摘要
为研究典型高聚物黏结炸药(PBX)在温度影响下剪切带和脆韧转换行为的关系,结合DIC数字图像技术,测试PBX在323~363 K下的剪切带参数,对剪切带演变规律与破坏模式进行深入分析。基于裂纹滑移理论模型,分析温度效应下PBX的脆韧转换机制,得到翼裂纹失稳扩展和塑性滑移区的判定条件。结果表明:在323~363 K下,PBX的剪切带宽度变化取决于剪胀和剪缩的竞争机制,其主要机制有4类:Ⅰ:剪胀与剪缩达到平衡;Ⅱ:剪缩占优;Ⅲ:剪胀占优;Ⅳ:剪胀间歇性占优。结合Griffith能量释放原理,发现试件的剪切强度、粘聚力、断裂韧度是脆韧转换的控制因素,并以此为基础,进一步获得了PBX脆韧转换的判定依据:当满足翼裂纹失稳的条件时,宏观破坏模式趋于劈裂破坏;当满足塑性滑移区的临界生成条件时,多个滑移区相互连接形成塑性滑移面,宏观破坏模式以剪切裂纹滑移的韧性断裂为主导。
图文摘要
The evolution law of the shear band was explored, and the internal relationship between the change law of the characteristic parameters of the shear band and the brittle‑ductile transition mechanism was deeply analyzed. Based on the crack‑slip theoretical model, taking into account the temperature effect, the brittle‑ductile transition mechanism and failure mode of PBX are analyzed.
随着现代高性能武器装备的迅速发展,弹药的安全性能问题受到广泛关注。周围环境的温度、湿度、振动等因素都可能会影响高聚物粘结炸药的感度,从而影响其安全
目前已有相关学者对高聚物粘结炸药的局部损伤变化及内部破坏情况进行研
此外,相关学者针对剪切带的演化及相应损伤行为展开了系统研究。Liu
由于以HMX为基的典型高聚物粘结炸药PBX为粘弹性材
本研究所用PBX由奥克托今(HMX)、粘结剂以及钝感剂等部分组成,其主要成分由
component | weight / % | volume / % |
---|---|---|
HMX | 95 | 94.15 |
binder | 4.3 | 4.64 |
desensitizer | 0.7 | 1.21 |

a. PBX specimen

b. global schematic diagram of specimen loading device

c. partial schematic diagram of specimen loading device
图1 PBX试件及准静态压缩试验的加载装置图
Fig.1 Diagram of PBX specimen and loading device for quasi‑static compression test
实验主要由加载系统、升温系统、图像采集系统3部分组成,实验装置如图
研究采用军标单轴压缩试验方法(GJB 772A.416.1-1997
work condition | T/ K | diameter / mm | height / mm | density / gc | mass / g |
---|---|---|---|---|---|
1 | 323 | 14.92 | 50.12 | 1.852 | 16.23 |
2 | 336 | 14.92 | 50.10 | 1.852 | 16.23 |
3 | 348 | 14.93 | 50.14 | 1.839 | 16.15 |
4 | 356 | 14.92 | 50.10 | 1.849 | 16.20 |
5 | 363 | 14.92 | 50.12 | 1.849 | 16.21 |
Note: T is ambient temperature under loading.
结合各工况变形特征和宏观破坏模式,采用DIC技术记录试件局部特征变形的演化情况,基于剪切带演化规律及相关竞争机制的研究,剖析剪切带特征参数与脆韧转换机制的内在联系;结合裂纹滑移模
为揭示温度作用下PBX的脆韧转化特性,结合典型工况应力‑应变关系,针对不同温度点的局部应变演化与试件脆韧转换间的有机联系,分析了不同温度下PBX的变形特征。对323~363 K下的PBX进行了力学性能测试,得到其应力‑应变关系,如

