te ways.
Steel bars are commonly held by wedge action. As the pull rises, the wedge drives serrated or knurled jaws harder into the specimen, so the hold tightens with the load. Wedge grips are made with capacities beyond 500 kN, and the round bar needs inserts shaped to its diameter to keep the load on the axis. Reinforcing bar, rod and wire have their own test method, ISO 15630-1.
Film needs the reverse. Faces that bite into steel can cut a thin strip, so film is held with smooth or rubber-coated faces. The usual choices are pneumatic grips, with the air pressure set to suit the material, and eccentric roller grips.
In a roller grip, an off-center roller pins the strip to a fixed face and turns tighter as tension builds. It keeps contact while the film stretches and thins, which helps prevent slip or tearing late in the test.
The plastic tensile testing standards for film write the same care into the method. ASTM D882 covers sheeting and film too thin for the dumbbell specimens of ASTM D638, so the material is pulled as a strip. D882 and ISO 527-3, the film and sheet part of the ISO series, are both run with controlled alignment and grip pressure so the strip does not tear at the jaws. Clamp a film strip in faces cut for steel and the likely outcome is a wasted specimen and a retest.
Capacity separates the two grip sets as well. Pneumatic side-action grips are commonly used up to around 20 kN, and heavier loads go to wedge or hydraulic grips. A look across the full range of tensile testing equipment puts the hardware for steel bar and the hardware for film at opposite ends of the capacity scale. A shared frame needs both sets, quick-change adapters and a place to store whichever set is idle.
Film Needs Stroke And Speed That Bar Testing Does Not
Force and grips draw most of the attention when a frame is specified. For film, stroke matters as much, and it is fixed when the frame is chosen.
Film belongs with rubber among the high-elongation materials, and the crosshead has to follow it all the way to break. Grips, fixtures and an extensometer all take up travel, and what remains is the stroke available to the specimen. Elastomers show how fast that margin goes: a specimen that starts at 100 mm can finish beyond 500 mm.
Low-force frames are built for that stretch, with 960 mm of crosshead travel on a 50 N to 5 kN benchtop frame and up to 1,600 mm on extended mid-range dual-column frames. A servo-hydraulic frame built for high force offers far less, and across one 300 kN to 3000 kN family the actuator stroke runs from 150 mm to 300 mm. A frame picked for its force rating alone can run out of travel while a high-elongation specimen is still intact.
Speed points the same way. Metal methods such as ASTM E8/E8M and ISO 6892-1 run slowly, while packaging film is pulled fast. Top speed also tends to fall as capacity rises. Within one floor-standing electromechanical series, it drops from about 850 mm/min on the 50 kN and 100 kN models to about 300 mm/min on the 1000 kN model.
Rate control adds a demand from the metal side. ISO 6892-1 Method A works from strain rate, either in closed loop or as an estimated rate over the parallel length. Closed-loop control runs from an extensometer on the specimen, so a shared frame has to hold a steady low strain rate on steel and still move fast enough for film. The useful question for a supplier is the usable speed range and how the machine behaves at each end of it, checked against the rate the film method calls for.
When One Frame Covers Both And When Two Fit Better
The decision turns on two forces, the strongest bar and the weakest film, measured against what one frame class can span.
One frame is a practical answer when the metal work stays within a mid-range frame. Dual-column frames rated up to 10 kN or 50 kN are specified for mixed workloads that list metals beside plastics, composites and films. The controller recognizes their interchangeable load cells automatically, so an operator can switch from a low-capacity cell for sensitive material to a high-capacity cell for structural samples without re-entering calibration data.
Steel wire, metal strip and machined coupons of modest sections fit this pattern, usually as one frame with two or more cells rather than one oversized cell. The cost is paperwork. Each cell that produces reported results needs its own entry in the machine’s force verification and calibration records. Annual calibration is the common minimum, with shorter intervals under heavy use or after a move.
Two frames make more sense once the bar pushes the metal work out of the frame class that suits film. Specimens that need more than roughly 50 kN belong on a floor-standing frame, and heavy structural metal on a servo-hydraulic system. One such system, built for metallic specimens, reads force through a single load cell from 1 percent of full scale upward without range switching. Film work then belongs on a low-force frame sized for it.
Workflow can tip the decision even when force does not. A second frame is worth pricing when the lab would swap cells, grips and extensometers more than once a day, or when both programs run continuously. Frequent reconfiguration tends to push the less convenient test off the schedule. A dual-space frame sits between the two options, since a lab that changes setups often can keep its most used fixtures and cells installed in each test space.
Most of the decision fits on one worksheet, filled in before any quotation is requested:
- Strongest bar: cross-sectional area times tensile strength, and whether the result stays below roughly 50 kN.
- Weakest film: expected peak force against the verified floor of the smallest cell and the 10 to 70 percent band.
- Grips: wedge action for the bar, smooth or rubber-coated faces at controlled pressure for the film.
- Travel and speed: crosshead travel left for the film once grips and extensometer are fitted, and the slowest and fastest rates the two methods need.
- Changeovers: how many times a day the setup would switch between bar and film.
If the first two entries land in different frame classes, the quotation is usually for two machines, and the remaining entries decide how each one is configured.
Questions Labs Ask Before Sharing One Frame
Five questions come up once a lab starts pricing a shared tensile tester for bar and film.
- Can A Load Cell Sized For Steel Bar Measure Plastic Film?
Not reliably, unless the film’s peak force falls inside the verified range recorded for that cell. A cell sized for bar leaves film low in its range, where its accuracy class gives little assurance, so a second, smaller cell is the common fix.
- Does Swapping Load Cells Mean Recalibrating The Machine?
No, as long as the cells carry digital identification, because the controller recognizes a newly fitted cell and loads its calibration data itself. Verification is a separate matter, and each cell configuration used for reportable work needs its own coverage in the machine’s force verification records.
- Can One Set Of Grips Hold Both Steel Bar And Film?
Generally not. Steel bar at high force calls for wedge action, while film calls for low, controlled clamping pressure on smooth or rubber-coated faces. At moderate loads, side-action grips with interchangeable smooth, serrated and rubber-coated faces let one grip body cover several materials.
- Is Plastic Film Tested To ASTM D638 Or ASTM D882?
ASTM D882. D638 covers rigid and semi-rigid plastics in dumbbell form up to a defined thickness, and thinner sheeting and film go to D882 as strips. On the ISO side, ISO 527-3 covers films and sheets and ISO 527-2 molding and extrusion plastics, both under ISO 527-1. ISO 527-3 also replaced the older ISO 1184 film method, now withdrawn.
- Does One Extensometer Work For Both Steel And Film?
Usually not. Yield and modulus on steel call for a contact extensometer of a tight class over a defined gauge length, the kind ASTM E83 assigns to modulus and proof strength. Film calls for a high-elongation unit that adds little mass to the specimen, long travel arms or a non-contact video system, and E83 places large-strain work in its wider classes.