What is Ultimate Tensile Strength?

Introduction

Strength is one of the basic properties of materials, usually referring to the ability of materials to resist deformation or breakage under external forces. There are many kinds of external forces, such as tensile force, compression force, etc. Correspondingly, materials also have tensile strength, yield strength, compressive strength and other characteristics. In application scenarios such as mechanical manufacturing, engineering structures, and building bridges, tensile strength is an important indicator used by researchers, engineers, and quality control teams to evaluate the mechanical properties of materials and products.

In this article, HYT will introduce you to the concept, calculation method, application and many other aspects of ultimate tensile strength. In this article, HYT will use a combination of popular explanations and scientific and rigorous explanations to explain so that newcomers to the materials industry can also understand the relevant knowledge.

Attached at the end of the article is a table of tensile strength and ultimate tensile strength of common materials.

What Is Ultimate Tensile Strength

In layman’s terms, in daily life, we have all tried to break a rubber band with our hands. After we applied a large enough tensile force, the rubber band was torn into two pieces by us. This is mainly because the tensile force applied by our hands to the rubber band exceeded the bearing limit of the rubber band material itself, and eventually caused the rubber band to break. This phenomenon indicates that the stress on the rubber band during stretching exceeds its ultimate tensile strength, which eventually leads to fracture. Therefore, the ultimate tensile strength (UTS) refers to the maximum stress that a material can withstand during the stretching process before it breaks.

Ductile Materials and Brittle Materials

Before discussing the types of tensile strength, we need to explain two concepts, ductile materials and brittle materials.

Ductile materials have a clear yield point. When the yield point is reached, they will change from elastic deformation to plastic deformation. After reaching the ultimate tensile strength, they will continue to deform, and necking will occur at this time. Metals such as steel, aluminum alloy, and copper are ductile materials.

Brittle materials do not have a clear yield point. They do not deform during tensile stress and do not undergo plastic deformation. After reaching the ultimate tensile strength, they will directly break, and the broken surface is generally granular. Materials such as ceramics, glass, and some high-strength alloy hardened steels belong to this category.

Tensile Strength Types

Material undergoes multiple stages of deformation before being stretched to fracture, and each deformation stage corresponds to different mechanical properties, so different types of tensile strength can be measured accordingly. Metallic materials are ductile materials, but their deformation changes before fracture are not very obvious. For the sake of easy understanding and imagination, we still choose rubber bands with large deformation changes as an example to illustrate the types of tensile strength, and we will also illustrate the changes of metals in the corresponding stages.

Stage 1: Proportional Limit

When we first pull the rubber band, the tension applied is relatively small. At this time, the tension on the rubber band is proportional to the actual deformation elongation length. In simple terms, the greater the force used, the longer the rubber band. In other words, the stress-strain curve of the rubber band is approximately linear within a certain range (in accordance with Hooke’s Law), that is, stress and strain are proportional. At this time, as long as we let go, the rubber band can be completely restored to its original state. At this stage, the material undergoes elastic deformation, and the deformation of the rubber band is reversible.

For metal materials, within the proportional limit, metal materials comply with Hooke’s Law, and their stress-strain curve is linear. The metal also only undergoes elastic deformation at this stage, and can return to its original state after the applied force is removed.

Stage 2: Yield Strength

We continue to apply greater force to stretch the rubber band, and we will find that when the rubber band is stretched long enough, even if we let go, the rubber band cannot return to its original length, and the rubber band becomes permanently longer. This means that the rubber band has undergone plastic deformation and has produced permanent deformation. If we apply a greater force than the previous time each time, the rubber band itself will produce more irreversible deformation and will become longer and longer.

For metal materials, this stage corresponds to the yield strength. We can get the minimum stress value for the material to enter plastic deformation from elastic deformation. Metals generally have a clear yield point, and once the yield point is exceeded, the material cannot return to its original state even if the external force is removed.

Stage 3: Ultimate Tensile Strength

Next, we continue to increase the tension on the rubber band, and we will find that the rubber band material has begun to become locally thinner. This phenomenon indicates that the rubber band has approached the maximum stress it can withstand during the stretching process. At this point, we can also measure the ultimate tensile strength.

