ASTM D6115 describes a standard test method for the number of cycles (N) required to start delamination growth in unidirectional fiber-reinforced polymer matrix composites. The method uses the Double Cantilever Beam (DCB) specimen and applies constant-amplitude tension-tension fatigue loading to measure the opening-mode I cyclic strain energy release rate (G). When technicians test multiple specimens at various G-levels, they develop a G-N curve showing the relationship between cyclic strain energy release rate and the number of cycles to delamination-growth onset. The standard applies to composites made of unidirectional carbon fiber tape laminates with single-phase polymer matrices, consistent with experience from the round-robin testing.
ASTM D6115 Standard Test Method for Mode I Fatigue Delamination Growth Onset of Unidirectional Fiber-Reinforced Polymer Matrix Composites
What is ASTM D6115?
What is the Scope of ASTM D6115 Standard?
ASTM D6115 specifies the number of cycles required for the onset of delamination growth under opening-mode I cyclic loading of a DCB specimen. This standard-
- Calculates the number of cycles required for the onset of delamination growth.
- Uses the opening-mode I cyclic strain energy release rate (G).
- Uses a Double Cantilever Beam specimen.
- Imparts constant-amplitude, tension-tension fatigue loading.
- Applies to continuous fiber-reinforced composite materials in the scope.
- Specifically applies to unidirectional carbon-fiber tape laminates using single-phase polymer matrices.
- Allows results from multiple G levels to be presented as a G-N curve.
What are the Uses of ASTM D6115 Testing?
ASTM D6115 testing helps laboratories assess composite laminate resistance to the onset of delamination growth under fatigue loading. The results help with material selection, product development, and composite durability analysis. This standard-
- Supports a comparison of the relative G-N behavior of various composite materials.
- Assists in assessing processing and environmental impacts.
- Assists in the assessment of surface treatment of the fibers and variations in the fiber-volume-fraction at local levels.
- Supports development of criteria for onset of delamination growth avoidance.
- Provides information for composite damage-tolerance and durability analyses.
What Materials Can Be Tested Under ASTM D6115?
ASTM D6115 applies to unidirectional carbon-fiber tape laminates with a polymer matrix of one phase. ASTM indicates that the method applies to other types of composites; however, some interferences have been observed for materials outside the method’s scope.
Why is ASTM D6115 Important?
Delamination is an important failure mode for many laminated composite structures, and ASTM D6115 is important. Understanding delamination initiation under cyclic loading can aid engineers in understanding the behavior of composite materials when exposed to repeated fatigue stresses. The comparative relationship between cyclic strain energy release rate and cycles to delamination-growth onset offers additional information to aid in the selection of composite structure materials and damage-tolerance and durability evaluations.
ASTM D6115 Equipment and Sample Preparation Guide
ASTM D6115 uses the Double Cantilever Beam (DCB) specimen described in ASTM D5528. The specimen includes loading hardware, including piano hinges, and the fatigue test system applies controlled cyclic displacement.
| Sample and Specimen Details | Technicians use unidirectional fiber-reinforced composite specimens. The Double Cantilever Beam (DCB) specimen is the required test specimen configuration |
| Specimen Preparation | Technicians prepare DCB specimens from composite laminates with a pre-crack inserted at one end. They attach piano hinges to the specimen ends for load introduction |
| Specimen Dimensions | Typical specimen dimensions are about 5 inches (127 mm) long, up to 1 inch (25.4 mm) wide, and 0.118 to 0.197 inches (3 to 5 mm) thick. |
| Instrumentation | Technicians use a universal testing machine capable of applying constant-amplitude, tension-tension fatigue loading, a DCB test fixture, extensometers or displacement gauges for crack opening measurement, and a data acquisition system for recording load-displacement data |
Testing Procedures and Requirements for ASTM D6115
The technician mounts the DCB specimen to the fatigue test fixture and runs the fatigue test at a predetermined minimum and maximum displacement level and selected test frequency. The displacement ratio equals the R-ratio for linear-elastic behavior and small deflections. The researchers then subject the specimen to cyclic loading in the lab and record the number of cycles at which delamination growth onset.
| Specimen Preparation and Measurement | Technicians prepare DCB specimens with a pre-crack and attach piano hinges. They measure specimen dimensions and verify the pre-crack length. |
| Specimen Mounting and Fatigue Loading | Technicians mount the DCB specimen in the testing machine using the piano hinges. They apply constant-amplitude, tension-tension fatigue loading at a specified frequency and stress ratio. |
| Crack Monitoring | Technicians monitor crack growth and record the number of cycles (N) at the onset of delamination growth. |
| Data Analysis | Technicians calculate the mode I cyclic strain energy release rate (G) from the load-displacement data and determine the G–N relationship. |
| Result Reporting | Laboratories report the G–N curve or the number of cycles to delamination onset at specific G-levels in SI units as the standard. |
ASTM D6115 Testing Process and Data Collection
Technicians prepare the DCB specimen and mount it in the fatigue testing machine. The machine oscillates the specimen between the minimum and maximum displacement at the selected oscillation frequency. The laboratory monitors delamination development and records the number of cycles at which growth begins. The analyst selects the appropriate cyclic strain energy release rate and relates it to the recorded number of cycles. When laboratories test several specimens at different G levels, they develop a G-N curve for the material.
Common Challenges and Troubleshooting
Accurate results require appropriate DCB specimen preparation, correct fixture installation, stable cyclic loading, and reliable identification of delamination-growth onset. Technicians maintain consistent test frequency, displacement conditions, and specimen geometry since these factors affect the response to fatigue.
ASTM D6115 Analysis Results and Interpretation
The laboratory reports the number of cycles (N) at the onset of delamination growth and the cyclic strain energy release rate (G) for the opening mode (I).
- The technician records the cyclic loading conditions and identifies the number of cycles at which delamination growth begins.
- The analyst calculates the mode I cyclic strain energy release rate using the measured specimen response and geometry.
- The laboratory relates the measured strain energy release rate to the number of cycles required for delamination-growth onset.
- The engineer uses G-N results to compare the relative resistance to fatigue delamination growth of the specimens or materials.
- Engineers use the resulting G-N relationship to support material selection, product development, and damage-tolerance and durability analyses.
Get Certified ASTM D6115 Testing for Reliable Composite Analysis
ASTM D6115 offers a controlled approach to determine the resistance to initiation of delamination of laminated composite materials under fatigue loading. Fatigue resistance to interlaminar fracture is a critical issue in many advanced laminated composite structures, and understanding interlaminar fracture resistance under fatigue loading supports material selection and product development. G-N relationship is used for developing damage-tolerance design allowables as well. The method is used to compare composite materials and to study the effects of fiber surface treatment, fiber volume fraction, process variables, and environmental conditions.
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