Effect of porosity defects on crack initiation and propagation behavior in SiC/AZ91D composites
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摘要: 采用内聚力模型及有限元分析方法,在含真实形貌SiC颗粒增强AZ91D镁基复合材料中引入孔隙缺陷,分析不同孔隙率及孔隙形状在单轴拉伸过程中对SiC/AZ91D复合材料力学行为的影响。结果表明:孔隙长径比为1时,孔隙率为0%、0.5%、1.0%、1.5%的复合材料的抗拉强度分别为351.214 MPa、339.452 MPa、325.735 MPa、306.791 MPa,抗拉强度随孔隙率的增加逐渐降低,复合材料中裂纹萌生和裂纹扩展时间均随孔隙率增加而提前。孔隙长径比越大,其尖端部位应力集中越严重,复合材料抗拉强度也越低。无孔隙缺陷的SiC/AZ91D复合材料裂纹萌生扩展机制是颗粒与基体交界处萌生微裂纹,微裂纹相互连接形成主裂纹绕开颗粒进行扩展致使材料断裂,含孔隙的SiC/AZ91D复合材料裂纹萌生扩展机制为微裂纹在孔隙周围萌生,与颗粒和基体交界处产生的微裂纹相互连接,汇集成主裂纹绕开颗粒扩展使材料断裂。Abstract: Using the finite element analysis method, this study introduced porosity defects into SiC/AZ91D magnesium matrix composites with realistic SiC particle morphology, and analyzed the influence of different porosity rates and shapes on the mechanical behavior of SiC/AZ91D composites during uniaxial tensile process. The results show that when the aspect ratio of the pore length to width is 1, the tensile strengths of the composites with void contents of 0%, 0.5%, 1.0%, and 1.5% are 351.214 MPa, 339.452 MPa, 325.735 MPa and 306.791 MPa, respectively. The tensile strength gradually decreases with the increase of porosity rate, and the initiation and propagation time of cracks in the composite material advances with the increase of porosity rate. As the aspect ratio of the pore length to width increases, the stress concentration at the tip of the pore becomes more severe, resulting in lower tensile strength of the composite material. The crack initiation and propagation mechanism in the SiC/AZ91D composite material without porosity defects involves the initiation of microcracks at the particle-matrix interface, followed by their interconnection to form a main crack, which propagates around the particles leading to material fracture. In the case of SiC/AZ91D composites with porosity, microcracks initiate around the pores and interconnect with microcracks generates at the particle-matrix interface, ultimately converging into a main crack that propagates around the particles, causing material fracture.
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Key words:
- SiC/AZ91D composites /
- crack initiation /
- crack extension /
- finite element analysis /
- porosity /
- cohesive model
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图 6 双线性内聚力模型
Figure 6. Bilinear cohesive zone model
$ {\delta }_{\mathrm{m}}^{\text{max}} $—Maximum value of the effective displacement; $ {\delta }_{\mathrm{m}}^{\mathrm{f}} $—Effective displacement at complete failure; $ {\delta }_{\mathrm{m}}^{0} $—Effective displacement at the initiation of damage; $ {\tau }_{\mathrm{m}}^{0} $—Maximum separation stress; K—Elasticity coefficient or spring constant; D—Damage amount
表 1 排水法测量的SiC/AZ91D复合材料孔隙率(VC)
Table 1. Void content (VC) measurement for SiC/AZ91D composite material using drainage method
Sample serial number VC/% 1 1.56 2 0.84 3 0.85 4 1.62 5 0.83 6 0.41 7 1.65 8 0.94 9 0.48 10 1.18 表 2 AZ91D镁合金和SiC颗粒的基本参数
Table 2. Basic parameters of AZ91D magnesium alloy and SiC particles
Material $ \rho $/(kg·m−3) E/GPa $ \mu $ σb/MPa AZ91D 1800 45 0.33 164 SiC 3215 450 0.17 2000 Notes: $ \rho $—Material density; E—Modulus of elasticity; $ \mu $—Poisson's ratio; σb—Tensile strength. 表 3 SiC/AZ91D颗粒-界面的本构模型参数
Table 3. Constitutive model parameters of SiC/AZ91D particle-interface
$ {t}_{\mathrm{n}}/\mathrm{M}\mathrm{P}\mathrm{a} $ $ {t}_{\mathrm{t}}/\mathrm{M}\mathrm{P}\mathrm{a} $ $ {\delta }_{\mathrm{m}\mathrm{a}\mathrm{x}}/\mathrm{m}\mathrm{m} $ $ {\delta }_{\mathrm{f}}/\mathrm{m}\mathrm{m} $ 400 400 0.00015 0.00005 Notes: $ {t}_{\mathrm{n}} $—Interface normal nominal stress; $ {t}_{\mathrm{t}} $—Interfacial tangential nominal stress; $ {\delta }_{\mathrm{m}\mathrm{a}\mathrm{x}} $—Destruction displacement; $ {\delta }_{\mathrm{f}} $—Material complete failure separation. 表 4 AZ91D镁合金的Johnson-Cook (J-C)本构参数
Table 4. Johnson-Cook (J-C) constitutive model parameters for AZ9ID magnesium alloy
A/MPa B/MPa n C ${u}_{{\rm{f}}}^{\mathrm{p}\mathrm{l} }/{\rm{mm} }$ 164 600 0.283 0.021 0.00015 Notes: A—Yield strength of AZ91D matrix under static load; B—Hardening constant; n—Hardening exponent; C—Strain rate constant; $ {u}_{\mathrm{f}}^{\mathrm{p}\mathrm{l}} $—Failure displacement. 表 5 SiC颗粒本构模型及失效参数
Table 5. Constitutive model and failure parameters of SiC particles
Parameter Value G/GPa 193 A0 0.96 B0 0.35 C0 0.009 M 1 N 0.65 T1/MPa 750 SFMAX/MPa 1300 LHE/MPa 11700 PHEL/MPa 5130 D1 0.48 D2 0.48 K1 220000 K2 361000 K3 0 Notes: G—Shear modulus; A0—Strength parameter before damage; B0—Strength parameter when damage occurs; C0—Strain rate constant; M—Pressure index when damage occurs; N—Pressure index when no damage occurs; T1—Cut-off pressure; SFMAX—Maximum fracture strength; LHE—Hugoniot elastic limit; PHEL—Hugoniot elastic limit pressure; D1, D2—Fracture constant; K1, K2, K3—Material parameter. -
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