The following are the fundamental mechanical properties of materials:
(i) Tensile Strength (ii) Hardness (iii) Impact Strength (iv) Fatigue (v) Creep
(ii) Hardness :
It is ability to resist abrasion, indentation (or penetration) and scratching by harder bodies.
There are several methods of hardness testing : Brinell , Rockwell , Vickers , and Knoop s hardness tests .
Brinell hardness number is the hardness index, calculated by pressing a hardened steel ball (indenter) into test specimen .
The Rockwell hardness index is measured by the depth of penetration by a small indenter. By selecting different loads and shapes of indenter, different Rockwell scales have been developed.
Moh’s scale has been developed for comparison of hardness. In order of increasing
hardness, the following ten minerals are arranged: Talc, Gypsum, Calcite, Flourite, Apatite, Orothoclase, Quartz, Topaz, Corundum and Diamond.
Sometimes, hardness is expressed in terms of macro-hardness and micro-hardness. The macro hardness of material relates to its resistance to larger volume displacements in plastic deformation, whereas microhardness is the hardness of the materials in microscopically small volumes e.g. in grain boundaries.
Hardness of materials is of importance for dies and punches, limit gauges , cutting tools, bearing surfaces etc. Softness of a material is opposite extreme of hardness.
(iii) Impact Strength: The resistance of a material to fracture under dynamic loads. Impact strength of a material is a complex characteristic which takes into account both the toughness and strength of a material.. It is defined as the specific work required to fracture a test specimen with a stress concentrator (notch) in the mid when broken by a single blow of striker in pendulum-type impact testing machine:
KC = K/So (where K is the work of the fracture and So is the cross-sectional area of the specimen at the notch).
There are two standard test methods:
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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