hello everyone,
I have tested a sample of bulk fill dental composite for mechanical features and the results are confusing. The compressive strength should be lower than the flexural strength in majority of materials including dental composites and it has been reported in previous researches. Testing conditions are mentioned below. have anyone faced this problem? does anyone know how to fix it?
3 point flexural test: (as mentioned in ISO4049)
Results: 114MPa
sample dimensions: 2*2*25mm
distance between fixtures: 20mm
cross head speed: 1mm/min
curing protocol: 9 points on each side, each 15s
compression test:
Results:80MPa
sample dimensions: cylinder d=4, h=6
cross head speed: 1mm/min
curing protocol:15s on each side, 15s on two points of lateral sides
When compared to flexural strength, compressive strength should be lower if the materials you choose are entirely homogeneous and independent of directional orthagolity; nevertheless, if the materials have a tendency to act more brittle, compressive strength should be higher. What exactly does the composite consist of?
There is no physical meaning in a higher bending strength of a brittle isomorphic material than in compression strength. The compressive strength simply has to be higher. Could it be that the cross section of the compressive specimen is larger than that of the flexural specimen and therefore the cylindrical specimen is not fully cured? In order to exclude the effect of unequal curing on the strength measurement, both types of specimens would have to be made from the same piece of pre-cured material.
please read
https://www.quora.com/Why-is-tensile-strength-smaller-than-flexural-strength
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