Materials Science: Mechanical Properties Quiz

Materials and Manufacturing Processes (Nuclear Engineering) 4EN514

• Course code: 4EN514
• Relevant Key Skills and Behaviors (KSBs): SK2, SK3, SK7, SK8, B3, B5

Mechanical Behavior

• Objectives:

  • Determine the strength of structural materials
  • Assess the deformation capacity of structural materials under specific loads
  • Investigate how materials deform, elongate, compress, twist, or break as influenced by load, time, temperature, and other conditions
  • Understand standard testing methods and terminology for mechanical material properties

• Examples:

  • Aircraft materials (aluminum, carbon fiber-reinforced composites) need to be strong, lightweight, and resistant to cyclic loading
  • Sports equipment (bats, golf clubs) must be light and resilient to impact forces

Mechanical Properties of Solids - Terminology

• Material selection for components is based on matching mechanical properties to service conditions
• Material characteristics considered include stiffness, strength, and ductility, along with operation at elevated or low temperatures
• Mechanical tests determine the material's response to applied forces

Things Related to Mechanical Behavior

• Distinguishing ductile and brittle fracture responses
• Differentiating between static and cyclic loading conditions
• Analyzing material deformation mechanisms
• Evaluating creep deformation (stress and temperature impact)
• Determining fatigue fracture behaviors (high vs. low cycle)
• Understanding notch sensitivity and loading directions

Mechanical Testing of Materials

• Test types/loading: Tension, compression, hardness, bending, torsion
• Test specimens: Un-notched, notched
• Environmental conditions: Test ambient conditions of specimens
• Test standards: Industry, company
• Test equipment: Detailed description of equipment and procedures

Types of Mechanical Loadings

• Diagrams illustrating tensile, compressive, shear, and torsional loading conditions
• Figures showcasing relevant testing methodologies

Standard Test Methods

• Mechanical testing considers material properties to drive design processes, and ensure quality control
• Standardized tests ensure consistency and effective communication between producers and users
• Key professional societies include ASTM (American Society for Testing and Materials) and ISO (International Organization for Standardization)

Standard Test Methods (continued)

• ASTM standards cover a substantial range mechanical testing procedures and include several volumes covering different material types (metals, concrete, plastics, rubber, and glass, etc.).
• Clear specifications detail procedures for specimen description, loading conditions, reporting values, and statistical analyses.

Uniaxial Tension Test

• Method/equipment to assess resistance to material deformation under static load
• Universal testing machine to measure stress-strain response
• Strain gauge or extensometer to measure applied strain
• Load cell to measure applied force

Concept of Stress

• Stress is defined as force per unit area (σ = F/A)
• Stress is a key parameter for predicting the performance of structures and materials
• Length and shape of the specimen do not affect the failure stress for the same material and cross section

Concept of Stress (continued)

• Different shaped specimens with same material and cross sectional area exhibit similar failure loads or tensile force
• Stress is the intensity of the force applied over a unit area of the material.

Concept of Stress

• Stress describes the amount of force per unit area, and is a quantifiable parameter indicating a material's capacity to withstand and carry forces
• Emphasis is placed on the force per unit area
• Emphasis on importance of unit area for measuring stresses correctly.

Uniaxial Tension Test Sample

• Detailed description of sample design for tensile testing
• Specific dimensions and shapes of specimen for tensile tests
• Identification of specimen parts - important for consistent results.

Engineering Strain and Stress

• Formulas define engineering strain (ε) and stress (σ)
• These parameters are crucial for analyzing and comparing test results across specimens with differing dimensions (area, length).

Uniaxial Tension Test

• Strain or load control methodologies, typically constant crosshead speeds
• Record stress-strain curve
• Use dedicated strain measuring tools for reliable results
  • Extensometer
  • Digital image correlation (DIC)

Mechanical Properties of Solids - Terminology

• Understanding elastic deformation (elastic strain), including recovery following stress removal
• Explanation of Young's Modulus (modulus of elasticity) representing the linear region slope of the stress-strain curve, and its inverse (compliance)
• Defining plastic deformation (plastic strain), the permanent deformation after stress removal.
• Identification of yield strength, the point where plastic deformation starts.

