Sensitivity of a disc rupture strength test to air bubble pores in phosphate-bonded investment materials at elevated temperatures.
OBJECTIVE
To investigate the sensitivity of a modified disc rupture test to variables affecting the strength of four phosphate-bonded investment materials under conditions closely resembling the in-service environment. This study examined the influence of air bubble pores on the strength of phosphate bonded investment materials at a temperature of 900 degrees C.


METHODS
A modified disc rupture configuration was used to test a circular investment diaphragm with clamped edges, in tension, initiated by bending. A sequentially-varying molten metal mass was used to apply the load using an electronic centrifugal casting machine. The staircase method was used to determine the increase or decrease of the applied load with the standardized increment of load being no greater than 1 or 2g. The mean load applied to 30 samples defined the transition of survival to failure of the investment disc and was taken as a measurement of investment strength. Two material handling techniques, one giving virtually no pores and one showing a wide pore size distribution were used.


RESULTS
The diaphragm clearly either failed or survived the test with the pattern of fracture indicating brittle failure. A statistically significant difference in strength (P<0.050) of diaphragms with and without air bubble pores was demonstrated for three materials. Fewer pores resulted in increased strength of those materials and produced a much reduced scatter of strength in the fourth.


CONCLUSIONS
The modified disc rupture test is sufficiently sensitive to identify variations in strength of phosphate bonded investment materials caused by differing pore size distributions.
Juszczyk
Andrzej S
AS
Division of Prosthetic Dentistry, GKT Dental Institute, King's College, London SE1 9RT, UK. andrzej.s.juszczyk@kcl.ac.uk
Radford
David R
DR
Curtis
Richard V
RV
D000388:Air; D000497:Alloys / Q000737:chemistry; D000704:Analysis of Variance; D003734:Dental Casting Investment / Q000737:chemistry*; D003735:Dental Casting Technique / Q000295:instrumentation; D004548:Elasticity; D004581:Electronics / Q000295:instrumentation; D006358:Hot Temperature; D006801:Humans; D016014:Linear Models; D008422:Materials Testing / Q000379:methods; D016011:Normal Distribution; D010710:Phosphates / Q000737:chemistry*; D018583:Pliability; D016062:Porosity; D011312:Pressure; D013314:Stress, Mechanical; D013499:Surface Properties; D013718:Tensile Strength; D016474:Weight-Bearing
D000497:Alloys; D003734:Dental Casting Investment; D010710:Phosphates