Study on mechanical properties and damage characteristics of booster explosives under static compression
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摘要: 为研究聚黑-14C(JH-14C)传爆药静态压缩力学性能及损伤特性,开展准静态压缩实验,获得了不同应变率下的应力-应变曲线,建立了描述不同应变率下JH-14C力学行为的非线性本构模型;利用扫描电镜(SEM)对回收试样进行细微形貌观测,获得了准静态压缩JH-14C损伤特性的表征。结果表明:JH-14C压缩强度随应变率的升高而提高;实验与计算结果对照验证了本构模型的有效性;准静态压缩实验中,JH-14C主要损伤模式为脱湿和穿晶断裂。Abstract: In order to study the static compression mechanical properties and damage characteristics of the JH-14C booster explosive, quasi-static compressive experiments were performed on a testing machine equipped with an environmental chamber (INSTRON). According to the GJB 770B–2005 powder test method, dimensions of the cylindrical specimen were set as
∅ 12.5 mm×12.5 mm in the static compressive experiments. During compression, only one extensometer was used. All experiments were performed at a crosshead speed of 0.012 5, 0.062 5, 0.125, 0.625 and 1.25 mm/s at room temperature (25 °C), which led to a nominal strain rate of 0.001, 0.005, 0.01, 0.05 and 0.1 s−1, respectively. The average stress-strain values and standard deviations were calculated using five replicable experiments for each condition. The experimental results were compared with X0242 and PBX-9501, and the mechanical properties of JH-14C were analyzed. According to the mechanical properties of JH-14C at low strain rates, the original Ramberg-Osgood constitutive relationship was modified, and a nonlinear constitutive model including the strain rate term was established to describe the mechanical behavior of JH-14C at low strain rates. The micro morphologies of the recovered samples was observed by a scanning electron microscope (SEM) and compared with that of PBX-9501. The damage mode was analyzed to characterize the damage characteristics of JH-14C under quasi-static compression. The results show that the compressive strength of JH-14C increases with the increase of strain rate. The validity of the constitutive model was verified by comparing the experimental and calculated results. In the quasi-static compression experiments, the energetic particles and the binder were debonded. With the increase of the pressure, the original crack and the micro-crack formed by debonding on the energetic particles were converged and coalesced to form a macro crack, which led to the rupture and failure of the explosive.-
