ASTM D1938: Tear Propagation Resistance Test for Plastic Film & Thin Sheeting
ASTM D1938 trouser tear propagation test on plastic film measuring tear force per unit thicknessWhat Is ASTM D1938?
ASTM D1938 is the Standard Test Method for Tear-Propagation Resistance (Trouser Tear) of Plastic Film and Thin Sheeting by a Single-Tear Method. It measures the force required to propagate an existing tear through a plastic film or thin sheeting specimen — a critical mechanical property for films subjected to tearing forces during use, handling, or accidental damage. The test is widely used across the packaging films, agricultural films, and flexible packaging industries, where tear resistance directly determines product durability, consumer usability, and resistance to damage during distribution.
Principles of the Trouser Tear Test
Test Specimen Geometry
The trouser-tear specimen is a rectangular strip with a longitudinal slit extending from one end to approximately the midpoint, creating two “trouser legs.” The legs are gripped in the upper and lower jaws of a tensile testing machine and pulled apart — propagating the tear from the slit tip through the unslit portion of the specimen.
This geometry applies a predominantly Mode III (out-of-plane shear) tearing stress at the propagating crack tip, simulating the type of tear that occurs when film is accidentally snagged or deliberately torn open along a seam.
Test Parameters
- Specimen dimensions: Typically 25 mm wide × 150 mm long with a 75 mm initial slit
- Crosshead speed: 250 mm/min (standard for most flexible films)
- Measurement: Mean tear force (N) over the propagation length, excluding initial and terminal portions of the force-displacement trace
- Reported result: Tear force (N) or tear resistance normalized per unit thickness (N/mm)
Machine Direction vs. Transverse Direction
Tear propagation resistance is highly anisotropic in oriented films. Oriented films tear much more easily in the machine direction (MD) than the transverse direction (TD) — a feature exploited in easy-open food packaging (MD tear strips) and a vulnerability to avoid in structural containment applications. Both MD and TD tear resistance are measured and reported separately.
Comparison with Other Tear Test Methods
Method | Standard | Geometry | Failure Mode |
Trouser Tear | ASTM D1938 | Split strip | Mode III propagation |
Elmendorf Tear | ASTM D1922 | Pendulum impact | Dynamic propagation |
Tongue Tear | ASTM D2261 | Single tongue | Combined modes |
ASTM D1938 trouser tear is preferred for fundamental characterization of tear propagation energy and correlation with molecular structure. Elmendorf tear (D1922) is faster and more reproducible for routine QC at higher throughput.
Industry Applications
Flexible packaging producers use ASTM D1938 to qualify multilayer barrier films, stretch wrap, and agricultural mulch films for tear resistance specifications. Agricultural film manufacturers measure both MD and TD trouser tear to verify minimum performance for manual field handling and mechanical winding. Industrial stretch film producers optimize molecular weight distribution and LLDPE blend ratios by monitoring trouser tear as an indicator of film structure. Medical packaging developers use tear data alongside seal strength to ensure peelability without uncontrolled tearing.
Conclusion
ASTM D1938 trouser-tear testing provides manufacturers and film developers with a direct, reproducible measure of tear-propagation resistance — essential for qualifying flexible packaging, agricultural films, and industrial stretch films. Testing both machine and transverse directions captures anisotropic behavior critical to real-world performance, helping engineers optimize polymer formulation, film structure, and end-use reliability before product release.
Why Choose Infinita Lab for ASTM D1938 Tear Testing?
Infinita Lab is a trusted USA-based testing laboratory offering ASTM D1938 trouser-tear testing and comprehensive mechanical testing of flexible films across an extensive network of accredited facilities, with fast turnaround and full project management.
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Frequently Asked Questions
What is the difference between ASTM D1938 trouser tear and ASTM D1922 Elmendorf tear? ASTM D1938 measures quasi-static force to propagate a pre-existing slit under controlled crosshead displacement, providing fundamental tear energy data sensitive to molecular orientation. ASTM D1922 measures pendulum energy absorbed during dynamic tear propagation — faster and better suited for routine production quality control monitoring.
Why does tear resistance differ between machine direction and transverse direction in oriented films? Machine direction orientation aligns polymer chains during film production. MD tearing propagates perpendicular to chain alignment, requiring disentanglement rather than scission — giving lower resistance. TD tearing breaks oriented chains directly, giving higher resistance. This anisotropy is intentionally exploited in easy-open packaging and controlled in agricultural containment films.
How is mean tear force calculated from ASTM D1938 test data? The force-displacement trace shows an initial rise, propagation region, and terminal region. Mean tear force is calculated from the middle 50–75% of the propagation trace, excluding initial stress concentration and terminal edge effects. Testing software automatically calculates this from user-defined integration limits applied to the propagation region.
What film materials are most commonly tested by ASTM D1938? DPE, LLDPE, HDPE, cast and oriented polypropylene, PET, polyamide, EVA copolymer, PVDC barrier films, and multilayer co-extruded packaging films are most frequently tested. Agricultural films — typically LLDPE or metallocene PE blends — are routinely characterized by trouser tear in both MD and TD directions.
How does film thickness affect trouser tear results? Thicker films require greater absolute tear force but not necessarily greater normalized tear resistance. Comparing films of different thickness requires reporting normalized tear force in N/mm rather than absolute force values. Crystallinity and orientation differences at equivalent thickness also significantly affect tear performance independently of thickness alone.
