임팩트 해머법을 이용한 소나무 옹이재의 비파괴적 휨강도 성능평가
Nondestructive Evaluation of Bending Strength Performances for Red Pine Containing Knots Using Impact Hammer Technique
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초록
본 연구에서는 소나무 옹이재에 대한 휨강도 성능을 비파괴적인 방법으로 평가하기 위하여 다양한 크기의 옹이가 있는 시험편을 준비하였고, 임팩트해머로 시험편 상단을 타격하여 얻은 공진주파수로부터 동적탄성계수를 구하여 정적 휨 강도성능과의 상관회귀를 실시하여 소나무 옹이재의 정적 휨 강도성능을 예측하였다. 임팩트해머의 휨 진동에 의한 동적 탄성계수와 정적 휨 시험에 의한 정적 휨 탄성계수사이에는 0.856~0.886의 높은 상관계수를 나타내었고, 동적 탄성계수와 휨강도와의 상관계수의 범위는 0.792~0.821로 1%수준에서 유의성이 인정되는 높은 상관관계를 나타내었는데, 이 값은 정적 휨 탄성계수와 휨 강도간의 상관계수 0.850 와 비슷한 값을 나타내었다. 이 값들은 진동의 수신방법 및 임팩트해머의 타격면(정목면, 판목면)에 따른 명확한 차이를 나타내지는 않았고, 옹이가 차지하는 비율과 동적 탄성계수와의 상관회귀에서는 ASTM D 3737에 규정된 한 면 폭에 대한 옹이직경의 비율인 KO(%)보다는 시험편 전체 네 면의 폭에 대한 옹이직경 전체의 합의 비율인 KT(%)가 소나무 옹이재의 휨강도 특성을 평가하는데 더 효과적이었다. 따라서 임팩트 해머의 휨 진동법은 소나무 옹이재의 비파괴적 휨강도성능 평가에 유용한 것이 확인되었다.
Abstract
To evaluate static bending strength performances of red pine containing knots using nondestructive evaluation technique, red pine specimens with various knots of different diameter were prepared. Bending strength performances of red pine containing knots were predicted from dynamic modulus of elasticity(MOE) which calculated from resonance frequency obtained by tapping the top of specimen with an impact hammer. Correlation coefficients between dynamic MOE obtained by tapping of an impact hammer and static bending MOE showed 0.856~0.886, and the values between dynamic MOE and static bending strength showed 0.792~0.821. Both values were similar to that(r=0.850) between static bending MOE and bending strength, and were significant at 1% level. The differences of the values for vibration receive methods (accelerometer, microphone) or tapping planes(quarter-sawn plane and flat-sawn plane) were very small. In regression analyses between dynamic MOE and the ratios of knots in red pine, the correlation coefficient between KT(%) which expressed as the percentage of total knot diameter to total width of red pine specimen and dynamic MOE was higher than that between KO(%) value calculated using ASTM D 3737 and dynamic MOE. Therefore, it was concluded the flexural vibration method of impact hammer was very useful to predict static bending strength performances.
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