| 1. |
- Karimi Bakhshandi, Reza, et al.
(author)
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Failure analysis of two cylindrical impact pistons subjected to high velocity impacts in drilling applications
- 2022
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In: Engineering Failure Analysis. - : Elsevier. - 1350-6307 .- 1873-1961. ; 140
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Journal article (peer-reviewed)abstract
- Top hammer drilling is a common method to drill holes in rock formations in mining and civil engineering applications. Failure of drilling machine components has a significant impact on the cost and period of the operation. Internal components of percussive hammers experience extreme loading conditions during their service life. The focus of the present case study is to characterize failure mechanisms of two cylindrical impact pistons subjected to impact loading. The investi-gated components were manufactured from two different steel grades, a surface hardened low alloyed high strength steel and a through hardened cold work tool steel.Failure of both pistons started with degradation of the impact surfaces in term of cavitation erosion and localized surface fatigue phenomena. Subsequently, chipping and removal of material from impact surfaces resulted in formation of semi-spherical holes and craters on both surfaces.Radial and hoop cracks started to develop from cavities on the impact surface. The radial cracks then propagated parallel to the impacting surface in the longitudinal direction of the piston. Once the cracks formed at the impact surface, the damage was controlled by impact fa-tigue. Fatigue beach marks were identified on the fracture surface of failed component.
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| 2. |
- Oesterle, Bastian, 1986-, et al.
(author)
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Intrinsically Selective Mass Scaling with Hierarchic Structural Element Formulations
- 2021
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Conference paper (peer-reviewed)abstract
- Hierarchic shear deformable Reissner-Mindlin shell formulations possess the advantage of being intrinsically free from transverse shear locking [1], [2]. Transverse shear locking is avoided a priori through reparametrization of the kinematic variables. This reparametrization yields beam, plate and shell formulations with distinct transverse shear degrees of freedom. The efficiency of explicit dynamic analyses of thin-walled structures is limited by the critical time step size, which depends on the highest frequency of the discretized system. If Reissner-Mindlin type shell elements are used for discretization of a thin structure, the highest transverse shear frequencies limit the critical time step in explicit dynamic analyses, while being relatively unimportant for the structural response of the system. The basic idea of selective mass scaling is to scale down the highest frequencies in order to increase the critical time step size, while keeping the low frequency modes unaffected, see for instance [3]. In most concepts, this comes at the cost of non-diagonal mass matrices. In this contribution, we present recent investigations on selective mass scaling with hierarchic formulations. Since hierarchic formulations possess distinct transverse shear degrees of freedom, they offer the intrinsic ability for selective mass scaling of the shear frequency modes, while keeping the bending dominated modes mostly unaffected and retaining the diagonal structure of a lumped mass matrix. We discuss the effects of transverse shear parametrization, locking and mass lumping on the accuracy of results and a feasible time step. REFERENCES[1] R. Echter, B. Oesterle and M. Bischoff, A hierarchic family of isogeometric shell finite elements. Computer Methods in Applied Mechanics and Engineering, Vol. 254. pp. 170-180, 2013.[2] B. Oesterle, E. Ramm and M. Bischoff, A shear deformable, rotation-free isogeometric shell formulation. Computer Methods in Applied Mechanics and Engineering, Vol. 307, pp. 235-255, 2016.[3] G. Cocchetti, M. Pagani and U. Perego, Selective mass scaling and critical time-step estimate for explicit dynamics analyses with solid-shell elements. Computers and Structures, Vol. 27,pp. 39-52, 2013
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| 3. |
- Tkachuk, Anton, 1986-, et al.
(author)
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Design of truss structures with multiple eigenfrequency constraints via rank minimization
- 2024
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In: Computers & structures. - : Elsevier. - 0045-7949 .- 1879-2243. ; 299
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Journal article (peer-reviewed)abstract
- Rank deficiency of the dynamic stiffness matrix is an indicator for resonance of a structure at a given frequency. This indicator can be exploited as a heuristic optimization objective to achieve resonance at several frequencies. Log-det heuristic provides a tractable surrogate function for matrix rank in the case of affine dependency of stiffness and mass matrices on design parameters, which applies to truss structures. Reducing the rank of the dynamic stiffness matrix for higher frequencies implies that the matrix is not semi-positive definite. For this case, the log-det heuristic is valid with a combination of interior-point methods and Fazel’s semi-definite embedding via linear matrix inequalities. Further constraints on the fundamental frequency and compliance can be easily added within the framework as linear matrix inequalities. Several successful numerical examples illustrate the performance of the approach.
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| 4. |
- Tkachuk, Anton, 1986-, et al.
