1. Introduction
Pultruded glass fibre reinforced polymer (GFRP) profiles are being increasingly considered for civil engineering structural applications, as a replacement of conventional materials, such as reinforced concrete and especially steel. This is mostly due to their lightweight, high strength, and non-corrodibility. Among the various applications of GFRP profiles are building structures and bridges, both vehicular and especially pedestrian. In all these applications, one of the main design requirements is the comfort of users and, for that purpose, the dynamic properties of fibre-polymer composite materials are of paramount importance. In the specific case of pedestrian bridges, design is very often governed by vibration criteria.
To allow for a comprehensive design of pedestrian bridges made of pultruded GFRP profiles, it is necessary to obtain an in-depth understanding of their dynamic properties, namely compared to steel, which is the main alternative material it aims at replacing. The high damping capacity of polymer-based materials is widely known and used in the mechanical, automotive, and aerospace industries. In fact, the vibrations produced in structures and mechanical components are the main sources of problems in high precision instruments, machines, automobiles or aircrafts, and the use of polymeric composites as passive damping elements allows mitigating vibrations in such structures [
1,
2]. The frequency-dependent stiffness behaviour of a viscoelastic material directly affects the modal characteristics of the structural system, resulting in complex vibration modes and differences in the relative phase of vibration [
3].
In what concerns the use of material damping as the sole damping source of a structural system, Vasques et al. [
4] pointed out that the viscoelastic damping has been used mainly as distributed surface mounted or embedded damping treatments, utilizing passive viscoelastic materials alone. In the same manner, Nashif et al. [
5] introduced fundamentals of both vibrations and shock damping, focusing only on passive treatments for vibration attenuation without the effective use of the structure itself as a damping agent. Finegan and Gibson [
6] summarised the research done on the damping of polymer-based composites in two levels: (a) At the macromechanical level, the research efforts aimed to study the properties of the laminate layers, the inertial effects and the contact surfaces; (b) at the micromechanical level, the focus has been on the effects of the orientation and ratio of the reinforcing fibres, the fibre/matrix interface and the properties of the fibres and the matrix. In the same study, the authors state that material damping can contribute to passive vibration control using the inherent ability of polymeric materials to dissipate energy. This ability was explored by Adams and Bacon [
7] who examined the effect of fibre orientation and lamination geometry on the flexural and torsional damping and dynamic moduli of fibre reinforced polymer (FRP) composites.
As mentioned above, polymer-based composites are now finding increasing interest in civil engineering to be used as structural members (pultruded profiles, laminated shells and plates, and sandwich panels). Nevertheless, most of the research carried out in this area has focused on the evaluation of the static behaviour of composite structures, and less often on the dynamic behaviour of specific structures [
8,
9]; comprehensive information about the dynamic characteristics of polymer-based composites is not yet available, namely with respect to the material damping that is necessary to conduct dynamic analysis (in the time domain) under pedestrian loads, which are sometimes necessary to assess/design in a comprehensive way the comfort of users. This lack of information is well reflected in available design standards for composite structures, which provide very limited guidance in this respect (ASCE LFRD Standard [
10]; Prospect for New Guidance of FRP Structures [
11]; Italian Code [
12]). In fact, none of the above-mentioned design guidelines give detailed information on material damping to be used in the dynamic analysis and design of fibre-polymer composite structures.
In this respect, Boscato et al. [
13] stated that structures composed of pultruded FRP profiles have been extensively addressed in their static aspects, but the same attention has not yet been given to their dynamic behaviour. Regarding the dynamic behaviour of FRP materials, some studies have been conducted in the last two decades, such as that of Boscato et al. [
14], who presented a modal identification of an all-FRP two dimensional frame in free vibration. The results show that the GFRP structural system presents a better dynamic performance compared to systems comprising steel and aluminium members. Stankiewicz et al. [
15] presented dynamic in situ tests of a cable-stayed all-GFRP footbridge under human excitation. They found out that the analysed footbridge fulfilled the vibration comfort criteria elaborated by the technical guide Sétra [
16].
