Paweł Hołobut, Ph.D., Eng.

Department of Mechanics of Materials (ZMM)
Division of Micromechanics of Materials (PMM)
position: senior specialist
telephone: (+48) 22 826 12 81 ext.: 195
room: 109
e-mail: pholob

Recent publications
1.Majewski M., Kursa M., Hołobut P., Kowalczyk-Gajewska K., Micromechanical and numerical analysis of packing and size effects in elastic particulate composites, COMPOSITES PART B-ENGINEERING, ISSN: 1359-8368, DOI: 10.1016/j.compositesb.2017.05.004, Vol.124, pp.158-174, 2017
Abstract:

Effects of particle packing and size on the overall elastic properties of particulate random composites are analyzed. In order to account for the two effects the mean-field Morphologically Representative Pattern (MRP) approach is employed and an additional interphase surrounding inclusions (coating) is introduced. The analytical mean-field estimates are compared with the results of computational homogenization performed using the finite element (FE) method. Periodic unit cells with cubic crystal-type arrangements and representative volume elements with random distributions of particles are used for verification purposes. The validity of the MRP estimates with respect to the FE results is assessed.

Keywords:

Composite materials, Elasticity, Micro-mechanics, Packing and size effects

Affiliations:
Majewski M.-IPPT PAN
Kursa M.-IPPT PAN
Hołobut P.-IPPT PAN
Kowalczyk-Gajewska K.-IPPT PAN
2.An Y., Błachowski B., Zhong Y., Hołobut P., Ou J., Rank-revealing QR decomposition applied to damage localization in truss structures, STRUCTURAL CONTROL AND HEALTH MONITORING, ISSN: 1545-2255, DOI: 10.1002/stc.1849, Vol.24, No.2, pp.e1849-1-15, 2017
Abstract:

The purpose of this work is the development of an efficient and high-sensitive damage localization technique for truss structures, based on the rank-revealing QR decomposition (RRQR) of the difference-of-flexibility matrix. The method is an enhancement of the existing techniques of damage detection, which rely on the set of so-called damage locating vector (DLV). The advantages of the RRQR decomposition-based DLV (RRQR-DLV) method are its less computational effort and high sensitivity to damage. Compared with the frequently used stochastic DLV (SDLV) method, RRQR-DLV offers higher sensitivity to damage, which has been validated based on the presented numerical simulation. The effectiveness of the proposed RRQR-DLV method is also illustrated with the experimental validation based on a laboratory-scale Bailey truss bridge model. The proposed method works under ambient excitation such as traffic excitation and wind excitation; therefore, it is promising for real-time damage monitoring of truss structures.

Keywords:

damage localization, rank-revealing QR decomposition, damage sensitivity, truss structure, structural health monitoring

Affiliations:
An Y.-Dalian University of Technology (CN)
Błachowski B.-IPPT PAN
Zhong Y.-Dalian University of Technology (CN)
Hołobut P.-IPPT PAN
Ou J.-Dalian University of Technology (CN)
3.Lengiewicz J., Kursa M., Hołobut P., Modular-robotic structures for scalable collective actuation, ROBOTICA, ISSN: 0263-5747, DOI: 10.1017/S026357471500082X, Vol.35, No.4, pp.787-808, 2017
Abstract:

We propose a new class of modular-robotic structures, intended to produce forces which scale with the number of modules. We adopt the concept of a spherical catom and extend it by a new connection type which is relatively strong but static. We examine analytically and numerically the mechanical properties of two collective-actuator designs. The simulations are based on the discrete element method (DEM), with friction and elastic deformations taken into account. One of the actuators is shown to generate forces proportional to its volume. This property seems necessary for building modular structures of useful strength and dimensions.

