Flexural strength, also known as modulus of rupture, or bend strength, is a material property, defined as the stress in a material just before it yields in a flexure test.
When an object is formed of a single material (like a wooden beam or a steel rod) is bent, it experiences a range of stresses across its depth. At the edge of the object on the inside of the bend (concave face) the stress will be at its maximum compressive stress value.
At the outside of the bend (convex face) the stress will be at its maximum tensile value. These inner and outer edges of the beam or rod are known as the 'extreme fibres'. Most materials generally fail under tensile stress before they fail under compressive stress, so the maximum tensile stress value that can be sustained before the beam or rod fails is its flexural strength. So higher the flexural strength provides the highest resistance to cracking.
TAGLUS material has very high flexural modulus of 330GPa so it makes the best crack free aligners and retainers.