Adsorption and Kinetic Effects on Crack Growth in MnZn Ferrites
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L.J.M.G. Dortmans TNO Institute of Applied Physics, P.O. Box 595, 5600 AN, Eindhoven, The Netherlands
G. de Withb) Laboratory of Solid State and Materials Chemistry, Eindhoven University of Technology, P.O. Box 513, 5600 MB, Eindhoven, The Netherlands (Received 21 September 1999; accepted 27 March 2000)
The variation of the fracture toughness of MnZn ferrite ceramics with varying loading rate and humidity was determined with the aid of the single edge notched beam (SENB) test. A strong decrease with increasing humidity and decreasing loading rate was observed. A model for subcritical crack growth incorporating kinetic and adsorption effects was formulated to analyze the data. The value of the adsorptioncontrolled fracture toughness was determined independently by double torsion experiments and agreed favorably with the values as determined from the SENB data using the model. The strength of the material was determined, and analysis showed a strength behavior similar to the fracture toughness behavior, as predicted by the model. The analysis presented can be used to assess the subcritical crack growth behavior using a limited number of SENB specimens. I. INTRODUCTION
Thanks to their ferrimagnetic properties, combined with a high electric resistivity, MnZn ferrites find many applications; for example, in power transformers and filters, which are used in many consumer, communications, and automotive products. For many of these applications, it is expected that in the near future the ferrites will be exposed to higher mechanical stresses. This is caused by an increase in power throughput, combined with miniaturization, leading to higher power losses relative to the size of the component; by automated printed circuit board mounting and soldering; or by mechanical vibrations as found in the growing field of automotive applications. This has quite recently brought about an interest in the mechanical reliability of ferrites. MnZn ferrites obey linear elastic fracture mechanics, controlled by defects generally caused by processing, machining, or handling steps. One of the important material parameters to be determined is the fracture toughness. It relates the size of the defects present and the resulting strength of a body. An important aspect of the mechanical behavior of MnZn ferrites is the influence humidity has on their strength, as shown in earlier work.1 The usual explanaa)
Present address: Philips Center for Manufacturing Technologies, P.O. Box 218, 5600 MD, Eindhoven, The Netherlands. b) Address all correspondence to this author. J. Mater. Res., Vol. 15, No. 6, Jun 2000
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tion for this phenomenon is subcritical crack growth (SCG), whereby in the presence of a reactive species, a crack can grow at stresses below the critical value determined from the fracture toughness. In this paper, it will be shown that measurement of the fracture toughness of an SCG-susceptible material is possible, but that the analysis of the experiments has to account for subcritical
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