【作者】 胡文军；

【导师】 刘占芳；

【作者基本信息】 重庆大学 ， 固体力学， 2008， 博士

【摘要】 聚碳酸酯是一种无定形的热塑性材料,具有优良的物理机械性能,尤其是耐冲击性优异,性能与一般金属材料十分相似,被称为“透明金属”。利用其透明性和高抗冲击强度,可制作防弹玻璃、飞机的挡风罩、座舱盖等。因此,开展聚碳酸酯力学行为研究,掌握温度、应变率对聚碳酸酯力学性能的影响规律,为聚碳酸酯材料作为冲击防护结构设计提供技术支持;聚合物材料屈服、变形机制的研究也是高分子科学的前沿研究领域,具有较高学术价值,另一方面,在战斗部的设计中,需了解穿靶过程中弹体的动态响应以及影响弹体破坏的主要因素等,从而设计出合理的弹体结构。由于战斗部弹体一般由高强度金属材料制成且弹体结构复杂,难以从现有试验中观察、分析撞靶过程中弹体结构参数对其响应的影响规律,也给相关理论分析和数值模拟工作的模型校验带来较大难度。而用聚碳酸酯材料制作的透明弹丸,可以直观的观测不同弹体的撞击变形或破坏现象,认识弹体在冲击载荷作用下的变形或破坏规律。也可将试验数据能直接用于相关理论分析及数值模拟工作中模型校验和修正,研究弹体撞靶过程的动态破坏规律,建立相应的研究方法,为战斗部的结构设计提供一定的理论依据和技术支撑。本文在分析国内外有关无定形聚合物力学行为及聚合物材料Taylor撞击响应研究的基础上,设计实施了聚碳酸酯材料在宽温度范围、宽应变率范围的力学行为实验研究,对其屈服和塑性变形机理进行了分析,建立了无定形聚合物材料屈服的本构模型;开展了聚碳酸酯弹丸的撞击和穿甲实验研究,采用光塑性分析方法,对聚合物弹丸碰撞的塑性变形行为进行了分析,给出了不同弹丸头形撞击后的变形或破坏的影响规律。论文的主要研究内容、方法和结论如下:①无定形聚合物的屈服和塑性变形机理非常复杂,在不同的变形阶段、不同的实验条件下有不同的机理,在变形过程中构象改变、位错运动和扭结传播同时存在。分子动力学模拟表明:在弹性区域,体系的总势能和各势能分量随应变的增加发生不规则的波动;在屈服点附近,总势能和键伸缩振动能将突然增加,非键作用能(范氏作用能)将减少;在稳定的塑性流动区域,各项能量分量变化不大;在应变硬化阶段,各项能量随应变增加而增大;在拉伸过程中,当应变接近屈服区域时,材料内部的变形是不均匀的,局部出现空隙,进入塑性流动区域时,分子开始发生取向。②聚碳酸酯是一种应变率和温度敏感材料,其流动应力随应变率增加而增大,随温度升高而降低;屈服应力随应变率的变化满足幂律函数关系;其屈服应力在应变率为102/s发生转折,比金属材料低两个数量级;在低于玻璃化温度时,屈服应力与温度的关系满足多项式指数方程,在温度低于-75℃或高于100℃时,有突变现象,屈服可看作是一种应力产生的相变,相当于热力学过程;加载弹性模量随温度升高而降低,随应变率增加而升高;在屈服点前的加、卸载循环压缩表明,加、卸载弹性模量随循环次数的增加而增加,即在粘弹性区域,循环加、卸载也会使聚合物材料发生硬化。③在准静态拉伸条件下,在接近屈服点时表面温度开始上升,在应变软化阶段温度上升最快,进入稳定的塑性流动区域后,最高温度变化跟随颈缩区域移动,几乎保持不变,既材料进入塑性区域温度到达最高;在动态压缩下,试样中心温度最高,由中心向外逐渐降低,而且温升率和上升的最高温度与应变率有关的。在应变率为2624s-1时,有59%的塑性功转变为热。④聚合物材料在断裂之前,要先发生分子链的滑移,引起局部变形,呈现出变形的不均匀性:对于钝角边缺口试样,断裂之前,应变场沿载荷45度-60度方向到达试样的承受载荷的中心线位置;断面形貌与裂纹传播的速度有关。⑤通过对不同头形的PC弹丸进行了弹速在118m/s～278m/s范围内撞击刚性靶实验研究,表明可以通过测量弹丸撞击刚性靶的压力反映弹体的载荷时间历程;其脉冲宽度主要取决弹丸的长度,弹丸头部越尖,其波形越平滑,弹丸头部越接近平头,其波形振荡越严重。冲击载荷的最大峰值和弹丸的变形受弹丸头部形状的影响较大。⑥通过对三种不同头部形状PC弹丸对纯铝靶和A3钢靶的穿甲实验结果分析表明,PC弹丸穿甲变化规律与金属弹丸穿甲实验的规律相似,三种头部形状的弹丸中,平头弹丸的过载系数最大,半球头弹丸的次之,截锥型相切尖拱弹丸的过载系数最小。高长径比的截锥型相切尖拱弹丸有利于穿甲。⑦不同头形的弹丸在撞击后的塑性变形分析表明,不同头形弹丸内部的变形模式不同,撞击刚性靶和穿甲后引起的弹丸变形模式也不相同,无论是撞击刚性靶还是穿甲,从头部开始逐渐远离头部,都存在变形模式的转变;弹丸在碰撞时主要是头部承受冲击载荷,柱段承受的冲击载荷较小,因此在弹丸设计的选材时,柱段可以选用强度较低的材料。⑧在对一维DSGZ模型分析的基础上,将其转化为三维模型,并以用户子程序的形式将该模型嵌入到大型通用有限元分析程序ABAQUS/Explicit中,不仅较好模拟了聚碳酸酯弹丸的撞击和穿甲响应,也可以直接应用于实际工程分析,增强了DSGZ模型的实用性。更多还原

【Abstract】 As a amorphous thermal-plastic material, polycarbonate is also called“transparent metal”because of its excellent physical mechanical properties, especially the great impact resistance fairly similar with that of general metal materials. With the high transparence and impact strength, it could be applied in bulletproof glass, windshield, cockpit of airplane and so on. Therefore, studying the dynamic behavior of polycarbonate can provide technology support for the design of anti-impact structure using polycarbonate. Furthermore, for the design of warhead, the dynamic response of Penetrator during penetration target process and the main factor influencing the damage of projectile-body must be learned to design reasonable structure. As the body of warhead is usually made from high strength metal and its structure is very complicated, it is difficult to analysis that how its structure parameter influence the response during impacting process from current experiment, which put great difficulty on related theoretic analysis and model validation of computational simulations. By using