TWI Industrial Member Report Summary 852/2006
By P Bastid and P Jackson
In many applications involving high temperature processes (gas turbines, aero-engines, automotive engines, chemical plants), greater efficiency can be achieved by increasing the temperature. However, the mechanical and environmental properties of current metal alloys limit the maximum process temperature to about 700°C. Ceramic materials have satisfactory properties above this value, providing they do not experience excessive tensile stress, since they have relatively low fracture resistance. Increased process efficiency might be obtained using components with ceramic materials, where the temperature is high and applied stress is low, joined to metallic materials, where the temperature is lower and the stress is higher.
Ceramic and metal parts can be joined by using a brazing process at high temperature. Due to the mismatch in coefficients of thermal expansion, high residual stresses are generated during the cooling stage of the brazing cycle. Very high stresses, increasing as braze thickness decreases, build up at the edge of the joint and may cause the ceramic to fracture. The probability of fracture is known to increase as the size of the bonded surface becomes larger. However, conventional stress analysis of the manufacture of joints does not show a clear correlation between the observed failure and the predicted stress.
In this work, a Weibull statistical failure model is used to calculate the probability of fracture of the ceramic part from the results of finite element stress analysis of the brazing cycle and experimental calibration. Previous studies have shown that care must be taken regarding singularities, but that more reliable failure prediction can be obtained that is less dependant on mesh refinement than conventional stress analysis. The work described below investigates the reliability of ceramic/metal joints of different sizes and materials.
- To examine the feasibility of the applying finite element analysis and a Weibull failure model to determine the probability of failure of ceramic/metal joints.
- To apply finite element analysis with the Weibull model to predict the probability of failure of ceramic/metal joints of simple cross section from stresses produced during manufacturing cycle of brazing.
- To compare the predictions with the results of a series of experimental manufactured joints of different