What is fracture toughness?
Fracture toughness – or crack resistance – is a measure that describes the ability of a material with a crack in it to resist further fracture. This measure, therefore, indicates the amount of force that needs to be applied to cause crack extension in, for example, a lithium disilicate glass-ceramic or zirconium oxide. In other words:
The higher the value, the lower will be the probability of the crack spreading in a material.
A high value, therefore, is an indicator of superior clinical long-term performance – and consequently a longer lasting material.
That is why fracture toughness is a clinically significant value. It is expressed as MPa· m1/2.
A comparison of fracture toughness
Titanium alloys and steel show very high values in terms of their fracture toughness. They reach values of more than 50 MPa· m1/2. In comparison, concrete with which houses and bridges are built has a fracture toughness of only 0.2-1.4 MPa· m1/2. This means that the material is quite brittle. Therefore, it is generally reinforced with steel.
Zirconium oxide is the dental ceramic that shows the greatest fracture toughness. Nevertheless, enormous differences are found within this group of materials. For example, Vsmile HT Zirconia Disc has a fracture toughness of 5 MPa · m1/2, while other zirconium oxide materials show values in the region of 2-4 MPa · m1/2.
Interestingly enough, opaque zirconium oxide exhibits higher fracture toughness values than translucent zirconium oxide. Vsmile HT zirconia Disc and Vsmile ST zirconia Disc demonstrate relatively high fracture toughness.
Important: Don't compare apples with oranges!
Please note: As is the case with the related topic of flexural strength, comparisons of the fracture toughness of materials are only meaningful if the same measuring method is used to test all the specimens. When different measuring methods are used to evaluate various specimens, the results will not be comparable.
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