Dart Impact Test Procedure: Step-by-Step ASTM D1709 Method A & B Guide

A dart impact test procedure is a standardized sequence in which a weighted dart with a hemispherical head falls from a fixed height onto a clamped plastic film specimen. The pass/fail outcomes across multiple specimens are used to calculate the F50 value: the dart mass in grams that causes 50 percent of specimens to fail. The test takes under 30 minutes to run, yet most unreliable results trace back to the 40 hours of conditioning that happened before a single dart dropped.

Governed by ASTM D1709, this procedure applies to PE, PP, PET, and multilayer films used in packaging, agriculture, and industrial liners.

Why Procedure Precision Determines Your F50

The F50 value is only reproducible when conditioning, clamping, drop height, and weight increments all follow the standard. A test that skips even one of these controls produces a number that looks valid but cannot be compared against any other lab's result.

Before reviewing any dart impact data from a supplier or internal lab, confirm that the report states the method (A or B), specimen count, and conditioning time. Missing any of these makes the F50 unreliable.

Equipment Setup — Method A vs Method B Parameters

ASTM D1709 defines two methods. Method A is the default for most thin blown films. Method B exists for thicker or tougher films where Method A dart masses exceed the practical ceiling of the weight set. If you are setting up a test for the first time, start with Method A unless your film's expected F50 exceeds the heaviest dart assembly available for the 38 mm head.

Method A — 660 mm Drop, 38 mm Dart (Default for Thin Films)

Most blown PE and PP films between 15 and 80 microns fall squarely into Method A territory — it is the procedure cited in the majority of film supplier specifications. Set the drop height to 660 mm and install the 38 mm hemispherical dart head. Confirm that the specimen extends at least 25 mm beyond the clamping ring on all sides and that clamping pressure is uniform — uneven clamping shifts the failure point away from the dart contact zone, inflating the F50 reading.

Method B — 1524 mm Drop, 51 mm Dart (Thicker and Tougher Films)

When Method A requires dart masses above the practical ceiling of the weight set, Method B provides a higher drop height and larger dart head to deliver more energy per gram of added weight. This is common for multilayer barrier films and thick HDPE sheets. Set the drop height to 1524 mm and install the 51 mm dart head. The larger dart head distributes force over a wider area, so Method B F50 values cannot be compared directly to Method A values.

Before either method, measure specimen thickness at a minimum of five points per specimen per the referenced thickness method (ASTM D5947 or D6988). Record each measurement. Thickness variation beyond the standard tolerance flags a specimen that should be discarded before testing begins.

If your film consistently passes Method A at the lowest available dart mass, you are likely under-testing. Switch to Method B to get a meaningful F50 that differentiates between production lots.

Running the Test — Staircase (Bruceton) Method Step by Step

The Bruceton staircase method is the statistical engine behind the dart impact test. It works by letting each specimen's result dictate the dart mass for the next specimen, systematically converging on the true F50. Here is the sequence:

1. Select a starting dart mass close to the estimated F50 for your film. If no estimate exists, begin at a mid-range mass and expect the first several specimens to be exploratory.
2. Drop the dart onto the first clamped specimen from the correct height (660 mm for Method A, 1524 mm for Method B).
3. Record the outcome — pass (specimen intact) or fail (specimen ruptured).
4. Adjust the dart mass: if the specimen passed, increase mass by one weight increment for the next specimen. If it failed, decrease mass by one increment.
5. Repeat for a minimum of 20 specimens.
6. Check validity: the staircase sequence must contain both passes and failures with enough reversals (pass-to-fail or fail-to-pass transitions) for the formula to produce a meaningful result.

What happens if your first ten specimens all pass? That means the starting mass was set too low. The staircase has not begun converging, and those specimens are wasted time and material. A good starting estimate saves specimens — ask the film supplier for a historical F50 range, or run a quick three-specimen scouting sequence at increasing masses before committing to the full 20-specimen run.

Create a recording template with columns for specimen number, dart mass, and pass/fail before you start. Fill it in real time. Reconstructing the staircase sequence from memory after the run introduces transcription errors that distort the F50 calculation.

