Fracture crack healing

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. 

Total crack to be healed = pre-notch + fracture crack... 

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

The machined specimens were subjected to crack-healing treatment by heat-treating in air at 1373 - 1673 K for 1 or 10 h. The specimens are here called as machined specimen healed. Also the smooth specimens were crack-healed at 1573 K for 1 h in air. These specimens are defined as healed smooth specimen. Even the smooth specimen involved minute flaws such as surface-crack and surface-pore. The flaws can be completely healed by heat-tteatment Thus, all the fractures of the healed smooth specimens Initiate from largest embedded flaw such as pore. 

Figure 3 shows the effect of the crack-healing conditions on the strength recovery behavior of the machined specimen. The cut depth by one pass, d was 10 pm. The symbols in left column and in right column show the fracture stress (ay) of as-machined specimen and healed smooth specimen, respectively. The average Oy of these specimens were found to be 177 MPa and 628 MPa, respectively, indicative of an 100% strength recovery upon crack-healing. 

Furthermore, it was found that the machining cracks were completely healed by crack-healing as shown in figure 5 (a). Figures 5 (a-1) and (a-2) show the SEM images of fracture origin and detail of point A, respectively of the machined specimen healed at 1673 K for 10 h. As shown in these figures, all machined specimen healed fractured from sites other than the... 

Figure 7 shows the temperature dependence of the bending strength of the healed machined specimens. The symbol shows the Gf of the machined specimen healed at 1673 K for 10 h (t/ = 10 pm/pass). The Gf of the healed machined specimens decrease almost linearly in the temperature range from room temperature to 1573 K, and these values were found to be more than approximately 350 MPa. Above 1573 K, Gp decrease slightly with the increase of the test temperature. Moreover, SEM observations confirmed that all of the machined specimens did not fracture from examples of the healed machining-cracks. Thus, it could be concluded that all crack-healed zones exhibited the same levels strength to the base material (mulite / SiC... 

Fracture stress (or) of machined specimen healed at 1673 K for 10 h was almost equal to that of healed smooth specimens. From these results, the optimized crack-healing conditions for machining cracks was defined as heating at 1673 K for 10 h. 

Mechanical properties of SiC-AlN-Y203 composites (SiC 50%wt-AlN 50%wt), pressureless-sintered with an innovative and cost-effective method, were determined before and after oxidation performed at 1300°C for 1 h. As a consequence of the oxidative treatment, fracture toughness increased from 4.6 MPa m to 6.6 MPa m, flexural strength from 420 MPa to 488 MPa, Weibull modulus from 4.5 to 5.3 and thermal shock resistance (expressed as critical temperature difference) from 3I0°C to 380°C. First of all, these results demonstrated that a pre-oxidation treatment is needed to increase the mechanical resistance and reliability of SiC-AlN-Y203 components. Secondarily, the beneficial effects of the oxidation on the mechanical properties could be explained in terms of compressive residual stresses and crack healing ability. 

In the present study, a hybrid surface reinforcement consisting of crack-healing and low energy electron beam irradiation has been proposed. The usefulness of the method was investigated from the surface hardness and the fracture strength. 

Figure 3 Fracture strength of the hybrid surface reinforced specimen (closed triangle n-3) as a function of the irradiation dose, with that of EB irradiated specimen (closed circle w=3), crack-healed specimen (open triangle n= 9) and smooth specimen (open circle n=4). where n is the number of measurement.

K. Ando, B.S. Kim, M.C. Chu, S. Saito and K. Takahashi, Crack-healing and Mechanical Behaviour of Al203/SiC Composites at Elevated Tetrqtetature. Fatigue and fracture of Engineering Materials and structures. 2004,27, 533-541... 

Fig. 8.73 Effect of crack-healing temperature on the bending strength of Si3N4/ SiC at room temperature. Data marked with an asterisk indicate that fracture occurred outside of the crack-healed zone [10]. With kind permission of John Wiley and Sons...

Fig. 8.76 Fracture patterns of crack-healed SiaNVSiC a Fracture occurred outside of the crack-healed zone. (Healing conditions 1100 °C for 5 h in air, (Tmsx = 210 MPa, R = 0.2, f = 5 Hz, bending strength at room temperature 653 MPa.) b Specimen fractured across the crack-healed zone. (Healing conditions 1100 °C for 15 h in air,

Fig. 9.42 Fracture surface of AI2O3/S1C crack-healed a Macro-SEM photograph of crack initiation site (Nf = 3900 cycle) b crack initiation site and flaw (Nf = 3900 cycles) c crack initiation site of Nf = 15885 cycles sample, no-flaw can be seen at the crack initiation site. With kind permission of Professor Ando for the authors of Ref. [1] and Professor Liu of Ref. [2]...

SCG rate steeply increases as Ki approaches the critical value Kic, leading to spontaneous fracture. In the case of sintered silicon nitrides at high temperatures, the fatigue limit is considered as the threshold between the crack healing and the crack growth. 

Suprapolix have visually demonstrated crack-healing in the surface of the supramolecular material by heating the sample to 140 °C. At this temperature, the material is able to flow and fill the fracture-void without further external intervention (Figure 3), regenerating the flat surface of the pristine material. 

From a kinetics point of view, formation of new surfaces, fracture, and subsequent crack healing have a certain symmetry, with chains variously being pulled out or diffusing into the fracture surface. The symmetry is far from complete, however, because of chain breakage, the chemical and physical state of the surfaces at the time of crack healing, and so on (53-60). 

So far, we have assumed that the threshold value Gq for vanishing crack speed is the Dupre energy of adhesion w for adherence, or twice the surface energy y for fracture of homogeneous bodies. Indeed, for adherence of glass on polyurethane, crack propagation was observed for G > w and crack healing for G < vi. (4,63,64)... 

Nature provides different types of mechanisms to repair fractures in order to be able to cope with different mechanical environments about a fracture [Hulth, 1989 Schenk, 1992]. For example, incomplete fractures (cracks), which only allow micromotion between the fracture fragments, heal with a small amount of fracture-line callus, known as primary healing. In contrast, complete fractures which are unstable, and therefore generate macromotion, heal with a voluminous callus stemming from the sides of the bone, known as secondary healing [Brighton, 1984 Hulth, 1989]. 

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