Failure Analysis Plastics Products, VDI Guideline 3822

Sprachauswahl/Language selection

Dieser Artikel ist auch auf Deutsch verfügbar

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

Fatigue analysis plastics products, VDI Guideline 3822

Introduction

Plastic products are designed and manufactured with economic considerations in mind to ensure functional, safe and hazard-free use throughout their intended service life. Despite careful design, planning and production, damage and defects in plastic products cannot always be avoided. These damages and defects must be analysed in order to uncover the causes of defective changes in materials and products, including failure (see: fracture). This is the field of failure analysis.

Targeted measures to remedy and prevent damage can only be initiated if the causes of damage and influencing factors are clarified through systematic investigations. Therefore, one of the main tasks of failure analysis is to select suitable investigation methods and to evaluate the individual results in a scientifically sound manner [1–3]. Guidance on conducting damage analyses is provided by the VDI 3822 guideline ‘Failure Analysis’.

Within the scope of this guideline, damage to thermoplastic products is addressed in a series consisting of ten sheets: VDI 3822 Sheet 2.1.x ‘Failure Analysis – Defects of Thermoplastic Products’. The authors were responsible for drafting the sheet ‘Failure Analysis – Defects of Thermoplastic products Made of Plastics Caused by Mechanical Stress’ (VDI 3822 Blatt 2.1.4).

This article describes typical damage patterns that occur on thermoplastic components or products using practical examples.

Damage analysis based on VDI 3822

A systematic approach (see: failure analysis – basics) to performing failure analyses on plastic products (i.e. materials, semi-finished products and components) is essential for their success [4]. This approach is described and outlined in the VDI 3822 basic data sheet [5].

The first step is to examine the defect product and the visible external phenomena (e.g. cracks, deformations, discolouration, fractures, surface anomalies).

After a comprehensive description of the damage and an inventory, damage hypotheses are formulated and scenarios developed, taking into account the damage environment. This leads to the selection of suitable analysis methods. Based on the results of the investigation, the hypotheses are verified, the causes of the damage are determined, and measures to remedy the damage are initiated or proposed.

The damage report includes details that are important for determining the cause, which are documented with pictures and measurement results.

Damage to thermoplastic products

Damage to thermoplastic products can be caused by errors in design (including the implementation of requirements), in the material, in processing and during use. The VDI 3822 Sheet 2.1.x series of guidelines was structured accordingly [6–15].

The most common causes of failure are errors that occurred during development and production. In contrast, a comparatively small proportion of plastic damage is attributable to stress, environmental influences and ageing.

In the series of guidelines, a separate sheet was devoted to each individual cause of failure. Three guidelines describe possible causes that can occur before use (design, processing and material or material selection [6–8]). Six further guidelines describe the causes of failure that arise during use. A distinction is made between the individual types of stress: mechanical, thermal, tribological, media, microbial and weathering stress [9–14]. Numerous instrumental analysis methods are available for performing damage analyses on plastic products. These are explained in a supplementary guideline [15].

Damage due to faulty design

It is often the case that plastic components were previously made from metallic materials and are therefore purely substitutes in terms of design. If the design rules for plastic components are not taken into account and no material-appropriate adaptation or redevelopment of the components has taken place, faulty design can be the cause of plastic component failure.

VDI Guideline 3822 Sheet 2.1.1 "Failure Analysis – Defects of Thermoplastic Products Made of Plastics Caused by Faulty Design" lists, among other things, design rules for plastic components [6]. The most important design rules are:

make wall thicknesses as thin as possible and ensure that they are uniform,
avoid accumulations of material and flat surfaces,
round off corners and edges,
avoid undercuts,
ensure sufficient conicity.

Figure 1 shows a section of a component manufactured using injection moulding from a polycarbonate/acrylonitrile butadiene styrene (abbreviation: PC/ABS) blend. This component has a complex geometry with wall thickness variations, stiffening ribs and small radii. Indentations can be seen on the surface of this moulded part (marked with red arrows in Fig. 1). These depressions are sink marks in areas where material has accumulated and are caused by material shrinkage during the cooling process. The sink marks are usually clearly visible. They can be avoided by using a plastic-compatible design that prevents material accumulation.

If the location of the cut-off is in areas with a lower wall thickness, the after-pressure can no longer take effect after the mould has been filled, as the moulding compound has already solidified here. The cut-off should be moved to the area with a higher wall thickness.

Fig. 1:

Sink marks in an injection-moulded PC/ABS component in areas with thick walls (stiffening ribs))

Damage due to faulty processing

One of the most common processing methods for thermoplastics is injection moulding. Numerous sources of error can be found here in the pressure and temperature control in the cylinder and in the mould. An incorrectly selected flow velocity and excessive shear stress (see also: shear viscosity) on the melt during the processing process can also cause damage.

