Effect of gradient temperature rolling process on promoting crack healing in Q500 heavy plates

To ensure the quality of heavy plate products as determined by ultrasonic inspection, it is necessary to effectively control defects such as cracks and shrinkage cavities in heavy plates. Generally, some defects such as large size cracks exist due to insufficient deformation in the center of traditionally rolled plates. Compared with the traditional rolling process, gradient temperature rolling(GTR) process can effectively increase deformation inside heavy plates. In this study, the effect of GTR on crack healing was analyzed through a comparison experiment with the uniform temperature rolling(UTR). The results show that the GTR process could increase the plastic strain inside the heavy plate and effectively promote the healing process of the preset cracks. The degrees of crack healing at the center and quarter thickness position of the steel plate via GTR were greater than twice those of the plate via UTR. The GTR process can significantly reduce the internal defects of heavy plates and improve the defect detection level of heavy plate products. Also, The GTR process results in the formation of new crystal grains in the crack region, which is crucial to crack healing.

Combined studies of surface evolution and crack healing for the suppression of negative factors during CO_(2) laser repairing of fused silica

In order to reveal the evolution mechanism of repaired morphology and the material's migration mechanism on the crack surface in the process of CO_(2) laser repairing surface damage of fused silica optics, two multi-physics coupling mathematical models with different scales are developed, respectively. The physical problems, such as heat and mass transfer,material phase transition, melt flow, evaporation removal, and crack healing, are analyzed. Studies show that material ablation and the gasification recoil pressure accompanying the material splash are the leading factors in forming the Gaussian crater with a raised rim feature. The use of low-power lasers for a long time can fully melt the material around the crack before healing, which can greatly reduce the size of the residual air layer. Combined with the experimental research, the methods to suppress the negative factors(e.g., raised rim, deposited debris, air bubbles) in the CO_(2) laser repairing process are proposed.

Preparation and Healing of Cracks in Al2O3 - MgAION Composite

The Al2 O3 - MgAION specimens with dimensions of Ф20 mm × 10 mm and 3 mm × 4 mm× 36 mm were prepared using corundum grains （ w （ Al2 O3 ） 〉 98%, particle size： 3-1 mm, ≤1 mm, ≤85μm and ≤3μm, respectively） and MgAlON （particle size ≤ 2 μm ） as starting materials, PVA as binder, designing 3 formulations containing aggregate and 4 special formulations without aggregate, pressing under 150 MPa and keeping for 5 s, firing at 1 500 ℃ for 2 h in nitrogen under normal pressure, and the cracks were prepared on the specimens by thermal shock method （air cooling at 1 100 ℃ for 10 cycles） and by Vickers method （29. 4, 49. 0 and 98. 0 N, keeping for 25 s）, then holding at 1 100 ℃ and 1 550 ℃ for 6 h in nitrogen atmosphere for cracks healing, respectively. The influences of crack prefabricated method, grain composition, pressure of Vickers and pressure holding time on the shape and dimension of cracks were studied. Effects of healing temperature on healing degree and MOR of specimens were investigated and the crack healing rate was calculated. The results showed that： （1） the cracks of MgAlON specimens prepared by thermal shock method under normal pressure were of irregular shape and uncertain length; （2） the cracks prepared by Vickers method was approximate diamond-shape without slender cracks emitting from every vertex angle; but if composite specimens were fabricated by particles of several microns, the crack size, shape and its site, especially that obtained under 98. 0 N for 25 s, could be controlled exactly under a Vickers hardometer, which made the crack healing research easy to conduct; （3） after healing treatment at 1 100 ℃ or 1 500℃, the cracks became shallower and smaller even disappeared, the strength of the specimen was recovered finally; （4） a formulation for crack heal ing rate expressed by the cracked capacity was： V/ t=1/3 Cv^2（1＋ctg θ ） Cv/ t.

Influence of Mineral Admixtures on the Electrodeposition Healing Effect of Concrete Cracks

Two types of solutions（Zn SO4, Mg SO4） were selected to study the influence of mineral admixtures on the electro-deposition healing effect of concrete cracks. Four parameters（i e, rates of weight gain, surface coating, crack closure and crack filling depth） were measured. The mineral composition of electro-deposits in the cracks was analyzed. The study shows that the healing effect of mortar specimens with 10% fly ash is the worst, while the healing effect of mortar specimens with 20% fly ash is better than that of the specimens without fly ash. The rates of weight gain, surface coating, crack closure and crack filling depth decrease with increasing content of the ground granulated blast-furnace slag（GGBS）. The mineral admixtures have no influence on the composition of deposits.

Generic principles of crack-healing ceramics

Ceramic materials able to heal manufacture or damage induced microstructure defects might trigger a change in paradigm for design and application of load bearing ceramics.This work reviews thermodynamic and kinetic aspects governing the regeneration of solid contact able to transfer stress between disrupted crack surfaces in ceramics.Major crack healing processes include perturbation of crack-like pores followed by sintering of isolated pores,as well as reaction with an environmental atmosphere and filling of the crack space with an oxidation product.Since thermally activated solid state reactions require elevated temperatures which may exceed 1000℃,processes able to trigger crack healing at lower temperatures are of particular interest for transferring into engineering applications.Generic principles of microstructure modifications able to facilitate crack repair at lower temperatures will be considered:(i)acceleration of material transport by grain boundary decoration and grain size reduction,and(ii)reduction of thermal activation barrier by repair filler activation.Examples demonstrating crack healing capability include oxidation reaction of low energy bonded intercalation metal from nano-laminate MAX phases and catalyzed surface nitridation of polymer derived ceramics containing repair fillers.

Solid state crack repair by friction stir processing in 304L stainless steel

Friction stir processing （FSP） was investigated as a method of repairing cracks in 12 mm thick 304L stainless steel plate. Healing feasibility was demonstrated by processing a tapered crack using a PCBN/W- Re tool with a 25 mm diameter shoulder and a pin length of 6.4 mm. The experiment showed that it was possible to heal a crack that begins narrow and then progressively grows up to a width of 2 mm. Bead on plate experiments were used to find the best parameters for creating a consolidated stir zone with the least amount of hardness difference compared to the base metal. Grain refinement in some specimens resulted in much higher stir zone hardness, compared to base metal, A plot of grain size versus microhardness showed a very strong inverse correlation between grain size and hardness, as expected from the Hall- Perch relationship. Corrosion testing was carried out in order to evaluate the effect of FSP on potential sensitization of the stir zone. After 1000 h of intermittent immersion in 3.5% saline solution at room temperature it was found that no corrosion products formed on the base material controls or on any of the friction stir processed specimens.