Fractography Basics

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Fractography Basics

4.1 Fracture Surface Features and Interpretation The formation of a specific fracture surface results from the complex interdependence of the prevailing conditions of the applied stress, the component geometry, the local material properties in the fracture path, and the local environment. Things such as the rate and configuration of loading, the magnitude and nature of stress (tensile, compression, or shear), and the state of stress (plane stress or plane strain) are important considerations, as are the composition and microstructure of the material and environmental factors such as temperature, chemical exposure, etc. Some fracture surface features are associated with fast, brittle fracture, while others are primarily associated with slow moving fractures like slow crack growth (SCG). Some features may be observed to some extent in both fast fracture and slow moving fractures. The fracture surface features can also change dramatically depending on whether the material failed above or below its glass transition temperature (Tg). This chapter will describe common features that one often observes on plastic fracture surfaces in order to help the investigator arrive at the root cause of the failure. When conducting an examination of a fracture surface, it is often helpful to try to answer a few questions regarding the fracture. Some of these questions are posed below and then explored in greater detail later in the text.

4.1.1 What Failure Characteristics Are Normally Associated with This Material? This is a very basic and important question that should be kept in mind when examining a fracture surface as part of a failure analysis. The material properties and underlying structure of the polymer—in addition to other issues such as stress state, geometry, and environment—determine to a large extent what is observed on the fracture surface. One would expect differences in fracture surface features for different materials with the same type of failure. Fracture surfaces due to creep rupture failure, for example, will look very different in a ductile semicrystalline

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polymer such as polyacetal compared to a brittle amorphous polymer such as polystyrene (PS). There will also be differences between similar polymers, for example, more or less microductility on the fracture surface, depending on their type and quality. Knowledge of fracture surface features that are typical for a particular fracture mechanism may provide strong evidence of that fracture mechanism if they are observed. On the other hand, differences between the typical fracture surface features expected for a certain mechanism and those actually present on the subject fracture surface may provide important clues that other processes are at work.

4.1.2 What Is the Location and Nature of the Fracture Origin? Locating the fracture origin is one of the primary goals when a fracture surface is examined. The location of the fracture origin or origins will answer several important questions regarding the possible cause of the fracture, such as: Is the fracture origin at a point of known high stress in the part? Is the origin at a stress concentration? Is there a flaw in the material at this location? Is there evidence of degradation? Is there evidence of a chemical on the surface at the origin location? Are there witness marks associated with the fracture origin area?

4.1.3 Is the Fracture Surface Brittle or Ductile—How Ductile? The amount of ductility evident on the fracture surface is an important clue as to what stress or environment the material may have been exposed to in service. In general, higher stresses will lead to more ductility and roughness on the fracture surface. Lower stresses usually will have less ductility and roughness. Examples of this include environmental stress cracking (ESC) failures in high-density polyethylene (HDPE) where the origin area exhibits very little microductility, with greater levels of microductility evident at points more remote from the origin. The amount of ductility can be important in determining whether the fracture is related simply to stress, environment, or a combination of the two, as will be discussed later. If resources are available, a study to calibrate the level of ductility versus stress or versus environment on exemplar samples of a particular material could be helpful.

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4.1.4 Is the Fracture Surface Smooth or Rough, Dull or Glossy? This is a basic but important question similar to the brittle versus ductile question. The answer to this question can help determine the speed of fracture and the conditions under which it occurred, depending on the material. Smoother fracture surfaces are generally associated with lower stresses and brittle fracture. Smooth fracture surfaces can also result due to severe ESC. Very smooth, glossy fracture surfaces can also occur in SCG failures due to ruptured crazes in amorphous polymers. As the stress and/or speed increases, the fracture surface generally becomes progressively rougher in brittle fractures. However, the smoothness and roughness of the surface may change, transitioning to a smoother surface at very high crack speeds, as in rapid crack propagation failures. The surface roughness in these type of failures may change somewhat as the crack speed and direction change.

4.1.5 Is Stress Whitening Present Anywhere on the Fracture Surface? This question is related to the previous question in that it is a strong indicator of the stress at which the polymer failed. Stress whitening occurs in many plastic materials when they are stressed beyond their yield strength. The formation of microvoids or crazes gives the material a whitish color, by scattering transmitted light. It is most apparent in transparent or translucent plastics [1]. The nature of stress whitening is dependent on the type of material, ranging from a very white appearance to a slight lightening of the color of a material. Figure 4.1 is an example of stress whitening that occurred on a portion of a fracture surface in a part molded from a polyphenylene ether
polystyrene (PPE-PS) material. The stress whitening occurred in this part near the end of the failure due to the high stress that resulted when the crack grew large enough, causing the ligament stress to exceed the yield strength of the polymer. While most plastics can exhibit some level of stress whitening, some such as polyphenylsulfone do not. In these cases, the extent of ductility on the fracture surface must be evaluated to assess the presence of yielding and, therefore, high stress.

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Figure 4.1 ESC failure in PPE/PS material with stress whitening near the end of the fracture.

4.1.6 What Is the Nature of Striations and Other Marks on the Fracture Surface—Was the Fracture Fast or Slow? Striations or lines on fracture surfaces appear due to a variety of conditions including fatigue, SCG, and fast brittle fracture. They are distinct curved lines on a fracture surface that may be related to crack arrest and/or progression of a crack through the material. Interpretation of striations on a fracture surface often requires integration of other information (fractographic and otherwise) about the specific failure, including whether or not the part might have been subjected to cyclic stresses and whether or not the fracture occurred at high speed, based on other fractographic features such as hackles. These considerations will be discussed in greater detail later in this chapter.

