Tensile Testing and Stress-Strain Curves

Tensile Test

• The tensile test, also known as the tension test, is a fundamental mechanical test performed on materials.
• This test provides significant information about the material, quantifying important properties.
• Lifting equipment examiners need this information to understand material specifications and relate them to lifting equipment suitability.
• A standard specimen, typically round, square, or rectangular, is machined to a predetermined size.

Tensile Test Importance

• Material properties determined through the test include elastic limit, yield point, ultimate tensile strength, and elongation/reduction in cross-sectional area.
• A tensile load is applied to the specimen until it fractures.
• During the test, the load required for specific elongation is recorded, constructing a load/elongation curve. This curve reveals the material's tensile behavior.

Local Necking

• Local necking refers to a reduction in material cross-sectional area under tensile load exceeding its maximum tensile strength.
• This reduction is visible when a material reaches its elastic limit.

Tensile Test Definitions

• Limit of Proportionality: The point where the stress-strain graph deviates from a straight line (stress and strain are initially proportional).
• Elastic Limit: The point up to which a material deforms elastically (i.e., the material returns to its original dimensions if the load is removed).
• Yield Point: When a sudden permanent extension in the material occurs without a corresponding increase in load (potentially a slight drop).
• Tensile Strength: The point of maximum load reached during the test.
• Ultimate Breaking Stress: The maximum stress value achieved before the material fractures. A reduction in material cross-sectional area ('necking') occurs.

Material Types in Tensile Testing

• Brittle materials: Fracture at relatively high stress levels with little deformation.
• Ductile materials: Yield at lower stress levels than brittle materials, showing significant elongation and deformation before fracture.

Stress Types

• Single Shear: Forces acting across a material at a single point (e.g., a lifting lug).
• Double Shear: Forces acting across a material at two points (e.g., a shackle pin).
• Compression: A pushing force (e.g., a jack body under load).
• Tension: A pulling force (e.g., a chain sling under load).
• Torsion: A twisting force (e.g., a rotating gearbox shaft).

Units of Measure

• Units of measure are standardized quantities of physical properties, used to determine multiple quantities of a given property.
• Different systems of units are based on different fundamental units (e.g., the International System of Units or SI). SI has 7 base units.
• Common units include weight, length, mass, and force.

Verification

• Verification is the process used to confirm the safety and suitability of lifting equipment.
• Verification methods depend on the specific equipment and standard (e.g., testing, calculations, measurements, and examinations).
• Newly manufactured equipment is verified for compliance with relevant standards, legal requirements, and specifications.
• Verification is used to ensure equipment meets requirements and is safe to operate.

Test Machines and Force/Load Measuring Equipment

• Many standards and codes dictate the accuracy levels for test loads or forces applied to equipment.
• ISO 7500-1 specifies various classes/grades of testing machines based on accuracy. Each machine's accuracy is stated on its calibration certificate.
• Calibration and verification of test equipment are crucial for accurate results and must be performed regularly. Requirements for calibration and verification are in the LEEA Technical Requirements for Members and Guidance document (LEEA-042).