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Compressive Testing of Materials

Significance and Purpose

Compressive Testing evaluates a material’s behavior under uniaxial compressive loading, determining properties such as stiffness, strength, and deformation characteristics. This type of testing is essential for materials that experience compression in real-world applications, such as polymers, composites, elastomers, and foams. Standardized test methods ensure consistent and reliable measurement of compressive properties across different materials and loading conditions.

Compressive testing is used for:

• Material Characterization: Determines fundamental mechanical properties, including compressive modulus and strength.
• Quality Control: Ensures materials meet design and performance specifications.
• Failure Analysis: Identifies potential failure modes due to compressive stresses.
• Engineering Design: Supports structural and mechanical design of load-bearing components.
• Research & Development: Helps in optimizing material formulations for improved compressive performance.

Relevant ASTM and ISO Standards

General Compressive Testing

• ASTM D695 / ISO 604: Standard methods for determining the compressive properties of rigid plastics, including high-speed and lubricated compressive stress-strain testing.

Elastomers and Rubbers

• ASTM D575: Compressive properties of elastomers.
• ASTM D395: Compression set of rubber materials (constant deflection).

Foams and Soft Materials

• ASTM D695 / ISO 604: Compressive properties of rigid foams, including unloading behavior.
• ASTM D575: High-speed compressibility of foams and elastomers.

Composite Materials

• ASTM D6641/D6641M: Combined loading compression (CLC) for filled plastics and laminate composites.
• ASTM D6484: Open hole compression of composites.

DatapointLabs Tests for Compressive Testing

Tests in the DatapointLabs test catalog that reference compressive testing are as follows:

General Compressive Testing (inquire regarding material suitability)

Test ID	Test Description	Standards
M-012	Compressive Modulus	ASTM D695, ISO 604
M-031	Compressive Strength	ASTM D695
M-210	Compressive Stress-Strain, Strength and Modulus	ASTM D695, ISO 604
M-233	High Speed Compressive Stress-Strain	ASTM D695

Compressive Testing Specific to Elastomers, Foams

Test ID	Test Description	Standards
M-212	Compressive Properties of Elastomers	ASTM D575
M-211	Lubricated Compressive Stress-Strain (Biaxial Tension)	ASTM D695
M-260	Compression Set of Rubbers (Constant Deflection)	ASTM D395
M-213	Confined Compression Stress-Strain (Hydrostatic, for rubbers)	DPL M-213*
M-215	Compressive Properties of Foam with Unloading	ASTM D695
M-231	High Speed Compressibility of Foams and Elastomers	ASTM D575, ISO 604

* Internal DatapointLabs Standard

Compressive Testing Specific to Filled Plastics, Laminate Composites

Test ID	Test Description	Standards
M-209	Combined Loading Compression (CLC)	ASTM D6641/D6641M
M-209C	Combined Loading Compression (CLC) for composite materials	ASTM D6641/D6641M
M-209C_strength	Compressive Strength using Combined Loading Compression (CLC) for composite materials	ASTM D6641/D6641M
M-051	Open Hole Compression (OHC)	ASTM D6484

Principle of Operation

Compressive testing involves applying a uniaxial compressive force to a specimen until failure or a predefined strain level. The test setup typically includes:

1. Specimen Preparation: The sample is prepared per the relevant standard (cylindrical, rectangular, or cubic specimens are common).
2. Mounting in the Testing Machine: The specimen is placed between parallel compression platens in a universal testing machine (UTM).
3. Application of Load: A controlled compressive force is applied at a defined strain rate.
4. Measurement of Force and Deformation: Load cells record the applied force, while extensometers or digital image correlation (DIC) track specimen deformation.
5. Data Collection and Analysis: Stress-strain curves are generated to determine key material properties.

Typical Procedure

1. Specimen Preparation
   • Shape and dimensions conform to relevant standards.
   • Surface preparation may include lubrication to reduce friction effects (for lubricated compression tests).
2. Test Setup
   • Specimen placed between compression platens.
   • Strain measurement devices (extensometers, DIC) applied.
3. Loading and Testing
   • Compressive force applied under controlled displacement or force rate.
   • Load and deformation continuously recorded.
4. Post-Test Analysis
   • Stress-strain curves generated.
   • Key properties such as modulus, strength, and strain at failure extracted.

Specimen Types

Specimens used by DatapointLabs in various types of compressive testing are as follows:

Specimen Type	DatapointLabs Test IDs
Prisms [Details]	M-212, M-211, M-260, M-231
Hydrostatic Test Cylinders [Details]	M-213
Discs (50mm Diam.) [Details]	M-215

Extensometry Techniques

Extensometry techniques typically employed by DatapointLabs in various types of compressive testing are as follows:

Extensometry Technique	DatapointLabs Test IDs
Crosshead Displacement (Axial)	M-012, M-031, M-210, M-233, M-212, M-211, M-260, M-213, M-215
Non-Contact Extensometry (Axial)	M-209, M-209C, M-209C_strength
Optical 2D Digital Image Correlation (DIC)	M-051
Drop Tower Load Cell Accelerometer	M-231

Characterization Measurements

Compressive Modulus (E_c)

• Definition: Slope of the initial linear portion of the compressive stress-strain curve.
• Significance: Indicates material stiffness under compression.

