壓滾座工藝及鉆M8螺紋底孔夾具設(shè)計(jì)（全套含CAD圖紙）,壓滾座,工藝,m8,螺紋,羅紋,底孔,夾具,設(shè)計(jì),全套,cad,圖紙 Hardness and hardness testing The hardness of a metal is its resistance to surface indentation under standard test conditions. Two main test methods are used, Brinell and Rockwell. The Brinell test In the Brinell test an indenter consisting of a hardened steel ball, mounted in a suitable holder, is forced into a prepared surface of the test-piece using a suitable load, which is maintained for fifteen seconds. The diameter of the circular indentation left in the surface after removal of the load is measured in two directions at right angles using a low-power graduated microscope. The average diameter is taken. The Brinell hardness number may be calculated formula HB=Load/Area of curved surface of indentation However, to avoid this tedious calculation, the Brinell hardness number may be found form tables relating hardness to the diameter of the indentation. each table of such relationships refers to a specific to a specific load and ball diameter. The Rockwell test In the Rockwell test the depth of the indentation is measured by the instrument and this is directly on a dial as a hardness value. No subsequent measurement of the indentation is involved. The test-piece is placed on the table of the machine and the indenter is brought into contact with the prepared surface under a minor load of 10 kfg. This takes up the slack in system and indicator is set to zero. The major load is then applied, and when the reading of the dial indicator is steady the minor load is taken off. The test-piece remains under the minor load while the hardness value is read directly form the dial indicator. Nine sales of hardness are available but the most commonly used are the B and C scales, the details of which are given in table. The dial is divided into 100 divisions, each representing one point on the hardness scale, the symbol HR is supplemented by a letter which denotes the particularly suitable for rapid routine checks on finished material. The C scale is used mainly for hardened steels, while the B scale is used for unhardened stells and the harder non-ferrous alloy. (B) Stress and strain When a force is applied to a material , it produces a stress in the material. The stress acting on the material the force exerted per unit area: Stress= Force/Area Stress may be tensile , compressive or shear in nature. Figure1.3 shows a metal block in tension, the load F is a stretching force which thus increases the length of the block and reduces its cross-section. If the metal block has a cross-sectional area A, then the tensile stress is F/A. The dimensional change caused by a stress is called strain. In tension , the strain is the ratio of the change in length to the original length. Tensile strain =Extension in length/Original length=l/L Being a ratio, strain is a number without units, but change both in length and original length must be expressed in the same units. Strain may also be expressed as a percentage. In figure 1.4 , the force F compresses the metal ., thus reducing its length and increasing its cross-section . in this case, the compressive stress is F/A and Compressive strain = Redution in length / Oringinal length = l/L . In elastic behaviour , the strain produced in a stressed material is completely removed as soon as the stress is removed. ,so that the material returns to its original dimensions. Some metallic materials show elastic properties up to fairly high stresses , while others have little , if any , elasticity. When an elastic material is loaded progressively in tension , the elastic strain produced is directly proportional to directly proportional to the stress causing it . This relational is known as Hookes law. The graph of stress against strain will be a straight line passing through the origin . the slope of this straight line is a constant for a given material . This constant is known as Youngs modulus , or the modulus of elasticity , and is denotrd by E, so that Modulus of elasticity E = Stress / Strain . Since strain is a dimensionless quantity , E has the same units as stress, The value of E is governed by the nature of the material , for steel it is about 2*105N/mm2 and I not much affected by composition or heat treatment , but decreases with increase in temperature. The higher the value of E the more apringy a material is .