以實驗與模擬探討含脫層之複合材料三明治結構破壞行為

Abstract

本研究主要目的在於探討具表層與芯材間脫層之複合材料三明治結構(Sandwich structure)的挫屈模式及破壞強度。當脫層長度大於臨界脫層長度時，三明治結構具有局部挫屈的破壞模式，相反地，則為總體挫屈的破壞模式。表層厚度對臨界脫層長度的影響將是本研究討論的重點。三明治結構試片主要由碳纖維複合材料表層及Polymethacrylimide(PMI)發泡芯材所黏合而成，為了製作脫層區域，在表層與芯材間埋入離型布。本研究分別選用[(0/90)2]s及[(0/90)3]s複合材料疊層板做為複材三明治結構試片的表層。根據壓縮實驗結果觀察，當脫層長度較短時，主要的挫屈模式為總體挫屈(Global buckling)，挫屈發生後結構迅速破壞。相反地，當脫層長度較長時，主要的挫屈模式為局部挫屈(Local buckling)，結構發生挫屈後仍可承受一定的負載，直到表層與芯材分離，而總體挫屈的挫屈負載與破壞強度比局部挫屈的還要高。此外，挫屈負載與破壞強度將隨著脫層長度越長而降低。 為了分析含脫層之複材三明治結構的挫屈模式與破壞機制，使用有限元素分析(Finite element analysis)進行模擬。藉由線性挫屈分析得到挫屈模態，並以此模態當作初始缺陷(imperfection)進行非線性分析。將分析結果和實驗的數據進行比較，可以預估含脫層複材三明治結構試片存在的缺陷大小。分析結果顯示，當表層較厚或脫層長度較短時，對應的缺陷將較小。此外，根據最大應力準則，從非線性分析可以得到三明治結構的破壞負載。經由有限元素分析得到的破壞應力及挫屈行為與實驗的觀察相吻合。最後，藉由各種脫層長度下的非線性分析結果，得到[(0/90)2]s及[(0/90)3]s複材三明治結構的臨界脫層長度。

This research aims to investigate the buckling behaviors and failure strength of the composite sandwich structure with a local debonded interface between face sheet and core. When debonding length is longer than critical debonding length, the composite sandwich would fail in local buckling, on the other hand, it would fail in global buckling. The effect of the face-sheet thickness on critical debonding length would be of concern in this study. The composite sandwich samples were fabricated by adhering the composite face-sheet to the Polymethacrylimide(PMI) foam core, while for the generation of the debonded interface, the release fabric film was inserted between face-sheets and foam core. The [(0/90)2]s and [(0/90)3]s composite laminates were considered respectively as the face sheet in the composite sandwich samples. Experimental results indicated that for the short debonding length, the failure is dominated by global buckling, while, for long debonding length, the failure is initiated by local buckling. Once global buckling occurs, the structure fails rapidly. On a contrary, after local buckling, the sandwich structure can sustain for a while until the face sheet and core completely separates. It is noted that the sandwich structures with global failure exhibit higher strength than those with local buckling. Moreover, the buckling load and failure strength in sandwich structures decreases with the increment of the debonding length. In order to characterize the buckling behaviors as well as the failure mechanisms, the finite element analysis (FEM) was performed on the sandwich structures with debonding defect. The buckling mode obtained from the linear analysis was regarded as the initial imperfection in the nonlinear analysis. By comparing the numerical analysis with the experimental data, the size of initial imperfection associated with different debonding length in the FEM model was estimated. It was found that when the face-sheet is thick or the debonding length is short, the corresponding imperfection is relatively small. In addition, based on the maximum principal stress criterion, the failure load in the sandwich structures was predicted from the nonlinear FEM analysis. Both failure stress and buckling behaviors obtained from FEM analysis is coincided with the experimental observations. Finally, by FEM analysis of different debonding length, the critical debonding lengths of [(0/90)2]s and [(0/90)3]s composite sandwich structure are obtained.