Polymers fracture mechanics

Fracture mechanics is now quite weU estabHshed for metals, and a number of ASTM standards have been defined (4—6). For other materials, standardization efforts are underway (7,8). The techniques and procedures are being adapted from the metals Hterature. The concepts are appHcable to any material, provided the stmcture of the material can be treated as a continuum relative to the size-scale of the primary crack. There are many textbooks on the subject covering the appHcation of fracture mechanics to metals, polymers, and composites (9—15) (see Composite materials). 

J. G. WiUiams, fracture Mechanics of Polymers, Ellis Horwood, Chichester, UK, 1984. 

Marshall, G.P. Design for toughness in polymers - Fracture Mechanics, Plastics and Rubber Proc. and Appl. 2(1982) p 169-182. 

Williams JG (1984) Fracture mechanics of polymers. Ellis Horwood, Chichester, p 46... 

J G. Williams, Fracture Mechanics of Polymers. Wiley, New York. 19H4. 

Owing to the chain orientation distribution the fracture mechanism of oriented polymer fibres is different from that of isotropic fibres. The presence of this distribution leads to a non-uniform distribution of the strain energy between the domains. The strain energy is defined by... 

These pressure tests should be seen in the context of the mechanical stresses and subsequent failures in the field. In spite of all precautions on installation it must be assumed that pipes will be damaged, backfill and trenches will differ from those specified, welds may be imperfect and there will be bending stresses. Poor installation practice has been the principal cause of service failures, particularly of PVC pipe, and when installed correctly pipes taken from service (now 40 years) show no evidence of deterioration. In addition, polymers have improved greatly since pipes were first manufactured. Marshall and co-workers [2] describe the situation in more detail and recommend an approach to testing based on fracture mechanics. 

Hourston, D. J., Lane, S. and Zhang, H. X., Toughened thermoplastics Part 2 impact properties and fracture mechanisms of rubber modified PBT, Polymer, 32, 2215-2220 (1991). 

Moore, D., Pavan, A., and Williams, J. 2001. Fracture Mechanics Testing Methods for Polymers, Adhesives, and Composites. Elsevier, New York. 

Thermosetting epoxy polymers are widely employed in structural engineering applications and thus a knowledge of the mechanics and mechanisms of the fracture of such materials is of vital importance. The present Chapter discusses the fracture of epoxy polymers, concentrating on the use of a continuum fracture mechanics approach for elucidating the micromechanisms of crack growth and identifying pertinent failure criteria. 

Most of the recent advances in the understanding of the fracture behaviour of epoxy polymers has been through the application of fracture mechanics 2) and the present Chapter is therefore concerned with the study of the mechanisms and mechanics of crack growth in crosslinked epoxies using fracture mechanics. 

The basic tenet of continuum fracture mechanics is,- therefore, that the strength of most real solids is governed by the presence of flaws and, since the various theories enable the manner in which the flaws propagate under stress to be analysed mathematically, the application of fracture mechanics to crack growth in polymers has received considerable attention. Two main, inter-relatable, conditions for fracture are proposed. 

In the case of linear-elastic-fracture-mechanics, and nearly all epoxy polymers obey the requirements for LEFM to be employed, a simple relationship exists between KIc and G,c... 

The analysis of brittle fracture is the very domain of Linear Elastic Fracture Mechanics (LEFM). A comprehensive introduction to its fundamentals and the validity of its application to polymers has been given by Williams [25] and more recently by Grellmann and Seidler [26]. The fracture criteria and relevant test procedures elaborated by the ESIS technical committee TC4 can be found in [27]. 

For all further questions of definition, mathematical analysis, limits of applicability, and experimental procedure the reader is referred to the specialized literature. The fracture mechanics of polymers are particularly well treated in [25-27]. 

In order to get a quantitative characterisation of fracture, different concepts of fracture mechanics have been developed and we will focus on two of them (i.e. the K c and Gic approaches) which will be defined and briefly considered. More information about polymer fracture is given in the first review of this volume and in [4,5]. 

EPR spectroscopy is also frequently used in polymer science. Applications involve the detection of free radicals in cross-linking, high-energy irradiation, photochemical degradation and oxidation, and mechanical fracture of polymer chains. 

The best approach, however, consists of controlling the defect size and geometry and taking into account the corresponding stress-field inhomogeneity. This is realized in the frame of linear elastic fracture mechanics (LEFM), which was first applied to metals and ceramics and then adapted with success to polymers (Williams, 1984). 

Vallo Cl, Frontini PM, Williams RJJ, Polymer Gels and Networks, 1, 257-266 (1993). Williams JG, Fracture Mechanics of Polymers, Ellis Horwood, Chichester, UK, 1984. Williams JG, Cawood MJ, Polym. Testing, 9, 15-26 (1990). 

The fracture behaviour of polymers, usually under conditions of mode I opening, considered the severest test of a material s resistance to crack initiation and propagation, is widely characterised using linear elastic fracture mechanics (LEFM) parameters, such as the plane strain critical stress intensity factor, Kic, or the critical strain energy release rate, Gic, for crack initiation (determined using standard geometries such as those in Fig. 1). LEFM... 

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