6 - Fracture
Introduction
The content of this course is primarily based on "Materials Science and Engineering: An Introduction" by Callister & Rethwisch.
Failure & Fracture
Types of Failure
Different types of failure occur under various conditions/situations.
Understanding the mechanism associated with each type is crucial for assessment.
Material resistance to fracture is quantified using specific parameters.
Various measures can be taken to reduce the likelihood of each failure type.
Causes of Failure
Major causes of failure in aerospace materials:
Pilot Error: 50% of accidents related to operational mistakes.
Mechanical Failure: 20%, second most common cause.
Failures can endanger human life, cause economic losses, and interfere with services.
If the cause and mechanism of failure are known, prevention remains challenging.
Usual Causes of Failure
Improper material selection and/or processing.
Inadequate design: Design must incorporate potential failure modes.
Misuse of components: Components should be used within their intended design limits.
Engineers have the responsibility to anticipate potential failures and take preventive measures.
Understanding Fracture
Definition of Fracture:
The separation of a body into two or more parts due to applied static stress, typically at low temperatures.
Fractures can occur in different modes: tension, compression, shear, and torsion (focus on tensile loads).
Fracture is a two-stage process:
Crack Formation
Crack Propagation
Additional types of fracture to explore in future lectures include fatigue (cyclic loading) and creep (time-dependent deformation at high temperatures).
Fracture Modes
Ductile Fracture:
Characterized by slow crack propagation and significant plastic deformation.
Fails with warning, allowing for preventative action.
Brittle Fracture:
Characterized by rapid crack propagation with little or no plastic deformation.
This mode fails catastrophically without warning.
Factors like temperature, strain rate, and stress state influence fracture mode selection.
Fracture Profiles: Macroscopic Aspect
Fracture characteristics can be divided into:
Very Ductile
Moderately Ductile
Brittle
Fracture Surface Characteristics
Cup-and-Cone Fracture:
Typical of moderately ductile materials.
Totally Brittle Fracture:
Exhibits flat surfaces with less deformation.
Distinguishing Ductile and Brittle Fractures
Ductile Fracture Characteristics:
One piece, large deformation.
Brittle Fracture Characteristics:
Multiple pieces, small deformations.
Detailed Examination of Ductile Fracture
Extensive Plastic Deformation:
Occurs near advancing cracks, resulting in stable crack propagation.
Crack propagation is resisted unless stress increases.
Evidence of gross deformation includes twisting and tearing.
Highly Ductile Fracture
Found in materials like pure gold and lead at room temperature, characterized by a neck narrowing down to a point.
Occurs in metals, polymers, or inorganic glasses at elevated temperatures, not common in practice.
Moderately Ductile Fracture
Most frequent in ductile metals and alloys.
Observes moderate necking and typically occurs in stages:
Microvoid nucleation.
Microvoid growth and coalescence.
Crack propagation, ultimately leading to fracture.
Cup-and-Cone Fracture Dynamics
Final fracture stage involves rapid crack propagation around the perimeter at a shear angle of 45°, where shear stress is maximized.
Fractographic Studies
Microscopic studies using scanning electron microscopy (SEM) can reveal:
Fracture mechanisms, modes, stress states, and crack initiation sites.
SEM has higher resolution compared to optical microscopy (OM).
Ductile fracture exhibits characteristic parabolic dimples and spherical dimples.
Brittleness in Fracture
Characteristics of Brittleness:
No substantial plastic deformation before fracture.
Crack propagation initiates spontaneously, continuing without increased stress.
Propagation direction is nearly perpendicular to the applied tensile stress, resulting in flat fracture surfaces.
Brittle Failure Surface Morphology
V-shaped "Chevron" Markings:
Present in some steel fracture surfaces, indicating crack initiation points.
Radial Lines/Ridges:
Common in very hard and fine-grained metals, identifiable to the naked eye.
Crack Propagation in Brittle Fracture
Cracks may propagate through:
Transgranular: along crystallographic planes.
Intergranular: along grain boundaries, often around flawed or embrittled zones.
Examples of Brittle Fracture Surfaces
Amorphous Materials:
Exhibit shiny, smooth surfaces upon fracture.
Principles of Fracture Mechanics
Fracture mechanics quantifies the relationships among:
Material properties, stress levels, crack-producing flaws, and propagation mechanisms.
Actual measured fracture strengths are often several magnitudes lower ($ rac{E}{100}$ to $ rac{E}{10,000}$) than theoretical predictions due to the existence of microscopic flaws.
Cohesive Forces and Fracture Strength
Theoretical strength depends on the cohesive forces holding atoms together.
Stress Concentration Effects
Fracture strength is altered due to stress concentration at crack tips.
Stress concentration $ aum$ at crack tips is calculated using:
Where:
$
ho_t$ = radius of curvature$ au_0$ = applied stress
$a$ = length of a surface crack.
Effects of Crack Tip Geometry on Stress
Stress Concentration Factor (Kt):
A sharp crack significantly amplifies the applied stress near its tip.
Griffith’s Theory of Brittle Fracture
Suggested that all brittle materials have a range of defects that can lead to fracture under applied stress.
Energy balance during crack propagation involves:
Elastic strain energy is released as material transfers load.
New surfaces created require energy input, leading to a derived critical stress $ auc$ for crack propagation as:
Stress Intensity Factor
The stress intensity factor is calculated based on applied stress and crack size.
Under high-stress conditions, this factor approaches a critical threshold $K_{IC}$, beyond which materials will fail catastrophically.
Design Considerations in Fracture Mechanics
Designing components requires understanding crack propagation and mitigating failure risks.
Specific standards for designs include:
Maximum allowable flaw sizes determined by fracture toughness and applied stress.
Non-detectable flaws must not exceed specific sizes determined by design specifications.
Impact Testing and Effects of Strain Rate
Impact Testing:
Charpy and Izod tests employed to assess resistance to fracture under sudden load.
Important for evaluating brittle failure potential, and influence of temperature on ductile-to-brittle transition.
Temperature Influence on Fracture Behavior
Ductile-to-brittle transition (DBTT): varies with temperature, impacting toughness and material choice for engineering applications.
Criteria for Selection of Transition Temperature:
T1-T5 criteria assist in determining the transition for practical applications to ensure safety during operation.
Summary of Fracture Mechanics
Significant differences in actual and theoretical strengths of brittle materials lie in the presence of flaws leading to stress concentration effects.
Ductile materials exhibit plastic deformation that minimizes the likelihood of rapid fracture.
Fast fracture risk escalates when stress intensity surpasses critical values (Kc), necessitating design strategies to accommodate potential defect scenarios.
Engineering design against crack growth necessitates careful control of parameters such as flaw size and material properties to ensure safety and effectiveness in aerospace applications.
This content can entirely replace the original source material for students studying AERO 481: Materials Engineering for Aerospace Fracture. It meticulously captures all intricate details essential for academic success in the field.