4.1.7 Do the Mating Halves of the Fracture Show the Same Crack Direction? This question is important in that it addresses the mode of fracture. For the majority of cracks, the fracture direction will be the same for mating parts. This is true for crack opening, or Mode I fractures and fractures that are predominantly Mode I (includes most fractures that are evaluated). However, if the crack directions for mating parts are traveling in opposite directions, this indicates a shearing failure of the

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Figure 4.2 Schematic of fracture modes.

material in the plane of the fracture related to Mode II (in-plane shear) or Mode III (out-of-plane shear) fracture. Cracks that tilt involve Mode I with a smaller Mode II contribution. In Mode I/Mode III loading, the crack appears to twist, producing steps on the surface [2]. Figure 4.2 is a schematic that describes these three modes of fracture.

4.1.8 Is the Crack Straight or Curved? Whether a crack is straight or curved can give clues to the stresses present in the failed part, the direction of the loading, the microstructure of the material, the presence of defects, and the speed of the fracture. Curved cracks often are related to mixed mode fractures (Mode I/Mode II or Mode I/Mode III) where multiaxial forces are involved.

4.1.9 Are There Branches, Bifurcations, or T-Junctions of the Crack in the Part? Whether or not there is crack branching in a failed part gives several important clues about the failure. If cracks are branching it indicates that the stress is high and the material is unable to dissipate the energy that is driving the failure with a single crack. High-energy release (and high stress in the part) will cause a greater number of bifurcations of the crack. T-junctions can occur when independent cracks intersect or when a crack turns back on itself and arrests at an earlier portion of the crack. These features are typically associated with impact or other dynamic events.

4.1.10 Are Both SCG and Fast Fracture Areas Present on the Fracture Surface? Many times, fractures initiate and grow slowly into the material and then transition to a fast fracture mode. The size of the SCG region can

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be used to estimate the stress on a part using fracture mechanics and the crack length at which point the fracture becomes unstable. (See Mirror Zone discussion below.)

4.1.11 Is There Any Foreign Material or Chemical Evident on the Surface? This is a very important observation to make with respect to failures that may be caused by exposure to chemicals or other aggressive environments. However, in some cases, the material may not be visible to the naked eye or even under magnification. If ESC failure is suspected, analytical testing of material excised from the surface or obtained by solvent washing the surface may be performed in order to identify any residual chemicals that may be present. Typical analytical tests include Fourier Transform Infrared spectroscopy and gas chromatography
mass spectrometry. The answers to these questions and any other observations that can be noted often are critical in performing a thorough root cause failure analysis. Several of these questions are addressed in greater detail below.

4.2 Brittle Versus Ductile Failures in Polymers There are two general failure modes that are observed in materials: brittle failure and ductile failure. Fracture analyses most often involve unexpected brittle failure of normally ductile polymers. “Brittle” refers to the lack of significant deformation of the material and is generally associated with cracking and fracture. Brittle fractures show very little deformation of the material around the fracture. The fracture surface is very smooth and does not exhibit a lot of ductility or “stretching” of the material. On the other hand, ductile failures can exhibit a lot of deformation around the fracture area. This deformation may be limited or less obvious if the material had a notch or deep scratch that precipitated the failure. Ductile failures in polyethylene (PE) materials, as well as other plastics, exhibit distortion, stretching, and shearing (in-plane extension of the material due to the material being forced in opposite directions). Ductile fracture is characterized by drawing of the material and occurs when the material is stressed beyond its yield strength. Ductile failures are less common as subjects for failure analysis, although they do occur when materials are severely stressed or subjected to severe environments, such as high service temperatures or chemical attack by a solvent [1].

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Stress

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Strain

Figure 4.3 Schematic of tensile test showing elongation of tensile specimen at different strains.

Tensile test samples, for example, may exhibit varying degrees of ductility or distortion, depending on the test conditions. Figure 4.3 illustrates the increase in deformation for a ductile sample as the strain increases during a tensile test. Brittle materials will have a small strain to failure and corresponding low toughness, while ductile materials will have a large strain to failure and corresponding higher toughness. One measure of toughness is the area under the stress
strain curve, which can be calculated by integration as follows: ð Brittle: σdA small ð Ductile:

σdA large

These two cases feature dramatically different sample transformations: (i) crazing in brittle materials and (ii) necking in ductile materials. Figure 4.4 shows multiple crazes that occurred in a polymethyl methacrylate (PMMA) coffee mug during service. There is almost no distortion of the part at all even though there are numerous crazes of varying severity in the part. Figure 4.5 shows a typical ductile failure of a PE 3408 tensile specimen, with extensive deformation and extension of the material (necking) at failure ( .500% elongation at failure). Figure 4.6 shows a ductile failure of a PE 3408 tensile specimen that was notched with a razor blade prior to testing. The behavior is still ductile, exhibiting tearing and some

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Figure 4.4 Example of multiple crazes in a PMMA coffee mug.

Figure 4.5 HDPE tensile test specimen before and after testing.

stress whitening (whitish color due to stretching of the material), but the overall deformation is very limited in comparison to the un-notched specimen. Figure 4.7 shows a brittle fracture of the same PE 3408 material, which was notched and cooled in liquid nitrogen before testing. This specimen has a very smooth, brittle fracture surface and almost no distortion of the surrounding material—the fractured pieces fit together perfectly, displaying their original, pretest geometry. Brittle fractures generally occur at stresses well below the yield stress of the polymer. Brittle fracture is favored by: • high strain rates (e.g., impact), • low temperatures,

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Figure 4.6 HDPE tensile test specimen that was tested with a razor notch.

Figure 4.7 HDPE tensile test specimen that was razor notched and cooled in liquid nitrogen prior to testing.

