Ductility

Ductility is a critical mechanical performance indicator, particularly in applications that require materials to bend, stretch, or deform in other ways without breaking.

[1][2] This formula helps in quantifying how much a material can stretch under tensile stress before failure, providing key insights into its ductile behavior.

It defines a material's suitability for certain manufacturing operations (such as cold working) and its capacity to absorb mechanical overload like in an engine.

This brittleness primarily stems from their strong ionic or covalent bonds, which maintain the atoms in a rigid, densely packed arrangement.

The significant difference in ductility observed between metals and inorganic semiconductor or insulator can be traced back to each material’s inherent characteristics, including the nature of their defects, such as dislocations, and their specific chemical bonding properties.

[6][7] Malleability, a similar mechanical property, is characterized by a material's ability to deform plastically without failure under compressive stress.

[4][11] Ductility is especially important in metalworking, as materials that crack, break or shatter under stress cannot be manipulated using metal-forming processes such as hammering, rolling, drawing or extruding.

The delocalized electrons allow metal atoms to slide past one another without being subjected to strong repulsive forces that would cause other materials to shatter.

[13][14] When highly stretched, such metals distort via formation, reorientation and migration of dislocations and crystal twins without noticeable hardening.

An important point concerning the value of the ductility (nominal strain at failure) in a tensile test is that it commonly exhibits a dependence on sample dimensions.

There have been both experimental studies[20] and theoretical explorations[21][22][23][24] of the effect, mostly based on Finite Element Method (FEM) modelling.

Nevertheless, it is not universally appreciated and, since the range of sample dimensions in common use is quite wide, it can lead to highly significant variations (by factors of up to 2 or 3) in ductility values obtained for the same material in different tests.

This point can be difficult to identify on a (nominal) stress-strain curve, because the peak (representing the onset of necking) is often relatively flat.

While the true strain at fracture is a genuine indicator of "ductility", it cannot readily be obtained from a conventional tensile test.

Furthermore, it is sensitive to exactly what happens in the latter stages of necking, when the true strain is often becoming very high and the behavior is of limited significance in terms of a meaningful definition of strength (or toughness).

The temperature where the material changes from brittle to ductile or vice versa is crucial for the design of load-bearing metallic products.

This leads to each of these structures having different performances as they approach failure (fatigue, overload, and stress cracking) under various temperatures, and shows the importance of the DBTT in selecting the correct material for a specific application.

This famously resulted in serious hull cracking in Liberty ships in colder waters during World War II, causing many sinkings.

The Charpy V-notch test determines the impact energy absorption ability or toughness of the specimen by measuring the potential energy difference resulting from the collision between a mass on a free-falling pendulum and the machined V-shaped notch in the sample, resulting in the pendulum breaking through the sample.

The DBTT is determined by repeating this test over a variety of temperatures and noting when the resulting fracture changes to a brittle behavior which occurs when the absorbed energy is dramatically decreased.

At a certain temperature, dislocations shield[clarification needed] the crack tip to such an extent that the applied deformation rate is not sufficient for the stress intensity at the crack-tip to reach the critical value for fracture (KiC).

If experiments are performed at a higher strain rate, more dislocation shielding is required to prevent brittle fracture, and the transition temperature is raised.