·Table of Contents
·Materials Characterization and testing
Transpyramidal Indentation Viewing - New possibilities for mobile hardness testingAuthor: Dr. Stefan Frank, KRAUTKRÄMER GmbH & Co. oHG, D - Hürth
What is hardness?
As applied to metals, hardness has always been a subject of much discussion among technical people , resulting in a multiplicity of definitions. Hardness properties include such varied attributes as resistance to abrasives, resistance to plastic deformation, high modulus of elasticity, high yield point, high strength, absences of elastic damping, brittleness or lack of ductility.
To a metallurgist hardness is a material's resistance to penetration. In general, an indenter is pressed into the surface of the material to be tested under a specific load for a definite time interval, and a measurement is made of the size or depth of the indentation
Hardness is not a fundamental property of a material, but a response to a particular test method. Basically hardness values are arbitrary, and there are no absolute standards for hardness. Hardness has no quantitative value, except in terms of a given load applied in a specific, reproducible manner and with a specified indentor shape.
Static indentation tests in which a ball, cone or pyramid is forced into the surface of the material being tested are widespread. The relationship of load to the area or depth of indentation is the measure of hardness, such as in common bench-top Brinell, Rockwell, Vickers or Knoop hardness testers.
The different methods and differently shaped indenters used by e.g. Brinell and Rockwell produce dissimilar responses of the material under test. Tables relating to HRC and HB values are only approximations - there exists no mathematical equation to transfer measurements from one scale into the other. So called conversion tables have to be determined empirically by experimental evaluation of a specific material's hardness with the different test methods. To compare the hardness of two different samples, both must be measured to the same hardness scale, or a scale must be developed to convert from one measurement to the other. Hardness scales are only in relationship to themselves!
Why hardness testing?
In manufacturing applications, materials are primarily tested for two reasons: either to research the characteristics of a new material or as a quality check to ensure that the sample meets a particular specification.
On-site hardness testing?
Conventional hardness testers like Rockwell, Brinell or Vickers machines require the test piece be brought to the testing device; but this is not always possible. Portable testing devices have been developed that permit in-situ hardness measurements.
One popular device measures the frequency shift of a resonating rod with a Vickers-diamond tip, which occurs when the diamond penetrates into the test material by applying a specific test load. The frequency shift is evaluated and electronically converted in a LCD readout of the hardness value. The MICRODUR 10 instrument (Krautkrämer) works according this method, the so called UCI (Ultrasonic Contact Impedance) method .
Another well-known principle for portable hardness testers is the rebound method. The DynaMIC (Krautkrämer), for example, measures the velocity of a propelled impact body directly before and after the impact onto the test material's surface. The ratio between both velocities indicates the hardness of the material, which can be converted into different scales by using conversion tables stored in the instrument for different materials.
While both methods are successfully used in the field and solve many on-site hardness testing applications, there are limitations concerning the kind of material under test and its size and weight, respectively. Furthermore, because of the influence of the Youngs-Modulus, most conventional testing methods do not allow to measure different materials without firstly calibrating or adjusting the instrument.
What are the advantages of the TDT method?
With the Through Diamond Technique we overcome this "handicap": Practically all kinds of material from steel to rubber and from aluminum to plastics can be tested without the necessity of instrument calibration. The disturbing influence of Youngs-Modulus on TDT hardness measurements is non-existent.
While testing under load by viewing through the diamond the TDT instruments even allow to measure the hardness of elastic or soft materials. Other types of tests, such as Brinell, Vickers or Knoop tests, have their difficulties. The problem with trying to apply some of the older types of tests, is the indentations themselves can at times almost completely recover, and there is no permanent impression left. The measurements then become impossible to make. The TDT method eliminates that problem. It involves pressing a diamond punch of a known geometry into the surface of a material. The indentation size will be monitored under load during the experiment.
In some industries, aluminum or soft metal alloys such as solder would be considered "soft" materials. But as the testing of rubbers, plastics, and polymers becomes more commonplace, even the softest metals will seem comparatively hard. It is a relative term. Applications for testing soft materials are nonetheless widespread. The automotive industry tests the hardness of paints and tires. The microelectronics and photonics industries test low-dielectric constant films, chemical and mechanical polishing pads, bond pads, solders, and electronic packaging materials. The biomaterials industry tests polymer joint-implant materials, nail polish and drug particles. The medical field even tests biological samples such as liver, cartilage, and arterial tissues. Determining meaningful hardness values for soft materials has always been challenging, and despite recent advances in methods and instruments, continues to be so.
The TDT method not only offers on-site hardness testing of different materials, it also
allows - depending on the test load - to measure the hardness of coils, coatings or layers.
|Fig 1: Schematic diagram of the TDT probe|
In order to obtain the highest resolution of the indentation picture it is necessary to match the wavelength of the LED light and the spectral sensitivity characteristics of the CCD chip. A special lens system was developed and adjusted to the red color LED to ensure maximum resolution. Computer assisted evaluation of the indentation and determination of the diagonal's length occurs in three steps. A first step locates the approximate position of the indentation. After that the exact course of the indentation's border is determined in local vicinities (so-called Areas of Interest) by applying suitable "transition filters" for determination of any grey scale transition. Finally the indentation surface or the diagonals are determined using the intersections of the calculated borders and the edges of the Vickers diamond. According to the definition of Vickers, the HV value is calculated for the applied test load.
|Fig 2: Typical Vickers diamond indentations obtained by TDT measurements on a) steel b) coiled steel c) Teflon and d) ceramics (Al2O3)|
|© AIPnD , created by NDT.net|||Home| |Top||