What causes the pressure plate to fail

Pressure test arrangement

Pressure test arrangement

Metrological requirements

For the exact performance of the compression test to assess the material behavior under uniaxial pressure loading (see: uniaxial stress condition), a number of metrological requirements must be observed [1].
On the one hand, the test setup appears to be relatively simple and straightforward, on the other hand, test practice shows that the results obtained are often flawed, larger measurement-related standard deviations can occur and the absolute values ​​can be incorrect [2].
For the exact and standard-compliant execution of the pressure test, the manufacturers of materials testing machines offer movable pressure plates that can be used, for. B. in the variant of the company Instron are ball bearings (Image 1) [3].

Image 1: Pressure test arrangement with ball-bearing pressure plates [3]

The movable pressure plates can be mounted above or below and have a centering plate that uses springs to set an exact position in the load line. Since the friction of the bearing has a high influence, these variants of the pressure test arrangement are mostly only used for materials with a high compressive strength (metals and concrete), but sufficient ductility, since high centering forces are required. In the case of brittle or non-ductile materials, such as ceramics or high-performance composites, with very low elongations, special pressure testing devices are used. Since these materials react sensitively to additional loads (bending, torsion or shear), special testing machines (see: Testing of composite materials - Requirements for material testing machines) with mechanical centering devices in the x, y and z directions and online display of errors in the Load line used [4]. Affected here are z. B. Shear loading or combined loading pressure tests and the Compression After Impact (CAI) test.
In the case of thermoplastics, pressure test arrangements of this type are usually not used, since the forces required to center the test device cannot be achieved with these materials. As a rule, rigidly fixed pressure plates are used here, which are equipped with centering aids (concentric rings on the plates). In this case, the operator of the test facility is responsible for calibrating the exact plane-parallel alignment of the pressure plate in the load line, or this task is taken over by a certified test facility, since otherwise stress concentrations (see: fracture mechanics) can occur during the pressure tests that lead to premature Failure (see: breakage) can result.

Another problem is the centering of the test body, since an eccentric position on the pressure plate or an offset of the test body position relative to the load line of the force measurement chain leads to the introduction of a bending moment and thus influences the measurement result.

Requirements for the specimen geometry

The main problem of pressure testing, however, is due to the manufacture of test specimens. When preparing the specimen, if the plane parallelism is but not the perpendicularity (Fig 2a), then in addition to the compressive stress there is a bending component that significantly influences the strength and deformation behavior [2]. If both requirements are not met (Pic 2b), then there is only partial contact between the test body and the pressure plate. Since the contact area A is not known, it changes with the increasing duration of the compression test and the stress calculation using the total area A0 occurs, the strength values ​​will generally be too low. Of course, this effect also has an impact on the calculation of the modulus of elasticity, since the position of the extensometer has a significant influence.

picture 2: Influence of faulty specimen geometry on the compression test

In the right-hand position of the test body (Pic 2b) a strong start-up behavior occurs, followed by an increasing compression (Image 2c). However, the module calculation provides incorrect parameters, as the voltage is not calculated correctly due to the incorrect area. If the transducer is attached to the left side of the test body, then a tensile strain is measured due to the bending stress. This means that either no module value is calculated or nonsensical values ​​are output.

Friction and specimen positioning

Another problem is the friction between the end face of the test specimen and the pressure plates in connection with the HERTZ pressure at the position of the force introduction (picture 3). This leads to a hindered transverse expansion in the contact area, which has only a minor effect on brittle test specimens, but causes a central bulge in ductile materials.

picture 3: Influence of surface roughness on the printing test

To reduce friction, greases or oils can be applied to the end faces, but this must be noted in the protocol in each case. These measures do not help if the roughness of the end faces is too great (Fig 3a) and the contact area is undefined. In this case, a pronounced start-up behavior occurs and the measured moduli of elasticity result in values ​​that are too low. In the case of ductile materials, this effect decreases with increasing compression, but it makes the evaluation considerably more difficult.
This problem can be avoided with the aid of a cold-curing resin (embedding agent) with a short pot life by applying it to the end faces of the test specimens and positioning it between the pressure plates. A small preload is then applied, which displaces the excess resin. After the pot life has expired, the experiment can then be carried out (Picture 3b). This delays the execution of the test somewhat and must also be noted in the protocol, but the results of the pressure test are significantly improved.