Tensile strength vs yield strength is a common comparison when evaluating the mechanical behavior of materials. Yield strength indicates the stress at which a material begins to undergo permanent plastic deformation, while tensile strength represents the maximum engineering stress reached during a tensile test.Understanding these two key metrics is crucial for material selection and for assessing the safety of structures and components.
This article will explore tensile strength and yield strength in depth and explain the similarities and differences between these two key metrics.
What Is Tensile Strength?
Tensile strength, also commonly referred to as ultimate tensile strength (UTS), refers to the maximum stress a material can withstand under a tensile load before it fractures. Once a material reaches this maximum stress, continuing to increase the tensile load typically leads to further failure and eventual fracture.
Tensile strength is usually expressed in MPa (N/mm²), and is calculated by dividing the maximum tensile force by the original cross-sectional area of the specimen:
Tensile Strength = Maximum Tensile Force / Original Cross-Sectional Area
It is important to note that tensile strength represents the maximum stress a material can withstand during tensile testing and is not equivalent to the yield strength, which is the stress at which the material begins to undergo permanent deformation.
What is Yield Strength?
Yield strength is the stress at which a material begins to undergo permanent plastic deformation. If the force applied to the material is below this value, it remains in the elastic deformation stage; when the external force is removed, the material returns to its original state. However, if the applied force exceeds this critical value, the material undergoes permanent deformation and cannot return to its original state even after the external force is removed.
Yield strength is typically expressed in MPa (N/mm²), and the basic formula for calculating it is: the load at the yield point divided by the original cross-sectional area of the specimen:
Yield strength = Yield force / Original cross-sectional area
Unlike tensile strength, yield strength focuses on when a material begins to undergo permanent deformation, rather than the maximum stress the material can withstand before fracture.
Tensile Strength vs Yield Strength: Key Differences
| Property | Yield Strength | Tensile Strength |
|---|---|---|
| Definition | Stress at which permanent plastic deformation begins | Maximum engineering stress reached during a tensile test |
| Main difference | Tells you when the material starts to deform permanently | Tells you the maximum tensile stress the material can withstand |
| Material behavior | End of primarily elastic behavior and beginning of plastic deformation | Peak stress, followed by necking in ductile materials, occurs after significant plastic deformation has developed |
| Curve position | Before tensile strength | Maximum point on the engineering stress-strain curve |
| For ductile materials | Usually lower | Usually higher |
| Practical question | “When will the material start to deform permanently?” | “What is the maximum tensile stress the material can reach?” |
For most ductile materials, the tensile strength is typically higher than the yield strength. This is because, after deformation occurs, the material can still withstand a certain amount of stress until it breaks.
Where Do They Appear on a Stress-Strain Curve?
When an initial tensile load is applied to a material, it enters the elastic deformation stage. At this stage, the material typically returns to its original state as long as the load is removed.
As the load continues to be applied, the material gradually approaches the yield point. For materials exhibiting a distinct yield phenomenon, when the curve displays a clear yield characteristic and the deformation transitions from elastic to plastic, the material enters the yield stage. The stress corresponding to this yield point is the yield strength. Once this strength is exceeded, the material undergoes permanent deformation and cannot return to its original shape.
If the load is continued after the yield strength is reached, the material enters the plastic deformation stage. The peak point that appears on the stress-strain curve corresponds to the tensile strength. After this, the material undergoes necking and eventually fractures.
Why Is Yield Strength Usually Lower Than Tensile Strength?
The yield strength is typically lower than the tensile strength because, once a material reaches its yield strength, it begins to undergo permanent deformation but can still continue to withstand the applied load and does not fracture immediately. It represents the stress at which the material begins to undergo permanent deformation. If a tensile load is continued to be applied thereafter until the maximum stress the material can withstand during the tensile process is reached, the stress corresponding to the peak point at that moment is the tensile strength, which represents the maximum stress the material can bear before fracture.
Common Mistakes When Comparing Tensile Strength and Yield Strength
When comparing tensile strength and yield strength, it is easy to treat the two values as interchangeable or assume that a higher value always indicates better material performance. Common mistakes include confusing yield strength with the point of fracture, comparing results obtained under different test conditions, and overlooking the material’s deformation behavior. Tensile strength describes the maximum stress a material can withstand during a tensile test, while yield strength indicates when permanent deformation begins. Therefore, both values should be interpreted together with properties such as elongation, tensile modulus, material type, and the applicable test standard rather than evaluated separately.
How to Choose the Right Tensile Test Method?
The correct tensile test method depends on several factors:
- Material Type
- Specimen Geometry
- Material Thickness
- Required Tensile Properties
- Applicable Standards
- Test Speed
- Expected Force Range
- Required Strain or Elongation Measurement
For metals, ASTM E8/E8M may be applicable when determining tensile strength, yield strength, elongation, and related properties.
For plastics, ASTM D638 or ISO 527 may be appropriate for standard specimens, while ASTM D882 or ISO 527-3 may be applicable to thin films and sheets depending on the material and specification.
The test standard should be selected based on the actual material and testing requirement rather than simply choosing a commonly used method.
What Should You Consider When Selecting a Tensile Testing Machine?
When selecting an appropriate tensile testing machine, the following factors should be considered:
- Load Capacity
- Force Measurement Accuracy
- Test Speed Range
- Displacement or Strain Measurement
- Fixture Configuration
- Travel Distance
- Data Acquisition
- Stress-Strain Curve Generation
- Automatic Calculation of Test Results
- Software Features Required for Test Programs
- Compliance with Standards
Selecting equipment that performs well and meets the required standards ensures the accuracy and reliability of test results. Saicheng’s tensile testing machines not only comply with international standards but are also equipped with high-precision sensors. Test speeds range from 0.1 to 800 mm/min and can be adjusted continuously. If you’d like to learn more about tensile testing machines or customize a solution tailored to your needs, please contact Saicheng.
For more information on how to select a tensile testing system for different materials and applications, see our Tensile Testing Machine Guide.
Conclusion
The load-bearing capacity and deformation behavior of materials are evaluated through yield strength and tensile strength to ensure the safety of structures and components. Yield strength provides insight into the maximum stress a material can withstand before permanent deformation, allowing for the establishment of reasonable load limits; tensile strength evaluates a material’s load-bearing capacity as it approaches failure, thereby preventing the risk of failure due to insufficient material strength.
Both yield strength and tensile strength can be measured using a tensile testing machine. By selecting an appropriate tensile testing machine, it is possible to verify whether the performance of materials and components meets the expected requirements.