Crazing
Polypedia > Defects in polymers
| Crazing | ||
Definition: Crack formation (application of load to irreversible damage in the micro-field, crazing). When polystyrene or polycarbonate is stretched, a line is sometimes visible, perpendicular to the applied force. In some cases, they may be accompanied by whitening. Cracking and cracking can be induced by stress or combined stress and the action of the solvent. For general polymers, they show cracking and cracking problems. They appear to be a fine, microscopic network of cracks that generally move in a direction at right angles to the maximum principal voltage. The suppression of stress production has been observed for some polymers by high pressure imposition. The "lines" when examined under a large number of very small gaps, occupy up to 50% of the affected volume. The voids scatter light. Crazing (crazing) is the formation of zones that contain small gaps, often gathered in a series of bands, giving the appearance to the naked eye of a series of weak parallel lines The long polymer molecules form bundles or oriented fibrils that extend over the crazed region, while at high tensile stresses, one or two of the fibrils can break, starting to crack. The craze will become "crack", but the impact resistance increases. Any pre-existing orientation of the polymer molecules will affect the tendency to craze, since the direction of an applied tension can be parallel to the predominant orientation (inhibit cracking) or normal (fostering cracking). The crazes increase the volume of the affected part, without changing the cross section This distinguishes the cracking of the shearing, where the opposite occurs. |  | |
| Tensión | ||
El agrietamiento por corrosión por tensión (SCC) en los termoplásticos de ingeniería se observa comúnmente en forma de una colonia de microcrack dentro de una capa superficial de polímero degradado expuesto a una acción combinada de tensiones mecánicas y un entorno químicamente agresivo. |  | |
| Estrés y tornillos | ||
Internal stress can also break the component. Badly designed screws, metal inserts and snaps are all features that usually fail in stress cracking. Causes of internal stress and cracking. |  | |
| Aqueous solutions | ||
Aqueous solutions of surfactant materials, such as detergents, can produce brittle cracks, particularly in stressed materials. The term environmental stress cracking (ESC) was introduced to describe such situations. ESC is now more widely applied to describe the promotion of slow and fragile failures in materials stressed by organic substances. The fastening of polymeric materials to organic liquids and gases can also promote the formation of networks of small voids (follies), particularly in amorphous polymers where voids may be empty or occupied by smaller molecules, which are easily eliminated. These gaps are places of opportunity for organic materials to be exploited even more. Both in the ESC and in the attack on the environment, the direct chemical attack on the polymer chain is minor or not involved. It appears that the organic liquid and other promoting substance is locally absorbed or dissolved in the defects to help further failure, possibly by plasticizing the stress material or modifying the surface energy. |  | |
| Mechanisms | ||
The three mechanisms result in a difference in the optical properties of polymeric materials due to preferential reorientation, with the realignment of the polymer chains resulting in a change in optical properties, such as the refractive index, which allows detection through various optical methods, including visual examination and microscopy, and infrared spectroscopy of films. Therefore, the crazed and cracked sites of optically transparent materials appear opaque, while the shear sites may appear "wavy" when viewed correctly with the naked eye and even partially refracted light is employed. |  | |
| More sensitive materials | ||
Amorphous materials such as ABS, PS, PMMA or PC are more sensitive to continuous charge, as well as chemicals than semicrystalline plastics / polymers. If transparency is not a key issue, move away from them in applications that involve a continuous load. Not the temperature, not the temperature of the room. Tension cracking failures are probably common because designers are not aware of their existence. |  |