Engineering materials

Mechanical Properties

They describe how a material responds to forces during manufacturing, influencing equipment selection, force estimation, and failure prevention.

Manufacturing Processes Affected by Material Response

Forging, extrusion, drawing, and rolling depend on how a material responds to deformation.

Factors Affecting Material Behavior in Manufacturing

Material condition, temperature, deformation rate, surface condition, and environment can change material behavior.

Strength

The resistance of a material to yielding or failure.

Stiffness

The resistance of a material to elastic deformation, represented by the elastic modulus E.

Ductility

The amount of plastic deformation a material can withstand before fracturing.

Toughness

The energy absorbed per unit volume before fracture, dependent on both strength and ductility.

Difference between Strength and Stiffness

Strength refers to resistance to yielding or failure, while stiffness refers to resistance to elastic deformation.

Difference between Ductility and Toughness

Ductility indicates the extent of plastic deformation before fracture; toughness refers to energy absorption prior to fracture.

Tension Test Purpose

To determine key mechanical properties by applying a load to a standardized specimen.

Mechanical Properties from Tension Test

Elastic modulus, yield strength, ultimate tensile strength, ductility, toughness, and strain-hardening information.

Engineering Stress

Applied load divided by the specimen's original cross-sectional area: σ = P/A₀.

P in Engineering Stress Equation

The applied load.

A₀ in Engineering Stress Equation

The original cross-sectional area.

Engineering Strain

Change in gage length divided by the original gage length: e = (l − l₀)/l₀.

l₀ in Engineering Strain Equation

The original gage length.

l in Engineering Strain Equation

The current or final gage length.

Convenience of Engineering Stress and Strain

They use original dimensions, facilitating data comparison and testing.

Elastic Region of Stress-Strain Curve

Deformation is reversible, allowing the specimen to return to original dimensions when unloaded.

Hooke's Law

In the linear elastic region, stress is proportional to strain: σ = Eε.

Elastic Modulus E

The ratio of stress to strain in the linear elastic region, indicating material stiffness.

High Elastic Modulus Meaning

Indicates higher material stiffness; requires more stress for the same elastic strain.

High Stiffness vs. High Strength

High stiffness does not imply high strength; materials can be stiff without high yield strength.

Yield Strength

The stress level where permanent deformation begins.

Deformation Below Yield Strength

Recoverable deformation occurs when load is removed.

Deformation Beyond Yield Strength

Permanent strain remains post-unloading.

0.2% Offset Method Strain

Uses a strain of 0.002, which equals 0.2%.

Yield Strength's Importance in Manufacturing

Forming processes exceed yield strength for creating permanent shape changes.

Ultimate Tensile Strength (UTS)

The maximum engineering stress reached during a tensile test.

Necking After UTS

Localized reduction in cross-sectional area starts after UTS is met.

Decrease in Engineering Stress After Necking

Engineering stress decreases since it is based on the original area while necking occurs.

Tensile Specimen Fracture Region

Fracture occurs in the necked region.

Ductile Behavior

Significant plastic deformation before fracture, often with necking.

Brittle Behavior

Minimal or no plastic deformation before fracture.

Percent Elongation Measurement

A measure of ductility based on increase in specimen length.

Percent Reduction of Area Measurement

Quantifies ductility based on decrease in cross-sectional area.

Stress-Strain Curve Strength Representation

Vertical extent of the curve indicates strength.

Stress-Strain Curve Ductility Representation

Horizontal extent of the curve shows ductility before fracture.

Toughness on Stress-Strain Curve

Represented by the area under the curve up to fracture.

True Stress Measurement

Uses instantaneous cross-sectional area instead of original area.

Difference Between Engineering and True Strain

Engineering strain accounts for original gage length; true strain adjusts for changing length.

When Engineering vs. True Strain Diverges

At small strains, they are nearly equal; they differ significantly at large deformations.

Use of True Stress-True Strain Data

More accurately reflects material behavior during large plastic deformation.

Strain Hardening

Increase in a metal's strength and hardness due to plastic deformation, also known as work hardening.

Flow Curve Equation

σ = Kεⁿ.

K in Flow Curve Equation

The strength coefficient.

n in Flow Curve Equation

The strain-hardening exponent.

Larger Strain-Hardening Exponent n Implication

Indicates greater strain-hardening capacity and more uniform stretching before necking.

Ductility and Temperature Relationship

Ductility generally increases with rising temperature.

