Walter Harrer

Chairside CAD/CAM materials. Part 2: Flexural strength testing

OBJECTIVE Strength is one of the preferred parameters used in dentistry for determining clinical indication of dental restoratives. However, small dimensions of CAD/CAM blocks limit reliable measurements with standardized uniaxial bending tests. The objective of this study was to introduce the ball-on-three-ball (B3B) biaxial strength test for dental for small CAD/CAM block in the context of the size effect on strength predicted by the Weibull theory. METHODS Eight representative chairside CAD/CAM materials ranging from polycrystalline zirconia (e.max ZirCAD, Ivoclar-Vivadent), reinforced glasses (Vitablocs Mark II, VITA; Empress CAD, Ivoclar-Vivadent) and glass-ceramics (e.max CAD, Ivoclar-Vivadent; Suprinity, VITA; Celtra Duo, Dentsply) to hybrid materials (Enamic, VITA; Lava Ultimate, 3M ESPE) have been selected. Specimens were prepared with highly polished surfaces in rectangular plate (12×12×1.2mm3) or round disc (Ø=12mm, thickness=1.2mm) geometries. Specimens were tested using the B3B assembly and the biaxial strength was determined using calculations derived from finite element analyses of the respective stress fields. Size effects on strength were determined based on results from 4-point-bending specimens. RESULTS A good agreement was found between the biaxial strength results for the different geometries (plates vs. discs) using the B3B test. Strength values ranged from 110.9MPa (Vitablocs Mark II) to 1303.21MPa (e.max ZirCAD). The strength dependency on specimen size was demonstrated through the calculated effective volume/surface. SIGNIFICANCE The B3B test has shown to be a reliable and simple method for determining the biaxial strength restorative materials supplied as small CAD/CAM blocks. A flexible solution was made available for the B3B test in the rectangular plate geometry.

Strength Tests on Silicon Nitride Balls

For some years ceramic bearing balls based on silicon nitride have been routinely used in technical practice. An important property of bearing balls is their strength, but appropriate testing methods are still missing. In this paper four different methods for strength testing are applied to commercial bearing balls. Each of the tests needs a different type of specimen, their preparation needs a very different effort, and the stress state applied to the specimens is also very different. This causes pros and cons, which are discussed in detail. The conventional 4-point bending test characterises the material in the interior of the balls. The applied stress state is uniaxial. The machining of the bending bars out of the balls is time intensive and costly. The ball on three balls test also characterises interior of the balls. The stress state is biaxial. The machining of the disc shaped specimens out of the balls is less expensive than the production of bending bars, but the finish of the tensile loaded surface needs special care. The data of both types of tests can be converted into each other using Weibull theory. The specimens in the triple ball crush test are as-received bearing balls, which are squeezed together. This causes some kind of contact loading, as will also occur in service. Failure is caused by the creation and growth of contact cracks, followed by a collapse of the compressed and cracked material. A detailed analysis of test results is complicated. It can be speculated that the component’s behaviour is mainly influenced by the toughness of the material and that the flaws in the material or at the component’s surface are of less significance. In the newly developed notched ball test the highest stressed region is a part of the original surface of the balls. Machining of the notch is straightforward. The stress state is almost uniaxial. The strength depends on size of flaws in the surface region. Therefore the notched ball test is a relevant measure to characterize the quality of the bearing balls.

Failure Analysis of Si3N4 Rolls for Wire Hot Rolling by Numerical Simulation of Thermal and Mechanical Stresses

Silicon nitride rolls for wire hot rolling have been tested in the rolling mill. After short employment for processing of ultra-high strength materials, cracks appeared in the roll calibre, which limited further application of the rolls. In order to find out possible causes for the damage, relevant mechanical properties of the roll have been determined and thermally and mechanically induced stresses were assessed by numerical simulation. It has been found that thermal stresses are of little relevance, whereas mechanically induced stresses are high enough to enhance subcritical crack growth resulting in the development of the macroscopic cracks, so that the observed damage of the rolls could have been adequately explained. One further result of the analysis was that flaws induced by grinding are decisive for the service time of ceramic rolls, so that special attention has to be paid to the machining of the roll calibre.

Silicon nitride tools for hot rolling of high-alloyed steel and superalloy wires : load analysis and first practical tests

Abstract For hot rolling wires of high-alloyed steels or superalloys tools are nowadays made of cemented carbides. In service they suffer from roughening of the surfaces and severe wear, which deteriorates the surface quality of the wires and restricts the lifetime of the tool. Due to their high hardness and good high-temperature properties, improvements of tool behaviour can be expected by the use of ceramic. In this paper the suitability of silicon nitride as material for rolls is investigated. The thermal and mechanical loads in silicon nitride rolls during the hot rolling of steel and superalloy wires are analysed. Although the working temperature can be up to 1100 °C the tensile thermal stresses in the rolls reach only a few percent of the materials strength. But mechanical stresses due to contact stresses may become severe. When rolling wires of superalloys tensile contact stresses in the rolls can reach up to 600 MPa – about 60 % of the characteristic bending strength of the silicon nitride material. Experiments in the rolling mill of Boehler in Kapfenberg confirm these theoretical findings. When rolling high-speed tool steels the silicon nitride rolls were superior to the common hard metal rolls. But when rolling superalloys cracks in the rolls arise. For less demanding applications (driving rollers, guiding rollers) silicon nitride rolls are still routinely used by Boehler in Kapfenberg. In summary, silicon nitride ceramics are well suited as tool material for rolling steel wires, if the rolls are properly manufactured and used. For rolling superalloy wires the ceramic material is at its limit, and a safe operation can only be expected for rolls with a material-based design.

