Surface crazing

A crack in the wall. This type of hole differs by having a much greater surface area of the hole channel. It can also show up as crazing of the surface (crazing is a collection of many small cracks). 

Mechanically stressed polystyrene without the application of any chemical compound can also produce crazing. Figure 23 shows crazing and its diffraction pattern attributed to bending the polystyrene sheet surface crazing appears on only one side. This contrasts the development of crazing in depth by simple tensile stress (see Figure 24) where crazes developed on both sides of, and sometimes in between, the surfaces. 

Surface crazing that is clear, on items of amber jewellery of even colour - either transparent or opaque - could indicate pressed amber from the end of the nineteenth century. Typical examples are beads, or brooches with a motif of flowers or fruits in clear material, on an opaque background (Fig. 1.33). 

The material is slightly porous and is therefore affected by its surroundings. This can lead to surface crazing but seldom to a complete breakdown. 

The most susceptible of the plastics used to simulate organics is cellulose nitrate. Signs of instability and imminent degradation are patches of discolouration, surface crazing or blistering, weeping, and fumes. 

The growth behavior of surface crazes has been intensively studied in various polymers (e.g. 6-80) jjjg fracture mechanics approach provides a basis for an analytical description of the growth in front of a crack tip (e.g. In this Section... 

Differences in the growth behavior between surface crazes and the single craze at a crack tip have been reported. 

The growth behavior of crazes in unnotched samples has been widely investigated. Figure 21 shows examples of the observed growth of surface crazes in terms of the... 

Fig. 21. Growth of surface crazes in unnotched specimens inair ...

Fig. 8. Surface craze formed by elastic dilatational stress at the root of notch under plane strain...

Crazes in amorphous polymers can be classified into three types depending on where crazing occurs (1) surface crazes, (2) crazes at a crack tip, and (3) internal crazes. Surface crazes occur on the surface of a sample. An example is shown in Figure 14.15. The length of crazes can reach values around 10 mm, while their thickness is very small, between 0.1 pm and several micrometers. Environmental factors have an important influence on this type of craze. 

Sternstein and Ongchin (28) considered that if cavitation occurs in crazes the criterion for crazing initiation should include the dilative stress component. They proposed the criterion to fit the experimental data for surface craze initiation in PMMA when the polymer is subjected to biaxial tension. The segmental mobility of the polymer will increase due to dilative stresses, thus provoking cavitation and the orientation of molecular segments along the maximum stress direction. 

The surface crazes observed are typically on the order of a micron in depth, so the entire sample bar thickness does not to be Invaded for the test to be useful. Even rather large molecules can penetrate to the depth of a micron in a matter of hours or days, and smaller molecules like CO2 can do so in a matter of minutes or seconds. 

Photo Ic is a photomicrograph of IPS that contained no light stabilizers which was exposed outdoors in Florida for six months. Note the total loss of surface integrity including deep surface crazes. This photo also shows the mildew that was visually present along with their typical hyphal structures. 

Figure 3. KLY-to-visual surface crazing (high density polyethylene. 08-in. injection-molded 0.3% LS 45° south direct Arizona)...

Optical KLenents. Problems which are common to many solar-related optical elements Include dirt retention, cleaning, surface abrasion, and photodegradation. A common feature of some of these problems Is that the deleterious effects occur at an Interface. Ultraviolet radiation, atmospheric components, mechanical stress, etc., can have a profound effect on performance by changing surface characteristics. The lifetimes of UV stabilizers can be limited by exudation permeability can cause harmful reactions at Interfaces and mechanical properties can be Influenced by surface crazing. In other applications mechanical behavior of the bulk polymer Is critical and virtually all applications require that the polymer system withstand multiple environmental stresses simultaneously. 

Polyacetal is markedly unstable towards lights because even UV radiation of wavelengths as high as 365 mn may initiate its degradation. Polyacetal cannot therefore be used outdoors if it does not contain any light stabilizers. Even after a short weathering, surface crazes and pronounced chalking are observed. 

A surface craze in a PMMA product breaks down and becomes a crack. Treat it as an edge crack of length a = 0.5 mm in a body of width w >a. What tensile stress stress intensity factor K = 1.12[Pg.498]

To illustrate the effect of sample thickness, we can compare adhesives to sealants, which in many cases can be viewed as thick adhesives. As the sample thickness is increased, the benefit of the BTZ is clearly demonstrated. In Table 3 a SEBS/hydrogenated hydrocarbon tackifier sealant formulation was prepared as a hot melt and poured into shallow petri dishes. Although both of these polymers have good inherent stability, sealant applications may require extended exposure to UV radiation. The discoloration data show that the BTZ prevents yellowing of the sealant. However, examination of the sealant surface shows surface crazing and cracking when not protected by incorporation of the HALS. The combination of the two classes of light stabilizers provides the best overall performance. 

Such polymer molecular weight reduction becomes apparent as surface crazing and cracking. 

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