Many wood composites are known to poorly resist the effects of moisture and temperature extremes over time. Degradation of adhesive bonds, irreversible thickness swell and dimensional instability result in a decrease of properties and a possible failure of the material. Accelerated weathering has become an essential tool for assessment of the service-life of existing and newly developed wood-based panels. Predictions are typically based on intermittent measurements, with little to no yield of information on the progression of degradation during the accelerated weathering exposure. For a more robust test, and retrieval of more information on the progression of degradation, the goal of this study was to develop a non-destructive and continuous monitoring system.
The spectral composition of continuous pulse-echo measurements were analyzed for statistical correlation with periodic stress-wave velocity measurements of three types of wood-based panels that were subjected to accelerated weathering. Mechanical testing and dimensional monitoring were conducted to provide complimentary empirical evidence of degradation.
Acoustic properties of wood are highly influenced by moisture content. A central objective of this study was the continuous, non-destructive monitoring of moisture content in two dimensions during accelerated weathering. Following the principle of multi-wavelength sensing, a capacitive sensor was developed. An efficient combination and alignment of electrodes provided advanced capabilities of moisture content localization and monitoring. Two-dimensional moisture content maps were generated to track the progression of moisture adsorption and desorption throughout the accelerated weathering exposure.
Experiments were conducted on small- and full-scale specimens using different types of accelerated weathering devices. Three types of wood panels were tested as part of a full-scale floor setup, using the Multi-Chamber Modular Environment Conditioning (MCMEC) System, a new, state of the art accelerated weathering device.
To provide a proof-of-concept, a multi-linear regression was conducted to determine the relationship between two types of cross-correlated frequency spectra comparisons with stress wave velocity and moisture content measurements. A root mean square deviation index was used. Correlations were based on the deviation of the power spectral density from its initial condition at the start of the treatment.
Changes in the root mean square deviation index can be explained as a function of stress wave velocity and moisture content of the upper layer in the panel. Apart from demonstrating the feasibility of continuously monitoring degradation and moisture content under accelerated weathering, important and useful experience was gained about the moisture-related behavior of small- and full-scale panels. A future impact on the design of accelerated weathering schedules can be expected.