Compressive Strain at Yield (ε_c)

• Definition: Strain at which the material first deviates from linear elasticity.
• Significance: Marks the onset of plastic deformation.

Compressive Strength at Yield (σ_c)

• Definition: Stress at the yield point in compression.
• Significance: Critical for materials that exhibit distinct yielding behavior.

Engineering Compressive Stress-Strain Curves

• Definition: Relationship between engineering stress and strain based on initial cross-sectional area and height.
• Significance: Commonly used for material selection and design.

True Compressive Stress-Strain Curves

• Definition: Accounts for actual material deformation by considering instantaneous cross-sectional area.
• Calculation:
  ${\mathrm{\sigma }}_{\mathrm{t}}={\mathrm{\sigma }}_{\mathrm{e}}(1+{\mathrm{\epsilon }}_{\mathrm{e}});{\mathrm{\epsilon }}_{\mathrm{t}}=\ln (1+{\mathrm{\epsilon }}_{\mathrm{e}})\text{ [Hencky strain]}$
• Significance: More accurate for materials that experience large deformation.

Offset Yield Strain in Compression (ε_0.2c)

• Definition: Strain corresponding to an offset yield stress (e.g., 0.2% strain).
• Significance: Used when materials lack a clear yield point.

Offset Yield Stress in Compression (σ_0.2c)

• Definition: Stress at 0.2% plastic strain offset.
• Significance: Common in testing metals and plastics.

Compressive Strain at Ultimate (ε_cu)

• Definition: Strain at peak compressive stress before failure.
• Significance: Indicates material ductility in compression.

Compressive Ultimate Strength (σ_cu)

• Definition: Maximum stress sustained before failure.
• Significance: Important for materials used in load-bearing applications.

Cyclic Engineering Stress vs. Strain Curves

• Definition: Stress-strain response under cyclic loading.
• Significance: Evaluates material fatigue and viscoelastic behavior.

Compression Set with Final and Initial Thickness (S_c)

• Definition: Permanent deformation of elastomers and rubbers after prolonged compression.
• Calculation:
  ${\mathrm{S}}_{\mathrm{c}}=\frac{{\mathrm{t}}_0-{\mathrm{t}}_{\mathrm{f}}}{{\mathrm{t}}_0}\bullet 100$
  where t₀ is the initial thickness, and t_f is the final thickness after unloading.
• Significance: Measures the material’s ability to recover after compression.

Bulk Modulus (K)

• Definition: Resistance to uniform compression under confined conditions.
• Calculation:
  $\mathrm{K}=-\frac{\mathrm{\Delta }\mathrm{P}}{\mathrm{\Delta }\mathrm{V}\mathrm{/}\mathrm{V}}$
  where ΔP is the pressure change, and ΔV/V is the volumetric strain.
• Significance: Important for materials subjected to hydrostatic pressure.

Typical Data Reported (see test descriptions for exact details)

• Compressive Modulus: Relationship between stress and strain in the elastic region.
• Poisson’s Ratio: Ratio of lateral strain to axial strain in the elastic region.
• Compressive Yield Strength and Strain: Stress and strain at the first deviation from linearity.
• Compressive Ultimate Strength and Strain: Stress and strain at peak stress before failure.
• Compressive Break Strength and Strain: Stress and strain at moment of fracture.
• Compressive Stress-Strain Curves (Engineering and True): Stress-strain relationship based on initial or instantaneous cross-sectional area, respectively.
• Compression Set: Permanent deformation, for elastomers and rubbers.
• Bulk Modulus: Relationship between stress and volumetric strain in the elastic region, for confined compression tests.

Suitable Material Types

• Plastics: ASTM D695, ISO 604.
• Elastomers: ASTM D575, ASTM D395.
• Foams: ASTM D695, ISO 604.
• Composites: ASTM D6641/D6641M, ASTM D6484.

Suitable Applications

• Material Selection: Evaluating compressive strength, stiffness, and deformation behavior to select suitable materials for load-bearing applications.
• Component Design: Assessing the performance of structural elements such as foams, gaskets, and cushioning materials under compression.
• Quality Control: Ensuring consistency and reliability in products such as plastic components, and elastomeric seals.
• Research & Development: Investigating new materials, composites, and formulations for improved compressive properties.
• Failure Analysis: Identifying causes of material failure such as buckling, crushing, or permanent deformation.
• Product Certification: Verifying compliance with standards for compressive strength, impact resistance, and durability.
• Process Optimization: Evaluating the impact of manufacturing techniques on material compression performance.
• Environmental Testing: Assessing material behavior under conditions such as temperature extremes, moisture exposure, or cyclic loading.

Conclusion

Compressive testing is essential for material characterization and ensuring performance in real-world applications where compression loading occurs. It provides critical insights into stiffness, strength, deformation, and failure mechanisms across plastics, composites, foams, and elastomers. By following ASTM D695, ISO 604, ASTM D575, ASTM D395, and ASTM D6641, engineers can optimize material performance, ensuring reliable product design, structural integrity, and regulatory compliance across multiple industries.