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SAE J1638-2015: Measuring Compression Set in Thermoset Elastomeric Hoses
Stabilized Notice: SAE J1638-2015 has been declared stabilized by the SAE Non-Hydraulic Hose Committee. The underlying technology is mature and the document is no longer subjected to periodic reviews. Users are responsible for verifying referenced documents and continued suitability of the technical requirements.
Purpose and Scope
Compression set is a critical property for thermoset elastomeric hoses used in automotive and industrial applications. Under securing clamps, the hose wall is subjected to sustained compression. Over time, the material may fail to recover, leading to a loss of clamping force and eventual leakage of fluids or gases. SAE J1638-2015 provides a standardized test method to measure the compression set of a hose material (or a solid disc cut from a generic elastomer) and thereby predict sealing performance.
The test reproduces the type of indentation that a clamp imparts to the hose wall. By measuring how much the material recovers after a defined period of compression and thermal aging, engineers can assess whether the hose is likely to maintain a tight seal in service. This recommended practice is applicable specifically to thermoset elastomers, not thermoplastic materials.
Test Procedure and Default Parameters
🛠️ The test requires a compression fixture (as shown in Figures 1–4 of the standard), a dial micrometer conforming to ASTM D3767 Method A1, feeler gages or gage blocks, and an air-circulating oven meeting ASTM E145 Type IIA.
Sample Preparation: Using a suitable die, stamp a 13 mm ± 0.5 mm diameter disc from the wall section of the hose or test material (three samples per test). If the hose cover has an irregular surface, it must be lightly buffed smooth prior to cutting.
Test Steps:
- Measure the initial thickness of each sample (A).
- Calculate the required compressed thickness (B) to achieve the specified deflection (typically 25–50%).
- Set the indentors in the compression fixture using feeler gages or gage blocks.
- Insert the samples under the preset indentors and tighten the fixture.
- Place the fixture in the oven at the specified temperature for the required time.
- Remove the fixture and allow it to cool for 3.0 ± 0.25 hours.
- Remove each sample and measure its recovered thickness (C) after a precise recovery time (±0.1 h).
If no test parameters are specified by the user, the standard provides the following defaults:
| Parameter | Default Value |
|---|---|
| Test Temperature | 125 °C |
| Test Duration | 70 hours |
| Compression Deflection | 25% |
| Sample Diameter | 13 mm ± 0.5 mm |
| Cooling/Recovery Time | 3 hours ± 0.25 hours |
Evaluation: The compression set percentage is calculated using the formula:
% Compression Set = (A – C) / (A – B) × 100
where A = initial sample thickness, B = compressed thickness before aging, and C = compressed thickness after aging and recovery.
Engineering Insights and Frequently Asked Questions
Engineering Design Insight: The test is designed to simulate real-world clamp conditions. The 25% deflection represents a typical compression from a tightened clamp. However, for hoses that will experience higher or lower compression, the deflection may be adjusted within the 25–50% range. Surface preparation is critical—any irregularity on the cover can lead to erroneous thickness readings and poor repeatability. Always buff the test area if the hose cover has a fabric impression, ribs, or other texture. Additionally, the recovery time and temperature control are crucial: the standard mandates a specific cooling cycle and a recovery measurement window to ensure consistent results.
⚠️ Common Mistakes to Avoid:
- Using the wrong die size for sample cutting.
- Failing to buff irregular surfaces before measurement.
- Incorrectly setting the compression distance (e.g., not using feeler gages or gage blocks).
- Allowing insufficient recovery time after aging.
- Using an oven that does not meet ASTM E145 Type IIA requirements.
- Misapplying the formula (e.g., confusing A, B, or C).
- Assuming the test applies to all elastomers (it is only for thermoset materials).
Frequently Asked Questions
Q1: Why is compression set important for hose performance?
Excessive compression set reduces the ability of the hose wall to maintain a tight seal under a clamp, leading to fluid or gas leakage. This test provides a quantitative measure to predict such failures.
Q2: What are the default test conditions if I don’t have a specific requirement?
The standard recommends 70 hours at 125 °C with a 25% deflection. These conditions are suitable for many typical thermoset elastomeric hose applications.
Q3: How is the compression set percentage formula applied?
Measure the initial thickness (A), calculate the compressed thickness (B) from the desired deflection, and measure the final recovered thickness (C). Plug these values into the equation: (A – C) / (A – B) × 100.
Q4: Can this test be used for solid rubber discs not cut from a hose?
Yes. The standard explicitly covers “solid discs” as well. The same procedure applies; simply stamp the disc from a sheet of the thermoset elastomer of interest.
