PORTABLE HARDNESS TESTING – PRINCIPLES AND APPLICATIONS (2023)

Abstract

Conventional hardness testers, like Rockwell, Brinell or Vickers machines, require the testpiece be brought to the testing device; but this is not always possible. Portable testingdevices have been developed that permit in-situ hardness measurements thus offeringquick and economical supplements to conventional, stationary testing machines. There aretwo different physical methods particularly recognized in the practical field and which areaccepted tools for many applications.

This paper explains the basic principles of both test methods and compares, usingexamples from the practical field, the application possibilities of both methods. Thesubjects critically discussed are the factors of influence on hardness testing, such as surfacepreparation or the wall thickness of parts to be tested, e.g. pipelines.

In addition to these successfully applied methods, Krautkramer now introduces acompletely new technique: The Through Diamond Technique. This optical mobilehardness tester measures, for the first time, real Vickers hardness under load.

Introduction

What is “hardness”?
With regard to metals, hardness has always been a subject of much discussion amongtechnical people, resulting in a wide range of definitions. Hardness properties include suchvaried attributes as resistance to abrasives, resistance to plastic deformation, high modulusof elasticity, high yield point, high strength, absence of elastic damping, brittleness or lackof ductility.

To a metallurgist, hardness is a material’s resistance to penetration. In general, an indenteris pressed into the surface of the material to be tested under a specific load for a definitetime 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 testmethod. Basically hardness values are arbitrary, and there are no absolute standards forhardness. Hardness has no quantitative value, except in terms of a given load applied in aspecific, reproducible manner and with a specified indenter shape.

Static indentation tests in which a ball, cone or pyramid penetrates into the surface of thematerial being tested are widespread. The relationship of load to the area or depth ofindentation is a measure of hardness, such as that found in common bench-top Brinell,Rockwell, Vickers or Knoop hardness testers.

The different methods and differently shaped indenters used by, for example, Brinell (HB)and Rockwell (HRC) produce dissimilar responses of the material under test. Conversiontables relating to e.g. HRC and HB values have to be determined empirically byexperimental evaluation of a specific material’s hardness with the different test methods.There exists no mathematical equation to transfer measurements from one scale to another.To compare the hardness of two different samples, both must be measured using the samehardness scale, or a scale must be developed to convert from one measurement to the other.

Why hardness testing?
In manufacturing applications, materials are primarily tested for two reasons: either toresearch the characteristics of a new material or as a quality check to ensure that thesample meets a particular specification.

(Video) Portable hardness tester Phase II PHT-1900 portable hardness tester rockwell brinell vickers Revised

How to measure hardness on-site?
Conventional hardness testing machines require the test piece be brought to the testingdevice; but this is not always possible. Portable testing devices have been developed thatpermit 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 applyinga specific test load. The frequency shift is evaluated and electronically converted to ahardness value displayed on the LCD. The MICRODUR instruments (Krautkramer) workaccording this method, the so-called UCI (Ultrasonic Contact Impedance) method.

Another well-known principle for portable hardness testers is the rebound method. TheDynaMIC and DynaPOCKET (Krautkramer), for example, measure the velocity of apropelled 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 beconverted into different scales by using conversion tables stored in the instrument fordifferent materials.

See Also
11 Revenue Models, Examples & Tips To Pick The Right OneList Of 13 Best Open Source & Free Monitoring Tools In 2022List Of Estates in Lekki – InformationTop Tips: Obtaining and Using Feedback

UCI and Rebound hardness testing with just one instrument!

The brand-new Krautkramer MIC 20 combines the two most successfully applied portablehardness testing principles in one instrument. Whether you want to use the UCI principleor the dynamic Rebound testing method – the MIC 20 offers both possibilities. All youneed is just one instrument and all current UCI probes and impact devices can be pluggedin and used.

The UCI Method

As in standard Vickers or Brinell hardness testing, thequestion as to the size of the test indentation in thematerial generated by a certain test load also arises inVickers hardness testing according to the UCI(Ultrasonic Contact Impedance) method. However,the diagonals of the test indentation, which have to beknown in order to determine the Vickers Hardnessvalue, are not evaluated optically as usual, but theindentation area is electronically detected bymeasuring the shift of an ultrasonic frequency

A UCI probe typically consists of a Vickers diamond attached to the end of a metal rod(Fig. 1). This rod is excited into longitudinal oscillation at about 70 kHz by piezoelectrictransducers.

Fig 1: Schematic descriptionof the UCI probe.

When the test load is applied, a frequency shift occurs as the diamond penetrates into thematerial. This frequency shift will become greater when the test indentation becomeslarger, that means when the diamond penetrates deeper into “soft” material. Analogously,the smallest frequency shift is produced by hard test materials; the diamond penetrates onlyslightly into the material and leaves a small indentation.

