材料高速受力行為研究

Abstract

材料在高應變率變形下其降伏強度以及抗拉強度等性質會與靜態變形有所不同，因此本論文主要即針對如何取得材料在高應變率變形下之應力應變曲線以及應變率之影響進行研究。 高應變率之試驗方法種類繁多，常見的主要以霍普金森桿法、直接撞擊法以及以伺服液壓系統進行試驗，其中又以伺服液壓系統較適合於應變率低於500s-1，而本論文選擇以伺服液壓系統進行試驗。 本論文使用荷重計以及黏貼應變規之方式以取得試驗之數據，隨著應變率之增高，荷重計之應力訊號震盪之現象隨之增大，因此討論其產生之原因，而應變規所取得之訊號較不受影響。本論文亦討論在試驗過程中工程應變率與試片之真實應變率之差異，而後使用Cowper-Symonds之經驗公式以描述材料在不同應變率下之應力應變曲線，以便於輸入有限元素軟體進行應變率影響之模擬分析。 最後使用CAE模擬分析在試驗過程中夾具之受力情形，以及對於試驗過程所產生之問題進行討論，並且以一液壓管件碰撞之問題為例探討有無考慮應變率之影響其結果之差異。

The finite element method has been widely applied to simulate the crashworthiness tests in the automotive industry. However, in the high strain-rate deformation, the yield strength and ultimate tensile strength of a material may be changed. In order to obtain accurate results, the stress-strain relations of the material in high strain rates are required for the simulations of the crashworthiness tests. There are various high strain-rate tests available to obtain the stress-strain relations, such as the split Hopkinson bar system, direct impact method, and servo hydraulic system. Each test method is applicable in certain strain-rate range. In the present study, the servo hydraulic system MTS819 was adopted to implement the high rate tests in a strain-rate range below 500s-1. The testing equipment including the machine frame, load cell, and data acquisition system was fine tuned first to make it suitable for the tests. As strain rate increasing, the amplitude of the stress vibration acquired from load cell increases. Hence, the efforts to determine the cause of the stress vibration and the remedy approaches were made. The actual strain rate measured in specimen during the test was considered. It was found that there is an acceleration zone in the beginning of the test. So it is important to determine when the strain rate comes to a constant strain rate. The stress-strain curves acquired from the experiments conducted in the present study were fitted by the Cowper-Symonds equation and then input to the finite element software for simulations. Through the finite element simulations, the actual strain rates in specimen and stress distributions in the grip during experiment were investigated. The finite element simulations were also performed to examine the strain rate effect on the impact of a hydro-formed engine cradle to a rigid wall. The experimental approach and the finite element simulations results obtained in the present study could be valuable references for the future researches in field of deformation on the high strain rates.