Our micromechanical test setups are optimized for the investigation of samples in the micrometer range. This enables us to examine the smallest components including the determination of the local properties of macroscopic components. This allows us to characterize mechanical scale and scaling effects, which cause additional scattering in the mechanical properties during miniaturization. Micromechanical tests are used to statistically map these scattering effects.
Quasistatic tensile tests on steel samples - strain controlled fatigue tensile tests
We determine the material properties of our microsamples using quasistatic and cyclic tensile and bending tests. These tests can be carried out under force or displacement control. We also offer strain-controlled fatigue tensile tests. These are often used in the plastic section, i.e. outside the range of validity of the technical stress.
10For fatigue tests at high and very high numbers of load cycles, up to ten billion (1010), we have a specially developed test setup in the form of the multiaxial resonance apparatus.
This is characterized by a pronounced sensitivity for detecting early fatigue damage, from slip band formation to the growth of microstructurally short cracks.
On the one hand, our microtests can be based on DIN standards from the macro world. On the other hand, the modularity of our setups enables us to continuously develop and respond to customer requests for setup optimizations and new developments.
Customization options exist, for example, for the mechanics and regulation, the test procedure and the sensors used. Our test setups are also used to determine the properties of our in-house developed metamaterials.
On this page:
Microsamples: preparation, quality control and measurement
Non-contact strain measurement using digital image correlation (DIC)
Multiaxial resonance fatigue tests to determine the operational and fatigue strength
Fraunhofer IWM video series: Resonance scenarios of the multiaxial resonance apparatus
Test parameters Tensile, bending and bending resonance tests
Development and optimization of micromechanical test rigs (incl. software tools)
In addition to component tests, we extract samples from relevant sections of your components to determine local mechanical properties. Various methods are used to produce samples. We can test microsamples/components with a thickness of between 10 µm and 1000 µm, with the usual sample thickness being around 200 µm. The sample geometry is adapted to the respective project requirements and the materials to be tested. The same applies to sample production. The following sample production methods are available:
Before micromechanical tests are carried out, the samples are examined under a microscope. On the one hand, the surface quality is checked for pores, scratches and other defects, and on the other, the sample geometry is measured automatically. Even if the sample production is very reproducible, if required, e.g. in the case of new materials, the quality test can also include the recording of roughness parameters, among other things. The determination of geometry is crucial for the subsequent calculation of mechanical material parameters (e.g. technical stress) based on the results of bending and tensile tests. The sample geometry is measured at various positions in order to obtain statistics for averaging or potential fluctuations.
We carry out quasistatic and cyclic microtensile tests in a linear test setup.
While one specimen grip is statically connected to the load cell, the other is moved by a piezo actuator.
This enables maximum spatial resolution. If the movement range of the piezo actuator is not sufficient, larger movements can be realized with the help of a stepper motor. This allows us to achieve defined experimental boundary conditions.
We can offer both force-controlled and displacement-controlled tensile tests. Fatigue tensile tests with load cycles in the section of low cycle fatigue (LCF, up to approx. ten thousand load cycles) and structural durabiity (high cycle fatigue, HCF, up to approx. two million load cycles) are carried out as standard. The simultaneous regulation of the sample frequency and vibration amplitude creates controlled load conditions. These can be maintained in a frequency range between 0.1 - 120 Hz and at positive medium voltages (depending on geometry).
We can offer both quasistatic tensile tests and tensile fatigue tests in a temperature range from room temperature to approx. 800 °C (tests up to approx. 1100 °C are planned).
We have developed special software for fatigue tensile tests in the plastic section, which enables us to carry out these tests in a strain-controlled manner.
Determinable material parameters:
The microsamples we examine often have dimensions of only a few tens or hundreds of micrometers. To prevent additional damage to the sample during bending or tensile tests, we measure the strain without contact using a special image correlation method (DIC). Two different variants of image correlation are used for quasistatic and cyclic tensile and bending tests.
We also carry out quasistatic and cyclic three- and four-point bending tests in a linear test setup. The specimen holder with the two outer bearings is firmly grouped with the load cell. By moving the piezo actuator vertically, a force is exerted on the specimen via the single (three-point bending) or double compression fin (four-point bending). The resulting deflection is recorded with the aid of a camera. If the movement range of the piezo actuator is not sufficient, larger movements can be realized with the aid of a stepper motor. The distance between the outer bearings can be varied depending on the specimen size and material properties. Bending tests can be carried out under force or displacement control.
Fatigue bending tests with load cycles in the section of low cycle fatigue (LCF, up to approx. one hundred thousand cycles) and structural durabiity (high cycle fatigue, HCF, up to approx. two million cycles) are carried out as standard.
Determinable material parameters:
We offer fatigue tests in the high cycle and very high cycle fatigue regime (HCF or VHCF), i.e. at load cycles of one hundred thousand (105) to around ten billion (1010). With our self-developed multiaxial resonance apparatus, micro specimens can be fatigued in bending and torsion as well as in mixed resonance.
Two piezo actuators indirectly excite the microsample to vibrate at its natural frequency (100-2000 Hz, depending on the sample geometry), while the vibration amplitude is measured and controlled via a laser system. Thus, a symmetrical load (R=-1) is set in this fatigue apparatus. Fatigue phenomena such as extrusion formation and crack nucleation in individual grains or the growth of microcracks can be detected very early by a characteristic drop in the resonance frequency. In addition to grain boundaries and precipitations, extrusions are failure-relevant precursors of cracks in many materials. Imaging techniques can be used to record a series of images during the course of fatigue, from which information on the kinetics of damage accumulation can be obtained.
Determinable material parameters and damage analysis
Detection of early stages of damage via a characteristic drop in the natural frequency
Early stages of damage can be detected at the bending resonance setup via a characteristic drop in the natural frequency. The figure on the left shows the characteristic curve of the relative change in resonance frequency during the fatigue of a nickel sample.
Initial changes in the resonance frequency can be observed from load cycles of approx. 105 due to the formation of extrusions. These contribute to the formation of superficial or internal microcracks within individual grains. From 106 cycles, microcracks also spread across grain boundaries and form the first longer cracks.
Due to the formation of a long crack over the entire sample, the resonance frequency finally drops very sharply.
The table on the left provides an overview of the performance parameters of the available technical equipment. Please do not hesitate to contact us if you have specific questions about micromechanics. Since we write our control software ourselves and our test setups and hardware are modular, we can develop customized test methods for your applications.
We carry out all tests on test setups we have developed ourselves. By default, we base our test technology and protocols on macroscopic mechanics standards. If required, we can adapt our modular setups to the specific requirements of new projects at any time, e.g. to create more application-like load conditions. The customization options range from selecting the appropriate load cell and integrating additional sensors to adapting the sample geometry and sample clamping to developing furnaces for different temperature ranges. In addition, modifications can be made to the control software at short notice in order to run new test protocols, excite with other waveforms or create images at specific points in the load cycle.
Various methods such as incremental step tests, creep tests and stress relaxation tests have been implemented in the past.
Fraunhofer Institute for Mechanics of Materials IWM