For metal materials, UTS represents the maximum stress that the metal can withstand. After reaching UTS, the metal will experience the phenomenon of necking, that is, a certain area of ​​the material will become thinner, and this area will become the starting point of the fracture. Although the metal material has not completely broken at this time and the deformation is still continuing, the material’s bearing capacity has actually begun to decline.

Stage 4: Fracture Strength

Further increase the force, and the rubber band will eventually break because it cannot withstand the greater tensile force. The stress value at the time of fracture is the Fracture Strength we want to measure.

In metal materials, fracture strength refers to the stress value that the material withstands at the moment of fracture. For brittle materials, the fracture strength is basically close to UTS. For ductile materials, because of the necking phenomenon, the fracture strength is often lower than UTS.

Fatigue Strength and Endurance Limit

In addition to the above four tensile strengths, there are also fatigue strength and endurance limit.

They are both used to describe the maximum stress that a material can withstand under cyclic loads. The difference is that fatigue strength indicates the maximum stress at which a material will not suffer fatigue failure under a given number of cycles, and fatigue strength decreases with the increase in the number of cycles; while endurance limit indicates the maximum stress at which fatigue failure will not occur under an infinite number of cyclic loads, so endurance limit is also called fatigue limit.

From the S-N curve diagram, if the material has an endurance limit, the fatigue strength will approach a stable value as the number of cycles increases. If the material does not have an endurance limit, the fatigue strength will continue to decrease as the number of cycles increases.

How to Calculate and Measure the Ultimate Tensile Strength

Calculation Formula for UTS

In the previous article, we explained that UTS refers to the maximum tensile stress that a material can withstand in a tensile test. Therefore, the formula for calculating UTS is

Tensile strength (σ) = maximum tensile force (F) / specimen cross-sectional area (A)

Where is the maximum tensile force when the material breaks in a tensile test (unit: N).

is the original cross-sectional area of ​​the material (unit: mm²).

In the International System of Units, tensile strength is expressed in Pascals or Megapascals (MPa), which is equivalent to Newtons per square meter (N/m²).

In the American System of Units, tensile strength is expressed in pounds per square inch (lbf/in² or psi).

In the imperial system, tensile strength is expressed in kilo pounds per square inch (ksi).

The conversion relationship between them is

1 MPa = 1 N/mm² = 0.145 ksi = 145 psi

UTS Measurement

The UTS of metal materials is generally measured by tensile testing, and the tensile strength of materials is measured in accordance with international standards such as ASTM E8 and ISO 6892.

Sample Preparation

Prepare metal specimens of standard shape according to the test standard, generally cylindrical or dumbbell-shaped. The surface of the metal specimen should be smooth and free of defects.

Equipment Selection

Tensile tests are generally carried out using Universal Testing Machine (UTM), which is usually equipped with a hydraulic system to ensure that the machine can provide a large tensile force.

Test Process

First, clamp the metal specimen in the fixture of the testing machine, and gradually apply tensile force according to the strain rate required by international standards such as ASTM E8 and ISO 6892. Use an Extensometer or displacement sensor to measure the strain change of the specimen and record the applied force at the same time.

Determine UTS

Finally, we will get a Force-Displacement Curve. Through this graph, we can get the tensile strength values ​​of the material in three stages from elastic deformation to plastic deformation to final fragmentation.

You can watch the stainless steel tensile test video below produced by Solid Mechanics UCL MechEng to see the change in tensile strength of the material in three stages.

Through this video, we can also verify the previous statement that ductile materials such as metals have a clear yield point, which is the stress at which the material begins to deform plastically or irreversibly.

Why Tensile Strength Is Important

Tensile strength, especially UTS, is very important. Because determining the specific UTS range of different materials can help engineers determine whether the material is suitable for specific applications and projects.

Aerospace

In the aerospace industry, aircraft need to withstand strong aerodynamic loads when flying at high altitudes. During flight, the uneven distribution of air pressure on the aircraft surface will cause tensile stress on the fuselage and wings; secondly, any fuselage movement of the aircraft during flight will cause tensile stress in the structural components due to centrifugal force and dynamic loads; and the high-altitude environment is relatively low temperature, and the material may produce internal stress due to thermal expansion and contraction, which in turn exacerbates the tensile effect.

