Mechanical and microwave absorbing properties of Mn-Zn ferrite/polylactic acid composites formed by fused deposition modeling
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摘要: 3D打印技术在快速制造复杂形状零件方面获得了越来越多的关注。将锰锌铁氧体(MZF)作为增强体填充到聚乳酸(PLA)中,通过球磨混合和熔融挤出法制备出MZF/PLA复合线材,利用熔融沉积成形(FDM)制备出MZF/PLA复合材料。采用XRD、 SEM和矢量网络分析仪对不同复合比例的MZF/PLA复合材料的微观形貌、力学性能和电磁性能进行表征,并计算不同厚度的反射损耗,研究MZF的含量对复合材料吸波性能的影响。结果表明:当MZF含量为10wt%时,MZF/PLA复合材料的拉伸强度相比纯PLA提升了17.6%,随着MZF含量的提升,复合材料的吸波性能随之增强。当MZF的含量达到50wt%,在12.7 GHz处,厚度为7.4 mm时反射率达到最小值−55.3 dB,在厚度为7.9 mm时,有效吸波频带宽为4.5 GHz。因此,基于FDM制备的3D打印MZF/PLA复合材料具有良好的吸波性能和承载能力,是一种非常有前途的3D打印微波吸收材料。
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关键词:
- 复合材料 /
- 锰锌铁氧体 /
- 吸波性能 /
- 力学性能 /
- 熔融沉积成形(FDM)
Abstract: 3D printing technology has received more and more attention in the rapid manufacturing of complex shape parts. Mn-Zn ferrite (MZF) was filled into polylactic acid (PLA) as reinforcement, the MZF/PLA composite wire was prepared by ball milling mixing and melt extrusion, and the MZF/PLA composites was prepared by fused deposition modeling (FDM). The micro morphology, mechanical properties and electromagnetic properties of MZF/PLA composites with different composite ratios were characterized by XRD, SEM and vector network analyzer, and the reflection loss of different thickness was calculated to study the effect of MZF content on the microwave absorption properties of the composites. The results show that when the MZF content is 10wt%, the tensile strength of MZF/PLA composite is 17.6% higher than that of pure PLA. With the increase of MZF content, the microwave absorption performance of the composite enhanced. When the content of MZF reaches 50wt% at 12.7 GHz, when the thickness is 7.4 mm, the reflectivity reaches the minimum value of −55.3 dB, and when the thickness is 7.9 mm, the effective microwave absorption band width is 4.5 GHz. Therefore, the 3D printed MZF/PLA composite prepared based on FDM has good microwave absorbing properties and bearing capacity, and it is a very promising microwave absorbing material for 3D printing. -
