Greenhalgh Failure Analysis and Fractography of Polymer Composites

1. Auflage 2009
ISBN: 978-1-84569-681-8
Verlag: Elsevier Science & Techn.
Format: EPUB
Kopierschutz: 6 - ePub Watermark

E-Book, Englisch, 608 Seiten

Reihe: Woodhead Publishing Series in Composites Science and Engineering

The growing use of polymer composites is leading to increasing demand for fractographic expertise. Fractography is the study of fracture surface morphologies and it gives an insight into damage and failure mechanisms, underpinning the development of physically-based failure criteria. In composites research it provides a crucial link between predictive models and experimental observations. Finally, it is vital for post-mortem analysis of failed or crashed polymer composite components, the findings of which can be used to optimise future designs.Failure analysis and fractography of polymer composites covers the following topics: methodology and tools for failure analysis; fibre-dominated failures; delamination-dominated failures; fatigue failures; the influence of fibre architecture on failure; types of defect and damage; case studies of failures due to overload and design deficiencies; case studies of failures due to material and manufacturing defects; and case studies of failures due to in-service factors.With its distinguished author, Failure analysis and fractography of polymer composites is a standard reference text for researchers working on damage and failure mechanisms in composites, engineers characterising manufacturing and in-service defects in composite structures, and investigators undertaking post-mortem failure analysis of components. The book is aimed at both academic and industrial users, specifically final year and postgraduate engineering and materials students researching composites and industry designers and engineers in aerospace, civil, marine, power and transport applications. - Examines the study of fracture surface morphologies in uderstanding composite structural behaviour - Discusses composites research and post-modern analysis of failed or crashed polymer composite components - Provides an overview of damage mechanisms, types of defect and failure criteria

Dr Emile Greenhalgh works in the Department of Aeronautics at Imperial College London, UK. He is widely renowned for his expertise on the failure mechanics of composite materials.

Autoren/Hrsg.