Yield Strength

• Method for yield points identification in materials (offset method)
• Procedure for determining the point at which plastic deformation becomes visible.

Offset Yield Strength

• Method to pinpoint yield strength using a strain offset
• Calculation and procedure for determining the Proof Strength.

Young's Modulus

• Explanation of Young's modulus to measure material stiffness
• Relationship between Young's modulus and atomic binding energies in materials
• Significance of Young's modulus in assessing material resistance to permanent deformation

More Terminology

• Definition of tensile strength (ultimate strength), the highest stress during a test
• Explanation of the process of necking, a localized narrowing of a tensile specimen.
• Explanation of how necking is relevant in ductile materials

More Terminology

• Description of residual stress, the lingering stress after loads are removed
• Definition of strain hardening, including increasing strength with increasing deformation.
• Explain cold working as a form of strain hardening.

True Stress versus Engineering Stress

• Differentiating true and engineering stress from the viewpoint of change in cross-sectional area
• True stress-strain curve showing change in area
• Relation between true stress-strain curves and engineering stress-strain curves

True Stress versus Engineering Stress: Strain

• Relationship between true stress, engineering strain, and strain hardening exponent in the log-log graph
• Strain hardening exponent (n) is a material property in the log-log graph
• Relationship between strain hardening exponent, material deformability, and shaping processes.

Strain Hardening

• Process of increased material strength due to plastic deformation.
• Dislocation interaction as the mechanism for strain hardening.
• Effects on mechanical properties (strength, ductility) at various stages of deformation

Strain Hardening

• Explanation of strain hardening using stress-strain curves showing the material yield and tensile strength ranges
• How strain hardening relates to stress levels in the new curve, strength, and ductility aspects during the process.

Necking

• Localized deformation (necking) in ductile materials
• Importance of true stress-strain curves in accounting for area reductions
• Correlation between true stress-strain curves and necking during the testing process.

True Stress vs. Engineering Stress

• Comparison of true and engineering stress/strain definitions, emphasizing the effect of actual versus initial area/lengths
• Applicability of engineering stress/strain for small dimensional changes in specimens

Ductility and Toughness

• Definition of ductility (percent elongation at failure)
• Definition of toughness, material's capacity to absorb energy
• Relation between ductility and toughness

Ductility and Temperature

• Importance of high ductility in resisting crack propagation in materials.
• Effects of temperature on mechanical properties, especially for cryogenic applications.
• Description of material ductility behavior at different temperature ranges (high and low.)
• Detailed explanation of ductility behavior across different material types (bcc metals, FCC metals, plastics/elastomers, Ceramics/glasses.)

Upper and Lower Yield Points for Low Carbon Steel

• Identification of upper and lower yield points for low-carbon steel, with relation to material properties, including interstitial carbon's influence on dislocation pinning and stress concentration near specimen grips.
• Understanding the implications on engineering stress-strain behavior during plastic deformation.

Other Elastic Constants

• Defining Poisson's ratio, shear modulus, and related concepts, along with important formulas
• Importance of these elastic constants in material science

Poisson's Ratio and Shear Modulus

• Values for Poisson's ratio and shear modulus for different material types like alloys (Table).

Brittle vs. Ductile Behavior

• Stress-strain behaviors, and differences between brittle and ductile materials
• Examples of each behavior (cast iron, polymers, glass, etc.)
• Correlation between material behavior and failure mechanisms, and their visual characteristics.

Ceramics and Glasses

• Overview of ceramics and glasses, and their brittle fracture behavior

Brittle Fracture: Ceramic and Glasses

• Stress-strain curves for ceramics and glass are presented, illustrating brittle fracture behavior and significant differences when tested in tension vs. compression.
• Emphasis is placed on the stress-strain behavior, under conditions of tension versus compression in these brittle materials.

Brittle Materials

• Overview of brittle material fracture point (stress at maximum load), noting equivalence with yield, tensile, and breaking points
• Explanation of why yielding does NOT occur by dislocation movement (but by planar defects, like cracks) in brittle materials.

