複合塑膠汽車頭燈之有限元素分析與振動實驗研究

Abstract

本文提出一簡化有限元素模型來研究車燈整體及局部之振動行為，此有限元素模型可作為車燈的先期工程分析研究。本研究先結合自然模態及強制激振二種實驗與有限元分析進行夾治具模型驗證，其次再導入車燈有限元素模型對於強制激振進行實驗與分析驗證；對於車燈模型建構採取曲面外型平面化以及非結構外觀省略二種模型簡化原則，並藉由調整有限元素模型材料參數及網格與實驗頻譜及模態進行驗證；研究結果顯示，有限元素模型修飾的關鍵為對後燈座元件於固定處剛性元素連結強化、增加簡化處厚度與楊氏係數的二種調整方法可獲得一車燈等效有限元素模型。 本研究也進行車燈在實車與夾治具安裝方式上模態分析比較與設計改善，研究結果顯示，夾治具採用「ㄇ」型整體式設計與實車情境接近，可避免低頻模態誤判。而由車燈模態振型分析與失效試驗進行驗證發現，車燈主要失效皆為中間飾板反射鏡局部振型所引起，經由增加固定點可衰減加速度響應以及改善車燈失效的發生；此外，透過本研究車燈模態振型分析結果發現，對於SAE J575振動頻率規格若修正至350Hz時，可獲得同樣失效模式下加速65%疲勞試驗時間與實驗成本。

This study proposes a simplified FE model for global/ local vibration analysis of vehicle headlamp. This FE model can be used for preliminary design and failure assessment of vehicle headlamp. In the FE model, the curve surfaces are replaced by plane surfaces and the non-structural parts are omitted. The first step of this study is to verify an equivalent FE model of the fixture used in the vibration test via the comparison between the theoretical and experimental results of the fixture subjected to force vibration. The next step is to verify the proposed FE model of the headlamp. The headlamp was subjected to force vibration testing. From the measured frequency response spectrum, the natural frequencies and vibration shapes of the headlamp are determined and used to verify the accuracy of the FE model. The results of the study reveal that it is important in the FE model to construct appropriate rigid beam links at the fixing holes and use suitable thickness and Young’s modulus in the simplified area. This study also consider the effects of the mounting method on the vibration behavior of the headlamp. The result of this study reveals that the fixture will be equivalent to the real mounting condition if the shape of the fixture is a “ㄇ” shape. Such shape will avoid incorrect prediction of low frequency modes. For the failure mode verification, this study also compares the theoretical failure mode with that identified in the test. It shows that both approaches identify the same failure area. The response of the failed part can be significantly lowered if an additional restraint is placed at the middle frame part. Furthermore, the fatigue test period can be shorten 65% for the same failure mode if its maximum frequency of 250Hz is revised to 350Hz when carrying out the SAE J575 specification testing.