Crack healing

The randomization stage refers to the equilibration of the nonequilibrium conformations of the chains near the surfaces and in the case of crack healing and processing, the restoration of the molecular weight distribution and random orientation of chain segments near the interface. The conformational relaxation is of particular importance in the strength development at incompatible interfaces and affects molecular connectivity at polymer-solid interfaces. 

Fig. 1.85 Oxidation of high-purity iron in oxygen at differing pressures. All figures on curves are in N m . At 1-3 x 10 tol-3 x 10 N/m torr the rate is controlled by the impact of molecular oxygen at I - 3 x 10 N/m torr the initial rate of oxidation is sufficiently high to give void precipitation and the rate decreases with pressure increasing to l-3N/m at pressures greater than this the crack-heal mechanism becomes operative and the rate again increases with pressure (after Hussey and Cohen...

Fig. 1.86 Stills from a scanning electron microscope study by time-lapse photography of iron oxidation showing the results of the crack-heal mechanism. Left, Immsl/tm right, 1 mm 0-5 /tm (courtesy Central Electricity Research Laboratories)...

Once intimate contact is achieved, bonding of the ply interfaces can occur. The mathematical relationships between interply bond formation and processing temperature and time were discussed in Section 7.3. The analyses are based on the theories explaining strength development of a polymer-polymer interface and crack healing in polymers. 

Cracks heal due to transport of silica from wallrock to cracks wallrocks become depleted in silica adjacent to cracks... 

Ignoring the reverse process (crack healing), classical... 

Fujii, Y., Tanifuji, K., Stafford, C, Takahashi, T. Hashida, T. 2001. Crack healing and microcrack formation in granite under the presence of high temperature and pressurized water. Proc. of Asian Pacific Conf. on Fracture and Strength 01 and Int.Conf. on Advanced Technology in Experimental Mechanics 01 Sendai, Oct. 20-22, 2001 pp. 1041-1046. 

One of the most important and useful applications of the reptation concept concerns crack healing, which is primarily the result of the diffusion of macromolecules across the interface. This healing process was studied particularly by Kausch and co-workers [76]. The problem of healing is to correlate the macroscopic strength measurements to the microscopic description of motion. The difference between self-diffusion phenomena in the bulk polymer and healing is that the polymer chains in the former case move over... 

Crackle glass has been produced for its artistic appearance for over 150 years (perhaps since the sixteenth century in Venice). The glass is heated to 1000°C and then plunged into water causing cracks to form across the surface if reheated and blown further the cracks heal on the interior a similar effect is produced in glazes. ... 

Crack healing (Step 2) Molten CP molecules adhere across the fractured interface, and when cooled down, bond the fractured surface together... 

Gilman concentrated at separation of the crystal planes by the crack he did not make any observations of the crack healing to produce adhesive joining of the cleaved surfaces. So he did not fully demonstrate reversibility. However, he restricted plasticity to low levels and obtained surface energies comparable with those estimated from elastic modulus, as shown in Table 4.1. 

Once the interface crack has initiated inside a composite material, then traveled some distance along the interface, there may be a tendency for the crack to heal, if the crack surfaces come into close proximity, so that the molecular adhesion forces can pull the smooth crack faces together again. The simplest situation in which this interface crack healing arises is shown in Fig. 16.17. This schematic diagram illustrates the observations made on a model composite laminate made from four strips of smooth, transparent rubber. 

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