In this paper, the identification of modal characteristics of free-supported beams is carried out to determine the internal damping of two types of materials: (i) Steel and (ii) a polymer-based composite. With this purpose, forced vibration tests using hammer excitation are conducted on two sets of similar free-supported beams, considering the lengths of 250, 500, and 1000 mm. A free-supported beam scenario, in which the beams are placed on a low-density polyurethane foam, is considered to avoid damping added by external sources and to minimise the interference from boundary conditions. In this way, it is possible to extract only the material damping ratio. These types of support conditions are common (and well-established) in biomechanical (body in free fall), naval (submarines), and aerospace (aircraft in flight) engineering. The different lengths aimed at verifying the relevance of the structure scale. The application of identification algorithms to the collected data enables the identification of modal properties, namely damping ratios, which are further correlated with damping factors extracted using an energy-based method analytically deduced in this research.
The remainder of this paper is organized as follows: In
Section 2, a brief review of the free-supported beam theory is presented first, and the technique employed to identify modal properties is discussed next. The experimental programme is described in
Section 3 and the results obtained are presented in
Section 4. In
Section 5, a comparison is made between modal parameters identified in the tests and the ones analytically calculated. In
Section 6, damping estimates obtained for the two materials, GFRP and steel, are compared.
Section 7 summarises the main findings of this study.
5. Comparison of Identified and Calculated Modal Parameters
This section presents a comparison between the modal parameters obtained from the analytical formulae and the experimental results. For this purpose, the Euler-Bernoulli theory [
30] and the values of the corresponding wavenumber are used.
Table 13 summarizes the elastic properties of the beams in which the “Ratio EI/m” column emphasizes the relationship between the bending stiffness and the mass of the specimen. It is possible to verify that the relationship “stiffness × mass” for specimens of the same geometry is similar, with a relative difference of only 8% between both (with the calculated ratio for the GFRP composite being higher than that for steel).
Table 14 shows the analytical values of natural frequencies,
, given by Equation (9).
According to the Euler-Bernoulli theory, the natural frequency is inversely proportional to the square of specimen length. In this sense, as the ratios are similar between specimens with the same geometry, it is expected that the natural frequencies of the specimen C-1000 mm are lower than those of the specimen S-1000, since the length of the latter is 3.85% smaller than that of the former, it did not occur with the other specimens.
A comparison between the analytical and experimental values of natural frequencies is made to assess the accuracy of the analytical modelling and the test approach, and further, to validate the accuracy of the proposed model in simulating the free-supported beam boundary conditions.
Table 15 summarises the ratios between the experimental and numerical values of the natural frequencies for the different materials-lengths and for each mode.
In what concerns the accuracy aspect mentioned above, a value of ±0.10 was defined as an acceptable threshold. In this sense, the first mode of specimens C-1000 and S-1000 should be discarded from the analysis. Except for the first mode of the 1000 mm-length specimens, the good agreement between the experimental frequencies and those estimated with the analytical models shows that the hypothesis of the free-supported condition was achieved with the use of low-density foam as beams supports. It was not possible to unequivocally identify a clear cause for the differences between experimental and analytical results.
6. Damping Analysis
In this section, the damping ratios extracted for each pair of specimens with an identical length is analysed and compared. The procedure described in
Section 2 is applied to calculate the damping ratios.
From the power spectrum of amplitude response plots, the half-power bandwidth method is used and the damping ratio is calculated. The values obtained are shown in
Table 16.
The values given in
Table 17 show that for a given mode, the damping ratio,
, of the GFRP composite is higher than that of the corresponding steel-based specimen. It must be noted that for the same structure, at the same frequency, different response amplitudes will generate different levels of damping [
31]. In this sense, it is important to assess the damping ratio concerning the ratio of the response amplitudes of the structure.
To confirm the conclusion on the enhanced damping ability of the GFRP composite with respect to steel, the evaluation of the relative response amplitudes associated with each mode component for the two different materials is performed using the power spectrum amplitude response.
