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).
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+?)) ; 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
المادة المعروضة اعلاه هي مدخل الى المحاضرة المرفوعة بواسطة استاذ(ة) المادة . وقد تبدو لك غير متكاملة . حيث يضع استاذ المادة في بعض الاحيان فقط الجزء الاول من المحاضرة من اجل الاطلاع على ما ستقوم بتحميله لاحقا . في نظام التعليم الالكتروني نوفر هذه الخدمة لكي نبقيك على اطلاع حول محتوى الملف الذي ستقوم بتحميله .
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