Material Testing for CAE: General Considerations
Finite Element Analysis (FEA) relies critically on accurate and representative material data. While materials testing is widespread, much of it is not designed with FEA in mind. Instead, tests are often conducted for quality control using international standards like ASTM or ISO. These standards emphasize repeatability over scientific precision, and the resulting data — though useful for process control — may not capture the nuanced, quantitative behavior needed for advanced simulation.
In contrast, material testing for FEA aims to precisely measure the properties required to build a reliable material model under well-controlled conditions. The accuracy of this data is paramount, as it directly influences simulation fidelity. Here, we address the methods, standards, instrumentation, and environmental considerations involved in testing materials specifically for CAE simulation purposes.
To learn more regarding material testing and characterization in support of material models for CAE, see our TestPaks or contact us to talk with our materials testing experts.
Measuring Stress-Strain Behavior
The tensile test is the cornerstone of mechanical characterization for FEA, accounting for the majority of material models built for simulation. Using a Universal Testing Machine (UTM), a standardized test specimen is subjected to controlled tension. The UTM records the applied load and resulting deformation, from which stress-strain curves are derived.
Test specimens are typically dog-bone shaped to ensure a uniform stress distribution in the gauge section. Force is measured via calibrated load cells, and deformation (strain) is most commonly captured using extensometers.
UTMs may also be adapted to various other test modes, including compression, shear, and torsion, depending on the desired material model. These more advanced modes are used to characterize complex behaviors such as anisotropy or multi-axial stress responses.
Test Speeds and Strain Rate Considerations
Most FEA assumes quasi-static, rate-independent material behavior. Therefore, testing should ideally be performed at constant strain rate, mimicking the theoretical basis of FEA solvers. However, most labs operate tests at constant crosshead speed (displacement), which approximates constant strain rate only for materials with small elongation.
For materials with large deformation (e.g., elastomers or soft plastics), the changing geometry means the strain rate is not truly constant throughout the test. Nevertheless, the rate dependency of many materials is weak within narrow speed ranges, making constant displacement tests sufficient for many purposes.
Still, for FEA simulation of applications involving impact, crash or drop, high-strain-rate testing becomes necessary. In such cases, rate effects become pronounced, and special high-speed test equipment and procedures are required.
Test Equipment and Instrumentation
Sensors, tools and fixtures used in tensile testing include load cells to determine applied stress as well as a number of possible implementations — strain gauges, contact and non-contact extensometers — to determine induced strain, and various grips and related fixturing to hold specimens in place.
Load Cells
- Used to measure force; must be properly sized and calibrated (ASTM E4).
- Over-sizing reduces resolution and sensitivity.
Strain Gauges
- Bonded to the specimen to measure local strain via changes in electrical resistance.
- Sensitive and accurate up to about 3% strain, but can detach during large deformation.
- Less commonly used today due to challenges with placement and interaction with soft materials.
Contact Extensometers
- Attach to the specimen’s gauge section using clips or bands.
- Provide clean, robust data, but can induce deformation or miss strain localization.
- Not ideal for weak materials or post-yield behavior.
Non-Contact Extensometers
- Use video or laser tracking of markers on the specimen.
- Avoid specimen interference; good for soft, thin, or delicate materials.
- Newer technologies include full-field 2D and 3D camera systems, which may be used in conjunction with digital image correlation (DIC) to visualize complex full-field strain data (e.g., necking, crack growth).
- These systems are expensive, generate large datasets, and require post-processing.
Grips and Fixtures
- Must firmly secure the specimen without causing grip-region failure.
- Materials and textures vary (e.g., diamond, sandpaper, rubber) to accommodate different sample types.
- Advanced grips include hydraulic or pneumatic systems for dynamic or cyclic testing.
Standards for Tensile Testing
Standardized test methods are vital for repeatability and benchmarking and often form excellent starting points for characterization in service of FEA. Standards of relevance for tensile testing include:
Metals
- Standards: ASTM E8, ISO 6892.
- Good for linear and nonlinear FEA up to yield, as well as post-yield hardening, but poorly capture necking (strain localization onset).