纤维增强复合材料具有比强度高、比模量大等优点,已被广泛应用于航空航天等工业领域[1]。但复合材料对冲击敏感性较高,内部受低速冲击后会产生不可见损伤,造成安全隐患。碳纤维-玻璃纤维混杂复合材料是将碳纤维及玻璃纤维在同一种基体内成型的复合材料,可有效提高复合材料的力学性能[2-3]。目前对混杂复合材料低速冲击性能的研究主要集中在混杂结构和纤维种类上[4-5]。Swolfs等[6]研究发现,将低伸长率纤维放在层合板中间位置,可有效提高抗侵彻性能。Hung等[7]分析了碳纤维-玻璃纤维层间混杂复合材料低速冲击性能,发现将碳纤维置于冲击面时抗冲击性能更强。Manikandan等[8]研究发现,将韧性纤维置于复合材料背面可吸收更多冲击能量,原因是由于下部韧性层可为上部脆性层提供更大的变形。Sarasini等[9]对玻璃纤维-玄武岩纤维混杂复合材料进行低速冲击研究表明,玄武岩纤维为芯层的夹芯混杂结构吸收冲击能量更多,玻璃纤维为芯层时弯曲性能较好。有限元分析是分析低速冲击的有效手段,Liu等[10]对比了Puck、Hashin及Chang-Chang失效准则对低速冲击预测的区别,结果表明,三种准则在低速冲击响应和能量耗散方面的预测结果基本一致,在基体和分层损伤的预测上有所不同。Hou等[11]基于连续介质损伤力学(CDM)建立了含修补区域的复合材料低速冲击模型,采用基于断裂韧性的损伤变量,研究了修补区厚度及铺层结构对低速冲击性能的影响。Ebina等[12]对不同铺层结构的碳纤维复合材料进行低速冲击模拟,面内损伤采用增强连续介质力学(ECDM)模型,纤维损伤采用裂缝模型(SCM),层间采用界面单元(CZM),模拟结果与实验数据拟合度较高。Chen等[13]建立了碳纤维-玻璃纤维-玄武岩纤维复合材料低速冲击损伤模型,定义指数型损伤变量,研究发现,碳纤维为芯层的夹芯结构抗冲击性能较好,玄武岩-碳纤维混杂结构与碳纤维-玻璃纤维混杂结构的冲击响应类似。Wu等[14]建立了纱线尺度的三维正交碳纤维-玻璃纤维混杂复合材料低速冲击模型,采用代表性体积单元(RUC)计算复合材料层合板宏观力学参数,研究发现,冲击面为碳纤维时抗冲击性能更好,破坏主要为冲击面碳纤维断裂和上层纤维-基体分层损伤,冲击面为玻璃纤维时破坏主要为复合材料层合板背部分层损伤。由于冲击速度较低,应变率效应不明显,大多数复合材料低速冲击研究会忽略应变率效应[15-16]。部分低速冲击数值模拟研究考虑了应变率效应,Wang等[17]建立了应变率相关的碳纤维复合材料低速冲击三维损伤模型,采用修正的复合材料应力-应变关系及考虑应变率效应的层内层间损伤模型,模拟结果与实验数据拟合程度较好。
本文以碳纤维-玻璃纤维混杂复合材料为研究对象,分析混杂结构和冲击面纤维种类对低速冲击性能的影响。采用商业有限元软件ABAQUS建立了层间和层内两类混杂复合材料低速冲击模型,层内混杂复合材料采用纱线尺度模型。编写VUMAT子程序定义指数型渐进损伤因子及刚度退化方案,考虑纤维断裂、基体开裂、分层等损伤,通过分析实验数据及损伤形貌,揭示了碳纤维-玻璃纤维混杂复合材料低速冲击损伤破坏机制。
1. 实验及仿真方法
1.1 原材料及试样制备
单向经编织物(NCF)采用碳纤维(CF,TORAY T620SC-24K-50C)和玻璃纤维(GF,CPIC ECT469L-2400)制成,包括纯CF和纯GF的NCF织物及两种层内混杂织物,织物规格如表1所示,其中CF-GF和CF-CF-GF-GF为层内混杂织物,织物结构示意图如图1所示。环氧树脂,型号为2511-1A/BS风电叶片真空灌注专用树脂,主剂与固化剂质量比为100∶30,上纬(天津)公司。
表 1 碳纤维-玻璃纤维(CF-GF)单向经编织物(NCF)规格Table 1. Specifications of carbon fiber-glass fiber (CF-GF) non-crimp fabric (NCF)Fabric type Areal density/(g·m−2) Mass ratio of CF to GF CF GF CF 728.3 0 1∶0 GF 0 944.9 0∶1 CF-GF 364.2 472.4 1∶1 CF-CF-GF-GF 364.2 472.4 1∶1 本文设计层间和层内两类混杂结构,层内混杂层合板由CF-GF和CF-CF-GF-GF两种层内混杂织物铺层而成。各层合板的CF-GF混杂比均为1∶1,铺层方式为(0°/90°)4S,共8层,根据混杂结构及冲击面纤维种类进行命名,如表2所示,其中S-C和S-G采用夹芯式层间铺层形式。采用真空辅助树脂传递模塑(VARTM)工艺成型,固化条件为80℃、8 h。
