Institute of Fundamental Technological Research

This study investigates thermal strains in fibre reinforced polymeric samples manufactured using a modified Fused Deposition Modelling (FDM) method. The investigated material was a composition of polylactic acid (PLA) resin and continuous carbon fibres. Each test sample was equipped with two Bragg grating (FBG) sensors, one embedded inside and the other bonded to the surface. Both sensors monitored temperature-induced deformations
during the conditioning of the specimens in a thermal chamber. Multiscale, analytical and finite element method based models were implemented to quantify the temperature deformations. Research has revealed that in investigated samples, bending occurs due to thermal loading. This can result in an inaccurate estimation of the coefficient of thermal expansion when relying on surface deformation measurements. A proposed solution involves the use of one FBG sensor embedded inside the specimen or two FBG sensors placed symmetrically, capable of measuring axial thermal deformation and averaging the effects associated
with bending.

Continuous Carbon Fibre Reinforced Polymer Composites, Fibre Bragg gratings, Thermal expansion , Additive manufacturing, Multiscale modelling

Composite materials are widely used in many engineering applications and fields of technology. One of the main defects, which occur in fiber-reinforced composite materials, is delamination. It manifests itself in the separation of layers of material and the damaged structure once subjected to mechanical loads degrades further. Delamination results in lower stiffness and the decrease of structure’s carry load capability. Its early detection is one of the tasks of non-invasive structural health monitoring of layered composite materials. This publication discusses a new method for delamination detection in fiber-reinforced composite materials. The approach is based on analysis of energy signal, calculated with Teager–Kaiser energy operator, and comparison of change of the weighted instantaneous frequency for measurement points located in- and outside of delamination area. First, applicability of the developed method was tested using simple models of vibration signals, reflecting considered phenomena. Next, the authors’ weighted instantaneous frequency was applied for detection of deamination using signals obtained from FEM simulated response of the cantilever beam. Finally, the methods effectiveness were tested involving real experimental signals collected by the laser Doppler vibrometer (LVD) sensor measuring vibrations of the delaminated glass-epoxy specimens.

delamination, Teager–Kaiser energy operator, instantaneous frequency, fiber-reinforced composite material

This study presents and tests a method for semi-active control of vibrations in sandwich-type beam structures. This method adapts a strategy called prestress accumulation release. The prestress accumulation release strategy is based on structural reconfiguration: it uses short time, impulsive and localised changes of actuator properties (such as stiffness or damping), which are applied to a part of the system in the moments, when its strain energy attains a local maximum. The method has been earlier applied as a global control scheme to mitigate the fundamental vibration mode of a cantilever beam (by stiffness control) and in the task of mitigating the first four modes of a frame structure (by damping control). This study proposes a prestress accumulation release strategy and tests its effectiveness for the case of a three-layered sandwich structure, with the internal layer fabricated from a material with dissipative characteristic locally controllable through the material damping coefficient. In contrast to the earlier research, the control is applied thus at the level of material characteristics instead of a discrete set of dedicated actuators. Based on the finite element method, a numerical experiment involving a passively damped, as well as prestress accumulation release-controlled, three-layered cantilever beam excited by initial displacements was performed. The effectiveness of the approach was studied for a broad range of internal layer damping parameters. The presented results revealed a high potential of the prestress accumulation release strategy in semi-active damping of vibrations of sandwich-type structures.

vibration control, sandwich structure, semi-active control, decentralised control, smart structures, constrained layer method

This paper proposes an on/off semi-active control approach for mitigation of free structural vibrations, designed for application in 2D smart frame structures. The approach is rooted in the Prestress-Accumulation Release (PAR) control strategies. The feedback signal is the global strain energy of the structure, or its approximation in the experimental setup. The actuators take the form of on/off nodes with a controllable ability to transfer moments (blockable hinges). Effectiveness of the approach is confirmed in a numerical simulation, as well as using a laboratory experimental test stand.

structural reconfiguration, structural control, semi-active control, frame structures, controllable nodes‎

This paper studies the effect of prestress force magnitude on natural frequencies and dynamic behaviour of eccentrically prestressed glass fibre reinforced polymer composite beams, including the theoretical background, numerical results and experimental verification. The term prestress indicates the initial tensile stress applied to the fibres embedded in selected external layers of the composite material. First, the paper presents the theoretical background of the finite element method modelling of prestressed composites. Then, the results of numerical simulations conducted for a five-layered glass-epoxy composite beam are presented. The natural frequencies corresponding to three initial bending modes are analyzed for different prestressing force levels and for different fibre volume content. Finally, the results are verificated by experimental modal analysis conducted on three different glass-epoxy composite specimens of various mechanical parameters. Both the numerical results obtained from finite element method and the experimental results obtained from experimental modal analysis reveal that the first bending frequency increases and the two subsequent bending frequencies decrease due to the prestressing force. The comparison of numerical and experimental data confirms the effect and allows to quantify the influence that the prestress force has on the natural frequencies of composites, which is an interesting and practically relevant phenomenon.

