Tensile strength is one of the most fundamental properties in any building material. This mechanical property is frequently used to assess the suitability of materials in various engineering applications. Tensile strength values are often inputted into various formulas, calculations and computer software to help predict the behavior of structural members under different types of loading. Due to its importance, this property is often clearly stated in material specification documents.
Testing a Material's Tensile Strength
One of the most popular methods used to determine the tensile strength of a material is the tensile test (also known as a tension test). During this procedure, a cylindrical test specimen is loaded into a machine that grips it on one end and applies an axial tensile force on the other. The machine then slowly and continuously stretches the specimen at a standardized rate until failure. The opposing force in the test specimen due to the imposed stretching is recorded and plotted on a graph against the applied elongation.
The resulting force-elongation graph (or stress-strain graph) for a steel specimen displays three distinct regions that represent the three different types of tensile strength: yield, ultimate and fracture strength. In this article, we will discuss these three tensile strength parameters in detail to give an idea of how they are applied in engineering applications.
Tensile Strength #1: Yield Strength
The yield strength is defined as the maximum stress a material can withstand without undergoing permanent deformation. (Stress is discussed in more detail in the article Why Understanding the Stress Concentration Factor (Kt) is Important When Evaluating Corrosion in Metal Structures.) The value of the yield strength can be observed as the end point of the linear part of the stress-strain graph.
As the specimen is elongated in the initial stages of the test, the initial slope of the stress-strain graph is linear, i.e., the stress in the material is directly proportional to the applied strain. This first phase is referred to as the linear-elastic region because the material still obeys Hooke’s Law. At this point, the material is said to behave elastically. Therefore, should the test load be removed, the specimen is expected to spring back to its original shape and length.
As the machine continues to elongate the test specimen, a point is reached where the metal is stretched beyond its ability to return to its original length. In other words, the material is said to have yielded, and the value of the stress at this point is called the yield strength.
Tensile Strength #2: Ultimate Strength
The ultimate tensile strength (UTS), or simply, ultimate strength, is defined as the maximum stress that a material can withstand before failure. After the material yields, it enters the plastic region. At this stage, the material is stretched to the point where it deforms permanently, i.e., the test specimen will not return to its original shape and length when the load is removed. A good analogy is an overstretched spring.
In the plastic region, the opposing force continues to increase as the test subject resists elongation in a non-linear manner. This apparent strengthening of the material occurs due to a phenomenon known as strain hardening (also known as work hardening). During strain hardening, the crystalline structure within the material’s microstructure undergoes permanent dislocation and rearrangement. (Learn more about the crystalline structure in The Crystalline Structure of Metals.)
As a result, the specimen strain hardens up to a maximum point, after which the resistive force or stain decreases. The value of this maximum stress is termed the ultimate tensile strength.
The ultimate tensile strength is a crucial parameter in the design and analysis of many engineered buildings and bridges. In most ductile materials, the ultimate strength is usually around 1.5 to 2.0 times higher than the reported yield strength.
Tensile Strength #3: Fracture Strength
The fracture strength, also known as the breaking strength, is the value of the stress at the point of rupture. In the tensile strength test, it is the stress value at which the test specimen separates into two distinct pieces.
In ductile materials, such as steel, once the ultimate strength is reached the value of the opposing force in the material gradually drops with continued elongation. This drop in resistance is due to necking in the test subject shortly before fracture.
During necking, a prominent decrease in local cross-sectional area occurs in the metal, giving it a "V" or "neck" shape. All further plastic deformation as a result of continuous elongation now occurs at the neck. The neck eventually becomes the location of fracture when enough strain is applied to the test subject.
Ductile vs Brittle Behavior
The stress-strain graph illustration and the different types of tensile strengths defined in this article were in relation to ductile materials. This was done deliberately because ductile materials best illustrate the distinction between yield, ultimate and fracture strengths.
Brittle materials, such as cast iron, masonry and glass, however, act a bit differently. A brittle fracture in brittle materials is relatively sudden, i.e., there is typically no noticeable change in cross-section or rate of elongation prior to fracture.
Most brittle materials do not have a well-defined yield point, nor do they strain harden. Their ultimate strength and fracture strength are, therefore, the same. The stress-strain graph for brittle materials is mostly linear. As also evident in the graph, brittle materials do not exhibit plastic deformation behavior and fail while the material is basically elastic.
Another characteristic of brittle materials that distinguishes them from ductile behavior is that there is little to no reduction in cross-sectional area during fracture. In other words, a neck does not form. As a consequence the two broken parts can be reassembled to produce the same shape as the original component. (Enjoying this article? You might want to read How to Get Started in a Career as a Materials Scientist.)
Conclusion
The yield, ultimate and fracture strength of materials are essential engineering properties that help determine how components will perform when subjected to various applied loads. The value of these strengths is dependent on several factors, including the material type, temperature, molecular structure and chemical composition.
Yield, ultimate and fracture strengths are easily identified in the stress-strain graphs of ductile materials. Brittle materials, on the other hand, only exhibit fracture strengths. The distinction between these two types of behaviors is crucial in engineering applications where the ductility and brittleness of materials can have a profound influence on the design and analysis process.