Yield point phenomenon

Summary

This video explains the yield point phenomenon in materials like low carbon steels, copper, and aluminum alloys during tensile testing. It covers the different stages of stress-strain behavior, from elastic elongation to plastic deformation, highlighting how the stress drops when dislocations break free from foreign atoms. The video discusses the role of Cottrell atmospheres in yielding and how aging, temperature, and diffusion rates impact the material's behavior. It also covers the formation of stretcher strain marks, their undesirability in forming processes, and the conditions under which the yield point phenomenon disappears or reappears.

Takeaways

• 😀 The yield point phenomenon refers to the sudden onset of plastic deformation in materials like low carbon steels, copper, and aluminum alloys during tensile testing.
• 😀 When a material is stretched beyond its elastic limit, it undergoes permanent deformation, which initially involves a brief drop in stress followed by nearly constant stress before further elongation.
• 😀 The yield point phenomenon is caused by the interaction between foreign atoms and dislocations in the material, creating dilatation zones that pin the dislocations in place.
• 😀 Once the dislocations break free from the foreign atoms, the material experiences a sharp drop in stress, which is followed by a reduction in force required for further deformation.
• 😀 The region between the onset of plastic deformation and strain hardening is referred to as yield point elongation or looter strain.
• 😀 Looter strain is characterized by the formation of slip steps, or matte, stripe-shaped marks on the material's surface, typically seen at a 45-degree angle to the tensile axis.
• 😀 These stretcher strain marks are undesirable in forming technology because they result in an unsightly surface appearance.
• 😀 Yield point elongation occurs inhomogeneously, meaning that the looter bands appear gradually, moving from one area of the specimen to another.
• 😀 After unloading and reloading the specimen, the yield point phenomenon no longer occurs because the dislocations are no longer pinned by the foreign atoms.
• 😀 To prevent yield point elongation, deep drawing sheets are often plastically deformed by rolling to eliminate the occurrence of looter strain and stretcher strain marks.
• 😀 The yield point phenomenon only occurs at relatively low temperatures, and as temperature increases, the diffusion of foreign atoms prevents the formation of Cottrell atmospheres, thus eliminating the yield point phenomenon.

Q & A

What is the yield point phenomenon?

The yield point phenomenon occurs when a material deforms plastically, characterized by a sudden drop in stress followed by continued deformation. This happens due to the movement of dislocations in the material, which are initially pinned by foreign atoms.

What are dislocations and how do they relate to the yield point phenomenon?

Dislocations are defects in the crystalline structure of materials. They move when the material is stressed. In the context of the yield point phenomenon, dislocations are temporarily pinned by foreign atoms, causing a brief reduction in stress when they break free.

How does the stress-strain curve behave during the yield point phenomenon?

During the yield point phenomenon, the stress-strain curve initially drops when plastic deformation begins, followed by a period where the stress remains nearly constant. Afterward, stress must be increased for further elongation, leading to a necking effect and eventual fracture.

What are Cottrell atmospheres, and what role do they play in the yield point phenomenon?

Cottrell atmospheres are accumulations of foreign atoms that gather around dislocations. These atmospheres prevent dislocations from moving, effectively pinning them in place. The yield point phenomenon occurs when the stress is high enough to break the dislocations free from these atmospheres.

What is strain hardening, and why does it occur after the yield point phenomenon?

Strain hardening occurs after the yield point phenomenon when the dislocations, now free from foreign atoms, encounter other obstacles. This increases the material's resistance to further deformation, requiring more stress to continue elongation.

What are Luder bands, and how are they related to the yield point phenomenon?

Luder bands are surface markings that appear during the yield point elongation. These bands are visible as matte, stripe-shaped patterns on the surface of the specimen and are typically oriented at about a 45-degree angle to the tensile axis.

Why are Luder bands undesirable in forming technology?

Luder bands are undesirable in forming technology because they cause unsightly surface markings that affect the material's appearance and potentially its structural integrity, especially in processes like deep drawing.

How does temperature affect the yield point phenomenon?

At higher temperatures, the yield point phenomenon disappears because the diffusion of foreign atoms increases. This diffusion prevents the formation of Cottrell atmospheres, allowing dislocations to move freely without being pinned.

What is the Portevin-Le Chatelier effect?

The Portevin-Le Chatelier effect occurs when the rate of diffusion of foreign atoms is similar to the speed of dislocation motion. This leads to a zigzag pattern in the stress-strain curve as dislocations are temporarily captured and released by foreign atoms.

What is the significance of aging in relation to the yield point phenomenon?

Aging refers to the process where foreign atoms accumulate again over time, forming Cottrell atmospheres. This reintroduces the yield point phenomenon, even in materials that previously experienced free dislocation movement after initial deformation.