How much force does it take to snap a piece of wood? At what point would a piece of spaghetti break if you pulled it from both ends?

The point at which any material is literally "stretched to the breaking point" is an important, and a fascinating concept called tensile strength.

Read on to find out more. 

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Tensile strength, or ultimate tensile strength to give it its full name, is the maximum load that a material can withstand without fracturing or being stretched, divided by the original cross-sectional area of the material in question. 

It is usually used to describe a material's ability to withstand a pulling (tensile) force until a point is reached where many strands of the material break simultaneously at a constant rate of extension or load. In other words, it is the maximum (or minimum depending on your point of view) stress a material can resist before permanently deforming or breaking. 

It is usually recorded as a dimension of force divided by unit area in a given system of measurement, such as pounds per square inch (psi). 

In the SI system, tensile strength is usually determined in pascals (pa) or one newton per square meter (nm). For reference, one newton is the force needed to accelerate one kilogram of mass at the rate of one meter per second squared in the direction of the applied force.

When stress forces that are lower than a material's tensile strength limit are removed, the material will generally completely, or partially, return to its original shape or size. As stress forces approach the value of a material's tensile strength, however, the material, if ductile, will begin to flow plastically and will deform, or stretch rapidly into a constricted region called a neck before eventually fracturing or snapping. 

By way of example, let's take a metal rod that is one square inch (2.5 cm2) in cross-sectional area. If it can withstand 1,000 pounds before breaking it would, therefore, have a tensile strength of 1,000 pounds per square inch. Some common tensile strengths include 400 MPa (58,015 psi) for structural steel and 841 MPa (121,977 psi) for carbon steel. 

This is a very important and widely used metric for materials that are used for structural applications. 

While tensile strength usually refers to the amount of stretching a material can handle before failing, the actual definition of failure can differ depending on the materials used and on their structural applications.

"Reduction in ductility and an increase in brittleness are associated with an accelerated corrosion rate, which in turn can change the failure of the material from a ductile failure to a much more dangerous brittle failure." - corrosionpedia. 

Tensile strength is an incredibly important concept in engineering as well as in related fields like materials science, mechanical engineering, and structural engineering. Tensile strength tends to be more important for brittle materials than ductile materials, though not always.

Tensile strength tends to also be referred to in one of three other ways. These are: -

Once a limit is met, or exceeded, the material will tend to fail in one of two ways, depending on its physical properties.

Breaking strength is defined as "the ability of a material to withstand a pulling or tensile force. It is customarily measured in units of force per cross-sectional area". In this respect, it is effectively a synonym for tensile strength. 

Breaking strength is typically reported, like tensile strength, in pounds per square inch or newtons per meter. 

Tensile strength is usually determined through a series of formal tests of materials. These tests will determine how much a material will elongate before reaching its ultimate tensile strength and how much load a particular material can resist before losing its structural integrity. 

Such tests may involve taking a small sample of the material with a fixed cross-sectional area, fixing both ends to "grips", and slowly, but steadily, pulling them apart -- often using a weight fixed to one end while the other remains in one place. The weight is gradually increased and measurements are made about the change in length of the sample.

More complex testing regimes are carried out for quality assurance purposes for materials like metal under laboratory conditions using a machine called universal testing machines (UTM). Sometimes also called tensometers, these machines are able to perform standardized testing of various materials, components, and structures. 

Such machines are also used to perform compressive strength testing of materials. Most machines are modular and can be readily adapted to fit a particular material's dimensions or a customer's needs.

They can accommodate many kinds of materials ranging from hard samples (like steel or concrete) to more flexible things like rubber or textiles. 

"The UTM is a versatile and valuable piece of testing equipment that can evaluate materials properties such as tensile strength, elasticity, compression, yield strength, elastic and plastic deformation, bend compression, and strain hardening. Different models of Universal Testing Machines have different load capacities, some as low as 5kN and others as high as 2,000kN." - worldoftest. 

Tensile strength testing can also be conducted under a variety of environmental conditions by placing the machine in an environmental room or chamber. UTM's can range widely in size from small tabletop versions to very large machines with greater than 53 mega newtons (12 million lbs) capacity. 

As previously mentioned, tensile strength is a very important concept in various fields, especially engineering. It is used to guarantee the quality of components, materials, and finished products within a wide range of industries. 

Some typical industries include: 

Tensile strength testing, as we have seen, is an incredibly important process. While simple in concept, assessing the tensile strength of something can be fairly involved and sophisticated.