汽車引擎蓋急閉撞擊之動態有限元素分析

Dynamic Finite Element Analysis For The Impact of An Engine Hood to A Radiator Support Frame

洪廷芳(Ting-Fang Hung)

指導教授 ： 陳復國

國立臺灣大學/工學院/機械工程學研究所/碩士(2006年)

https://doi.org/10.6342/NTU.2006.00930

摘要

由於安全考量與法規的要求，汽車結構強度必須通過一連串的測試。然而，原形車的製造與實驗的進行均需要花費相當多的時間，在節省時間與成本的考量下，通常會在進行實驗前利用有限元素法模擬車身的結構強度，然而，模擬分析的精確度，尤其是動態撞擊分析，仍然需要加以改進。

本論文選擇某車型的引擎室作為研究載具，利用有限元素法軟體LS-DYNA模擬引擎蓋自由落下撞擊散熱器支架之行為，並在模型建立的過程中，討論在模型建立時所必須考量的重點，例如元素的連結性，焊點的模擬以及材料模型的選擇等。

在模型建立方面，必須特別注意CAD圖檔的品質，以免發生不連續的元素外，本文亦針對修補CAD圖檔提出了可行的方法。此外，本文亦比較了兩種建立旋轉機構的方式，分別為直接建立旋轉接頭與利用剛接連結旋轉接頭，並在所建立的模型中採用了適當的模擬方式。
在焊點方面，本文比較了拘束式焊點與接觸式焊點兩種模擬焊點方式之效果，並且發現拘束式焊點往往會產生較高的應力，此外，本研究比較了不同的焊點排列方式對於模型與模擬分析的影響性。針對運算速度方面，本研究將引擎蓋旋轉至撞擊瞬間之高度，並利用本文所推導之引擎蓋落下之質心角速度公式，建立可節省模擬分析時間的簡化模型。
最後利用實際的引擎室，進行引擎蓋自由落下撞擊散熱器支架之應變與受力實驗，而模擬結果亦確認了模擬模型的正確性。本論文所探討的動態有限元素模型建立方式，亦可應用於整車結構件之模擬分析。

關鍵字

動態有限元素法分析 ； 撞擊分析 ； 引擎蓋 ； 旋轉機構與焊點模型 ； 實驗

並列摘要

The structural strength of an automobile must pass through a series of tests due to safety concerns and regulations. However, it often takes a long time to manufacture a prototype and conduct a series tests. In order to save time and cost, the finite element simulations are usually performed to evaluate the strength of the designed car-body structure before the actual tests are conducted. However, the accuracy of finite element simulation for the strength analysis of a car-body structure, especially the dynamic impact analysis, is still being improved by researchers.

In this thesis, the strength of a radiator support frame impacted by a free-falling engine hood was examined with the use finite element code, LS-DYNA. The entire engine room of a recreational vehicle was modeled for the analysis. Key points of building a reliable model, such as connectivity of elements, modeling of spot welds and selection of material models, were determined first.

In the construction of a finite element model, disconnected elements were found due to poor quality of CAD files imported. An appropriate approach to remedy the CAD files was developed for the subsequent mesh generation. In order to model the hinge connecting the hood to the engine room, two different elements, such as revolute joints with rigid elements and revolute joints with rigid links, were thoroughly examined and the suitable element type was selected to represent the hinge in the finite element model.

In modeling spot welds, the efficiency in using the constrained spot weld approach and contact spot weld approach was compared. Constrained spot weld was found resulting in higher stress. In addition, the effects of different arrangements of constrained spot welds in the finite element model on the simulation accuracy were also examined. In order to further reduce the computation time, a theoretical model was also proposed in the present study to calculate the instantaneous angular velocity of the free-falling engine hood at the incipient impact to the radiator support frame, which was used to replace the simulation of the free falling of engine hood.

A series of experiments were also conducted in the present study to measure the strains and impact forces during the impact of the engine hood to the radiator support frame. The experimental data were found consistent with those obtained from the finite element simulation results. The accuracy of the finite element model built for the dynamic analysis for the impact of an engine hood to the radiator support was then validated. The approaches developed in the present study for the dynamic finite element model of impact analysis could be extended to the whole car-body structure.

Keywords: dynamic finite element analysis, impact analysis, engine hood, hinge and spot-weld models, experiments.

並列關鍵字

dynamic finite element analysis ； impact analysis ； engine hood ； hinge and spot-weld models ； experiments

參考文獻

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