Many people frequently use strength and hardness interchangeably; however, they are completely different. Strength is the ability of a material to resist deformation caused by external load. The more external loading a material can withstand, the higher strength it will have. Based on the level of deformation, strength can be defined in two distinct regions—yield strength and ultimate strength. Yield strength basically indicates maximum load that the concerned material can withstand in its elastic limit; while, ultimate strength defines the maximum load the material can withstand before fracture. Both are properties of solid and are useful for design considerations. Strength is expressed by N/mm2, so it is always measured per unit area basis.
While strength is the property of the solid, hardness is not one property of solid as the later one is measured with respect to or relative to another material. Basically, hardness is the ability of a material to resist indentation, penetration or scratching caused by another material. It is always relevant to the surface of a solid component, not the entire component. It determines whether one material can scratch or indent another material. A harder material can penetrate into a softer material only. Hardness can be measured in various standard scales, each having different indenter. While measuring hardness, capability of a standard indenter to indent or scratch the workpiece surface is measured. Since harder material offers more resistance to scratch, so it also shows better wear resistance (mechanical wears only, such as abrasion, erosion). Various differences between strength and hardness are given below in table form.
Differences between strength and hardness
| Strength | Hardness |
|---|---|
| Strength is defined as the ability of the solid material to withstand external load. Based on the region it is measured, strength may be either yield strength (maximum stress in elastic region) or ultimate strength (maximum stress before fracture). | Hardness is defined as the resistance that a solid material offers against penetration or scratching. |
| Consequently strength indicates the maximum value of the external load that a material can withstand either in elastic or in plastic limit. | Hardness indicates the ease of making an indentation on the solid surface by another material. |
| Strength is a basic property of the solid material. | Hardness is not a property of solid material; in fact, it is the property of solid surface. |
| Strength of a solid material can be determined by standard tensile or compressive testing in Universal Testing Machine (UTM). | Hardness can be measured by several testing such as:
|
| Strength is expressed by N/mm2; by the by, strength is nothing but stress. | Hardness is expressed by Hardness Number based on testing method. |
| Strength is an important parameter for designing purpose as it limits the maximum allowable load on the material. | Hardness is an important parameter when mechanical wear resistance is considered (any area where two solid surfaces, having relative motion, are in contact). Harder materials exhibit good mechanical wear resistance. |
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