表 2 CF-GF混杂复合材料层合板铺层结构Table 2. Stacking configurations of CF-GF hybrid composite laminatesHybrid structure Stacking sequence Nomenclature Non-hybrid (CFCFCFCF)2s C (GFGFGFGF)2s G Interply-hybrid (CFGFCFGF)2s I-C (GFCFGFCF)2s I-G Sandwich-hybrid (CFCFGFGF)2s S-C (GFGFCFCF)2s S-G Intralayer-hybrid (CF-GF) fabric CN-1 (CF-CF-GF-GF) fabric CN-2 1.2 性能测试
按照ASTM D7136M—05[18],采用INSTRON-9250HV落锤试验机进行低速冲击实验,通过改变冲击速度控制冲击能量,设置两个冲击能量分别为30 J和50 J,试样尺寸为100 mm×150 mm×6 mm,每组测试5个试样。利用NAUT21空气耦合式超声波C扫监测复合材料冲击后分层损伤,采用Bruker SkyScan1072进行Micro-CT测试,观察内部损伤形貌。
1.3 有限元模型
1.3.1 复合材料损伤准则
Hashin失效准则被广泛应用于分析预测复合材料损伤破坏[19],但复合材料在损伤过程中冲击点附近的应力变化剧烈,应力形式的Hashin失效准则无法准确描述材料破坏过程,而应变在冲击过程中的变化较为平缓,更适合作为复合材料的失效判据。因此,本文采用基于应变形式的Hashin失效准则[20],具体表达式如下:
纤维拉伸断裂 (
ε11⩾ ):{e}_{\rm{f}}^{\rm{t}}={\left(\frac{{\varepsilon }_{11}}{{\varepsilon }_{11}^{\rm{T}}}\right)}^{2}+{\left(\frac{{\varepsilon }_{12}}{{\tau }_{12}}\right)}^{2}+{\left(\frac{{\varepsilon }_{13}}{{\tau }_{13}}\right)}^{2}\geqslant 1 (1) 纤维压缩断裂 (
{\varepsilon }_{11}\leqslant 0 ):{e}_{\rm{f}}^{\rm{c}}={\left(\frac{{\varepsilon }_{11}}{{\varepsilon }_{11}^{\rm{C}}}\right)}^{2}\geqslant 1 (2) 基体拉伸断裂(
{\varepsilon }_{22}\geqslant 0 ):{e}_{\rm{m}}^{\rm{t}}={\left(\frac{{\varepsilon }_{22}}{{\varepsilon }_{22}^{\rm{T}}}\right)}^{2}+{\left(\frac{{\varepsilon }_{12}}{{\tau }_{12}}\right)}^{2}+{\left(\frac{{\varepsilon }_{23}}{{\tau }_{23}}\right)}^{2}\geqslant 1 (3) 基体压缩断裂(
{\varepsilon }_{22}\leqslant 0 ):\begin{split} e_{\rm{m}}^{\rm{c}} = &{\left( {\frac{{{E_{22}}{\varepsilon _{22}}}}{{2{G_{12}}{\gamma _{12}}}}} \right)^2} + {\left( {\frac{{{\varepsilon _{22}}}}{{\varepsilon _{22}^{\rm{T}}}}} \right)^2}\left[ {\left( {{{\left( {\frac{{{E_{22}}\varepsilon _{22}^{\rm{C}}}}{{2{G_{12}}{\tau _{12}}}}} \right)}^2} - 1} \right)} \right] + \\& {\left( {\frac{{{\varepsilon _{12}}}}{{{\tau _{12}}}}} \right)^2} + {\left( {\frac{{{\varepsilon _{13}}}}{{{\tau _{13}}}}} \right)^2} \geqslant 1 \end{split} (4) 式中:E和G分别为材料的杨氏模量和剪切模量;
