![Ceramics glaze crazing](https://upload.wikimedia.org/wikipedia/commons/e/e5/Ceramics_glaze_crazing.jpg)
Crazing is the phenomenon that produces a network of fine cracks on the surface of a material, for example in a
glaze layer. Crazing frequently precedes fracture in some
glass
Glass is a non-crystalline, often transparent, amorphous solid that has widespread practical, technological, and decorative use in, for example, window panes, tableware, and optics. Glass is most often formed by rapid cooling ( quenching ...
y
thermoplastic
A thermoplastic, or thermosoft plastic, is any plastic polymer material that becomes pliable or moldable at a certain elevated temperature and solidifies upon cooling.
Most thermoplastics have a high molecular weight. The polymer chains associate ...
polymer
A polymer (; Greek '' poly-'', "many" + ''-mer'', "part")
is a substance or material consisting of very large molecules called macromolecules, composed of many repeating subunits. Due to their broad spectrum of properties, both synthetic a ...
s. As it only takes place under tensile stress, the plane of the crazing corresponds to the stress direction. The effect is visibly distinguishable from other types of fine cracking because the crazing region has different
refractive indices
In optics, the refractive index (or refraction index) of an optical medium is a dimensionless number that gives the indication of the light bending ability of that medium.
The refractive index determines how much the path of light is bent, or ...
from surrounding material. Crazing occurs in regions of high
hydrostatic
Fluid statics or hydrostatics is the branch of fluid mechanics that studies the condition of the equilibrium of a floating body and submerged body "fluids at hydrostatic equilibrium and the pressure in a fluid, or exerted by a fluid, on an imme ...
tension, or in regions of very localized
yielding, which leads to the formation of interpenetrating microvoids and small
fibril
Fibrils (from the Latin ''fibra'') are structural biological materials found in nearly all living organisms. Not to be confused with fibers or filaments, fibrils tend to have diameters ranging from 10-100 nanometers (whereas fibers are micro ...
s. If an applied tensile load is sufficient, these bridges elongate and break, causing the microvoids to grow and coalesce; as microvoids coalesce,
cracks begin to form.
Polymers
Crazing occurs in polymers, because the material is held together by a combination of weaker
Van der Waals force
In molecular physics, the van der Waals force is a distance-dependent interaction between atoms or molecules. Unlike ionic or covalent bonds, these attractions do not result from a chemical electronic bond; they are comparatively weak and th ...
s and stronger
covalent bonds. Sufficient local stress overcomes the Van der Waals force, allowing a narrow gap. Once the slack is taken out of the backbone chain, covalent bonds holding the chain together hinder further widening of the gap. The gaps in a craze are microscopic in size. Crazes can be seen because light reflects off the surfaces of the gaps. The gaps are bridged by fine
filament called fibrils, which are molecules of the stretched backbone chain. The fibrils are only a few
nanometers
330px, Different lengths as in respect to the molecular scale.
The nanometre (international spelling as used by the International Bureau of Weights and Measures; SI symbol: nm) or nanometer (American and British English spelling differences#-re ...
in diameter, and cannot be seen with a light microscope, but are visible with an electron microscope.
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The thickness profile of a crazing is like a sewing needle: the very tip of the crazing may be as thin as several atoms. As the distance from the tip increase, it tends to thicken gradually with the rate of the increase diminishing with distance. Therefore, the growth of crazing has a critical distance from the tip. The opening angle of the crazing lies between 2° to 10°. The boundary between crazing and surrounding bulk polymer is very sharp, the microstructure of which can be scaled down to 20Å or less, which means it can only be observed by electron microscopy.
A craze is different from a
crack in that it cannot be felt on the surface and it can continue to support a load. Furthermore, the process of craze growth prior to cracking absorbs fracture energy and effectively increases the
fracture toughness of a polymer. The initial energy absorption per square meter in a craze region has been found to be up to several hundred times that of the uncrazed region, but quickly decreases and levels off. Crazes form at highly
stressed regions associated with scratches, flaws,
stress concentrations and molecular inhomogeneities. Crazes generally propagate perpendicular to the applied tension. Crazing occurs mostly in amorphous, brittle
polymer
A polymer (; Greek '' poly-'', "many" + ''-mer'', "part")
is a substance or material consisting of very large molecules called macromolecules, composed of many repeating subunits. Due to their broad spectrum of properties, both synthetic a ...
s like
polystyrene (PS),
acrylic
Acrylic may refer to:
Chemicals and materials
* Acrylic acid, the simplest acrylic compound
* Acrylate polymer, a group of polymers (plastics) noted for transparency and elasticity
* Acrylic resin, a group of related thermoplastic or thermosett ...
(PMMA), and
polycarbonate; it is typified by a whitening of the crazed region. The white colour is caused by light-scattering from the crazes.
The production of crazing is a reversible process, after applied compressive stress or elevated temperature (higher than glass transformation temperature), it may disappear and the materials will return to optically homogeneous state.
Shear banding is the narrow region with high level of shearing strain from local strain softening; it is also very common during the deformation of thermoplastic materials. One of the main differences between crazing and
shear band A shear band (or, more generally, a 'strain localization') is a narrow zone of intense shearing strain, usually of plastic nature, developing during severe deformation of ductile materials.
As an example, a soil (overconsolidated silty-clay) specime ...
ing is that crazing occurs with an increase in volume, which shear banding does not. This means that under compression, many of these brittle, amorphous polymers will shear band rather than craze, as there is a contraction of volume instead of an increase. In addition, when crazing occurs, one will typically not observe "necking," or concentration of force upon one spot in a material. Rather, crazing will occur homogeneously throughout the material.
