Micro-Tensile Tests
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Mikrozugprüfung
Development of miniature test specimens
The increasing miniaturisation of components in microelectronics and microsystems engineering requires a precise understanding of strength and deformation behaviour, as well as deformation and fracture mechanisms, in order to ensure the functionality of such microsystems and components [1] (see also: micromechanics & nanomechanics). Furthermore, for the design and construction of these microcomponents, the optimisation of the properties of composite materials or medical applications, reliable micro- or nanoscale material values are required to accurately simulate these properties. As the standard test specimens used in materials and polymer testing are unsuitable for determining the material properties of microcomponents, whilst at the same time geometry-independent characteristic values (see: geometry criterion) are required, specialised miniature test specimens have increasingly been developed; however, these impose high demands in terms of both testing methodology and technical capabilities. These test specimens are also subject to high preparatory and geometric requirements, as the quality of the surface and geometric consistency can have a significant influence on the level of the characteristic values. At the same time, residual stresses and orientations, as well as structural and morphological inhomogeneities, have a greater influence due to the stress state of the miniature test specimens than is the case when using standard test specimens. Consequently, established analysis and evaluation concepts from continuum and fracture mechanics lose their validity for microspecimens, which is why the dependence of material properties on specimen dimensions is of essential importance.
In-situ experiments in the ESEM
These miniature test specimens therefore place very high demands on handling and testing techniques. Whilst micro-testing set-ups are already standard as accessories for in-situ experiments in the ESEM (Environmental Scanning Electron Microscope) (Fig. 1), external testing equipment has only been designed and built in recent years.
| Fig. 1: | MT5000 micro-tensile testing system from Deben, Suffolk, UK, as an attachment to the ESEM Quanta 600 FEG from FEI, Eindhoven, Netherlands |
Micro-testing machines
Micro testing machines are testing systems with a nominal load of <100 N which, depending on the load cell used, produce reliable measurement results from at least 0.5 N with an accuracy class of 0.5 per cent. These testing machines are available as optional systems for conventional universal testing machines or as stand-alone units. A typical example of a stand-alone micro-testing machine is the inspekt micro LC100N from Hegewald & Peschke GmbH, Nossen, which can be used for both quasi-static and dynamic tests (see: fatigue) (Fig. 2).
| Fig. 2: | The inspekt-micro LC100N micro-inspection system from Hegewald & Peschke, Nossen |
In addition to its internal length measurement unit, this testing system features integrated video recording and can be optionally upgraded with an ESPI (laser speckle extensometer). The micro-testing system allows for quasi-static tensile, compression and bending tests, as well as shear tests, at a maximum test speed of 120 mm/s over a measurement range of 10 mm. The displacement resolution is 20 nm with a measuring accuracy of 3 µm. The smallest force transducer, with a capacity of 10 N, has an accuracy of 0.05 N. This enables tensile tests to be carried out on microspecimens and bond wires, shear tests on solder joints, as well as instrumented hardness measurements and compression tests on a wide variety of materials. In dynamic test mode at up to 50 Hz, component tests can also be carried out, for example on microswitches or biomedical materials.
The micro-tensile test
The micro-tensile testing machine marketed by Dr. Wazau Mess- und Prüfsysteme GmbH, Berlin, represents a specialised variant of micro-testing technology, as it was designed as an add-on device for a universal testing machine (Fig. 3).
| Fig. 3: | MZP micro-tensile testing machine, adapted for use with an INSTRON 1362 |
The tensile test on micro-tensile specimens (length: 15 mm, diameter: < 0.5 mm, cross-sectional area: 1 mm2) using conventional testing equipment is difficult to carry out due to the predominant occurrence of clamping effects such as transverse forces and bending influences (see: specimen clamping). Perfect axial positioning of the test specimen along the load line can only be achieved with considerable effort, which is why paper templates with test specimens glued into them are used as clamping assistance (Fig. 4).
| Fig. 4: | Micro-tensile test specimens used |
Once clamped in place, the paper template is separated down the centre on both sides, after which the tensile test can be carried out.
Using the measurement setup for a micro-tensile testing machine shown in Fig. 3, it is possible not only to measure small forces in the mN range using inductive length transducers, but also to determine small strain values with a measurement resolution of 100 nm. The measurement systems allow for precise, reproducible, force- and strain-controlled testing under quasi-static loading (Fig. 5).
| Fig. 5: | ZWICK Z020 with adapted MZP Wazau micro-tensile testing machine |
This micro-tensile testing machine can be used to efficiently determine the modulus of elasticity as well as the strength and deformation behaviour of micro-tensile test specimens, fibre bundles or individual fibres, for example to obtain data for simulation calculations. Cyclic tests at low frequencies (LCF) on micro-components or to investigate the bonding behaviour of very fine wires can also be carried out in the tensile threshold range (see: fatigue), whereby the drives for material testing machines must comply with the mechanical requirements of the micro-tensile testing machine in every load case.
See also
- In-situ tensile test in ESEM with AE
- In-situ tensile test in NMR
- Micro-damage limit
- In-situ ultramicrotomy
- Bend test and light microscopy
References
| [1] | Grellmann, W., Seidler, S. (Eds.): Polymer Testing. Carl Hanser, Munich (2022) 3rd Edition, pp. 682–686 (ISBN 978-1-56990-806-8; E-Book: ISBN 978-1-56990-807-5; see AMK-Library under A 22) |