Developmental Research on Uniaxial Impact Testing Machines with Arbitrary Input Pulse Conrol Capability for Characterization of Impact Damage Progress in Composites

本研究的目的是开发具有任意输入脉冲控制能力的单轴冲击试验机，用于表征复合材料的冲击损伤过程，并验证其在详细观察单向纤维增强复合材料冲击压缩损伤方面的有效性。虽然这些机器主要基于传统的分离式霍普金森压杆技术，但通过仔细控制波的反射和传输，可以获得任意的单轴输入冲击脉冲形状和大小。冲击载荷可以在简单的动态压缩应力-应变曲线中的任意点停止，复合材料试件可以恢复用于详细的表面/亚表面损伤发展的微观观察。针对单向玻璃纤维增强不饱和聚酯基复合材料的冲击压缩损伤，得到了以下结果：(1)压缩强度随应变率的增加而增加，随温度的升高而降低；(2)压缩模量值和面内剪切模量值也随应变率的增加而增加，但随温度的升高而减小。(3)回收的复合材料试件表明，纤维微滑移是主要的损伤机制。首次利用本文的实验技术阐明了冲击损伤的产生和发展过程。(4)在实验结果的基础上，提出了考虑非线性面内剪切特性的单向复合材料纤维微细观损伤理论。该理论成功地预测了实验获得的抗压强度与应变率和温度的关系，所开发的试验机被认为是一种有用的表征工具，可用于新型抗冲击材料的研发。

The purpose of the present investigation is to develop uniaxial impact testing machines with arbitrary input pulse control capability for characterization of impact damage progress in composites, and to demonstrate its validity for detailed observation of impact compression damage in unidiectionally fiber reinforced composites. Although the machineswere principally based on conventional split Hopkinson pressure bar techniques, wave reflection and transmission were carefully conrolled to obtain arbitrary uniaxial input impact pulse shapes and magnitudes. Impact loading can be stopped atany arbitrary point in a simple dynamic compression stress-strain curve, the composite specimens can be recovered for detailed microscopic observation of surface/sub-surface damage development. Both system hardware and software were developed for friendly use.Specifically for impact compression damage in unidiectionally glass fiber reinforced unsaturated polyester matrix composites, the following results were obtained :( 1 ) Compressive strength increases with increasing strain rates, but decreases with increasing temperatures.( 2 ) Compressive modulus and in-plane shear modulus also increase with increasing strain rates, but decrease with increasing temperatures.( 3 ) Recovered composite specimens revealed that fiber micro-bucling is a major damage mechanism. The impact damage initiation and progress were clarified for the first time using the present experimental technique.( 4 ) Based on the experimental results, a new theory for fiber micro-bucling in unidirectional composites was developed by considering nonlinear in-plane shear properties. The theory successfully predicted the strain-rate and temperature dependence of compressive strength obtained experimentally.The developed testing machines are believeded to be a useful characterization tool for R&D studies of novel impact-resistant materials.