Key takeaways
- A cross-section is the only routine check that shows voids and strand damage inside the barrel.
- Cut strands usually come from the die, the strip length or the insertion depth, not from the operator alone.
- The bellmouth relieves the stress concentration at the conductor entry and should be visible and even.
- Acceptance limits belong in the drawing or the control plan, with a photograph of a good and a bad section.
What a cross-section shows
A pull test tells you whether the joint held. A cross-section tells you why. It shows the barrel wall, the strand bundle, the voids between strands, the compaction at the crimp and the shape of the bellmouth. It is the only routine check that opens the joint without destroying the evidence.
The section is also the best way to see strand damage. A cut strand can sit inside a crimp that passes a pull test, because the remaining strands carry the load. The joint may then fail in vibration or after a few thermal cycles, when the damaged strand starts a crack.
A section is a sample, not a guarantee for the whole lot. It shows one plane through one joint. Use it with the crimp height, the pull test and the visual check, and take it at a frequency that reflects the criticality of the part.
Preparing a section that can be read
Cut the joint through the middle of the crimp, perpendicular to the barrel axis. A cut that is off-centre or angled shows a distorted section and can be mistaken for a void. Mark the cut line on the sample before sectioning so that the plane is repeatable.
Mount the sample in a resin that fills the voids, then grind and polish in stages. A coarse final polish pulls metal across the voids and hides them. Etch only if the grain structure matters; for routine acceptance, a polished section is usually enough.
Keep the magnification and the lighting the same for every sample. A photograph at one magnification cannot be compared with a section viewed at another. Add a scale bar and label the sample with the die, the crimp height and the lot.
Strand damage: what it looks like and where it comes from
Cut strands show a flat or sheared end inside the barrel. They usually come from a die with a sharp edge, a die that is too small for the barrel, or a conductor that was forced into a barrel that is too tight. A burr at the bellmouth can also cut strands as the conductor is inserted.
Crushed strands are flattened or deformed but not separated. They can come from over-crimp or from a conductor that is not fully inserted, so the die closes on the strand bundle at the wrong point. Displaced strands are pushed to one side, which leaves a void on the other side and an uneven crimp.
Count the damaged strands and note the position. One cut strand at the bellmouth is a different problem from several cut strands at the crimp centre. The first points to entry or die flash, the second to crimp depth or die wear. The position is part of the finding.
- Cut strands: sharp die edge, tight barrel or a burr at the entry
- Crushed strands: over-crimp or incomplete insertion
- Displaced strands: uneven die closure or a conductor that crossed the bundle
- Voids at one side: barrel off-centre or a die that is out of alignment
The bellmouth at the conductor entry
The bellmouth is the small flare at the conductor entry. It gives the strands room to spread as they leave the crimp and it moves the sharp transition away from the point where the strands are most highly stressed. Many specifications call for a visible, even bellmouth on a closed barrel crimp.
A bellmouth that is closed flat creates a sharp edge against the strands. Under vibration, that edge can cut strands or start a fatigue crack. A bellmouth that is too large means the barrel is not gripping the conductor over its full length, and the effective crimp area is shorter than the drawing assumes.
Inspect the bellmouth on the finished joint as well as on the section. It should be symmetric, free of cracks and free of a fold. On an insulated terminal, check that the sleeve has not been pushed into the bellmouth during crimping.
A closed bellmouth is not a neat crimp
A die that closes the entry flat can make the joint look tidy and still cut strands at the first flex. Check the flare, not the appearance of the barrel.
Voids, compaction and crimp ratio
Voids are the gaps between strands inside the crimp. A small amount is normal in a stranded conductor, because the strands cannot pack into a solid circle. A large void means the barrel was not compressed enough, the conductor was undersized, or the strands were displaced to one side.
Compaction is the reduction in the bundle area after crimping. It is a useful process indicator, but it is not a substitute for the section. Two joints can have the same compaction and different void patterns, because one has a uniform bundle and the other has a void at the barrel wall.