Prestressed structures, laminated composites, prestressed reinforced composites, glass fibre reinforced polymer composite materials, vibrations, finite element method

This paper presents a preliminary research aimed at developing a comprehensive approach to modeling, manufacturing and optimal design of prestressed FRP composite structures. A simple and effective analytical model of prestressed composite is derived and further verified by two numerical models and the results of the experimental tests conducted on manufactured prestressed composite samples. The model reveals beneficial influence of prestress on strain and stress distribution in particular plies and resulting improvement of the global response of the composite. A design case-study of prestressed composite is presented and challenges related to design and application of more complicated composite prestressed structures are discussed.

Prestressed structures, Laminated composites, Prestressed FRP reinforced composites

The proposed approach assumes that the composite structure is equipped with a specially designed electrical circuit with a 3D grid layout, composed of high resistivity elements embedded in the structure. The special layout of the electrical circuit activated by small currents provides scattered sources of thermal field in the laminate. It is assumed that the circuit elements exhibit failure which is coincident with the commencement of delamination. These breaks in the electrical circuit cause variations in the thermal field which can be observed by a long-wave thermovision camera. The paper is focused on two research aspects. First, some numerical simulations are presented to examine the potential of the idea itself. Next, a simple experiment using a composite sample with a hand-fabricated electrical grid is described. The performance of the grid for an arbitrarily selected position of defect in the investigated composite shows high potential for damage detection.

New method for semi-active control of vibrating structures is introduced. So-called Prestress Accumulation-Release (PAR) strategy aims at releasing of the strain energy accumulated in the structure during its deformation process. The strain energy is converted into kinetic energy of higher modes of vibration which is suppressed with structural damping or by means of a damping device. The adaptation process essentially affects the first mode vibrations by introducing an elastic force that opposes the movement. Numerical simulations as well as experimental results prove that the strategy can be very effective in mitigating of the fundamental mode of a free – vibrating structure. In a numerical example 95% of the vibration amplitude was mitigated after two cycles. An experimental demonstrator shows 85% reduction of the amplitude in a cantilever free- vibrations. In much more complex practical problems smaller portion of total energy can be released from the system in each cycle, nevertheless the strategy could be applied to mitigate the vibrations of, for example, pipeline systems or pedestrian walkways.

The problem of modelling and identification of delamination in double-layer beams has been undertaken within the framework of the virtual distortion method. For delamination modelling, a concept of the contact layer has been proposed, assuming simple but effective truss connections. The laminate layers have been modelled with Bernoulli beams. Good correspondence of the delamination model with experiments has been observed despite disregarding the friction between layers. An algorithm for off-line identification of delamination, solving an inverse problem with the use of gradient optimization, has been proposed. For double cantilever beam examples, two co-existing delamination zones have been successfully detected. An idea of on-line identification of delamination has been put forward, too.

Delamination, Double-layer beams, Identification, Inverse problem

The paper analyzes the influence of prestressing force on the natural frequencies on Glass Fibre Reinforced Polymer (GFRP) composite beams. Prestress is introduced by applying initial tensile forces to the fibres embedded in selected layer of the composite material during the manufacturing process. Release of prestressing forces results in deformation and self-equilibrated state of stress of the entire composite which changes both its static properties and dynamic characteristics. The paper is focused on analysis of shifts in natural frequencies corresponding to initial bending modes of the composite beams of various fibre volume fraction and prestressed layer location. The problem is analyzed with the use of finite element simulations and experimental modal analysis. The conducted numerical and experimental work reveals that shifts in the natural frequencies caused by non-axial prestressing can be significant and they are important phenomenon which has to be taken into account during design of the composite material.

