Special Solutions for Research and Development

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Along with customized solutions developed by Zwick in cooperation with various institutions, Zwick also offers many standard solutions specific to applications in research. For scientists, a key concern is the flexibility of a material testing machine. They encounter new testing requirements on a regular basis, and these requirements need to be met using their existing machinery, if possible.

System interfaces are of particular importance. On the one hand, new sensors are often used, which must be recorded together with the machine's measurement-signals. On the other hand, it must be possible to easily process the recorded data. This can be achieved directly in testXpert II or by exporting the data to an external evaluation software.

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1. Integrating Strain Gages

In principle, it is necessary, from a measurement technology standpoint, to synchronously record all signals with a strain gage—from force, to deformation, and the strain.  

Zwick offers two connection options for connecting strain gages.

Direct connection to testControl measurement and control electronics

 The Wheatstone bridge completion is implemented by means of an external connection box, which connects various types of strain gages (120Ω, 350Ω, etc.) to the testing system. An applied strain gage can be connected in this interface connection by means of 4- and 6-conductor technology. Temperature compensation is also possible . Up to 4 DMS channels can be integrated, depending on how testControl is equipped. All channels are recorded synchronously by testControl and can be used for strain-controlled tests as well. testXpert II and testControl options allow you to formulate all channels, so they can be controlled using the mean value of 4 strain gages. The new testControl bus extension makes it possible to integrate 6, 12, or even more strain gages.

Connection via an HBM measurement amplifier

 The Wheatstone bridge completion is implemented by means of an external connection box, which connects various types of strain gages (120Ω, 350Ω, etc.) to the testing system. An applied strain gage can be connected in this interface connection by means of 4- and 6-conductor technology. Temperature compensation is also possible . Up to 4 DMS channels can be integrated, depending on how testControl is equipped. All channels are recorded synchronously by testControl and can be used for strain-controlled tests as well. testXpert II and testControl options allow you to formulate all channels, so they can be controlled using the mean value of 4 strain gages. The new testControl bus extension makes it possible to integrate 6, 12, or even more strain gages.

4. Creep Tests and Cyclical Characteristics

Along with purely high temperature characteristics, other mechanical characteristics, which must also be determined under high temperatures, are also important in power plant technology. In addition to high temperature resistance, the modified operation mode of many power plants caused by a fluctuating supply from wind and solar power plants is a factor, placing special requirements on materials.

Many power plants must be able to be started up and shut down again in a flexible way, and in short intervals. This leads to material thermo-mechanical fatigue (TMF), where the material expands thermally with every start up and shut down. Most power plants of the 20th century were not designed for such stresses and must be recalculated and converted accordingly in retrospect.

Another issue for all steam power plants, but specifically for A-USC power plants operated at temperatures up to 760 °C and vapor pressures up to 380 bar, is material corrosion. The way in which the materials react (relaxation or retardation) is observed at a constant strain or stress over a longer period of time at elevated temperatures. This is also done cyclically (creep fatigue).

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5. Low Cycle Fatigue (LCF Tests)

During a low cycle fatigue test, the material is cyclically loaded at a specific temperature (usually a heightened temperature) until it reaches minimal plastic deformation. The specimen (material) withstands only a few thousand load changes under this kind of load. During this process, the demands placed on the testing machine or the machine controller are particularly high, because during the transition from elastic to plastic deformation, the specimen's stiffness dramatically changes and therefore the controller must respond very quickly to guarantee a constant rate of strain increase.
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6. Thermo-Mechanical Fatigue (TMF)

Thermo-mechanical fatigue (TMF) is the simulation of the mechanical load as a result of the thermal expansion of the material. It occurs every time a power plant (but also any thermal engine) is started up or shut down. During startup, the temperature of all components rises from room temperature to operating temperature, which is accompanied by an expansion of the material. This expansion creates a stress in the material, which must be determined with accuracy to prevent component damage.

Thermo-mechanical fatigue involves heating a specimen cyclically; the testing machine superimposes a mechanical strain in the same or inverse phase.

7. High Cycle Fatigue (HCF)

In contrast to an LCF test, the load change is driven only in the linear-elastic range of the material in a high cycle fatigue test. One key application is the determination of the fatigue limit of a material or component. Determination of the fatigue limit is usually determined using a Wöhler curve (s/n curve). Various stress and strain amplitudes are applied to the specimen until it fails. The Wöhler curve is determined using constant amplitudes; the Gassner curve is determined using variable amplitudes.

These characteristic curves are also determined at various temperatures.

HCF Vibrophore

8. Instrumented Hardness Ranges from Nano to Macro

Instrumented indentation depth measurement has been used in research for many years. It is used to determine the mechanical characteristics of very thin layers or coatings. A force indentation depth measurement can determine the hardness (Martens) and the elastic and plastic characteristics.

As a rule of thumb for determining the coating properties, the indenter (Vickers or Berkowitch) may not penetrate more than 10% of the coating thickness. The Zwick UNAT system can be used for indentation depths between 10 nm and 30 µm, and the ZHU/Zwicki System for indentation depths above 6 µm. This means Zwick has a solution for all nano, micro, and macro hardness ranges.

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Soft robot mimics caterpillar movement

Researchers at Tufts University in the US are using a Zwick biaxial testing machine to test biological and engineered biocompatible materials for the development of the world's first soft-bodied robot. Based on the neuromechanical system of the caterpillar Manduca sexta, the softbot prototype is approximately 30 cm long and made of silicone elastomer. The mechanical material characterization is performed at the Soft Materials Characterization Laboratory at Tufts' Advanced Technology Laboratory, led by Professor Luis Dorfmann.
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