Cyclic Loading Testing of Materials
Note: For fatigue testing of materials, see the following section: Fatigue Testing of Materials.
Significance and Purpose
Cyclic Loading Testing evaluates how materials respond to repeated loading and unloading cycles, assessing their ability to withstand fatigue, plastic deformation, damage accumulation, and stress-strain behavior over time. It is crucial in industries where materials experience dynamic loads, such as automotive, aerospace, biomedical, and structural engineering.
This testing method helps in:
- Assessing Onset of Plasticity: Predicting failure under repetitive loading.
- Characterizing Material Hardening/Softening: Understanding cyclic plasticity and stress-strain evolution.
- Quantifying Mullins Effect in Elastomers: Evaluating the softening behavior of rubbers under cyclic loading.
- Analyzing Plastic Damage Accumulation: Understanding stress-strain evolution and fracture in thermoplastics, metals, and composites.
- Studying Hysteresis and Energy Dissipation: Important for damping materials and elastomers in dynamic applications.
Relevant ASTM, ISO, and DatapointLabs (DPL) Standards
For General Materials
- DPL M-035: Plastic Point in Tension.
- DPL M-036: Plastic Point in Flexure.
- DPL M-037: Plastic Point in Compression.
The plastic point provides a more accurate determination of the strain level in polymeric materials denoting the onset of irrecoverable plastic strain. For a technical description of the significance, theory, and test procedure regarding the determination of the plastic point, see Hubert Lobo & Juan A. Hurtado, “Characterization and Modeling of Non-linear Behavior of Plastics”.
For Elastomers
- ASTM D412: Standard Test Methods for Vulcanized Rubber and Thermoplastic Elastomers — Tension.
- ASTM D638: Standard Test Method for Tensile Properties of Plastics.
- ISO 527-1: Determination of tensile properties of plastics.
- Unspecified Standard: For Dynamic Hysteresis Loop testing.
For Plastics
- ASTM D638, ASTM E8, ISO 527-1: For Cyclic Damage of Plastics.
For Plastics and Metals
- Unspecified Standard: For Cyclic Hardening Stress-Strain characterization.
DatapointLabs Tests for Cyclic Loading Testing
Tests in the DatapointLabs test catalog that reference cyclic loading testing are as follows:
General Cyclic Loading Testing (inquire regarding material suitability)
| Test ID | Test Description | Standards |
|---|---|---|
| M-035 | Plastic Point in Tension | DPL M-035* |
| M-037 | Plastic Point In Compression | DPL M-037* |
| M-036 | Plastic Point in Flexure | DPL M-036* |
* Internal DatapointLabs Standard
Cyclic Loading Testing Specific to Elastomers
| Test ID | Test Description | Standards |
|---|---|---|
| M-621 | Cyclic Mullins Effect | ASTM D412, ASTM D638, ISO 527-1 |
| M-620 | Dynamic Hysteresis Loop | Unspecified Standard |
Cyclic Loading Testing Specific to Plastics
| Test ID | Test Description | Standards |
|---|---|---|
| M-623 | Cyclic Damage of Plastics | ASTM D638, ASTM E8, ISO 527-1 |
Cyclic Loading Testing Specific to Plastics, Metals:
| Test ID | Test Description | Standards |
|---|---|---|
| M-622 | Cyclic Hardening Stress-Strain | Unspecified Standard |
Principle of Operation
Cyclic loading testing involves repeatedly applying a load to a sample while measuring its stress-strain response over time. The test typically follows these steps:
- Sample Preparation:
- Specimens are prepared according to standard geometries (dogbone, cylindrical, rectangular) and specific material requirements.
- Instrument Setup:
- A servo-hydraulic or electromechanical test frame is used to apply controlled cyclic loads.
- Extensometers measure strain, while load cells measure force.
- Test Execution:
- The sample is subjected to tension, compression, or flexural cyclic loading at controlled amplitudes and frequencies.
- The test can be strain-controlled or stress-controlled, depending on material behavior and standard requirements.
- Data Collection & Analysis:
- True Yield Strain and Stress (Plastic Point Testing).
- Cyclic Stress-Strain Behavior (fatigue, hardening, damage accumulation).
- Hysteresis Loop and Energy Dissipation (for elastomers).
- Damage Evolution (for plastics and metals).
Typical Procedure
- Set Initial Load Conditions:
- Define the loading waveform (e.g., sinusoidal, triangular, sawtooth).
- Choose frequency and amplitude based on expected material performance.
- Perform Cyclic Loading:
- Apply repeated loading-unloading cycles under controlled stress or strain.
- Monitor stress-strain evolution, damage progression, and hysteresis behavior.
- Assess Material Behavior:
- Track changes in modulus, yield point, and maximum plastic strain.