Rubber toughening
Rubber particles are often used to toughen thermoplastic materials. Particles with a greater compliance which are accordingly softer than the surrounding matrix act as stress concentrators. These regions of concentrated stress initiate crazes, propagating normal to the direction of applied force. This describes a phenomenon termed "multiple crazing", which, such as in HIPS, is a source of ductility for otherwise brittle polymer matrices.
After modification, the ability of absorbing energy will be increased significantly. For some brittle plastic materials, they can even go through brittle-ductile transformation. Previously, the rubber particles were considered as the main contributor to the increased energy absorption. It was proposed that rubber particles might gather around crack tips under tension and impede the growth of crack, or the contraction of rubber particles induced the decline of glass transformation temperature of the matrix. Nevertheless, experiments showed that the energy absorbed by rubber particles made up only 10% of the total energy, and the decrease of glass transformation temperature caused by rubber was only around 10 K, which was not enough for the matrix to yield at room temperature.
Schmitt and Bucknall developed the mechanism of rubber toughening according to the existence of
stress whitening and shear yielding when the stress is lower than fracture strength. They proposed that the rubber particles served as the center for stress concentration, hence initiated the brittle-ductile transformation and yielding of the matrix material. To specify, yielding happens in the form of crazing or shear band, which can consume a large portion of deformation energy.
Environment effect
Crazing can take place in glassy polymers under environmental effects. It is problematic because it requires a much lower stress state and sometimes happens after a long delay, which means it is hard to detect and avoid. For example, PMMA containers in daily use are quite resistive to humidity and temperature without any visible defects. After machine-washing and left in air for one or two days, they will shutter abruptly when wet with gin. During the process, the stress applied is negligible, but crazing is still found on the containers.
There are many theories that tried to explain the environmental effects upon formation of crazing, among which surface energy reduction and plasticization are widely accepted and well developed. To eliminate the environmental crazing and cracking, many methods like surface coating, stress reduction are adopted. However, due to the complicity of the environmental effects, especially the effects in organic environment, it's hard to find a general solution and remove the effect completely.
Construction
Crazing is also seen on single ply roofing membranes, joint sealant, and on
concrete
Concrete is a composite material composed of fine and coarse aggregate bonded together with a fluid cement (cement paste) that hardens (cures) over time. Concrete is the second-most-used substance in the world after water, and is the most wid ...
when good concrete practices are not followed.
Ceramics
Crazing is a
glaze defect
Glaze defects are any flaws in the surface quality of a ceramic glaze, its physical structure or its interaction with the body.
Body/glaze interaction problems
Glaze defects can be as a result of the incompatibility of the body and the selected ...
of
glazed pottery
Pottery is the process and the products of forming vessels and other objects with clay and other ceramic materials, which are fired at high temperatures to give them a hard and durable form. Major types include earthenware, stoneware and ...
. Characterised as a
spider web
A spider web, spiderweb, spider's web, or cobweb (from the archaic word '' coppe'', meaning "spider") is a structure created by a spider out of proteinaceous spider silk extruded from its spinnerets, generally meant to catch its prey.
Spi ...
pattern of cracks penetrating the glaze, it is caused by
tensile stress
In continuum mechanics, stress is a physical quantity. It is a quantity that describes the magnitude of forces that cause deformation. Stress is defined as ''force per unit area''. When an object is pulled apart by a force it will cause elonga ...
es greater than the glaze is able to withstand. In pottery a distinction is often made between crazing, as an accidental defect, and "
crackle
Crackle or crackling may refer to:
Foods
* Cracklings, the tissue remaining after lard and tallow have been extracted from animal fats
** Pork rinds in American English, pork scratchings in British English when served in small pieces as a snac ...
", when the same phenomenon, often strongly accentuated, is produced deliberately. The Chinese in particular enjoyed the random effects of crackle and whereas in
Ru ware
Ru ware, Ju ware, or "Ru official ware" () is a famous and extremely rare type of Chinese pottery from the Song dynasty, produced for the imperial court for a brief period around 1100. Fewer than 100 complete pieces survive, though there are ...
it seems to have been a tolerated feature of most pieces, but not sought, in
Guan ware
Guan ware or Kuan ware () is one of the Five Famous Kilns of Song dynasty China, making high-status stonewares, whose surface decoration relied heavily on crackled glaze, randomly crazed by a network of crack lines in the glaze.
''Guan'' mean ...
a strong crackle was a desired effect.
Odontology
Crazing is also used as a term in
odontology
Dentistry, also known as dental medicine and oral medicine, is the branch of medicine focused on the teeth, gums, and mouth. It consists of the study, diagnosis, prevention, management, and treatment of diseases, disorders, and conditions o ...
to describe fine cracks in the
enamel of teeth.
Metaphor
The root sense of “crazy” in English, meaning “to shatter, crush, or break,” dates to the 1300s. The metaphorical senses familiar today derive from crazing in pottery: “crazy” meaning “diseased or sickly” dates to about 1570; “of unsound mind,” to about 1610.
References
External links
Understanding CrazingCrazing Concrete Surfaces{{Webarchive, url=https://web.archive.org/web/20060517063338/http://www.nrmca.org/aboutconcrete/cips/03p.pdf , date=2006-05-17
Images of crazing via Google Images(look for the images of fine 'cracks')
Glass engineering and science
Roofs
Polymers
Pottery
Ceramic engineering
Ceramic glazes