Vibration Fracture
Schwingungsbruch | A service provided by |
|---|
|
| Polymer Service GmbH Merseburg |
| Tel.: +49 3461 30889-50 E-Mail: info@psm-merseburg.de Web: https://www.psm-merseburg.de |
| Our further education offers: https://www.psm-merseburg.de/weiterbildung |
| PSM on Wikipedia: https://de.wikipedia.org/wiki/Polymer Service Merseburg |
Vibration fracture
General information
Vibration fractures occur during dynamic or vibrating stress on components in practical applications or on test specimens in so-called fatigue tests [1, 2]. Compared to static, quasi-static or impact stress, failure under vibrating loads results in fracture surfaces with a different appearance. In general, the fracture surfaces of metallic materials and plastics under dynamic stress have similar fracture surface characteristics (see: types of fracture), which are significantly influenced by the ductility and toughness of the materials under investigation.
However, differences in the fracture patterns of metals and plastics are also caused by the type of fatigue test. While the tensile threshold range and low test frequencies (< 50 Hz) are preferred for plastics due to the thin geometric shape of the components or test specimens, higher frequencies can be used for metals and both the alternating and compressive threshold ranges can be used to characterise the fatigue properties (Fig. 1).
| Fig. 1: | Stress areas in continuous vibration testing (fatigue testing) |
Fatigue fractures in plastics
The starting point for the development of fatigue fractures in plastics can be internal discontinuities (voids or inclusions) or microcracks, as well as other surface defects caused by the manufacture or processing of the components. On a microscopic scale, no surface is perfectly smooth and flat, but rather exhibits minimal roughness and fine cracks, which cause localised three-dimensional stress concentrations due to the notch effect.
As a result, the yield strength is exceeded in the local area and microplastic deformations occur, which in turn can lead to strengthening effects. The sum of these local deformation processes then forms an initial crack, which spreads deeper into the interior of the material with each load cycle due to energy input and crack propagation. This crack propagation characterises the onset of fatigue fracture. Depending on the test temperature, media exposure, mean stress σm and stress amplitude σa, this crack propagation will take a different amount of time until final total fracture.
As a result of the change in load and the cyclic propagation of the crack, typical vibration or oscillation stripes form in the material during the crack propagation phase, but these are only visible with high-resolution microscopic testing methods (scanning electron microscopy (SEM) or scanning tunnelling microscopes).
If there are arrest or recovery phases or periods of greatly increased stress during loading, local stress relaxation (see: Relaxation plastics) or increased crack propagation occurs, and the fatigue stripes can be seen as arrest lines (see: fracture types, fractography and waves and arrest lines) using a loupe or light microscopy.
The arrest lines also become visible because the change in multi-stage stress is always associated with a variation in the crack propagation speed. Since the arrest lines always run perpendicular to the direction of crack propagation, the point of crack initiation can be determined relatively easily based on their arrangement and course. If the time intervals between the stress changes or the arrest lines are known, the number of arrest lines can be used to determine the time at which crack propagation began [3].
Fatigue fractures in brittle plastics
In the case of plastics with very low deformability, such as polystyrene (abbreviation: PS) or polymethyl methacrylate (abbreviation: PMMA), and the presence of micro-notches on the surface or inside the material, a crack tip can form under dynamic stress, which, after initial blunting, exhibits stable crack propagation behaviour (see also: fracture mechanics and crack resistance curve). The opening and closing of the crack flanks provides the energy necessary for stable crack propagation under cyclic stress (Fig. 2a). The structural explanation for this behaviour is the low extensibility of the molecular chains, which are oriented in the stress field in front of the crack tip (fibrillation) and then separated by the advancing crack front (Fig. 2b), causing the structural integrity of the component or test specimen (compliance) to continuously decrease.
| Fig. 2: | Fracture propagation in brittle plastics with vibration stripes according to [4] |
As the crack progresses, parallel vibration stripes form in the material; the characteristics thereof depend on the type of plastic and the load parameters (frequency, mean stress and stress amplitude). Brittle plastics exhibit vibration stripes with very flat profiles and sharp-edged steps between the fracture paths, with inclusions acting as obstacles and only partially visible on the fracture surface surface (Fig. 3a). The vibration stripes without secondary cracks shown schematically in Fig. 3a are clearly visible in polyamide 12 in Fig. 3b. Hard inclusions in the base material are usually bypassed by the crack front, causing them to be covered by the fracture (Fig. 4a) or to lie exposed and separated from the matrix on it (Fig. 4b). Very soft particles are usually separated from the crack front and are then invisible [4].
| Fig. 3: | Brittle vibration fractures (a) Schematic diagram and (b) scanning electron microscope image of a vibration fracture with arrest lines in spray-dried polyamide 12 (abbreviation: PA12) |
| Fig. 4: | Brittle vibration fractures of (a) compressed styrene-acrylonitrile (abbreviation: SAN) and (b) a polystyrene-rubber compound with embedded polystyrene particles (0.1 to 0.5 µm) (abbreviation: PS) according to [4] |
|}
As the crack propagates, the remaining load-bearing cross-section decreases, causing the stress to be distributed over an increasingly smaller area. If the tensile strength is exceeded, violent or brittle fracture occurs (see: types of fracture). No vibration striations or notch lines are visible in the area of unstable brittle fracture.
Fatigue fractures in tough plastics
In tough plastics, the vibration stripes are usually not as clearly defined due to high plastic deformation, but instead have a wrinkled or rounded appearance. In terms of volume (Fig. 5a), the vibration fracture paths can also spread out in parallel from different crack initiation positions. The open fracture surface (see also: fractography and component failure) shows the typical vibration strips (Fig. 5b) with the fracture steps and exposed inclusions on the surface.
| Fig. 5: | Schematic diagram of ductile vibration fractures (a) in volume and (b) typical habitus of the brittle fracture surface according to [4] |
A characteristic vibration fracture surface for a standard-moisture polyamide 66 (abbreviation: PA 66) is shown in Fig. 6 with clearly visible vibration stripes and a wave-like structure.
| Fig. 6: | Ductile fracture of a polyamide 66 test specimen after [4] |
As can be seen, fatigue fracture can be identified in two typical fracture areas: the area of the arrest lines or vibration stripes and the residual fracture surface. This vibration fracture surface is usually relatively smooth, while the brittle fracture surface is generally heavily fractured.
See also
References
| [1] | Höninger, H.: Fatigue Behaviour. In: Grellmann, W., Seidler, S. (Eds.): Polymer Testing. Carl Hanser, Munich (2022) 3rd Edition, pp. 156–166 (ISBN 978-1-56990-806-8; E-Book: ISBN 978-1-56990-807-5; see AMK-Library under A 22) |
| [2] | Bierögel, C., Grellmann, W.: Fatigue Loading. In: Grellmann, W., Seidler, S. (Eds.): Mechanical and Thermomechanical Properties of Polymers. Landolt-Börnstein. Volume VIII/6A3, Springer, Berlin (2014) pp. 241–285, (ISBN 978-3-642-55165-9; see AMK-Library under A 16) |
| [3] | Ehrenstein, G. W., Engel, K., Klingele, H., Schaper, H.: Scanning Electron Microscopy of Plastics Failure. Carl Hanser, Munich (2011), (ISBN 978-3-446-42242-1; see AMK-Library under D 5) |
| [4] | Ehrenstein, G. W.: Schadensanalyse an Kunststoff-Formteilen. VDI Publishing Düsseldorf (1981), (ISBN 3-18-404068-2; see AMK-Library under D 3) |