Maya Ellison
Crazing, derived from the Middle English term crasen, meaning to break, is a phenomenon observed in polymers and polymeric materials, resulting in a network of fine cracks on their surfaces. This process involves localized plastic strain and cavitation, allowing the material to achieve plastic strain faster. Crazing is often associated with the presence of cracks or defects, which act as initiation points for the formation of microvoids and subsequent craze fibrils. These fibrils enable the material to withstand substantial tensile forces but are susceptible to shear forces. The distinction between crazes and cracks lies in their ability to support stress and their response to unloading and rejuvenation. Understanding the mechanisms of crazing is crucial for evaluating material resistance to fracture and predicting crack growth behaviour, especially in the context of toughened thermoplastics and epoxy materials.
| Characteristics | Values |
|---|---|
| Definition | Crazing, derived from the Middle English term "crasen" meaning "to break", is a network of fine cracks in the surfaces of glasses and ceramics. |
| Occurrence | Crazing occurs in polymers when excessive tensile stress is applied, leading to microvoid formation in a plane normal to the stress. |
| Crack vs Craze | Craze may disappear upon unloading and rejuvenating the sample above the glass transition temperature, unlike cracks. Crazes can support stress, unlike cracks. |
| Toughened Polymers | Toughened polymeric materials may contain initial defects such as cracks, which serve as initiation points for crazing. |
| Inelastic Deformation | Crazing is a special type of inelastic, plastic deformation of the polymer, accompanied by rapid interfacial surface development. |
| Deformation Mechanisms | Crazing and shear yielding are the two principal deformation mechanisms in polymers. Crazing is more localized and associated with brittle failure. |
| Prevention | Products that relax or flow after application are less likely to craze. |
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What You'll Learn
Crazing in polymers
Crazing is a phenomenon observed in glassy amorphous polymers, as well as some semicrystalline polymers. It is less common in thermosetting polymers due to the difficulty of crosslinked molecules to undergo significant molecular stretching. Polymers like polystyrene, poly(methyl methacrylate), acrylics, and polycarbonate are particularly prone to crazing.
Crazes differ from cracks in that they contain polymer fibrils (5-30 nm in diameter) and can transmit load between their two faces. They are stabilised by fibrils of plastically deformed polymer chains. The refractive index of crazes is lower than that of the surrounding material, causing light scattering and a ""stress-whitened" appearance.
Crazing involves a localised plastic strain of the material, allowing for plastic deformation at stress levels below those required for shear yielding. It is enhanced by hydrostatic tension and can be promoted by the addition of micro-stress concentrators within the polymer. This process increases the energy absorption capacity of the material.
Crazing is an underlying mode of polymer fracture that has been widely studied. It is a significant failure mechanism in toughened thermoplastics, though its presence in epoxy materials is a subject of debate. Understanding crazing helps predict crack growth and evaluate a material's resistance to fracture under various loading conditions.
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Crazing vs cracking
Crazing and cracking are two distinct phenomena that can occur in polymers, although they are often confused with each other. Crazing is a yielding mechanism in polymers, characterised by the formation of a fine network of microvoids and fibrils, which develop due to excessive tensile stress. These microvoids and fibrils, known as crazes, appear as linear features and precede brittle fracture. On the other hand, cracks are deep fissures that occur when a material is overloaded and the binder system cannot hold it together, resulting in a complete break in the material.
Crazes and cracks differ in several ways. Firstly, crazes contain polymer fibrils, which are absent in cracks. These fibrils enable crazes to transmit load between their two faces, making them load-bearing and able to support stress. In contrast, cracks are unable to support a load as the space within them is completely empty. Additionally, crazes may disappear upon unloading and rejuvenating the sample above the glass transition temperature, whereas cracks cannot be reversed.
The formation of crazes and cracks is also influenced by different factors. Crazes are favoured by the presence of triaxial tensile stresses and can be inhibited by applying hydrostatic pressure. They are commonly observed when two flat plates with a layer of liquid between them are forced apart. In contrast, cracks are the result of overloading a material with solids, such as marble dust, exceeding the capacity of the binder system. They can also be caused by environmental factors such as temperature, humidity, and airflow.
In terms of appearance, crazes are fine, thin cracks that may extend in an unreinforced or reinforced plastic network on or under the surface of a material. They are formed perpendicular to the applied stress and are parallel to the stress direction. On the other hand, cracks are identified by their crisp, sharp-edged breaks, known as "platelets".
