Key takeaways
- A shear test measures the joint between the contact and the carrier, not the contact material itself.
- The fixture has to support the carrier and apply the load parallel to the joint plane, with a defined edge distance.
- The failure mode matters as much as the peak load, because a carrier tear and a bond failure mean different things.
- State the sample size, the test speed and the acceptance value on the drawing or the control plan.
What the test measures
A contact is joined to a carrier by welding, brazing, staking or a similar process. That joint carries the current and holds the contact against force and vibration. The shear test applies a load parallel to the joint plane and records the load at which the joint fails. It is a direct check on the process, not on the contact material.
The test is often confused with a pull test. A pull test loads the joint in tension and is useful where the service load pulls the contact away from the carrier. A shear test loads it sideways and is useful where the contact is pushed along the carrier by the mechanism or by a mating part. The two tests answer different questions, and some drawings need both.
The result is a peak load and a failure mode. The peak load alone can mislead, because a joint that tears the carrier at a high load may be stronger than a joint that separates at the bond line at a lower load. The failure mode tells you which part of the process is weak.
The specimen
The specimen should be a production part, made with the production process and the production material. A laboratory sample welded by an engineer is not evidence of what the line does. If the part is too small or too delicate to clamp, the test can use a carrier of the same material and thickness with a contact joined by the same process, but the drawing has to say so.
Record the lot and the process settings with the result. A shear value without the welding current, the electrode count or the braze lot is a number that cannot be reproduced. Take the sample after the process has settled, not from the first few parts while the electrodes are cold.
The specimen has to be clean and undamaged. A contact that was dropped or a carrier that was bent during handling will fail early and tell you nothing about the joint. Inspect the specimen before the test and photograph it, so the failure can be compared with the starting condition.
The fixture and the load path
The fixture has two jobs: hold the carrier rigidly and apply the load to the contact in the right direction. A carrier that flexes under load absorbs energy and the recorded peak is lower than the joint strength. A load that is applied at an angle introduces a peel component and the result is no longer a pure shear.
The load should be parallel to the joint plane and applied as close to the joint as the geometry allows. The edge distance, the punch width and the support span all affect the result. A punch that is too wide loads the carrier as well as the contact; a punch that is too narrow can indent the contact and start a crack.
Document the fixture with a drawing or a photograph and keep it with the test method. Two labs with different fixtures will produce different numbers on the same part. If the test is used for acceptance between a supplier and a customer, the fixture should be agreed, not assumed.
The fixture is part of the specification
A shear value means little without the support span, the punch width and the load direction. Put them in the test method and keep the fixture drawing with it.
Running the test
Mount the specimen so that the carrier is fully supported and the contact is free. Align the punch with the joint and set the crosshead speed. A slow speed, often in the range of 1 mm per minute for a small contact, gives a stable reading. A fast speed can add a dynamic effect and raise the peak. The speed is illustrative and should be fixed in the method.
Zero the load cell after the specimen is mounted and before the punch touches the part. Apply the load until the joint fails or the load drops sharply, then stop. Record the peak load, the displacement at failure and the failure mode. Photograph the fracture surface at a useful magnification.
Run the agreed number of specimens and report every result, not only the average. A single low value can show a process drift that an average hides. A single high value can show a carrier tear rather than a stronger joint. The spread is part of the result.
Reading the failure mode
A bond line failure means the joint separated at the interface. It points to a surface condition, a heat input or a filler problem. A contact body failure means the contact material itself sheared, which suggests the joint is stronger than the contact and the design may need a different face thickness or geometry.
A carrier tear means the carrier deformed or tore before the joint failed. That can be a good sign for the joint and a bad sign for the carrier, especially if the carrier is thin or soft. A mixed failure is common and should be described, not forced into one category.
The table below lists the modes and what they usually point to. It is a guide for the investigation, not a substitute for a cross section.
| Mode | Appearance | Likely cause |
|---|---|---|
| Bond line | Clean separation at the interface | Surface contamination or low heat |
| Cohesive in filler | Failure inside the braze layer | Filler strength or joint gap |
| Contact body | Shear through the contact material | Joint stronger than the face |
| Carrier tear | Carrier deformed or torn | Thin carrier or punch misalignment |
| Mixed | More than one mode on the surface | Process at the edge of its window |
The mode should be recorded with the load and a photograph, because the same load can mean different things.
Setting the acceptance value
The acceptance value should come from the design, not from the first batch. Start with the load the joint sees in service and add a factor for vibration, thermal cycling and the scatter of the process. Then check that the value is achievable with the chosen process and material.
A minimum load is the usual call, often with a sample size per lot. For illustration, a control plan might require five parts per lot with no result below the minimum. The number and the rule are illustrative; the drawing or the control plan has to set them, and the sample size should reflect the risk of the part.
Review the value when the process changes. A new electrode, a new braze lot or a new carrier supplier can move the distribution. A limit that was right for the old process may be too tight or too loose for the new one. The test is a control, and a control has to be revalidated when the process moves.
Records and the first article
Keep the peak load, the displacement, the failure mode, the specimen lot and the process settings for every test. A first article should include the shear results with the photographs, and the production control plan should say how often the test is repeated.
If the joint is joined by a subcontractor, the records have to travel with the parts. A shear test done at the assembly plant on a joint made elsewhere is a check on the incoming part, not on the joining process. The process owner should provide the result, and the assembly plant should audit it.
A shear test is a small, cheap check compared with a field failure. It is worth setting up properly, with a fixture drawing and a written method, so that the number means the same thing in every lab that runs it.
Frequently asked questions
What does a contact shear test measure?
It measures the strength of the joint between the contact and the carrier when a load is applied parallel to the joint plane. It also records the failure mode, which shows where the joint is weak.
How is a shear test different from a pull test?
A shear test loads the joint sideways, while a pull test loads it in tension. The right test depends on the direction of the service load, and some parts need both.
How many parts should be tested?
The sample size should be set by the control plan and the risk of the part. A common starting point is a small sample per lot with a minimum load, but the rule and the number have to be written down.
Why does the failure mode matter?
Because the same peak load can come from a strong joint that tears the carrier or a weak joint that separates at the bond line. The mode tells you which part of the process to investigate.
Sources and further reading
- ASTM B539, Standard test methods for measuring resistance of electrical connections
- ISO 17672, Brazing, filler metals
- Qumek contact joint test method and first article notes
Qumek Engineering Desk
Engineering team
The engineering team behind Qumek drawings, DFM notes, tolerances and inspection standards. Every guide is reviewed against the drawings and processes used on live programmes.
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