Heiko Thoemen

Effects of panel density and particle type on the ultrasonic transmission through wood-based panels

Air-coupled ultrasonic inspection for the detection of delaminations inside panels is a widely applied technique in the wood-based panel industry. Additionally, the ultrasonic signal is used to monitor the constancy of the production process as unsteady material properties and process parameters cause variations in the transmitted ultrasonic signal. This article examines the physical effects of air-coupled ultrasonic testing and analyses the influences of panel density and particle type on the transmitted ultrasonic signal. The experiments were carried out for a broad range of wood-based panels. The experimental results indicate that the tested properties influence the transmission in two ways. Firstly, density affects the reflection and transmission factors at the material boundaries, and secondly, particles and pores act as scattering centres inside the material, leading to an attenuation of the ultrasonic signal. The results of this study may be used to efficiently include the ultrasonic signal into a trending system for the wood-based panel production process.

Observation of interference effects in air-coupled ultrasonic inspection of wood-based panels

The air-coupled ultrasonic inspection method is a widely applied non-destructive measuring technique in the wood-based panel industry. The technology is mainly applied to detect panel delaminations by analyzing the transmitted signal. Recent research deals with the use of ultrasonic techniques not only for the qualitative but also for the quantitative characterization of wood-based panels. To achieve a fundamental understanding of the behavior of ultrasonic waves in wooden panels, it is necessary to study the mechanisms that affect ultrasonic transmission and velocity during testing. Impedance and attenuation effects have been examined in previous studies. This article focuses on the interferences of ultrasonic waves. The interferences can be detected in experiments where the ultrasonic transmission is tested against the panel thickness. The results are verified with a mathematical model that explains the interferences due to multiple reflections inside the tested panels. By fitting the experimental data to the model predictions, the ultrasonic velocity and attenuation can be determined. So far, interference effects have not been considered for the non-destructive testing of wood-based panels. This research is a contribution to a better understanding of the mechanisms influencing the air-coupled ultrasonic methods.

Liquid CO2 processing of solid polylactide foam precursors

Diffusion of CO2 in polylactide was modelled by assuming the diffusion coefficient to depend on CO2 concentration, c, according to D[c] = D[0]exp[Ac], where D[0] and A are empirical constants, with the aim of optimizing impregnation of nominally amorphous and semicrystalline polylactide/CO2-based precursors for physical foaming. Numerical simulations provided a consistent description of desorption at different temperatures, T, from polylactide impregnated with liquid CO2 at 10℃ and 5 MPa, and D[0, T] could be represented analytically using Arrhenius or Williams–Landel–Ferry-type expressions, allowing interpolation and extrapolation. Sorption was argued on this basis to involve a step-like diffusion front, such that the CO2 content of a plate of thickness l increased as (D[0]t)1/2l−1F[Aco], where co is the value of c at saturation and F is a function of Aco only. A major practical concern with polylactide/CO2 precursors is that the glass transition temperature, Tg, decreases strongly with c, so that amorphous polylactide saturated with CO2 at 10℃ and 5 MPa degasses spontaneously at room temperature and pressure. However, it was inferred from the models and confirmed experimentally that partial impregnation in liquid CO2 for relatively short times could provide a relatively rapid means of preparing precursors with a roughly uniform CO2 content of around 0.1 g/g that were stable with respect to rapid CO2 loss on heating to room temperature. The resulting precursors gave satisfactory foam morphologies and densities on foaming at 100℃. Moreover, it was also possible to adapt the impregnation conditions so as to obtain partially foamed structures from semicrystalline polylactide under these conditions, in spite of its tendency to undergo cold crystallization during impregnation in liquid CO2, which suppressed expansion of saturated specimens at 100℃.

Ultra-light particleboard: characterization of foam core layer by digital image correlation

Increasing markets for internet-traded furniture, but also economic concerns are main driving forces to considerably reduce the weight of wood-based furniture panels. Recent research and technological developments have led to an innovative one-step process which simplifies the typical multi-step process for production of foam core panels. Three layered sandwich panels (with particleboard faces and polymeric in situ expanded foam as core layer) can be produced by a one-step process without additional gluing between the face and core layers. As the morphology of the foam and hence its mechanical properties strongly depend on its chemical composition, as well as on the process parameters during expansion, there are no data available, so far, describing the foam of the novel panels. The aim of the proposed project is to determine the elastic properties of in situ expanded foams using 2D digital image correlation. The data can be used later on for the simulation of the elastic behavior of foam core particleboards by means of FEM to describe the short and long term behavior of the panels.