Table 17 summarises the amplitude response for each identified mode and the ratio between the measured amplitudes (GFRP/steel) as well as the ratio between the measured damping (GFRP/steel).
Table 17 shows that the response amplitude for the various identified modes has an average ratio of 1.4 between the GFRP composite and steel specimens, while the average ratio of damping between those two materials is 4.0. These two figures highlight the fact that the difference in measured amplitudes does not have a direct impact on the damping values. It should be noted that at certain modes the response amplitude for steel was higher than in the GFRP composite.
A consideration to be made refers to the scaling effect, which relates the length of the specimens to the damping variation, within the same frequency range, and which is directly related to the choice of specimen lengths adopted in this research. Furthermore, the natural frequencies of a full-scale system can be related to those of a reduced model, relating the two sets of frequencies by a scaling law taking into account the physical parameters of the model and the full-scale system [
32]. In this sense, the damping ratio as a function of the length (scale) was evaluated to identify whether the scale influences material damping.
Figure 17 shows the variation of the damping ratio with the frequency for all tested specimens. The damping ratio decreases as the frequency increases, up to a frequency of approximately 700 Hz. At high frequencies (above 800 Hz), the damping ratio starts to increase.
It may be noted that the material damping varies as a function of frequency but is not explicitly affected by the length of the specimens. As far as only material damping is concerned, the damping ratio is similar for specimens with different lengths, at the same frequency range. This is corroborated by a curve fitting in power law terms that exhibits a very-strong correlation between the damping ratio values and frequency for different lengths, as depicted in
Figure 18.
The curve fittings depicted in
Figure 18 can be compared to those obtained based on the energy method. From Equation (15), one may evaluate the damping ratio as a function of a constant
, in terms of exponential decay. In this sense, the results obtained for the GFRP composite, depicted in
Figure 19, show that the damping ratios estimated based on the energy ratio are similar to the ones calculated from the curve fitting described above.
7. Conclusions
This paper presents a modal analysis of the dynamic characteristics of free-supported beams, made of pultruded GFRP and steel, to identify and compare their resonance frequencies, damping ratios, and vibration modes.
For the modal identification process, an experimental campaign was conducted by applying the input-output method with the hammer impact and measuring the responses in terms of acceleration of specimens with various lengths. This approach allowed evaluating the damping variation as a function of resonance frequencies as well as achieving a wide range of frequencies, from 27 to 1800 Hz.
The comparison between the identified frequencies and the results obtained from the analytical model showed that the use of low-density foam as support enable a free-support beam condition, also allowing the retrieval of the material damping ratios from the test specimens. The same comparison showed a resemblance between the identified frequency for specimens with similar lengths but made of different materials. Furthermore, the order of magnitude of the relative differences between the frequencies identified for the same mode in the two different materials, did not exceed 3%.
The half-power bandwidth method was applied to the PSD plots and values of material damping ratios could be identified. For similar lengths, the values of the damping ratios of the two materials presented significant differences, with the GFRP composite presenting higher values, between two and six times higher than steel. This better damping behaviour of GFRP was confirmed through the evaluation and comparison of the measured acceleration amplitudes: The ratio of acceleration amplitudes between GFRP and steel was relatively small compared to the corresponding ratio of damping ratios. It is noteworthy that, for certain modes, the response amplitude of steel was higher than that of the GFRP. These results reflect the better performance of the polymer-based composite material in terms of energy dissipation and vibration attenuation.
Finally, an evaluation of the importance of the beam length was assessed to identify its influence on the material damping ratios. It was observed that the material damping ratio varies as a function of frequency but is not explicitly affected by the length of the specimens.
For the materials and geometries used in this study, it was shown that longer specimens exhibited modal characteristics that resemble those of the shorter specimens. Despite the limitations of this study, this points to the feasibility of estimating the material damping of full-scale structures based on small-scale tests at the material level.