Understanding Property Differences Between Mil-I-24768 Tube and Sheet Specification
When selecting materials for industrial applications, it’s essential to consider how the shape of a material impacts its properties. While sheets and tubes made from the same material grade (e.g., G-10 epoxy) might appear to share similar characteristics, they are tested differently, resulting in property variations. This post explains the key differences between Mil-I-24768 tube and sheet specifications, with a focus on how these differences might impact a customer’s choice of material for applications like bolt insulation.
Key Differences Between Sheets and Tubes
Test Procedures for Different Shapes
Material properties can vary significantly based on whether the material is in tube or sheet form due to the differences in test procedures. Here are some specifics:
Tensile Testing:
Sheets: Tensile strength for sheets is tested across the grain (transverse direction) and along the grain (longitudinal direction). Since sheets are pressed during manufacturing, they have a defined grain or fiber orientation.
Tubes: In the case of convolute-wound tubes, the tensile test is conducted axially, meaning along the length of the tube. This difference in testing direction can lead to variations in reported tensile strength.
Compressive Testing:
Sheets: Compression tests for sheets are performed perpendicular to the laminations, which provides insight into how the sheet performs under pressure across its layers.
Tubes: For tubes, compressive tests are conducted axially, i.e., in the direction of the tube’s length. This can result in different values compared to sheets because of the shape and structural arrangement of the material in tube form.
Fewer Specifications for Tubes Compared to Sheets
The standards and specifications (e.g., NEMA or Mil-I-24768) for tubes are more limited compared to sheets. Sheets have been widely tested and certified under many different conditions, which makes finding detailed and precise engineering data easier. Tubes, however, have fewer formal specifications, making it critical to rely on specific manufacturer data or custom testing for more specialized applications.
Pressure During Manufacturing Impacts Strength
Sheets: Sheets are produced under high pressure, which helps to align the fibers and increase mechanical strength. The pressing process also removes excess resin and creates a denser, stronger material.
Tubes: Tubes, on the other hand, are formed by winding the material around a mandrel and applying much lower pressures compared to sheets. As a result, tubes tend to have lower mechanical properties (like tensile and compressive strength) than sheets of the same material grade. However, this does not mean tubes are inherently weaker; their specific geometry often compensates for these differences in strength.
For example, the burst strength of a tube might be higher than the bond strength of a laminated sheet of the same material, depending on the application.
Impact of B-Stage Materials
In some cases, manufacturers use a B-stage (partially cured) material intended for tube production to make sheets. However, these sheets would be pressed at lower pressures to accommodate the high resin flow rate typical of B-stage materials. This results in lower mechanical properties compared to fully pressed sheets made specifically for high-pressure environments.
Additional Engineering Data in NEMA Standards
The NEMA book provides additional engineering data for all material grades, including values that aren’t required for material certification. These are average values collected from manufacturers over time, but many of these manufacturers are no longer in business. Therefore, while the data can serve as a useful reference, it may not always be completely up-to-date or fully applicable to modern manufacturing techniques.
Understanding the differences between tubes and sheets made from the same material grade, such as G-10 epoxy, is critical when selecting materials for specific applications. The key factors influencing these differences include test direction, manufacturing processes, and the geometry of the material. In some applications, like bolt insulation, tubes may offer advantages in terms of burst strength and insulation capability, even though they might have lower tensile and compressive strength compared to sheets.
By carefully evaluating these property differences, engineers and sourcing professionals can make informed decisions when choosing between sheets and tubes for their applications.