Key words:
- JH-14C /
- booster explosive /
- quasi-static /
- strain rate /
- constitutive model /
- damage mode
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含能材料的力学性能及损伤机理研究是武器战斗部安全评估的一个重要环节。JH-14C作为引信传爆序列中的一种传统装药,在运输、贮存等过程中会因意外刺激而产生损伤。这些损伤将成为潜在的热点,影响引信传感序列安全性,因此,其力学性能及损伤机理研究对引信传爆序列安全评估具有重要意义。
目前,对钝感装药静态、动态力学性能已开展了大量研究工作。邓琼等[1]利用Hopkinson压杆实验装置,对含能材料力学行为相关问题进行了研究,给出了冲击片雷管的动态力学响应特性。Rae等[2-4]利用改进的光学显微镜,对PBX9501进行准静态压缩实验,并对回收试样进行了细观形貌观测,发现在准静态压缩下,PBX9501主要有黏结剂与含能颗粒脱黏,同时伴随含能颗粒原始微裂纹开裂。Heider等[5-6]利用准静态和SHPB压缩实验,研究了PBX KS32的力学行为,给出了一种确定材料黏弹性松弛函数的方法,并运用该方法构造了描述PBX KS32动态力学性能的本构关系。Dinens等[7]给出了包括裂纹开裂、剪切、扩张和聚合的统计裂纹力学模型,基于此,Bennett等[8]建立了黏弹性统计微裂纹损伤模型。
JH-14C是战斗部常用传爆药,对JH-14C的物理化学性质和爆炸特性已有了详细研究[9-10],但对其力学性能尤其在不同加载条件下的力学性能的报道不多。张子敏等[11-12]采用分离式Hopkinson压杆,对JH-14C传爆药在不同冲击载荷下的动态力学性能进行了研究,并利用扫描电镜(SEM)对回收试样的细观形貌进行了观察,发现JH-14C呈现明显的应变率效应,在外界载荷下JH-14C的主要损伤模式为黏结剂与含能颗粒的脱黏,但研究仅针对动态载荷,并没有涉及静态载荷下JH-14C传爆药的力学性能。
本文中,研究JH-14C传爆药准静态力学性能及损伤特性:利用静态压缩实验,获得不同应变率下的应力-应变曲线;采用修正的Ramberg-Osgood关系,描述JH-14C在低应变率下的力学行为;结合扫描电子显微镜,研究JH-14C静态压缩损伤模式。
1. 实 验
1.1 材料
JH-14C试样由西安近代化学研究所提供,密度约为1.70 g/cm3,其成分RDX、黏结剂和石墨的质量分数分别为96.5%、3.0%和0.5%。图1为JH-14C的细观形貌,可见其内部含能颗粒端面清晰不规则散布于聚合物黏结剂中,直径主要在50~200 μm。
1.2 单轴准静态压缩实验
采用配备环境箱的INSTRON试验机进行单轴准静态压缩实验,根据GJB 770B–2005《火药试验方法》,设计JH-14C试样尺寸为
∅ 12.5 mm×12.5 mm。在室温(25 ℃)下,共进行5次实验,压缩速率分别为0.012 5、 0.062 5、0.125、0.625和1.25 mm/s,应变率分别为 0.001、0.005、0.01、0.05、0.1 s−1。1.3 准静态压缩力学性能
图2为JH-14C传爆药在应变率0.01 s−1下的变形过程,其中点A、B、C、D对应不同时刻试件的压缩应力-应变状态。在准静态压缩实验中,JH-14C传爆药试件变形过程呈均匀变化,随着压缩应变增大,试件表面裂纹逐渐增多:当到达点C(7 s)后应力开始减小,这是因试件产生宏观裂纹,导致承载能力降低;当到达点D(11 s)后,JH-14C传爆药发生明显断裂,表面裂纹贯穿上下表面,试件破坏。
图3为准静态压缩实验时JH-14C、X0242(HMX、Estane和BDNPA/F的质量分数分别为92%、4%和4%)[13]和PBX9501(HMX、Estane和BDNPA/F的质量分数为95.0%、2.5%和2.5%)[14]的应力-应变曲线。可见:(1)PBX9501、JH-14C和X0242的压缩强度都随着应变率升高而提高,3种材料均符合材料应变率效应规律;(2)PBX9501的准静态压缩强度显著高于JH-14C和X0242的,主要原因为PBX9501的含能颗粒含量高于JH-14C和X0242的,通过对比JH-14与PBX9501的细观扫描结果,发现PBX9501细观结构致密,因此细观结构对含能材料强度具有重要影响;(3)准静态实验中,相同压缩强度下PBX9501的应变比JH-14C和X0242的小,这是因内部黏结剂含量及细观结构差异导致,虽然3种材料都是脆性材料,但是PBX9501强度更高、脆性效应更明显。以上分析,可为钝感炸药配方设计、装药等提供参考。
