Degradation fracture

Gall, B.L. and Raible, C.J. "Molecular Size Studies of Degraded Fracturing Fluid Polymers," SPE paper 13566, 1985 International Symposium on Oilfield and Geothermal Chemistry, Phoenix, April 9-11. 

Besides the common polyhydroxybutyrate (PHB), other polymers of this class are produced by a variety of organisms such as poly-4-hydroxybutyrate, PHV, and polyhy-droxyhexanoate. PHV is a naturally occurring bacterial polyester, which was first isolated by Wallen and coworkers [172,173]. Also, PHV/PHB copolymers have been studied to make a wide range of thermally processable polyesters, which exhibit the necessary long-term degradation profile required for a degradable fracture fixation device [174]. 

Polyacrylamides are used in many other oilfield appUcations. These include cement additives for fluid loss control in well cementing operations (127), viscosity control additives for drilling muds (128), and fracturing fluids (129). Copolymers [40623-73-2] of acrylamide and acrylamidomethylpropanesulfonic acid do not degrade with the high concentrations of acids used in acid fracturing. 

Examination of oven-aged samples has demonstrated that substantial degradation is limited to the outer surface (34), ie, the oxidation process is diffusion limited. Consistent with this conclusion is the observation that oxidation rates are dependent on sample thickness (32). Impact property measurements by high speed puncture tests have shown that the critical thickness of the degraded layer at which surface fracture changes from ductile to brittle is about 0.2 mm. Removal of the degraded layer restores ductiHty (34). Effects of embrittled surface thickness on impact have been studied using ABS coated with styrene—acrylonitrile copolymer (35). 

At a holdup time longer than 10—15 min at a high temperature, resin degradation is avoided by keeping the rear of the cylinder at a lower temperature than the front. At short holdup times (4—5 min), cylinder temperatures are the same in rear and front. If melt fracture occurs, the injection rate is reduced pressures are in the range of 20.6—55.1 MPa (3000—8000 psi). Low backpressure and screw rotation rates should be used. 

Polyamides, like other macromolecules, degrade as a result of mechanical stress either in the melt phase, in solution, or in the soHd state (124). Degradation in the fluid state is usually detected via a change in viscosity or molecular weight distribution (125). However, in the soHd state it is possible to observe the free radicals formed as a result of polymer chains breaking under the appHed stress. If the polymer is protected from oxygen, then alkyl radicals can be observed (126). However, if the sample is exposed to air then the radicals react with oxygen in a manner similar to thermo- and photooxidation. These reactions lead to the formation of microcracks, embrittlement, and fracture, which can eventually result in failure of the fiber, film, or plastic article. 

Such degradation of the surface causes little effect on either flexural strength or flexural modulus of elasticity but the influence on the impact properties is more profound. In such instances the minute cracks form centres for crack initiation and samples struck on the face of samples opposite to the exposed surface show brittle behaviour. For example, a moulded disc which will withstand an impact of 12 ftlbf without fracture before weathering will still withstand this impact if struck on the exposed side but may resist impacts of only 0.75 ftlbf when struck on the unexposed face. 

S. Sato, K. Hirakawa, A. Kurumada, S. Kimura and E. Yasuda, Degradation of Fracture Mechanics Properties of Reactor Graphite Due to Bum-off, Nuclear Engineering and Design, Vol 118, 1990, pp 227 241... 

For cross-ply laminates, a knee in the load-deformation cun/e occurs after the mechanical and thermal interactions between layers uncouple because of failure (which might be only degradation, not necessarily fracture) of a lamina. The mechanical interactions are caused by Poisson effects and/or shear-extension coupling. The thermal interactions are caused by different coefficients of thermal expansion in different layers because of different angular orientations of the layers (even though the orthotropic materials in each lamina are the same). The interactions are disrupted if the layers in a laminate separate. 

The mechanisms by which polymers undergo degradation in the human body are not yet completely understood. Examples of breakdown of these materials are illustrated by the embrittlement and excessive wear of polyester sockets exposed to the mechanical, biochemical and thermal stresses of the physiological milieu, as well as by the fatigue fractures, excessive wear and additional cross-linking (embrittlement) that have been observed in polyethylene sockets. 

Bundy, K. J. and Desai, V. H., Studies of Stress-Corrosion Cracking Behaviour of Surgical Implant Materials using a Fracture Mechanics Approach , in Corrosion and Degradation of Implant Materials, second symposium , (Eds) A. C. Fraker and C. D. Griffin, 73-90, ASTM Publication STP 859, Philadelphia (1985)... 

In flow-induced degradation, K is strongly dependent on the chain length and on the fluid strain-rate (e). According to the rate theory of molecular fracture (Eqs. 70 and 73), the scission rate constant K can be described by the following equation [155]... 

It should be stressed that the observed critical strain-rate for bond fracture (sf) in the case of a polydisperse fraction refers to the longest chain present in the sample. This quantity is significantly different from the critical strain-rate (r ) defined with respect to an average molecular mass whose value could be determined only after careful consideration of the degradation kinetics. 

The parameter R in Eq. (92) can be derived empirically from the experimental SEC traces with a minimum of computational effort and without regard to the details of the degradation kinetics by using the following arguments. Once a macromolecule is fractured, the moieties are immediately driven into another region of space. Due to the decrease in MW, a considerably higher strain rate... 

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