Crazing structural failures

When the stress that can be bom at the interface between two glassy polymers increases to the point that a craze can form then the toughness increases considerably as energy is now dissipated in forming and extending the craze structure. The most used model that describes the micro-mechanics of crazing failure was proposed by Brown [8] in a fairly simple and approximate form. This model has since been improved and extended by a number of authors. As the original form of the model is simple and physically intuitive it will be described first and then the improvements will be discussed. 

Previous studies have shown that the formation and failure of the craze structure ahead of the crack tip is the precursor to fracture in polyethylene (PE). A knowledge of the craze development and its structure should lead to an understanding of the crack growth behaviour. However, to date there have been very few studies of the craze behaviour from its initiation and growth to eventual breakdown. 

It is well documented [2-4] that the precursor to fracture in PE is the failure of the craze structure ahead of the crack tip during SCO, The formation of the craze and the mechanism that leads to craze breakdown have been described frequently. The craze nucleation is characterised by the formation of a highly localised zone ahead of the crack tip which consists of multiple voids. Their growth and subsequent coalescence leads to the formation of a fibrous structure. Depending on the stability of the craze structure, the craze may widen by drawing material from the craze-bulk interface into the craze fibrils and eventually rupture at the midribs, or fail at the craze-bulk interface with little or no signs of material fibrillation [5],... 

Owing to their high Tg and the hydrolytic resistance of the aromatic sulfone backbone structure, polysulfones display reliable long-term performance in hot water and steam even under autoclave conditions. Unlike the other high-Tg, transparent polymers, such as polycarbonate (PC), polycarbonate-ester (PCE), and polyetherimides (PEI), the sulfone polymers are not prone to crazing and failure... 

The conditions for the formation of kink-bands within HM-HT fibres are the first part of the problem. The second part is what happens in repeated cycling. The axial compressive force causes the molecular buckling, and superficially the internal kink appears to be pulled out on retensioning. However, it seems likely that there is some residual structural disturbance, which becomes more severe after repeated cycling and leads to what appears to be crazing. Eventually failure occurs in the characteristic angled form of kink-band breaks, being the Achilles heel of HM-HT fibres. 

Preparation of crazed polymers for TEM is quite difficult. First, the whole specimen must be stressed to failure, resulting in crazes which are weak and full of voids. Worse yet, the craze structure is imstable in the absence of applied... 

In Fig. 26, we schematically illustrate four stages of failure in epoxies under an increasing tensile load. In each stage we document the craze/crack structure, the stress at the craze/crack surface and the resultant fracture topography. 

Finally, the effect of the cross-tie fibril structure on craze breakdown is unknown. We have emphasized here a very simplified picture of the fibril breakdown process in which the nucleation event of a breakdown is the failure of a single transfer length of a main load-bearing fibril. Yet we have excellent evidence that cross-tie fibrils exist and that they can transfer stress between main fibrils. These cross-tie fibrils may have to be considered in developing more exact models of craze fibril breakdown. 

To obtain an upper limit for Q0 Oprii is set equal to the failure stress Ojlhrij = Z-efffl, of the continuum fibril structure which gives a failure criterion for the critical crack opening displacement <5 = Sc at the crack tip. Here Z(, is the number of effective connector strands per nominal unit craze area can be less than Z due to the scission of chains during formation of the craze fibrils. 

In addition to the separate or combined effects of heat, oxygen, and radiation, polymers may deteriorate due to exposure to water (hydrolysis) or different types of chemical agents. Condensation polymers like nylons, polyesters, and polycarbonates are susceptible to hydrolysis. Structural alteration of some polymers may occur as a result of exposure to different chemical environments. Most thermoplastics in contact with organic liquids and vapors, which ordinarily may not be considered solvents for the polymers, can undergo environmental stress cracking and crazing. This may result in a loss of lifetime performance or mechanical stability and ultimately contribute to premature mechanical failure of the polymer under stress. 

---