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Title: | 以打音法進行製材多重模式振動模態之研究 Studying on multiple modes modal test of lumber vibration by tap tone method |
Authors: | Zi-Yu Chen 陳子育 |
Advisor: | 林法勤(Far-Ching Lin) |
Keyword: | 打音法,振動模態,有限元素法, Tap tone method,vibration mode,finite element method, |
Publication Year : | 2020 |
Degree: | 碩士 |
Abstract: | 本研究使用6種針、闊葉樹進行多重複打音法試驗,並以有限元素分析模擬各試材之振動頻率與振動模態,研究各試材之打音模態分布及多重複試驗對打音頻率變化之影響。試材以目視分等與抗彎彈性模數試驗做基本性質之評估,計算出各試材之抗彎彈性模數,並與打音法所量測之動彈性模數進行比較,最後比較打音法所量測之打音頻率與有限元素分析之模擬值比較,探討兩者間的相關性與差異。
多重複打音試驗顯示頻率變化主要分為4種類型,分別為穩定型、上升型、驟降型與隨機型,影響頻率變化的原因有試材密度、紋理、敲擊點硬度等,穩定型在每個樹種間皆有出現,上升型與驟降型出要發生於針葉樹;隨機型則是多集中於闊葉樹,隨著打音次數上升至300次時,頻率變化將會逐漸穩定,在頻率的標準差與變異係數的部分,針葉樹之標準差與變異係數皆較闊葉樹低,因此得到之敲擊頻率也較為穩定。 本次試驗主要能敲擊出試材之3種振動模態,分別為縱向振動第一模態、縱向振動第二模態以及橫向振動第一模態,有時還會敲擊出不在預期的異常頻率,當敲擊試材之中央點時,以縱向振動模態為主要出現模態,而敲擊試材角落點時則因樹種不同而有不同的模態分布,針葉樹仍以縱向振動模態作為主要出現模態,而闊葉樹則是以橫向振動模態為主要出現模態;縱向振動第二模態與異常之模態出現率則是闊葉樹高於針葉樹。 有限元素分析可模擬試材之縱向振動模態、橫向振動模態與扭轉振動模態,縱向振動頻率與有限元素模擬之結果相關性高,R2值皆高於0.99,針葉樹實驗值較模擬值頻均低16.6 %,闊葉樹則是低14.0 %;橫向振動頻率之實驗值與有限元素模擬之相關性低,因此僅能以有限元素模擬之結果推算試材縱向振動頻率。 打音法可以量測到許多模態,但並非所有模態皆會出現,本次試驗所使用之儀器仍有改進之空間,應改善試材敲擊時的旋轉,以及敲擊槌敲擊力道過小等問題,才可以取得更多模態之振動頻率。 This study uses the multi-repetition tap tone method to test on 6 kinds of softwood and hardwood, and uses finite element analysis to simulate the vibration frequency and vibration mode of each specimens. We studied the tap tone modal distribution of the specimens and the tap tone frequency influenced by the multi-repetition test. We first used visual grading method and static bending test to evaluate each specimen. Next, follow by comparing the modulus of elasticity that were calculated with the dynamic modulus of elasticity measured by tap tone method. In the end, evaluate the frequency measured by tap tone method and the analog value of finite element analysis to discuss the correlation and difference between them. Multi-repetition tap tone method shows that the frequency changes can be mainly categorize into 4 types, them being stable, increasing, steep decreasing and random type. The potential reasons affecting the frequency change are the density and texture of the specimens, hardness of the impact point and instrument abnormal; stable type occurs in each tree species; increasing type and steep decreasing type always occur in softwood; random type mostly concentrated in hardwood. As the sample size approaching 300, the result will gradually become stable. In terms of the standard deviation and the coefficient of variation for the results, both two values are lower for softwood comparing to hardwood, therefore the softwood has more consistence result while using the tap tone method. Through this study there are three main vibration modes of the specimens while tapping, which are first mode of longitudinal vibration, second mode of longitudinal vibration and first mode of transverse vibration. Also, sometimes it taps out of mode abnormal frequency. When taping the central point of the specimens, the longitudinal vibration is the main mode, when taping the corner point, the tree species have different modal distributions, the softwood still has the longitudinal vibration as the main mode, while the hardwood has the transverse vibration as the main mode. For longitudinal second mode vibration and abnormal mode, the occurrence rate of hardwood is higher than that of softwood. The finite element analysis can simulate the longitudinal vibration mode, transverse vibration mode and torsion vibration mode of the specimens. The longitudinal vibration frequency is highly correlated with the results of the finite element simulation, with all R2 value that are higher than 0.99. As for analog value mean, softwoods’ mean is lower than 16.6 %, while hardwoods’ mean is lower than 14%. For transverse vibration’s result, the correlation with finite element simulation are not significant, therefore we could only use the finite element simulation to calculate specimens’ longitudinal vibration. Tap tone method can definitely measure multiple modes of vibration, however yet to cover all possible modes, indicating there is still room for the equipment used in this study to improve. The rotation of the specimen while taping, and the force for each tap would need to be optimized in order to collect more modes of vibration. |
URI: | http://tdr.lib.ntu.edu.tw/jspui/handle/123456789/66660 |
DOI: | 10.6342/NTU202000293 |
Fulltext Rights: | 有償授權 |
Appears in Collections: | 森林環境暨資源學系 |
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