Fractography

Kausch [289,290]. Basic texts on fracture, such as by Liebowitz [291], discuss fracture mechanics and fractography. In this section, discussion will focus on the techniques required to prepare fractured or deformed specimens for microscope observation. 

In the first case, mechanical deformation is part of the sample preparation, where the sample is formed by testing used to obtain mechanical data. The purpose of conducting microscopy on the fractured sample is normally to determine the mode and propagation of failure and to correlate the mechanical data and sample microstructure. SEM of the fracture surfaces provides these observations for comparison with standards of similar materials [292]. The fracture could simply be used to provide access to an internal surface of the polymer for microstructural investigation. In this case, it must be remembered that the fracture surface is not a random section through the material, but one where the fracture required the least energy. 

In the second case, the mechanical deformation method will not be standard, but suitable samples will be designed to fit the microscope. The in situ deformation of polymers is almost always conducted in the SEM at low magnification and is most often used for fibers and fabrics. Other microdeformation methods are used to prepare thin films for TEM observations, to model fiber 

In the third case, where the failure and fracture occur outside the laboratory, microscopy is often used in a forensic manner to determine the failure mode and its point of initiation. Microanalysis may be used to pinpoint a specific cause and assign responsibility for the failure. Here the special features of specimen preparation relate to the nature of the work and optical photography may be required to document the relation of the specific SEM samples to the original object. 

Microscopic features on fracture surfaces are also often useful for determin- 

Fractured bulk polymers and composites require only coating with a conductive layer (Section 4.7.3) before observation in the SEM, although some composite fracture surfaces are so rough as to make deposition of a thin continuous conductive film very difficult. High resolution is rarely required in these materials so the common solution is to use a thick coating, and often carbon is evaporated followed by metal coating. Fibers, particularly textile fibers and thin films, have such a small cross sectional area that the main difficulty is in handling the broken sample. 

Hearle and Cross [357] broke thermoplastic fibers at normal rates of extension in an Instron and examined them in the SEM. They developed a special stub for mounting the fiber ends. A step was cut from the circular stub and an elliptical hole cut in the remaining stub and a screw placed in the cut step. This provides a space in the center of the stub for fibers to be mounted on double sided sticky tape. The screw is used to attach the cut step portion to the remainder of the stub. 

In marked contrast. Lynch and Trevena [98] for pnre Mg in a NaCl -I- K2Cr04 solution, reported that the fracture surfaces consisted of flutes containing equiaxed dimples on 10IX planes and grain boundaries as well as cleavage-like features on 0001 planes, with the flutes and dimples being smaller and shallower than those occurring in air. 

The bulk fracture surface may come from one of three general sources deformation of the sample in a standard mechanical testing device, such as a tensile tester (e.g., Instron, Norwood MA), an impact testing machine (e.g., Charpy or Izod), or a tear tester deformation by frac- 

The visual inspection and the study of broken samples by means of optical or scanning electron microscopy (SEM) are important tools of fracture analysis. They obviously aid 

The appearance of a fracture surface undoubtedly is the most convincing evidence that the fracture process has reached the phase of non-homogeneous deformation. 

On the basis of his extensive investigations Hearle [85] has established a classification of the main features of fiber fracture morphologies  

This is the classic form of failure in elastic materials and consists of a relatively smooth mirror zone of crack propagation leading to a rougher zone of final failure due to multiple crack-initiation (comparable in morphology to Fig. 1.7). 

This type of break runs perpendicularly across the fiber, with a rough texture and no evidence of any propagating cracks the whole structure appears to be ready to fail at the same time, and the breaks are very similar in appearance to lower-magnification views of the fracture of fiber-reinforced composites (Fig. 7.8 and 7.9). 

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Roulin-Moloney A C 1989 Fractography and failure mechanisms of polymers and composites (London Elsevier)... 

Most fractography can be conducted with simple instmments such as a pocket magnifying eyepiece or an optical microscope. As with any detective work, it is important to maintain careful records and to pay close attention to details in the reconstmctions of the conditions under which fabrication and failure occurred. Seemingly unimportant details of fabrication, service, and/or the conditions under which failure occurred can frequently be the key to determining the cause of failure. 

