BRITTLE FRACTURE IN CERAMICS AND GLASSES

Fracture is the separation of a material into two or more pieces under the action of an applied stress. A material may undergo one of two major types of fracture modes depending on its mechanical properties: ductile and brittle.

Materials undergoing ductile fracture first experience plastic deformation, i.e., the material resists the fracture by stretching itself. Imagine pulling on two ends of a plastic bag. The bag stretches by a considerable amount before it eventually tears. This plastic deformation, which is not limited to polymers, is also seen in metal alloys. 

Materials that undergo brittle fracture, on the other hand, will fracture with negligible plastic deformation. In other words, they break without warning.

Regardless of the type of fracture, during failure a material will experience:

• Crack formation, where all fractures start, and
• Propagation of the crack, in response to the applied stress

In ductile fractures, this crack is stable, i.e., it will undergo continuous deformation, only propagating when more stress is applied. As such, ductile materials will typically deflect by a significant amount before they fail, thus giving warning before they fracture entirely.

On the other hand, when cracks form under brittle fracture, they propagate across the material instantaneously; thus, failure can occur with little to no warning. This is one of the characteristics that makes brittle failure so undesirable, especially in applications such as building construction.

BRITTLE FRACTURE

Brittle vs. Ductile Fracture

Fracture involves the forced separation of a material into two or more parts. Brittle Fracture involves fracture without any appreciable plastic deformation (i.e. energy absorption). Ductile Fracture in the converse and involves large plastic deformation before separation. The difference between brittle and ductile fracture is illustrated in figures 1 and 2. Remembering that the area under the σ - ε curve, Fig. 1, represents energy, we can see that much less energy is expended in brittle fracture than in ductile fracture.   

 

The Mechanics of Brittle and Ductile Fracture

Ductile Fracture (Cup-and-cone Fracture)

Most metals and metal alloys are ductile by nature. One of the main characteristics of ductile fracture is a phenomenon known as necking. During necking, the material's cross-section gradually reduces before fracture occurs. The separated ends of the fractured material adopt a concave and convex shape. Hence, ductile fracture is commonly called the cup-and-cone fracture. The stages in the cup-and-cone formation are:

1. The ductile material undergoes necking
2. Micro-voids start to form perpendicular to the stress direction
3. The micro-voids coalesce to form a crack
4. The crack propagates
5. The material eventually fractures (a cup and cone form at the top and bottom respectively)

Brittle Fracture

In brittle fracture, no noticeable deformation is observed since crack propagation is instantaneous. This propagation is almost purely perpendicular to the direction of the tensile stress, compared to ductile fracture, which produces cup- and cone-like shapes.

While brittle fracture has a distinctive shape, different materials may exhibit unique characteristics. For example, in brittle steel, V-shaped markings are usually found in the center of the fractured cross-section. On the other hand, for amorphous materials like ceramic glasses, the surface of the fractured cross-section may have a smooth, shiny appearance.

Factors That Affect the Fracture of a Material

Engineers must understand the mechanics of different types of fractures to inform material selection and mitigate potential catastrophic failures. Some of the critical factors that affect material fracture include:

Stress Concentrations

For most brittle materials, the measured fracture strength is much lower than the predicted theoretical value based on the atomic bonding energies in the material. This is due to the presence of microscopic cracks and other flaws in the material's cross-section. These stress risers must, therefore, be accounted for when calculating fracture strength. (Related reading: Calculating Tensile Stress: Why It Needs to Be Done Now.)

The location of the applied load must also be taken into account. Consider a cylindrical material with an elongated crack (flaw) in its cross-section. The maximum stress it can handle before it undergoes fracture is measured on the crack tip and is dependent on the applied stress and the physical properties of the crack. However, applying the load away from the crack may alter the measured fracture strength.

Fracture Toughness

Fracture toughness is the resistance of a material to applied stress when a crack is present. This parameter is dependent on:

• The critical stress for crack propagation
• The crack length
• The crack geometry

For thin specimens, fracture toughness also depends on its thickness. However, fracture toughness becomes less of an influential factor in thick materials. 

How to Test Impact Fractures

The two most commonly used impact tests are the Charpy impact test and Izod impact test.

To perform these tests, a hammer is attached to a pendulum, which applies an impact force to the test specimen. The hammer is first raised to its starting position then released, hitting the specimen during its downward motion. Because some of the hammer's energy is absorbed by the sample, the pendulum swings to a smaller maximum height on the opposite side. The difference between the maximum heights of the hammer before and after impacting the specimen is used to calculate the impact energy. The only difference between Charpy and Izod is the way the specimen is loaded.

Conclusion

A fracture is simply the separation of a material into several pieces due to an applied stress. Fractures are subdivided into two major types: ductile fractures and brittle fractures. The type of fracture that a material will undergo depends mainly on its ability to deform before it cracks.

For both these fractures, failure begins with crack formation, which progresses to crack propagation and eventually separation.

It is crucial for engineers and designers to understand the mechanics behind material fractures to ensure that selected materials perform as intended in their given environment.

Types of fracture mechanisms: ductile failure, brittle fracture, intergranular fracture, and fatigue.

Three modes of fracture:

• Mode I – Opening mode (a tensile stress normal to the plane of the crack),
• Mode II – Sliding mode (a shear stress acting parallel to the plane of the crack and perpendicular to the crack front), and
• Mode III – Tearing mode (a shear stress acting parallel to the plane of the crack and parallel to the crack front).

