不同製程之複材膠合接口預衝擊濕熱老化破壞監控

Abstract

本研究探討碳纖維複材單搭接膠合接口受衝擊破壞之濕熱老化監控，分別以兩種製程製作碳纖維複合材料，比較傳統熱壓成型製程與真空袋製程之單搭接試片差異，並解釋預衝擊試片濕熱老化後之機械性質破壞特徵。 為監控膠合接口之破壞情形，本研究使用非破壞檢測(Non-destructive Technique, NDT)中之結構健康監測(Structural Health Monitoring, SHM)，該方法可實時監控試片之變化，藉由電性評估試片之破壞程度，操作簡單且成本較低。由於需透過電性監控，因此在搭接劑中加入多壁奈米碳管，透過碳管間接觸導電與無接觸碳管間隧穿效應導電，並提供一定的機械強度，受到破壞時碳管網絡改變使電性改變，能即時顯現出其中破壞。 兩製程製作之試片存在強度差異，真空袋製程試片較熱壓製程試片拉伸強度低13%，由於製程之環境不同，真空袋製程之複材含較多孔洞缺陷，因此導致單搭接試片之強度差異，然而由於此缺陷，受到衝擊時能吸收較多之衝擊能量，傳遞給接口之衝擊能量較少，受相同能量衝擊後之殘餘拉伸強度與疲勞強度下降較少。 電性監控方面，衝擊時電壓瞬間下降，由於大部分能量由表層基材吸收，接口主要受到壓縮，使碳管網絡更緻密導致導電度變佳，透過電性之變化可以觀測到接口受到衝擊傷害，但電性之變化無法量化成受到衝擊之程度。預衝擊拉伸試驗之電性監控多呈現雜訊，與力量變化無一定之趨勢，顯示出電性監控之失效；反之疲勞試驗可見電性大幅變化，其中可見電壓陡升，破斷面可見兩種破壞機制，其電性表現為兩種破壞之綜合表現。高溫高濕環境之電壓監控皆上升，大致可分成兩種趨勢，其中一種趨勢之試片在後續機械性質測試後存在兩種表現，為試片受到之衝擊破壞差異，在破壞後之顯微觀測可見受到衝擊破壞之程度差異。 綜合上述，本研究提供碳纖維複材單搭接膠合接口受衝擊之電性表現與預衝擊濕熱老化後機械試驗破壞特徵，可見衝擊破壞對接口之影響，後續可透過其他檢驗方式探討電性變化之破壞表徵。

This study investigates the monitoring of hygrothermal aging in pre-impacted single-lap joints in carbon fiber reinforced plastic. Two fabrication methods were employed to produce the composite specimens: autoclave molding and vacuum bag only (VBO) process. The study compares the differences between single-lap joint specimens fabricated by these two processes and explains the mechanical failure characteristics of pre-impacted specimens after hygrothermal aging. To monitor the damage in the joint, a non-destructive testing (NDT) method known as structural health monitoring (SHM) was utilized. SHM allows for real-time monitoring of changes in the specimens by evaluating electrical signals to assess damage progression. This method is characterized by its simplicity and low cost. For electrical sensing, multi-walled carbon nanotubes (MWCNTs) were incorporated into the adhesive layer. These nanotubes provide electrical conductivity through contact and tunneling resistance between nanotubes, while also contributing to mechanical strength. When damage occurs, changes in the CNT network lead to variations in electrical conductivity, which reflect the extent of damage in real time. Single-lap joint Specimens fabricated by the two processes exhibited differences in mechanical strength. The VBO specimens had 13% lower tensile strength compared to those made using the autoclave process. This difference is attributed to higher porosity in composites made by the VBO process. However, due to these voids, VBO specimens were able to absorb more impact energy, resulting in less energy being transmitted to the joint. Consequently, after being subjected to the same impact energy, VBO specimens showed smaller reductions in residual tensile and fatigue strength. In terms of electrical monitoring, a sharp voltage drop was observed at the moment of impact. Since most of the energy was absorbed by the surface matrix, the joint mainly experienced compression, densifying the CNT network and improving conductivity. Although this electrical change indicates that the interface suffered impact damage, it does not quantitatively reflect the severity of the impact. During pre-impacted tensile tests, electrical monitoring signals often appeared as noise with no clear correlation to load changes, indicating a failure of the electrical monitoring method in that context. In contrast, significant electrical changes were observed during fatigue testing, including sharp voltage increases. Fractographic analysis revealed two distinct failure mechanisms, and the electrical response was a combined representation of both. Under hygrothermal environment, the monitored voltage values increased and could be categorized into two distinct trends. Post-mechanical testing showed that specimens following one trend exhibited two different performance profiles, corresponding to varying degrees of impact damage. Microscopic observations confirmed differences in the extent of damage between specimens. In summary, this study provides insights into the electrical behavior of carbon fiber single-lap joints subjected to impact and the failure characteristics of mechanically tested specimens after pre-impact hygrothermal aging. The findings highlight the influence of impact damage on bonded interfaces and suggest that further investigations using alternative inspection methods are warranted to better interpret electrical signal variations as indicators of specific damage modes.