Failure Analysis of Brazed Joints Using the CZM Approach

Abstract

Brazing, as a type of joining process, is widely used in manufacturing industries to join individual components of a structure. Structural reliability of a brazed assembly is strongly dependent on the joint mechanical properties. In the present work, mechanical reliability of low carbon steel brazed joints with copper filler metal is investigated and a methodology for failure analysis of brazed joints using the cohesive zone model (CZM) is presented. Mechanical reliability of the brazed joints is characterized by strength and toughness. Uniaxial and biaxial strengths of the joints are evaluated experimentally and estimated by finite element method using the ABAQUS software. Microstructural analysis of the joint fracture surfaces reveals different failure mechanisms of dimple rupture and dendritic failure. Resistance of the brazed joints against crack propagation, evaluated by the single-parameter fracture toughness criterion, shows dependency on the specimen geometry and loading configuration. Fracture of the brazed joints and the subsequent ductile tearing process are investigated using a two-parameter CZM. The characterizing model parameters of the cohesive strength and cohesive energy are identified by a four-point bend fracture test accompanied with corresponding FE simulation. Using the characterized CZM, the joint fracture behavior under tensile loading is well estimated. Predictability of the developed cohesive zone FE model for fracture analysis of brazed joints independent of geometry and loading configuration is validated. The developed cohesive zone FE model is extended to fatigue crack growth analysis in brazed joints. A cyclic damage evolution law is implemented into the cohesive zone constitutive model to irreversibly account for the joint stiffness degradation over the number of cycles. Fatigue failure behavior of the brazed joints is characterized by performing fully reversed strain controlled cyclic tests. The damage law parameters are calibrated based on the analytical solutions and the experimental fatigue crack growth data. The characterized irreversible CZM shows applicability to fatigue crack growth life prediction of brazed joints.