• cyclic loading (fatigue), • temperatures below the glass transition temperature for the material, • a stress state known as plane strain (high constraint). The mode of fracture in plastics frequently depends on the state of stress experienced by the material. Uniaxial and biaxial stress (“plane stress”)

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states are more favorable conditions for ductile fracture, which results from shear yielding. Shear yielding is a plastic flow process involving no volume change in the material, and therefore it can occur more readily when lateral contractions are unrestrained. Plane stress ductile fractures are therefore accompanied by gross plastic deformation such as shape changes and distortions. Brittle fractures in polymer materials frequently are initiated by a process of craze formation, craze growth, and eventual rupture of the craze, producing a crack. Crazing in polymers is a cavitation process involving an increase in the volume of material under triaxial tensile stresses (“plane strain”). Thus, under these conditions, a polymer is more likely to fracture in a brittle manner. A more detailed discussion of these concepts and their relevance to fractography follows.

4.2.1

Plane Stress and Plane Strain

Plane stress refers to the condition in which the only non-zero components of stress lie in a single plane (i.e., a biaxial state of stress). This stress state is common in thin-walled plastic parts, where σ3 ,,, σ1, σ2. The largest stresses developed in the thickness direction are only a small fraction of those acting parallel with the surface. For argument’s sake, let σ3 5 0 be the stress acting in the thickness direction. Then, the thickness contraction is given by Hooke’s law as: ε3 5 2ν

ðσ1 1 σ2 Þ E

where ν 5 Poisson’s ratio, E 5 Young’s modulus, ε3 5 strain in the thickness direction, σ1, σ2 5 stress in the 1 and 2 directions, respectively. If, on the other hand, contraction is constrained in the thickness direction, that is, ε3 5 0, a tensile stress, σ3 develops. This condition is called plane strain (non-zero strains in two directions) or triaxial stress (stress acts in all three directions). From Hooke’s law, the stress acting in the thickness direction is: σ3 5 νðσ1 1 σ2 Þ

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Whether fracture occurs under a condition of plane stress or plane strain depends in part, on the thickness of a part: • Thin Part: normal stress  0; plane stress condition, which favors ductile yielding or failure. • Thick Part: normal stressc0; plane strain condition, which favors craze formation and brittle fracture. The state of stress near a crack or notch is multiaxial even if the applied stress is only uniaxial. On the external free surfaces near the crack tip, a biaxial state of stress exists, since the stress component normal to the free surface must vanish (σ3 5 0). Across the thickness of the specimen just ahead of the crack tip, σ3 must vary from zero on one surface, through a maximum at the center, and then diminishes to zero again on the other external surface, as shown in Figure 4.8. For thin specimens, σ3 at the center is small and the state of stress is essentially plane stress. For thicker specimens, the plane strain stress state in the interior of the specimen causes the crack front to take an elliptical shape with the crack at the interior advancing through the thickness faster than the portions of the crack near the free surfaces. Thus, applying an axial force to a notched or cracked body produces a non-uniform, multiaxial stress state in front of the notch. The axial stress σ1 decreases rapidly as the distance from the notch tip increases. Elements

Figure 4.8 Schematic of stress state ahead of a cracked or notched material.

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Figure 4.9 Schematic of stress state in a notched material.

above and below the crack plane are subjected to a lower stress than those lying on the crack plane. Adjacent elements near the notch tip which experience different stresses would tend to contract in different amounts. However, strain compatibility requires that the strains at the element boundaries be identical. Thus, an element subjected to a higher stress than its neighbors is restrained from lateral contraction. This leads to the development of lateral tensions resulting in a state of triaxial tension at the notch tip, or plane strain, illustrated in Figure 4.9.

4.2.2

Cautions

Even though brittle fractures are without gross plastic deformation, localized plasticity is sometimes observed on these fracture surfaces. Alternately, a fracture surface with a macroscopic brittle appearance may exhibit microductility, suggesting that the failure involves localized yielding of the material as well. Brittle fracture in many polymeric materials includes those failures that exhibit microductility on the fracture surface due to crack initiation and SCG. SCG is considered a “brittle” failure mode due to the fact that gross deformation of the plastic part does not occur, but ductile deformation does occur and is limited to a very small volume of material on the fracture surface. SCG occurs due to cavitation and microvoid formation that, in the case of plastics, leads to the formation of crazes. This differs from ductile failure in metals wherein microvoid coalescence results in a crack and quickly leads to failure. The microvoids in plastics do not coalesce into a crack immediately, but instead become stabilized by fibrils containing oriented polymeric material [1]. Crack growth occurs

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Figure 4.10 Low-magnification view of SCG in PE.

Figure 4.11 High-magnification fracture surface in PE with microductility/ fibrillation that is typical of creep/SCG failures.

when these fibrils break and the crack advances. Figures 4.10 and 4.11 are photographs depicting the typical microductility or fibrillation that is found on the fracture surface of an SCG failure in PE. These fibrils are the remnants of the oriented material that bridged the microvoids in the crazed plastic. Of course, brittle fracture can be related to gross overloading of the material as well if the loading conditions include high strain rate and/or low temperature, depending on the type of material.

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4.3 Crack Path Analysis As discussed in Chapter 3, a great deal of information about a part failure can be gleaned from macroscopic features in addition to the microscopic details touched on above (and discussed in greater detail in the following section). For instance, examination of the crack patterns on a broken part can reveal important information about the loading that induced the cracking. In particular, one can differentiate between crack branching and “T-junctions.” Crack branching or forking is associated with fast fracture in brittle materials and results when the release rate of stored energy in the material exceeds the amount of energy released due to the increase of surface area that occurs in crack formation. This leads to formation of crack branches or bifurcations in order to account for this excess energy release. Crack branching can also occur in slow moving fractures when the stress field is complex (e.g., large bending moments). Figure 4.12 depicts a series of branching cracks and shows the crack direction away from the origin area. Recognition of this pattern can be extremely useful in locating the section of the part that contains the fracture origin. For example, if you are investigating the failure of a long section of a pipeline that has fractured, it would be a daunting task to have to look at the fracture surfaces of several feet of pipe, but often one can trace the crack branching back to the origin area and begin the fracture surface examination there. Figures 4.13 and 4.14 are examples of branching cracks that can be used to locate the crack origin area in a sample. On the other hand, “T-junctions” occur when a secondary (later initiating) crack intersects with an existing crack. Fractures propagate normal to (i.e., at right angles to) a principal stress, and due to the stress field around a primary crack, a secondary crack will change direction to terminate perpendicular to the primary crack. “Crossing fractures” can also occur when the primary fracture is not fully penetrated through the thickness of the material, and the secondary crack passes over it. These

Figure 4.12 Determination of crack growth direction from crack branching.

