半结晶聚合物损伤演化的实验表征与数值模拟

EXPERIMENTAL CHARACTERIZATION AND NUMERICAL SIMULATION OF DAMAGE EVOLUTION IN SEMI-CRYSTALLINE POLYMERS

损伤本构模型对研究材料的断裂失效行为有重要意义,但聚合物材料损伤演化的定量表征实验研究相对匮乏.通过4种高密度聚乙烯(high density polythylene,HDPE)缺口圆棒试样的单轴拉伸实验获得了各类试样的载荷−位移曲线和真应力−应变曲线,采用实验和有限元模拟相结合的方法确定了HDPE材料不同应力状态下的本构关系,并建立了缺口半径与应力三轴度之间的关系;采用两阶段实验法定量描述了4种HDPE试样单轴拉伸过程中的弹性模量变化,并建立了基于弹性模量衰减的损伤演化方程,结合中断实验和扫描电子显微镜分析了应力状态对HDPE材料微观结构演化的影响.结果表明缺口半径越小,应力三轴度越大,损伤起始越早、演化越快;微观表现为:高应力三轴度促进孔洞的萌生和发展,但抑制纤维状结构的产生;基于实验和有限元模拟获得的断裂应变、应力三轴度、损伤演化方程等信息提出了一种适用于聚合物的损伤模型参数确定方法,最后将本文获得的本构关系和损伤模型用于HDPE平板的冲压成形模拟,模拟结果与实验结果吻合良好.

The damage constitutive model is of great significance for studying the fracture and failure behavior of materials,but very few studies have been conducted to characterize damage evolution in polymeric materials quantitatively.In this study,notched round bar specimens with four different notch radii,made from high density polyethylene(HDPE)are stretched under uniaxial tension until fracture to obtain load-displacement curves and true stress-strain curves.The constitutive equations for HDPE materials under different stress states are determined through a combination of experimental testing and finite element(FE)simulation.The FE model,which can successfully regenerate the experimentally determined load-displacement curves,is then applied to establish the relationship between notch radius and stress triaxiality.A two-stage test method is proposed to quantify the variation of elastic modulus in HDPE specimens under uniaxial tension.The damage evolution equations for four types of HDPE specimens are established based on the degradation of elastic modulus.In addition,microstructure evolution in HDPE specimens under different stress states has been analyzed using interrupted tests and scan electron microscopy(SEM).The results show that the smaller the notch radius,the higher the stress triaxiality.Additionally,damage initiates earlier and develops fasters in HDPE specimens with higher stress triaxiality.From the microstructural point of view,higher stress triaxiality facilities the initiation and evolution of cavities in HDPE specimens,while suppresses the formation of fibrillar structures.A new approach for the identification of parameters in damage evolution models has been proposed base on the information of fracture strain,stress triaxiality and damage evolution equations determined from experimental testing and FE simulation.The constitutive equation and damage evolution model determined using the proposed methods are applied to simulate the deformation and fracture behavior of HDPE plate subjected to punch loading.The simulation results are in good agreement with the punch test results.