Lecture_7

MECH 390: Engineering Materials Lecture 7: Failure

Instructor

  • Dr. Manar Almazrouei

  • Department of Mechanical and Aerospace Engineering

  • United Arab Emirates University

  • Fall 2024

Learning Objectives

  • Describe crack propagation mechanisms.

  • Explain low strength of brittle materials.

  • Define fracture toughness and distinguish types.

  • Identify impact fracture tests and define fatigue & its conditions.

  • Analyze fatigue plots, define creep & its conditions, and interpret creep plots.

  • Apply concepts to real-world scenarios, mastering key concepts to understand material failure.

Introduction to Failure of Engineering Materials

  • Key Concepts:

    • Failure is often undesirable, leading to economic losses and interruptions in services or products.

    • Factors contributing to failure include material selection, processing, design, and misuse.

    • Understanding failure mechanisms is crucial for safe and reliable engineering design.

    • Prevention is challenging despite known failure causes and includes regular inspections and timely repairs.

  • Chapter Topics:

    • Simple fracture (ductile and brittle).

    • Fundamentals of fracture mechanics.

    • Fracture toughness testing.

    • Ductile-to-brittle transition.

    • Fatigue and creep.

Failure Examples

  • Plastic Package Opening: Failure due to crack propagation.

  • Boeing 737-200: Case of explosive decompression due to assembly flaws.

  • Oil Tanker: Instantaneous fracture under stress concentration due to notches and flaws.

    • Highlights the importance of understanding failure mechanisms for engineering design.

Fundamentals of Fracture

Simple Fracture

  • Definition: The separation of a body into pieces due to static stress usually at low temperatures relative to the melting point.

  • Can result from:

    • Fatigue: Failure due to cyclic stresses.

    • Creep: Time-dependent deformation at elevated temperatures.

Fracture Modes

  • Ductile Modes: High energy absorption; exhibit plastic deformation.

  • Brittle Modes: Low energy absorption; exhibit little to no plastic deformation.

Material Behaviors in Fracture

  • Metals: Often behave in a ductile manner.

  • Ceramics: Typically brittle.

  • Polymers: Exhibit a range from brittle to ductile behaviors.

Importance of Ductile versus Brittle

  • Ductile materials provide a warning sign before failure, feature high energy absorption, and allow preventive measures.

  • Brittle materials fail suddenly and catastrophically without any warning.

Comparison of Ductile and Brittle Failure

  • Distinctive Features:

    • Ductile failure displays cup-and-cone fractures, preceded by necking.

    • Brittle failure shows little to no plastic deformation, marked by flat, smooth surfaces.

Stages of Moderately Ductile Failure

  1. Necking: Localized deformation occurs.

  2. Void Nucleation: Formation of microcavities.

  3. Void Growth and Coalescence: Enlargement and merging of microvoids.

  4. Crack Propagation: Formation of larger cracks.

  5. Fracture: Final shear fracture occurring at a 45° angle to the tensile direction.

Comparison of Ductile and Brittle Fracture

  • Features:

    • Ductile fracture often reveals fibrous and dull surfaces, exhibiting significant plastic deformation.

    • In contrast, brittle fracture presents a granular texture, often with a flat surface and crystalline appearance.

    • Understanding these behaviors is crucial for appropriate material selection.

Fatigue Definition and Characteristics

  • Overview:

    • Fatigue is defined as failure under dynamic and fluctuating stresses, typically occurring at stress levels lower than yield strength.

    • It is the most significant cause of material failures, especially in metals (contributing to 90% of failures).

  • Nature of Fatigue Failure:

    • Exhibits brittle-like characteristics even in ductile metals, often showing rapid crack propagation with little gross plastic deformation.

Cyclic Stresses Types

  • Axial (Tension-compression) and Flexural (Bending).

S-N Curve Introduction

  • S-N Curve Features:

    • Maximum stress (S) plotted against the number of cycles to failure (N).

    • Some materials exhibit a fatigue limit, below which failure does not occur.

Creep and Its Effects

Definition and Importance

  • Creep is the time-dependent permanent deformation occurring under constant load or stress at elevated temperatures, crucial for materials used in high-temperature applications (e.g., turbine rotors).

  • Stages of Creep:

    • Primary creep with decreasing slope, secondary creep with constant slope, and tertiary creep with an increasing slope leading to rupture.

Key Parameters in Creep Testing

  • The steady-state creep rate and rupture lifetime are key considerations for material performance in applications with significant thermal and mechanical stresses.

Design Principles in Engineering Fracture

Factors Influencing Material Selection

  • General Trends: Preference for materials with higher melting temperatures, better elastic modulus, and finer grain sizes to improve creep resistance.

  • High-Temperature Alloys: Stainless steels and superalloys are resilient to creep, benefitting from advanced processing techniques.

Summary of Failure

  • Failure can occur due to simple crack propagation, with distinctions between ductile (slow propagation with deformation) and brittle fractures (rapid propagation without deformation).

  • Understanding fracture toughness, impact tests, fatigue, and creep is essential for designing safe and reliable engineering components.