Physical Address
304 North Cardinal St.
Dorchester Center, MA 02124
D6115-97 – Standard Test Method Technical Guide
📐 Specimen Geometry and Material Scope
ASTM D6115‑97 (Reapproved 2019) specifies the use of the standard Double Cantilever Beam (DCB) configuration, identical to that shown in Fig. 1. This specimen features an embedded non‑adhesive insert film at the mid‑plane to serve as a pre‑existing flaw. The test determines the number of fatigue cycles (N) required for the onset of delamination growth based on the opening Mode I cyclic strain energy release rate (G).
Per Section 1.2, the method is strictly limited to unidirectional carbon fiber tape laminates with single‑phase polymer matrices. This limitation reflects the dataset and experience gained during the interlaboratory round‑robin testing program. Applying the method to other material systems—such as woven fabrics or toughened matrices—may introduce interferences that violate the pure Mode I fatigue onset assumptions. Users exploring such systems are directed to the interference discussion in Section 6.5 of Test Method D5528.
⚙️ Test Procedure and Parameter Selection
The core procedure subjects the DCB specimen to constant amplitude, tension‑tension fatigue loading. By testing multiple specimens at various cyclic G‑levels, a G–N curve (illustrated in Fig. 2) is generated. The onset of delamination growth is detected visually (e.g., using a traveling microscope) or through a defined change in specimen compliance. All loads and displacements must be recorded to accurately calculate the applied strain energy release rate.
The selection of the stress ratio (R), waveform, and test frequency is critical to obtaining valid data. Force verification of the test system must comply with Practices E4 and E467 to ensure dynamic accuracy throughout the test.
| 🟦 Fatigue Test Parameter | 📏 Standard Requirement / Recommendation |
|---|---|
| Loading Mode | Mode I – Constant Amplitude Tension‑Tension |
| Primary Output | Cycles to Onset (N) at a specified Gmax |
| Stress Ratio (R = Pmin / Pmax) | 0.1 (typical starting point) |
| Load Waveform | Sinusoidal |
| Frequency Range | 1 Hz – 10 Hz (must avoid hysteretic heating) |
| Onset Criterion | Visual crack growth or compliance change threshold |
| 🎯 Applied Gmax Level (% of GIC) | ⚡ Typical Cycles to Onset (N) |
|---|---|
| 90 – 100 % | Quasi‑static failure / Very Low Cycle (N < 10²) |
| 70 – 90 % | Low Cycle Fatigue (10² < N < 10⁴) |
| 50 – 70 % | Intermediate Cycle Fatigue (10⁴ < N < 10⁵) |
| 30 – 50 % | High Cycle Fatigue (10⁵ < N < 10⁶) |
| < 30 % | Run‑out / No Onset (N > 10⁶) |
📊 Key Measured Properties and Data Analysis
The fundamental data generated by this method are paired (G, N) values describing the cyclic strain energy release rate and the corresponding cycles to delamination onset. The statistical analysis of these fatigue data should follow the procedures outlined in Practice E739 for linearized stress‑life (S‑N) data. It is common to plot the G–N relationship on a semi‑log or log‑log scale to develop design curves and thresholds.
Calculation of the applied G follows the Modified Beam Theory (MBT) or Compliance Calibration methods adapted directly from Test Method D5528. The static Mode I interlaminar fracture toughness (GIC) should be reported alongside the fatigue data, as the applied cyclic G levels are typically selected as a percentage of this baseline property.
💡 Experimental Best Practice: To obtain a sharp, natural crack tip before beginning the fatigue test, perform a static precrack in displacement control per the procedures in D5528. This creates a short increment of stable crack growth from the insert film and provides more conservative, repeatable onset data by eliminating the effect of the blunt starter film.
⚠️ Critical Interference Warning: Fiber bridging across the crack wake is a known interference in the DCB test. While it provides apparent toughening under static loading, it complicates the interpretation of fatigue onset and can artificially inflate the number of cycles to onset. Strict monitoring of the actual crack tip (not just the visible end of the bridging zone) is required.
❓ Frequently Asked Questions
🔍 What is the G–N curve and how is it used?
The G–N curve plots the applied cyclic Mode I strain energy release rate (typically Gmax or ΔG) versus the number of cycles to delamination onset (N). It serves as a critical design tool for fatigue‑loaded composite structures, directly analogous to the S‑N curve used in metal fatigue, and quantifies the material’s resistance to delamination initiation under cyclic loads.
💡 How does D6115 differ from a delamination growth rate test?
D6115 specifically measures the onset of delamination growth from an existing flaw or insert film. It does not measure the propagation rate (da/d