{\varepsilon }_{11} 和{\varepsilon }_{22} 为单元材料主方向的应变分量;{\varepsilon }_{12} 、{\varepsilon }_{13} 和{\varepsilon }_{23} 为单元材料主方向的剪切应变分量;{\varepsilon }_{{{i}}{{i}}}^{\rm{T}} 和{\varepsilon }_{{{i}}{{i}}}^{\rm{C}} 分别为i 方向对应的拉伸和压缩强度的失效应变;{\tau }_{{{i}}{{j}}} 为单元剪切强度对应的剪切失效应变。当某单元内应变分量满足上述某一条件时,即认为该单元发生损伤。失效应变与材料强度之间的关系如下:\begin{split} &{X_{\rm{T}}} = {E_{11}}\varepsilon _{11}^{\rm{T}},{X_{\rm{C}}} = {E_{11}}\varepsilon _{11}^{\rm{C}},{Y_{\rm{T}}} = {E_{22}}\varepsilon _{22}^{\rm{T}},\\ &{Y_{\rm{C}}} = {E_{22}}\varepsilon _{22}^{\rm{C}},{Z_{\rm{T}}} = {E_{33}}\varepsilon _{33}^{\rm{T}},{S_{12}} = {G_{12}}{\tau _{12}},\\ &{S_{13}} = {G_{13}}{\tau _{13}},{S_{23}} = {G_{23}}{\tau _{23}} \end{split} (5) 式中,
{X}_{\rm{T}} 、X_{\rm{C}} 、Y_{\rm{T}} 、Y_{\rm{C}} 、{S}_{{{i}}{{j}}} 分别为层合板轴向拉伸、轴向压缩、横向拉伸、横向压缩和各方向的剪切强度。相关材料属性如表3和表4所示。表 3 用于数值模拟的CF/2511-1A/BS环氧树脂复合材料和GF/2511-1A/BS环氧树脂复合材料弹性参数Table 3. Elastic parameters of CF/2511-1A/BS epoxy composites and GF/2511-1A/BS epoxy composites used in numerical simulationMaterial E11/GPa E22=E33/GPa G12=G13/GPa G23/GPa {\mu }_{12} {\mu }_{13} {\mu }_{23} {G}_{{\rm{f}}}/(kJ·m−2) {G}_{{\rm{m}}}/(kJ·m−2) CF/epoxy 110 8.3 4.6 3.4 0.303 0.303 0.38 80 1 GF/epoxy 40 8.4 4.3 3.2 0.315 0.315 0.39 65 1 Notes: E11, E22, E33—Elastic modulus (direction 11, 22, 33); G12, G13, G23—Shear modulus (direction 12, 13, 23); {\mu }_{12} , {\mu }_{13} , {\mu }_{23} —Poisson’s ratio (direction 12, 13 and 23). 表 4 用于数值模拟的CF/2511-1A/BS环氧树脂复合材料和GF/2511-1A/BS环氧树脂复合材料的强度参数Table 4. Strength parameters of CF/2511-1A/BS epoxy composites and GF/2511-1A/BS epoxy composites used in numerical simulationMPa Material XT XC YT=ZT YC=ZC S12=S13 S23 CF/epoxy 1600 640 48 150 80 60 GF/epoxy 860 550 48 140 65 60 单元产生损伤后需进行材料性能退化,即刚度折减,本文引入指数形式的损伤状态变量d定义渐进损伤刚度折减方案。相比于参数型损伤变量将材料宏观属性直接折减,指数型损伤变量更接近实际情况且损伤过程连续[21]。将Hashin准则中的失效因子(