Toughness and Temperature Relationship

Toughness generally increases as temperature rises.

Yield Strength and Temperature Relationship

Yield strength usually decreases with an increase in temperature.

Elastic Modulus and Temperature Relationship

Elastic modulus typically decreases with increasing temperature.

Rationale for Warm/Hot Manufacturing Operations

Increased temperature enhances ductility and formability while lowering strength needed for deformation.

Deformation Rate Definition

Speed of change in a material's dimensions.

Strain Rate Definition

Deformation per unit dimension per unit time.

Strain-Rate Hardening Definition

Increase in material strength with rising strain rate.

Effect of Deformation Speed on Manufacturing Forces

Material strength increases with strain rate, affecting required deformation forces.

Necessary Condition for Permanent Shaping

Applied stress must exceed yield strength but should avoid causing premature fracture.

Compression Testing Purpose

Provides information for estimating forces and power needs in forging, rolling, and extrusion.

Barreling in Compression Test Cause

Result of friction between specimen and platens.

Torsion Testing Measurement

Assesses material behavior under shear deformation.

Shear Modulus G Definition

Ratio of shear stress to shear strain in the elastic region.

Manufacturing Operations Involving Shear Deformation

Punching, swaging, and machining necessitate shear analysis.

Bending Tests Utility for Brittle Materials

Bending tests are preferable due to the difficulty of gripping brittle materials for tensile testing.

Lower Surface during Bending Test

Subjected to tension.

Upper Surface during Bending Test

Subjected to compression.

Hardness Definition

Resistance of a material to permanent indentation.

Usefulness of Hardness in Manufacturing

Hardness tests are fast and often done directly on components.

Indicators of Hardness

General indicators of strength, scratching resistance, and wear resistance.

Hardness Not Fundamental Property Reason

Depends on factors like indenter geometry and applied load.

Indenter Used in Brinell Hardness Test

A steel or tungsten-carbide ball.

Rockwell Hardness Test Measurement

Measures penetration depth.

Vickers Hardness Test Apparatus

Uses a diamond pyramid.

Use of Knoop Hardness Test

Applicable for thin, small specimens, brittle materials, and microhardness.

Shore Hardness Common Testing Material

Used for rubbers, plastics, and soft elastic materials.

Fatigue Definition

Failure from repeated or fluctuating loads.

Fatigue Failure Under Lower Stress Possibility

Yes, fatigue can fail below static stress limits.

Fatigue Failure Mechanism

Cracks initiate and grow due to repetitive loading until a critical size is reached.

S-N Curve Definition

Illustrates the relationship between stress amplitude (S) and cycles to failure (N).

Endurance Limit Definition

Stress level below which fatigue failure does not happen, notably in certain steels.

Materials Without Distinct Endurance Limit

Aluminum alloys do not exhibit a clear endurance limit.

Fatigue Strength for Non-Endurance Limit Materials

Specified stress level at a particular cycle count.

Surface Condition Impact on Fatigue Strength

Imperfections like scratches and notches can weaken fatigue resistance.

Beneficial Residual Surface Stress Type

Compressive residual stress improves fatigue life.

Residual Stress Inducing Processes

Processes like shot peening and surface rolling can improve surface compressive residual stress.

Detrimental Residual Stress Type

Tensile residual stress typically worsens fatigue life.

Creep Definition

Permanent deformation from a constant static load applied over time.

Creep Importance in Metals

Significant at elevated temperatures.

Stages of Creep Curve

Primary, secondary, and tertiary stages characterize the creep process.

Charpy and Izod Tests Purpose

Measure impact toughness under rapid loading.

Charpy Specimen Support Type

Supported at both ends.

Izod Specimen Support Type

Supported at one end.

Impact Toughness Measure in Tests

Energy absorbed in breaking a notched specimen is measured.

Ductile Fracture Characteristic

Fracture involved significant plastic deformation, including necking.

Brittle Fracture Characteristic

Fracture with minimal plastic deformation, allowing for rapid crack propagation.

Importance of Defects in Fracture Behavior

Defects like scratches and voids serve as stress concentrators, impacting fracture.

Brittle Materials Defect Sensitivity

Sharp defects increase local stresses and foster rapid crack growth.

Residual Stresses Definition

Stresses that persist after external forces are removed from a part.

Residual Stress Causes

Can arise from nonuniform plastic deformation and thermal gradients during processing.

Machining Effects on Residual Stresses

Machining can disrupt the balance of residual stresses, potentially causing part warping.