Calculating and Interpreting Your F50 Result

Once the staircase run is complete, the F50 calculation uses the less-frequent event (pass or fail) in the converged portion of the sequence. ASTM D1709 Section 10 provides the full formula, which accounts for the lowest mass level of the less-frequent event, the weight increment (step size), and a correction factor whose sign depends on whether failures or passes are the less-frequent event. The exact calculation requires working through the formula in the standard; an overview article cannot substitute for it, and spreadsheet templates from instrument vendors can automate the arithmetic.

Does a higher F50 always mean a better film? Only within the same method and dart geometry. Cross-method comparisons are invalid because the different drop heights and dart head sizes deliver different energy profiles.

For QC decisions, compare the F50 against the customer specification or internal control limit. A result above the minimum spec is a pass. A result below is a fail — but a single below-spec run may reflect a conditioning error rather than a material defect. Track F50 values over time: a gradual downward trend across multiple lots signals process drift in extrusion, blending, or raw material quality, and warrants investigation before the value crosses the rejection threshold.

Specimen Conditioning and Common Procedural Errors

Conditioning is the most underestimated step in the entire dart impact test procedure. ASTM D618 requires specimens to equilibrate at 23 °C and 50 percent relative humidity for a minimum of 40 hours before testing. Moisture content and temperature both affect polymer chain mobility and, by extension, impact resistance. An unconditioned specimen tested straight from a warehouse at 35 °C and 30 percent RH will return an F50 that cannot be reproduced by any lab following the standard.

Check your lab log — if conditioning time is recorded as a round number every single run, someone is estimating instead of measuring. Use a calibrated data logger inside the conditioning chamber and tie specimen release to actual recorded conditions, not a wall clock.

Pre-Test Checklist

Use this checklist before every dart impact test run:

• [ ] Specimens conditioned ≥ 40 h at 23 °C / 50% RH — verified by data logger, not estimated
• [ ] Thickness measured at 5 points per specimen per ASTM D5947 or D6988
• [ ] Specimen extends ≥ 25 mm beyond clamping ring on all sides
• [ ] Dart head inspected for nicks, dents, or surface damage
• [ ] Minimum 20 specimens prepared and labeled
• [ ] Recording template ready (specimen number, dart mass, pass/fail)
• [ ] Correct method selected (A or B) and drop height verified

Print this checklist and tape it to the tester. Sixty seconds before each run eliminates the five most common sources of unrepeatable F50 values.

Frequently Asked Questions

Q: What is the difference between ASTM D1709 Method A and Method B?

Method A uses a 38 mm hemispherical dart head dropped from 660 mm and suits most thin blown films. Method B uses a 51 mm dart head dropped from 1524 mm, designed for thicker or tougher materials where Method A dart masses become impractically large. Results from the two methods are not interchangeable because the dart geometry and energy profiles differ.

Q: How many specimens do I need for a dart impact test?

A minimum of 20 per ASTM D1709. Fewer specimens prevent the staircase from converging on a valid F50. Prepare extras in case any are discarded for thickness non-conformance.

Q: What does the F50 value mean in dart impact testing?

F50 is the dart mass in grams at which 50 percent of specimens fail under the Bruceton staircase method. It is the standard metric for comparing impact resistance across production lots and supplier films.

Q: Why is specimen conditioning required before the dart impact test?

Films must equilibrate to 23 °C and 50 percent relative humidity for at least 40 hours per ASTM D618. Moisture content and temperature both affect polymer impact resistance, so unconditioned specimens produce F50 values that cannot be reproduced by another lab following the same standard.

Q: Can I compare dart impact results tested under Method A with results tested under Method B?

No. The different dart geometries and drop heights deliver different energy profiles. Both results must come from the same method for a valid comparison.

Q: What is the Bruceton staircase method in dart impact testing?

It is the statistical procedure used to converge on the F50 value. After each specimen is tested, the dart mass is increased (if the specimen passed) or decreased (if it failed) by a fixed increment. Over 20 or more specimens, this up-down sequence narrows in on the 50 percent failure threshold, producing a statistically grounded F50 result.