The presentation describes the information on deformation bands and kink bads

1. A presentation on
“DEFORMATION BANDS
AND KINK BANDS”
PREPARED BY : RAGHAVENDRA DARJI(385)
GUIDED BY: Dr. V.V. MATHANE.
M.E.-WELDING TECHNOLOGY,
FACULTY OF TECH. & ENGG.
THE M.S.UNIVERSITY, BARODA

2. stress v/s strain curve
Non Uniform
Deformation

3. Re orientation of planes
 In a tensile test, the plastic deformation of the single crystal occurs by the
slip on certain plane and particular directions.
 But the grip of the tensile machine (cross head) does not permit uniform
deformation by glide, instead the central portion of the crystal changes its
orientation. The rotation occurs of both the slip plane and the slip
direction into the axis of tension the comparison of free and constraint
deformation is shown in the figure below.

4. Deformation Bands:
 In case of poly crystal, the initial grain structures break down into
differently oriented sub grains. A common feature associated with the
microstructure of such cold worked metals is a banded appearance of
grains.
 The slip line may removed after sub sequent polishing of the surface, but
the bands due to re orientation can not be removed after polishing and
etching.
 Such regions of different orientations observed in deformed metals are
called deformation bands.
 Deformation bands are irregular in shape and appeared stretched in the
direction of principal strain.
 Both FCC and BCC metals show tendency for deformation bands but not
HCP.

5. Deformation Bands:
 In FCC crystals, because of 12 no. of slip systems , the critical
resolved shear stress can be achieved on more than one slip
system. Multiple slip may be operative from the beginning or it
may become operative soon after some initial deformation on the
primary slip system.
 The tendency for the formation of deformation bands is greater in
polycrystalline specimens because the restraint imposed by the
grain boundaries make it easy for orientation differences to arise
in a grain during deformation.

6. Kink bands
 The equation for the critical resolved shear stress shows that it will be
difficult to deform a hexagonal crystal when the basal plane is nearly
parallel to the stress axis.
 Orowan found that if a cadmium crystal of this orientation were loaded in
compression, it would deform by a localised region of the crystal of the
crystal suddenly snapping into tilted position with a sudden shortening of
the crystal.
 Suppose an HCP crystal is loaded in compression in the orientation
illustrated in fig 1. The resolved shear stress on the slip plane
(0 0 0 1) is zero. { Cos Ф=0} , and thus , deformation by slip mode
cannot be occur.

7. Kink bands
 Fig 1 : Kink Formation, (starting from left) 1) unreformed Cd crystal.
Basal plane parallel to stress axis, 2) kink formation after deformation, 3)
twinning results in relative displacement of the end, 4) Deformation
constrained

8. Kink bands
 Two important results of kink formation are:
1. The crystal decreases its length and thus accommodates the applied
stress. Kink are thus also called accommodation kinks.
2. Kinked region has different orientation, some slip system may become
operative as the resolved shear stress may become critical. Thus, the
process of slip can occur now in the new orientation of kinked region.
The restraint imposed by the grip in testing machine, at the end of the
crystal produces accommodation kink band.

9. Factors controlling the kink band
formation
 Density,
 Bulk Modulus: Resistance to compression
 Shear Modulus: Elastic Modulus
 Cohesion: The part of shear strength that is independent of the normal
effective stress in mass movement. OR the interatomic attraction
between like molecules.
 Tensile strength,
 Friction angle,
 Dilation angle.

10. Kink band
 Kink band formation is the most common failure mode found in fibre
reinforced composites under axial compression.
 The SEM proved to be the most effiencient equipment to use for kink
band observation.
Fig 2: kink band initiation without fibre failure and band broadening and fibre failure

11. Kink band in composite and kink band
geometry

15. Conclusions
 Inhomogeneous deformation of a crystal results in regions
of different orientations called a deformation bands.
 The crystallographic study of kink band gives the slip
direction and the orientation of stresses and their
concentration.
 Unresolved shear stress, in layered surfaces and twin re-
orientation inhibits slip or induces buckling stresses.

16. References:
 G.E.Dieter, “Mechanical Metallurgy”, Page No. 137.
 Vijendra Singh, “Physical Metallurgy”, Page No. 315-320.
 Shear bands: Role of friction and dilation angles, Roberto Weinberg,
Monash University, Australia.
 Deformation bands and kink bands, 2013-2014 Seminar, M.E-Welding
Technology, Faculty of Technology and Engineering, M.S.University,
Baroda, Gujarat, India.
 Micromechanics of kink-band formation, Department of Mechanical
Engineering and Industrial Management Faculty of Engineering, University
of Porto, Soraia Pimenta, March-2008.