Microscopic cracks

When fabricated polycarbonate parts are exposed to ultraviolet light, either in laboratory equipment or by outdoor exposure, a progressive dulling is observed on the exposed surface. The dullness is due to microscopic cracks on the surface of the resin. If the surface resin is analysed it is observed that it has a significantly lower molecular weight than the parent polymer. 

It was proposed by Griffith in 1920 that microscopic cracks exist both within and on the surface of all real materials, which are deleterious to the strength of any material that does not possess ductility. The presence of cracks whose longest dimension is perpendicular to the direction of the applied tensile stress gives rise to especially large stress concentrations at the crack tip, where the real localized stress can approach the theoretical strength of the material due to a small area over which it is applied (see Figure 5.38). It can be shown that the maximum stress at the tip of a crack, is... 

Long before a brittle hair crack has come to its complete development, it is already present as a nucleus, and the material has already undergone irreversible damage. Microscopic cracks are often partially filled with material in the form of very thin fibres, forming bridges between the fracture surfaces. These cracks are denoted as... 

A material that is subjected to cyclic application of stresses may fail after a large number of load cycles without nearly reaching the maximum failure stress of direct loading. The effect of such cyclic stresses is to initiate microscopic cracks at centres of stress concentration within the material or on the surface, and subsequently to enable these cracks to propagate, leading to eventual failure. For high stress amplitudes less cycles are needed for failure than for low stress amplitudes. For high frequencies less cycles are needed for... 

Because particles may be hard and smooth in one case and rough and spongy in another, one must express densities with great care. Density is universally defined as weight per unit volume. Three types of densities—true density, particle density, and bulk density—can be defined, depending on the volume of particles containing microscopic cracks, internal pores, and capillary spaces. 

Shankoff used thin gelatin films ( 1 ym) and discovered that a new development process gave vastly improved holograms. The film is washed in water and then immersed in isopropyl alcohol. This second step causes rapid dehydration of the water-swollen film Which shrinks and forms microscopic cracks in the uncross-linked regions. The result is a substantially increased... 

One major problem with structural materials is that they crack and weaken as they age. The human body has mechanisms for healing itself if the skin is cut or a bone is broken. However, inanimate materials have had no such mechanisms— until now. Scientists at the University of Illinois at Ur-bana-Champaign (UIUC) have invented a plastic that automatically heals microscopic cracks before they can develop into large cracks that would degrade the usefulness of the material. This accomplishment was achieved by an interdisciplinary team of scientists including aeronautical engineering professors Scott White and Philippe Geubelle, applied mechanics professor Nancy Sottos, and chemistry professor Jeffrey Moore. 

The analysis by Inglis showed that the local stress at sharp notches or corners of the microcrack can rise to a level several times that of the applied stress. This shows how the microscopic cracks or flaws within the solid might become potential sources of weakness of the solid. 

As shown in Fig. 15, the mode II fracture toughness, G], was more than twice as high as the mode I fracture toughness, G q. However, the difference was much smaller than that of the other materials such as polymer matrix composite materials. This would be the consequence that the microscopic fracture of the present specimen was not pure mode II owing to the discrepancy between the macroscopic crack face and microscopic crack path even though the mode II loading was applied macroscopically, as suggested by the results shown in Fig. 14. 

The role of nonsoluble impurities in a solidification process was considered in detail by Danilov et al. [60], Kazachkovskii and Danilov [60] have shown that microscopic cracks and imperfections on the surface of solid particles are of great importance. Metallic crystals formed by any way inside those cracks can be preserved with heating above the melting temperature of free crystals. This interpretation offers an explanation for the role of overheating in decreasing the activity of the formation of solidification sites and is supported by the majority of scientists. 

Bones are constantly subjected to forces that cause microscopic cracks. These microcracks (1) attach blood monocytes circulating within the periosteum and bone marrow and (2) induce adjacent osteoblasts to produce cytokines (Sect. 3.3.2) that cause these monocytes to proliferate, fuse, and differentiate into large multinucleated cells called osteoclasts. Osteoclasts cause bone resorption by acid demineralization and digestion of its proteins by enzymes that are optimally active in an acidic environment. These proteases and other hydrolytic enzymes are stored in a specialized, membrane-sealed compartment (lysosomes) into which they are guided by possessing terminal mannose 6-phosphate residues on N-linked glycans. 

Fracture surfaces of the cyclically deformed fatigue specimens were observed at low magnification in order to identify the zones of fatigue initiation and final failure, at higher magnifications in order to identify the regions of microscopic crack formation and growth and microscopic plastic mechanisms. 

Figure 8. Microscopic cracks observed in the sample tested at 120 MPa.

V.I. Betekhtin, A.G. Kadomtsev, Evolution of microscopic cracks and pores in solids under loading, Phys. Solid State, 47, 825-831 (2005). 

To further discuss failure, one must first decide what constitutes failure. Although it is common to associate failure with a part breaking into two or more pieces, conditions other than fracture can make a part inoperative or render its operation unsatisfactory. Other forms of failure include excessive elastic deformation, plastic flow and deformation, wear, or even loss of attractive appearance. At times several of these modes may combine to produce the resultant failure. For example, plastic deformation often precedes fracture and may, in fact, nucleate the microscopic cracks that ultimately coalesce into a macroscopic fracture. This chapter will concentrate largely on fracture and will only touch briefly on other types of failure. 

II. Real structure of solid, determined by a number and type of defects, including the size of grains, availability and sizes of primary microscopic cracks, etc. 

Stress-strain curves developed during tensile, flexural and compression tests may be quite different from each other. The moduli determined in compression are generally higher than those determined in tension. Flaws and sub-microscopic cracks significantly influence the tensile properties of brittle polymeric materials. However, they do not play such an important role in compression tests as the stresses tend to close the cracks rather than open them. Thus, while tension tests are more characteristic of the defects in the material, compression tests are characteristic of the polymeric material as it is. The ratio of compressive strength to tensile strength in the case of polymers is in the range 1.5 to 4.0 [Dukes, 1966]. 

Fatigue failure may occur when a specimen fractured into two parts, was softened or its stiffness significanfly is reduced by thermal heating and/or cracking. Sometimes, for different reasons, a large number of cycles elapses from the first formation of microscopic cracks to complete frac-mre. In this case, the fatigue failure is arbitrarily defined as having occurred when the specimen... 

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