Microcrack formation

A similar mechanoehemieal chain reaction was proposed to explain the microcrack formation in the glassy state [143] ... 

Figure 25. Effect of radiation fluence on microcrack formation in composite specimens subjected to 100 thermal cycles between 80 C and -150 C (Reproduced from reference 19.)...

Other studies have produced apparently conflicting results on the relationship between local composition and microcrack formation [56, 57]. In 2005, Akkus and coworkers used Raman microspectroscopy and human male femurs to examine... 

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. 

MICROCRACK FORMATION AND FRACTURE CHARACTERISTICS IN GRANITE UNDER SUPERCRITICAL WATER CONDITIONS... 

Better candidates for the study of acoustic emission are composite materials. The extremely brittle polyvinyltoluene sample which showed easily detectable acousic emission (6) was indeed to some extent such a composite material since it was a sample used for scintillation counting of nuclear radiation. The crystalline particles of the inorganic scintillator embedded in the rather rigid polymer matrix differ enough in elastic properties from those of the matrix that a substantial stress enhancement occurs on the interface between the two components. One has about twice the bulk stress on the poles and one third on the equator of a perfectly rigid spherical particle. Such a stress increase in the poles leads rather early to adhesion failure of the particle-matrix boundary and to microcrack formation. This finally makes the sample fail at small strain-to-fracture, cb = 0.5%. The microcracks act as nuclei for crazing. The opening of a fissure between the particle and the matrix is sufficiently... 

Such a microcrack formation occurs as soon as the strain enhancement at any microfibril end is high enough for material separation. The higher the bulk strain the higher the number of such defects which are deformed so much that a microcrack can be opened. Their number increases almost exponentially with strain as can be concluded from the dependence of radical concentration on bulk strain. But the sample itself is still strong and will hold the load up to the point where the microcrack coalescence yields the first critical size crack which will start to grow catastrophically and will make the sample fail. 

GPa, cb =. 3, V = 10 A, 13 =. 1 (10% of chains in amorphous layers are taut tie molecules). It seems unlikely that the acoustic emission associated with such an elementary act could be detected. The chances are better with microcrack coalescence along the outer boundary of fibrils which involves a rapid sequence of microcracks opening and eventual full separation of the fibril from a fraction of adjacent fibrils. On the other hand one can expect that with increasing strain the frequency of microcrack formation will be so high that the cumulative acoustic emission from a finite volume of the sample will be detectable above the noise background. 

The strength development increases maikedly with curing temperature. However, after 28 days the strength and elasticity modulus are higher for the medium (40-50 °C or lower temperatures (Fig. 4.61). This phenomenon is explained by Al-exanderson as the effect of microcracks formation in concrete, resulting from thermal expansion of humid air [206]. 

The main goal of fillers addition is, due to their physical properties arrd fineness, to improve the concrete mixtirre workabihty, the increase of compactness, tlius the reduction of permeability, primarily due to capillary pores volirme decrease, as well as the reduction of trend to microcracks formation. 

Fabrication of SiC composites by second-phase dispersion is widely applied to improve material toughness. The various toughening mechanisms [93] that have high potential to reduce crack extension in SiC- composite materials are crack deflection, microcrack formation, crack bridging by reinforcement with metallic ligaments (e.g. TiC, TiB2), and crack bridging and pull out by platelet- or fiber-reinforcement. 

Figure 14. Potential toughening mechanisms for SiC based ceramics 1 crack deflection, 2 microcrack formation, 3 crack bridging with metallic ligaments, e.g. TiB2, 4 crack bridging and pullout by platelet or fiber - reinforcement.

The ends of the microfibrils create about Itf m point vacancies in the microfibrillar superlattice (Fig. 11). Under applied tensile load they may fail first, eventually by microcrack formation so that the adjacent microfibrils have to carry a heavier load than the rest of the sample. Hence they are first candidates for rupture detectable by the rascals formed at the rupture of tie molecules in at least one amorphous layer of the microfibril affected. Depending on the ratio of axial strength to lateral adhesion of the microfibrils the microcracks will grow parallel (high ratio) or perpendicular (low ratio) to the fibre axis yielding a large number of broken, chains and radicals in the former and a small one in the latter case. Nylon is an example of the former and linear polyethylene of the latter type. 

---