What Fraction of the Base Material: Making a Pull Test Mean Something

"How strong is your weld?" is hard to answer directly, because a pull figure is not a material property but a product of the test method. Change the gripping on the same batch and the number changes.

Fix the test conditions first

For comparable data, at least four things must be fixed.

Gripping. How far the grips sit from the joint and whether the fixture introduces a bending moment set the stress state at the weld directly. Grip slightly off and the joint sees tension plus bending, and the peak drops noticeably. Every test on a given product must use the same fixture.

Loading rate. Plastics are viscoelastic, so the faster the pull, the higher the apparent strength. Without a fixed rate the numbers are not comparable.

Temperature. Plastic strength is temperature sensitive, particularly near the glass transition. A summer shop floor and an air-conditioned lab give different results.

Time since welding. A freshly welded part is still shrinking and crystallising, and strength changes with time. Specify a delay before testing, for example a uniform 24 hours.

Where it broke matters more than the number

The essential information is in the fracture.

Broke in the base material, joint intact. The joint is at least as strong as the material and the process is right. This is the best outcome, and the recorded force reflects the material rather than the weld.

Broke at the interface with a smooth face. Classic lack of fusion. The halves were merely touching, with no real entanglement of polymer chains. Add energy, add force, review the joint design.

Broke in the base material next to the interface, with a torn layer attached. Fusion was good but the heat-affected zone was weakened, usually by excessive energy or time causing degradation. Reduce energy.

Part open, part carrying material along the joint. Uneven fusion, most often from poor seating, flatness, or uneven amplitude distribution. Check the fixture and the horn.

So the report must state the failure mode. A report carrying only a force value throws away most of the information.

How to use the "percentage of base material" figure

The industry often quotes weld strength as a percentage of the base material. The figure is usable with its conditions attached: the same material on both sides, a suitable joint design, and optimised parameters. Under those conditions a good ultrasonic weld reaches the point where the interface is no longer the weakest link and the part breaks in the material.

Do not expect that outcome with glass-filled grades, where fibres do not cross the interface; with dissimilar materials of limited compatibility; with non-painted metallic grades, where pigment concentrates at the interface; or with batches carrying a high regrind fraction. In those cases the ceiling is set by the material and the process can only approach it.

So design values should be based on measurement rather than on a discount applied to base material data.

Passing a pull test is not passing as a product

A pull test measures static load in one direction. The product may see shear, peel, torsion, impact or alternating load. Peel is the most severe, concentrating stress along a line, and many joints that pass a pull test open easily under peel.

Design the validation around how the product is actually loaded rather than defaulting to tension.

How the line holds it

Pull testing is destructive and cannot be applied to every part. The workable route is process data as the in-line criterion and pull tests as periodic verification: record energy, power and post-weld height per part with limits; sample regularly for pull tests to confirm the window has not drifted; and archive each pull result against the process data of that part to keep the chain traceable.

Joint design, failure mode interpretation and test methods are in the handbooks on the downloads page. Per-part monitoring of energy, power and post-weld height is on the ultrasonic welding machine pages.

Send the grade, the load direction and the target strength to 1427498429@qq.com, or call +86 769 8202 9510 / +86 137 6001 0932.