The paper introduces the concept of prestressing selected layers of laminated composite in order to increase its stiffness and improve its overall mechanical response. Both analytical and numerical models of prestressed composites are proposed and utilized to optimize applied prestressing forces. Further, dedicated laboratory stand and developed methodology of experimental evaluation are described. The final part of the paper briefly discusses prospective applications of prestressed composites.

prestressed structures, laminated composites, prestressed reinforced composites

Additive Manufacturing is one of the most rapidly developing production technologies in the last decade. In case of polymeric materials the Fused Deposition Modelling (FDM) provides low cost and high versatility, however is limited by low accuracy and mechanical properties of applied polymeric materials. One of the ways of overcoming the latter deficiency is the modification of the FDM process allowing for the reinforcement of the printed composites with continuous fibres. The objective of the research is to present the performance of additively manufactured Carbon Fibre Reinforced Polymer (CFRP) sample with embedded Fiber Bragg Grating sensors (FBG) under the influence of mechanical loading and temperature changes. FBG sensors were introduced to the middle of the specimen during manufacturing process. Experimental results show high nonlinearity due to a relatively low glass transition temperature of the PLA material. Additionally, the numerical modelling using the Finite Element Method provides more complex insight into the influence of embedded fibre optic on the AM composite material.

This contribution reviews a recently proposed control strategy for mitigation of vibrations based on the Prestress-Accumulation Release (PAR) approach [1]. The control is executed by means of semi-actively controllable truss-frame nodes. Such nodes have an on/off ability to transfer bending moments: they are able to temporary switch their operational characteristics between the truss-like and the frame-like behaviors. The focus is not on local energy dissipation in the nodes treated as friction dampers, but rather on stimulating the global transfer of vibration energy to high-order modes. Such modes are high-frequency and thus highly dissipative by means of the standard mechanisms of material damping. The transfer is triggered by temporary switches to the truss-like state performed at the moments of a high local bending strain. A sudden removal of a kinematic constraint releases the locally accumulated strain energy into high-frequency and quickly damped vibrations.
The first formulation investigated global control laws [1]. Recent approaches generalized it to decen-tralized control with a local-only feedback, which was tested in damping of free vibrations [2] as well as forced vibrations [3]. Recently, a global formulation was proposed that aims at a targeted energy transfer between specific vibration modes [4], and attempts were made to go beyond skeletal struc-tures [5]. Numerical and experimental results will be presented to confirm the high effectiveness of the approach in mitigation of free, forced random and forced harmonic vibrations.

The concept of increasing strength capacity of structural elements by introducing preliminary stresses, counteracting the exploitation stresses, is known for years. Large number of applications of pre-stressed materials in civil engineering proves that proper compression of material can effectively increase the strength of structural elements. Because of the rapid development of composite materials, and growing demand for light materials with particularly high stiffness and strength properties, the pre-stressed FRP composites application in industry seems to be a question of time. This assumption is confirmed by increasing number of publications concerning the problem of mechanical characteristics for such materials. This paper presents the results of preliminary research on the pre-stressing of the FRP composite structures, while the term pre-stress indicates initial tensile stress applied to the fibres embedded in selected layers of the composite material. Manufacturing process and shape forming possibilities as well as short-term static and dynamic behaviour of the pre-stressed composites are discussed. Presented results are achieved by the use of the experimental methods (three-point bending tests and Experimental Modal Analysis) and experimentally verified Finite Element Method model of pre-stressed structure.

The paper introduces the concept of eccentrical prestressing of fiber reinforced polymer materials in order to improve their mechanical properties and global mechanical behaviour. Prestressing is here understood as application of the initial tensile stress to the fibres embedded in selected external layers of the composite material. The objective of prestressing is to increase stiffness of the composite and to obtain desired response to applied external loading. The main objective of this research work is to develop a comprehensive approach to analysis of prestressed composites, which includes analytical and numerical modelling of the static behaviour, optimal composite design, as well as, simulation of dynamic response. Initially, we derive simple and effective analytical model of prestressed composite based on models of individual prestressed plies and their homogenization. The analytical model is used to reveal beneficial influence of prestress on strain and stress distribution in particular plies and resulting increase of the composite stiffness. Further, three options for the FEM-based numerical modelling of prestressed composites are proposed and thoroughly compared with each other. Developed analytical and numerical models are used to propose methods of prestressed composite design in which optimization of prestressing forces is used to minimize required composite thickness or fiber volume fraction. Eventually, FEM simulations are applied to assess the influence of prestress force magnitude on natural frequencies and modal shapes of eccentrically prestressed composite beams of various fibre volume fraction. The final part of the paper summarizes the potential advantages of the prestressed composites and unveils their superiority in comparison to the standard ones. Potential applications of prestressed composite materials in civil engineering and aerospace industry are briefly discussed. In addition, challenges related to design and manufacturing of structures made of prestressed composite materials are presented.