For structural components, strength and toughness are two important mechanical properties. Yield strength is the measure of the stress a metal can withstand before deforming. Tensile strength is a measure of the maximum stress a metal can support before starting to fracture. Fracture toughness is a measure of the energy required to fracture a material that contains a crack.
As a metal's yield strength increases, the amount of stress the metal can support without deforming increases. Alternatively, as yield strength increases, a smaller cross-section of metal is required to support a given load without deforming. As tensile strength increases, the amount of stress a metal can support without cracking and fracturing increases.
As a metal's fracture toughness increases, the energy required to cause a crack to grow to fracture increases. For a component with a crack of a certain length, as the fracture toughness decreases there is a decrease in the component’s ability to support the load without fracturing. Conversely, for a certain load, as fracture toughness increases, a component can tolerate a longer crack before fracturing.
Metal toughness decreases as strength increases. For any particular alloy, mechanical and/or thermal treatments are used to modify the alloy's strength. For many alloys, it is possible to use different processes to get different toughness vs. strength curves.
Designers are often tempted to use a material that is as strong as possible to enable them to minimize component cross-section. However, this can inadvertently lead to using a material with insufficient fracture toughness to withstand fracturing if a crack forms in the component during manufacturing or use.
There are a few options when a component, made of any particular alloy and fabrication process, does not have the toughness and strength needed - use a different alloy and/or use different fabrication processes. Fabrication processes include the mill or casting processes used to make the metal, subsequent mechanical processing, and heat treating. For example, the toughness vs. strength for a cold-rolled carbon steel alloy is different than for the quench and tempered alloy.
To Learn More
Look into the following resources to learn more about fatigue and fracture toughness:
Improving Fatigue Resistance - an article about how to improve a component's resistance to failing due to due fatigue. www.imetllc.com/improving-fatigue-resistance/
Preventing Metal Fatigue - webinar video recording. Teaches the physics of the fatigue mechanism, design factors that influence component fatigue life, and practical ways to improve fatigue resistance. www.imetllc.com/product/preventing-metal-fatigue-video/
Metal Failure Analysis course. www.imetllc.com/product/failure-analysis-metals/