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Figure 4.13 Example of cracks branching in both directions away from the fracture origin area (center) in cross-linked PE (PEX) pipe.

Figure 4.14 Example of cracks branching away in both directions from fracture origin area in a PVC pipe fitting.

behaviors are described in more detail in Refs. [3,4]. By properly interpreting these features, one can often determine the sequencing of crack formation and propagation.

4.4 Fracture Features In order to analyze the fracture surface of a plastic failure and clearly communicate any observations derived from the examination, it is useful

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Table 4.1 Basic Fracture Features in Polymeric Materials Fracture Surface Feature

Fast Fracture

Slow Fracture

Origin(s) Mirror zone Mist region Rib markings Hackles Twist hackle Wallner lines Fatigue striations or beach marks Conic markings Crack branching Ratchet marks or ledges River line patterns Chevron markings

x x x x x x x

x xa xa x

x x x x x

x x x x x x x

a

Analogues of mirror and mist may be present in ductile failures, but will look very different compared to brittle fractures.

to have a list of terms that relate to the features observed on the fracture surface. The terms used by metallurgists to describe some fracture surface features are also used for plastics, but in many cases, different terms have been used by plastics engineers, causing confusion. For the purposes of this text, a short list of terms will be developed to describe the fracture surface features that are important with respect to plastics. Terms that are typically used for metallurgical failures will be noted and, when appropriate, will be applied to plastic fracture surfaces as well. Table 4.1 lists basic fracture surface features of polymeric materials and notes whether or not they are generally present in slow and fast fractures. Many of these features are described schematically in Figure 4.15. The next section will address these features and their application to polymeric material fracture surfaces.

4.4.1

Fracture Origin(s)

In order to determine the root cause of the failure, a determination of where the fracture initiated is typically the first step in a laboratory examination of the fracture surface. The location of the fracture origin will help determine the cause of failure, whether it is located in an area of

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Figure 4.15 Schematic of various fracture surface features.

high stress, at a void or inclusion in the material, or in an area where degradation has taken place. Fracture origins often occur at surfaces, but they also occur frequently in the interior of parts for various reasons. When the fracture surface has a single origin, it is usually at the location of the maximum tensile stress in the part or at an area of minimum material resistance to fracture. Secondary origins may arise at other initiation points that are not necessarily on the same fracture plane. Conic markings (discussed below) occur when secondary crack origins are created ahead of the growing crack tip due to an elevation of stress at that point. A fracture surface may also be created when crack initiation occurs at multiple origins and the subsequent crack extensions merge into a single crack plane. Multiple crack initiation is common for surfaces that exhibit degradation due to UV exposure, chemical attack, and/or ESC. The number of fracture origins is thus an important observation that can guide the failure analysis. Multiple fracture origins can also occur due to the distribution of high long-term stresses in a part and may not be related to ESC or degradation, but simply are related to the stress field at that location in the part. Multiple crack origins are common in fatigue failures, for example. Fracture origins are commonly found at regions of stress concentration, maximum tensile stress, local material weakness, an inclusion, or a void. Figures 4.16
4.22 depict various examples of fracture origins including fracture initiation due to: • voids (Figure 4.16), • inclusions of foreign material (Figure 4.17),

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Figure 4.16 Fracture origin at a void in a PEX tube.

Figure 4.17 Fracture origin at an inclusion in PVC.

• stress concentrations resulting from the geometry of the part (Figure 4.18), • areas of localized loading on a part (Figure 4.19), • embrittled and degraded portions of a surface (Figure 4.20), • ESC damage (Figure 4.21), • areas of localized ductile overload of the material (Figure 4.22).

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Figure 4.18 Fracture origin at a geometric discontinuity in PMMA.

Figure 4.19 Point loading of a PE pipe causing crack initiation on the pipe inner surface (bottom of photograph).

4.4.2 Mirror Zone The mirror zone is a region of the fracture surface found near the fracture origin that is smooth, flat, and mirrorlike (reflects light). A mirror zone is typically found in glass and amorphous, brittle plastics such as PS and PMMA, and also in thermoset resins like polyester and epoxy. Figure 4.23 depicts a mirror zone in the origin area of a brittle fracture in a PMMA part. More ductile polymers, including semicrystalline materials

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Figure 4.20 Cracks initiating at degraded outer surface of nylon part (at bottom of photograph).

Figure 4.21 ESC failure in ABS, note multiple origins and very glossy fracture surface (top of photograph).

like polyolefins, may have regions that are analogous to a mirror zone, but are not as mirrorlike, exhibiting some surface roughness. Semicrystalline materials that fracture below their glass transition temperature (Tg) or due to impact may have features in the origin area that are much closer to the flat, featureless mirror zones found in glassy, amorphous plastics. Mirror zones usually appear featureless, even at high magnifications under the microscope. In many polymers, the mirror zone is the remnant of a ruptured craze. Figure 4.24 is a schematic of a crack in a polymer and a craze (precrack structure) just ahead of the crack tip.

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Figure 4.22 Ductile fracture origin, followed by fast, brittle fracture in ABS (at arrow).

Figure 4.23 Example of mirror zone at the origin area of a brittle fracture in PMMA.

