Auswahl der wissenschaftlichen Literatur zum Thema „Ball-on-three-balls (B3B) test“

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.

A newly developed process chain enables the production of active structural components. Thereby lead zirconate titanate (PZT) fibres are integrated in metal sheets by joining by forming. For the further development of the process chain to a high volume production the fracture strength of PZT should be increased. This paper pursues the approach to increase the strength by selected electrode layers. For this purpose, various electrode layers are tested on PZT plates in strength tests with the ball on three balls test (B3B-test). The evaluation of the recorded values is carried out according to statistically methods. The results show that the strength of PZT can be increased by electrode layers up to 30%. In addition, it was established that a high tensile strength and a high Young’s modulus of the electrode layers do not have a positive effect on the strength of PZT.

Open cell ceramic foam filters are utilized to reduce non-metallic inclusions during casting of metals and therefore to enhance the quality of cast parts. A new generation of multifunctional filters made of carbon bonded alumina (Al2O3-C) has been developed within the scope of the collaborative research center CRC 920 [1]. The assessment of the resistance against high thermal and mechanical loads requires a mechanical characterization of the ceramic filter material. The mechanical properties show a distinctive size dependency, that’s why the specimen dimensions should be similar to the strut size of the real filter structures. The tensile fracture behavior is investigated by means of the Small Punch Test (SPT) using miniaturized disk-shaped specimens. During the mechanical tests a load-displacement curve is measured until failure occurs and a fracture stress is calculated from the experimental results. An estimation about the failure probability by means of Weibull statistics is performed because of the large scatter of the strength of the material. Furthermore, a modified version of the SPT, the so called Ball On Three Balls Test (B3B), is applied and compared to the SPT. In a final step numerical simulations of the B3B tests are performed by means of the finite element method to identify fracture mechanical material parameters like the fracture toughness.

In this work, the effect of the test configuration and temperature on the mechanical behaviour of cemented carbides (WC-Co) with different carbide grain sizes (dWC) and cobalt volume fractions (VCo), implying different binder mean free paths (λCo), was studied. The mechanical strength was measured at 600 °C with bar-shaped specimens subjected to uniaxial four-point bending (4PB) tests and with disc specimens subjected to biaxial ball-on-three-balls (B3B) tests. The results were analysed within the frame of the Weibull theory and compared with strength measurements performed at room temperature under the same loading conditions. A mechanical degradation greater than 30% was observed when the samples were tested at 600 °C due to oxidation phenomena, but higher Weibull moduli were obtained as a result of narrower defect size distributions. A fractographic analysis was conducted with broken specimens from each test configuration. The number of fragments (Nf) and the macroscopic fracture surface were related to the flexural strength and fracture toughness of WC-Co. For a given number of fragments, higher mechanical strength values were always obtained for WC-Co grades with higher KIc. The observed differences were discussed based on a linear elastic fracture mechanics (LEFM) model, taking into account the effect of the temperature and microstructure of the cemented carbides on the mechanical strength.

Tato diplomová práce se zabývá numerickou simulací B3B zkoušky. Byla provedena rešerše v souvisejících oblastech. Poté byly provedeny numerické simulace této zkoušky pro mnoho různých konfigurací (rovný, zakřivený a zakřivený vrstvený vzorek) pro disk a desku. Byla zjištěna hyperbolická závislost mezi tloušťkou a f faktorem. Závislost f faktoru na křivosti byla lineární. Byla provedena demonstrace možnosti prasknutí na rozhraní vrstev. Poslední část ukázala, jak by zanedbání vlivu zbytkového napětí mohlo vést ke špatnému vyhodnocování experimentů