Keywords:

Modular robots, Self-reconfiguration, Programmable matter, Actuators, Mechanical strength

Affiliations:
Lengiewicz J.-IPPT PAN
Kursa M.-IPPT PAN
Hołobut P.-IPPT PAN
4.Sobczyk K., Hołobut P., Information-theoretic approach to dynamics of stochastic systems, PROBABILISTIC ENGINEERING MECHANICS, ISSN: 0266-8920, DOI: 10.1016/j.probengmech.2011.05.007, Vol.27, No.1, pp.47-56, 2012
Abstract:

In the paper we show how some basic informational quality measures (e.g. the Shannon entropy and the relative entropy/Kullback-Leibler divergence) defined for stochastic dynamical systems change in time and how they depend on the system properties and intensity of random disturbances. First, the Liouvillian systems (when randomness is present in the initial states only) are discussed and then various (linear and nonlinear) systems with random external excitations are treated in detail. Both, general and specific systems are considered, including numerical and graphical illustrations.

Keywords:

stochastic dynamics, information, entropy, relative entropy, information gain, non-Gaussian prediction, maximum entropy approximation

Affiliations:
Sobczyk K.-IPPT PAN
Hołobut P.-IPPT PAN
5.Hołobut P., Random hydrogen-assisted fatigue crack growth in steel plates, PROBABILISTIC ENGINEERING MECHANICS, ISSN: 0266-8920, Vol.26, No.1, pp.61-66, 2011
Abstract:

A stochastic analysis of hydrogen-assisted fatigue crack growth in steel plates is presented. First, a simplified deterministic model of the process is proposed. It captures the basic empirical property that the influence of hydrogen diminishes, as the crack growth rate increases. However, it only applies to cases, when diffusion is rate-limiting. Next, the model parameters are randomized to reflect the uncertainty inherent in the physical situation. On the basis of the obtained stochastic equation, probabilistic moments of the time, in which the crack reaches a critical length, are computed. Theoretical results are illustrated by a numerical example.

Keywords:

random fatigue, crack growth model, hydrogen embrittlement, hydrogen diffusion, probabilistic moment, time to failure

Affiliations:
Hołobut P.-IPPT PAN
6.Hołobut P., Fatigue crack growth model for a thin steel plate containing hydrogen, INTERNATIONAL JOURNAL OF FATIGUE, ISSN: 0142-1123, Vol.32, No.12, pp.1895-1903, 2010
Abstract:

A method of predicting fatigue crack growth in hydrogen-charged thin steel plates is proposed. It consists in computing modified stress intensity factors which reflect the presence of hydrogen, and using them in a fatigue crack growth equation for an uncharged plate. The modification accounts for both, hydrogen embrittlement of crack tips and hydrogen-induced residual stresses in the plate. The new stress intensity factors are derived for two special cases - uniform and stationary hydrogen distribution in the plate. With their use bounds are established on the rate of hydrogen-assisted fatigue crack growth. Theoretical results are illustrated by numerical calculations.

Keywords:

hydrogen fatigue, crack growth model, hydrogen embrittlement, residual stresses

Affiliations:
Hołobut P.-IPPT PAN
7.Hołobut P., Sobczyk K., Random hydrogen - induced stresses and effects on cracking, ARCHIVES OF MECHANICS, ISSN: 0373-2029, Vol.59, No.6, pp.1-21, 2007
Abstract:

The paper presents a method for quantitative characterization of random hydrogen-induced stresses. The method is based on randomized diffusion-elasticity equations. Also a stochastic parametric model, suitable for representing relevant empirical data, is outlined. The general considerations are illustrated by two particular examples. The first one concerns the effect of random hydrogen concentration on material failure time in a half-space, whereas the second one shows its effect on the Mode-I stress intensity factor for a crack in a circular cylinder.

Affiliations:
Hołobut P.-IPPT PAN
Sobczyk K.-IPPT PAN
8.Hołobut P., Time-optimal control of hydraulic manipulators with path constrains, JOURNAL OF THEORETICAL AND APPLIED MECHANICS, ISSN: 1429-2955, Vol.43, No.3, pp.523-538, 2005
Abstract:

A method of optimization intended to speed up motions of non-redundant hydraulic manipulators along prescribed paths of their end-effectors is presented. A parametric path of the end-effector of a non-redundant manipulator determines the corresponding path in the manipulator joint-space. The optimization problem therefore reduces to finding the optimum distribution of the parameter of the path in time. The proposed optimization scheme is based on discretization of the distribution of the parameter into a fixed number of points, and finding their optimum locations by methods of constrained nonlinear programming. Incompressibility of the hydraulic fluid is assumed throughout for greater effectiveness of the procedure. Results of sample optimizations performed on a three-link hydraulic excavator are presented.

