Download Compression The mechanical properties of a ductile metal are

Compression
The mechanical properties of a ductile metal are generally obtained from a tension test.
Compression behavior is of interest in the metal forming industry, since most processes:
rolling and forging involve compression deformations of the metal.
In compression an elastic range is exhibited as in tension and the elastic modulus,
proportional limit and yield point or proof stresses have closely corresponding values for the
two types of deformation. The real problem arises in a compression test when the metal
enters the plastic range.
The test piece has to be relatively short to avoid the possibility of instability and buckling.
The axial compression is accompanied by lateral expansion, but this is restrained at the ends
of the specimen owing to the friction between machine platens and the end faces, and
consequently on a shorter specimen marked barreling occurs as in Fig. (1).
Fig. (1) : deformation during a compression test.
This causes a non uniformity of stress distribution, and conical sections of material at each
end are strained and hardened to a lesser degree than the central region. The effect on the
load – compression curve, after the smaller values of plastic strain have been achieved, is a
fairly rapid rise in the load required to overcome friction and cause farther compression.
Owing to the barreling effect, only an average stress can be computed from the load –
compression curve, based on an average area determined from considerations of constant
volume.
Hence (as for tension): - 𝐴 =
𝐴°
(1+𝜀)
; compressive strains are negative (A > A ͦ ) → various
methods have been attempted to overcome the effects of barreling, none of which is
completely successful. The most satisfactory appears to be the technique of using
several cylinders of the same metal having different diameter – to – length ratios.
Incremental compression tests are conducted on the set of cylinders aat a series of
loads of increasing magnitude, measuring the strain for each of the cylinders at each
load. Extrapolation of curves of D/L against strain with load as parameter to a value
of D/L = 0 , representing an infinitely long specimen where barreling would be
negligible, enables the true compressive stress- strain curve to be determined (Fig.
2). Failure of a ductile metal in compression only occurs owing to excessive barreling
causing axial splitting around the periphery.
For brittle materials, such as flake cast-iron, concrete which would not normally be used
intension, the compression test is used to give quantitative mechanical properties. Although
end fraction still occurs, which affects the stress values some what, owing to the absence of
ductility in these materials the barreling condition is barely achieved. Fracture takes place on
planes of maximum shear stress as in Fig. (3).
Fig. (2); compressive stress – strain curves for various Diameter / length ratios.
Concrete
Flake iron
Timber