PG 04 A - Stress Strain

Summary

This lecture on seismology explores the study of earthquakes and the Earth's interior. It explains the relationship between stress, strain, and deformation in geological materials, using concepts like Hooke’s law, elastic limits, and plastic deformation. The video introduces key terms like the modulus of elasticity and Poisson’s ratio, which describe the behavior of rocks under stress. The lecture also touches on the principles of wave propagation through different rock types, the factors influencing their deformation, and how these properties relate to seismic activity. A practical application of these concepts is seen in the study of earthquake mechanics and rock behavior.

Takeaways

• 😀 Seismology is the study of earthquakes and the waves that propagate through the Earth's medium.
• 😀 The term 'seismology' comes from the Greek words 'seismos' (earthquake) and 'logos' (study).
• 😀 Earthquakes occur due to various factors and can be measured and predicted to reduce risks.
• 😀 Stress applied to objects leads to deformation, which is a change in shape or length (e.g., spring or rubber).
• 😀 Deformation can be categorized into elastic (temporary), plastic (permanent), and breaking (failure) phases.
• 😀 Hooke's Law applies to elastic deformation, where stress causes strain in a linear relationship until the elastic limit is reached.
• 😀 When force exceeds the elastic limit, the material enters the plastic phase, resulting in permanent deformation.
• 😀 The relationship between stress (force per unit area) and strain (deformation) defines an object’s elasticity or modulus.
• 😀 The coefficient of elasticity can be divided into Young’s modulus, Poisson's ratio, and bulk modulus, each describing different material behaviors under stress.
• 😀 In practical terms, hard rocks require a greater force to deform compared to softer materials, which affects wave propagation speeds in seismology.
• 😀 Young's modulus, Poisson's ratio, and density of rocks play significant roles in understanding seismic wave propagation and material behavior during earthquakes.

Q & A

What is the definition of seismology according to the transcript?

Seismology is the science that studies earthquakes, including their origin, propagation of seismic waves, distribution, measurement, and consequences.

What are the main causes of earthquakes mentioned in the lecture?

Earthquakes occur due to stress accumulation in the Earth's crust, leading to deformation and eventually fracture when the stress exceeds the strength of rocks.

How does stress affect an object according to the lecture?

Stress applied to an object causes deformation or a change in shape. If stress is applied continuously, the object may reach its fracture point and break.

What is Hooke's Law and how does it relate to stress and strain?

Hooke's Law states that within the elastic limit, the change in strain is proportional to the applied stress. When stress exceeds the elastic limit, the object deforms plastically.

What is the elastic limit and why is it important?

The elastic limit is the maximum stress an object can withstand while still returning to its original shape after the stress is removed. Beyond this point, permanent deformation occurs.

What is the difference between elastic, plastic, and inelastic deformation?

Elastic deformation is reversible, plastic deformation is permanent but occurs before breaking, and inelastic deformation refers to changes that are not reversible and often lead to fracture.

How is the coefficient of elasticity defined in the lecture?

The coefficient of elasticity is the relationship between stress (force per unit area) and strain (deformation) of an object. It quantifies how much force is needed to deform the material.

What are the different types of elastic moduli discussed?

The lecture discusses three types of moduli: Young's modulus (tensile stress vs strain), Bulk modulus (pressure vs volumetric change), and Shear modulus (shear stress vs shear strain).

How does the lecture describe the relationship between rock stiffness and wave propagation?

Stiffer and harder rocks have higher Young's modulus and can transmit seismic waves faster, while softer rocks require less force to deform and have slower wave velocities.

What is Poisson's ratio and how is it defined in the lecture?

Poisson's ratio is the negative ratio of lateral strain to axial strain when an object is subjected to stress. It describes how materials change dimension in directions perpendicular to the applied force.

How does continuous stress lead to breaking or fracture in materials?

When stress is applied continuously beyond the elastic and plastic limits, the material cannot sustain the deformation and reaches a breaking point, resulting in fracture.

Why is understanding stress-strain relationships important in seismology?

Understanding stress-strain relationships helps predict how rocks in the Earth's crust deform under stress, which is crucial for understanding earthquake mechanics and assessing seismic risks.