Keywords:

hydraulic manipulators, time-optimal control, trajectory optimization

Affiliations:
Hołobut P.-IPPT PAN

Conference papers
1.Ortiz A.R., Błachowski B., Hołobut P., Franco J.M., Marulanda J., Thomson P., Modeling and Measurement of a Pedestrian’s Center-of-Mass Trajectory, 35th IMAC, XXXV International Modal Analysis Conference, A Conference and Exposition on Structural Dynamics 2017, 2017-01-30/02-02, Garden Grove, CA. (US), DOI: 10.1007/978-3-319-54777-0_20, pp.159-167, 2017
Abstract:

This paper presents the measurement and model updating of a pedestrian’s center of mass trajectory. A mathematical model proposed by the authors is updated using the actual trajectory of a pedestrian. The mathematical model is based on the principle that a human’s control capability tries to maintain balance with respect to the pedestrian’s center of mass (CoM), independently of the surface type. In this research, the human is considered as a mass point concentrated at CoM. The parameters of the models are updated using experimental identification of the human walking trajectory on a rigid surface. The proposed measurement technique uses a depth sensor, which enable skeletal tracking of the pedestrian walking on rigid or flexible structures. Experiments were performed using a mobile platform with the time-of-flight commercial camera Microsoft Kinect for Windows 2.0. The velocity of the mobile platform is set to maintain a 1 m separation from the pedestrian in order to provide high resolution. The results of the measurement technique allowed the identification of the human’s CoM trajectory. The results of the model updating process present the probability density function of the parameters which could be used for modeling the CoM’s trajectory of the pedestrian.

Keywords:

Human-structure interaction, Pedestrian’s trajectory, Human-induced vibrations, MS Kinect sensor

Affiliations:
Ortiz A.R.-Universidad del Norte (CO)
Błachowski B.-IPPT PAN
Hołobut P.-IPPT PAN
Franco J.M.-Universidad del Valle (CO)
Marulanda J.-Universidad del Valle (CO)
Thomson P.-Universidad del Valle (CO)
2.Hołobut P., Lengiewicz J., Distributed computation of forces in modular-robotic ensembles as part of reconfiguration planning, ICRA 2017, IEEE International Conference on Robotics and Automation, 2017-05-29/06-03, Marina Bay Sands (SG), DOI: 10.1109/ICRA.2017.7989242, No.7989242, pp.2103-2109, 2017
Abstract:

We discuss selected mechanical aspects of self-reconfiguration of densely-packed modular robots. The change of connection topology and transport of modules are fundamental mechanisms for these systems, which determine their desired emergent behavior, e.g., movement, shape change or interaction with their surroundings. At the same time, reconfiguration affects the forces between modules. We present a distributed procedure by which a robot can predict if the next planned reconfiguration step will overstress intermodular connections. We use a Finite Element model of a modular robot, with one-node-per-module discretization and beam elements representing intermodular connections. The analysis is restricted to static loads and linear elasticity. We present a distributed procedure of aggregation of the stiffness matrix and iterative solution of the resulting equations of elasticity. The procedure is illustrated with numerical examples and analyzed in terms of its efficiency. © 2017 IEEE.

