International Journal for Numerical Methods in Engineering. - : John Wiley & Sons. - 0029-5981 .- 1097-0207. ; 42:1, s. 145-173
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Tidskriftsartikel (refereegranskat)abstract
- This paper presents two algorithms for solving the discrete, quasi-static, small-displacement, linear elastic, contact problem with Coulomb friction. The algorithms are adoptions of a Newton method for solving B-differentiable equations and an interior point method for solving smooth, constrained equations. For the application of the former method, the contact problem is formulated as a system of B-differentiable equations involving the projection operator onto sets with simple structure; for the application of the latter method, the contact problem is formulated as a system of smooth equations involving complementarity conditions and with the non-negativity of variables treated as constraints. The two algorithms are numerically tested for two-dimensional problems containing up to 100 contact nodes and up to 100 time increments. Results show that at the present stage of development, the Newton method is superior both in robustness and speed. Additional comparison is made with a commercial finite element code.
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| 18. |
- Ciavarella, M., et al.
(författare)
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Interaction of thermal contact resistance and frictional heating in thermoelastic instability
- 2003
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Ingår i: International Journal of Solids and Structures. - 0020-7683 .- 1879-2146. ; 40:21, s. 5583-5597
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Tidskriftsartikel (refereegranskat)abstract
- Thermoelastic contact problems can posess non-unique and/or unstable steady-state solutions if there is frictional heating or if there is a pressure-dependent thermal contact resistance at the interface. These two effects have been extensively studied in isolation, but their possible interaction has never been investigated. In this paper, we consider an idealized problem in which a thermoelastic rod slides against a rigid plane with both frictional heating and a contact resistance. For sufficiently low sliding speeds, the results are qualitatively similar to those with no sliding. In particular, there is always an odd number of steady-state solutions, if the steady-state is unique it is stable and if it is non-unique, stable and unstable solutions alternate, with the outlying solutions being stable. However, we identify a sliding speed V0 above which the number of steady states is always even (including zero, implying possible non-existence of a steady-state) and again stable and unstable states alternate. A parallel numerical study shows that for V > V0 there are some initial conditions from which the contact pressure grows without limit in time, whereas for V < V0 the system will always tend to one of the stable steady states. © 2003 Elsevier Ltd. All rights reserved.
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| 19. |
- Daghighi, Abtin, et al.
(författare)
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F.E.M. Stress-Investigation of Scolios Apex
- 2018
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Ingår i: Open Biomedical Engineering Journal. - : Bentham Open. - 1874-1207. ; 12, s. 51-71
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Tidskriftsartikel (refereegranskat)abstract
- In scoliosis, kypholordos and wedge properties of the vertebrae should be involved in determining how stress is distributed in the vertebral column. The impact is logically expected to be maximal at the apex.
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| 20. |
- Eriksson, Robert, et al.
(författare)
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OPTIMIZING THERMAL BARRIER COATING DESIGN USING STRUCTURAL OPTIMIZATION METHODS
- 2017
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Ingår i: PROCEEDINGS OF THE ASME TURBO EXPO: TURBINE TECHNICAL CONFERENCE AND EXPOSITION, 2017, VOL 6. - : AMER SOC MECHANICAL ENGINEERS. - 9780791850916
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Konferensbidrag (refereegranskat)abstract
- Thermal barrier coatings (TBCs) are used in gas turbines to reduce creep, thermo-mechanical fatigue, and oxidation, or to allow for reduced air cooling. TBCs may fail due to fatigue. Structural optimization methods were applied to optimize the. TBC thickness in such a way as to increase the life of the TBC. The TBC thickness was varied for three cases: 1) minimizing TBC volume, 2) minimizing TBC maximum effective stress, and 3) minimizing compliance (minimizing the strain energy). The results from the optimization were used to estimate the relative change in TBC life via a strain energy based failure criterion and a Coffin-Manson-like expression. Minimization of volume had limited use due to limitations in the current implementation. Minimization of effective stress did not give any significant increase in life. The minimization of compliance increased the estimated TBC life at highly stressed regions.
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| 21. |
- Fredricson, Harald, et al.
