Selection Efficiencies of Nondestructive Strength Tests

The research explores the power of vibrational measurement of elastic modulus tests to identify wood of acceptable strength after ensuring rejection of wood of unacceptable strength. It proposes a measure of selection efficiency defined as the percentage of the acceptable material actually recovered after the rejection of low strength material is ensured. The research is based on 92 clear, straight-grained specimens of sugar maple, 1 by 1 by 16 inches. The specimens are vibrated in transverse bending by an air-coupled audio frequency oscillator. Vibration measurements are made with photo-cell pick-up, amplifier, oscilloscope, and electronic counter. Specimen density, fundamental resonant frequency, and dimensions are used to determine dynamic modulus of elasticity. Log decrement was determined at frequencies above and below resonance for one-half amplitude at resonance. Specimens were then destructively tested to obtain static modulus of elasticity and modulus of rupture. The research showed that dynamic modulus of elasticity measurement produced the highest selection efficiency for strength, followed by static modulus of elasticity. Other parameters, such as specific gravity, logarithmic decrement, and ratio of dynamic modulus of elasticity to logarithmic decrement did not provide good selection efficiency. It was noted that selection efficiency diminishes as the strength level for exclusion is raised. Dynamic modulus of elasticity as a parameter for measuring bending stiffness had a selection efficiency comparable to that for measuring modulus of rupture.

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