Question

The letters following each Review Question refer to the corresponding Learning Objective from the Chapter Opener. What is the difference between brittle and plastic deformation? (C)

Short answer

The difference between brittle and plastic deformation lies in their response to stress and ductility of materials. Brittle deformation occurs in materials with low ductility and results in fracture or breakage without significant change in shape or size before breaking, whereas plastic deformation occurs in materials with high ductility and leads to a permanent change in shape or size without breaking. Brittle deformation happens suddenly with little energy absorption, while plastic deformation is gradual and absorbs energy as the material changes shape, providing resistance against failure.

Step-by-step solution

Definition of Brittle Deformation

Brittle deformation refers to the type of deformation in which materials break or fracture when subjected to stress. This occurs without any significant change in the shape or size of the material before breaking. Brittle deformation typically occurs in materials with low ductility like glass, ceramics, or some hard minerals.

Definition of Plastic Deformation

Plastic deformation occurs when a material undergoes a permanent change in shape or size due to an applied stress. During plastic deformation, the atoms or molecules in the material rearrange themselves, resulting in a new shape that persists after the stress is removed. Materials with high ductility like metals, polymers, and some minerals exhibit plastic deformation.

Key Differences

1. Brittle deformation results in fracture or breakage, while plastic deformation causes a permanent change in shape or size without breaking. 2. Materials with low ductility experience brittle deformation, and materials with high ductility undergo plastic deformation. 3. Brittle deformation occurs suddenly and with little warning, whereas plastic deformation is gradual and can be observed as the material deforms. 4. During brittle deformation, there is little or no energy absorption by the material, leading to failure. In contrast, plastic deformation absorbs energy as the material changes shape, providing a measure of resistance against failure.

Key concepts

Brittle Deformation

Imagine bending a piece of chalk until it snaps. This sudden failure under stress without significant prior deformation is a hallmark of brittle deformation. Materials that undergo brittle deformation, such as glass, ceramics, and some hard minerals, break when subjected to forces beyond their capacity to absorb or redistribute that force. These materials typically have low ductility, meaning they can't be drawn into wires or hammered into sheets without breaking.

In brittle materials, cracks can propagate quickly, leading to an abrupt failure. This property means that brittle items are not particularly forgiving when it comes to handling stress. They might hold up well right until their breaking point, but they offer little warning or margin for error before they fracture. Understanding this behavior is crucial in engineering and construction, where material failure can have serious consequences.

Plastic Deformation

If you've ever bent a paperclip back and forth, you have induced plastic deformation. This term describes the permanent distortion of a material subjected to stress beyond its elastic limit. Unlike brittle materials, those that exhibit plastic behavior, like metals and polymers, can absorb more energy and undergo large strains before failing.

During plastic deformation, molecules within the material rearrange themselves to accommodate the imposed forces. Even after the force is removed, the material does not return to its original shape. This characteristic is pivotal in many manufacturing processes such as forging, where the ability of a metal to be hammered or pressed into different shapes without cracking is essential.

Material Ductility

Ductility is a measure of a material's ability to undergo significant plastic deformation before rupture. The term is often associated with the stretching of a material into a wire. High ductility is a desirable property in materials subjected to tensile stresses, where they are pulled apart, like metals in structural applications, because they can endure greater deformation without failure.

Assessing a material's ductility involves evaluating its elongation or the amount of strain it can handle during a tensile test. Materials with high ductility, such as gold or copper, have extensive applications in industries like electronics and construction, where flexible yet strong materials are needed. On the other hand, low-ductility materials are more likely to fail in brittle fashion, which limits their use in applications where flexibility and failure resistance are critical.

Stress and Strain

The concepts of stress and strain are foundational when discussing material deformation. Stress is the force applied to a material divided by the area over which that force is distributed, while strain is the measure of deformation experienced by the material as a result of that stress.

Stress can be tensile (pulling apart), compressive (pushing together), or shear (sliding layers past each other), and the way a material responds to these different types of stress determines its mechanical properties. Strain, on the other hand, is usually a change in length divided by the original length, often expressed as a percentage or fraction. Materials show different stress-strain behaviors; for instance, brittle materials have a high stress and low strain at failure, while ductile materials have a lower stress and high strain at failure. Understanding these properties helps engineers select the appropriate materials for various applications.