Introduction

In this chapter, the reader is introduced to the topic of fractography, the study of fracture surfaces in order to determine the root cause of the fracture and to establish the connection between the crack growth mechanisms and the microstructure of the material. The concept of the structure–property relationship is reviewed briefly, and the basic steps to follow in conducting a plastic product failure analysis investigation are introduced. It is stressed that information regarding the mechanism of part failure learned via fractography should be considered in conjunction with other information and data about the failure in order to perform a proper root cause failure analysis.

Keywords

Fractography; plastics; part failure; structure–property relationship; thermoplastic; thermoset; rubber; root cause

1.1 Motivations

It should come as no surprise to an individual interested in reading Fractography in Failure Analysis of Polymers that plastic products do occasionally fail. This possibility exists partly because of the sizeable number of factors that influence product performance, including material formulation, processing, design, and implementation. Small changes in these variables can have a dramatic effect on the performance of the product. Although plastics, when used properly, are not any more prone to failure than other materials, the number of failures has undoubtedly increased over the last few decades as the volume of plastic production has increased by 300% in the United States alone (as measured by US dollars) [1,2]. Today, plastics are one of the most utilized materials in the world, and their use in demanding engineering applications continues to increase. Thus, the increase in failures can be attributed not only to an increase in sheer volume of plastic parts in service but also to their use in new innovative applications, which push the boundaries of design, material performance, and processing.

When a plastic product does fail, it may lead to property damage, environmental damage, personal injury, or death. Failures can also be damaging to the manufacturer, impacting its brand credibility, financial strength, and ability to function without a loss in employee productivity. A high profile or catastrophic product failure can also be a distraction to the company, requiring a shift of resources from research and development and manufacturing to warranty claim work, failure analysis, customer relations, and possibly even litigation. In addition, a proper fix must be developed and deployed to eliminate the failures. Hopefully, one positive outcome will be an increase in the collective awareness of the potential causes of failure and an improvement in quality control processes, so that similar failures can be avoided in the future.

The determination of how and why a plastic part fails is important for all parties involved in bringing the product to market—not just the manufacturer. This includes raw material suppliers, compounders, designers, and distributors. It has been reported that in the United States alone, approximately 900,000 individuals are directly employed in the plastic industry in over 16,200 facilities [2]. The goal of this book then is to provide a tool to some of these plastics professionals in order to enable faster and more accurate failure analysis and, ultimately, a reduction in product failures. The specific question that this book addresses is: What information can be obtained from a fracture surface and does it help determine how and why the product failed? This technique is termed fractography.

1.2 What Is Fractography?

Fractography is the study of fracture surfaces in order to determine the relation between the microstructure and the mechanism(s) of crack initiation and propagation and, eventually, the root cause of the fracture. Fractography is arguably the most valuable tool available to a failure analyst investigating plastic product failures. The term “fractography” was coined in 1944 to describe the science of studying fracture surfaces, although fracture surfaces have been studied for centuries [2–5]. The majority of the early work on fractography in the mid-twentieth century was performed on metals, ceramics, and glass materials. The widespread use of fractography in analyzing plastic material failures began decades later, as the use of plastics in load bearing applications and the frequency of associated failures increased.

Fractography of plastic failures is generally applied to cracks in parts that exhibit little deformation. The goal is to characterize the features of the fracture surface, including topography, morphology, and color, as these features are a result of the material structure and its response to stress and crack propagation. In fact, there is a direct relationship between a plastic material’s solid-state structure and its physical properties, and thus, its service performance. We cannot begin a discussion on the fracture process without a short introduction to this concept. While the concept of the structure–property relationship (a broad topic on which many books have been written) is not the focus of this book, it is explored briefly here for the benefit of the reader, who may not be familiar with fundamentals of polymers.

1.3 Plastic Material Structure–Property Relationship

A plastic product is manufactured using a compound that contains, as an essential ingredient, one or more organic synthetic polymers, which are long-chain molecules of high molecular weight (i.e., “hundreds of thousands”) [6]. It is typically solid in its finished state and, at some stage in its manufacture or processing into the finished articles, was shaped by flow [2,7]. Polymers are generally classified into three categories: thermoplastic, thermosets, and rubbers. Due to the chemical cross-linking in thermosets and rubbers, these polymers do not develop an organized microstructure in their solid state. Rather, they respond as an interconnected network of large molecular chains.

Thermoplastics or “plastics,” on the other hand, are un-crosslinked polymers. Their lack of networking allows for greater mobility of the polymer chains, which limits strength, thermal stability, and creep resistance. Plastics can be further divided into two groups based on their solid-state microstructure: semicrystalline polymers and amorphous polymers. An amorphous thermoplastic is one in which the molecular chains are hindered from developing any organized (crystalline) structure, due to the chemical makeup of the chain. Thus, the plastic material remains completely amorphous in its solid state. A semicrystalline thermoplastic is one in which the long molecular chains of the polymer can fold and form mixtures of crystalline (ordered) structures within amorphous (non-ordered) material.

Regardless of polymer classification, it is through some process of flow that raw plastics are converted into useful products with desirable properties. Because of their unique microstructure, all polymers display both elastic and viscous type behaviors at ordinary temperatures and loading rates. It has been well documented that the mechanical behavior and the deformation process by which cracking develops in products is ultimately determined by the response of the product’s material solid-state microstructure (or lack thereof) to its surroundings [8,9]. Due to their unique long chain nature and solid-state microstructure, the plastic material’s response to its surrounding is dependent on the stress, time, and temperature, as well as many other variables of the service environment. It then follows that, in root cause failure investigations for plastic products, each of these factors need to be considered and evaluated. Fractography plays a key role in this assessment.

1.4 Components of a Failure Investigation

When a plastic product fails, the entities involved in bringing that product to market will many times initiate a failure analysis investigation. As discussed in detail in the following chapters of this book, fractographic analysis will typically lead to hypotheses regarding the root cause of the failure. In order to develop and test these hypotheses, the basic components of a failure investigation should be followed [2,10]:

To this end, Chapters 2 and 3 of this book provide information as to how the failure investigation should be conducted in order get to the root cause of the failure efficiently. Specifically, Chapter 2 provides context for the use of fractography in the broader objective of failure analysis. Some definitions for key failure analysis terms are provided for clarity’s sake, and common mistakes or pitfalls are reviewed. In Chapter 3, the tools and techniques used to document a fracture surface are reviewed. Other key analytical methods that are used to provide supporting or complementary data are also discussed. In Chapter 4, the fundamental concepts and terms used to describe fracture features are introduced and discussed in detail. The four basic root causes of failure in plastic products are discussed at length. Chapter 5 provides a detailed discussion of long-term failure modes involving slow crack growth. The authors felt it...