Crazing is not a constant volume process like ductile fracture. As the craze grows in a triaxial stress state (plane strain), the void volume increases. As loading continues, there is a coalescence of the voids and extension of the fibrils that form at the boundaries of these voids. A crack forms when the fibrils of material between these coalesced voids break, creating two surfaces. Frequently in transparent polymers such as PS, PMMA, and polycarbonate (PC), interference colors can be observed in the mirror zone. This is due to a thin layer of ruptured craze fibrils on the fracture surface with

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Figure 4.24 Schematic of a crack and a craze in a polymer.

Figure 4.25 PC fracture surface showing mirror region with multiple ruptured crazes at root of notch.

a different refractive index than that of the bulk polymer. The size of the mirror zone depends on both the prevailing stress at the time of fracture and the fracture toughness, K1C, of the material. Figure 4.25 is a microphotograph of a PC fracture surface that depicts a mirror region that contains the remnants of ruptured crazes, followed by fast, brittle fracture of the remaining sample. This fracture initiated at the root of a notched izod impact specimen that was subjected to a very high bending stress that put the material into tension at the notch. The specimen was relatively thick at 0.4 in., leading to a plane strain condition in the interior of the specimen at the notch root. The part failed after approximately 15 min under stress at room temperature. The stress intensity factor, K1, at the tip of a crack can be written as: pffiffiffi K1 5 Yσ a where Y is a geometric parameter dependent on specimen and crack geometry, as well as loading configuration, σ is the applied stress, and

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a is the crack length. Subcritical crack growth occurs when K1 , K1C, meaning that crack growth is stable and does not cause immediate failure. On the other hand, unstable brittle fracture takes place when pffiffiffiffiffi K1 5 K1C 5 Yσf ac where σf is the stress at fracture and ac is the critical crack size at the onset of brittle fracture. Solving for the critical crack size ac gives ac 5 ½K1C =ðYσf Þ2 Accordingly, the mirror zone size should be inversely proportional to the square of the fracture stress. In practice, this relationship is of limited use since K1C is also dependent on many variables that are unknown to the failure analyst (e.g., temperature), and sometimes the fracture toughness itself, K1C, is not readily available to the analyst. However, this type of analysis has been successfully used to estimate the stress on a part at the time of failure with good correlation to the stress rupture properties of the material [5]. Case Study 15 in Chapter 6 demonstrates how to calculate the stress at fracture based on the size of the SCG zone prior to brittle fracture in PVC.

4.4.3 Mist Region The mist region is commonly observed in glass and several glassy polymers. It is usually found adjacent to the mirror zone. The mist region differs from the mirror zone only in a microscopic change in the surface texture. Instead of being mirror smooth, it has a misty appearance. Its formation may be associated with crack acceleration just prior to rapid crack growth. In many semicrystalline materials, the mirror region may not be apparent at all and may appear only as a mist region (i.e., not perfectly flat or mirrorlike). An example is shown in Figure 4.26 for a PC sample. Figures 4.27 and 4.28 depict the growth of crazes in a notched HDPE test specimen ahead of a crack after fatigue cycling. The fracture surfaces of these samples have the same fibrillar features on them as seen in the SCG failure shown previously in Figure 4.11.

4.4.4 Rib Markings/Beach Marks Rib markings are prominent curved crack front markings that are often visually observable. They are also known as “beach marks” or

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Figure 4.26 Mist region (marked by ovals) in vicinity of fracture origin.

Figure 4.27 Crazing ahead of fatigue crack in HDPE.

“crack arrest marks,” particularly with respect to fatigue fracture surfaces in metals. These markings occur during crack arrest or sudden changes in crack velocity. The spacing is typically random. Outward normals point to crack growth direction and the fracture origin can be found somewhere on the concave sides of rib markings, as shown schematically in Figure 4.29. Figure 4.30 is an example of a HDPE gas pipe fracture showing rib markings. Rib markings can be present close to the origin or very far away, depending on the fracture. The shape of the rib markings as they

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Figure 4.28 Close-up of crazing of HDPE fatigue specimen from Figure 4.27.

Figure 4.29 Schematic of rib markings showing crack direction.

intersect the part surface on the concave side can also yield information concerning degradation of that surface. Normally these markings will curve in tightly as they approach the surface (as in Figure 4.31) due to a change in stress state, that is, plane strain (triaxial stress) in the interior of the part and plane stress (biaxial stress) at the surfaces of the part. As we have discussed previously, brittle fracture is favored by the plane strain condition at the crack tip in the interior of parts and is inhibited near the surfaces where the stress state changes to plane stress, which favors ductile failure. This phenomenon was illustrated in Figure 4.8 (see previous discussion on plane stress-plane strain), showing greater crack propagation in the interior of a specimen where plane strain exists. However, if the surface of the part is degraded, the brittle

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Figure 4.30 Rib markings on HDPE pipe fracture surface.

Figure 4.31 Point load failure in HDPE pipe showing rib markings curving inward toward inner surface.

crack propagates much more easily due to the lower material strength. Hence, these rib markings will flare out as they approach the surface and not curve inward (Figure 4.32). Figures 4.31 and 4.32 depict this phenomenon, contrasting a point load failure of an HDPE pipe whose inner surface was not degraded with one where the inner surface had been degraded due to long-term exposure to water disinfectants and elevated temperatures (see Case Study 12).

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Figure 4.32 Failure in HDPE pipe with a degraded inner surface showing flaring out of rib markings at the inner surface.