Keywords:

Finite element method, Modular robots, Stiffness matrix, Distributed computations, Iterative solutions, Reconfiguration planning

Affiliations:
Hołobut P.-IPPT PAN
Lengiewicz J.-IPPT PAN
3.Hołobut P., Chodkiewicz P., Macios A., Lengiewicz J., Internal localization algorithm based on relative positions for cubic-lattice modular-robotic ensembles, IROS, IROS 2016 - IEEE/RSJ International Conference on Intelligent Robots and Systems, 2016-10-09/10-14, Daejeon, South Korea (KP), DOI: 10.1109/IROS.2016.7759473, pp.3056-3062, 2016
Abstract:

Module localization is an important aspect of the operation of self-reconfigurable robots. The knowledge of spatial positions of modules, or at least of the overall shape which the modules form, is the usual prerequisite for reconfiguration planning. We present a general, decentralized algorithm for determining the positions of modules placed on a cubic grid from local sensor information. The connection topology of the robot is arbitrary. We assume that a module can sense the presence of its immediate neighbors on the grid and determine their positions in its own local coordinate system, but cannot sense the orientations of the coordinate systems of its neighbors. Since orientation cannot be directly communicated between modules, the modules can only exchange information about the relative positions of their neighbors. The algorithm aggregates this information over the entire network of modules and narrows down the set of valid positions for each module as far as possible. If there exists a unique locally-consistent assignment of coordinates to all modules then it is found.

Affiliations:
Hołobut P.-IPPT PAN
Chodkiewicz P.-Warsaw University of Technology (PL)
Macios A.-Warsaw University of Technology (PL)
Lengiewicz J.-IPPT PAN
4.Błachowski B., Hołobut P., Ortiz A., Caicedo J.M., Simple human-structure interaction model of walking on a flexible surface, ISMA 2016 / USD 2016, International Conference on Noise and Vibration Engineering / International Conference on Uncertainty in Structural Dynamics, 2016-09-19/09-21, Leuven (BE), pp.559-570, 2016
Abstract:

We present a new human-structure interaction (HSI) model of walking on a flexible surface. A human is considered as a mass point, located at the body’s center of mass (COM). The mass moves along a predefined trajectory, which deforms together with the surface on which the human walks. The forces of motion, equal to the sum of inertial and gravitational forces acting on the mass, are transfered to the surface at prescribed foot positions. The motion of the surface is described using a few selected mode shapes, corresponding damping ratios, and natural frequencies. The equations of motion of the system are time-dependent and discontinuous. They can be written in the form of the second order differential equations of structural dynamics, but with the right-hand forcing dependent on the deformation of the surface. We present a numerical example of a human walking on a long beam structure. The motion of the beam is described by three mode shapes, representing its vertical, lateral, and torsional deflections.

Keywords:

Human induced vibrations, Dynamics of bridges, Human walking model

Affiliations:
Błachowski B.-IPPT PAN
Hołobut P.-IPPT PAN
Ortiz A.-Universidad del Norte (CO)
Caicedo J.M.-University of South Carolina (US)
5.Hołobut P., Kursa M., Lengiewicz J., Efficient modular-robotic structures to increase the force-to-weight ratio of scalable collective actuators, IROS 2015, IEEE/RSJ International Conference on Intelligent Robots and Systems, 2015-09-28/10-02, Hamburg (DE), DOI: 10.1109/IROS.2015.7353836, pp.3302-3307, 2015
Abstract:

A collective actuator is a self-reconfigurable modular-robotic structure which produces useful mechanical work through simultaneous reconfiguration of its constituent units. An actuator is additionally called scalable if its force-to-weight ratio does not depend on the number of its member modules. In this work, we consider scalable collective actuators built from spherical catoms with two connection types: strong but fixed and weak but mobile. We investigate how to construct these actuators in such a way, as to maximize their force-to-weight ratio. We present a number of designs of high strength, whose force capacities significantly exceed those of similar actuators reported previously.

