Torsion Test• Ductile material twist• Brittle material fractures

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Impact Test (testing fracture characteristics under high strain rates)

Notched-bar impact tests are used to measure the impact energy (energy required to fracture a test piece under impact load), also called notch toughness. It determines the tendency of the material to behave in a brittle manner.

Due to the non-equilibrium impact conditions this test will detect differences between materials which are not observable in tensile test.

We can compare the absorption energy capacity before fracture ofdifferent materials.

Two classes of specimens have been standardized for notched-impact testing, Charpy (mainly in the US) and Izod (mainly in the UK)

CharpyIzod

h’h

Energy ~ h - h’

Charpy v-notch TestA 10mm square section material is tested, having a 45o notched, 2mm deep.

The impact toughness is determined from finding the difference in potential energy before and after the hammer has fractured the material.

Units are J (Joules) when testing Metals, J/cm2 when testing polymers (Polymers will stretch, metals will snap).

Impact Test Examples

6Magnesium4Low-Grade Plastic

14Aluminum20Titanium20Steel

Charpy Impact Strength, in JoulesMaterial

As temperature decreases a ductile material can become brittle - ductile-to-brittle transition.FCC metals show high impact energy values that do not change appreciably with changes in temperature.

Ductile-to-brittle transition

BCC metals, polymers and ceramic materials show a transition temperature, below which the material behaves in a brittle manner. The transition temperature varies over a wide range of temperatures. For metals and polymers is between -130 to 93oC. For ceramics is over 530oC.

In low alloy and plain carbon steels, the transition temperature is set to an impact energy of 20J or to the temperature corresponding to 50% brittle fracture.

Low temperatures can severely embrittle steels. The Liberty ships, produced in great numbers during the WWII were the first all-welded ships. A significant number of ships failed by catastrophic fracture. Fatigue cracks nucleated at the corners of square hatches and propagated rapidly by brittle fracture.

Ductile-to-brittle transition

1912: Titanic on its maiden voyage from Southampton April 10, 1912. credit: THE BETTMANN ARCHIVE

HardnessHardness: a measure of a material’s resistance to localized plastic deformation (eg. Small

dent or scratch).

Hardness: Different Techniques1. Scratch hardness 2. Indentation hardness3. Rebound hardness

Scratch HardnessEarly hardness test were based nature minerals with a scale constructed

solely on the ability of one material to scratch another (Mohs scale –German Friedrich Mohs).

Mohs scale ranges from 1 on the soft end for talc to 10 for diamond.

More accurate quantitative hardness techniques have been developed over the years in which a small indenter is forced into the surface of the material to be tested under controlled conditions of load and rate of application.

Mohs Hardness Mineral Absolute Hardness1 Talc (Mg3Si4O10(OH)2) 12 Gypsum (CaSO4·2H2O) 33 Calcite (CaCO3) 94 Fluorite (CaF2) 215 Apatite(Ca5(PO4)3(OH-,Cl-,F-)) 486 Orthoclase (KAlSi3O8) 727 Quartz (SiO2) 1008 Topaz (Al2SiO4(OH-,F-)2) 2009 Corundum (Al2O3) 40010 Diamond (C) 1500

The fundamental “physics” of hardness is not yet clearly understood.All hardness measures are functions of interatomic forces.There is no single measure of hardness has been devised that is

universally applicable to all materials. Hardness is arbitrarily defined.

Hardness – Some Basic Knowledge

Indentation HardnessResistance to permanent indentation under static or dynamic loadsExamples

Brinell Hardness Test (ASTM E 10) - Commonly used.Rockwell Hardness Test (ASTM E 18) - Commonly used. Indentor and

loads are smaller than with the Brinell test.Vickers Hardness Test (ASTM E 92) - Similar to Rockwell. Uses a

square-based diamond pyramid for the indentor.Knoop (Tukon) Hardness Test - used for very thin and/or very small

specimens.Rebound Hardness

Energy absorbed under impact loadsExamplesShore Scleroscope (ASTM E 448) - Measures the rebound of a small pointed device dropped from a 254mm height.Schmidt Hammer - Measures rebound of a spring loaded hammer. The test has been correlated with concrete compressive strength.

Brinell Hardness (BHN)A Load applied to a 10mm diameter ball.Measure diameter of the indentation to the nearest 0.02 mm under a

microscope.Compute the Brinell Hardness Number (BHN)

– D = ball diameter (mm) D = 10mm– Di = indentation diameter (mm)– F = load (units = kg)

Rockwell Hardness (HR)Widely used in the USA diamond cone shape indenter is used for hard metals or hard spherical steel ball for softer materials.Different combinations of loads and indenter (Rockwell scale).

A minor load (10 kg) is applied firstA major load (60, 100, 150 kg) is applied laterHardness is determined from the difference in penetration depthSeveral scales are used (A, B, C, etc.)The depth of the indentation is measured by the machine.No measurement is made by the operator other than dial reading of hardness.

A. Depth reached by indenter after preliminary test force (minor load).B. Position of indenter under total test force.C. Final position reached by indenter after elastic recovery of the material. D. Position at which measurement is taken.

Widely used in EuropeA square base diamond pyramid indenter is used for hard materials. The diagonals of the square indentation are measured.

Opposing indenter faces are set at a 136 degree angle to each other

2854.1DFHV =

Long side faces are set at a 172 degree, 30 minute angle to each other. Short side faces are set at a 130 degree angle to each other

Knoop Hardness (HK)

22.14DFHK =

Vickers Hardness (HV)

Pyramidal diamond shape indenter

Correlation between Hardness and Tensile Strength

TS (MPa) = 3.45xHB

TS (psi) = 500xHB

Note:No method of measuring hardness uniquely indicates any other single mechanical property.Some hardness tests seem to be more closely associated with tensile strength, others with ductility, etc.