Threads, Tips and Films
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Threads, tips and films or filaments, ears and films
General information
Component failure is usually initiated by microscopic crack formation processes. Depending on the material behaviour of the plastics, stable crack propagation then occurs, usually culminating in unstable crack propagation, also known as brittle fracture or ultimate fracture [1].
In the subsequent failure analysis, it is essential to determine the location where the crack originated and the direction of crack propagation in order to draw conclusions about the failure mechanism and stress parameters [2, 3].
Microscopic examination of the fracture surfaces provides additional information on the type and magnitude of stress, as well as on the influence of temperature and media, the loading speed application, and on ageing effects or manufacturing defects.
The main objective of the failure analysis is to determine the location where the crack originated, the course of the fracture and the direction in which it propagated, as well as the crack propagation velocity, the fracture type (ductile or brittle) and any factors that may have promoted the fracture.
The VDI Guideline VDI 3822, sheet 2.1.4 [3], summarises and explains the typical features observable on plastic fracture surfaces; however, only a few specific fracture surface features can provide information about the direction of crack propagation and the location of crack initiation. These are the so-called fracture parabolas or hyperbolas, also known as U- or V-ramps, and the ramps, bars or steps, which are used as synonymous terms [2–6].
Fracture surface features: threads, tips and films
Threads, tips and films are only observed in ductile materials and result from severe local plastic deformation, which typically appears on the fracture surface in combination with fracture parabolic curves, as well as ramps, clods or steps. These secondary fracture features do not normally allow conclusions to be drawn regarding the location of crack initiation or the direction of crack propagation and are characterised in the VDI guideline VDI 3822 using the symbols shown in Fig. 1 [3].
| Fig. 1: | Symbolic fracture surface features for (a) a tip, (b) a film and (c) threads according to [3] |
Tips, which occur specifically at the edges of ramps as a result of plastic deformation, are three-dimensional local bulge-like deformations (Fig. 1a). These are significantly thicker and more compact than, for example, threads or foils, and the tip generally points against the direction of the principal tensile stress (Fig. 2a). Tips are often observed symmetrically at the edges of ramps.
| Fig. 2: | Fracture surface features: tips and films for ductile material behaviour for (a) polyethylene (abbreviation: PE) tips at T = –20 °C and (b) polyethylene films at 23 °C according to [3] |
Films are local two-dimensional deformations (Fig. 1b) that can result from biaxial stretching processes. Their thickness is significantly smaller than their other dimensions, which is why the films can fold over and buckle (Fig. 2b). Threads (Fig. 1c) are one-dimensional plastic deformations resulting from fibrillation processes and are usually very clearly visible on the fracture surface (Fig. 3). The films often serve as the starting point for branching thread structures (Fig. 3b).
Given the localised nature of the tips, films and threads, these fracture surface features can be regarded as secondary fracture surface characteristics, which allow only limited conclusions to be drawn. The fracture features of threads, tips and films therefore do not provide any significant information regarding the location of crack initiation and rarely indicate the direction of crack propagation.
| Fig. 3: | Fracture surface features of threads in ductile materials for (a) polyethylene at 23 °C in the near-notch region and (b) polyethylene at T = 23 °C under impact loading [3] |
See also
- Fracture
- Fracture types
- Fracture parables
- Ramps, bars and steps
- Vibration fracture
- Brittle fracture promoting factors
- Waves and arrest lines
References
| [1] | Grellmann, W.: Beurteilung der Zähigkeitseigenschaften von Polymerwerkstoffen durch bruchmechanische Kennwerte. Habilitation (1986), Technische Hochschule Leuna-Merseburg, Wiss. Zeitschrift TH Merseburg 28 (1986), No. 6, pp. 787–788 (Content, Summary) |
| [2] | Kotter, I., Grellmann, W.: Die Fraktografie als Hilfsmittel in der Schadensanalyse an Kunststoffprodukten. 24th Internationale Fachtagung Technomer an der Technischen Universität Chemnitz, (2015), Proceedings V 8.6 |
| [3] | VDI 3822 Blatt 2.1.4 (2024-06): Failure Analysis – Defects of Thermoplastic Products Made of Plastics Causedd by Mechanical Stress |
| [4] | VDI 3822 Blatt 2.1.2 (2024-06): Failure Analysis – Defects of Thermoplastic Products Made of Plastics Caused by Faulty Processing |
| [5] | Ehrenstein, G. W.: Schadensanalyse an Kunststoff-Formteilen. VDI Public House Düsseldorf, (1981), (ISBN 3-18-404068-2; see AMK-Library under D 3) |
| [6] | Ehrenstein, G. W., Engel, K., Klingele, H., Schaper, H.: Scanning Electron Microscopy of Plastics Failure / REM von Kunststoffschäden. Carl Hanser, Munich (2011), (ISBN 978-3-446-42242-1; see AMK-Library under D 5) |