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Toughness is the strength with which the material opposes rupture. One definition of material toughness is the amount of energy per unit volume that a material can absorb before rupturing. This measure of toughness is different from that used for fracture toughness, which describes the capacity of materials to resist fracture. [2]
Fracture toughness varies by approximately 4 orders of magnitude across materials. Metals hold the highest values of fracture toughness. Cracks cannot easily propagate in tough materials, making metals highly resistant to cracking under stress and gives their stress–strain curve a large zone of plastic flow.
Nonetheless, toughness should be improved without sacrificing its strength in metal. Designing a new alloy or improving its processing can achieve this goal. Designing a new alloy can be explained by different toughness in several ferrous alloy.18%Ni-maraging steel has a higher toughness than the martensitic steel AISI 4340.
The impact energies of high-strength materials other than steels or BCC transition metals are usually insensitive to temperature. High-strength BCC steels display a wider variation of impact energy than high-strength metal that do not have a BCC structure because steels undergo microscopic ductile-brittle transition.
Toughness as defined by the area under the stress–strain curve. Materials that are both strong and ductile are classified as tough. Toughness is a material property defined as the area under the stress-strain curve. Toughness can be determined by integrating the stress-strain curve. [3]
The ultimate strength is the maximum stress that a material can withstand before it breaks or weakens. [12] For example, the ultimate tensile strength (UTS) of AISI 1018 Steel is 440 MPa. In Imperial units, the unit of stress is given as lbf/in 2 or pounds-force per square inch. This unit is often abbreviated as psi.
The ultimate tensile strength of a material is an intensive property; therefore its value does not depend on the size of the test specimen.However, depending on the material, it may be dependent on other factors, such as the preparation of the specimen, the presence or otherwise of surface defects, and the temperature of the test environment and material.
Very hard steel (e.g. chisels, quality knife blades): HRC 55–66 (Hardened High Speed Carbon and Tool Steels such as M2, W2, O1, CPM-M4, and D2, as well as many of the newer powder metallurgy Stainless Steels such as CPM-S30V, CPM-154, ZDP-189. There are alloys that hold a HRC upwards 68-70, such as the Hitachi developed HAP72.