(author)
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Dispersion design of 1D periodic discrete systems using log-det heuristic
- 2021
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In: COMPDYN Proceedings. - 9786188507234 ; , s. 1922-1933
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Conference paper (peer-reviewed)abstract
- A new class of performance functions for dispersion design of 1D periodic discrete systems using matrix rank is considered, and its regularization using existent log-det heuristics is proposed. As an input, the desired dispersion dependency of a branch is used. The design space is defined by a finite set of continuous parameters such as mass or stiffness constants. The mass and stiffness matrices are assumed to depend linearly on these parameters. Ideally, the representative dynamic stiffness matrix is singular at every point of the desired branch. Instead, the sum of ranks for the representative dynamic stiffness matrix evaluated at several discrete points is minimized using a surrogate log-det objective. This approach avoids ordering or tracking of eigenfrequencies and reduces the design problem to a sequence of quadratic programming problems. The considered periodic discrete systems are simplified objects for method development with a future goal of dispersion design of acoustic metamaterials. Also, a combination with reduced-order techniques is conceptually tested.
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| 5. |
- Tkachuk, Anton, 1986-, et al.
(author)
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Heuristic dispersion design of discrete periodic systems
- 2021
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Conference paper (peer-reviewed)abstract
- We consider a new class of performance functions for dispersion design of 1D periodic discrete systems using matrix rang and its regularization using log-det heuristics [1]. As an input, the desired dispersion dependency of a branch is used. Ideally, the representative dynamic stiffness matrix (RDSM) is singular at every point of the desired branch. Instead, the sum of ranks for RDSM evaluated at several discrete points is minimized using a surrogate log-det objective [2]. An example of a periodic system with a side branch is given in the Figure. The system has four free stiffness and three free mass parameters with admissible ranges given in Figure. The desired dispersion relation should have a constant frequency branch at 1.58. Thank of RDSM is evaluated at 12 discrete points. The obtained design satisfies dispersion requirements. This approach avoids ordering or tracking of eigenfrequencies and reduces the problem to a sequence of quadratic programming problems. The considered periodic discrete systems are simplified objects for method development with a further dispersion design goal for acoustic metamaterials.[1] FAZEL, M.; HINDI, H.; BOYD, S.P. Log-det heuristic for matrix rank minimization with applications to Hankel and Euclidean distance matrices. In: Proc. of the ACC2003. IEEE, 2003. PP 2156-2162.[2] TKACHUK, A. Customization of reciprocal mass matrices via log‐det heuristic. IJNME, 2020, 121., PP.690-711.
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| 6. |
- Tkachuk, Anton, 1986-, et al.
(author)
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Weighted Eigenvalue Counts on Intervals for Spectrum Optimization
- 2023
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In: Advances in Mechanical and Power Engineering. - Cham : Springer. - 9783031184864 ; , s. 228-237
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Conference paper (peer-reviewed)abstract
- Power equipment is prone to vibrations. Removing eigenfrequencies of a structure from the interval of working frequencies reduces the probability of resonance during operation. In this contribution, a structural optimization problem is formulated whose objective at a minimum removes all eigenfrequencies from a given frequency interval. We consider systems without damping whose mass and stiffness matrices depend continuously on real parameters. The approach relies on the identity proposed by Futamura for eigenvalue count on intervals. The identity uses a contour integral in a complex plane of a trace of a specially constructed matrix. The contour integral is evaluated numerically using the trapezoidal rule over a circular path. The latter expression is differentiable. Present contribution extends the identity by adding a concave weighting function strictly positive in the interval. Furthermore, an explicit expression for the gradient of the objective and a simple optimization strategy are presented. Finally, a multi-degree of freedom example illustrates the performance of the approach.
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| 7. |
- Tkachuk, M. M., et al.
(author)
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Large deformation of cable networks with fiber sliding as a second-order cone programming
- 2024
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In: International Journal of Solids and Structures. - : Elsevier. - 0020-7683 .- 1879-2146. ; 298
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Journal article (peer-reviewed)abstract
- A new model for irreversible large deformations of fiber networks is developed. The fibers are considered as inextensible cables that slide relative to each other in the frictional junctions. This sliding is constituted by a rate-independent flow rule. The nonsmooth dissipation potential for each sliding system is defined as a product of the yield strength and the absolute value of the fiber sliding. The response of the cable segments is nonsmooth as well, since it shows asymmetry with respect to tension and compression. A principle of minimum incremental potential and a pure complementary energy principle are derived for the equilibrium incremental loading of the network at large deformations. They form a pair of primal and dual second-order cone programming problems with matching sets of displacement-based and force-based variables. These problems can be effectively solved by interior point methods that have many advantages compared to the gradient-based methods or dynamic relaxation. The model is extended by a simple mechanism of fiber pull-out resulting from fiber sliding at the free unconstrained ends. This can be used for the microstructural analysis of failure of needle-punched nonwoven materials.
