Using experimental modal analysis to assess the behaviour of timber elements

Abstract

Timber frameworks are one of the most important and widespread types of structures. Their configurations and joints are usually complex and require a high level of craftsmanship to assemble. In the field of restoration, a good understanding of the structural behaviour is necessary and is often based on assessment techniques dedicated to wood characterisation. This paper presents the use of experimental modal analysis for finite element updating. To do this, several timber beams in a free supported condition were analysed in order to extract their bending natural characteristics (frequency, damping and mode shapes). Corresponding ABAQUS finite element models were derived which included the effects of local defects (holes, cracks and wood nodes), moisture and structural decay. To achieve the modal updating, additional simulations were performed in order to study the sensitivity of the mechanical parameters. With the intent to estimate their mechanical properties, a procedure of modal updating was carried out in MatLab with a Python script. This was created to extract the modal information from the ABAQUS modal analysis results to be compared with the experimental results. The updating was based on a minimum of unconstrained multivariable function using a derivative-free method. The objective function was selected from the conventional comparison tools (absolute or relative frequency difference, and/or modal assurance criterion). This testing technique was used to determine the dynamic mechanical properties of timber beams, such as the anisotropic Young's Moduli and damping ratio. To verify the modulus, a series of static 4-point bending tests and STS04 classifications were conducted. The results also revealed that local defects have a negligible influence on natural frequencies. The results demonstrate that this assessment tool offers an effective method to obtain the mechanical properties of timber elements, especially when on-site and non-destructive techniques are needed, for example when retrofitting an existing structure.

An Investigation of the Vibration Modes of an External Gear Pump through Experiments and Numerical Modeling

“…Few studies have addressed the uncertainties of material properties by using numerical modal analysis. Relevant examples are the works by Kouroussis [22] and Tabatabaei [23], who utilized FE modal analysis in comparison with experimentally measured modal parameters to estimate realistic material properties of timber beams and functionallygraded materials. However, except for one study by Xu [17], this topic has not been often discussed in the existing hydraulic noise and vibration studies.…”

Editorial: Special Issue on Experimental Vibration Analysis

“…The International Journal of Mechanics & Industry compiles a Special Issue on Experimental Vibration Analysis and makes a focus on the recent challenging innovation and advances in vibration engineering after the last special issue in 2013 [1]. These analyses are necessary to calculate the modal parameters of a system, a structure [2][3] or a mechanism [4], and the expected response to the given excitation [5] (harmonic, random, shock, etc.). In this way, it can be determined whether a particular system, structure or mechanism will fulfill its intended function; in addition, the results of the dynamic loadings acting on a structure can be predicted [6], such as the dynamic stresses, fatigue life [7] and vibration levels.…”

Non-Destructive Assessment of the Dynamic Elasticity Modulus of Eucalyptus nitens Timber Boards

“…The traditional way to calculate E x of a timber board is through a four-point pseudo-static bending test, performed in certified laboratories [ 20 ]. However, in recent years, several researchers have looked for faster, non-destructive techniques based on short-duration dynamic excitations [ 21 , 22 , 23 , 24 , 25 , 26 , 27 , 28 , 29 ]. The so-called dynamic modulus of elasticity ( E xd ) determined by this vibration technics is, then, correlated to the E x obtained by applying the pseudo static bending test.…”

“…Among the advantages of these techniques, their application speed stands out, since sensors are only located at the ends of the timber boards, without the need for special support conditions. In contrast, in the investigations developed in [ 26 , 27 , 28 , 29 ], dynamic excitations were applied in the transverse direction of timber boards. These techniques are an excellent alternative to estimate E xd when it is impossible to access the ends of the wooden boards (for example, for in-situ field testing), or when its dynamic properties need to be known for numerical model calibration.…”