This is the secret of UCI hardness testing: the frequency shift is proportional to the size ofthe test indentation produced by the Vickers diamond. Equation (1) describes this basicrelation in comparison to the definition of the Vickers hardness value.

(Video) Portable Hardness Tester (Hindi)

(1)

Equation 1:

The Frequency shift as a function of the indentation size of a Vickersindenter. Df= frequency shift, A = area of indentation, Eeff = effectiveelastic modulus (contains the elastic constants of both the indenter andthe test piece), HV = Vickers hardness value, and F= Force applied in theVickers hardness test.

To carry out the UCI principle, a probe containing a rod with a Vickers diamond attachedto the contact end is resonated by piezoelectric ceramics at an ultrasonic frequency. Aspring applies the load and the frequency of the rod changes in proportion to the contactarea of the indentation produced by the Vickers diamond. Therefore, the hardness value isnot visually determined by the diagonals of the indent, but by an electronic measurementof the frequency shift within seconds.

The instrument constantly monitors the frequency, performs the calculation andinstantaneously displays the hardness value.

The frequency shift, nevertheless, also depends on the Young's modulus of elasticity,which is a material constant. For the practical application of the UCI-method, the Young'smodulus therefore has to be considered. The instrument has to be calibrated when thehardness of different materials with different values of the Young's modulus has to bedetermined.

After completing the calibration, the UCI method can be used for all materials showingthis modulus of elasticity. When being manufactured, the probes are calibrated on low-alloyed or unalloyed steels; however, modern test instruments can be calibrated quickly,also at the test location, to other materials as well, such as titanium or copper.

The Rebound method

Hardness testers using the Rebound method operate in a slightly different manner.Although the size of the test indentation generated is connected with the material hardnesseven in this case, it is indirectly measured via the loss of energy of a so-called impactbody. A mass is accelerated to the surface of the test object and impinges on it at a definedspeed, i.e. kinetic energy. The impact creates a plastic deformation of the surface, i.e. anindentation, due to which the impact body loses part of its original speed - or energy. Itwill lose more velocity when creating a bigger indentation on softer material. Technically,this principle of measurement is implemented by means of an impact body which has aspherical tungsten carbide tip and which is accelerated onto the test surface by springforce. The velocities after and before the impact are each measured in a non-contact mode.

This is done by a small permanent magnet within the impact body (Fig. 2) which generatesan induction voltage during its passage through a coil, with this voltage being proportionalto the speed.

Fig 2: Cross-cut of a typicalimpact device.

The inventor of this method, D. Leeb, defined “his own” hardness value, the Leeb hardnessvalue. The Leeb hardness value, HL, is calculated from the ratio of the impact and reboundspeed according to:

(2)

Equation 2:

The Hardness value according to Leeb (HL) is defined as the ratiobetween the rebound velocity (vR) after and the impact velocity (vI)before the impact of the tungsten carbide ball onto the test piecesurface.

You might ask yourself: "Who wants to measurethe hardness value in Leeb?" The answer is: as amatter of fact, anybody who uses the reboundhardness testing method does it, because the Leebhardness value is, by definition in the equation(2), the actual physical measurement value behindthis method. However, nearly no user indicates theLeeb hardness value HL in his specifications ortest reports. We mostly convert into the requiredhardness scales (HV, HB, HS, HRC, HRB,N/mm²). For this reason, only conversion bringsthe rebound hardness method to life. Empiricallydetermined conversion tables for differentmaterial groups are stored in the Krautkramerhardness testing instruments.

(Video) Steel Hardness Test (Leeb Rebound)

To apply the principle, an impact device uses aspring to propel an impact body through a guide tube towards the test piece. As it travelstowards the test piece, a magnet contained within the impact body generates a signal in acoil encircling the guide tube. After the impact, it rebounds from the surface inducing asecond signal into the coil. The Krautkramer instrument calculates the hardness valueusing the ratio of the voltages and analyzes their phases to automatically compensate forchanges in orientation. Due to the patented signal processing, there is no need for anymanual correction for the impact direction. Only Krautkramer hardness testers offer thisautobalancing feature.

Application solutions are determined by the force and ball size of the impact body. Theoperator can select between different impact devices for the DynaMIC hardness testers(Dyna D, Dyna E and Dyna G) as well as the DynaPOCKET instrument.

The Through-Diamond-Technique

While both methods – UCI- and Rebound – are successfully used in the field and solvemany on-site hardness testing applications, there are limitations concerning the kind ofmaterial under test and its size and weight, respectively. Furthermore, because of theinfluence of Youngs-Modulus, most conventional testing methods do not allow to measuredifferent materials without firstly calibrating or adjusting the instrument.

What are the advantages of the TDT method?

With Transpyramidal Indenter Viewing, oralso known as Through DiamondTechnique, we overcome this “handicap”:practically all kinds of material from steelto rubber and from aluminum to plasticscan be tested without the necessity ofinstrument calibration. With thisinstrument we developed an optical mobilehardness tester, allowing real Vickershardness measurement under load withouthaving to face the disturbing influence ofthe elastic properties of the test material,i.e. Youngs-Modulus.

