A recent customer question brings to light a frequent challenge in material fabrication:
“Do you think there is a high probability of circumferential cracks occurring in this material?”
This question arose after we flagged the potential for hairline circumferential cracking due to heavy wall thickness—a scenario that underscores the importance of balancing material properties with application requirements. Considering the potential impact, let’s explore what causes circumferential cracking, its likelihood under specific conditions, and how to reduce these risks.
What Causes Circumferential Hairline Cracking?
Grade G3 tubes are manufactured using glass fabric coated with a semi-cured phenolic resin. This process involves solvents—also known as volatiles—that are essential to the lamination process. During the oven curing phase, these solvents are released through the ends of the tube lamination.
For tubes with wall thicknesses greater than 0.50 inches, the situation becomes more complex. Gases trapped inside the tube during curing can become pressurized, leading to hairline cracks as they are forced out through the tube ends. These cracks, known as circumferential hairline cracks, should not be confused with delamination between laminations, which is a completely separate issue and typically a cause for material rejection.
Why the Disclaimer?
Our quotes include a standard disclaimer for all glass fabric tubes with a wall thickness to tube ID ratio greater than 0.25. In the case of the customer’s tube—measuring 2.00” OD x 0.634” ID—the ratio was 0.45, meaning it fell within the range where cracking might be theoretically possible. However, based on our experience manufacturing similar G3 tubes, we determined that the likelihood of hairline cracking for this specific size and grade of material was minimal.
For reference, the chart below provides a guide for assessing the risk of circumferential hairline cracking based on wall thickness and ID ratios:
| Wall Thickness to Tube ID Ratio | Risk of Circumferential Cracking |
| 0.25 or below | Low Risk |
| 0.25 to 0.50 | Moderate Risk |
| Above 0.50 | Elevated Risk |
Assessing the Impact on Functionality
To better understand how circumferential cracking might impact performance, we visited the customer’s facility to learn about their specific application. The tubes were being used to protect wires during the assembly of nearby metal components. This application involved no exposure to temperatures above 200°F and no electrical insulation requirements.
Given these conditions, we concluded that circumferential hairline cracking would not affect the functionality of the material for this particular use case. The cracks, while visible, do not compromise the tube’s structural integrity or its ability to protect the wires.
For reference, the guidelines below may apply if you are concerned about cracking. These cracks are likely to occur when one of the criteria listed here is exceeded:
| Tube Grades | Ratio between wall thickness and ID exceeds: | Wall thickness is bigger than: |
|---|---|---|
| Cotton Fabric Grades | 0.4 | 30 mm (1.181″) |
| Paper Grades | 0.4 | 12 mm (0.472″) |
| Glass Fabric Grades | 0.25 | 12 mm (0.472″) |
| Glass Silicone | 0.2 | 6 mm (0.256″) |
| Ratio is determined by wall thickness divided by tube ID. |
While circumferential hairline cracking might sound concerning, it is often a minor cosmetic issue rather than a functional defect. By understanding the manufacturing process, assessing risk factors, and considering the application’s specific requirements, it’s possible to make informed decisions about the suitability of these materials.
For questions about circumferential cracking or selecting the right material for your application, feel free to reach out to our team of material and machining experts. We’re here to help you navigate the complexities of composite materials with confidence.
Key Takeaways
- Circumferential Cracking is Rare but Possible: Hairline cracks are most likely to occur in tubes with a wall thickness greater than 0.50 inches due to trapped gases during curing.
- Not All Cracks Are Rejection-Worthy: It’s crucial to distinguish circumferential hairline cracks from delamination, which is a more serious issue.
- Functionality Matters Most: In many cases, these cracks do not impact the material’s performance in its intended application. For instance, in low-temperature, non-insulating applications, circumferential cracks are unlikely to cause issues.
- Experience Counts: Leveraging historical data and expertise in manufacturing similar products can provide a realistic assessment of cracking risks for specific sizes and grades.