Multiple cracks

Multiple cracks Multiple individual cracks are initiated, and these subsequently coalesce into a unified fracture. Numerous crack origins and the corresponding unions are illustrative of an ESC failure mechanism. 

Even in the loading regime in which inherent flaw effects dominate the fracture process, further clarification of the fracture activation and growth process is needed. For example, dynamic crack branching leading to multiple fracturing is expected to constitute an important part of the breakage process. Such a cooperative and collective fracture process does not fit well within a... 

The above-mentioned models differ in the relation that is derived between the rate of pull-out of the individual chain and the crack velocity. These models also differ in their interpretation of the threshold stress and the threshold toughness (Go). Also, V is expected to be dependent on the configuration of the connector chain at the interface. The value of v when connector chain crosses the interface just once is higher than the value when the chain forms multiple stitches, even though Go is not altered. When the chain forms multiple stitches, the block and tackle effect ensures that the viscous processes dominate even at lower velocities, and V is reduced by a factor of N from the value obtained from the single crossing case. These models are discussed by Brown and coworkers [45,46]. 

The energy release rate (G) represents adherence and is attributed to a multiplicative combination of interfacial and bulk effects. The interface contributions to the overall adherence are captured by the adhesion energy (Go), which is assumed to be rate-independent and equal to the thermodynamic work of adhesion (IVa)-Additional dissipation occurring within the elastomer is contained in the bulk viscoelastic loss function 0, which is dependent on the crack growth velocity (v) and on temperature (T). The function 0 is therefore substrate surface independent, but test geometry dependent. 

Given the relatively rare appearance of oxetanes in natural products, the more powerful functionality of the Patemo-Biichi reaction is the ability to set the relative stereochemistry of multiple centers by cracking or otherwise derivitizing the oxetane ring. Schreiber noted that Patemo—Btlchi reactions of furans with aldehydes followed by acidic hydrolysis generated product 37, tantamount to a threo selective Aldol reaction. This process is referred to as photochemical Aldolization . Schreiber uses this selectivity to establish the absolute stereochemistry of the fused tetrahydrofuran core 44 of the natural product asteltoxin. ... 

Many engineering thermoplastics (e.g., polysulfone, polycarbonate, etc.) have limited utility in applications that require exposure to chemical environments. Environmental stress cracking [13] occurs when a stressed polymer is exposed to solvents. Poly(aryl ether phenylquin-oxalines) [27] and poly(aryl ether benzoxazoles) [60] show poor resistance to environmental stress cracking in the presence of acetone, chloroform, etc. This is expected because these structures are amorphous, and there is no crystallinity or liquid crystalline type structure to give solvent resistance. Thus, these materials may have limited utility in processes or applications that require multiple solvent coatings or exposures, whereas acetylene terminated polyaryl ethers [13] exhibit excellent processability, high adhesive properties, and good resistance to hydraulic fluid. 

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