壓電陣列激發圓管導波之設計及環境條件監測

Abstract

超音波導波檢測是非破壞檢測的一種，時常應用於結構的缺陷檢測。在離岸風力發電機系統中，因樁柱結構深埋於海床中，一般傳統的超音波非破壞檢測法無法直接對結構進行檢測，需藉由導波法才能檢測深埋於海床中的包覆狀況。由於海床中的土壤具有黏滯力與高含水量的特性，容易使導波在傳遞時產生洩漏而造成能量衰減，隨樁柱在海床中的深度和土壤的含水量，其能量衰減情形亦會有不同的情形。因此，本研究在探討導波L(0,1)縱向模態檢測圓管結構時，於不同的土壤包覆壓力與含水飽和度的情況之下，土壤特性對導波之波傳行為、回波訊號的衰減影響為主要目標。 本論文成功利用PZT壓電片設計出雙環換能器陣列，透過陣列的對稱性和梳狀結構抑制導波中大部分的非軸對稱模態，並成功激發L(0,1)模態在圓管中傳遞且用於檢測表面包覆情形。同時，也嘗試了不同壓電片數量的換能器陣列，其結果以雙環八顆換能器能夠激發出最好的L(0,1)模態。利用超音波陣列系統激發的縱向模態在不同的包覆壓力和土壤含水飽和度的情況之下，因導波的洩漏程度不同，所產生的波傳特性與回波訊號的改變，來判定圓管結構在海床中不同的包覆情形與健康度狀況。 由實驗結果顯示，導波會因為洩漏而產生衰減。衰減率會受到土壤含水量和土壤包覆力的不同而改變。當土壤含水量增加時，導波衰減率會跟著增加。而當土壤包覆力增加時，導波衰減率也會跟著增加。而土壤含水度和包覆力都是離岸風力發電機的樁柱健康指標，以此論文的定量分析，提供樁柱即時監控的參考指標，判定樁柱的健康度，達到定期維修、降低損壞機率與維修成本等目的。

Ultrasonic detection is a general type of nondestructive testing. It always be used to detect defects in the structure. As offshore wind turbines based structures are located in the ocean, the traditional ultrasonic non-destructive testing can’t be used in offshore wind turbines power system. Guided wave method is a faster and more effective method to detect wind turbine pile surface condition buried in the sea floor. Due to the sand in the sea floor have high viscosity and high water content, guide wave leakage into the sand and lost energy with propagation distance. With the pile buried depth and water content different in the sand, guide wave will have different attenuation. Therefore, we plan to detect different pile surface condition by using guide wave L(0,1) modes. By measuring the attenuation of L(0,1) echo signal from the pile and realize the propagation characteristic, we can monitor the health of pile buried in the sea floor. The thesis successfully build a double rings transducer array by PZT piezoelectric. We can excite L(0,1) mode propagating over four meter and suppress non-axisymmetric mode on the pipe by using axisymmetric transducer arrays and comb structure. At the same time, we know that sixteen transducers double rings structure is the best method to excite L(0,1) mode from the experiment. We can use this propagating mode excited by transducer array system to know the different pipe surface condition and health situation. From the experiment result, the guided wave will attenuate by leakage into surround. Attenuation of guide wave will change by the water content and cover force on the pipe surface. When water content increase in the sand, the attenuation will increase. Same as the cover force on the pipe, attenuation will increase with the cover force. The water content and cover force is the health critical point in the offshore wind turbines pile. The thesis provide a reference for the health monitor on the pile.