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There is an engineering stress strain curve and a true stress strain curve. The engineering stress strain curve assumes that the member cross section area remains constant at all levels of tensile load, but in actuality, cross section starts to significantly reduce (called necking) at high stress values beyond the yield point, in which case if you plot true stress, which accounts for the reduced cross section area , versus strain, the true stress value always increases up to rupture, whereas if you use engineering stress, you get a peak on the curve prior to significant necking, and beyond that, the stress gets lower because it is assumed that cross section remains constant . The value of the engineering stress at this peak is called the ultimate tensile strength, whereas the breaking strength is the rupture stress at point of failure .
There is an engineering stress strain curve and a true stress strain curve. The engineering stress strain curve assumes that the member cross section area remains constant at all levels of tensile load, but in actuality, cross section starts to significantly reduce (called necking) at high stress values beyond the yield point, in which case if you plot true stress, which accounts for the reduced cross section area , versus strain, the true stress value always increases up to rupture, whereas if you use engineering stress, you get a peak on the curve prior to significant necking, and beyond that, the stress gets lower because it is assumed that cross section remains constant . The value of the engineering stress at this peak is called the ultimate tensile strength, whereas the breaking strength is the rupture stress at point of failure .
Two vises apply tension to a specimen by pulling at it, stretching the specimen until it fractures. The maximum stress it withstands before fracturing is its ultimate tensile strength.
Ultimate tensile strength (UTS), often shortened to tensile strength (TS), ultimate strength, or F tu {displaystyle F_{text{tu}}} 
The ultimate tensile strength is usually found by performing a tensile test and recording the engineering stress versus strain. The highest point of the stress–strain curve is the ultimate tensile strength and has units of stress. The equivalent point for the case of compression, instead of tension, is called the compressive strength.
Tensile strengths are rarely of any consequence in the design of ductile members, but they are important with brittle members. They are tabulated for common materials such as alloys, composite materials, ceramics, plastics, and wood.
Definition
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The ultimate tensile strength of a material is an intensive property; therefore its value does not depend on the size of the test specimen. However, depending on the material, it may be dependent on other factors, such as the preparation of the specimen, the presence or otherwise of surface defects, and the temperature of the test environment and material.
Some materials break very sharply, without plastic deformation, in what is called a brittle failure. Others, which are more ductile, including most metals, experience some plastic deformation and possibly necking before fracture.
Tensile strength is defined as a stress, which is measured as force per unit area. For some non-homogeneous materials (or for assembled components) it can be reported just as a force or as a force per unit width. In the International System of Units (SI), the unit is the pascal (Pa) (or a multiple thereof, often megapascals (MPa), using the SI prefix mega); or, equivalently to pascals, newtons per square metre (N/m2). A United States customary unit is pounds per square inch (lb/in2 or psi). Kilopounds per square inch (ksi, or sometimes kpsi) is equal to 1000 psi, and is commonly used in the United States, when measuring tensile strengths.
Ductile materials
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- Ultimate strength
- Yield strength
- Proportional limit stress
- Fracture
- Offset strain (typically 0.2%)
Figure 1: “Engineering” stress–strain (σ–ε) curve typical of aluminum
Many materials can display linear elastic behavior, defined by a linear stress–strain relationship, as shown in figure 1 up to point 3. The elastic behavior of materials often extends into a non-linear region, represented in figure 1 by point 2 (the “yield point”), up to which deformations are completely recoverable upon removal of the load; that is, a specimen loaded elastically in tension will elongate, but will return to its original shape and size when unloaded. Beyond this elastic region, for ductile materials, such as steel, deformations are plastic. A plastically deformed specimen does not completely return to its original size and shape when unloaded. For many applications, plastic deformation is unacceptable, and is used as the design limitation.
After the yield point, ductile metals undergo a period of strain hardening, in which the stress increases again with increasing strain, and they begin to neck, as the cross-sectional area of the specimen decreases due to plastic flow. In a sufficiently ductile material, when necking becomes substantial, it causes a reversal of the engineering stress–strain curve (curve A, figure 2); this is because the engineering stress is calculated assuming the original cross-sectional area before necking. The reversal point is the maximum stress on the engineering stress–strain curve, and the engineering stress coordinate of this point is the ultimate tensile strength, given by point 1.
Ultimate tensile strength is not used in the design of ductile static members because design practices dictate the use of the yield stress. It is, however, used for quality control, because of the ease of testing. It is also used to roughly determine material types for unknown samples.[4]
The ultimate tensile strength is a common engineering parameter to design members made of brittle material because such materials have no yield point.[4]
Testing
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Round bar specimen after tensile stress testing
Aluminium tensile test samples after breakage
The “cup” side of the “cup–cone” characteristic failure pattern
Some parts showing the “cup” shape and some showing the “cone” shape
Typically, the testing involves taking a small sample with a fixed cross-sectional area, and then pulling it with a tensometer at a constant strain (change in gauge length divided by initial gauge length) rate until the sample breaks.
When testing some metals, indentation hardness correlates linearly with tensile strength. This important relation permits economically important nondestructive testing of bulk metal deliveries with lightweight, even portable equipment, such as hand-held Rockwell hardness testers.[5] This practical correlation helps quality assurance in metalworking industries to extend well beyond the laboratory and universal testing machines.
Typical tensile strengths
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^a^b[38] The first nanotube ropes (20 mm in length) whose tensile strength was published (in 2000) had a strength of 3.6 GPa.[39] The density depends on the manufacturing method, and the lowest value is 0.037 or 0.55 (solid).[40]^c[41] The value shown in the table, 1000 MPa, is roughly representative of the results from a few studies involving several different species of spider however specific results varied greatly.[42]^d
Typical Properties of annealed elements
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Typical properties for annealed elements[43]ElementYoung’s
modulus
(GPa)Yield strength
(MPa)Ultimate
strength
(MPa)Silicon1075000–9000Tungsten411550550–620Iron21180–100350Titanium120100–225246–370Copper130117210Tantalum186180200Tin479–1415–200Zinc85–105200–400200–400Nickel170140–350140–195Silver83170Gold79100Aluminium7015–2040–50Lead1612
See also
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References
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Further reading
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Disclaimer: all data and information obtained viathe Polymer Selector including but not limited to material suitability, materialproperties, performances, characteristics and cost are given for information purposeonly. Although the data and information contained in the Polymer Selector are believedto be accurate and correspond to the best of our knowledge, they are provided withoutimplied warranty of any kind. Data and information contained in the Polymer Selectorare intended for guidance in a polymer selection process and should not be consideredas binding specifications. The determination of the suitability of this informationfor any particular use is solely the responsibility of the user. Before workingwith any material, users should contact material suppliers in order to receive specific,complete and detailed information about the material they are considering. Partof the data and information contained in the Polymer Selector are genericised basedon commercial literature provided by polymer suppliers and other parts are comingfrom assessments of our experts.
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What Does Breaking Stress Mean?
Breaking stress is the maximum force that can be applied on a cross sectional area of a material in such a way that the material is unable to withstand any additional amount of stress before breaking.
Breaking stress is calculated with the formula:
Breaking Stress = Force / Area
Breaking stress testing for metals determines how much a particular alloy will elongate before it reaches its ultimate tensile strength and how much load a particular piece of metal can accommodate before it loses structural integrity. Therefore, it is a very important concept in material science and for safety considerations.
Breaking stress may also be known as ultimate tensile stress or breaking strength.
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.
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.
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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.
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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.
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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.
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