The effect of °Co y-ray irradiation on the mechanical properties, surface morphology, and fractography of blends of plasticized PVC and thermoplastic copolyester elastomer, Hytrel (E.I. Du Pont de Nemours Company, Inc., Wilmington, Delaware), have been studied by Thomas et al. [445]. Radiation has two major effects on the blend cross-linking of the Hytrel phase and degradation of PVC phase. Both effects are found more prominent at higher radiation dose. 

Bailey, J. E., Ellis, B., Howarth, L. G. Oldfield, C. W. B. (1991). The fracture of glass-ionomer cements. Conference on the Fractography of Glasses and Ceramics II, New York State College of Ceramics, Alfred University, New York, July 1990. American Ceramic Society. 

The cracking susceptibility of a micro-alloyed HSLA-100 steel was examined and compared to that of a HY-100 steel in the as-received condition and after heat treatment to simulate the thermal history of a single pass weld. Slow strain rate tensile tests were conducted on samples of these alloys with these thermal histories in an inert environment and in an aqueous solution during continuous cathodic charging at different potentials with respect to a reference electrode. Both alloys exhibited reduced ductilities at cathodic potentials indicating susceptibility to hydrogen embrittlement. The results of these experiments will be presented and discussed in relation to the observed microstructures and fractography. 

Denison, P., Jones, F.R., Brown, A., Humphrey. P. and Paul, A.J. (1988b). Scanning SIMS fractography of CFRP. In Interfaces in Polymer. Ceramic and Metal Matrix Composites (H. Ishida ed.), Elsevier. New York, pp. 239-248. 

Friedrich, K. and Karger-Kocsis, J. (1989). Unfilled and short fiber reinforced semi-crystalline thermoplastics. In Fractography and Failure Mechanisms of Polymers and Composites, (A.C. Roulin-Moloney ed.), Elsevier Appl. Science, London, pp. 437-494. 

Masters. J.E. (1987b). Characterisation of impact damage development in graphite/epoxy laminates. In Fractography of Modern Engineering Materials Composites and Metals, ASTM STP 948, ASTM. Philadelphia, PA, pp. 238-258. 

Fractography is the examination of fracture surfaces with no sample preparation other than cleaning to determine the fracture mechanism. 

Fagan, A. F., Bell, J. M., and Briggs, G. A. D. (1989). Acoustic microscopy of polymers and polymer composites. In Fractography and failure mechanisms of polymers and composites (ed. A. C. Roulin-Moloney), pp. 213-30. Elsevier Applied Science, London. [204]... 

Tensile stress-strain curves were generated using an electro-mechanical universal testing machine with specially designed flat-ended fixtures that were machined in order the grip the specimens carefully. All the samples were tested for failure under displacement control with a prescribed displacement rate of 1.5 mm min-1. Fractography of the tested samples was carried out using a SIRION field emission SEM. 

E. E. Glickman and V. I. Igoshev, Micromechanism of Solid Metal Induced Embrittlement Fractography and Kinetic Model, Surface Physics, Chemistry, Mechanics, No. 3, 104—112, (1989) (in Russian). 

Fiber Fractography and Morphology. Fibers broken in tensile tests were examined in the scanning electron microscope to gain a better understanding of their failure mechanism. Such insight can be used to guide the development of consolidants that prevent such failures. 

Fig. 7.11 (a) An example of a SiC whisker bridging the faces of a fatigue crack in an alumina/SiC composite (R = 0.15, T = 1400°C, and vc = 0.1 Hz), (b) Scanning electron fractography showing pulled-out SiC whiskers which have been oxidized to form glass. From Refs. 4 and 61. 

Rowe, E. H., Fractography of Thermoset Resins Toughened with Hycar... 

BRESEE AND GOODYEAR Fractography of Historic Silk Fibers... 

Methods and Capabilities of Electron Microscope Fractography , Springfield Armory TR 20-2411 (1966) 36) J.E. Mapes, Crystal... 

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