Affiliations:
Hołobut P.-IPPT PAN
Kursa M.-IPPT PAN
Lengiewicz J.-IPPT PAN
6.Hołobut P., Kursa M., Lengiewicz J., A class of microstructures for scalable collective actuation of Programmable Matter, IROS 2014, IEEE/RSJ International Conference on Intelligent Robots and Systems, 2014-09-14/09-18, Chicago (US), DOI: 10.1109/IROS.2014.6943113, pp.3919-3925, 2014
Abstract:

The term Programmable Matter (PM) describes the class of future meta-materials of programmable and controllable properties and behavior, e.g., able to autonomously transform into an arbitrary shape. The robotic approaches towards PM are based on the concept of cooperation of millions of micro-robots (modules), acting at a very fine length-scale and collectively imitating deformation of a macroscopically continuous material. Recent ideas about reconfiguration of a collective of modules to obtain a desired overall mechanical response are promising. However, they are limited by the strength of individual connections between modules. In the present work, we propose a way of arranging spherical modules into microstructures, in which some connections are fixed and mechanically stronger, and the rest are active (reconfigurable) but weaker. If the fixed connections are sufficiently strong, the proposed microstructures perform the function of collective actuation by exerting forces proportional to their volumes. Two variants of a linear-actuator microstructure are presented and studied in more detail. A rotary-actuator microstructure is also introduced.

Affiliations:
Hołobut P.-IPPT PAN
Kursa M.-IPPT PAN
Lengiewicz J.-IPPT PAN

Conference abstracts
1.Lengiewicz J., Kursa M., Hołobut P., Two-domain contact model of volumetric actuators, CMIS 2016, Contact Mechanics International Symposium, 2016-05-11/05-13, Warszawa (PL), No.P047, pp.90-91, 2016
2.Lengiewicz J., Kursa M., Hołobut P., Two-domain model of volumetric actuators, ICTAM XXIV, 24th International Congress of Theoretical and Applied Mechanics, 2016-08-21/08-26, Montréal (CA), pp.2632-2633, 2016
3.Majewski M., Hołobut P., Kursa M., Kowalczyk-Gajewska K., Micromechanical modelling of packing and size effects in particulate elastic-plastic composites, SolMech 2016, 40th Solid Mechanics Conference, 2016-08-29/09-02, Warszawa (PL), No.P099, pp.1-2, 2016
4.Majewski M., Hołobut P., Kursa M., Kowalczyk-Gajewska K., Description of packing and size effects in particulate composites by micromechanical averaging schemes and computational homogenization, PCM-CMM 2015, 3rd Polish Congress of Mechanics and 21st Computer Methods in Mechanics, 2015-09-08/09-11, Gdańsk (PL), pp.571-572, 2015
Abstract:

Different approaches to model packing and size effects are studied to model overall properties of particulate composites of different morphological features of phase distribution. The micromechanical schemes originating in the composite sphere model and its extension by morphologically-based pattern approach are taken as a basis. Analytical predictions are compared with results of computational homogenization performed on the generated representative volume elements of prescribed statistical characteristics.

Keywords:

micromechanics, morphologically representative pattern, computational homogenization, size and scale effect

Affiliations:
Majewski M.-IPPT PAN
Hołobut P.-IPPT PAN
Kursa M.-IPPT PAN
Kowalczyk-Gajewska K.-IPPT PAN
5.Lengiewicz J., Kursa M., Hołobut P., Actuation by reconfiguration—modular active structures to create Programmable Matter, PCM-CMM 2015, 3rd Polish Congress of Mechanics and 21st Computer Methods in Mechanics, 2015-09-08/09-11, Gdańsk (PL), pp.799-800, 2015
Abstract:

We examine, analytically and numerically, forces produced by collective actuators–possible future building blocks of Programmable Matter. The actuators are composed of tiny spherical robotic modules which can strongly attach to their neighbors, and move by rolling over one another using electric or magnetic local propulsion mechanisms. An actuator works through collective reconfiguration–a coordinated motion of its constituent modules–which results in a global deformation of the structure. The simulations are performed using specially adapted discrete element method software, and account for friction and elastic deformations of modules.

Keywords:

programmable matter, active materials, actuators, mechanical strength, modular robots

Affiliations:
Lengiewicz J.-IPPT PAN
Kursa M.-IPPT PAN
Hołobut P.-IPPT PAN
6.Hołobut P., Kursa M., Macios A., Lengiewicz J., Evolving microstructures for scalable actuation in programmable matter, SolMech 2014, 39th Solid Mechanics Conference, 2014-09-01/09-05, Zakopane (PL), pp.209-210, 2014