transparent projectiles made from polycarbonate, we could observe the deformation and damaging process intuitively and understand the deformation and damaging rules. Moreover, the experimental data can be directly used in related theoretic analysis, checking and modifying models of computational simulations, studying the dynamic damage rules during impacting process, establish corresponding research method, which could provides certain theory base and technology support for the structure design of warhead.In this paper, based upon the analysis of the dynamic behavior of polycarbonate and Taylor impact response of polymeric materials, a series of experiments about the dynamic behavior in wide temperature and strain rate range were designed and performed. By analyzing the yield and plastic deformation mechanism, the yield constitutive model of amorphous polymeric materials was founded. By using polycarbonate as model material, impacting and penetrating experiments were carried out with polycarbonate projectiles. Combining photoplastic analysis, the influence rule of projectile on deformation or damage during impact process was provided. The main study content, methods and conclusions of this thesis are as follows:①The yield and plastic deformation mechanisms of amorphous polymers are very complicated. At different deformation stage and testing conditions there are different mechanisms such as the conformation changing、dislocation movement and kink pair propagation simultaneity existing in the deformation process at the same time. The molecule dynamic simulation result indicated that at elastic region, the system total potential energy and potential component irregularly fluctuated with the increase of strain. Close to the yield point, total potential energy and bond stretching energy abruptly increased and non-bond energy (Van der Waals energy) decreased. At steady plastic flowing region, each kind of energy had no apparently changing. At strain hardening stage, each kind of energy rose with the increase of stain. Analyzing the rapid snapshot on molecule chain during drawing process, it could be found that near yield region the internal deformation of materials was asymmetry. Interspaces appeared partially; after going into flowing region, molecules began to orientate.②Polycarbonate is sensitive against strain rate and temperature. Its yield stress increases with the increase of strain rate and decreases when temperatures rise. The dependence of yield stress against strain rate agrees with power-law function;Under glass transition temperature, the relationship of yield stress and temperature agree with polynomial Index equation. Below -75℃or above 100℃, there is a sudden change phenomena occurring. Yield can be looked as a kind of phase transition caused by stress, which could be equivalent to a thermodynamics process. The loading elastic modulus decreases with the rise of temperature and increases with the rise of strain rate. Cyclic loading and unloading compression experiment before yield point indicate that loading and unloading elastic modulus rise with the of increase cyclic times. Namely, cyclic loading and unloading will also cause the hardening of materials in viscoelastic region.