Figure 2 shows a defect at the cut-off as a record effect. The record effect occurred on a polyoxymethylene (abbreviation: POM) component as a result of an excessive cooling rate. The edge layer is frozen and can no longer be transported to the mould wall. The following melt flows around this area and also solidifies on the wall. Grooves or notches are formed on the surface. Due to the rapid cooling, the mass has solidified, and the grooves can no longer be closed during the holding pressure phase.

Fig. 2:

Error at the cut – record effect around a point cut on a POM component

If the cut is located in an area of the component that is subject to high tensile stress, a notch effect can be caused in the grooves, which will then lead to failure due to crack formation or fracture of the component.

The record effect can be prevented by increasing the injection speed and pressure with which the material is filled into the mould, as well as the melt or mould temperature.

Figure 3 shows cavities with fissured surfaces inside the material. The fissured surfaces indicate that there is negative pressure or even a vacuum inside these cavities. This phenomenon is referred to as vacuoles. Vacuoles occur in areas of material accumulation when the outer layer has already solidified during mould filling and the centre is still in a molten state. Processing-related causes for the formation of vacuoles include, for example, too low a mould temperature, too low an effective holding pressure or too short a holding pressure time.

Fig. 3:

Vacuoles in an injection-moulded POM component in areas with thick walls

Damage due to faulty material selection or material defects

The right choice of material plays an important role in the successful use of plastics as construction materials. The choice of material is usually made on the basis of tables and material data sheets. The material values contained in these data sheets are determined using standardised test methods on test specimens specially manufactured and optimised for the determination of characteristic values [16]. Components, on the other hand, are subject to complex stress regimes and usually have different wall thicknesses and geometric features. This means that the actual stress capacity of the component is not identical to the stress applied to the test specimen to determine the material values. This must be taken into account when selecting materials, as must the creep, relaxation and ageing behaviour of plastics.

The property profile of plastics is determined by the polymer material itself, but also by the additives. Additives include fillers and reinforcing materials as well as additives such as stabilisers or processing aids.

Material defects also include defects in both the polymer material and the additives.

In highly hygroscopic plastics, such as polyamides, insufficient pre-drying and inadequate degassing during the processing stage can result in the formation of bubbles inside the material (see: gas bubbles). Unlike vacuoles, the bubbles are distributed across the entire wall thickness and have a smooth, usually glossy surface (see Figure 4).

Fig. 4:

Bubbles in a component made of PA6 with mineral filling due to insufficient pre-drying

Damage occurring during use

Stresses that occur during the use of plastic components can cause various types of damage.

Purely mechanical stress can lead to deformation, white discolouration, crack formation or even fracture.

In most cases, however, mechanical stress is an indicator of damage caused by other types of stress or by faulty processing, design or materials, or by the wrong choice of materials. In these cases, the level of mechanical stress is comparatively low. If the actual cause of the damage can be traced back to mechanical stress, then the failure was caused by overuse or mistreatment.

Damage caused by thermal stress can be recognised, for example, by deformations ranging from loss of geometric contour to discolouration, changes in gloss or blooming.

Tribological stress manifests itself in scratches, grooves, chatter marks, wall thickness reductions due to material removal or changes in gloss.

The scratched surface of a component made of PE-LD is shown in Fig. 5. In this case, friction with a harder friction partner leads to partial abrasion of the material. The visual appearance of the component is severely impaired as a result of a loss of gloss (see: gloss measurement).

Fig. 5:

Surface of a container made of PE-LD with heavy signs of use (scratches due to tribological stress)

Media exposure of plastic products leads, for example, to discolouration, loss of transparency, reduction in mechanical properties or loss of gloss, i.e. an increase in roughness. A distinction is made here between pure media exposure and media exposure in conjunction with the effect of external or internal mechanical stresses. Furthermore, a distinction is made between physical (swelling (see: water absorption) and extraction) and chemical (molecular degradation) media effects.

An example of damage caused by weather-related loading is shown in Fig. 6. A PE-LD cover that was fixed to a cardboard sleeve and exposed to direct sunlight behind window glass for a period of approximately five years shows yellowing and crack formation. Minor mechanical stress causes brittle fracture failure in this material, which is very flexible and deformable when not exposed to weathering (see: types of fracture). The PE-LD cover, which was fixed to the underside of the cardboard sleeve and was therefore not exposed to sunlight, is undamaged. Sunlight causes the material to become brittle as a result of cross-linking, chain degradation, stress cracking (see: environmental stress cracking resistance) or the degradation of impact modifiers.

