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Transpyramidal Indentation Viewing - New possibilities for mobile hardness testing Author: Dr. Stefan Frank, KRAUTKRÄMER GmbH & Co. oHG, D - Hürth Contact

Abstract

Mobile hardness testing is on the advance in the field of on-site application within the modern production process and plays an ever increasing part when testing large geometry parts and positions which are difficult to access. The test piece no longer needs to be cut and to be brought to the hardness tester - today mobile handheld instruments allow measurements to be made on the spot.
Several instruments based on different physical methods are particularly recognized in the practical field. Each method is limited - more or less - to a specific application area and therefore, the decision which method and instrument to use strongly depends on the test application. It is true that those instruments solve plenty of mobile hardness testing tasks, but because of the influence of material properties in hardness testing, i.e. the Youngs-Modulus, most conventional testing methods do not allow to measure different materials without firstly calibrating or adjusting the instrument.
With Transpyramidal Indentation Viewing or also known as Through Diamond Technique (TDT) we developed an optical mobile hardness tester, allowing real Vickers hardness measurement under load without having to face the disturbing influence of Youngs-Modulus. The system consists of the portable base instrument including a graphical LCD display and the TDT probe. By applying a certain test load (50 N) the diamond penetrates into the material. The indentation size of the Vickers diamond, i. e. the lengths of the diagonals, is automatically measured under load by viewing through the diamond with an optical system having a CCD camera. Data evaluation is then made in the instrument. As Vickers hardness is simply defined as the ratio between test load and the indentation size (i.e. the diagonals of the indentation) the TDT-measurement of the diagonal length immediately produces a Vickers hardness value for the applied test load. The live picture of the indentation displayed on the instrument's LCD also allows immediate characterization of measurement reliability, i.e. the quality of the Vickers diamond indentation.
The physical method of TDT hardness testing - traced back to Vickers hardness - allows mobile testing of different materials without the necessity of calibrating the instrument.
By viewing through the diamond under load, TDT opens up mobile hardness testing not only to new applications like coils, thin layers and coatings but also to different materials like plastics, glass and high-tech materials like ceramics or intermetallics.

Introduction

What is hardness?
As applied to metals, hardness has always been a subject of much discussion among technical people [1], 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 [2].
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.

The Through Diamond Technique (TDT)

For the first time the TDT hardness testing method allows immediate evaluation of the Vickers diamond under load by using a video camera. The determination of the indentation size, i.e. the measurement of the surface and the diagonal's length, respectively, is made by a computer-assisted, digital evaluation of the cameras picture. For that purpose the transition in color (grey scale) from the diamonds indentation to the unaffected surface of the material under test has to be detected. A reliable hardness measurement, therefore, is based on an exact determination of the indentation border.
Based on the geometry of the diamond indenter, the Through Diamond Technique is traced back to conventional Vickers hardness testing. The aim of both methods is to determine the indentation surface for a specific test load. But there are basic differences between both methods. Vickers hardness is defined as the ratio of test load to indentation size (i.e. diagonal length) after the test load has been applied. In this case the obtained hardness value is based on the plastic deformation of the material under test.
The TDT measures the "Vickers hardness" under load and considers the plastic as well as the elastic component - similar to hardness testing according to the Universal Hardness Method, which evaluates the penetration depth in dependence on the load.
The Through Diamond Technique, therefore, can be described as Vickers hardness testing under load.
The innovation with this technique is the evaluation of the Vickers diamond indentation, which takes place by viewing through the Vickers diamond using a CCD camera. For that purpose it is necessary to light up the inner surfaces of the diamond using light-emitting diodes (LED) geometrically arranged (see Fig.1.).

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.

Practical application

The PC-based TDT system consists of a PC and the TDT probe which is connected to the computer via an interface located on the ISA or PCI slot. This interface serves as power supply for the probe as well as connector for all control functions The interface also feeds the BAS signal from the CCD camera to the frame grabber, which is locked on the PCI bus. With special software the data can be evaluated, the diagonals measured and the hardness value calculated. The live picture through the diamond can be displayed, enabling the diamond's indentation process to be viewed, i.e. the growth of the indentation by applying the test load. It also allows an in situ quality characterization of the diamond and the diamond's indentation, respectively.
Depending on the resolution and the test load, different hardness ranges can be analyzed. The standard TDT probe with a test load of 50 N allows a measurement range from about 100 HV5 to 900 HV5. For softer materials a lower test load has to be applied.
In principle, all kind of materials can be tested as long as the hardness value is in the range of the TDT probe used for the measurement.
Fig. 2 shows some typical Vickers diamond indentations obtained by the TDT instrument on different materials under test. TDT - for example - allows to determine the hardness of bulk material
• (a), enables to measure the hardness on coils
• (b) and also opens up hardness testing to new applications like high-tech materials
• (c), rubber and plastics.

Fig 2: Typical Vickers diamond indentations obtained by TDT measurements on a) steel b) coiled steel c) Teflon and d) ceramics (Al2O3)

Summary

By viewing through the diamond under load the Through Diamond Technique opens up new applications for hardness testing. The portable instrument not only allows for on-site measurements but also enables hardness measurements on all kind of materials. The obtained hardness value corresponds to Vickers testing, with the exception that TDT measures under load, which enables to measure also elastic materials where usual indentation hardness failed.

References

1. H.E. Boyer, in: Hardness Testing, ASM International, Metals Park, Ohio, 1987
2. J.Kising, W. Weiler and I. Winckler, VDI-Berichte, 583 (1986), p. 371-391.

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