Lüders bands, is type of
slip bands in metals or stretcher-strain marks which are formed due to localized bands of
plastic deformation
In engineering, deformation refers to the change in size or shape of an object. ''Displacements'' are the ''absolute'' change in position of a point on the object. Deflection is the relative change in external displacements on an object. Strai ...
in metals experiencing tensile stresses, common to low-carbon steels and certain Al-Mg alloys. First reported by
Guillaume Piobert, and later by
W. Lüders, the mechanism that stimulates their appearance is known as
dynamic strain aging
Dynamic strain aging (DSA) for materials science is an instability in plastic flow of materials, associated with interaction between moving dislocations and diffusing solutes. Although sometimes dynamic strain aging is used interchangeably with t ...
, or the inhibition of
dislocation
In materials science, a dislocation or Taylor's dislocation is a linear crystallographic defect or irregularity within a crystal structure that contains an abrupt change in the arrangement of atoms. The movement of dislocations allow atoms to ...
motion by interstitial atoms (in steels, typically
carbon
Carbon () is a chemical element with the symbol C and atomic number 6. It is nonmetallic and tetravalent—its atom making four electrons available to form covalent chemical bonds. It belongs to group 14 of the periodic table. Carbon makes ...
and
nitrogen
Nitrogen is the chemical element with the symbol N and atomic number 7. Nitrogen is a nonmetal and the lightest member of group 15 of the periodic table, often called the pnictogens. It is a common element in the universe, estimated at seve ...
), around which "
atmospheres" or "zones" naturally congregate.
As internal stresses tend to be highest at the shoulders of tensile test specimens, band formation is favored in those areas. However, the formation of Lüders bands depends primarily on the microscopic (i.e. average
grain
A grain is a small, hard, dry fruit ( caryopsis) – with or without an attached hull layer – harvested for human or animal consumption. A grain crop is a grain-producing plant. The two main types of commercial grain crops are cereals and legu ...
size and
crystal structure
In crystallography, crystal structure is a description of the ordered arrangement of atoms, ions or molecules in a crystalline material. Ordered structures occur from the intrinsic nature of the constituent particles to form symmetric patterns t ...
, if applicable) and macroscopic geometries of the material. For example, a tensile-tested steel bar with a square cross-section tends to develop comparatively more bands than would a bar of identical composition having a circular cross-section.
The formation of a Lüders band is preceded by a yield point and a drop in the flow stress. Then the band appears as a localized event of a single band between plastically deformed and undeformed material that moves with the constant cross head velocity. The Lüders Band usually starts at one end of the specimen and propagates toward the other end.
The visible front on the material usually makes a well-defined angle typically 50–55° from the specimen axis as it moves down the sample.
During the propagation of the band the nominal
stress–strain curve is flat.
After the band has passed through the material the deformation proceeds uniformly with positive
strain hardening
In materials science, work hardening, also known as strain hardening, is the strengthening of a metal or polymer by plastic deformation. Work hardening may be desirable, undesirable, or inconsequential, depending on the context.
This strength ...
. Sometimes Lüders band transition into the
Portevin–Le Chatelier effect
The Portevin–Le Chatelier (PLC) effect describes a serrated stress–strain curve or jerky flow, which some materials exhibit as they undergo plastic deformation, specifically inhomogeneous deformation. This effect has been long associated wi ...
while changing the temperature or
strain rate
In materials science, strain rate is the change in strain ( deformation) of a material with respect to time.
The strain rate at some point within the material measures the rate at which the distances of adjacent parcels of the material change ...
, this implies these are related phenomena
Lüders bands are known as a strain softening instability.
If a sample is stretched beyond the range of the Lüder strain once, no Lüder strain occurs any more when the sample is deformed again, since the dislocations have already torn themselves away from the interstitial atoms. For this reason, deep drawing sheets are often
cold rolled in advance to prevent the formation of stretcher-strain marks during the actual deep drawing process.
The formation of Lüder bands can occur again with a deformation over time, since the interstitial atoms accumulate by diffusing processes called
precipitation hardening
Precipitation hardening, also called age hardening or particle hardening, is a heat treatment technique used to increase the yield strength of malleable materials, including most structural alloys of aluminium, magnesium, nickel, titanium, and so ...
(or ''aging'').
See also
*
Portevin–Le Chatelier effect
The Portevin–Le Chatelier (PLC) effect describes a serrated stress–strain curve or jerky flow, which some materials exhibit as they undergo plastic deformation, specifically inhomogeneous deformation. This effect has been long associated wi ...
*
Adiabatic shear band
In physics, mechanics and engineering, 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. ...
*
Persistent slip bands
Formation of slip bands indicates a concentrated unidirectional slip on certain planes causing a stress concentration. Typically, slip bands induce surface steps (e.g., roughness due persistent slip bands during fatigue) and a stress concentration ...
References
Further reading
*
Richard W. Hertzberg
Richard is a male given name. It originates, via Old French, from Old Frankish and is a compound of the words descending from Proto-Germanic ''*rīk-'' 'ruler, leader, king' and ''*hardu-'' 'strong, brave, hardy', and it therefore means 'str ...
, ''Deformation and Fracture Mechanics of Engineering Materials, 4th Edition'', pp. 29–30
* W. Mason, "The Lüders' Lines on Mild Steel", 1910 Proc. Phys. Soc. London 23 3
{{DEFAULTSORT:Luders Band
Materials degradation
Mechanical failure