Metal strength and fracture toughness are important mechanical properties for structural components. This article explains the trade-offs between strength and toughness when designing for applications requiring high strength and toughness.
For structural components, strength and fracture 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.
As shown in the figure, 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.
One common source of cracking during use is exposure to fatigue conditions. When designing components in which fatigue cracks are expected to form, knowledge of metal fracture toughness is required to determine how long the component can remain in service before a crack grows too long and the component fractures. This applies to aerospace components and pressure vessels such as boilers.
For structural components exposed to fatigue conditions, designers must be concerned with both the strength and the toughness. The strength must be large enough so that the material can withstand the applied loads without deforming. The toughness must be sufficient for the metal to withstand the formation and growth of fatigue cracks, without fracturing when the cracks are small.
For more information read Deformation and Fracture Mechanics of Engineering Materials by R.W. Hertzberg. This book contains lots of information about the relationship between fracture toughness and strength.
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