4.4.5 Hackles Hackles are typically rough in appearance, consisting of divergent lines radiating outward from the fracture origin. They are perhaps the most reliable fracture surface features to use to locate the fracture origin. Hackled regions are associated with high-energy dissipation due to localized plastic deformation on the fracture surface. They are regions of violent activities involving high crack velocity, rapid changes in the stress field, and non-coplanar fracture paths (Figure 4.33). Hackled regions exhibit large, irregularly oriented facets and are related to micro-bifurcation of the crack as it propagates at high speed [2]. Hackles also may curve toward the surfaces of a part, particularly when the crack is turning. These markings clearly show the crack propagation direction and point back toward the origin area, as shown in Figure 4.34. Hackles may curve toward both surfaces as the crack is running if the crack is propagating uniformly along the length of the part, as shown in Figure 4.35 for an acrylic fracture surface away from the origin. Some investigators may also refer to these marks as chevron or herringbone markings. For example, according to the ASM Handbook Dictionary of Metals [6], these “nested Vs” are typically found on brittle fracture surfaces “in parts whose widths are considerably greater than their thicknesses.” Some features, also often referred to as hackles, are observed in the region of a specimen where the nature of the stress field is rapidly

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Figure 4.33 Hackles diverging along crack direction in a PC sample.

Figure 4.34 Hackles radiating out along crack direction (left to right), curving toward outer surface of a PVC pipe.

changing (mixed Mode I/III loading). Hull reports that some investigators refer to this as a “twist hackle,” since it is due to the changing stress field [2]. Figure 4.36 depicts this feature near the final fracture at the surface of a PC izod impact specimen. It can be noted that the features in Figure 4.36 are very similar to river lines (discussed below) and in fact may be characterized more appropriately by that term.

4.4.6

River Patterns or River Markings

River patterns are the most characteristic feature of the fracture surfaces of brittle materials. Similar patterns occur on the fracture surfaces

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Figure 4.35 Fracture surface from an acrylic part showing hackles curving toward both surfaces (crack direction is right to left).

Figure 4.36 “Twist hackle” or river lines near final ligament fracture of PC izod impact specimen (crack direction is bottom to top).

of semi-brittle materials [2]. They were originally dubbed river lines due to the appearance of small lines on the surface (tributaries) that merge into larger lines which in turn merge into even larger lines as the crack grows [2], as in the twist hackle example above. These features may be present on the fracture surfaces of plastics where there is some component of mixed mode loading (e.g., Mode I/III). The intensity of the river markings depends on the material’s microstructure and the amount of mixed mode loading. River markings tend to be less

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Figure 4.37 Example of river patterns/hackles on a fracture surface from a failed PVC fitting.

prominent initially when crack growth is predominantly Mode I with a small Mode III component. They become much more prominent as the Mode III component increases. River markings have been observed on fracture surfaces in both fast, brittle fractures and in slower, more ductile fractures. They can appear like the example shown in Figure 4.36 for a “twist hackle” or as in Figure 4.37, an example of river lines or hackles emanating from the origin area in a PVC fitting. The crack growth direction is in the direction of progressive roughening of the fracture surface. River patterns are less pronounced and less numerous as truly brittle fracture is approached. Due to the similarities in surface roughness that occur under conditions of increased stress intensity and that caused by mixed mode loading, these features are often simply referred to as hackles. To avoid confusion, it may be appropriate to refer to roughness resulting from increased stress intensity as hackles and increased roughness resulting from mixed mode loading as river lines. However, as can be observed in Figures 4.34
4.37, a combination of these factors often exists on many fracture surfaces.

4.4.7

Wallner Lines

Wallner lines are faint striations on otherwise smooth fracture surfaces associated with fast moving fractures, unlike fatigue striations that occur in slow moving fractures (see next section). They resemble fatigue striations with periodic spacing, but are formed when stress waves reflected from the specimen boundaries or free surfaces interact with the propagating crack front. They are subtle changes in the fracture surface texture that result when the stress waves produce a slight

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Figure 4.38 Wallner lines and hackles on a polyphenylsulfone fracture.

perturbation of the stress field just ahead of the crack front. Wallner lines are unlike rib markings, which are much more distinct. At times, it can be difficult to see Wallner lines on a fracture surface using lowmagnification optical microscopy, as the lines can be very faint. It is quite common to see hackles along with Wallner lines, since the fracture is moving at high speed. In fact, the presence of hackles crossing the striations can help to rule out fatigue. Since the fracture is moving at high speed, microductility will be greatly suppressed or absent. Wallner lines are typically curved markings similar to crack front markings with the fracture origin located somewhere on their concave side. They are not true crack front markings, but may be considered to be “snap shots” of the crack front during crack propagation, since stress wave velocities are much higher than crack velocities. Figures 4.38 and 4.39 depict Wallner lines in polyphenylsulfone and PC polymers. A fracture surface may contain more than one set of Wallner lines with different orientations that intersect one another on the fracture surface. This phenomenon may occur in parts with complex geometries or when there are multiple sources of stress waves in the part and/or multiple origins. Figure 4.40 shows multiple sets of Wallner lines intersecting each other on a PET fracture surface.

4.4.8 Fatigue Striations Fatigue striations are true crack arrest markings that generally are observable at low magnifications in plastic materials. Fatigue striations

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Figure 4.39 Wallner lines and hackles emanating from origin area in a PC stress rupture failure.

Figure 4.40 Multiple sets of Wallner lines intersecting on a PET fracture surface.

are more distinct than Wallner lines, with each striation marking one or more cycles of crack growth and arrest. The striations have regular spacing if the applied stress intensity is also uniform and have wider spacing as stress intensity increases. Hackles are absent from the fatigue region since fatigue cracks grow at low velocity. Figure 4.41 depicts fatigue striations on a styrene acrylonitrile (SAN) field fracture. There are also a few river lines that occur as the fracture progressed due to tilting of the crack (mixed Mode I/Mode III loading). Figures 4.42 and 4.43

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Figure 4.41 Fatigue striations on the fracture surface of an SAN field fracture (origin at top left corner).