③Under quasi-static tension conditions, the surface temperature begins to rise when approaching the yield point and rise fastest at strain softening stage. After going into steady plastic flowing region, the highest temperature tailing after the necking region has no obvious change,that is, the temperature reaches the highest after going into plastic region. Under dynamic compress, the temperature of the specimen has the h -ighest value in the center and become to decrease from center to out side. The temperature rising rate and the highest rising value depend on strain rate. At strain rate of 2624s-1,about 59% plastic work transforms into heat.④Before the rupture of polymeric materials, slippage of molecular chains firstly occurs which will cause partial deformation and result in the asymmetry of deformation. For single edge notched specimen before rupture, strain field reaches centerline position enduring loads along 45°～60°direction. For double edge notched specimen, the strain field will focuses to centerline position along the same direction. The rupture shape is related to the crack propagation rate.⑤A serials of PC projectile with different nosed shape and speed of 118m/s～278m/s were used to impact rigid target, the experiment result indicated that the load-time history of projectile can be obtained by measure the interface pressure while projectile impacting target. The impact pulse width mainly lies on the length of projectile. The wavform is smooth when the acuter the projectile-nose and waveform is vibrational when the projectile-nose approach flated-end. And the deformation of projectile is deeply affected by the shape of projectile-nose.⑥The Penetrating experiment of 3 kind of PC projectile with different nose shape at speed of 150m/s～250m/s across pure aluminums target and A3 steel target shows that the changing laws of PC projectile are similar with that of metal projectile. The overload coefficient of these 3 specimens decease in the following trend: flated-end projectile>hemisphere nosed projectile>truncated conical tip projectile. And the truncated conical tip projectile with large length-diameter ratio is suitable for Penetration.⑦The photoplastic analysis after impacting experiment shows that the interior deformation models are deferent according to different nose-shape. And the deformation model of projectile impacting rigid target is also different from that of Penetrating one. The changing of deformation model exists from nose to end of projectile. When a projectile impacts target, cylindrical regions endure smaller impact loading than nose region. Thus, materials with lower strength such as composite materials can be used for cylindrical regions to raise mass ratio between energetic materials and penetrator.⑧After thorough analyzed, the one-dimensional DSGZ model was transformed into three-dimensional and the model which was then embedded into huge general FEM program ABAQUS/Explicit as user subroutine. Not only the impacting and penetrating response of polycarbonate projectile was well simulated, but also immediacy apply to engineering analysis and practicability of DSGZ model is enhanced.更多还原