(a) Hardness:
Hardness quantifies a material’s resistance to indentation or scratching. It indicates how easily the surface of the material can be permanently deformed by an external force. Common hardness tests include Brinell, Rockwell, and Vickers. Higher hardness values indicate a more resistant material.
(b) Proof Stress:
Proof stress, also known as yield strength in some contexts, is the stress level at which a material begins to deform plastically (permanently). This means that once stress exceeds the proof stress, the material will not return to its original shape after the stress is removed. It defines the practical limit of elastic behaviour for a material.
(c) Ultimate Tensile Strength (UTS):
UTS is the maximum stress a material can withstand before fracturing or breaking under tension (pulling force). It represents the strongest point the material can reach before ultimate failure. UTS is typically a higher value than yield stress, as it measures the breaking point rather than the point of permanent deformation.
(d) Young’s Modulus:
Young’s modulus, also known as elastic modulus, describes the stiffness of a material. It quantifies the relationship between stress and strain within the elastic range (region where deformation is temporary). A higher Young’s modulus indicates a stiffer material that requires more force to deform a given amount.
(e) Yield Stress:
Yield stress, as mentioned earlier, can be used interchangeably with proof stress in some contexts. It refers to the stress level at which plastic deformation begins. However, depending on the material and testing method, there might be slight variations in how these terms are defined and measured.
Here’s a quick table summarising the key differences:
| Term | Definition | Units | Importance |
| Hardness | Resistance to indentation/scratching | N/mm² or HV | Surface wear resistance, machining properties |
| Proof Stress (Yield Strength) | Stress at onset of permanent deformation | MPa or ksi | Structural integrity under load, design limits |
| Ultimate Tensile Strength (UTS) | Maximum stress before breaking | MPa or ksi | Material strength, safety margins |
| Young’s Modulus | Stiffness of the material | GPa or psi | Deformation under load, vibration response |