Bulk/Volumetric Testing of Materials
Significance and Purpose
Volumetric Mechanical Testing evaluates how materials respond to hydrostatic pressure and volumetric compression. It provides critical insights into a material’s compressibility, bulk modulus, and volumetric stress-strain behavior. These properties are essential for applications where materials experience multi-axial compression, such as in seals, gaskets, energy-absorbing structures, and hyperelastic materials.
Key objectives of volumetric mechanical testing include:
- Determining Bulk Modulus: Quantifying resistance to volumetric compression.
- Understanding Material Compressibility: Assessing how materials densify under pressure.
- Material Modeling for FEA: Inputting volumetric stress-strain behavior into hyperelastic models.
- Predicting Performance in Confined Environments: Evaluating material performance in constrained applications like automotive NVH components or biomedical implants.
Relevant ASTM and DatapointLabs (DPL) Standards
- DPL D-020: Bulk Modulus Testing (Up to 8 Temperatures).
- DPL M-214: Volumetric Stress-Strain Curves.
- ASTM D792: Standard Test Method for Density and Specific Gravity of Plastics by Displacement (used as a reference for solid density).
DatapointLabs Tests for Volumetric Mechanical Testing
Tests in the DatapointLabs test catalog that reference volumetric mechanical testing are as follows:
Volumetric Mechanical Testing Specific to Plastics, Thermosets and Elastomers
| Test ID | Test Description | Standards |
|---|---|---|
| M-014 | Bulk Modulus (Up To 8 Temperatures) | DPL D-020* |
| M-214 | Volumetric Stress Strain Curves | DPL M-214* |
* Internal DatapointLabs Standard
Principle of Operation
Volumetric mechanical testing is based on high-pressure indirect dilatometry (confining fluid method).
Key principles are as follows:
- Hydrostatic Compression: The specimen is subjected to uniform pressure from all sides via a confining fluid (mercury).
- Volume Change Measurement: A bellows spring connected to an LVDT (Linear Variable Differential Transformer) tracks changes in specific volume as pressure and temperature vary.
- Thermal Compensation: The apparatus compensates for thermal expansion of mercury, ensuring that volume changes are solely due to the specimen.
- Bulk Modulus Calculation: Derived from volumetric stress-strain curves.
Typical Procedure
Bulk Modulus Testing (DPL D-020)
- Sample Preparation:
- Dry approximately 1 gram of the sample.
- Measure solid density at room temperature using ASTM D792.
- Loading the Sample:
- Place the sample in the PVT cell, sealing it under vacuum with mercury as the confining fluid.
- Test Execution:
- Perform isothermal heating scans across up to 8 different temperatures.
- Apply pressure from 10 MPa to 200 MPa.
- Record volume change using the LVDT sensor.
- Data Analysis:
- Calculate bulk modulus from the slope of the volumetric stress-strain curve.
Bulk Modulus (K) is calculated as:
where:- V = initial volume
- ΔP = change in pressure
- ΔV = change in volume
Volumetric Stress-Strain Curves (DPL M-214)
-
Sample Preparation:
- Dry approximately 1 gram of the sample.
-
Loading the Sample:
- Place the sample in the PVT apparatus with mercury as the confining fluid.
-
Test Execution:
- Apply hydrostatic compression while measuring volume change.
- Record volumetric stress vs. strain over the full pressure range.
-
Data Analysis:
- Generate volumetric stress-strain curves for hyperelastic modeling.
Specimen Types
Specimens used by DatapointLabs in bulk/volumetric mechanical testing are as follows:
| Specimen Type | DatapointLabs Test IDs |
|---|---|
| Parts, Any Shape [Details] | M-014, M-214 |
Extensometry Techniques
Extensometry techniques typically employed by DatapointLabs in volumetric mechanical testing are as follows:
| Extensometry Technique | DatapointLabs Test IDs |
|---|---|
| Linear Variable Differential Transformer (LVDT) | M-014, M-214 |
Characterization Measurements
Bulk Modulus (DPL D-020)
- Bulk Modulus vs. Temperature: Change in compressibility across up to 8 temperature levels.
- Solid Density: Baseline density at room temperature.
- Specific Volume vs. Temperature: Inverse of density, showing thermal expansion behavior.
Volumetric Stress-Strain Curves (DPL M-214)
- Solid Density: Baseline density at room temperature.
- Volumetric Stress vs. Volumetric Strain: Tracks compressive response to hydrostatic pressure.
Typical Data Reported (see test descriptions for exact details)
- Bulk Modulus vs. Temperature: Quantifies compressibility variation with temperature.
- Solid Density: Reference measurement using ASTM D792.
- Specific Volume vs. Temperature: Tracks volumetric expansion.
- Volumetric Stress-Strain Curves: Plots volumetric stress against strain.
- Temperature-Pressure Dependence: Analysis of how pressure affects thermal expansion.
Suitable Material Types
- Thermoplastics: PE, PP, PC, PA, PET, ABS.
- Thermosets: Epoxies, phenolics, polyurethanes.
- Elastomers: TPU, TPE, silicone rubber.
- Composites: Fiber-reinforced polymers (FRPs).
Suitable Applications
- Injection Molding Simulation: Accurate volumetric shrinkage data for mold-filling predictions.
- Hyperelastic Material Modeling: FEA models for materials like rubber seals and gaskets.
- Thermal Stress Analysis: Understanding material performance during temperature cycling.
- Impact and Crashworthiness: Modeling material response under high-pressure loading.
- Biomedical Devices: Evaluating compressibility in soft tissue-mimicking materials.
- Automotive Components: Analyzing NVH (Noise, Vibration, Harshness) materials under compressive loading.
Conclusion
Volumetric mechanical testing provides essential data on compressibility, bulk modulus, and thermal expansion for plastics, thermosets, and elastomers. By using methods like DPL D-020 (Bulk Modulus) and DPL M-214 (Volumetric Stress-Strain), engineers can optimize material formulations, improve product designs, and enhance the accuracy of numerical models for high-performance applications in industries like automotive, aerospace, and medical devices.
