Adiabatic shear band
Encyclopedia
Adiabatic shear band is a term used in physics
, mechanics
and engineering
.
Since the 1960s adiabatic shear
bands have been studied extensively because of their importance as a failure mode in areas such as metal forming and cutting, various types of ballistic
impact, as well as vehicle crashes.
An adiabatic shear band is one of the many mechanisms of failure
that occur in metals and other materials that are deformed at a high rate in processes such as metal forming, machining
and ballistic impact. Adiabatic shear bands are usually very narrow, typically 5-500 μm and they consist of very highly sheared material.
"Adiabatic" is a thermodynamic term meaning an absence of heat transfer
– the heat produced is retained in the zone where it is created. (The opposite extreme, where all heat that is produced is conducted away, is "isothermal".)
It is necessary to include some basics of plastic deformation to understand the link between heat produced and the plastic work done. If we carry out a compression test on a
cylindrical specimen to, say, 50 % of its original height, the stress of the work material will increase usually significantly
with reduction. This is called ‘work hardening’. During work hardening, the micro-structure, distortion of grain structure and the generation and glide of dislocations all occur. The remainder of the plastic work done – which can be as much as 90% of the total, is dissipated as heat.
If the plastic deformation is carried out under dynamic
conditions, such as by drop forging, then the plastic deformation is localized more as the forging
hammer speed is increased. This also means that the deformed material becomes hotter the higher the speed of the drop hammer. Now as metals become warmer, their resistance to further plastic deformation decreases. From this point we can see that there is a type of cascade effect: as more plastic deformation is absorbed by the metal, more heat is produced, making it easier for the metal to deform further. This is a catastrophic effect which almost inevitably leads to failure.
It appears that the first person to carry out any reported experimental programme to investigate the heat produced as a result of plastic deformation was Henri Tresca
. These results were reported in a very long and useful paper in June 1878
Tresca forged a bar of platinum
(as well as many other metals); at the moment of forging the metal had just cooled down below red heat. The subsequent blow of the steam hammer, which left a depression in the bar and lengthened it, also reheated it in the direction of two lines in the form of a letter X. So great was this reheating, the metal along these lines was fully restored for some seconds to red heat. Tresca carried out many forging experiments on different metals. Tresca estimated the amount of plastic work converted into heat from a large number of experiments, and it was always above 70%.
Physics
Physics is a natural science that involves the study of matter and its motion through spacetime, along with related concepts such as energy and force. More broadly, it is the general analysis of nature, conducted in order to understand how the universe behaves.Physics is one of the oldest academic...
, mechanics
Mechanics
Mechanics is the branch of physics concerned with the behavior of physical bodies when subjected to forces or displacements, and the subsequent effects of the bodies on their environment....
and engineering
Engineering
Engineering is the discipline, art, skill and profession of acquiring and applying scientific, mathematical, economic, social, and practical knowledge, in order to design and build structures, machines, devices, systems, materials and processes that safely realize improvements to the lives of...
.
Since the 1960s adiabatic shear
Shearing (physics)
Shearing in continuum mechanics refers to the occurrence of a shear strain, which is a deformation of a material substance in which parallel internal surfaces slide past one another. It is induced by a shear stress in the material...
bands have been studied extensively because of their importance as a failure mode in areas such as metal forming and cutting, various types of ballistic
Terminal ballistics
Terminal ballistics, a sub-field of ballistics, is the study of the behavior of a projectile when it hits its target. It is often referred to as stopping power when dealing with human or other living targets. Terminal ballistics is relevant both for small caliber projectiles as well as for large...
impact, as well as vehicle crashes.
An adiabatic shear band is one of the many mechanisms of failure
Failure
Failure refers to the state or condition of not meeting a desirable or intended objective, and may be viewed as the opposite of success. Product failure ranges from failure to sell the product to fracture of the product, in the worst cases leading to personal injury, the province of forensic...
that occur in metals and other materials that are deformed at a high rate in processes such as metal forming, machining
Machining
Conventional machining is a form of subtractive manufacturing, in which a collection of material-working processes utilizing power-driven machine tools, such as saws, lathes, milling machines, and drill presses, are used with a sharp cutting tool to physical remove material to achieve a desired...
and ballistic impact. Adiabatic shear bands are usually very narrow, typically 5-500 μm and they consist of very highly sheared material.
"Adiabatic" is a thermodynamic term meaning an absence of heat transfer
Heat transfer
Heat transfer is a discipline of thermal engineering that concerns the exchange of thermal energy from one physical system to another. Heat transfer is classified into various mechanisms, such as heat conduction, convection, thermal radiation, and phase-change transfer...
– the heat produced is retained in the zone where it is created. (The opposite extreme, where all heat that is produced is conducted away, is "isothermal".)
It is necessary to include some basics of plastic deformation to understand the link between heat produced and the plastic work done. If we carry out a compression test on a
cylindrical specimen to, say, 50 % of its original height, the stress of the work material will increase usually significantly
with reduction. This is called ‘work hardening’. During work hardening, the micro-structure, distortion of grain structure and the generation and glide of dislocations all occur. The remainder of the plastic work done – which can be as much as 90% of the total, is dissipated as heat.
If the plastic deformation is carried out under dynamic
Dynamics (mechanics)
In the field of physics, the study of the causes of motion and changes in motion is dynamics. In other words the study of forces and why objects are in motion. Dynamics includes the study of the effect of torques on motion...
conditions, such as by drop forging, then the plastic deformation is localized more as the forging
Forging
Forging is a manufacturing process involving the shaping of metal using localized compressive forces. Forging is often classified according to the temperature at which it is performed: '"cold," "warm," or "hot" forging. Forged parts can range in weight from less than a kilogram to 580 metric tons...
hammer speed is increased. This also means that the deformed material becomes hotter the higher the speed of the drop hammer. Now as metals become warmer, their resistance to further plastic deformation decreases. From this point we can see that there is a type of cascade effect: as more plastic deformation is absorbed by the metal, more heat is produced, making it easier for the metal to deform further. This is a catastrophic effect which almost inevitably leads to failure.
It appears that the first person to carry out any reported experimental programme to investigate the heat produced as a result of plastic deformation was Henri Tresca
Henri Tresca
Henri Édouard Tresca was a French mechanical engineer, and a professor at the Conservatoire National des Arts et Métiers in Paris....
. These results were reported in a very long and useful paper in June 1878
Tresca forged a bar of platinum
Platinum
Platinum is a chemical element with the chemical symbol Pt and an atomic number of 78. Its name is derived from the Spanish term platina del Pinto, which is literally translated into "little silver of the Pinto River." It is a dense, malleable, ductile, precious, gray-white transition metal...
(as well as many other metals); at the moment of forging the metal had just cooled down below red heat. The subsequent blow of the steam hammer, which left a depression in the bar and lengthened it, also reheated it in the direction of two lines in the form of a letter X. So great was this reheating, the metal along these lines was fully restored for some seconds to red heat. Tresca carried out many forging experiments on different metals. Tresca estimated the amount of plastic work converted into heat from a large number of experiments, and it was always above 70%.