flexural strength
/ FLEK-shur-ul /
Flexural strength is how much bending a brittle bar can take before it snaps, measured by resting a small beam on two supports and pushing down in the middle until it breaks. We use a bend test rather than a pull test for ceramics for a very practical reason: a brittle sample is so sensitive to tiny flaws that gripping it in a tensile machine tends to crack it in the jaws before the real test even starts. Bending it is gentler and repeatable.
When you bend a beam, the top surface is squeezed (compression) and the bottom surface is stretched (tension), with a neutral line in the middle feeling nothing. Ceramics fail from the stretched face, so the flexural strength, also called the modulus of rupture, is the calculated tensile stress on that bottom surface at the instant of fracture. For a rectangular bar in three-point bending it is sigma = 3 F L / (2 b d^2), where F is the breaking force, L the span between supports, b the width and d the depth. A typical alumina reads about 300 to 400 MPa this way.
Treat that single number with care. Flexural strength is not a fixed material constant like Young's modulus; it depends on the specimen size, the surface finish, and above all on the largest flaw that happens to sit in the stretched region, so nominally identical bars scatter widely. Because only the thin outer layer sees peak tension, the measured modulus of rupture usually comes out higher than the true pure-tension strength. This is why ceramic strength is reported with Weibull statistics and a survival probability, not as one deterministic figure.
Three-point bend of a rectangular bar: sigma = 3 F L / (2 b d^2). A dense alumina typically breaks at a modulus of rupture of about 300 to 400 MPa.
Ceramics break from the stretched face; the bend test measures that tensile stress at fracture.
The modulus of rupture usually reads higher than the true tensile strength because only a thin surface layer sees the peak stress; it is not a size-independent constant, so always quote the specimen geometry.