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Use of Terms Yield Strength and Yield Point
This article summarizes the SAE Recommended Practice J450:2017, which defines and distinguishes between the terms yield strength and yield point. It provides guidance on their proper use in materials testing and specification, helping engineers select the correct term based on material behavior and avoid common reporting errors.
Definitions and Key Differences
According to SAE J450, yield strength is the stress at which a material exhibits a specified limiting deviation from stress-strain proportionality. It is expressed in units of stress, referenced to a particular strain (engineering or true). The deviation can be measured by the Offset method or Extension Under Load method as described in ASTM E8. Yield strength applies to both smooth stress-strain curves and discontinuous yielding.
Yield point is a special case of yield strength applicable only when yielding occurs discontinuously. An upper and lower yield point may be determined. Importantly, only ferrous metals exhibit this phenomenon under certain circumstances, so yield point is not applicable to nonferrous metals.
| Term | Definition | Applicable Materials | Comments |
|---|---|---|---|
| Yield Strength | Stress at specified limiting deviation from proportionality | Ferrous and nonferrous metals | Always state method and strain limits |
| Yield Point | Stress at discontinuous yielding (upper/lower) | Only ferrous metals with discontinuous yield | Not for nonferrous or smooth curves |
Recommended Usage and Engineering Insights
Nonferrous Metals
Only the term yield strength is applicable. Specifications and test reports must always state the method of test and limiting values of strain.
Ferrous Metals
Yield strength is the general term suitable for both smooth and discontinuous stress-strain curves. The term yield point should be used only when the material exhibits a clear upper and lower yield point. If a single value is reported, indicate whether it is upper or lower yield point. For materials with smooth stress-strain curves, yield strength is the appropriate term; yield point should be discouraged.
🔍 When reporting yield strength, engineers must specify the measurement method (offset or extension under load) and the limiting strain values. This ensures reproducibility and clarity. For nonferrous metals, yield point is never used. For ferrous metals, yield point should be reserved for materials with a sharp knee or distinct upper/lower yield behavior. Specifications that prescribe yield point for smooth stress-strain curves should be revised to use yield strength.
⚠️ Important: Using yield point for materials that do not exhibit discontinuous yielding can lead to ambiguity. Always refer to ASTM A370, E6, and E8 for detailed test methods.
Frequently Asked Questions
1. What is the difference between yield strength and yield point?
Yield strength is a general term for stress at a specified plastic strain offset. Yield point is a specific phenomenon where stress drops after initial yielding, occurring only in some ferrous metals. Yield strength can be measured for any metal; yield point is only defined for materials with discontinuous yielding.
2. Can yield point be used for nonferrous metals?
No. Yield point is not applicable to nonferrous metals because they do not exhibit discontinuous yielding. Only yield strength should be reported for nonferrous materials.
3. How should yield strength be reported?
Always state the method of test (e.g., 0.2% offset) and the limiting values of strain. Also specify whether engineering or true stress/strain is used.
4. What standards govern these definitions?
SAE J450 references ASTM A370, E6, and E8. These provide detailed test methods and terminology for mechanical testing.
🛠️ Key Takeaway: Proper terminology in tensile testing avoids confusion between material specifications and ensures consistent quality control. Use yield strength for general reporting; reserve yield point for ferrous metals that clearly exhibit discontinuous yielding.