Strength and Fractography of Piezoceramic Multilayer Stacks

Modern low-voltage piezoelectric actuators consist of a stack of piezoceramic layers (PZT) with metallic electrodes in between. Due to the use of these parts in automotive applications, a big but sensitive market is opened. During application mechanical stresses are an inherent loading of these electro-mechanical converter components. Therefore some strength of the actuators is necessary to guarantee a demanded life time. Bending and tensile tests were performed on commercial components to measure the strength in axial direction. Fracture surfaces were investigated with the methods of fractography to get information about the weakest links in the microstructure.

Silicon Nitride Tools for Hot Rolling of High-Alloyed Steel and Superalloy Wires

For hot rolling wires of high-alloyed steels or superalloys tools are nowadays made of ce¬mented carbides. In service they suffer from roughening of the surfaces and severe wear, which de¬teriorates the surface quality of the wires and restricts the lifetime of the tool. Due to their high hard¬ness and good high-temperature properties, improvements in tool behaviour can be expected by the use of silicon nitride tools. Experiments with several types of rollers were performed in commercial rolling mills. At modest and medium severe loaded positions (e.g. in the case of guidance rolls) silicon nitride rolls show superior performance to conventional steel or cemented carbide rolls. At the most severe loaded positions silicon nitride rolls were also superior to conventional rolls when rolling high strength steel wires. But for rolling superalloy wires, cracks, which limited further applications of the rolls, appeared in the roll surface profile (calibre). Cracks in the surface of the rollers are in general caused by Hertzian contact stresses, which can reach several hundred MPa. These cracks come into existence if a limiting load is exceeded. Then small flaws can quickly extend to a length of more then one millimetre, and then they stop again (pop in behaviour). Popped in cracks can slowly extend by cyclic fatigue up to a length where breaking out of large fragments of the rollers occurs. The critical load depends on the flow curve of the rolled materials and on the design of the rolls. For the analysed design it is exceeded when rolling superalloy wires, but it is not exceeded when rolling materials having a lower flow curve.

Biaxial Strength Testing on Mini Specimens

Biaxial strength testing of brittle materials using the ball on three balls (B3B) test is a new method for strength testing of disc or plate specimens [1] (see Fig. 1). The analysis of the stress fields and the calculation of the effective volumes and surfaces for several types of specimens and testing geometries has been performed recently. The ball on three balls testing method has several advantages compared to the common three- or four point bending tests: the results are very stable against small geometrical inaccuracies of the specimen or the test assembly, edge defects are not relevant and there exists only a very low influence of friction [2]. This makes this type of experiment ideal for testing very small specimens [3].

Failure Investigation of Forming Rolls Made of Si3N4

Tool surfaces for the forming of shaped steel strips are typically made from cemented carbides. Disadvantages of these tools are that they suffer from roughening of the surfaces and severe wear, which deteriorates the surface quality of the products and restricts the lifetime of the tool. Due to their high hardness and better high-temperature properties, improvements of tool behaviour can be expected by the use of silicon nitride. During a collaborative project between industrial partners, universities and research centres, forming rolls made of silicon nitride were tested at several positions in rolling mills. The suitability of Si3N4-rolls in rolling mills could be demonstrated at low and medium hard loaded positions. At Böhler-Profil in Waidhofen/Austria the rollers were used for the preparation of shaped steel strips from blank feedstock. During this very severely loaded application a pair of rollers failed. It could be shown that the rolls failed due to thermal stresses which can be reduced to a large extent by an improved design.

Influence of the Sample Size on the Results of B3B-Tests

The ball-on-three-balls (B3B)-test is a biaxial strength test for brittle materials. The results of B3B-tests are very stable against small geometrical inaccuracies of the specimens or the test support. In contrast to conventional bending tests there exists only a small influence of friction and edge defects are not relevant. These advantages, compared to beam tests, make the testing of mini-specimens with volumes of a few mm3 feasible. For this investigation silicon nitride specimens of different sizes were tested by use of the B3B-test. The maximum tensile stresses and the effective volumes and effective surfaces of the specimens were determined. The obtained results are compared directly and with the results of conventional 4-point-bending tests and are discussed in the framework of the Weibull-Theory. Additionally fracture surfaces of B3B-specimens and bending specimens were investigated fractographically to identify possible fracture origins.

Einfluss von Kontaktspannungen auf die Festigkeit im 4-Kugelversuch

Kurzfassung Der 4-Kugelversuch ist ein neuer Festigkeitstest zur Ermittlung der Zugfestigkeit spröder Werkstoffe unter bi-axialer Beanspruchung. Eine Scheibe wird auf drei Kugeln gelagert und auf der gegenüberliegenden Seite mit einer vierten Kugel zentrisch bis zum Bruch beansprucht. Wegen der gut definierten und stabilen Lasteinleitung eignet sich dieser Test auch zur Prüfung von Proben mit leicht gekrümmten Oberflächen. Die Prüfung gesinterter Proben ohne aufwendige und kostenintensive Hartbearbeitung ist somit möglich. Beim Kontakt von Kugeln und Platten entstehen in den Platten Zugspannungen, die in spröden Werkstoffen zur Entstehung von Rissen führen können (Hertz Risse). In dieser Arbeit wird der Frage nachgegangen, ob bei der Lasteinleitung solche Schäden entstehen und ob Kontaktrisse zu einer Verringerung der Probenfestigkeit führen. Die hier vorgestellten Arbeiten an Proben mit künstlich eingebrachten Rissen und an Proben ohne Vorschäden zeigen, dass Risse in der Probe im Bereich der Krafteinleitung (Kontakt zwischen Kugeln und Probe) keinen Einfluss auf die Festigkeit haben.