結構健康監測技術(Structural Health Monitoring, SHM)之目的為早期預警，結構損傷的擴大前及時發現並進行修補作業，避免造成結構的失效，甚至生命財產的損失。本研究使用的結構健康監測設備為SMART Layer®系統，使用PZT傳感器將系統輸入的電壓訊號利用壓電效應轉換，產生的應力波在結構中傳遞一段距離後，於平板中形成藍姆波(Lamb wave)，再由傳感器接收，將接收的訊號進行處理，分析基線訊號(Baseline signal)與量測訊號的變化，並以損傷指數(Damage Index, DI)量化，判斷結構健康狀況。
根據頻散曲線圖(Dispersion curve)，不同的激發頻率下會產生各種藍姆波模態，而藍姆波的基本模態(S0 mode和A0 mode)常用於結構健康監測的損傷判斷。根據材料性質，推導等向性材料與非等向性材料的藍姆波特徵方程式並求得頻散曲線圖，並與藍姆波頻散曲線計算軟體(GUIGUW)驗證。根據感測器設置距離除以理論群速度獲得包波抵達時間(TOF)，將電磁干擾訊號(EMI)與藍姆波高模態一同繪製成TOF圖，使作業人員判斷此結構的最佳檢測激發頻率區間。
在實驗之前，本研究以有限元素模型分析單剪力金屬搭接結構與複材補片修補結構，判斷主要破壞區域和計算各種裂紋長度下的應力集中因子，並針對此區域設置SMART Layer®感測器，再進行疲勞拉伸實驗，根據疲勞裂紋發生路徑進行藍姆波訊號分析，若檢測路徑上有缺陷區存在，會使訊號的S0模態能量的衰減 (Voltage下降)。
傳統DI值計算方式，會因訊號相位差的現象，造成DI值異常增大，誤判結構損傷位置，且相位差現象的主因並非由結構中裂紋引起，另外三個單剪力金屬搭接結構試件斷裂前的DI值差異很大，導致難有一個基準值表示結構處於危險狀態。對於較複雜的結構監測，本研究發展出一種穩定偵測裂紋的計算方式−HTB-DI值，並設置一個損傷警告值判定結構健康狀況。單剪力金屬搭接結構與複材補片修補結構的HTB-DI最高值皆對應試件的最大損傷(裂紋)處，於較早的拉伸週期時皆高於結構警告值(0.25)，達到早期預警目的，並於斷裂前高過結構危險值(0.5)，證實HTB-DI值能反映出結構中的損傷位置與損壞程度。
單剪力金屬搭接結構僅由扣件扭力上磅組合而成，非以用膠黏合兩材料，使得該結構之理論群速度不能以單層或雙層概之，但依據藍姆波特性，激發頻率越高，該結構群速度會越接近單層理論。
複材補片修補結構分析路徑因感測器黏貼方式分為兩種，一種感測器邊界條件較為單純的金屬面路徑；另一種為感測器的PZT振動端朝向金屬板面黏貼後，再將複材補片黏貼進行結構修補，稱為複材金屬夾層路徑，此黏貼方式使能量相對較集中於金屬板傳遞，若金屬結構疲勞裂紋繼續成長，與金屬面路徑相比，複材與金屬夾層路徑的HTB-DI值會較高，但會因補片與金屬黏合的好壞而有激發頻率的選用限制。

Structural Health Monitoring (SHM) aims at early warning that the damages in structures. When damage is found in the structure, maintenance staff repair before the expansion of structural damage. It can avoid structural failure, which may cause loss of life and property. In this study, the structural health monitoring equipment used is SMART Layer® system. In the system, the PZT sensor is used to convert the voltage signal into stress wave by the piezoelectric effect. Stress waves turn into Lamb waves after a distance propagation in the plate, and then received by the sensor. The received signal is analyzed, and then compare differences of baseline signal and measurement signal. The damage index (DI) is quantified the signal differences to determine the health status of the structure.
According to the Lamb wave dispersion curve, various Lamb wave modes will be generated from different excitation frequencies, and the Lamb wave basic modes (S0 mode and A0 mode) are commonly used for damage judgment of structural health monitoring. The Lamb wave characteristic equation of isotropic and non-isotropic materials is derived, and then obtain the dispersion curves which are verified by the calculation software of Lamb wave dispersion curve (GUIGUW). The packet arrival time is obtained by dividing the distance of the sensors by the theoretical group velocity. Draw the arrival time of electromagnetic interference signal together with the arrival time of Lamb wave modes into TOF diagram, which help the operators to determine the optimal detection excitation frequency range of this structure.
Before the experiment, this study carried out finite element model analysis on single shear metal lap structure and composite patch repair structure for the purposes of judgment of main failure zone and calculation of stress concentration factor under various crack lengths. Installing sensors in this zone and making the fatigue tensile test of specimens.
Lamb wave signal analysis is carried out according to the fatigue crack occurrence path. If there is a defect area on the detection path, the Lamb wave S0 mode energy of the signal will be attenuated (voltage attenuation).
The traditional DI value calculation method cause the abnormal increase of value due to the phenomenon of signal phase difference which mainly is not caused by crack damage. Finally lead to misjudgment of the location of structural damage. Another question is that the traditional DI values of the single shear lap structures are quite different before fracture, which makes it difficult to judge whether the structure is in a dangerous state.
For complex structures, more rigorous calculation methods are required. This study research and develop a calculation method - HTB-DI value for detection of cracks. The maximum HTB-DI value of single shear metal lap structure and composite patch repair structure is corresponding to the maximum damage (crack) of the specimens. The value of HTB-DI is higher than the structural warning value (0.25) in the early tensile cycle, which achieves the purpose of early warning, and continues to be higher than the structural risk value (0.5) before fracture. It is confirmed that HTB-DI value can be used to detect the damage location and damage degree in the structure.
The single shear lap joint structure only uses bolts to combine two materials without using glue. It makes the theoretical group velocity of this structure neither single-layer nor double-layer. However, according to the Lamb wave characteristics, the higher the excitation frequency, the closer the group velocity of the structure to the single-layer theory.
The analysis path of racked metal structure with composite bonding repair can be divided into two types according to the adhesive way of sensor. One is the metal side path with simple boundary condition of sensor. Another is the composite metal sandwich path, that the sensor is embedded between the composite material and the metal. The composite metal sandwich path's transducers stick to the metal surface to make the energy relatively concentrated on the metal plate. If the fatigue crack of the metal structure continues to increase, compared with the metal surface path, the HTB-DI value of the composite metal sandwich path will be higher, but the selection of excitation frequency will be limited due to the degree of bonding between the patch and the metal.

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