(författare)
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Topology optimization of frame structures with flexible joints
- 2003
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Ingår i: Structural and multidisciplinary optimization (Print). - : Springer Science and Business Media LLC. - 1615-147X .- 1615-1488. ; 25:3, s. 199-214
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Tidskriftsartikel (refereegranskat)abstract
- A method for structural topology optimization of frame structures with flexible joints is presented. A typical frame structure is a set of beams and joints assembled to carry an applied load. The problem considered in this paper is to find the stiffest frame for a given mass. By introducing design variables for beams and joints, a mass distribution for optimal structural stiffness can be found. Each beam can have several design variables connected to its cross section. One of these is an area-type design variable which is used to represent the global size of the beam. The other design variables are of length ratio type, controlling the cross section of the beam. Joints are flexible elements connecting the beams in the structure. Each joint has stiffness properties and a mass. A framework for modelling these stiffnesses is presented and design variables for joints are introduced. We prove a theorem which can be interpreted as the fact that the removal of structural elements, e.g. joints or beams, can be modelled by a small strictly positive material amount assigned to the element. This is needed for the computations of sensitivities used in the applied gradient based iterative method. Both two and three dimensional problems, as well as multiple load cases and multiple mass constraints, are treated.
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| 22. |
- Haveroth, G. A., et al.
(författare)
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Topology optimization including a model of the layer-by-layer additive manufacturing process
- 2022
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Ingår i: Computer Methods in Applied Mechanics and Engineering. - : ELSEVIER SCIENCE SA. - 0045-7825 .- 1879-2138. ; 398
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Tidskriftsartikel (refereegranskat)abstract
- A topology optimization formulation including a model of the layer-by-layer additive manufacturing (AM) process is considered. Defined as a multi-objective minimization problem, the formulation accounts for the performance and cost of both the final and partially manufactured designs and allows for considering AM-related issues such as overhang and residual stresses in the optimization. The formulation is exemplified by stiffness optimization in which the overhang is limited by adding mechanical or thermal compliance as a measure of the cost of partially manufactured designs. Convergence of the model as the approximate layer-by-layer model is refined is shown theoretically, and an extensive numerical study indicates that this convergence can be fast, thus making it a computationally viable approach useful for including AM-related issues into topology optimization. The examples also show that drips and sharp corners associated with some geometry-based formulations for overhang limitation can be avoided. The codes used in this article are written in Python using only open sources libraries and are available for reference.
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| 23. |
- Hilding, D., et al.
(författare)
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A computational methodology for shape optimization of structures in frictionless contact
- 2001
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Ingår i: Computer Methods in Applied Mechanics and Engineering. - 0045-7825 .- 1879-2138. ; 190:31, s. 4043-4060
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Tidskriftsartikel (refereegranskat)abstract
- This paper presents a computational methodology for shape optimization of structures in frictionless contact, which provides a basis for developing user-friendly and efficient shape optimization software. For evaluation it has been implemented as a subsystem of a general finite element software. The overall design and main principles of operation of this software are outlined. The parts connected to shape optimization are described in more detail. The key building blocks are: analytic sensitivity analysis, an adaptive finite element method, an accurate contact solver, and a sequential convex programing optimization algorithm. Results for three model application examples are presented, in which the contact pressure and the effective stress are optimized. cr 2001 Elsevier Science B.V. All rights reserved.
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| 24. |
- Hilding, Daniel, et al.
(författare)
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Optimization of structures in frictional contact
- 2012
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Ingår i: Computer Methods in Applied Mechanics and Engineering. - : Elsevier. - 0045-7825 .- 1879-2138. ; 205-208:S1, s. 83-90
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Tidskriftsartikel (refereegranskat)abstract
- This paper describes a new approach to optimization of linear elastic structures in frictional contact. It uses a novel method to determine an, in a specified sense, likely equilibrium state of the structure, using only the static equilibrium conditions. That is, no complex dynamic/quasi-static analyses have to be performed. The approach has the advantage that it is not necessary to know the complete load history, which is most often unknown for practical problems. To illustrate the theory, numerical results are given for the optimal design problem of sizing a truss to attain a more uniform normal contact force distribution.
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| 25. |
- Holmberg, Erik, et al.
(författare)
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Fatigue constrained topology optimization
- 2014
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Ingår i: Structural and multidisciplinary optimization (Print). - : Springer-Verlag New York. - 1615-147X .- 1615-1488. ; 50:2, s. 207-219
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Tidskriftsartikel (refereegranskat)abstract
- We present a contribution to a relatively unexplored application of topology optimization: structural topology optimization with fatigue constraints. A probability based high-cycle fatigue analysis is combined with principal stress calculations in order to find the topology with minimal mass that can withstand prescribed loading conditions for a specific life time. This allows us to generate optimal conceptual designs of structural components where fatigue life is the dimensioning factor.We describe the fatigue analysis and present ideas that makes it possible to separate the fatigue analysis from the topology optimization. The number of constraints is kept low as they are applied to stress clusters, which are created such that they give adequate representations of the local stresses. Optimized designs constrained by fatigue and static stresses are shown and a comparison is also made between stress constraints based on the von Mises criterion and the highest tensile principal stresses.
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