In the context of ceramics and pottery, the terms crazing and crackling are often used interchangeably to describe the appearance of fine cracks in the glaze. This can occur naturally over time or as a decorative technique. While crazing in ceramics was once considered a defect that could weaken the piece and allow bacteria and chemicals to leach into food, proper washing can mitigate these concerns.
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Crazing in paints
Crazing is a phenomenon that occurs in polymers, where narrow gaps develop in the material, resulting in many small cracks. This phenomenon is derived from the Middle English term "crasen", meaning "to break". Crazing is a form of plastic deformation, specifically observed in transparent glassy polymers. Under tensile stress, these polymers develop cracks on their surfaces, which propagate across specimens without causing immediate failure.
It is important to distinguish between crazing and cracking in paints. Cracks are characterised by empty spaces and sharp-edged individual pieces called "platelets". They occur when a hard, rigid paint layer is flexed beyond its physical capacity. In contrast, crazes may disappear upon unloading and rejuventating the sample above the glass transition temperature. Additionally, crazes can support stress, whereas cracks cannot.
To address crazing in paints, various corrective actions can be taken. For example, artists can use products that do not require relaxation or flow after application, such as Soft Gel, Heavy Body Acrylics, or Molding Paste, as these materials are highly unlikely to craze. Additionally, understanding the underlying causes of crazing can enable artists to take advantage of the paint's unique characteristics to create desired effects.
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Crazing in ceramics
Crazing is a phenomenon that occurs in polymers, resulting in many tiny cracks in the plane normal to the tensile stress. It is a special type of inelastic, plastic deformation of the polymer. Crazing in ceramics is specifically known as ceramic glaze crazing.
Ceramic glaze crazing is a complex interplay of chemistry, physics, and artistry. It is caused by a mismatch in the thermal expansion rates of the clay body and the glaze. As the ceramic cools, the glaze contracts more than the clay, resulting in stress and the formation of fine cracks. This happens because of a disparity in the thermal expansion properties of the glaze and clay body. When these rates are too different, crazing occurs.
The chemical makeup of a glaze affects its expansion and contraction with temperature changes. Different clay bodies, such as stoneware, porcelain, and earthenware, have varying thermal expansion properties, influencing their interaction with glazes. It is common for a glaze to craze on one type of clay body but not on another.
Crazing generally occurs with age, but there are other factors that can cause immediate crazing, including temperature and humidity changes, moisture getting into the glaze, repeated bumps or knocks, and the glaze shrinking more than the body of the ware.
To prevent crazing, potters can use thinner glaze applications, ensure compatibility between the thermal expansion rates of the glaze and clay body, and experiment with different recipes of glaze and clay.
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Crazing and shear yielding
Under tensile stress, polymers develop what appear to be cracks on their surfaces, either very gradually or after prolonged periods. These fine cracks, or crazes, are formed perpendicular to the applied stress and can propagate across specimens without causing immediate failure. Crazing involves a localized or inhomogeneous plastic strain of the material. It is a cavitation process that takes place with an increase in volume. Craze fibrils can endure substantial tensile forces across the craze but cannot withstand shear forces.
Shear yielding involves the material undergoing shear flow with minimal or no change in density. It is the more ductile failure mode because it involves the deformation of a significant volume of the material. It manifests as plastic deformation in the form of shear bands and is closely associated with the material softening that occurs immediately after yielding. With continued deformation, the material undergoes hardening due to molecular orientation, resulting in the multiplication and propagation of shear bands.
When analyzing the yielding behavior of polymers, it is crucial to differentiate between shear yielding and craze yielding due to their distinct microstructural characteristics. By combining the criterion for shear yielding and crazing, a region can be found in which no yielding can occur.
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Frequently asked questions
Crazing is a phenomenon where narrow microscopic gaps develop in a polymer material, resulting in many minute cracks in the plane normal to the tensile stress. It is a special type of inelastic, plastic deformation of the polymer that is accompanied by the rapid development of its interfacial surface.
Crazes are like cracks in that they are wedge-shaped and formed perpendicular to the applied stress. However, crazes are able to support stress, whereas cracks cannot. Crazes may also disappear upon unloading and rejuvenating the sample above the glass transition temperature, whereas cracks cannot do this.
Crazing often occurs in acrylic paints or mediums when they are applied and begin to form a skin while the material underneath is still fluid and wet. As the edges of the skin begin to dry, the centre of the film continues to stretch as water evaporates and the film shrinks. In some cases, the centre areas of the drying film can no longer take the stress of shrinking and a tear occurs in the upper part of the film.