{e}_{\rm{f}} 、{e}_{\rm{m}} )与损伤变量({d}_{\rm{f}} 、{d}_{\rm{m}} )相关联,具体形式如下:[22],\begin{array}{l} {d_{\rm{f}}} = 1 - \dfrac{1}{{\sqrt {{e_{\rm{f}}}} }}{{\rm{e}}^{\left( { - {E_{11}}{{\left( {\varepsilon _{11}^{{{{\rm{f}}i}}}} \right)}^2}\left( {\sqrt {{e_{\rm{f}}}} - 1} \right){L^{\rm{c}}}/{G_{\rm{f}}}} \right)}}\\ {d_{\rm{m}}} = 1 - \dfrac{1}{{\sqrt {{e_{\rm{m}}}} }}{{\rm{e}}^{\left( { - {E_{22}}{{\left( {\varepsilon _{22}^{{{{\rm{m}}i}}}} \right)}^2}\left( {\sqrt {{e_{\rm{m}}}} - 1} \right){L^{\rm{c}}}/{G_{\rm{f}}}} \right)}} \end{array} (6) 式中:
{G}_{\rm{f}} 和{G}_{\rm{m}} 分别为材料纤维纵向和横向断裂韧性;{L}^{\rm{c}} 为单元特征长度,加入{L}^{\rm{c}} 可降低网格密度对结果精度的影响;i 根据单元受拉或受压分别赋值为T 或C 。本文采用的刚度退化方案如下:\begin{split} &{C}_{11}^{{\rm{d}}}=\left(1-{d}_{\rm{f}}\right){C}_{11} \\ &{C}_{22}^{{\rm{d}}}=\left(1-{d}_{\rm{f}}\right){\left(1-{d}_{\rm{m}}\right)C}_{22} \\ &{C}_{33}^{{\rm{d}}}=\left(1-{d}_{\rm{f}}\right){\left(1-{d}_{\rm{m}}\right)C}_{33} \\ &{C}_{12}^{{\rm{d}}}=\left(1-{d}_{\rm{f}}\right){\left(1-{d}_{\rm{m}}\right)C}_{12} \\ &{C}_{23}^{{\rm{d}}}=\left(1-{d}_{\rm{f}}\right){\left(1-{d}_{\rm{m}}\right)C}_{23} \\ &{C}_{13}^{{\rm{d}}}=\left(1-{d}_{\rm{f}}\right){\left(1-{d}_{\rm{m}}\right)C}_{13} \end{split} (7) 采用双线性内聚力单元(Cohesive element)模拟相邻子层界面的分层损伤[23]。采用二次应力损伤准则(Quads)定义损伤起始,损伤演化采用B-K (Benzeggaph-Kenane)准则,分别如下:
{\left(\frac{{t}_{{\rm{n}}}}{N}\right)}^{2}+{\left(\frac{{t}_{{\rm{s}}}}{S}\right)}^{2}+{\left(\frac{{t}_{\rm{t}}}{T}\right)}^{2}=1 (8) {G}^{\rm{C}}\geqslant {G}_{{\rm{n}}}^{\rm{C}}+\left({G}_{{\rm{S}}}^{\rm{C}}-{G}_{{\rm{n}}}^{\rm{C}}\right){\left(\frac{{G}_{{\rm{s}}}}{{G}_{\rm{T}}}\right)}^{\eta } (9) 式中:
{t}_{{\rm{n}}} 、{t}_{{\rm{s}}} 、{t}_{\rm{t}} 分别为界面法向应力和两个剪切应力;N、S、T对应界面法向和两个剪切强度;{G}_{{\rm{n}}}^{\rm{C}} 和{G}_{{\rm{S}}}^{\rm{C}} 分别为法向和切向临界应变能释放率;{G}_{{\rm{s}}} 和{G}_{{\rm{n}}} 分别为{t}_{{\rm{s}}} 、{t}_{{\rm{n}}} 对应的能量释放率。相关材料属性如表5所示。表 5 CF/2511-1A/BS环氧树脂复合材料和GF/2511-1A/BS环氧树脂复合材料的层间界面参数Table 5. Material properties of interface cohesive elements for CF/2511-1A/BS epoxy composites and GF/2511-1A/BS epoxy compositesρ/(kg·m−3) kN/(GPa·mm−1) kS=kT/(GPa·mm−1) N/MPa S=T/MPa G_{\rm{n}}^{\rm{C}}/(J·m−2) G_{\rm{S}}^{\rm{C}}/(J·m−2) η 1200 15 1.2 30 60 0.28 0.8 1.5 1.3.2 层间/层内数值模拟模型
图2为层间及夹芯混杂复合材料低速冲击模型,层合板采用沙漏增强模式的减缩积分单元C3D8R,共8层,单层尺寸为150 mm×100 mm×0.75 mm,定义X轴为纤维方向,Y轴为基体方向,通过改变每层材料属性实现相应的混杂结构。界面为0厚度的COH3D8内聚力单元,共7层。夹具是内径为120 mm、宽为10 mm的圆环,冲头是直径为12.6 mm的半球形锤头,夹具和冲头定义为刚体,采用R3D4单元。细化层合板冲击区域网格,提高模拟精准度。
层内混杂复合材料采用纱线尺度模型,层合板、夹具和冲头尺寸与层间模型一致,如图3(a)所示。模型假设树脂对纤维完全浸润,复合材料中纱线系统以纤维束增强树脂的复合形式存在,纤维与树脂为一整体[24],单根纱线横截面尺寸为5 mm×0.75 mm,如图3(b)所示,通过改变每根纤维束材料属性,实现CN-1及CN-2混杂结构,复合材料的基体部分由布尔运算得到,如图3(c)所示。采用金相显微镜观察纤维分布情况如图4(a)和图4(b)所示,测得CF体积分数为68vol%,GF体积分数为70vol%,在ABAQUS中建立四边形RUC (Representative unit cell)单元[25],如图4(c)所示,计算纤维束增强树脂弹性参数,相关属性如表6和表7所示。
表 6 CF-GF层内混杂增强环氧树脂复合材料模型中CF、GF和2511-1A/BS环氧树脂的材料参数Table 6. Material paramenters of CF, GF and 2511-1A/BS epoxy for CF-GF intralayer hybrid reinforced epoxy composite modelMaterial E11/GPa E22=E33/GPa G12=G13/GPa G23/GPa {\mu }_{12} {\mu }_{13} {\mu }_{23} \rho /(kg·m−3) CF 234 20 9.2 7.4 0.3 0.3 0.34 1.77 GF 78.7 7 4.0 2.5 0.3 0.3 0.4 2.54 2511-1A/BS epoxy 3.1 — — — 0.3 — — 1.13 表 7 CF-GF层内混杂增强环氧树脂复合材料模型中CF和GF纤维束的力学性能Table 7. Mechanical properties of CF and GF fiber bundle for CF-GF intralayer hybrid reinforced epoxy composite modelMaterial E11/GPa E22=E33/GPa G12=G13/GPa G23/GPa μ12 μ13 μ23 CF fiber bundle 153 8.70 3.60 3.20 0.3 0.3 0.36 GF fiber bundle 52 5.27 2.48 1.88 0.3 0.3 0.40 2. 结果与讨论
2.1 CF-GF混杂增强环氧树脂复合材料低速冲击实验结果
极限载荷和吸收能量是表征低速冲击性能的主要数据。图5为CF-GF混杂增强环氧树脂复合材料的极限载荷和吸收能量。可以发现,不同能量下CF-GF混杂增强环氧树脂复合材料的极限载荷不同,CF-GF混杂增强环氧树脂复合材料50 J冲击能量的极限载荷较30 J冲击能量下更高,这是由于在同样配比下,较大的冲击能量对应更快的冲击速度,撞击时的应变率效应更明显[26]。其中,G结构抗冲击性能最弱,S-C结构吸收能量较大,I-C和CN-1结构极限载荷较高。随着能量的增加,极限载荷和吸收能量均增大,混杂结构可提高极限载荷和吸收能量。冲击面为CF时,极限载荷和吸收能量均较高。I-C结构极限载荷最高,相较于G结构提高了24%。I-G和S-G结构的极限载荷相差不大,均较小。两种层内混杂结构呈现出较高的极限载荷,相较于G结构提高了20%左右。冲击面纤维种类相同时,夹芯(S-C和S-G)结构的吸收能量大于层间混杂(I-C和I-G)结构。相同混杂结构中,冲击面为CF时吸收能量较大,S-C结构的吸收能量较C结构和G结构分别提高了10%和16%。CN-2结构的吸收能量大于CN-1结构,与面内混杂界面的分布有关系[27]。