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| 8. |
- Tkachuk, Mykola, 1983-, et al.
(author)
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Numerical and Analytical Analysis Methods for Radial Response of Flexible Ring Dampers
- 2021
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In: Machine and Industrial Design in Mechanical Engineering. - Cham : Springer. ; , s. 499-506
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Conference paper (peer-reviewed)abstract
- Radial stiffness of flexible ring dampers used in rotor supports is analyzed in this paper. Two major approaches are proposed for this purpose. The first is the conventional finite element modeling of this contact mechanics problem. The second is an analytical method that can be used as alternative to the costly numerical computations. This method is based on the Kalker’s principle of minimum complementary energy. A special variational formulation is developed in the closed form using Euler–Bernoulli beam approximation for the elastic ring and a simplified model of normal contact at the ring flanges. It has been shown that the surface tolerances of the parts have a substantial effect on the radial response of the flexible ring that may become nonlinear. The tight fit of the ring on both sides makes it much stiffer, while the loose fit results in free motion of the rotor and much weaker damping of its motion. Both methods produced results that are in excellent agreement for the considered cases.
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| 9. |
- Gade, Jan, et al.
(author)
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Redundancy distribution in elastostatic beam and surface structures
- 2021
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Conference paper (peer-reviewed)abstract
- The degree of statical indeterminacy is a fundamental property in structural mechanics of discrete truss and beam structures, exploitable in analysis and design. While further specifications, like.e.g. subdivision into an internal and external part or determination w.r.t. special load directions, are well-established, the property is today mainly understood as an integral property of an entire structure (or entire substructures), without quantified information about its distribution in space and w.r.t. load-carrying types. The redundancy matrix, introduced in [1, 2] and extended in [3], provides information about the distribution of statical indeterminacy in discrete truss and beam structures. This gives an additional valuable insight into the load-carrying behavior. In [3] also the redundancy distribution for one-dimensional continua is introduced and computed analogously to the redundancy matrix in discrete truss structures. A generalization of the redundancy concept to spatially continuous, linear, elastostatic representations of structures is given in [4]. The quantity cation of redundancy distribution considering geometrically non-linear behavior is approached in [2, 5]. These works are limited to discrete representations of truss structures with prestressing. We present an extension of the concept of redundancy to beam and surface structures using a finite element framework. We also discuss ideas on how to consider geometrically non-linear behaviour. There are numerous applications like e.g. robust design of structures, quantification of imperfection sensitivity, evaluation of adaptability, assessment of actuator placement as well as optimal control in adaptive structures.REFERENCES[1] Bahndorf, J.: Zur Systematisierung der Seilnetzberechnung und zur Optimierung vonSeilnetzen. Doctoral Thesis, Universitat Stuttgart, Stuttgart, 1991.[2] Strobel, D.: Die Anwendung der Ausgleichungsrechnung auf elastomechanische Systeme.Doctoral Thesis, Universitat Stuttgart, Stuttgart, 1995.[3] von Scheven, M., Ramm, E. and Bischoff, M.: Quanti cation of the Redundancy Distributionin Truss and Beam Structures. Int. J. Sol. Str., 2020.[4] Gade, J., Tkachuk, A., von Scheven, M. and Bischoff, M.: A continuum-mechanical theory of redundancy in elastostatic structures. Int. J. Sol. Str., under review, 2020.[5] Zhou, J., Chen, W., Zhao, B. and Gao, C.: A uni ed formulation for redundancy of cable-strut structures considering the effect of pre-stresses. Proc. IASS Ann. Symp. 2016, Tokyo, Japan.
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| 10. |
- Hoffmann, Moritz, et al.
(author)
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FINITE ELEMENT TECHNOLOGY-BASED SELECTIVE MASS SCALING FOR EXPLICIT DYNAMIC ANALYSES OF THIN-WALLED STRUCTURES USING SOLID ELEMENTS
- 2023
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Conference paper (peer-reviewed)abstract
- We present a novel class of selective mass scaling (SMS) concepts in the context of explicit dynamic analyses of thin-walled structures using solid elements. The novel SMS schemes are based on the discrete strain gap (DSG) method [1], a method from the field of finite element technology (FET). Thus, they are denoted as DSGSMS schemes and they extend the initial work of [2] for shear deformable structural element formulations to the application in thin solid elements. We show that these novel DSGSMS concepts naturally preserve both translational and rotational inertia and possess high accuracy. Additionally, having non-linear problems including large rotations in mind, we show how efficient isotropic DSGSMS concepts can be developed such that the need for reassembly of scaled mass matrices is avoided.
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