Fig. 6:

Brittleness and yellowing of a PE-LD cover after five years of exposure to sunlight behind window glass

Colonisation by microorganisms leads to discolouration of the material, loss of gloss, clouding or increased roughness (see: surface testing technology). In the case of colonisation by fungi, circular red or black discolouration can be observed (see Fig. 7).

Fig. 7:

Colonisation of fungi in a silicone joint on a washbasin

Summary

Plastic damage analyses serve to determine the causes of failure in plastic components or products, from which targeted measures for damage remediation and prevention can be initiated. Another objective of failure analysis is to find the optimum combination of material, design, manufacturing and component properties for a technical product, taking cost aspects into account. Damage can only be remedied and prevented if the causes of damage and influencing factors are clarified through systematic investigations. Damage analyses therefore serve to improve material development, material selection, design, manufacturing and modes of operation.

References

[1]	Kotter, I., Grellmann, W.: Schadensanalyse an Kunststoffprodukten – Beispiele aus der Praxis, In: Borsutzki, M. Moginger, G. (Eds.): Fortschritte in der Werkstoffprüfung für Forschung und Praxis; Tagung Werkstoffprüfung 2012, Bad Neuenahr-Ahrweiler, December 6–7, 2012, Publishing House Stahleisen GmbH Düsseldorf (2012), Proceedings pp. 317–322 (ISBN 978-3-514-00794-9; see AMK-Library under M 24)
[2]	Kotter, I., Grellmann, W.: Failure Analysis of Thermoplastic Products – Practical Examples, 15th Internationale Fachtagung "Polymerwerkstoffe 2012", Halle (Saale), September 12–14, 2012, Preoceedings (CD-ROM), P 37, pp. 1–10
[3]	Kotter, I., Grellmann, W.: Die Fraktographie als Hilfsmittel in der Schadensanalyse – Möglichkeiten und Grenzen. 14th Problemseminar "Deformation und Bruchverhalten von Kunststoffen", Merseburg, June 25–27, 2014, Proceedings pp. 18–26
[4]	Kotter, I., Grellmann, W.: Schadensanalyse an Kunststoffprodukten – Die VDI-Richtlinie in der praktischen Anwendung; 23th Fachtagung "Technomer 2009", Chemnitz, November 14–15, 2013, Proceedings (CD-ROM), KP V 8.6 pp. 1–6 (ISBN 978-3-939382-11-9)
[5]	VDI 3822: Failure Analysis – Fundamentals and Performance of Failure Analysis (2023-12)
[6]	VDI 3822 Blatt 2.1.1: Failure Analysis – Defects of Thermoplastic Products Made of Plastics Caused by Faulty Design (2024-06)
[7]	VDI 3822 Blatt 2.1.2: Failure Analysis – Defects of Thermoplastic Products Made of Plastics Caused by Faulty Processing (2024-06)
[8]	VDI 3822 Blatt 2.1.3: Failure Analysis – Defects of Thermoplastic Products Made of Plastics Caused by an Unfavourable Choice of Material and by Defects in the Material (2024-06)
[9]	VDI 3822 Blatt 2.1.4: Failure Analysis – Defects of Thermoplastic Products Made of Plastics Caused by Mechanical Stress (2024-06)
[10]	VDI 3822 Blatt 2.1.5: Failure Analysis – Defects of Thermoplastics Products Made of Plastics Caused by Thermal Stress (2024-06)
[11]	VDI 3822 Blatt 2.1.6: Failure Analysis – Defects of Thermoplastic Products Made of Plastics Caused by Tribology-induced Stress (2024-07)
[12]	VDI 3822 Blatt 2.1.7: Failure Analysis – Defects of Thermoplastic Products Made of Plastics Caused by Chemical Stress (2024-07)
[13]	VDI 3822 Blatt 2.1.8: Failure Analysis – Defects of Thermoplastic Products Made of Plastics Caused by Weather-induced Stress (2024-07)
[14]	VDI 3822 Blatt 2.1.9: Failure Analysis – Defects of Thermoplastic Products Made of Plastics Caused by Microbial Stress (2012-01)
[15]	VDI 3822 Blatt 2.1.10: Failure Analysis – Significant Instrumental Analysis Methods for Failure Analysis of Products Made of Plastics (2024-07)
[16]	Grellmann, W., Seidler, S.: Mechanical and Thermomechanical Properties of Polymers. Landolt-Börnstein, Volume VIII/6A3, Springer, Berlin (2014), (ISBN 978-3-642-55165-9; see AMK-Library under A 16)