2.2 CF-GF混杂增强环氧树脂复合材料低速冲击有限元结果
2.2.1 实验与数值模拟结果对比
图6为30 J能量冲击下C结构、I-C结构、S-G结构和CN-1结构的CF-GF混杂增强环氧树脂复合材料时间-载荷和时间-能量曲线。可知,极限载荷模拟结果略大于实验值,能量模拟数据略低。实验曲线随时间变化趋势与模拟曲线结果拟合度较高。
2.2.2 低速冲击损伤形态及破坏机制
图7为50 J能量冲击后CF-GF混杂增强环氧树脂复合材料C扫描及分层失效模拟(SDEG)结果。图8为50 J能量冲击后CF-GF混杂增强环氧树脂复合材料的分层损伤面积。图9为CF-GF混杂增强环氧树脂复合材料目视及模拟冲击面纤维损伤结果(SDV1)。图10为CF-GF混杂增强环氧树脂复合材料冲击背面基体目视损伤及数值模拟结果(SDV2)。可知,模拟结果与实验损伤形貌较一致,冲击面为CF (C、I-C、S-C)时,分层面积为菱形,纤维损伤沿Y方向,I-C结构背部基体损伤较少,S-C结构背部基体损伤区域较大且分层面积最大。冲击面为GF (G、I-G、S-G)时,分层面积接近椭圆形,纤维损伤集中于冲击点下方,冲击面损伤呈圆圈状,冲击背面表现为菱形损伤,I-G结构与S-G结构背面基体破坏形貌类似,I-G结构分层面积最小。层内混杂结构的冲击面纤维损伤形貌与C结构类似,但面内损伤传播连续性较低,CF纤维束内SDV1损伤较GF纤维束集中,且沿X轴有横向扩散,在远离冲击点处仍可观察到部分CF纤维束内SDV1失效,而相邻GF纤维束未出现破坏。SDV2损伤由冲击点正下方向四周扩散,经混杂界面后损伤程度明显降低,大多数损伤发生在CF纤维束内,说明CF纤维束对相邻的GF纤维束有保护作用。CN-1结构的纤维损伤较CN-2结构低,纤维束内基体损伤较CN-2高,混杂界面对面内损伤的抑制作用在CN-1结构中更明显。层内混杂结构的树脂损伤由冲击点向下扩展,应力分布呈菱形,底部断裂沿Y方向。30 J能量冲击后损伤形貌与其类似,不再赘述。
前期已对30 J能量冲击下C结构、G结构、I-C结构和S-C结构的CF-GF混杂增强环氧树脂复合材料内部损伤进行了分析[28]。图11为30 J能量冲击后I-G、S-G、CN-1、CN-2的CF-GF混杂增强环氧树脂复合材料Micro-CT扫描横截面损伤。可知,冲击方向沿Z轴向下,白色区域为GF层,黑色区域为CF层。I-G结构和S-G结构冲击区域形变不明显,这是由于GF韧性大,变形恢复性好。I-G结构冲击点附近及内部GF层出现纤维断裂损伤,CF层未观察到破坏。S-G结构上部及内部界面有分层损伤,冲击面GF纵向脆断明显,内部CF层损伤较少。CN-1结构中纤维损伤由冲击点向下传播,底部有少量分层。CN-2结构中冲击面GF受剪切力,损伤范围广,复合材料底部观察到分层损伤。
图12为不同时刻I-C和CN-1的CF-GF混杂增强环氧树脂复合材料纤维损伤(SDV1)情况。图13为不同时刻I-C和CN-1的CF-GF混杂增强环氧树脂复合材料基体损伤(SDV2)情况。可知,I-C结构显示横截面损伤,CN-1结构显示冲击面纤维损伤及冲击背面基体损伤。纤维损伤程度小于基体损伤,基体损伤首先出现在冲击点周围及复合材料背部,冲头达到最低位置时,底部界面有明显分层破坏。I-C结构的失效过程与CN-1结构有区别,I-C结构中纤维损伤主要集中在冲击面CF层及内部GF层,CN-1结构中CF束首先出现破坏,损伤方向沿Y轴,大多数损伤被限制在混杂界面之间。随着冲头下移,纤维损伤沿X轴增加,且GF纤维束内开始出现纤维破坏,CF纤维束中基体破坏呈三角形,沿X轴损伤严重,GF纤维束内损伤较少,部分CF纤维束在远离冲击点区域仍有损伤,而临近的GF纤维束内基体损伤较少。
3. 结 论
(1)相同混杂比条件下,碳纤维-玻璃纤维层间混杂增强环氧树脂复合材料抗冲击性能较好,I-C结构极限载荷较大,S-C结构吸收能量较多。I-G结构和S-G结构对冲击响应区别不明显,抗冲击性能差别不大。CN-1结构比CN-2结构具有更高的极限载荷,CN-2结构的损伤容限较高。
(2)混杂结构可有效降低冲击损伤,I-C结构和CN-1结构内部的纤维及基体损伤程度较其他混杂结构低,夹芯(S-C、S-G)结构表现出明显的分层损伤,其中S-C结构和CN-2结构分层损伤面积较大。CN-1结构的冲击面纤维损伤较CN-2结构轻,冲击背面基体损伤较CN-2结构严重。
