APLICACIÓN
What is Compression Testing of Metallic Materials?
Compression testing, as defined by ASTM E9, is a fundamental mechanical evaluation method used to determine the behavior of metallic materials under axial compressive loads at room temperature. Unlike tensile testing, which pulls a specimen apart, compression testing pushes the material together, causing it to shorten and expand laterally. This test is crucial for characterizing materials that are primarily used in load-bearing applications where they experience crushing forces, such as columns, bearings, and structural supports. It is particularly valuable for brittle materials that fracture before yielding in tension, as well as for ductile materials used in metal forming processes like forging and rolling, where large plastic deformation occurs without necking.
What is a typical Compression Test Result?
The results provide a comprehensive set of mechanical properties derived from the stress-strain curve generated during the test:
① Compressive Yield Strength ( Rp0.2 ): The stress at which the material exhibits a specified amount of permanent deformation (typically 0.2% offset). This indicates the limit of elastic behavior under compression.
② Compressive Yield Point: For materials with a distinct yield phenomenon, this is the maximum stress reached just before a sudden drop in load, marking the onset of plastic flow.
③ Modulus of Elasticity ( E ): The slope of the initial linear portion of the stress-strain curve, representing the material’s stiffness in compression. It is generally assumed to be equal to the tensile modulus for isotropic metals.
④ Compressive Strength: For brittle materials, this is the maximum stress at fracture. For ductile materials, which do not fracture but rather flatten indefinitely, compressive strength is often defined as the stress at a specific total strain (e.g., 50% reduction in height).
⑤ Stress-Strain Curve: The full plot showing the relationship between applied stress and resulting strain, revealing work hardening behavior and the absence of necking instability seen in tension.
⑥ Failure Mode: Description of how the specimen failed, such as barreling (bulging in the middle), buckling (lateral instability), shear failure, or shattering (for brittle materials).
Discovery and Evolution of Compression Testing
While tensile testing became the standard for structural design early in the industrial era, the need to understand material behavior under crushing loads grew with the expansion of heavy machinery, aerospace structures, and metal forming industries. Early compression tests were plagued by inconsistent results due to friction between the specimen and the machine platens, leading to “barreling” and inaccurate stress calculations. ASTM E9 was developed to standardize the mitigation of these effects through strict requirements for specimen geometry (length-to-diameter ratios), lubrication, and alignment devices (subpresses). Over time, the standard has evolved to include specific protocols for thin sheet materials using lateral support jigs and special annexes for ultra-hard materials like cemented carbides, ensuring reliable data across diverse material classes.
The standard defines specific mechanical properties for material acceptance:
Instability Modes: Addressing unique failure modes like buckling and barreling which are not present in tension.
Specimen Support: Requiring lateral support jigs for thin sheets and precise alignment devices (subpresses) for solid specimens to maintain axiality.
Ductile Failure Definition: Acknowledging that ductile metals do not fracture in compression; thus, strength is often defined at a specific strain rather than at rupture.
Friction Sensitivity: Placing a much higher emphasis on end-condition friction control due to its direct impact on the stress state.
The key contents covered by the standard include:
Specimen geometry requirements (cylindrical or prismatic) and aspect ratios (height-to-diameter) to prevent buckling.
Preparation of specimen ends (grinding/lapping) to ensure flatness and parallelism.
Lubrication techniques to minimize end friction.
Alignment procedures for the testing machine, including the use of spherical seats.
Extensometer attachment methods suitable for compressive loads.
Control of testing rates (strain rate or crosshead speed).
Calculation methods for stress (based on original or instantaneous area) and strain.
Criteria for identifying buckling failure vs. material failure.
Referenced Standards
ASTM E4: Practices for Force Verification of Testing Machines.
ASTM E6: Terminology Relating to Methods of Mechanical Testing.
ASTM E29: Practice for Using Significant Digits in Test Data to Determine Conformance with Specifications.
ASTM E83: Practice for Verification and Classification of Extensometer Systems.
ASTM E1012: Practice for Verification of Testing Frame and Specimen Alignment Under Tensile and Compressive Axial Force Application.
ASTM E111: Test Method for Young’s Modulus, Tangent Modulus, and Chord Modulus (often referenced for modulus calculation details).
ISO 13314: Mechanical testing of metals – Determination of compression properties of porous and cellular metals (Specific application).
ASTM E8/E8M: Tension Testing of Metallic Materials (The complementary tensile standard).
ASTM E290: Bend Testing of Material for Ductility.
ASTM E399: Fracture Toughness Testing.
ISO 6892-1: Tensile testing at ambient temperature.
EN 10002-1: (Superseded) Tensile testing of metallic materials.
Environment:
Temperature: Room temperature (typically 20℃ to 25℃).
Atmosphere: Ambient air.
Safety: Protective shields are mandatory, especially for brittle materials that may shatter or long specimens that may buckle violently and eject from the machine.
Test Procedure:
- Specimen Preparation: Machine specimens to specified dimensions (e.g., L/D = 2 for medium tests). Ensure ends are flat and parallel within 0.0005 in./in. and perpendicular to the axis. Polish surfaces to 63 µin. finish.
- Measurement: Measure diameter/width and thickness at multiple points to calculate the average cross-sectional area.
- Cleaning & Lubrication: Clean specimen ends and bearing blocks with solvent. Apply a high-pressure lubricant (e.g., MoS2, PTFE sheet) to minimize friction.
- Setup: Install alignment device or subpress if required. Place specimen between bearing blocks. For sheets, install in the lateral support jig with appropriate clamping pressure.
- Instrumentation: Attach an extensometer to the gauge length. Verify it is seated correctly and not slipping.
- Alignment Check: Ensure the load train is aligned. Use spherical seats if necessary to accommodate minor misalignments.
- Testing: Initiate the test at a controlled strain rate (0.005 in./in./min). Record load and strain continuously.
- Monitoring: Watch for signs of buckling or excessive barreling. For ductile materials, continue until the target strain is reached. For brittle materials, test to fracture.
- Post-Test: Remove specimen. Measure final dimensions if required for analysis. Inspect failure mode.
- Calculation: Convert load to stress (Load/Area). Determine yield strength (offset method), modulus, and compressive strength from the curve.
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