Why does a cable’s strain relief crack in flex testing?

Main answer · Wecent Editorial Team

Short answer: Strain relief cracks when the material is too hard or brittle, the shape ends abruptly, the moulding left stress in the part or the test is run cold. The fix is usually a softer or tougher compound, a longer tapered shape and better moulding control. Test small changes quickly on samples before you change the production tool.

What the strain relief does

The strain relief is the flexible part where the cable meets the connector. It spreads bending over a longer length, so the cable does not kink sharply at the plug, where wires would otherwise break.

Common causes of cracking

• Material too hard. A stiff compound cracks when bent repeatedly.
• Brittle recycled or filled material. Fillers and regrind reduce flexibility.
• Abrupt shape. A short, thick or blunt strain relief concentrates bending at its end.
• Thin walls or sharp corners. They create points where cracks start.
• Moulding stress. Wrong temperature or cooling leaves internal stress.
• Poor bonding. If the strain relief does not bond to the cable jacket, it moves and tears.
• Cold or ageing. Cold conditions and long ageing make materials stiffer.

How to improve it

1. Choose a softer, tougher compound, and check its flex-fatigue data.
2. Lengthen the strain relief and give it a gradual taper.
3. Round off corners and avoid thin sections.
4. Control moulding temperature, pressure and cooling.
5. Improve bonding between the strain relief and the jacket.
6. Limit regrind or ban it for this part.

Testing

• Use a repeatable flex test with a stated angle, weight and number of cycles.
• Test at room temperature and, if relevant, at cold.
• Check electrical continuity during the test, not only the appearance.
• Cut open failed samples to see where wires broke.

What to record

The compound, hardness, dimensions, process settings and result for each change, so you can see which change made the difference.

Quick summary

Use a tougher compound, a longer taper, good moulding and a test that matches real use.

Example specification wording

“The strain relief shall survive the stated number of flex cycles at the stated angle and weight, with no cracking or exposed conductor and with electrical continuity maintained throughout, and shall be tested on production-tool samples.” Clear wording like this avoids disputes about whether a sample has passed.

Quick checks on incoming parts

Bend a few strain reliefs by hand at room temperature and in a cold room. If they whiten or crack on a simple hand bend, they will not survive a real flex test. This check takes a minute and filters out poor lots before they reach the line.

Where this answer says to confirm with Wecent, the detail is not published on the product page and has to come from the datasheet or the factory. General facts come from the regulators, standards bodies or device makers named in the text.