Markku Tiitta

Combined acoustic and electric method for monitoring wood drying process: A review

Abstract The combined method is based on using an electrical method for moisture gradient monitoring and an acoustic emission method for detection of microcracking. In the method, electrodes are used to create an electric field in drying wood and to measure the electric complex spectrum using the impedance spectroscopy method, while at the same time measuring acoustic emissions from drying wood. The result can be used for calculating the parameters required for determining the stress state prevailing in the wood. The spectrum is affected by the properties of the wood, such as moisture content, moisture gradient, temperature, density and structure. When the electric complex spectrum and acoustic emission response are determined at the same time, it is possible to estimate both the main reason for the stresses (moisture gradient) and the outcome (microcracking and macrocracking). The results may be used to control drying to achieve wood products of superior quality. The method was studied and tested both in the laboratory and in industrial kilns.

Air-coupled ultrasound and electrical impedance analyses of normally dried and thermally modified Scots pine (Pinus sylvestris)

Abstract Wood is graded according to strength in various applications. The ultimate strength can only be determined by breaking the specimen, and thus other characteristics like density and modulus of elasticity (MOE) are used for estimation of the strength. In this study, the properties of normally dried and thermally modified Scots pine were studied using electrical impedance and air-coupled ultrasound (ACU) methods. Density, hardness, MOE and strength were analysed and compared with the electrical and ultrasonic responses. The measurements were conducted in stable laboratory conditions with well equilibrated samples and the moisture content was not determined. Both the ultrasonic and electrical parameters correlated with the density and mechanical properties. Using multivariate analysis, density could be estimated with accuracy of 21 kg/m3 (SD) for normally dried timber and 13 kg/m3 (SD) for thermally modified timber (TMT; N = 15). MOE could be estimated with accuracy of 0.7 kN/mm2 (SD) for normally dried timber and 1 kN/mm2 (SD) for TMT (N = 14). According to the study, electrical impedance spectroscopy combined with ACU measured across the grain is a potential non-destructive technique for the strength estimation of wood.

Study of stilbene and resin acid content of Scots pine heartwood by electrical impedance spectroscopy (EIS)

Abstract Scots pine (Pinus sylvestris L.) heartwood samples were measured with electrical impedance spectroscopy (EIS) at frequency range 1 Hz–10 MHz in green and relative humidity (RH) 65% conditioned moisture content (MC) after oven-drying. Complex impedance parameters were studied in relation to the density, moisture, resin acid and stilbene contents. The measurements were conducted in tangential (T) and longitudinal (L) directions with 36 samples in each analysis. For green MC, there were significant correlations between impedance phase angle and contents of stilbenes and resin acids at frequencies below 400 Hz. For the resin acid content, the strongest correlation with phase-angleT was -0.45 (P<0.01) at 100 Hz. Impedance magnitude correlated significantly with MC throughout the frequency range, e.g., at 10 kHz, r was -0.71 (P<0.001) for L-direction. In moisture conditioned state, the correlation between stilbenes and T-measured phase angle was strongest at 250 Hz, (r=-0.56, P<0.001). Equivalent circuit model of two ZARC-Cole elements in series was fitted for the measurements in green MC. Several model parameters correlated with MC (e.g., R1, L r=-0.64, P<0.001), but only one parameter correlated weakly with stilbene content (R2, T r=0.35, P<0.05). The study shows that EIS has a potential for independent determination of resin acid or stilbene contents and MC for green pine heartwood.

The effect of moisture content on electrical impedance spectroscopy response of natural fibre-polymer composite granules

Two types of natural fibre-polymer composite (NFPC) granules were measured with electrical impedance spectroscopy (EIS). The granules were immersed in water for 70 h, after which the excess water was removed and EIS measurements were conducted. Then, the granules were let dry in open containers at normal room temperature, and EIS measurements were repeated at increasing time intervals. The results show that the EIS response as a function of moisture content (MC) depends on the fibre content of the NFPC. In addition, the results indicate that the EIS could be used for the estimation of MC of certain type of granulate, especially at low MCs, which is relevant for the manufacturing of NFPCs. For single material type, a model with impedance modulus at a single frequency was able to predict 87–95% of the MC variation. Therefore, EIS as a non-destructive on-line technique would allow the evaluation of moisture in NFPC granules.

Predicting the bending properties of air dried and modified Populus tremula L. wood using combined air-coupled ultrasound and electrical impedance spectroscopy

Air-coupled ultrasound and electrical impedance spectroscopy are non-destructive measurement methods, which can be used, for example for quality assessment of sawn timber. Both methods may be used in through-transmission and one-sided reflection mode to measure internal properties and detect defects in wood materials. The ultrasound method is based on mechanical waves and is mainly affected by the mechanical properties of wood. Density affects both methods, and the electrical impedance method is especially affected by moisture content and the chemical properties of wood. In this study, the relations between the methods and the bending properties of air dried and modified aspen (Populus tremula L.) specimens were examined. The modification method was a combination of compression and thermal modification. According to the study, electrical impedance spectroscopy combined with air-coupled ultrasound measured across the grain is a potential non-destructive technique for the strength estimation of aspen wood.