2. 理论模型与计算
2.1 Ramberg-Osgood 本构关系
在准静态载荷下,一般采用修正的Ramberg-Osgood本构关系[15]来描述炸药的拉伸及压缩力学行为:
εε0 = σσ0+A(σσ0)m (1) 式中:A、m为材料常数;σ0、ԑ0分别为应力-应变关系初始线性段某处的应力和应变,其比为材料的弹性模量。
参考Johnson-Cook本构模型,对修正的Ramberg-Osgood本构关系进行改进,建立非线性本构模型描述低应变率下JH-14C的准静态压缩力学行为:
σ=f(˙ε)(ε−Bεn)(1+Cln(˙ε/˙ε0)) (2) 式中:
˙ε 、˙ε0 分别为应变率、参考应变率,˙ε0=1 s−1 ;f(˙ε) 为应变率相关的函数;B、C为材料常数。2.2 模型验证
通过最小二乘法拟合得到修正的Ramberg-Osgood本构关系:
{σ=f(˙ε)(ε−3ε1.536)(1+0.16ln˙ε)f(˙ε)=552.4˙ε0.249 6 (3) 式中:σ的单位为MPa,
˙ε 的单位为s−1。表1为修正后的Ramberg-Osgood本构关系拟合结果的相关系数。图4为修正后的Ramberg-Osgood本构关系与准静态实验的JH-14C传爆药的应力-应变曲线对比。改进后的模型与准静态实验结果较好吻合,准确描述了JH-14C在准静态压缩下的力学行为。
表 1 模型拟合结果的相关系数Table 1. Correlation coefficients of the model fitting results˙ε/s−1 R2 0.1 0.990 5 0.05 0.997 5 0.01 0.998 7 0.005 0.992 5 0.001 0.984 2 3. JH-14C静态压缩的损伤特性
图5为JH-14C传爆药与PBX9501[2]加载前的细观形貌,A、B、C、D为炸药中的含能颗粒。可见:JH-14C与PBX9501细观结构相似:(1)内部都存在大量的孔洞及微裂纹等缺陷,含能颗粒呈现菱角形状,且在大颗粒周围环绕着许多体积较小的含能晶体;(2)含能颗粒上都存在大量微裂纹;(3)部分含能颗粒相互接触,但他们间并不存在黏结剂。
图6为PBX9501细观形貌[4],准静态压缩中PBX9501的主要损伤模式为含能颗粒与黏结剂脱黏,同时伴随着穿晶断裂。
图7为JH-14C在准静态压缩实验中的细观形貌。当t=3 s时,应力约为1.8 MPa,JH-14C传爆药内部形成了很多微裂纹,这种应力状态下JH-14C的主要损伤机制为黏结剂的脱黏。当t=7 s时,应力约为3.4 MPa,此时不仅有黏结剂脱黏形成的微裂纹(红色虚线),同时内部还形成了新的裂纹扩展路径。即原本存在于含能颗粒上的裂纹(黄色虚线),因外力导致含能颗粒裂纹附近应力集中形成微裂纹区,这些微裂纹与黏结剂脱黏形成的微裂纹经过汇聚、贯通等而形成宏观裂缝,最终导致JH-14C发生断裂。通过对比JH-14C和PBX9501细观损伤形貌,发现两者类似:在外力作用下,首先产生脱湿,其次发生穿晶断裂,最后失效破坏。
图8为JH-14C在压缩实验后试样径向截面的细观形貌。由于石墨含量低,且分布不均匀,在制作过程中JH-14C内部含能颗粒形成团簇,图中白色位置无石墨。在压缩作用下,团簇体与整体界面处为主要损伤产生位置。
图9为JH-14C试样准静态压缩实验后的宏观断裂形貌。在试样表面裂纹的扩展方向与径向轴线方向成60°,试样断裂角度几乎相同,这表明JH-14C具有较好的内部结构。通常,采用与库仑相关的破坏模式准则[16],假设JH-14C摩擦角为30°,满足库仑准则的莫尔应力圈有两条对称的直线作为其包络线,则运用上述准则计算得到破坏角为60°,这与实验现象一致。
4. 结 论
(1)开展了准静态压缩实验,获取了JH-14C的强度、失效应变和应变率效应。
(2)修正了Ramberg-Osgood本构模型,对照实验结果验证了模型的有效性。
(3)准静态压缩下,JH-14C的主要损伤模式为黏结剂脱黏,并伴随穿晶断裂,JH-14C的裂纹拓展方向与径向轴线成60°。
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表 1 模型拟合结果的相关系数
Table 1. Correlation coefficients of the model fitting results
˙ε/s−1 R2 0.1 0.990 5 0.05 0.997 5 0.01 0.998 7 0.005 0.992 5 0.001 0.984 2 -
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