Uji kekerasan, uji impak dan uji keausan

Uji kekerasan dan uji impakUji kekerasanSecara umum kekerasan (hardness) didefinisikan sebagai ketahanan terhadap deformasi. Pada logam didefinisikan sebagai ketahanan terhadap deformasi plastis.Ada 3 jenis pengujian kekerasan :Scratch hardness.Identation hardness.Rebound (dynamic) hardness.

Hardness of MaterialsHardness test - Measures the resistance of a material to penetration by a sharp object.Macrohardness - Overall bulk hardness of materials measured using loads >2 N.Microhardness Hardness of materials typically measured using loads less than 2 N using such test as Knoop (HK).Nano-hardness - Hardness of materials measured at 110 nm length scale using extremely small (~100 N) forces.The hardness test measures the resistance to penetration of the surface of a material by a hard objectMany techniques have been used historically to determine hardness. The tests focused on here static indentation hardness test methods are widely used because of the ease of use and repeatability of the technique.Standard Static IndentationHardness Tests

Common Applications and Nomenclature for Hardness Tests

PRINCIPLES OF THE METHODThe basis of static indentation tests is that an indenter is forced into the surface of the material being tested for a set duration. When the force is applied to the test piece through contact with the indenter, the test piece will yield. After the force is removed, some plastic recovery in the direction opposite to the initial flow is expected, but over a smaller volume. Because the plastic recovery is not complete, biaxial residual stresses remain in planes parallel to the free surface after the force is removed. The hardness value is calculated by the amount of permanent deformation or plastic flow of the material observed relative to the test force applied. The deformation is quantified by the area or the depth of the indentation.Brinell Hardness TestIn the Brinell hardness test, the indentor is a hard steel sphere (usually 10 mm in diameter) that is forced into the surface of the material. The Brinell hardness value is calculated by dividing the test force by the surface area of the indentation. The test parameters taken into account are the test force and ball diameter while the indentation diameter is measured.Brinell Hardness TestBrinell hardness number (abbreviated as HB or BHN) is calculated from the following equation:

where F is the applied load in kilograms, D is the diameter of the indentor in millimeters, and Di is the diameter of the impression in millimeters. The Brinell hardness has the units of stress (e.g., kg/mm2).

Rockwell Hardness TestThe Rockwell hardness test uses a small-diameter steel ball for soft materials and a diamond cone, or Brale, for harder materials. The depth of penetration of the indentor is automatically measured by the testing machine and converted to a Rockwell hardness number: hardness Rockwell (HR). Optical measurement of the indention dimensions is not needed, the Rockwell test tends to be more popular than the Brinell test.Several variations of the Rockwell test are used (see : Common Applications and Nomenclature for Hardness Tests).A Rockwell C (HRC) test is used for hard steels, whereas a Rockwell F (HRF) test might be selected for aluminum. Rockwell tests provide a hardness number that has no units.

(c)2003 Brooks/Cole, a division of Thomson Learning, Inc. Thomson Learning is a trademark used herein under license.Indentors for the Brinell and Rockwell hardness testsThe Vickers TestA diamond square-based pyramid of 136 angle is used as the indenter, which gives geometrically similar impressions under differing loads (which may range from 5 to 120 kg). A square indent is thus produced, and the user measures the average diagonal length and again reads the hardness number (HV) from the tables. The Brinell and Vickers hardness values are identical up to a hardness of about 300 kgf /mm2 , but distortion of the steel ball occurs in Brinell tests on hard materials, so that the test is not reliable above values of 600 kgf/mm2.The Vickers Test

The Knoop TestThe Knoop test uses a diamond pyramidal indenter of apex angles 130 and 172.5. The Knoop Hardness Number (KHN, in kgf/mm2) is given by the relation:

UJI IMPAKImpact test : measures the total energy absorbed during specimen fracture.The standard specimen is hit by a pendulum at the opposite side of the notch and the energy required to break open is measured.

IMPACT TESTThe standard Charpy specimen is hit by a pendulum at the opposite side of the notch and the energy required to break open is measured.Standard specimen size is 10x10x55 mm3 with a V notch of 2 mm deep, 45o angle and 0.25 mm root radius.

Notch bar impact testNotches caused by poor machining, fabrication, or design concentrate stresses and reduce the toughness of materials. The notch sensitivity of a material can be evaluated by comparing the absorbed energies of notched versus unnotched specimens.Notched bar test specimens are used in different sizes and designs.The Charpy impact specimen is the most widely used.The parameter (energy absorption) obtained are not readily expressed in terms of stress level, so it is difficult for design.Can use the test result to indicate how brittle the materials are.Ductile to Brittlle Transition Temperature CurveOne of the primary functions of Charpy and Izod tests is to determine whether or not a material experiences a ductile-to-brittle transition with decreasing temperature and, if so, the range of temperatures over which it occurs. The ductile-to-brittle transition is related to the temperature dependence of the measured impact energy absorption.The absorbed energy (Joule) is plotted against testing temperature, giving a ductile to brittle transition temperature curve (DBTT curve).The curve represents a change in fracture behaviour from ductile at high.Ductile to Brittlle Transition Temperature Curve