图2 不同温度下的试件应力‑应变曲线关系
Fig.2 Stress‑strain curves of specimens at different temperatures
应变硬化区域(曲线a):有明显的弹性阶段和较大的弹性模量,结合
应变硬化‑热软化区域(曲线b):在此阶段,由于粘结剂刚度降低,材料发生一定程度软化,其应力极值和弹性模量大幅下降(39.4%以上),同一应力下,其应变值相较于曲线a更大。
热软化区域(曲线c、d、e):升温至348 K及以上时,粘结剂的储能模量降幅较
通过对样品进行回收分析,选取试件变形损伤及裂纹演化富集区域,利用CAD对试件局部显著的破坏特征进行勾描,获得其裂纹发展情况如

a. 323 K

b. 336 K

c. 348 K

d. 356 K

e. 363 K
图3 不同温度下试件终态的裂纹扩展情况
Fig.3 Crack propagation in the final state of specimens at different temperatures
当温度为323 K时,试件表现出脆性特征,由于泊松效
当温度处于336~348 K之间,材料软化导致剪切强度与粘聚力降低。在荷载及温度耦合作用下,试件内部出现软化点,当局部剪应力超过试件的剪切强度时就会发生剪切滑移。张拉裂纹与剪切滑移裂纹相互贯通、纵横交错形成宏观裂纹,导致材料的断裂韧度等力学性能迅速劣化,最终形成剪切破坏(如图
升温至356 K或363 K时,由于材料发生软化,试件刚度显著降低,局部应力集中区域造成较为密集的交叉剪切面,试件表面出现斜向交错泛白细纹,见图
通过试件的局部应变演化过程,探究其剪切带演变规律,进一步剖析剪切带特征参数的变化规律与脆韧转换机制的内在联系。选择

图4 各温度下试件局部应变演变规律
Fig.4 The evolution law of the local strain of the specimen at different temperatures
由于剪切带通常是材料内部具有高度剪切应变处在应力作用下材料局部产生应变软化形成的,剪切带的形成将引起材料发生失稳与破坏,因此需对试件局部的应变集中区域进行分析。结合
当升温至336 K时,高温软化使PBX的剪切强度和粘聚力降低,当局部应力超过临界切应力时会引发明显的局部剪切滑移,会沿着最大剪切力方向发展形成优势发展方向的平行剪切带(
在塑性变形过程中,剪切带内材料体积改变会引起不均匀的体积应变分布,为更好地分析局部剪切带的演变特征,需要对试件有无剪切带的形成进行预判。其中以具有典型应变集中带的336 K为例,结合336 K的应变云图,选取了试件中间位置失效区内4种代表性点P0、P1、P2、P3,其中P0为相邻两个方向剪切带的交点,P1、P2为交叉剪切带上的点,P3为相邻剪切带围成的平行四边形区域内的点,如

图 5 336 K试件体积应变随时间的演变
Fig. 5 Evolution of volumetric strain of 336 K specimen with time
为深入研究温度效应对PBX的脆韧转换机制的影响,获取应力集中带与剪切带密度、宽度以及倾角等参数,对各温度下应力集中带与剪切带参数的变化情况进行分析,将应力集中区域进行勾描,其中剪切带密度为测试面内剪切带面积与测试面的面积比值,剪切带倾角为剪切带与水平方向的夹角。在剪切带两侧布置测线后利用粒径分析软件对其宽度进行测量,为保证剪切带宽度值的精准度,剪切带宽度按如下方式测量:在剪切带上分别选取3,5,7,10,15个测量点进行平均值比较,采用回归分析,以15个点的平均值作为剪切带的宽度,其测量误差控制在10%左右,应力集中带参数的获取方法与剪切带测量方法相同,得到323 K破坏点云图应力集中带密度为53.37%,宽度为1.77 mm,倾角范围为71°~86°;剪切带的相关参数如
T / K | density of shear band / % | width of shear band / mm | angle range of shear band / (°) |
---|---|---|---|
336 | 75.42 | 1.45 | 55-78 |
348 | 80.42 | 1.42 | 61-66 |
356 | 77.56 | 1.31 | 62-64 |
363 | 78.35 | 1.20 | 55-61 |
根据

a. peak stress and density of shear band and stress concentration band

b. width of stress concentration band and shear band
图6 应力集中带与剪切带参数随温度的变化规律
Fig.6 Variation of parameters of stress concentration band and shear band with temperature
由于颗粒材料受到剪切作用以及温度等组合效应,试件的体积发生改变,出现剪胀和剪缩现象。目前研究表明,剪胀会引起剪切带宽度的增

a. category 1(basic balance relationship)

b. category 2(descending relationship)

c. category 3(ascending relationship)

d. category 4(uncertain relationship)
图7 剪切带宽度与轴向应变的关系(图例中第一个数值为根据工况表2中温度值所对应的工况编号,第二个数值为剪切带的编号)
Fig.7 Relationship between width of shear band and axial strain(The first number is the working condition number corresponding to the value of temperature in working condition shown in Table 2,The second number is the number of the shear band)
通过以上研究发现,脆韧转换机制在较大程度上取决于外部因素(如温度)和内部因素(如微观结构、剪切带竞争机制),与剪切强度、粘聚力等物理参数息息相关。本研究尝试利用剪切滑移模
材料内部存在的微裂纹在外载荷的作用将同时具有形成翼裂纹与形成塑性滑移区的2种趋势。当首先满足翼裂纹失稳的条件时,试样将沿轴向劈裂破坏;如果生成的塑性滑移区之间首先连接形成宏观滑移带,试样将沿塑性滑移区发生剪切滑移错动。当试件达到峰值应力时,在试件中将存在如