Types of fracture also categorized by the different forces like shear fracture, tensile fracture compressive fracture, etc.,

Why do ceramics and glasses generally have a low fracture toughness?

Ceramics are typically hard and brittle. While their strength in compression is very high, they are not suitable for loading in tension. Their brittle qualities mean that they fracture very easily.

What causes brittle fracture of ceramics?

Similarly, new developments in heat engines require ceramic parts in order to achieve the high temperatures that result in greater engine efficiency, and yet here, too, failure has been shown to occur by brittle fracture, caused by thermal shock as components are heated to and cooled from their operating temperatures.

Why is glass more brittle than ceramic?

Si linkages in glass, they are often characterized by ionic bonds between positive and negative ions. When they form crystals, the strong force of attraction between ions of opposite charge in the planes of ions make it difficult for one plane to slip past another. Ceramics are therefore brittle.

What is brittle fracture ceramic?

In ceramics, brittle fracture is controlled by the extension of small flaws which are dispersed in a material or component's surface and which behave like cracks. Flaws can arise from the production process, but also from handling and service.

What makes glass brittle?

Glass is brittle because it has many microscopic cracks in it which act as seeds for a fracture. If you can make glass without these cracks, as is done in fiberglass, then it is not so fragile. Even with the cracks, glass still has a higher observed strength than plastic.

What is the difference between glasses and ceramics?

Ceramics are hard, brittle, oxidation resistant, wear-resistant, thermal and electrical insulating, refractory, nonmagnetic, chemically stable and prone to thermal shock. Glass is hard, amorphous, inert, biologically inactive, fragile and transparent.

Glass vs Ceramics

Glass and ceramics are widely used for making household utensils. Apart from making household materials, glass and ceramics have found their place in many areas.

Glass can be called as a type of ceramic. Glass is known to be a non-crystalline material. It is an amorphous solid, which means that it has no long -range order of positioning of its molecules.

Ceramic can be termed as an inorganic material. Unlike glass, ceramics may have crystalline or partly crystalline structures. Ceramics may also be amorphous.

Silicon Dioxide is the main component of glass. Glass is a mixture of of two or more kinds of metallic silicates. Clay is the main component in Ceramics..

Both glass and ceramics are brittle and break at the instance of a small force. Glass is also transparent, which means light passes through it. Ceramics may be opaque, which means it does not allow light to pass through it. The earliest ceramic products were pottery made out of clay.

Ceramics are hard, brittle, oxidation resistant, wear-resistant, thermal and electrical insulating, refractory, nonmagnetic, chemically stable and prone to thermal shock.

Glass is hard, amorphous, inert, biologically inactive, fragile and transparent.

When comparing the two in terms of the price, ceramics is a bit costlier than glass.

In the manufacture of both glass and ceramics, there is a slight difference. A glass kiln will have heating elements on the top whereas a ceramic kiln will have heating elements on the sides.

What are the general properties of ceramics and glasses?

Ceramics and glasses are radically different materials than metals but are close cousins to each other. Both typically exhibit high strength, high hardness, high elastic modulus, unusually high chemical inertness, and are electrical and thermal insulators.

How are ceramics hard and brittle?

The two most common chemical bonds for ceramic materials are covalent and ionic. For metals, the chemical bond is called the metallic bond. The bonding of atoms together is much stronger in covalent and ionic bonding than in metallic. That is why, generally speaking, metals are ductile and ceramics are brittle.

Which is stronger glass or ceramic?

Ceramic is more expensive than tempered glass, but it is also stronger, more durable and is the choice people make when they want to be sure that they will not have to worry about replacing the glass.

Why is glass strong but brittle?

Most glass is made of silica, an amorphous solid in which atoms are arranged haphazardly. Silica glasses are strong, but they are also brittle. Frankberg says this is because of small gaps in the atomic structure. These defects prevent atoms from moving around when the material is stressed.

What are the factors which contribute to brittle failure?

There are three basic factors that contribute to brittle cleavage type of fracture in steels: a triaxial stress state, low temperature, and a high strain rate or rapid loading rate.

Which material is used in ceramics and glass making?

Glass is made up of natural and abundant raw materials like sand, soda ash and limestone that are melted at high temperature to form glass. Naturally occurring raw materials like silica, sand, quartz, flint, silicates and aluminosilicates are used in the manufacturing of ceramics.

What are the five main generic classes of ceramics and glasses?

Different Classifications of Ceramic Materials

• Glasses. Glasses are a familiar group of ceramics – containers, windows, mirrors, lenses, etc. ...
• Clay Products. Clay is one of the most widely used ceramic raw materials. ...
• Refractories. ...
• Abrasive Ceramics. ...
• Cements. ...
• Advanced Ceramics.

What are the physical factors of glass-ceramics?

• High strength.
• High impact resistance.
• Low co-efficient of thermal expansion, sometimes even negative co-efficient of thermal expansion.
• Good resistance to thermal shock.
• A range of optical properties, from translucent to opaque and sometime opalescence.

What type of ceramic is glass?

polycrystalline materials

Glass-ceramics are polycrystalline materials formed by controlling the nucleation and growth of crystal phases within the glass

What are the types of ceramics?

There are three main types of pottery/ceramic. These are earthenware, stoneware and porcelain.

What are the Classification of Ceramics?

Traditional Ceramics

Advanced Ceramics