Strength is a measure of the stress a metal without cracks can withstand before it deforms or breaks under a single applied load. Fracture toughness is a measure of the energy required to fracture a material containing a crack. The harder the material, the more energy is required to cause a crack to grow into a fracture. For a particular alloy, lower fracture toughness corresponds to lower ductility. Glass, for example, has very low toughness and is very brittle.
Effect of load on fracturing
For a component with a crack of a certain length, as the fracture toughness decreases, the ability of the component to withstand its load before fracture decreases. Conversely, for a given load, as the fracture toughness increases, the part can tolerate a longer crack before breaking. As shown in the graph below, for any particular alloy, toughness decreases as tensile strength increases. Therefore, when both high toughness and high strength are required, it is often necessary to switch from one alloy to a different alloy that meets both requirements.
Consider Strength and Toughness During Material Selection
Designers often prefer to use the strongest materials possible, allowing them to minimize component cross-sections. However, if small cracks form in the material during component manufacture or use, this can inadvertently lead to the use of materials with insufficient fracture toughness to withstand fracture. Fatigue stress is a possible cause of cracks.
Cracks often form in components exposed to fatigue conditions. In these cases, an understanding of fracture toughness is required to determine how long a component can last before a crack grows to the point where the full cross-section of the component cannot support the load and the component breaks. This applies to aerospace components and pressure vessels such as boilers.
For structural components exposed to fatigue conditions, designers need to focus on both strength and toughness. The strength needs to be great enough so that the material can withstand the applied load without deforming. Toughness needs to be sufficient for the metal to withstand the formation of fatigue cracks without catastrophic failure.