图8 滑移裂纹模型
Fig.8 Slip crack model
该裂纹在轴向载荷σ1下,既存在沿轴向生成翼裂纹(
(1) |
其中,a=sin2γ-μ(1‑cos2γ);=0.27c mm为裂纹长度;τc为粘聚力,MPa;KIC为断裂韧度,MPa·
同时,轴向载荷σ1与塑性滑移区长度lp有
(2) |
式中,τy为屈服强度,MPa;η为简化公式的替代参数。多个初始裂纹间的塑性滑移区相互连接后即形成了贯通试样的塑性滑移面,试样呈现塑性滑移破坏模式。当翼裂纹起裂时,其裂尖处的应力强度因子为:
(3) |
当θ=70.6°时,KI取到最大值。而当γ=55.3°时,微裂纹面上的剪切应力τ=τs-(μσs+τc)在轴向载荷σ1一定的情况下取最大值。综上所述,θ=70.6°且γ=55.3°时,翼裂纹最易起裂。裂纹半径c与粒径d之间满足0.2d<2c<d,且平均裂纹间距约为2倍颗粒直径。基于作者前期工作,测得内摩擦系数μ=0.377,并结合PBX粒度分析的结果,颗粒最大直径为0.148 mm,取2c=d,即c为0.074 mm。同时,当塑性滑移区长度lp=2d=4c时,即可认为塑性滑移面贯穿试样,试样进入应变软化。
将临界条件lt=0 mm,c=0.074mm,lp=4c=0.296 mm,θ=70.6°,γ=55.3°代入
(4) |
(5) |
联立
(6) |
其中,,,,K0=1 MPa·
KIC计算公式拟合及有效性验证: 根据实验结果,当T=323 K时,试件为轴向劈裂破坏,起裂应力阈值为23.19 MPa,由

图9 PBX炸药断裂韧度与温度的关系
Fig.9 Relationship between fracture toughness and temperature of PBX explosive
T / K | ||||
---|---|---|---|---|
323 | 0.185 | 1.52 | 4.77 | 6.97 |
336 | 0.162 | 1.41 | 4.38 | 4.24 |
348 | 0.141 | 1.31 | 4.02 | 3.82 |
356 | 0.128 | 1.24 | 3.78 | 3.41 |
363 | 0.115 | 1.18 | 3.57 | 3.22 |
当,材料为轴向劈裂破坏,,由轴向劈裂变为塑性滑移,并产生剪切带。由
为研究温度效应对PBX的脆韧转换机制影响,对323~363 K的PBX进行单轴压缩实验,得到以下结论:
(1)在323~363 K的单轴压缩实验中,随着温度升高,PBX的力学性能发生显著改变,按照温度将其分为三种效应区间(应变硬化区、应变硬化‑热软化区、热软化区),试件由脆性破坏转为韧性破坏,其主导试件变形的主控机制由应变硬化机制逐渐转换为热软化机制。
(2)剪切带的发展过程为:首先出现优势发展方向的平行剪切带,随后出现不同密度的多种交叉剪切带,其最大密度为80.42%,最终维持相对稳定。在336~363 K时,PBX的剪切带宽度变化取决于剪胀和剪缩的竞争机制,其主要机制为Ⅰ剪胀剪缩平衡;Ⅱ剪缩占优;Ⅲ剪胀占优;Ⅳ剪胀间歇性占优。
(3)基于剪切滑移模型与Griffith能量释放率准则,发现试件的剪切强度、粘聚力、断裂韧度是影响脆韧转换机制的控制因素,获得PBX脆韧转化的判定依据为。当时,满足塑性滑移区的临界生成条件,宏观破坏模式以剪切裂纹滑移的韧性断裂为主导;当时,满足翼裂纹失稳的条件,宏观破坏模式趋于劈裂破坏。
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