Brittle Materials

• How the bend test evaluates brittle materials, showing specimen setup and formula for elasticity in bending.
• Calculating the max flexural stress value for a three-point bend test based on the material and load condition.
• Relating flexural strength to tensile strength, specifically explaining how tensile failure occurs along the outer edge of the specimen during a bending test.

Brittle Materials

• Stress-deflection curve example for MgO testing and correlation between applied force and deflection of the material.

Modulus of Elasticity and Strength

• Modulus of elasticity and strength (modulus of rupture) data for some ceramics/glass materials

Poisson's Ratio for Ceramics and Glasses

• Poisson's ratio values for various ceramics and glasses.

Stresses at Crack Tips

• Explains how stress concentration happens at the crack tip of brittle materials, including the use of formulas to help determine stress at a crack tip in tension
• Relationship between the weaknesses of brittle materials under tension, and how a compressive load reduces these weaknesses by closing the flaws.

Why Ceramics are Brittle

• Explanation of the difficulty in creating obstacle-free slip planes in ceramics due to ion charges and repulsive forces.
• Stress concentrations around manufacturing defects contribute to brittle behavior, leading to fracture under relatively low stresses

Creep

• Plastic deformation at high temperatures and constant load given an overview of the phenomenon of creep and the processes by which it occurs
• Creep stages (primary, secondary, tertiary) are described with relation to strain rate, and the associated factors/causes, including dislocation mechanisms, and grain boundary changes

Creep (continued)

• Emphasis on factors like stress, temperature, and time in creep analysis
• Thermally activated process (Arrhenius equation) importance, showing relation between creep processes and strain rate

Creep of Ceramics

• Complex diffusion mechanisms in ceramics due to charge neutrality
• Grain boundary sliding as a significant creep mechanism in ceramics
• Importance of understanding creep for high-temperature ceramic applications, recognizing that creep is undesirable because these processes reduce the lifespan of the item (making it weaker at higher temperatures, etc.)

Creep

• Creep associated deformation mechanisms, discussing slip, diffusion to enable dislocation climb for deformation
• Solid solution hardening/precipitation hardening as effective prevention techniques at elevated temperatures for materials exhibiting creep

Creep (continued)

• Variation of the creep curve with elevated stress or temperature, illustrating the relationship between these factors
• Arrhenius behavior (log-log plot) in relation to the creep rate (strain rate) at different stress and temperatures
• Calculation of the Creep rate based on the Arrhenius equation.

Fatigue

• Definition of fatigue, a failure type associated with cyclic loading and stresses lower than static limits
• Importance of fatigue consideration in structures handling dynamic loading (e.g., aircraft, bridges)
• Description of the causes of fatigue in components that are often used in dynamic scenarios

Cyclic Stresses

• Types of cyclic stresses
• Definitions for completely reversed, purely tensile/compressive, and random cycles in relation to stress, time, and magnitudes

Important Fatigue Parameters

• Calculation of average (mean) stress, stress range, and stress amplitude using maximum and minimum stress values
• Definition of stress ratio, showing the ratio between minimum and maximum stress, and significance in fatigue analysis
• Plots of each stress cycle type to exemplify different cyclic stress scenarios

S-N Curve

• Plotting engineering fatigue data (stress-versus-number of cycles to failure)
• Stress values to plot on S-N curves are typically low for high-cycle fatigue tests
• Considerations for material behavior, local stress concentrations, and plasticity to avoid erroneous results

Fatigue

• Distinction between the two kinds of S-N curves found in materials (one for those with a fatigue limit/endurance limit, and one for those without.)
• Materials with a fatigue limit include those like steel and titanium.

Impact Energy

• Impact energy definition
• Impact energy testing techniques (Charpy/Izod for polymers.)

Impact Energy

• Impact energy values for different materials (e.g., polymers, metallic alloys,) as function of temperature

Impact Energy (continued)

• Relation between impact energy, temperature, and material ductility, emphasizing the concept of ductile-to-brittle transition temperature (DBTT).