Key takeaways
- Bend life is a test of the whole flexible section, including the terminals, not only the foils.
- Radius, travel and rate set the strain, so the test must use the installation's own values.
- A resistance change is the measure that catches a partly broken foil before it separates.
- A pass at one radius says little about another, so test the tightest bend in the route.
What bend life is measuring
A flexible busbar is used where a rigid bar would be loaded by movement. It may connect a battery pack to a chassis, a transformer to a cabinet or a moving arm to a fixed terminal. The flexible section bends to take up that movement. Bend life is the number of cycles the section survives before it fails.
The test is not a material test. It is a test of the construction: the foil or braid, the bond between the layers, the terminal joint and the insulation. A foil that is perfectly ductile on its own can still fail early if the terminal crimp is too tight, if the layers are bonded in a way that concentrates strain, or if the bend zone sits next to a rigid clamp.
The result is only useful if the test reproduces the installation. A test that bends a free length in the middle of a sample says little about a bar that is clamped at both ends and bent close to a terminal. The fixture, the radius, the travel and the rate all belong in the test report.
The test fixture
The usual arrangement fixes one end and moves the other through a controlled arc or a linear travel. A crank and connecting rod gives a simple sinusoidal motion. A linear actuator with a programmable profile can reproduce a specific movement. The moving end should be guided so the sample is bent, not twisted or pulled.
The bend zone should sit between two formers. The formers set the minimum radius and stop the sample from kinking. A former that is too small creates a tighter bend than the installation, which makes the test harsher than service. A former that is too large lets the sample bend over a longer length and gives an optimistic result.
The sample should be mounted with the same terminal hardware as the production part. A test sample with a soldered joint at each end does not prove a crimped or welded terminal. The insulation should be present if the production part is insulated, because the insulation changes the bend stiffness and can hide a foil break.
Radius, travel and rate
Strain in a bent foil is roughly proportional to the foil thickness divided by the bend radius. A thin foil can take a tight radius; a thick foil cannot. A laminated stack behaves as several foils bending together, so the outer foil sees the highest strain. That is why the radius should be called as a radius to the inside of the stack, not to the middle.
Travel sets the range of bend. A sample that bends through 30 degrees sees less strain than one that bends through 90 degrees at the same radius. The installation drawing should give the movement at the flexible section, including the tolerance and the worst case, and the test should use the worst case.
Rate matters less for a metal foil in the elastic range, but it matters for the insulation and for any bonded layer. A fast cycle can heat the bond line and give a different failure. A rate of about one cycle per second is a common starting point for a bench test, with the actual service rate noted in the report.
Test the tightest bend in the route
A flexible section often has one bend that is tighter than the rest because of the layout. Test that radius, not the average, because that is where the first foil will break.
What to measure
The primary measurement is resistance. A four wire measurement across the flexible section, taken before the test and at intervals during it, shows a rise when foils begin to break. The current used for the measurement should be low enough not to heat the sample. The result is a small change at first and a sharp rise when the last path fails.
A visual check under magnification looks for broken foils, exposed copper, cracks in the insulation and movement at the terminal. A dye penetrant or an eddy current check can find a crack that is not visible, but those methods are usually reserved for a failure investigation rather than a routine test.
Temperature is worth recording. A flexible section that is undersized for the current will run warm, and the insulation and the bond line will age faster. A thermocouple on the bend zone during a current load test gives a second view of the design margin.
Acceptance criteria
The acceptance should be written before the test starts. A resistance change limit, a visual limit and a terminal check are the usual three. The limits should relate to the function: a 5 per cent rise may be acceptable on a short link and unacceptable on a long run where the voltage drop is already tight.
The cycle count should match the service life with a margin. If the application moves once a day for ten years, the count is small. If it moves once a minute, the count is large. The test should state the count, the radius and the travel together, because a cycle count alone has no meaning.
A test that stops at the target count without a failure is a pass at that condition. It is not a statement that the part will never fail. The report should say what was tested, what was measured and what the margin was.
| Check | Method | Illustrative acceptance |
|---|---|---|
| Resistance | Four wire measurement before and during the test | Change under 5 per cent of baseline |
| Visual | Magnification at the bend zone | No broken foil or exposed conductor |
| Insulation | Insulation resistance at the stated voltage | No drop below the drawing limit |
| Terminal | Pull test on a sample after the test | No movement or strand break |
| Temperature | Thermocouple at the bend under load | Rise within the agreed band |
| Records | Cycle count, radius and travel | Traceable to the fixture and the sample |
Illustrative limits. Set the acceptance from the circuit and the service life, not from a default table.
Sample size and records
A bend test is destructive, so the sample size has to be decided with the cost in mind. A small number of samples gives a check on the process, not a statistical life estimate. If the part is critical, more samples and a longer test are justified.
Each sample should be identified with its lot, its foil thickness and its terminal process. The fixture should record the radius, the travel, the rate and the number of cycles. A photo of the failure, the resistance trace and the measurement positions complete the record.
The test should be repeated when the construction changes. A new foil thickness, a new bond adhesive, a new terminal process or a new insulation can all change the bend life. A test on the old construction does not cover the new one.
Failure modes and design fixes
The most common failure is a foil break at the edge of the bend zone, where the foil leaves the clamp. The strain is highest there and the foil has the least freedom to move. A longer flexible section, a larger radius or a softer clamp can move the failure away or reduce the strain.
A second failure is delamination of the bonded layers. The bond line cracks and the layers separate, which changes the stiffness and can expose the foil. A bond that is too stiff for the movement is the usual cause. A more flexible adhesive or an unbonded length in the bend zone can help.
A third failure is at the terminal. The joint between the foil stack and the lug is a stiffness change, and a terminal that is crimped too hard or welded with a brittle zone can crack. The terminal should be tested with the same movement as the flexible section, not only with a pull test.
Frequently asked questions
How many cycles should a flexible busbar bend test run?
It depends on the service. Work out the movement per cycle and the number of cycles in the design life, then add a margin. State the count with the radius and the travel, because the count alone is not a specification.
What counts as a failure in a bend test?
A broken foil, exposed conductor, insulation breach, a terminal that moves or a resistance rise beyond the agreed limit. Define the limit before the test starts.
Can a bend test be run without current?
Yes for a mechanical life test, but a current load test adds the thermal effect and checks the resistance under service conditions. The two tests answer different questions.
Why does a flexible busbar fail next to the terminal?
The terminal is a stiff point, so the strain concentrates where the flexible section leaves it. A longer flexible length or a gentler transition spreads the strain.
Sources and further reading
- IEC 61238-1:2018, Compression and mechanical connectors for power cables, test methods and requirements
- IEC 60068-2-6:2007, Environmental testing, test Fc: vibration, sinusoidal
- IEC 61439-1:2020, Low-voltage switchgear and controlgear assemblies, general rules
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