Christian Ullner

Instrumented indentation test for advanced technical ceramics

The standard for advanced technical ceramics ENV 843-4 of 1995 (Vickers, Knoop and Rockwell superficial hardness tests) was validated within the framework of the CERANORM EC-project. The paper reports on depth sensing hardness measurements done for comparison with the other hardness tests. The instrumented indentation test is a modern technique (recent issue ISO/DIS 14577) that has the potential to take into account the specific response of materials in a much better way. An evaluation has been made to establish whether the instrumented hardness technique is a appropriate method for advanced technical ceramics and offers potential for additional applications.

Hardness testing on advanced technical ceramics

To validate the European standard ENV 843-4 for hardness measurements on ceramics, three classes of ceramic materials, silicon nitride, silicon carbide, and aluminium oxide, involving 19 ceramics in total, were tested using the traditional techniques Vickers (HV1), Knoop (HK2), and superficial Rockwell (HR45N). The use of new ceramic reference blocks certified according to the standards ISO 4547 and ISO 6507-1 for metallic materials was studied. If the hardness response of the tested materials does not involve chipping and cracking the application of high hardness reference blocks for training users to obtain hardness values comparable with the certified HV1 and HK2 values improves the reproducibility from about 10% to 1 to 3%. The scatter between the laboratories is similar to the scatter within the laboratories. The measurement of the indentation geometry on typical commercial ceramic materials can be made only with higher scatter and reduced reproducibility compared with typical metallic materials, which is caused by the stochastic indentation response. For such materials involving chipping and cracking (for instance SiC), the application of reference blocks with well-shaped indentations does not provide improved comparability of results between the laboratories. The actual indentation response of the ceramic material tested must be considered before selecting the appropriate hardness technique and test force. There is no significant difference between the abilities of the hardness techniques HV1, HK2, and HR45N to discriminate sensitively between materials of closely similar properties.

Requirement of a robust method for the precise determination of the contact point in the depth sensing hardness test

Abstract The fundamental quantities in the depth sensing hardness test are force and displacement. The penetration depth used for the calculation of a certain hardness parameter (for instance HU, Eq. (1) ) makes it necessary to determine the contact point. As shown in the paper, fitting of the force–displacement curve, F ( h ), by a second order polynomial and extrapolating to F =0 is not always the right way to achieve a sufficiently small uncertainty of depth. The feature of that procedure is analysed by a simple mathematical model. In dependence of the testing force, the scatter of the experimental data, and the required limit of depth uncertainty, a profound selection of the fitting range, the fitting function, and the extrapolation function should be done.

New Characterization of Setting Times of Alkali Containing Calcium Phosphate Cements by Using an Automatically Working Device According to Gillmore Needle Test

This paper presents equipment for the measurement of initial as well as final setting times of calcium phosphate cements containing alkali to achieve higher solubility. Until now, the selfsetting process of cements also if alkali is integrated has been tested by using the “standard test method for time of setting of hydraulic-cement paste by Gillmore needles” in accordance to ASTM C 266 – 99. This procedure needs high man power and, furthermore, the results have been markedly influenced by individual experiences that are incorporated in the criterion “without appreciable indentation”.

Determination of local stress–strain properties of resistance spot-welded joints of advanced high-strength steels using the instrumented indentation test

For spot-welded joints, the resistance to mechanical stress depends on the local strength properties and gradients in the weld area. The commonly used methods for investigating the stress–strain behaviour across the weld area are connected with a high level of sample preparation and with considerable limitations in local resolution. A promising method for determining locally resolved stress–strain curves is the instrumented indentation test in connection with the method of representative stress and strain (RS) and the method of artificial neural networks (NNs). The stress–strain properties of the weld nugget and the base metal determined by these two methods are compared and discussed, additionally in relation to the stress–strain curves obtained from the tensile test. The measured Vickers hardness across the weld area is compared with the evaluated local stress–strain properties. Three steels used in automobile manufacturing are investigated: mild steel DC04 and two advanced high-strength steels (TRIP steel HCT690T and martensitic steel HDT 1200M). The results of the two methods (RS and NN) show good correspondence for the base metal area but some significant differences for the weld nugget. Comparing the data across the weld area, no evidence of the presence of residual stress (which would influence the results) was found.

Instrumentierte Eindringpruefung zur Bestimmung lokaler Festigkeitskennwerte an Punktschweissverbindungen

Kurzfassung Die lokalen Festigkeiten im Bereich einer Schweißverbindung sind maßgebend für deren Widerstandsfähigkeit gegenüber mechanischen Beanspruchungen. Im Gegensatz zu anderen präparationsaufwendigen Methoden, z.B. die Untersuchung von Mikro-Zugproben, liefert die instrumentierte Eindringprüfung mit deutlich geringerem Aufwand Verläufe von Festigkeiten mit sehr guter Ortsauflösung. Im Rahmen dieser Arbeit werden wahre Spannungs-Dehnungskurven aus zyklischen Eindringversuchen mittels Auswertungen auf der Basis repräsentativer Spannungen und Dehnungen und auf der Basis neuronaler Netze an Punktschweißverbindungen aus hochfesten Stählen, jeweils im Bereich des Grundwerkstoffs und der Schweißlinse, bestimmt. Die Ergebnisse werden exemplarisch für einen Stahltyp mit Ergebnissen von Zugversuchen am Grundwerkstoff und am thermisch behandelten Werkstoff mit Schweißgut ähnlicher Struktur verglichen und diskutiert.