- Analyze cyclic hardening/softening, Mullins effect (for elastomers), and fatigue damage accumulation.
- Evaluate Damage Mechanisms:
- Identify plastic deformation, microcracking, and residual strain.
- Calculate damage vs. plastic strain relationships.
Specimen Types
Specimens used by DatapointLabs in various types of cyclic loading testing are as follows:
| Specimen Type | DatapointLabs Test IDs |
|---|---|
| Tensile Bars [Details] | M-035, M-621, M-623 |
| Prisms [Details] | M-037 |
| Flex Bars [Details] | M-036 |
| Contact Us [Details] | M-620, M-622 |
Extensometry Techniques
Extensometry techniques typically employed by DatapointLabs in various types of cyclic loading testing are as follows:
| Extensometry Technique | DatapointLabs Test IDs |
|---|---|
| Contact Extensometry (Axial) | M-035 |
| Crosshead Displacement (Axial) | M-037, M-036 |
| Non-Contact Extensometry (Axial) | M-621, M-620, M-623, M-622 |
Characterization Measurements
For Plastic Point Testing (DPL M-035, M-036, M-037)
- True Yield Strain: Strain at the onset of plastic deformation (plastic point); see note regarding the plastic point under the ‘Standards’ section above.
- True Yield Stress: Stress at the plastic point in tension, compression, or flexure.
For Cyclic Mullins Effect (ASTM D412, ASTM D638, ISO 527-1) & Dynamic Hysteresis Loop
- Cyclic Stress-Strain Curves: Represents stress-strain response during multiple loading cycles. Used to evaluate material softening/hardening under cyclic loads.
For Cyclic Damage of Plastics (ASTM D638, ASTM E8, ISO 527-1)
- Offset Yield Strain in Tension: Strain at a specified offset stress (typically 0.2%).
- Offset Yield Stress in Tension: Stress corresponding to offset yield strain.
- Tensile Modulus: Initial slope of the stress-strain curve.
- Tensile Strain at Yield: Strain at the yield point.
- Tensile Strength at Yield: Maximum stress before plastic deformation.
- Damage vs. Plastic Strain: Monitors accumulation of permanent deformation.
- Engineering Tensile Stress-Strain Curves: Tracks cyclic variations in mechanical response.
- Maximum Plastic Strain: The highest strain recorded before failure.
- Unload Modulus: Slope of the stress-strain curve during unloading.
For Cyclic Hardening Stress-Strain (Plastics and Metals)
- Cyclic Stress-Strain Curves: Represents stress-strain response during multiple loading cycles. Shows material kinematic hardening behavior under cyclic loading due to Bauschinger effect.
Typical Data Reported (see test descriptions for exact details)
- True Yield Stress & Strain (Tension, Compression, Flexure): As determined from the plastic point.
- Cyclic Stress-Strain Curves: Stress vs. strain over multiple loading cycles.
- Tensile Modulus & Strength at Yield: As determined from the plastic point.
- First Cycle vs. Stabilized Cyclic Behavior: Initial vs. longer-term response to loading.
- Hysteresis Energy Dissipation: As determined by dynamic hysteresis loop testing.
- Damage vs. Plastic Strain: Damage curve derived from the slopes of the loading and unloading curves.
- Cyclic Hardening Trends: Kinematic hardening behavior under cyclic loading.
Suitable Material Types
- Plastics (Thermoplastics & Thermosets): Structural composites, high-performance polymers.
- Elastomers & Rubbers: Automotive tires, seals, medical elastomers.
- Foams & Cellular Materials: Cushioning applications.
- Biomaterials: Artificial joints, prosthetics.
- Metals & Alloys: Aerospace and automotive components.
Suitable Applications
- Plasticity Onset and Durability Studies: Structural materials for aerospace, automotive.
- Elastomer Performance Analysis: Predicting long-term use behavior.
- Plastics Processing Optimization: Enhancing polymer mechanical stability.
- Biomedical Device Testing: Evaluating the mechanical performance of implants and prosthetics.
- Energy Absorption Studies: Damping materials in vibration-sensitive applications.
- Kinematic Hardening Studies: Informing CAE material models for kinematic hardening behavior.
Conclusion
Cyclic loading testing, as outlined in ASTM D412, ASTM D638, ISO 527-1 as well as DatapointLabs DPL M-035, DPL M-036, DPL M-037 standards, is essential for evaluating the mechanical performance of materials under repeated stress. It provides insights into plastic deformation and true yield (plastic point), fatigue, hardening/softening, and damage accumulation, making it invaluable in automotive, aerospace, and biomedical applications. By assessing stress-strain evolution, hysteresis, and fatigue properties, this technique enables better material selection, processing, and failure prevention strategies.
