Material Testing and Mechanical Properties

Material Testing

• Testing materials helps determine if a product will perform as expected.
• Material properties are essential for understanding why objects are created from specific materials.
• Material properties are intensive and often quantitative properties of a solid.
• Quantitative properties can help assess the benefits of one material over another, aiding in material selection.

Mechanical Properties

• Mechanical properties describe how a material reacts to an applied load.
• These properties determine a material's usefulness and service life.
• Common mechanical properties include strength, ductility, hardness, impact resistance, and fracture toughness.
• Loading describes the application of a force to an object.
• Materials can undergo various loading scenarios, and performance depends on these conditions.
• Fundamental loading conditions include tension, compression, bending, shear, and torsion.

Tension

• Tension is a type of loading where forces pull sections of a material apart.

Compression

• Compression is the opposite of tension, pressing material together.

Bending

• Bending involves applying a load causing the material to curve, compressing one side and stretching the other.

Shear

• Shear involves applying a load parallel to a plane, causing one side of the material to slide across the other.

Torsion

• Torsion involves applying a force that causes twisting in a material.

Tensile Test Experiment

• Strength is a widely used material property.
• The tensile test is an experiment that provides information about a material's strength and mechanical behavior.

Stress-Strain Diagram

• A stress-strain diagram plots stress against strain, showing the material's behavior in tension.
• Elastic region: The material returns to its original shape when unloaded.
• Plastic region: The material does not return to its original shape when unloaded; deformation is permanent.
• Yield strength: The stress at which plastic deformation begins.
• Ultimate tensile strength: The maximum stress a material can withstand before fracture.
• Necking: A localized constriction in the material before fracture.
• Fracture: The point at which the material breaks.

Tensile Strength, TS

• The stress required to continue plastic deformation increases to a maximum point before decreasing to the fracture point.
• All deformation up to maximum stress is uniform.
• Necking begins at maximum stress; subsequent deformation is limited to this area.
• Fracture strength is the stress at the point of fracture.

Tensile Strength: Comparison

• Tensile strength values vary greatly depending on the material.
• Specific materials have variable tensile strengths depending on processing techniques.

Yield Strength

• Yield strength is the value of stress at the yield point.
• The yield point is found using the 0.002 strain offset method.

Yield Strength: Comparison

• Yield strength values vary significantly across different materials.
• The strength of a material is measured using specific processing techniques.

Elastic Deformation

• Elastic deformation is reversible; external forces do not permanently change the material's shape.
• External forces result in bonds stretching but bonds return to their original state when the force is removed.

Linear Elastic Properties

• Hooke's Law: Stress is equal to the product of the modulus of elasticity and strain
• Young's Modulus (E) is the slope of the linear portion of the stress-strain graph

Young's Moduli: Comparison

• The modulus of elasticity varies significantly by material type.
• The modulus differs by processing technique.

Plastic Deformation (Metals)

• Plastic deformation is permanent and involves bonds and planes shearing.
• The material no longer returns to its original shape when the external forces are removed

Ductility, %EL

• Ductility is the measurement of plastic deformation before fracture.
• Percentage elongation and percentage reduction in area are ways to measure ductility.
• Brittle materials have little plastic deformation; ductile materials experience significant deformation before fracture.

Toughness

• Toughness is the ability to absorb energy up to the point of fracture.
• It is represented by the area beneath the stress-strain curve.
• Toughness is a combination of strength and ductility.

Impact Fracture Testing

• Impact testing measures fracture characteristics during high-strain rates.
• Two common types are Charpy and Izod impact tests.
• The energy absorbed during fracture is measured.

Impact Test (Charpy) Data

• Impact energy correlates with toughness, the area beneath the total stress-strain curve.
• Impact testing provides a rapid method to gather complete tensile test results.

Impact Test: The Izod Test

• Izod testing, often used for polymers, differs from Charpy by the specimen configuration.

Material Variability

• Material variability includes the variability of materials within a sample, the variability introduced during sampling, and the variability introduced during testing.
• This variability can affect accuracy and precision.

Precision and Accuracy

• Precision describes the variability of repeat measurements.
• Accuracy describes how closely results match the true value.
• Bias is the tendency of an estimate to deviate in one direction.
• Measurements should be both precise and accurate.

Basic Statistics

• The arithmetic mean is the average, and standard deviation is the measure of spread.
• Both mean and standard deviation should be used to properly quantify material variability.
• Coefficient of Variation combines mean and standard deviation to provide a more useful depiction of material variability.

Hardness

• Hardness is a material's resistance to localized plastic deformation (dents or scratches).
• Quantitative hardness measurements use an indenter and measure the size of the indentation.
• Hardness is related to several other material properties, such as wear resistance.

Hardness Testing Techniques

• Brinell, Vickers, Knoop, and Rockwell are common hardness testing techniques.
• Different techniques use different indenters and loads to measure hardness.

Correlation between Hardness and Tensile Strength

• Hardness and tensile strength are related for some materials.
• Correlation can be used to predict materials properties.

Rockwell Hardness Scales

• Different Rockwell hardness scales use different indentation methods, and load values.

This quiz explores the fundamentals of material testing, including properties that determine material performance and their mechanical characteristics. Key concepts such as tension, loading conditions, and quantitative properties are covered. Understanding these elements is crucial for making informed material selections in engineering and manufacturing.