Thermal stresses in metal-ceramic specimens for the ISO crack initiation test (three-point flexure bond test).
OBJECTIVE
At the October 1996 meeting of the ISO/TC106/SC 2/WG 1 working group, a special three-point flexure test for the characterization of the metal-ceramic bond was incorporated in the standard (ISO CD 9693). Due to the fabrication process, like real porcelain-fused-to-metal (PFM) restorations, the specimens contain thermal (eigen-) stresses upon which the load stresses are superimposed in the actual test. This study is devoted to the determination of these residual thermal stresses.


METHODS
The residual thermal stresses in the specimen were calculated with the aid of the finite element method (FEM) using an especially fine mesh in the vicinity of the edge of initial debonding. Young's modulus, EM, of the alloy was varied within the interval 80 GPa < or = EM < or = 220 GPa which covers the spectrum of dental alloys. The analysis also allows the calculation of thermal stresses as a function of the difference delta alpha = alpha M - alpha C of the coefficients of thermal expansion of alloy and ceramic and the glass transition temperature theta G of porcelain.


RESULTS
The thermal shear and normal stresses at the bond interface concentrate at the end of the ceramic veneer and practically vanish over about three quarters of the central part of the layer. The larger the Young's modulus, EM, of the alloy, the higher both stresses.


CONCLUSIONS
The results permit a deeper comprehension of the debonding process in the test: shear stress induced by loading increases the overall shear stress at the end of the bond interface, whereas load tensile stress is buffered by thermal compressive stress.
Lenz
J
J
Institut für Theoretische Mechanik, Universität Karlsruhe (TH), Germany.
Kessel
S
S
D001840:Dental Bonding; D019232:Dental Restoration Failure; D003799:Dental Stress Analysis; D004548:Elasticity; D020342:Finite Element Analysis; D006358:Hot Temperature; D008422:Materials Testing / Q000379:methods*; D016876:Metal Ceramic Alloys / Q000737:chemistry* / Q000592:standards; D018583:Pliability; D012015:Reference Standards; D015203:Reproducibility of Results; D013314:Stress, Mechanical
D016876:Metal Ceramic Alloys