In materials science, fracture toughness is the critical stress intensity factor of a sharp crack where propagation of the crack suddenly becomes rapid and unlimited. It is a material property that quantifies its ability to resist crack propagation and failure under applied stress. A component's thickness affects the constraint conditions at the tip of a crack with thin components having plane stress conditions, leading to ductile behavior and thick components having plane strain conditions, where the constraint increases, leading to brittle failure. Plane strain conditions give the lowest fracture toughness value which is a material property. The critical value of stress intensity factor in mode I loading measured under plane strain conditions is known as the plane strain fracture toughness, denoted ${\displaystyle K_{\text{Ic}}}$. When a test fails to meet the thickness and other test requirements that are in place to ensure plane strain conditions, the fracture toughness value produced is given the designation ${\displaystyle K_{\text{c}}}$.

Slow self-sustaining crack propagation known as stress corrosion cracking, can occur in a corrosive environment above the threshold ${\displaystyle K_{\text{Iscc}}}$ (Stress Corrosion Cracking Threshold Stress Intensity Factor) and below ${\displaystyle K_{\text{Ic}}}$. Small increments of crack extension can also occur during fatigue crack growth, which after repeated loading cycles, can gradually grow a crack until final failure occurs by exceeding the fracture toughness.

Fracture toughness varies by approximately 4 orders of magnitude across materials. Metals hold the highest values of fracture toughness and ceramics holds the lowest. Cracks cannot easily propagate in tough materials, making metals highly resistant to cracking under stress and gives their stress–strain curve a large zone of plastic flow. Even though ceramics have a lower fracture toughness they show an exceptional improvement in the stress fracture that is attributed to their 1.5 orders of magnitude strength increase, relative to metals. The fracture toughness of composites, made by combining engineering ceramics with engineering polymers, greatly exceeds the individual fracture toughness of the constituent materials.^{[citation needed]}

Intrinsic toughening mechanisms are processes which act ahead of the crack tip to increase the material's toughness. These mechanisms operate at the atomic or microscopic level and are fundamental to the material itself, rather than being influenced by external factors. These will tend to be related to the structure and bonding of the base material, as well as microstructural features and additives to it. Examples of mechanisms include:

Any alteration to the base material which increases its ductility can also be thought of as intrinsic toughening.

The presence of grains in a material can also affect its toughness by affecting the way cracks propagate. In front of a crack, a plastic zone can be present as the material yields. Beyond that region, the material remains elastic. The conditions for fracture are the most favorable at the boundary between this plastic and elastic zone, and thus cracks often initiate by the cleavage of a grain at that location.

At low temperatures, where the material can become completely brittle, such as in a body-centered cubic (BCC) metal, the plastic zone shrinks away, and only the elastic zone exists. In this state, the crack will propagate by successive cleavage of the grains. At these low temperatures, the yield strength is high, but the fracture strain and crack tip radius of curvature are low, leading to a low toughness.

Additionally, the lack of plastic deformation at low temperatures results in minimal energy absorption before fracture, making the material highly susceptible to sudden and catastrophic failure. This brittle behavior is particularly critical in structural applications, where impact loading or stress concentrations can rapidly initiate fracture. The ductile-to-brittle transition temperature (DBTT) defines the temperature below which a material exhibits this brittle nature, and in BCC metals, it varies based on factors such as impurity content, grain size, and alloying elements. Engineering solutions, such as grain refinement or controlled heat treatments, are often employed to lower the DBTT and improve low-temperature toughness. Furthermore, external factors like strain rate and triaxial stress states can exacerbate brittleness, making careful material selection crucial for applications operating in cold environments.