Greenhalgh, Emile

Leseproben

2 Methodology and tools for failure analysis of polymer composites
Abstract
The basic methodologies, techniques and tools for conducting a failure analysis are presented. This information is pertinent for both laboratory and in-service failures, irrespective of the polymer composite architecture. The aim is to provide the reader with guidelines as to how to approach and undertake an analysis. Firstly, an overview of the health and safety issues associated with investigating composite components is presented. Following this, the general procedure for an analysis is described and then expanded upon in the following sections. Initial visual examination and non-destructive evaluation to determine the extent of any subsurface damage are described. Methods for photographing polymer composite fracture surfaces are then presented, followed by the important step of dissecting composites to expose the fracture surfaces. Verification of the materials and construction is described. The most important stages of the analysis involve microscopy, an overview of which is given, and detailed information on optical stereo, optical compound and scanning electron microscopy is presented. Other methods for material characterisation are described. The method of sequencing cracks and thus determining the sequence of failure from the information deduced during the analysis is described and finally guidelines for reporting are presented. Key words health and safety polymer composites fractography microscopy failure analysis non-destructive evaluation 2.1 Introduction
Failure of composite components generally falls into one of three classes; design failures, material and manufacturing defects, and in-service anomalies.1–3 Examples of these classes of failure are given in Chapters 8, 9 and 10 of this book, respectively. Whatever the class of failure, the methodology and techniques used to undertake the fractographic analysis are the same. Undertaking a fractographic analysis of a failed polymer composite component can be a challenging experience. The investigator is often presented with a complicated mixture of broken pieces, some of which amount to little more than piles of dust! Considerable time is spent at the beginning of any investigation trying to work out how everything fits together. Unfortunately, the brittle nature of polymer composites is such that there are usually numerous loose fractured pieces, and it may be difficult to identify where all these pieces had originated on the structure. However, before starting it is important to observe the following procedures. Information should be gathered about the history of the failed component and the conditions under which it had failed. If necessary, the scene of the failure should be visited, which can provide information about the layout of the failed parts prior to them being collected and brought for inspection. Secondly, it is important to protect the fracture surfaces and stabilise the components to ensure no post-failure damage is introduced.4 Some fracture faces, such as those generated under compression, tend not to separate, which makes protecting the surfaces relatively straightforward. However, tensile and interlaminar fractures tend to separate during failure, and are thus susceptible to post-failure damage. Fracture surfaces should therefore be protected from moisture, contamination and mechanical damage, such as fretting.5 A common problem is ‘finger grease’,6 which is usually introduced onto the fracture surfaces by preliminary handling, often with the phrase ‘I think the failure started here!’ being uttered! Finally, it is essential to take an objective point of view as to the cause of failure, even if the apparent cause may at first seem obvious. Analysis should be based around the facts, and not conjecture as to the most likely cause of failure; which can sometimes result in an attempt to ‘find evidence’ to support a particular failure scenario, whilst other important data is ignored or dismissed. During an analysis, it is vital to collate all the fractographic information in an objective and rigorous manner.7 If pressed to present ‘preliminary findings’ it is much better to be prudent and not provide any conclusions, rather than to suggest a failure mode, and have to retract it later. In this chapter, the basic methodologies, techniques and tools for conducting a failure analysis are described. This information is pertinent for both laboratory and in-service failures, irrespective of the polymer composite architecture. The aim is to provide the reader with guidelines as to how to approach and undertake an analysis. Firstly, however an overview of the health and safety issues associated with investigating composite components are presented. Following this, the general procedure for an analysis is described (Section 2.2). These steps are then expanded upon in the following sections. Initial visual examination and non-destructive evaluation, to determine the extent of any subsurface damage, are described. Methods for photographing polymer composite fracture surfaces are then presented, which precedes the important step of dissecting the fracture surfaces (Section 2.7). Verification of the materials and construction often needs to be undertaken during an analysis and so is described in Section 2.8. The most important stages of the analysis are perhaps the microscopy, an overview of which is described in Section 2.9, and then detailed information on optical stereo, optical compound and scanning electron microscopy is described in Sections 2.10, 2.11 and 2.12, respectively. Other methods of material characterisation are described in Section 2.13. The important aspect of sequencing cracks and thus determining the sequence of failure from the information deduced during the analysis is described in Section 2.14 and finally guidelines for reporting are described in Section 2.15. 2.2 Health and safety issues associated with composite fractography
2.2.1 Introduction
Unfortunately, failure of composite components can lead to exposure of the analyst to hazardous materials. Consequently, before embarking upon a fractographic analysis, it is important that the analyst has a good understanding of the risks associated with fractured composite materials and how to mitigate them. These risks and the means to avoid them are briefly described here. More detailed information is available,3,8,9 such as in composite repair manuals.10 The health issues associated with handling of composite failures can be partitioned into three levels of severity; components from an isolated environment, in-service failures, and in-service failures which have been exposed to fire.11 Furthermore, the description of the risks have been partitioned into those associated with handling of failed composite components and those associated with cutting and machining of failed composite components. 2.2.2 General issues
When presented with a failed component for investigation, the analyst should ensure he is provided with the background to the item, and details of any specific hazards relating to the incident. Furthermore, only allowing personnel who are undertaking the investigation to handle the items not only helps to preservation of the evidence, but also reduces the health and safety risks associated with the failure.11 Many of the general points related to handling of failed composites are common sense, such as not eating or drinking in the vicinity of failed composites, and draw on the recognised hazards associated with repair of composite materials.10 Finally, it may be prudent to ensure the analyst has been inoculated against tetanus, in case the skin is punctured by the component during an analysis. 2.2.3 Handing of single component from an isolated environment
The least hazardous failed components are those from an isolated or controlled environment, such as a mechanical test piece or defective production item. The key threat from such a component is that of splinters, loose fibres and dust which may have been liberated. Therefore, it is recommended that the analyst should always wear rubber gloves to avoid skin punctures. Glass and carbon fibre splinters do have a tendency to break when being extracted, so particular care needs to be taken when handling the specimens during all stages of the analysis to avoid injury. 2.2.4 Handing of failed in-service components
The next level of hazard is that associated with in-service failures, such as failed components from crashed aircraft or vehicles etc. Not only are splinters and loose fibres still a hazard, but the component may also have been contaminated from the surrounding systems such as fuel or hydraulic systems. Therefore, under these conditions, it is recommended that the appropriate personnel protection equipment (PPE) should be worn. A further issue which needs to be considered with in-service failures is that associated with biological hazards from injuries or fatalities that may have occurred during an accident/incident. Appropriate guidelines may need to be followed over and above standard laboratory practice in these cases. 2.2.5 Handing of failed in-service components exposed to fire
Polymer composite materials and components, which have been exposed...