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Understanding Tensile Strength, Its Importance in Engineering

The point at which any material is "stretched to the limit" is called tensile strength.

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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What is the definition of tensile strength?

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)

tensile strength tope
Tensile strength is a very important concept in many fields, for example testing the strength of ropes. Source: Bernard Spragg/Flickr

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. 

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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.

tensile strength filament
SEM image of a lightbulb filament. These have amazing properties like high melting point, low vapor pressure, and high tensile strength. Source: ZEISS Microscopy/Flickr

"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

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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.

What types of tensile strength are there?

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

  • Yield strength, which is the stress a material can withstand without permanent deformation
  • Ultimate strength, which is the maximum stress a material can withstand.
  • Breaking strength, which is the stress coordinate on the stress-strain curve at the point of rupture or failure. 

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

tensile strength breaking
Examples of some tensile strength testing samples that have failed. One brittle and two ductile. Source: Brendan Wood/Flickr

These are: 

  • Ductile failure - This is where a material first yields, then hardens, and finally breaks after plastically deforming. A neck may or may not form. 
  • Brittle failure - This is where the material will suddenly break into two or more pieces.

Is tensile strength the same as breaking strength?

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. 

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How do you calculate tensile strength?

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. 

tensile strength testing
Historical image of tensile strength testing. Source: BiblioArchives/Flickr

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. 

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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 of a carbon filament
Tensile strength testing of a carbon filament using a UTM. Source: Oak Ridge National Laboratory/Flickr

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. 

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Where is tensile strength used?

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: 

  • Aerospace Industry - For example, peel testing of airframes, shear, and tensile strength testing of bolts, nuts, etc. 
  • Automotive Industry - Quality assurance of interior and exterior fittings, bumper moldings, mirrors, etc.
  • Beverage Industry - Such as the peel strength of induction-sealed foils, ring pulls, cork extraction forces, etc. 
  • Construction Industry - For example, bond strength of adhesives, building components, and structural elements like steel beams, etc.
  • Electrical and Electronics Industry - Determining connector withdrawal forces, PCB material tensile strengths, etc.
tensile strength testing industry
Tensile strength testing is used in various industries including the aerospace industry. Source: Lukas Wunderlich/iStock
  • Medical Device Industry - Testing of the tensile strength of needles, medical tubing, suture materials, etc.
  • Packaging Industry - Adhesive/peel testing of adhesive bonds, elongation of plastic packaging, etc. 
  • Paper and Board Industry - Determining the operability of card and paper packaging, durability of documents, etc. 
  • Pharmaceuticals Industry - Testing the pull-off force of vial caps, etc. 
  • Plastics, Rubber, and Elastomers Industry - Testing the joint strength of interlocking plastic components, general determination of the tensile strength of materials, etc. 
  • Safety, Health, Fitness, and Leisure Industry - For example, testing the tensile strength of safety support netting or the elastic properties of racquet strings. 
  • Textiles Industry - Testing the pull-off characteristics of buttons, zippers, or strength testing clothing seams, etc. 

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. 

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