Industrial-Scale Timber Strength Grading

Original Language: German

The Challenge

Visual grading of structural timber relies on surface defects, knots, slope of grain, wane, as proxies for mechanical strength. But these indicators don’t capture the actual stiffness that determines a board’s load-bearing capacity. Two boards with similar visual grades can have significantly different modulus values, leading to either underutilization of strong material or, worse, acceptance of boards that don’t meet structural requirements.

The timber industry needed automated grading systems that could measure actual mechanical properties at production speeds, enabling strength-based sorting that maximizes the value extracted from each log.

The Solution

The Fagus Grecon system combines two complementary measurement stations for comprehensive timber characterization. The first station uses impulse excitation, a single impact on the board’s end face, to induce longitudinal vibrations. A sensor records the vibration response, and electronics calculate the oscillation period from which dynamic modulus of elasticity is derived.

The second station employs X-ray scanning to determine bulk density distribution and identify internal defects like knots across the board’s width and length. By integrating vibration-based modulus data with density measurements and board dimensions, the system computes a comprehensive strength indicator that accounts for both material stiffness and structural defects.

Results

The dual-measurement approach enables sorting accuracy that neither method could achieve alone. Vibration testing provides rapid, whole-board stiffness assessment, while X-ray imaging catches localized defects that might not affect overall resonant frequency but would compromise local strength. The combined system assigns each board to its optimal strength class, ensuring structural requirements are met while minimizing the waste of downgrading sound material based solely on visual appearance.

Key takeaway: Combining impulse excitation with X-ray density scanning assigns each board to its true strength class, eliminating both the safety risk of over-grading and the economic waste of under-grading visually sound timber.