When sufficient external load is applied on a solid material it undergoes deformation and consequently stress developed within the material to resist that deformation. The capability of a particular material to resist deformation is the measure of strength of the concerned material. So material having higher strength behaves more resistive under deformation and thus can retain its shape and size intact under comparatively higher loading. Deformation of any solid material occur in two distinct phases—elastic deformation and plastic deformation. Both phases are clearly palpable for ductile and semi-ductile materials; however, brittle materials display negligible plastic deformation.
Since deformation induces stress within the material, so capability to withstand elastic and plastic stresses is defined separately. Yield strength indicates maximum stress or load that a solid material can withstand when it is deformed within its elastic limit. On the other hand, ultimate strength indicates the maximum stress or load withstanding capability of a material when it is plastically deformed. In fact, ultimate strength is the maximum stress or load that a material can sustain before complete fracture under external load. Most engineering materials show an ultimate strength of 1.5 – 2.0 times higher than yield strength. Various differences between yield strength and ultimate strength are given below in table form.
- For highly brittle materials, yield strength and ultimate strength are also same (plastic deformation of brittle materials is negligible).
- Both are basically measurement of stress.
- Both are measured in same unit (N/mm2).
- Apart from material, its composition, crystal structures, imperfections, etc., temperature and inbuilt stress (residual stress) can influence both of them.
| Yield Strength | Ultimate Strength |
|---|---|
| Yield strength is defined as the maximum stress that a solid material can withstand when it is deformed within its elastic limit. | Ultimate strength is defined as the maximum stress that a solid material can withstand before its failure. |
| It is the stress corresponding to the yield point (upper) in the engineering stress-strain curve under tensile loading. | It is the stress corresponding to the ultimate tensile strength (UTS) point in the engineering stress-strain curve under tensile loading. |
| Brittle materials do not exhibit any yield point. So they do not have yield strength. | Ultimate tensile strength (UTS) is considered as the failure criteria for brittle material. |
| In ductile materials, yield strength is much lower than ultimate strength. | For ductile materials, ultimate strength is roughly 1.5 times higher than yield strength. |
| Yield strength is used while designing components or structures made of ductile materials. | Ultimate tensile strength is used while designing components or structures made of brittle material. It is also essential parameter in metal forming processes. |
References
- Book: Callister’s Materials Science and Engineering by R. Balasubramaniam (Wiley India). Buy this book
- Book: Introduction to Machine Design by V. B. Bhandari (McGraw Hill Education India Private Limited). Buy this book
- Book: A Textbook of Strength of Materials by R. K. Bansal (Laxmi Publications Private Limited). Buy this book
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