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Brittle materials, such as oxide glasses, are usually very sensitive to flaws, giving rise to a macroscopic fracture strength that is much lower than that predicted by theory. The same applies to metallic glasses (MGs), with the important difference that these glasses can exhibit certain plastic strain prior to catas- trophic failure. Here we consider the strongest metallic alloy known, a ternary Co 55 Ta 10 B 35 MG. We show that this macroscopically brittle glass is flaw-insensitive at the micrometer scale. This discovery emerges when testing pre-cracked specimens with self-similar geometries, where the fracture stress does not de- crease with increasing pre-crack size. The fracture toughness of this ultra-strong glassy alloy is further shown to increase with increasing sample size. Both these findings deviate from our classical under- standing of fracture mechanics, and are attributed to a transition from toughness-controlled to strength- controlled fracture below a critical sample size.
The worldwide spread of windfarms brings new challenges, especially for concrete structures as a part of towers, connecting joints and foundations of wind turbines. High-cyclic loadings in such structures lead to a high relevance of the subject of fatigue. A proper assessment of the fatigue strength of concrete demands therefore a basis of reliable experimental data and the development of standardized testing methods. This article presents first results of an ongoing research program of BAM (Bundesanstalt für Materialforschung und -prüfung) which is a part of a joint project (WinConFat) funded by the German Federal Ministry for Economic Affairs and Energy. The subproject investigates the effects of size and slenderness of the specimens on the fatigue behaviour of high strength concrete at different stress levels. Not only the fatigue strength, but also the fatigue process itself is monitored by means of several measurement methods. Strain measurements are used to calculate the load dependent elastic modulus in the fatigue hysteresis as indicators for fatigue development. Furthermore, the application of non-destructive methods like acoustic emission analysis and ultrasonic measurement in laboratory tests gives a deeper insight into damage processes under cyclic loading. The results shall be used to improve design rules for concrete members under fatigue load and to develop or improve non-destructive techniques for in-service structural health monitoring.
The worldwide spread of windfarms brings new challenges, especially for concrete structures as a part of towers, connecting joints and foundations of wind turbines. High-cyclic loadings in such structures lead to a high relevance of the subject of fatigue. A proper assessment of the fatigue strength of concrete demands therefore a basis of reliable experimental data and the development of standardized testing methods. This article presents first results of an ongoing research program of BAM (Bundesanstalt für Materialforschung und -prüfung) which is a part of a joint project (WinConFat) funded by the German Federal Ministry for Economic Affairs and Energy. The subproject investigates the effects of size and slenderness of the specimens on the fatigue behaviour of high strength concrete at different stress levels. Not only the fatigue strength, but also the fatigue process itself is monitored by means of several measurement methods. Strain measurements are used to calculate the load dependent elastic modulus in the fatigue hysteresis as indicators for fatigue development. Furthermore, the application of non-destructive methods like acoustic emission analysis and ultrasonic measurement in laboratory tests gives a deeper insight into damage processes under cyclic loading. The results shall be used to improve design rules for concrete members under fatigue load and to develop or improve non-destructive techniques for in-service structural health monitoring.
Within a current research project which is funded by the German Research Foundation (DFG) the influence of the size and the geometry of specimens on the spalling result in fire tests is going to be investigated at Bundesanstalt für Materialforschung und -prüfung (BAM). This paper describes some preliminary results and a newly developed test setup. The novel setup enables the efficient and precise quantification of spalling by using a special photogrammetric measurement system.
Polycrystalline Ni materials with grain sizes less than 100 nm (nano crystalline NC) and with grain sizes in the micrometer range (micro crystalline MC) in form of thin films have become important in many technologies due to their improved physical, chemical and mechanical properties. Usual the mechanical properties of such coatings are described by a Hardness value and a Young´s modulus measured by Instrumented Indentation Testing (IIT).The behavior of such coatings during indentation test is influenced by different size effects having their representative length scale – grain size, coating thickness, length that characterizes the depth dependence of the hardness (Indentation Size Effect ISE). To estimate realistic values for the intrinsic coating Hardness and Young´s modulus all size effects have to be considered.
For this work thin nano crystalline Ni – coatings (average grain size 30 nm) with thickness from 1 μm to 5 μm were electrodeposited on brass substrates. Indentation tests in the continuous stiffness measurement (CSM) mode were provided on as prepared Ni – coatings using a G200 Nanoindentation system (Fa. Keysight). For estimation of the intrinsic hardness of the coatings from composite hardness values calculated from the measured force –displacement curve using the Oliver & Pharr method, the model described by Z.S. Ma [1] was used.
It was found that the experimental data can be well described by the model. The fitted values for the intrinsic hardness and the fitting parameters will be given. The different values of intrinsic hardness and of length characterizing depth dependence of the hardness for different coating thickness are discussed as results of changes in the coating structure because of changes in deposition parameters.
[1] Z.S. Ma, Y.C. Zhou, S.G. Long, C. Lu: On the intrinsic hardness of a metallic film/substrate system: Indentation size and substrate effects. International Journal of Plasticity 34 (2012) 1-11.
This work is supported by European Metrology Program for Innovation and Research (EMPIR)
(JRP 14IND03 Strength – ABLE)
Within a current research project which is funded by the German Research Foundation (DFG) the influence of the size and the geometry of specimens on the spalling result in fire tests is going to be investigated at Bundesanstalt für Materialforschung und -prüfung (BAM). This paper describes some preliminary results and a newly developed test setup. The novel setup enables the efficient and precise quantification of spalling by using a special photogrammetric measurement system.