(3)层间混杂结构抗冲击性能受混杂界面数量影响,其中玻璃纤维层形变恢复性高而损伤较大,碳纤维层形变量小且损伤较低。层内混杂结构面内损伤具有取向性,面内混杂界面对抑制损伤传播具有积极作用,损伤扩展连续性较层间混杂结构低,应力在碳纤维束内传播速度快且广,碳纤维束承担了大部分冲击损伤,临近玻璃纤维束内损伤较小。
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图 9 不同MZF含量同轴环的电磁参数:在2~18 GHz频率范围内复介电常数的实部(a)和虚部(b);复磁导率的实部(c)和虚部(d);介电损耗角正切(e)和磁损耗角正切(f)
Figure 9. Electromagnetic parameters of coaxial rings with different MZF contents: Real (a) and imaginary (b) part of the complex permittivity; Real (c) and imaginary (d) part of the complex permeability; Tangent dielectric loss (e) and tangent magnetic loss (f) in the frequency range of 2-18 GHz
11 10%MZF/PLA (a)、20%MZF/PLA (b)、30%MZF/PLA (c)、40%MZF/PLA (d)和50%MZF/PLA (e)的反射损耗三维图和吸波曲线
{R_{{{\rm{L}}_{{\rm{min}}}}}} —Minimum reflection loss; EAB—Effective absorption bandwidth
11. 3D maps of reflection loss and microwave absorption curves of 10%MZF/PLA (a), 20%MZF/PLA (b), 30%MZF/PLA (c), 40%MZF/PLA (d) and 50%MZF/PLA (e)
表 1 MZF/聚乳酸(PLA)复合材料的组分
Table 1 Component of MZF/polylactic acid (PLA) composites
Sample number Mass fraction/wt% PLA MZF Pure PLA 100 0 10%MZF/PLA 90 10 20%MZF/PLA 80 20 30%MZF/PLA 70 30 40%MZF/PLA 60 40 50%MZF/PLA 50 50 表 2 MZF/PLA复合粉末DSC曲线对应的数据
Table 2 DSC data of MZF/PLA composite powders
Sample number Tm/℃ Tc/℃ Tg/℃ Pure PLA 114.07 97.71 80.81 10%MZF/PLA 110.85 97.71 81.17 20%MZF/PLA 112.17 97.98 81.89 30%MZF/PLA 110.86 97.88 81.62 40%MZF/PLA 111.12 97.71 81.35 50%MZF/PLA 111.11 98.52 81.89 Notes: Tm—Melting temperature; Tc—Crystallization temperature; Tg—Glass transition temperature. 表 3 不同MZF含量的MZF/PLA复合材料的拉伸强度和断裂延伸率
Table 3 Tensile strength and elongation at break of MZF/PLA composites with different MZF contents
Sample
numberTensile strength/MPa Elongation at break/% Pure PLA 34.40 26.12 10%MZF/PLA 40.40 21.76 20%MZF/PLA 35.60 20.56 30%MZF/PLA 25.42 15.28 40%MZF/PLA 16.07 8.93 50%MZF/PLA 14.22 6.68 -
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