Since dielectric elastomer actuators are commonly used as artificial muscles, different approaches in material parameters identification have been used. Most dielectric actuators are parametrized ...with the help of continuum mechanics. Alternatively, rheological models can be used. Unfortunately, in basic rheological models, frequency dependence of viscoelastic materials cannot be obtained with a single equation. In order to obtain frequency dependences of viscoelastic material, fractional Kelvin-Voigt model is used. Fractional Kelvin-Voigt model can be applied for parameters identification in case of dynamical or cyclical excitation with different frequencies and for creep and stress relaxation analysis. Basic Kelvin-Voigt is limited on specific frequency and on creep. It cannot provide stress relaxation. Fractional Kelvin-Voigt model can further be used for fractional control. All previous work dealing with this subject is focused on analysis of material properties. Our contribution and novelty is in preparing governing equations for development control algorithms for dielectric elastomer actuators.
Hybrid materials containing a light metal and CFRP are capable to make a relevant contribution in lightweight design and thereby in reducing greenhouse gases causing global warming. An aluminium ...CFRP-hybrid specimen with a thermoplastic interlayer that is suitable for application for the A-, B-, or C-pillar in a car is investigated in this work regarding the mechanical behaviour due to temperature variation. For this purpose, quasi-static as well as dynamic tensile tests are carried out not only for those hybrid specimens but also for their respective single-material components. Those are supported by various non-destructive testing (NDT) techniques such as thermography and CT-scans of X-ray tomography. The examination of the single materials as well as the hybrid specimens gives us the possibility to understand if a change in the damage process of the hybrid is caused by one of the single materials or the interaction of them. The use of the NDT techniques in combination with the mechanical experiments allows us to obtain a deeper look at the mechanisms causing the respective damage. It stands out that temperature changes affect the damage mechanisms in the hybrid significantly without having great influence on the single materials. In quasistatic testing, the maximum displacement of the hybrid specimens rises at elevated temperature, and in dynamic testing the initial stiffness and the sustained cycles decline significantly. It therefore can be concluded that the interfaces inside the hybrids are affected by temperature changes and play a major role concerning the damage mechanisms. The pure knowledge about the temperature behaviour of single materials is not sufficient for anticipating the behaviour of hybrid specimens under these restrictions.
Oligo(ethylene glycol)-based (OEG) hydrogel samples of varying cross-link densities and degrees of swelling were characterized through dynamic nanoindentation testing. Experiments were performed ...using a non-standard nanoindentation method, which was validated on a standard polystyrene sample. This method maximizes the capability of the instrument to measure the stiffness and damping of highly compliant, viscoelastic materials. Experiments were performed over the frequency range of 1 to 50Hz, using a 1mm diameter flat punch indenter. A hydration method was adopted to avoid sample dehydration during testing. Values of storage modulus (E′) ranged from 3.5 to 8.9MPa for the different OEG-hydrogel samples investigated. Samples with higher OEG concentrations showed greater scatter in the modulus measurements and it is attributed to inhomogeneities in these materials. The (E′) values did not show a strong variation over frequency for any of the samples. Values of loss modulus (E″) were two orders of magnitude lower than the storage modulus, resulting in very low values of loss factor (E″/E′<0.1). These are characteristics of strong gels, which present negligible viscous properties.
•Earthquake and cyclic tests on a full-scale RC flat-slab building structure.•Flexural and punching behaviour of slabs with and without transverse reinforcement.•Punching shear strengthening of RC ...flat slabs using post-installed bolts.
Full-scale testing of a two-storey flat slab structure is reported, undertaken in the SlabSTRESS research project; the construction and testing were planned and carried out at the ELSA laboratory of the European Commission’s Joint Research Centre. The dimensions are three bays by two, spans 4.5 and 5 m, slab thickness 0.2 m, interstorey height 3.2 m. Two different longitudinal reinforcement details were considered; welded studs shear reinforcement was provided only in the second floor slab. The testing program included seismic tests for service and ultimate actions, using the pseudodynamic technique with virtual walls. To this aim a building structure was designed with primary walls and the flat slab frame as secondary element. Cyclic loading tests followed up to ultimate drift capacity of the structure. The sequence of tests included strengthening of a set of damaged connections using bolted bars in holes drilled through the slab, followed by cyclic testing to failure. The instrumentation was provided for the global response and the connections with local rotations in the columns and slab; cracking around the columns was measured with through-crack sensors; a measurement system for internal forces and moments was included within the columns. The results show the response with deformations and damage for the different loading conditions up to failure. The results obtained on a full-scale structure extend and confirm the knowledge in the literature, mainly based on isolated connections and/or small-scale samples.