Figure 4.42 Fatigue striations on the fracture surface of a PS toothbrush failure (origin at top left corner).

depict fatigue striations from a PS toothbrush failure and a PC connector failure, respectively. The PC failure (Figure 4.43) shows wider spacing of the striations away from the origin due to higher stress intensity being present compared to the narrower spacing of the striations nearer the origin. Figure 4.44 shows fatigue striations on the fracture surface of an HDPE pipe that had been subjected to cyclic pressure in service over the course of many years. The fracture in this case initiated at a large inclusion in the pipe wall at the inner surface. Note also the several

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Figure 4.43 Micrograph showing striations in a PC connector due to low cycle fatigue (origin at bottom left corner).

Figure 4.44 Fatigue striations on an HDPE pipe fracture surface. The fracture initiated at a large inclusion in the pipe at the inner surface (note multiple ductile ledges near origin).

ductile ledges or ratchet marks that occurred in the origin area due to coalescence of multiple closely spaced cracks that initiated on slightly different planes. For polymers, some apparent fatigue striations may actually be discontinuous growth bands, in which each band includes hundreds to

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Figure 4.45 Discontinuous crack growth bands in PS fatigue test failure.

thousands of fatigue cycles. Thus, one cannot use striation count as an indication of fatigue cycles, as is sometimes possible in metals. In these cases, damage accumulates in the craze zone until the point where one additional cycle causes the crack to jump to the craze boundary, and the process starts all over again. Figure 4.45 is a photograph of the fracture surface of a PS part that failed in a fatigue test. This fracture surface exhibits a small number of discontinuous growth bands that do not equate to the 104 cycles to failure that was recorded for this specimen. Additional laboratory work is necessary in order to fully characterize the fatigue behavior of a particular polymer and to determine the resulting fracture surface features. It is also sometimes difficult to ascertain fatigue failures in certain instances because fatigue striations are not always visible. This can be due to local softening and melting caused by hysteretic heating of the sample, which can obliterate fatigue striations in less rigid plastics [7]. Figures 4.46 and 4.47 depict the fracture surface and fracture origin area, respectively, of a nylon axial fatigue test specimen that does not exhibit any fatigue striations when observed using an optical stereomicroscope. Although it was not performed in this case, SEM might have revealed fatigue striations, if indeed they exist. Figures 4.48
4.50 depict the fracture surface of a liquid crystal polymer (LCP) part that failed due to fatigue crack propagation. In this case, the striations were not readily visible with optical microscopy, and SEM was required in order to observe the fatigue striations.

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Figure 4.46 Fracture surface nylon axial fatigue test sample showing no fatigue striations.

Figure 4.47 Close-up of fracture origin from nylon axial fatigue test specimen showing no fatigue striations.

4.4.8.1 Fatigue Crack Growth Versus SCG It can be difficult to differentiate between fatigue and SCG on some polymer fracture surfaces. The presence of rib markings or striations on the fracture surface may be due to crack growth during either cyclic loading (fatigue) or sustained loading (creep). Figures 4.51 and 4.52 depict fracture surfaces of a CPVC pipe and a PC part, respectively, that were both subjected to constant stress leading to eventual failure.

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Figure 4.48 Fatigue failure in LCP part initiating near corner.

Figure 4.49 Origin area from the LCP part fatigue failure.

Many investigators would immediately assume, based on the appearance of the striations, that both of these fractures were fatigue-induced failures—even though there was no evidence of any cyclic stresses in either case. Rather, the CPVC pipe was held at constant pressure in service, and the PC failure was a laboratory failure in which the part was held at constant stress at elevated temperature until failure. In recent studies of SCG failures in PE, it has been demonstrated that SCG failures can also occur in a continuous fashion or in a stepwise fashion, leaving striations on the surface [8]. Dr. Chudnovsky demonstrated that the appearance of the SCG fracture surface is dependent upon the

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Figure 4.50 Close-up of fracture origin at the edge of LCP part.

Figure 4.51 Striations on a CPVC pipe fracture that was subjected to constant pressure.

stress and temperature at which the fracture occurred. Figures 4.53 and 4.54 depict continuous and discontinuous SCG in HDPE samples, respectively, both of which were tested at (different) constant stress conditions. The continuous SCG failure was tested at high stress at 23 C, while the discontinuous SCG failure was tested at lower stress at 80 C. If a stepwise SCG failure is considered at a micromechanics level, the stress also cycles at the crack tip, building up as damage occurs ahead of the crack tip. This stress is relieved when the crack jumps

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Figure 4.52 Striations on the fracture surface of a PC part that was subjected to constant stress at elevated temperature.

Figure 4.53 Continuous SCG in HDPE tested at 23 C [8]. Courtesy of SPE ANTEC 2010.

forward and arrests, and the process begins to repeat itself. The crack growth data in Figure 4.55 illustrates this phenomenon, showing stepwise and continuous crack growth at different conditions. It is apparent then that SCG and fatigue really are similar on many levels. Both are slow moving cracks. With fatigue failures, the applied stress is cycled, many times leading to striations on the fracture surface— but not always. Likewise in SCG, striations may or may not be present on

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Figure 4.54 Stepwise SCG failure in HDPE tested at 80 C [8]. Courtesy of SPE ANTEC 2010. 12

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Figure 4.55 Graph showing stepwise and continuous crack growth in HDPE at different constant stress test conditions [8]. Courtesy of SPE ANTEC 2010.

the fracture surface, depending on the conditions and material properties at the time of fracture. SCG/creep rupture failures may be considered as a limiting case of “fatigue” where the stress is held constant [9]. In fact, SCG and creep rupture have often been referred to as “static fatigue,” which, based on the preceding discussion, may be an appropriate term [10]. The concepts of creep and SCG are explored further in Chapter 5.