The system consists of the portable baseinstrument including a graphical LCDdisplay and the TDT probe (see Figure 3).

Fig 3: Schematic descriptionof the TDT probe.

By applying a certain test load (e.g. 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 thediamond with an optical system having a CCD camera. Data evaluation is then made in theinstrument. As Vickers hardness is simply defined as the ratio between test load andindentation size (the diagonals of the indentation), the TDT-measurement of the diagonallength immediately produces a Vickers hardness value for the applied test load. The livepicture of the indentation displayed on the instrument’s LCD also allows immediatecharacterization of measurement reliability, i.e. the quality of the Vickers diamondindentation.

The physical method of TDT hardness testing – traced back to Vickers hardness – allowsmobile testing of different materials without the necessity of calibrating the instrument. Byviewing through the diamond under load, TDT opens up mobile hardness testing not onlyto new applications like coils, thin layers and coatings but also to different materials likeplastics, glass and high-tech materials such as ceramics or intermetallics.

While testing under load by viewing through the diamond, the TDT instruments even allowto 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 ofthe “older types” of tests, is that the indentations themselves can at times almostcompletely recover, and there is no permanent impression left, thus making measurementsimpossible. The TDT method eliminates that problem. It involves pressing a diamondpunch of known geometry into the surface of a material. The indentation size will bemonitored under load during the test.

(Video) All-in-one solution for portable hardness testing I Equotip 550

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 morecommonplace, even the softest metals will seem comparatively hard. It is a relative term.Applications for testing soft materials are nonetheless widespread. The automotiveindustry tests the hardness of paints and tires. The microelectronics and photonicsindustries test low-dielectric constant films, chemical and mechanical polishing pads, bondpads, solders, and electronic packaging materials. The biomaterials industry tests polymerjoint-implant materials, nail polish and drug particles. The medical field even testsbiological samples such as liver, cartilage, and arterial tissues. Determining meaningfulhardness values for soft materials has always been challenging, and despite recentadvances in methods and instruments, continues to be so.

The Through Diamond Technique (TDT)

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 thatpurpose it is necessary to light up the inner surfaces of the diamond using light-emittingdiodes (LED) geometrically arranged.

In order to obtain the highest resolution of the indentation picture it is necessary to matchthe wavelength of the LED light and the spectral sensitivity characteristics of the CCDchip. A special lens system was developed and adjusted to the LED to ensure maximumresolution. Computer assisted evaluation of the indentation and determination of thediagonal’s length occurs in three steps. A first step locates the approximate position of theindentation. After that the exact course of the indentation’s border is determined in localvicinities (so-called Areas of Interest) by applying suitable “transition filters” fordetermination of any grey scale transition. Finally the indentation surface and the diagonalsare determined using the intersections of the calculated borders and the edges of theVickers diamond. According to the definition of Vickers, the HV value is calculated for theapplied test load.

Practical application

The PC-based TDT system consists of a hand-held PC and the TDT probe. The interfacebetween instrument and probe serves as power supply for the probe as well as connectorfor all control functions. The interface also feeds the BAS signal from the CCD camera tothe frame grabber. With special software the data can be evaluated, the diagonals measuredand 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 indentationby applying the test load. It also allows an in situ quality characterization of the diamondand 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 about100 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 rangeof the TDT probe used for the measurement.

Fig 4: Typical Vickers diamond indentations obtained by TDT measurementson a) steel b) coiled steel c) Teflon and d) ceramics (Al₂O₃).

Figure 4 shows some typical Vickers diamond indentationsobtained by the TDT instrument on different materials undertest. TDT – for example – allows to determine the hardness of bulk material

1. enables tomeasure the hardness on coils
2. and also opens up hardness testing to new applicationslike high-tech materials
3. ceramics
4. or rubber and plastics.

Summary

By viewing through the diamond under load the Through Diamond Technique opens upnew applications for hardness testing. The portable instrument not only allows for on-sitemeasurements but also enables hardness measurements on all kind of materials. Theobtained hardness value corresponds to Vickers testing, with the exception that TDTmeasures under load, thus also enabling measurement of elastic materials where usualindentation hardness fails.

(Video) Starrett 3811A - Portable Hardness Tester (product video presentation)

Conclusion

Mobile hardness testing instruments will not replace the conventional bench-top machines,but nevertheless, they became an indispensable addition for hardness testing units. Duringthe last decades several portable instruments based on different physical methods weredeveloped. Today portable units are widespread and accepted tools for portable, on-sitehardness testing applications.

Those instruments solve plenty of mobile hardness testing tasks, but however, each methodis limited – more or less – to a specific application area and, therefore, the decision as towhich method and instrument to use strongly depends on the testing application.