One of the most important and frequently performed tests on metal parts and components is the tensile test. Designers and engineers rely heavily on the value of a material’s tensile strength and its relationship to its physical properties.
Calculating these strength values is crucial for assessing the suitability of materials for different applications during the material selection process. In this article, we will discuss how tensile stresses are calculated and explain their importance in various industries.
How Tensile Strength is Measured and Calculated
Tensile capacity or tensile strength
One of the most pivotal developments in the history of material mechanics was the realization that the tensile strength of an axially loaded specimen was related to the size and shape of its cross-sectional area. This concept is reasonable, considering that tensile strength originates from the number of intermolecular bonds present in the material. As the number of bonds increases proportionally with the object’s area, so too does its tensile strength. (Further reading: The Crystalline Structure of Metals.)
One of the most popular methods of measuring the tensile strength of an object is the tensile test (also known as the tension test). During this procedure, the test specimen (usually cylindrically shaped) is loaded into an apparatus that clamps it at one end and applies an axial force on the other. This machine then gradually and continuously elongates the specimen at a standard rate until failure. (To learn about other tests, read 6 Tests to Measure a Material's Strength.)
The force in the sample that opposes the applied elongation is measured, recorded and plotted on a chart. This stress-strain (or force-elongation) graph (tensile curve) helps engineers determine the three essential types of tensile strengths:
- Yield strength – The maximum stress that a material can withstand without undergoing permanent deformation.
- Ultimate tensile strength – The maximum stress that a material can withstand before it fails or fractures.
- Fracture strength – The value of the stress in the material at the point of rupture. The fracture strength is also known as the breaking strength.
Applied tensile stress
Once the tensile capacity (or strength) of the material is assessed, engineers will typically calculate the applied stress in the component. The applied tensile stress in a body is defined as the sum of the internal forces per unit area of the object. This relationship is represented by the formula below:
σ = F / A
σ = Nominal stress
F = Applied force
A = Gross cross-sectional area of the material
This formula is ideal for calculating stresses in an object with a relatively uniform cross-section. In components with discontinuities (or stress risers), regions of irregularly high stress may exist. In this case, engineers will use a variety of computer modeling techniques, such as finite element analysis (FEA), to accurately determine the range of stresses in the material.
The tensile strength and the applied tensile stresses are then compared to determine whether the part or component is suitable for the applied loading. Typically, for a structure to be deemed safe the tensile capacity must be higher than the applied tensile stress by a specified margin of safety.
The Importance of Calculating Tensile Stress
As mentioned previously, the value of the tensile stress of a material can have a profound impact on its performance. In many industries, this parameter can even be the difference between life and death. Everyone involved in the design process, from engineers to metal fabricators are, therefore, responsible for ensuring that the chosen material possess adequate tensile properties for the given application.
Some industries that rely heavily on the accuracy of tensile stress calculations include, but are not limited to:
1. Construction Industry
Tensile stress parameters are extremely vital in the construction industry because building materials are required to withstand loads from a variety of sources without failure. (For more on this topic and the effects of corrosion, see Why Understanding the Stress Concentration Factor (Kt) is Important When Evaluating Corrosion in Metal Structures.) Additionally, the tensile strength limits of materials dictate how buildings and bridges will behave when subjected to dynamic loads, such as wind and seismic events.
The tensile strengths of materials are typically provided in specifications prepared by steel manufacturers. Structural engineers then use a variety of tools, such as computer modeling and hand calculations, to determine the applied stresses in structural members. The applied tensile stresses are compared to the corresponding strengths to assess the suitability of the selected materials.
The consequences can be serious if materials are improperly selected. For example, in April 2016, a six-story building in Uganda collapsed due to the use of counterfeit materials that possessed inadequate tensile strength properties.
2. Automotive Industry
In the automotive sector, tensile stress calculations are used to select materials for various vehicular components, from bumper moldings to interior fittings.
Material strength properties for automobile parts and components are tailored for specific applications. For example, the grade of metal used in engine framing is different than that used on the doors and pillars. Mechanical engineers will typically assess a material’s tensile properties to determine which combination of strength and ductility would be best for energy absorption and crash management.
Other automobile components in which tensile stress calculations play a crucial role include trims, number plates, electrical harnesses and exterior fittings.
3. Aerospace Industry
The aerospace industry requires materials that are suitable for high-performance and precise applications. Even the slightest miscalculation of tensile stresses can have catastrophic consequences. All materials used in this industry are, therefore, subject to strict testing and QA/QC processes.
Aerospace engineers must frequently select materials with the appropriate combination of tensile strength, ductility and weight to strike the perfect balance between safety and fuel efficiency.
4. Packaging Industry
While tensile testing and calculations are commonly associated with metals, they are also essential in assessing the performance of paper and plastics. In the packaging industry, wrapping needs to be elastic enough to stretch over products of different shapes and sizes while possessing enough strength to resist rupture and tears.
Some of the applications of tensile stress testing in the packaging industry include:
- Strain and elongation testing of packaging material
- Bond strength measurements of adhesives, seals, and labels
- Sealing force and durability of push/pull closures, pop-caps and snap caps
Tensile testing is one of the most widely used assessments performed on materials. By testing a specimen to its breaking point, engineers can use tensile capacity values to predict how materials will behave in their intended applications. Tensile stress calculations can also help to identify where specific components may require additional strengthening or improvement.
While generally associated with metals, the behavior of almost all materials, from steel and aluminum to concrete and plastics, are influenced by their tensile strengths. It is, therefore, essential that this parameter be tested and calculated as accurately as possible to avoid potential disasters.