ASTM D395
Rubber Compression Set

 ASTM D395 - Standard Test Methods for Rubber Property—Compression Set - PDF

Method A, B, and C Explained with Fixture Requirements and Test Setup

ASTM D395 is the standard test method used to determine the compression set of rubber materials. This test evaluates how well a rubber sample returns to its original thickness after being compressed for a fixed period under specific environmental conditions.

Compression set is an essential property for rubber materials used in gaskets, seals, O-rings, and other applications where the ability to maintain shape under load is critical.

ASTM D395 defines three methods:

• Method A – Compression set under constant force in air

• Method B – Compression set under constant deflection in air

• Method C – Compression set under constant deflection in air using a compression device (non-standardized, but often used in product testing or long-term research)

What is Compression Set?

Compression set measures the permanent deformation a rubber material retains after being compressed and allowed to recover. It's expressed as a percentage of the original deflection. Lower values indicate better elastic recovery.

Compression Set (%) is calculated as:
CS = [(t₀ - t₁) / (t₀ - tₛ)] × 100
Where:

• t₀ = original specimen thickness

• t₁ = final specimen thickness after recovery

• tₛ = spacer thickness (only applicable for Method B & C)

Method A – Constant Force in Air

Purpose: Simulates conditions where a rubber part is compressed by a constant force (spring or bolt pressure).

Specimen:

• One cylindrical test piece, typically 12.5 mm thick and 29 mm in diameter

• Thickness tolerance: ±0.5 mm

Test Setup:

• A compression fixture with two metal plates and a loading bolt/nut arrangement.

• The rubber specimen is compressed between the plates using a measured force, usually applied via calibrated weights or a universal testing machine with a force control function.

Procedure:

1. Measure initial thickness (t₀).

2. Apply constant force (e.g., 1.8 kN) using compression fixture.

3. Place the assembly in an oven at 70°C to 100°C for 22 to 70 hours.

4. After aging, remove and allow the sample to recover at room temperature for 30 minutes.

5. Measure final thickness (t₁) and calculate compression set.

Notes: This method is less commonly used than Method B, as constant-force setups are harder to maintain uniformly.

Method B – Constant Deflection in Air

Purpose: Most commonly used to evaluate sealing materials (O-rings, gaskets) under long-term load.

Specimen:

• Same as Method A (12.5 mm thick × 29 mm diameter)

Test Setup:

• A compression set fixture designed to apply a constant deflection, typically 25% of the specimen’s original thickness.

• Spacers (typically steel) ensure consistent deflection.

• The fixture must maintain deflection during heat exposure.

Procedure:

1. Measure initial thickness (t₀).

2. Insert the specimen between plates with spacers to achieve a 25% compression.

3. Place in an oven at the required test temperature for 22 to 70 hours.

4. Remove and allow the specimen to cool for 30 minutes.

5. Remove from the fixture and measure the final thickness (t₁).

6. Use the formula to calculate compression set.

Fixtures:
These fixtures typically include:

• Two flat plates (top and bottom)

• Threaded bolts and nuts

• Precision spacer bars or pins

The fixture must resist warping during heating, so stainless steel or hardened aluminum is ideal.

Method C – Custom Deflection in Air (Non-Standard Setup)

Purpose: Used in R&D and product testing where part geometry doesn’t allow standard specimens.

Specimen:

• Custom shape or real product component (e.g., actual O-ring or gasket section)

Setup:

• Test conducted using non-standardized fixtures, often designed to mimic the actual application.

• Can use precision jigs or devices that simulate the installed condition of a component.

Procedure:
Follows the same general steps as Method B, except the fixture and compression amount are specific to the real-world application. Recovery and measurement protocols remain similar.

Important: Method C is not part of the formal ASTM D395 standard but is referenced in practice for product-specific compression set evaluations.

Summary Table

MethodCompression TypeSpecimenFixture RequirementTypical Use

AConstant ForceCylindrical discLoad frame or fixture with weightsSpring-loaded gaskets

BConstant DeflectionCylindrical discPrecision spacer fixtureSealing applications (O-rings)

CConstant DeflectionCustom shapeApplication-specific setupR&D, product simulation

Final Thoughts

When performing ASTM D395 testing, accuracy in fixture setup and environmental control are essential.
Whether you're certifying material performance for aerospace seals or testing industrial O-rings, a properly configured compression set test gives you the data you need to evaluate long-term elasticity and durability.

If you need help sourcing or customizing a compression set fixture for ASTM D395 testing, we offer ready-to-use kits and custom setups that are compatible with most lab ovens and testing environments.