Crazing

Crazing is also called hairline craze. They can be fine, thin, tiny type cracks that may extend in an unreinforced or reinforced plastic network

Figure 3.6 Tensile stress-strain behavior of ductile plastics

Figure 3.6 Tensile stress-strain behavior of ductile plastics

Table 3.2 Tensile data

Plastic

Modulus MPa

Yield stress MPa

Elongation at yield, %

Elongation at break, %

ABS

2,700

55

2.5

75

Acetal homopolymer

3,100

69

12

75

Acetal copolymer

2,800

61

12

60

Acrylic

3,000

72

5.4

Nylon

2,400

82

5

60

Phenolic

19,300

62

8

90

Polyethylene

1,200

30

20

600

Polypropylene

1,400

35

12

400

Polystyrene

3,100

25

8

60

Polysulfone

2,500

70

6

100

on or under the surface or through a layer of a plastic material. Different conditions and effects occur depending on the type plastic, load conditions, and environment. The formation of crazes are like cracks in that they are wedge shaped and formed perpendicular to the applied stress. They differ from cracks by containing plastic that is stretched in a highly oriented manner perpendicular to the plane of the craze. They are parallel to the applied stress direction. Another major distinguishing feature is that unlike cracks, crazes are able to support stress.

With the application of static loading, the strain at which crazes start to form decreases as the applied stress decreases. In constant strain-rate testing crazes always start to form at a well-defined stress level. Crazes start sooner under high stress levels. When tensile stress is applied to an amorphous (Chapter 1) plastic such as acrylics, PVCs, PS, and PCs, crazing may occur before fracturing. Crazing occurs in crystalline plastics, but in those its onset is not readily visible. It also occurs in most fiber-reinforced plasdcs, at the time-dependent knee in the stressstrain curve.

Environmental stress cracking is the cracking of certain plastic products that becomes exposed to a chemical agent while it is under stress. This effect may be caused by exposure to such agents as cleaners or solvents. The susceptibility of affected plasdcs to stress cracking by a particular chemical agent varies considerably among plasdcs, particularly the TPs.

The resistance of a given plastic to attack may be evaluated by using either constant-deflecdon or constant-stress tests in which specimens are usually coated with the chemical or immersed in the chemical agent. After a specified time the degree of chemical attack is assessed by measuring such properties as those of tensile, flexural, and impacts. The results are then compared to specimens not yet exposed to the chemical. In addition to chemical agents and the environment for testing may also require such other factors as thermal or other energy-intensive conditions.

It is possible with solvents of a particular composition to determine quantitatively the level of stress existing in certain TP products where undesirable or limited fabricated-in stresses exist. The stresses can be residual (internal) stresses resulting from the molding, extrusion, or other process that was used to fabricate the plastic product. Stresses can also be applied such as bending the product. As it has been done for over a half century, the product is immersed in the solution that attacks the plastic for various time periods. Any initial cracks or surface imperfections provide information that stresses exist. Other tests conducted can be related to the stress-time information. Information on the solvent mixtures suitable for this type of test and how to interrupt them are available from plastic material suppliers or can be determined from industry test data which show solvents that effect the specific plastic to be evaluated.

TP cracking develops under certain conditions of stress and environment sometimes on a microscale. Because there are no fibrils to connect surfaces in the fracture plane (except possibly at the crack tip), cracks do not transmit stress across their plane. Cracks result from embrittiement, which is promoted by sustained elevated temperatures and ultraviolet, thermal, chemical, and other environments.

For the designer it is not important whether cracking develops upon exposure to a benign or an aggressive medium. The important considerations are the embrittlement itself and the fact that apparently benign environments can cause serious brittle fractures when imposed on a product that is under sustained stress and strain, which is true of certain plastics.

Crazing or stress whiting is damage that can occur when a TP is stretched near its yield point. The surface takes on a whitish appearance in regions that are under high stress. Crazing is usually associated with yielding. For practical purposes stress whiting is the result of the formation of microcracks or crazes, which is another form of damage. Crazes are not true fractures, because they contain strings of highly oriented plastic that connect the two flat faces of the crack. These fibrils are surrounded by air voids. Because they are filled with highly oriented fibrils, crazes are capable of carrying stress, unlike true fractures. As a result, a heavily crazed product can still carry significant stress, even though it may appear to be fractured.

It is important to note that crazes, microcracking, and stress whiting represent irreversible first damage to a material, which could ultimately cause failure. This damage usually lowers the impact strength and other properties of a material compared to those of undamaged plastics. One reason is that it exposes the interior of the plastic to attack and subsequent deterioration by aggressive fluids. In the total design evaluation, the formation of stress cracking or crazing damage should be a criterion for failure, based on the stress applied.

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