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Mechanical Testing Methods

What is Mechanical Testing?

Mechanical testing aims to describe the behavior of different materials or components, e.g. syringes, springs, micromotors or microwires under specific loading conditions. It provides information about the relationship between forces and the deformation that results from those forces acting on the components.

There are many standardized mechanical testing methods. The most used are: tensile testing, compression testing, flexural testing, peel testing, shear testing, tear testing, fatigue testing, creep testing and friction testing.

Universal Testing Machines

A Universal Testing Machine (UTM) is a device designed to test the mechanical properties of materials, components and products. It can perform many of the aforementioned standardized mechanical testing methods. This versatility is why it is referred to as ‘universal’.  

Most parts and components in different size scales can be tested in universal testing machines as long as the force/torque and deflection necessary for the respective test can be achieved in the instrument and the component can be properly fixed. 

UTMs are used in many industries (e.g. automotive, aerospace, electronics, packaging and biomedical) to make sure materials and final products meet the required specifications. They usually adhere to a series of standards (ISO, ASTM …) so consistency is achieved between test results, also regarding devices from different manufacturers, and so they can be used during several stages of product development.

What can UTM tests tell us?

UTMs can help analyze different mechanical properties of materials or components. Output values are, e.g. the Young’s Modulus, yield point, flexural modulus, ultimate tensile strength, strength of an adhesive bond and many more, depending on the material or part. They are therefore used to answer questions like, e.g., “How strong and stiff is my sample?”, “How much can my sample elongate or stretch?” or “How much can my sample compress or flex before breaking?” The important data derived from the answers is used by engineers in R&D and QC. Although UTMs are commonly used for testing of large-scale, robust components, they are not limited to high force/high torque applications. While classical UTMs can achieve forces up to 5000 kN depending on the model, small-scale, soft or fragile parts can also be characterized, as long as the forces and deflections involved can be accurately measured by the transducer.

Important terms and variables: (more possible)

 

TermSymbol
Young’s ModulusE
Shear ModulusG
Poisson’s ratioν
Extensional viscosityηE
Flexural ModulusEFlex
Bulk (Compression) ModulusK
Engineering strain
Engineering stress
Normal forceFN
TorqueM
Displacements
Deflection angleφ

How does mechanical testing with UTMs work?

A sample of the material or a component is placed in the UTM and held by grips or special fixtures. The sample is then subjected to a force or torque (stress), and the resulting deformation or deflection (strain) is measured or vice versa. As the dimensions of the sample are known, stress-strain curves can be generated from the recorded forces and deformation, which in turn allows prediction of the materials’ or components’ behavior in operating conditions.

Mechanical testing of small components

UTMs are commonly used for high-force/high-torque applications. Conventional UTMs do not provide suitable fixtures, torque/force presets are not precise enough, and sensors lack the necessary sensitivity to deliver useful data from the measurement of very small-scale parts and components. There is an increasing need to test smaller and softer devices and materials, but for most of these applications no commercial instruments exist and researchers therefore need to rely on self-made solutions in order to characterize their components.

Combining traditional mechanical testing methods with a rheometer set-up takes advantage of the high accuracy of the optical encoder and the excellent torque resolution of its EC drive to allow measurement of even the smallest torques and deflections. Due to its lift drive and the possibility of usage of an additional lower linear drive, it is also able to measure small normal loads and displacements. In combination, this enables characterization of parts and components previously not accessible to characterization. Furthermore, additional environmental parameters such as temperature or relative humidity can be set, allowing characterization of very small parts under a wide range of environmental conditions that can be experienced during operation.

Mechanical testing methods & measurement setups for small components

Testing of Medical Needles

Test Setup:

A: Sample Mount, B: Adapter sleeve, C: Stamp, D: Linear drive

Implemented setup with sample mount for syringes with optional adapter sleeve, stamp and a convection temperature device (CTD)

Results:

Figure 1: F-s diagram for unfilled glass and polymer syringes at 500 µm/s sliding speed and room temperature

Determination of breakaway forces and sliding forces of two commercially available syringes by use of a customized test setup on the UTM Micro.

Test performed at two different sliding speeds, and four different temperatures, with three different injection fluids. 

Picture 1: Test setup for medical needles

Determining the influence of ambient temperature, insertion speed and needle diameter on the force needed to penetrate the septum of a sealed vial.

Figure 2: Influence of insertion speed on insertion force of the 1.2 mm diameter needle.

Torsional Mechanical Properties of Nanocarbon Fibers

(Source: W. Eom et al. – Carbon nanotube-reduced graphene oxide fiber with high torsional strength from rheological hierarchy controlDOI: doi.org/10.1038/s41467-020-20518-0)

Test Setup:

Tests conducted in a UTM Micro equipped with the SRF (solid rectangular fixture) accessory. The fibers were mounted using a grid.

Step 1: Load single fiber onto grid

Step 2: Load specimen on fixture

Step 3: Cut grid on both sides

Step 4: Test fiber

Results:

Figure 3: Torsional stress-strain curves of the fibers. Inset shows close-up of strains from 0 % to 10 %.

Measuring the torsional strength of four different types of nanocarbon fibers:

GF: Graphene Oxide (GO) Fiber
HF: Hybrid GO/Carbon Nanotube (CNT) Fiber
D-GF / D-HF: Drawn GF/HF

To compare the results with other materials in terms of shear failure strength and density or tensile strength, Ashby plots can be used.

Cogging & Hysteresis Torque and Iron Losses of Sub-Fractional Horsepower Motors

Test Setup:

Picture 2: Test setup for sub-fractional horsepower motors

Special sample mount for attachment of permanent magnets to rotor cup in upper motor (ω ≠ 0) and motor stator to lower part (ω = 0).

Results:

Figure 4: Extraction of the cogging and hysteresis torque from the measured clockwise (CW) and counter-clockwise (CWW) no-load torque waveforms (Trheo in graph).

Measurement of the cogging torque and hysteresis torque waveforms of sub-fractional horsepower permanent magnet motors in sub-Nm range as well as the iron losses.Torque measurements in clockwise (CW) and counter-clockwise (CWW) direction at n = 1 rpm and increasing rotational speeds.

Figure 5: Measured offset torque vs rotational speed and torque separation.

First, no-load torque waveforms were measured. These waveforms represent the superposition of the cogging torque and hysteresis torque, which can be extracted mathematically.

An increase in no-load torque at higher rotational speeds can be observed due to eddy current effects. The measured offset torque can thus be used to determine the corresponding no-load iron losses.

Determination of Mechanical Properties of Balance Springs

Test Setup:

The balance spring was glued to a lower plate and the respective pin was glued to the upper measuring geometry (plate).

Results:

Deformation of the balance spring for one full revolution in both directions with a rotational speed of 0.25 rpm. Calculation of linear regression and degree of determination (R²) in software.

A distinction between different springs is possible via comparison of the torque residual as an indicator for deviation from ideal-linear progression for the purpose of quality control.

UTM Fixtures & Grips

A very wide range of materials and components can be tested with a UTM. Various fixtures and grips are needed to securely clamp the components during testing, and to prevent measurement errors. The choice of fixture depends on the test methods.

 

FixtureTest method
Parallel plate (PP)Compression testing, torsion testing
Solid rectangular fixture (SRF)Tensile testing, tear testing
Solid circular fixture (SCF)Tensile testing, tear testing
3-point bending (TPB)Bend testing
Universal extensional fixture (UXF)Peel testing
Standardized hole pattern flange to mount stages and self-made holders
Special and customized fixtures
(fixtures for syringes, micromotors, bearings, O-rings, springs etc.)