Therefore, we often use high UTS titanium alloy (Ti-6Al-4V) materials to make aircraft-related products, such as aircraft fuselage structures, landing gear components, engine blades, etc., which can allow the aircraft to fully exert its performance while ensuring flight safety;

Bridge Construction

Bridge construction engineering is a very common application in our lives. Bridges need to bear traffic loads on a daily basis, especially some bridges with very long spans. The suspension cables or inclined cables of the bridges will be continuously subjected to tensile forces. At the same time, some bridges may be affected by strong winds or even earthquakes for a long time due to their geographical factors. These external factors will impose tensile stress on the bridge structure.

Therefore, we often use high UTS alloy steels, such as AISI 4140, to produce the main cables and suspension cables of bridges, which can ensure the long-term stability and safety of bridges;

Automobile Manufacturing

The automobile industry is also an application field for high UTS materials. During the driving process of the car, the chassis and frame will be subjected to impact force from the ground and periodic tensile effects; when a high-speed car accelerates or brakes urgently, the structural parts will be subjected to tensile stress caused by dynamic loads.

Therefore, we will use ultra-high strength steels such as AHSS to produce automobile frames, B-pillars, and door sill beams, and use Aluminum 7075 to produce engine brackets and suspension systems, which can ensure the firmness and stability of the body structure and extend the service life of the car.

Advantages and Disadvantages of High Tensile Strength Materials

After explaining the applications of metals with higher tensile strength in the previous section, we can clearly see that these metals have the following advantages:

1. Strong Bearing Capacity

Tungsten alloy is a metal with ultra-high tensile strength and high density, and is often used in high-stress environments such as high-performance tools, armor-piercing bullets, and aircraft engine components;

2. Good Durability

Titanium alloy Ti-6Al-4V is recognized as a metal with excellent strength-to-weight ratio, with extremely high corrosion resistance and tensile strength, and is widely used in aerospace and biomedical implants and other applications;

3. Good Impact Resistance

17-4PH is a stainless steel material with high tensile strength, which can withstand high impact loads without breaking, and is used in military experts, load-bearing structures, industrial equipment and other applications;

4. High Material Utilization Efficiency

AISI 4140 has high tensile strength. At the same weight, AISI 4140 has higher rigidity than ordinary metals. Therefore, high-performance racing and motorcycle frames made of this steel can adopt relatively lighter and more compact structural designs. By reducing the mass of the frame and thus the mass of the entire vehicle, fuel efficiency can be improved, the burden of the frame on the suspension system and the brake system can be reduced, and the service life and performance of the vehicle can be improved.

But on the other hand, we must also see the other side of high tensile strength metal materials.

1. Strong load-bearing capacity, but also means that the material is brittle

Although tungsten alloy has extremely high tensile strength, it has poor toughness and is prone to brittle fracture, so it is not suitable for elastic structures;

2. Good durability, but difficult to process and high processing cost

Titanium alloy has high hardness and high tensile strength, so special processing technology is required, and its processing cost and difficulty are much higher than steel;

3. Good impact resistance, but high density and weight

Although 17-4PH stainless steel has high strength and excellent impact toughness, its density is much higher than titanium alloy, and it is not suitable for applications that require lightweight;

4. High material utilization efficiency, but high welding and forming costs

Although Chromium-Molybdenum Steel has a higher strength that can reduce material consumption and frame maintenance frequency, its material is harder, and the initial welding and bending forming are more difficult, which directly leads to higher production costs.

Summary

In short, the ultimate tensile strength is a key attribute in material science, indicating the maximum stress that a material can withstand before breaking. This property is also an important reference indicator for HYT to select materials for customer projects. If your project is also facing the challenge of material selection, please contact HYT’s technical team directly. HYT’s technicians will provide you with the most cost-effective material solution that best meets your project needs.

Tensile Strength and Ultimate Tensile Strength of Common Materials

The following values ​​are taken from Wikipedia and are for reference only.

Ductile Materials

Brittle Materials

Reference

Ultimate tensile strength – Wiki