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4.4.9 Conic or Parabolic Markings Parabolic or conic markings generally are microscopic in size but sometimes are observable without a microscope. Conic markings are commonly observed in ABS and PVC. These markings occur at random sites, such as filler particles or particles of rubber in a rubber-modified polymer. Conic markings resemble wakes created by a protruding stick in a stream of gently flowing water. They are formed by the interaction of the main crack front with a secondary crack nucleated just ahead of the main crack. They can result during either SCG or fast fracture. The secondary origin is found near the focus of the parabolic shape, and the main crack origin can be found somewhere on the convex side of the conic marking, as shown in Figure 4.56. Figure 4.57 is an example of conic markings in a PVC material.

Figure 4.56 Schematic of a parabolic marking.

Figure 4.57 Large parabolic markings visible on a PVC fracture surface (crack direction is left to right).

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Ratchet Marks or Ledges

Ratchet marks or ledges are common features on many fracture surfaces. They occur when multiple cracks are propagating from different origins in the same area, on slightly different planes. (The term “ratchet” is commonly used in metals to denote multiple crack origins on the surface of a part, especially under fatigue conditions.) As these cracks approach one another, the local stress increases until the material fails, causing these crack planes to come together and leave a ridge on the fracture surface. Many times these ridges exhibit ductility since the ligament of material yields and tears as the cracks coalesce (see Figure 4.44). Figure 4.58 depicts ratchet marks or ledges in a HDPE pipe failure. In the case of degraded materials and/or ESC failures, these ledges may exhibit sharp edges without any significant ductility, since in these cases the material fails below the yield strength of the material. Figure 4.59 is an example of sharp, truncated ledges that were observed in a CPVC ESC failure. One may also see slivers of material on the fracture surface when multiple cracks undercut one another. Figures 4.60 and 4.61 are examples of this phenomenon. These ledge areas may exhibit stress whitening or other color differences from the rest of the fracture surface due to differences in stress state and the thickness of the material. (The investigator must be careful not to confuse these features for material abnormalities such as contaminants, as discussed in Chapter 6, Case Study 11.)

Figure 4.58 Ratchet mark or ledge created when cracks on slightly different planes coalesce (HDPE).

Figure 4.59 Sharp, truncated ledges in a CPVC ESC failure.

Figure 4.60 Undercut ledges on fracture surface near origin in PEX tubing.

Figure 4.61 Undercut ledges in a PEX tank fracture near origin.

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4.5 Application of Fractography to Failure Analysis Obviously, not all of the features discussed above will be found on all fracture surfaces. The key observations needed include locating the fracture origin, determining the crack propagation direction, and determining the fracture mechanism. This is done by carefully observing the markings that are present on the fracture surface, such as rib markings, hackles, etc. as outlined above. Most failures result from multiple factors and are rarely related to a single cause. However, examination of polymer fracture surfaces in most cases will help determine the root cause of the failure. Fractography is both a science and an art, as the various case studies will show (Chapter 6). There are times when elucidation of the fracture mechanism can be quite challenging due to the variability in materials, loading conditions, and environment. Many times, features of interest are masked by other material or have been partially destroyed by erosion or wear. One of the most powerful tools in interpreting fractographic images is a library of reference images and case studies. In addition to the numerous examples provided through this text, several books listed in the references are excellent resources for additional information that is helpful to understand the use of fractography in analyzing fractures in polymer and composite materials. In particular, the following books are highly recommended: Derek Hull, Fractography: Observing, Measuring and Interpreting Fracture Surface Topography Hertzberg and Manson, Fatigue of Engineering Plastics ASM Handbook, Volume 12, Fractography ASM Engineered Materials Handbook, Volume 2, Engineering Plastics Lampman, ASM International, Characterization and Failure Analysis of Plastics Bradt and Tressler, Fractography of Glass Fractography of Modern Engineering Materials: Composites and Metals, ASTM STP 948 Ehrenstein, Engel, Klingele Schaper, SEM of Plastics Failure and the earlier An Atlas of Polymer Damage Roulin-Moloney, Fractography and Failure Mechanisms of Polymers and Composites

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Lustiger, Cassady, Uralil, and Hulbert, Field Failure Reference Catalog for Polyethylene Gas Piping, first ed., January 1980
December 1984, Gas Research Institute Myer Ezrin, Plastics Failure Guide: Cause and Prevention, second ed., (Cincinnati: Hanser Publications, 2013) John Scheirs, Compositional and Failure Analysis of Polymers (John Wiley & Sons Inc, 2000)

References [1] S. Lampman, Characterization and Failure Analysis of Plastics, ASM International, Materials Park, OH, 2003. [2] D. Hull, Fractography, Cambridge University Press, United Kingdom, 1999. [3] Kepple, J.B. & Wasylyk, J.S. Fracture of Glass Containers. 1994. Available at: http://books.google.com/books/about/Fractography_of_Glass. html?id5oS6nR9729XUC. [4] R.J. Parrington, Microsoft PowerPoint—ASM Practical Fractography Handout.ppt, ASM International, Materials Park, OH, 2014, pp.1
197. [5] L.J. Broutman, D.E.Duvall, P.K. So, Failure Analysis of a PVC water pipe, Proceedings of the Society of Plastics Engineers 47th Annual Technical Conference, 1989. [6] Anon, Dictionary of metals, Reference information library, ASM Handbooks Online: Desk Editions and General References, ASM International, Materials Park, OH, 2013. [7] R.J. Parrington, Fractography of metals and plastics, Practical Failure Analysis 2(5) (2002) 16
19, 44
46. [8] Z. Zhou, H. Zhang, A. Chudnovsky, W. Michie, M. Demirors, Temperature effects on slow crack growth in pipe grade PE, Proceedings of the Society of Plastic Engineers Annual Technical Conference, 2010. [9] R.W. Hertzberg, J.A. Manson, Fatigue of Engineering Plastics, Academic Press, Inc, New York, NY, 1980. [10] D.C. Wright, Environmental Stress Cracking of Plastics, RAPRA Technology Ltd, Shawbury, UK, 1996.