Look at the void distribution, not just the total area. A void at the centre of the bundle is usually less serious than a void that runs along the barrel wall, where it reduces the metal-to-metal contact and can become a leak path for corrosion. Note both the size and the position.
Setting acceptance criteria
The acceptance criteria should come from the product standard, the drawing or a written agreement with the customer. IEC 61238-1 and UL 486A-486B set out compression connector tests, and the drawing may add limits for strand damage, void area or bellmouth size. Do not invent a percentage and present it as a standard.
Where the standard leaves a judgement, define it with photographs. A photograph of an acceptable section and a rejected section, at the same magnification, removes most of the argument. Add a written rule for the number of damaged strands, the maximum void and the bellmouth shape.
The criteria should distinguish between a defect that affects function and one that affects appearance. A small void at the centre of a bundle is not the same as a crack in the barrel wall. Rank the defects so that the inspector knows which one stops the lot and which one needs a note.
| Defect | Appearance | Likely cause | Action |
|---|---|---|---|
| Cut strands | Flat or sheared strand ends inside the barrel | Sharp die, tight barrel or entry burr | Check the die edge and the insertion depth |
| Crushed strands | Flattened bundle with little space between strands | Over-crimp or incomplete insertion | Check the crimp height and the locator |
| Large void | Gap between the strand bundle and the barrel wall | Under-crimp, undersized conductor or displaced strands | Check the die, the conductor class and the barrel bore |
| Closed bellmouth | No flare at the conductor entry | Die profile or a worn die | Replace or reface the die and re-qualify |
| Barrel crack | Split or hairline at the bend or wall | Over-crimp, thin wall or a damaged die | Stop the lot and section a sample |
Use the product standard or drawing for the actual limits. The table describes the defect, not a numeric acceptance value.
Link the section to height and pull
A section is most useful when it is read with the crimp height and the pull result for the same joint. The height says how far the die closed. The pull says how much load the joint carried. The section says where the metal and the strands went.
If the pull is low and the section shows a large void, the height is probably too high or the barrel is too large. If the pull is low and the section shows cut strands, the die or the insertion is the problem. If the pull is good and the section shows a void at the wall, the joint has a hidden corrosion path and should be treated as a risk.
Keep the three records together for the qualification sample. For routine production, the height and the visual check can carry the load, with the section at a lower frequency. The section is the reference that proves the routine checks are still measuring the right thing.
How often to section
Section at setup, at a die change, at a new barrel lot and at the start of a new conductor type. Add a routine frequency for critical joints, and section a failed pull sample whenever the failure mode is not clear from the outside.
The frequency should reflect the consequence of a hidden defect. A joint in a control panel may need a section at qualification only. A joint in a battery pack or a high-current feeder may need a section per lot, because a void or a cut strand can become a hot spot.
Record the section image with the lot and the process settings. If a joint later fails, the image shows whether the defect was present at build. That is the difference between a supplier quality issue and a field condition, and it is worth the cost of a few sections.
Frequently asked questions
What does a good crimp cross-section look like?
A good section shows a barrel that is closed around a compact strand bundle, no cut strands, no crack in the wall, small voids distributed between strands and a visible bellmouth at the conductor entry. The exact limits come from the drawing or the product standard.
Why is a bellmouth needed on a crimped joint?
The flare gives the strands room at the point where they leave the crimp and moves the sharp transition away from the highest stress. A closed or damaged bellmouth can cut strands and start a fatigue crack under vibration.
Can a crimp pass a pull test and still have strand damage?
Yes. The remaining strands can carry the pull load, so a joint with a cut strand may pass. The damage can still reduce fatigue life or create a hot spot. That is why a cross-section is used alongside the pull test.
How often should a crimp be cross-sectioned?
At setup, after a die change, with a new barrel lot and at the start of a new conductor type. Add a routine frequency for critical joints, and section any failed pull sample where the cause is not visible from the outside.
Sources and further reading
- IEC 61238-1, Compression and mechanical connectors for power cables
- UL 486A-486B, Wire connectors and soldering lugs
- IEC 60228, Conductors of insulated cables
Qumek Engineering Desk
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