In a Nuclear Power Plant (NPP), the piping layout needs to have flexibility to reduce the operating stresses and nozzle loads due to thermal expansion. At the same time, the piping and equipment need ...to be supported adequately to sustain the dynamic loads viz., seismic loads, sudden opening/closing of valves etc. In Indian Pressurized Heavy Water Reactors (PHWRs), Snubbers are provided on nuclear equipment & piping to serve both purposes i.e. allow flexibility for thermal movement during operation and at the same provide requisite rigidity to take care of dynamic loads. Snubbers are subjected to harsh environment during operation including high temperature, radiation, vibration and corrosive condition, which may adversely affect its performance. Various degradations viz., high drag force, shift in dynamic characteristics (locking characteristic), jamming etc. have been observed. Periodic testing of Snubbers is required to ensure the proper functioning during normal operating condition as well as during dynamic event. A dedicated In-Service Inspection (ISI) program for snubbers exists in Indian PHWRs to ensure healthiness/functionality of Snubbers. Efforts have been made to reduce numbers of Snubbers progressively in PHWR units. Numbers of snubbers have been significantly reduced in latest 700MWe PHWR Design. This paper covers the various degradations observed in snubbers, ISI program for snubbers, design modifications, Snubber testing, assessment for fitness for service, replacement of Snubbers in older units and optimization of snubbers.
•Present applications of SHPB in characterization of various cement-based materials.•Details of experimental tests on different concrete and UHPC under high rate of loading.•Data and information ...about specimen and fiber geometries on experimental studies.•Summarized results of numerical simulations on cement-based materials under test by SHPB.
Our living standards are substantially influenced by the advent and development of new materials. The need for stronger building materials has led to useful innovations such as the fortification of concrete to produce Ultra-High-Performance Concrete (UHPC). Since numerous structures constructed by various types of concrete and UHPC, experience high rate of loading in their service life, reliable databehavior of this material is an important issue. In the current study, the behavior of different reinforced and unreinforced concrete, and also various UHPCs with and without fiber are comprehensively reviewed based on experiments by Split Hopkinson Pressure Bar (SHPB). The investigated parameters for the tests performed included matrix constituents, temperature condition, sample size, fiber material, and loading rate. The available results indicate that Ultra-High-Performance Fiber Reinforced Concrete (UHPFRC) is the most favorable material for utilization in impact-resistance structures. Since the fibers show crucial role in the dynamic strength, fibers material and their aspect ratio should be selected precisely. Applications of numerical simulations can significantly accelerate the material development process, therefore results of computational constitutive modeling are also presented in this paper. The summarized data can be used not only for comparison and the improvement of load carrying capacities of cement-based materials, but also can be beneficial in the development of new computational models.
Large‐scale multiaxial testing facilities mainly serve to experimentally examine the horizontal behavior of full‐scale critical structural members, such as columns and seismic isolation bearings, on ...which a large vertical compression load is exerted as they simultaneously undergo horizontal deformation. The system friction and inertia force play an important role in obtaining sufficiently reliable test results, and it is not easy to comprehensively grasp all the issues involved. To understand the system friction and inertia force of the Bi‐Axial Dynamic Testing System (BATS) at the National Center for Research on Earthquake Engineering Tainan laboratory and to avoid complexity caused by specimens as much as possible, a lubricated flat sliding bearing is chosen as the specimen to be tested under horizontal triangular and sinusoidal reversed loading together with a constant vertical compression load. When no specimens are installed, that is, without vertical compression loading, the system friction of BATS generated by the various sliding surfaces can be identified and mathematically characterized using the horizontal triangular reversed loading test results; then, the effective mass of BATS can be estimated using the horizontal sinusoidal reversal loading test results to solve the inertia force problem. When applying a vertical compression load, it is assumed that the system friction of BATS and the shear force of the specimen are simply related to the applied total normal force (or vertical compression load) and horizontal excitation rate. An iteration methodology is proposed to identify and mathematically describe the dependency of the friction performance of BATS and the specimen on total normal forces (or vertical compression loads) and horizontal excitation rates by iterating the horizontal triangular and sinusoidal reversed loading test results. Finally, a lead‐rubber bearing and a direct force measurement system are connected in series such that the measurement system precludes the system friction and inertia force and a series of tests are conducted. The reliability of the proposed mathematical model for BATS and the feasibility of the proposed direct force measurement strategy are further demonstrated by comparing the calibrated force response with the directly measured response.
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