How to prevent sample slippage during tensile testing of thick-walled PE pipes?

In tensile tests of PE (polyethylene) pipes, if the pipe wall is thick (e.g., ≥20 mm or even thicker), slippage or clamping damage can easily occur at the clamps. This is mainly due to the low elastic modulus of PE material, its smooth surface, and insufficient or uneven clamping force. Based on testing experience and equipment design, this can generally be addressed by considering the clamping method, clamp design, and clamp manufacturing process.

I. Key Points for Sample Clamping

1. Gradually Increase Clamping Force
    It is not recommended to use the maximum clamping force at once.

    It is recommended to clamp in two steps:

* Initial clamping to fix the sample;
* Preload (e.g., 50-100N) and then tighten the clamp again. This can avoid:
* Sudden slippage of the sample
* Uneven force on the clamp

2. Ensure the specimen is perfectly aligned. For medium-thickness specimens, misalignment will generate additional bending moment:

Consequences: Insufficient clamping force on one side; easier slippage.
Recommendation: Use self-aligning grips; or apply a slight preload after installation and then correct.

3. Keep the clamping area dry.

If the PE surface contains oil, coolant, water, etc., the coefficient of friction will decrease significantly.
Recommendation: Wipe with alcohol before clamping.

Fixture Structure Design Recommendations

For thick-walled PE specimens, the following fixture structures are recommended:

1. Wedge Grips

This is the most commonly used solution for universal testing machines.

Features: Greater tensile force → Greater clamping force
Automatic anti-slip
Key Design Points:
Wedge angle: 8°–12°
Surface: Serrated or diamond-coated surface
Advantages: Good anti-slip performance
Automatic pressure increase

Clamping Surface Design Recommendations (Critical)

2. Serrated Clamping Surface

1.Common Tooth Shapes:

V-shaped
Rhomboid

Parameter Recommendations:

Parameter Recommended Value

Tooth Depth 0.3–0.6 mm

Tooth Pitch 1–2 mm

Note:
Not too deep, otherwise it will damage the sample.

2. Emery Coating
Many imported equipment uses:
Tungsten carbide particle coating
Advantages:
High coefficient of friction
Wear resistance

3. Replaceable clamping blocks

Suggested design:
Modular clamping blocks

Advantages: Replaceable after wear; Adaptable to different samples

Fixture Manufacturing Process Recommendations

Fixture materials and machining are also crucial.

1. Fixture Materials
Commonly Used Materials:
42CrMo
H13
Cr12MoV
Heat Treatment Hardness:
HRC 50–55

3. Stiffness Design
For thick specimen tensile testing:
The fixture must have sufficient stiffness; otherwise:
The fixture will open up
The clamping force will decrease
Recommendation:
Fixture thickness ≥ 30 mm
High-strength structural steel