Study on Strength Evaluation of Ceramic/Metal Joints

对Si3N4/S45C和Si3N4接头光滑试件和预裂纹试件进行了四点弯曲试验。用有限元方法对焊接残余应力进行了分析研究。结果表明：(1)Si_3N_4/S45C接头的断裂强度降低，除了界面和过渡层的存在外，还有一个原因：(2)在连接后的机械切割过程中，固有缺陷容易产生裂纹；(3)在陶瓷界面附近存在裂纹和拉伸残余应力的情况下，强度急剧下降。应力强度因子可以用Si_3N_4的平面应变断裂韧性来评价接头的强度。(4)在靠近界面的陶瓷侧引入压痕预裂时，预裂纹会向界面弯曲，因此压痕方法不适合在界面附近引入预裂。(5)轴向…的结果对连接残馀应力进行了更多的计量分析，结果与三维分析结果吻合较好，两种平面应力分析结果的叠加可以近似估算堆积应力。(6)陶瓷/金属接头切割后残馀应力重新分布，峰值拉应力出现在陶瓷界面附近的新自由面。强调了切割后同一点处的残余应力有所下降，但最大值不变。(7)对Si3N4/S45C接头在切割前后用X射线法无损地测量了界面附近陶瓷界面的残余应力。本文所用的PSPC/RSF系统可以无损地测量界面附近的局部残余应力分布。切削过程中残余应力重新分布。相同位置的残余应力绝对值减小，但分布特征不变。将X射线测量结果与有限元分析结果进行了比较，定性吻合较好。(8)陶瓷和镶件材料的弹塑性特性表征了界面附近陶瓷残余应力的奇异性。(9)考虑了界面处陶瓷残余应力和外加应力的奇异性应力场，解释了Si3N4/S45C接头断裂强度比Si3N4/Si3N4接头的断裂强度降低的原因。较少

Four-point bend tests were conducted for Si_3N_4/S45C and Si_3N_4 joint smooth and precracked specimens. Analytical studies on joining residual stresses were also performed by the FEM method. The results obtained are as follows :(1) There is a reason for the decrease of the fracture strength in the Si_3N_4/S45C joint additional to the existence of the interface and interlayer.(2) Cracks are easily initiated from the inherent defect during mechanical cutting after joining.(3) In the case that there is a crack and tensile residual stresses at the ceramic near the interface, the strength decreases drastically. The strength of the joint can be evaluated by the stress intensity factor using the plane strain fracture toughness of Si_3N_4.(4) In the case that an indentation precrack is introduced at the ceramic side near the interface, the precrack is kinked toward the interface, so, the indentation method is not suitable to introduce a precrack near the interface.(5) The result of the axi-sy … More mmetric analysis on the joining residual stresses shows a good agreement with the results of the three dimensional analysis and the pack stress can be approximately estimated by superposing results of the two plane stress analysis.(6) The residual stresses redistribute after cutting of the ceramic/metal joint and the peak tensile stress occurs on the new free surface at the ceramic near the interface. It is emphasized that the residual stress at a same point decreases after cutting, but the maximum value does not change. The effects is almost the same as that of the changing the size and aspect ratio of the joint specimen.(7) Residual stresses at the ceramic near the interface were measured nondestructively by the X-ray method for the Si_3N_4/S45C joints before and after cuttings. The local residual stress distribution near the interface can be measured nondestructively by the PSPC/RSF system used in this study. The residual stresses redistribute by the cutting. The distribution characteristics do not change, although the absolute value of the residual stresses at the same position decreases. The comparison between the results of the X-ray measurement and the result of the FEM analysis shows a good agreement qualitatively.(8) The singularity of residual stresses at the ceramic near the interface is characterized by elastic-plastic properties of ceramic and insert materials. The exponent of the elastic-plastic singularity is larger than that of elastic singularity.(9) The decrease in the fracture strength for the Si_3N_4/S45C joints compared with those for the Si_3N_4 /Si_3N_4 joints is explained by considering the singular stress field of residual and applied stresses at the ceramic near the interface of the joint. Less