Plastics
- Standards: ASTM D638, ISO 527.
- Suitable for FEA but lacks sophistication for nonlinear elasticity, pre-yield plasticity and necking.
Rubbers
- Standards: ASTM D412, ISO 37.
- Good starting point but lacks guidance on Mullins effect or strain measurement techniques.
Composites
- Standards: ASTM D3039, ISO 527-4,5.
- Address anisotropy and shear, but testing is complex and difficult.
Thin Films
- Standards: ASTM D882, ISO 527-3.
- Difficult due to grip failures and strain measurement issues. Non-contact extensometry is often required.
Standards offer excellent starting points but must sometimes be modified to better reflect the complex behavior needed for simulation fidelity.
Specimen Preparation
The preparation of test specimens is critical and often underappreciated. Test samples must accurately reflect the material state and process conditions of the actual application. Differences in processing (e.g., molding method, thermal treatment) can drastically affect mechanical behavior.
Metals
- Bar stock: machined and polished to remove defects.
- Sheet metals: water-jet cut to avoid thermal effects; thickness and forming history must match final use.
Plastics
- Common methods: injection molding, machining from sheets.
- Injection molding is fast and reproducible, but induces flow-induced morphology (e.g., skin-core effects).
- Morphology varies with processing (e.g., blow molding, thermoforming), affecting final properties.
Composites
- Machined from sheet.
- Tabbing required to prevent grip-induced failures.
Rubbers and Films
- Typically punched from sheets.
- Films must avoid curling; require careful grip and strain measurement design.
Foams
- Rarely tested in tension; can be cut to shape when needed.
The closer the sample preparation process matches the actual product, the more accurate and relevant the data will be.
Testing for Environmental Effects
Materials often experience variable environmental conditions during service. These effects must be reflected in FEA material models.
Temperature Effects
- Metals generally exhibit temperature-insensitive properties across typical operating temperatures.
- Plastics, rubbers and composites exhibit properties that can vary dramatically across the typical lower and upper service limits (–40°C to 80°C).
- Proximity to or transition through critical temperatures such as glass transitions or melting points can lead to drastic changes in ductility or stiffness.
- Dynamic mechanical analysis (DMA) can identify temperature-sensitive transitions in a material.
Rate Effects
- Rate dependency becomes critical in impact, crash, or drop simulations.
- Most materials show little change until strain rate shifts by orders of magnitude.
- Rate-dependent material models require multiple stress-strain curves across different strain rates.
Moisture and Humidity
- Plastics, particularly hydrophilic types like Nylon, absorb moisture, altering stiffness and ductility.
- Moisture-conditioned testing is crucial to simulate in-service conditions.
- Standard methods include exposure to 50% RH for 40 hours or equilibrium conditioning via humidity chambers.
- Accelerated methods can speed up moisture uptake but may introduce stress relaxation artifacts.
Chemical Exposure and Aging
- Sunlight (UV), oil, gasoline, bodily fluids, etc., can alter material properties over time.
- Environmental exposure testing may require testing in situ (e.g., saline at 37°C for biomedical parts).
- Proper simulation of life-cycle performance requires use of data from aged, weathered or exposed materials.
Conclusion
Material testing for FEA is significantly more nuanced than standard quality control testing. While international standards form a strong foundation, FEA demands greater accuracy, environmental relevance, and deeper understanding of the material’s physical behavior.
Key Takeaways
- Tensile testing dominates mechanical data input for FEA but must be carefully executed.
- Instrumentation, test setup, and strain rate all influence data quality.
- Environmental factors such as temperature, moisture, and chemical exposure can cause significant property variation and must be accounted for.
- Specimen preparation must match real-world processes to ensure simulation relevance.
For meaningful and predictive FEA, testing must be tightly coupled to simulation goals. When performed correctly, this creates a virtuous loop between material data and virtual product design — reducing physical testing, accelerating development cycles, and increasing product reliability.
To explore the topics discussed on this page further, see Hubert Lobo (Founder, DatapointLabs) and Brian Croop (CEO, DatapointLabs), Determination and Use of Material Properties for Finite Element Analysis (NAFEMS, 2016), Ch.2.
