Universal Testing Machine Film Tensile Performance Testing: Standards, Methods, and Applications

Film tensile performance testing with a universal testing machine is the core quality-control method for evaluating tensile strength, yield strength, elongation at break, and elastic modulus of plastic and composite films. This article provides a complete technical guide covering applicable standards such as ASTM D882, ISO 527-3, and GB/T 13022, specimen preparation, instrument configuration, test procedures, and typical industry applications.

Technical Overview

Films are among the most widely used flexible materials in modern industry, appearing in food packaging, pharmaceutical blister laminates, lithium-ion battery separators, agricultural covers, and electronic optical components. Because films are thin, lightweight, and often highly extensible, their mechanical performance must be measured with specialized low-force instrumentation rather than heavy-duty frames. A universal testing machine (UTM), also known as a tensile testing machine, equipped with a high-accuracy small-capacity load cell and film-specific grips, is the standard solution for this task.

During a film tensile test, a rectangular strip or dumbbell specimen is clamped between two grips and stretched at a constant crosshead speed until it yields and breaks. The force-extension curve recorded by the software is then converted into stress-strain values using the specimen's original width and thickness. Key characteristics of a suitable system include:

• Load frame capacity typically between 100 N and 5 kN, driven by a precision ball screw for stable low-force control
• Interchangeable load cells (50 N, 100 N, 200 N, 500 N) with accuracy class 0.5 or better according to ISO 7500-1 or ASTM E4
• Pneumatic or mechanical film grips with rubber-faced jaws, or capstan (windlass) grips for slippery and highly extensible films
• Wide crosshead travel to accommodate elongation at break above 500 percent for materials such as LLDPE stretch film
• Software that automatically calculates tensile strength, yield strength, elongation, and modulus, and generates standards-compliant test reports

Testing Samples

Polyolefin Films

Polyethylene and polypropylene films, including LDPE, LLDPE, HDPE, CPP, IPP, and BOPP, represent the largest share of tested film materials. These films range from very soft and extensible (LLDPE stretch film) to stiff and highly oriented (BOPP), so both the grip type and the load cell range must be selected accordingly.

Polyester and Barrier Films

Biaxially oriented PET (BOPET), nylon (PA), PVDC-coated films, and EVOH co-extruded structures are tested for tensile strength and modulus, particularly because these materials serve as structural and barrier layers in laminates. Their strong orientation requires testing in both the machine direction (MD) and transverse direction (TD).

Composite and Laminated Films

Multilayer structures such as PET/AL/PE, OPP/CPP, and paper-foil-plastic laminates are tested to verify lamination integrity and overall mechanical strength. Test results help confirm adhesive performance, delamination resistance, and suitability for high-speed filling and sealing lines.

Functional and Specialty Films

This category includes lithium-ion battery separators, optical and protective films, shrink films, cling films, mulch and greenhouse films, and biodegradable films based on PLA and PBAT. Battery separators in particular demand precise MD/TD tensile data because they are typically only 10 to 25 micrometers thick.

Testing Items

A film tensile test delivers a complete set of mechanical parameters from a single stress-strain curve. The most commonly reported items are:

• Tensile strength: the maximum force sustained by the specimen divided by its original cross-sectional area, expressed in MPa
• Tensile strength at break: the stress recorded at the moment of specimen failure
• Yield strength: the stress at which the material begins to deform plastically, visible as a peak or plateau before break
• Elongation at break: the increase in gauge length at failure expressed as a percentage of the initial gauge length
• Elongation at yield: the strain corresponding to the yield point
• Tensile stress and strain at a specified strain level, for example stress at 1 percent or 5 percent strain
• Tensile modulus (elastic modulus): determined by ISO 527 as the chord slope between 0.05 percent and 0.25 percent strain, or by ASTM D882 as a secant modulus, reported in MPa
• Toughness or work to break: the area under the force-extension curve, indicating the energy absorbed before failure
• Break mode assessment: classification of ductile or brittle failure and identification of invalid breaks occurring at the grip line

Testing Methods

Applicable Standards

The three most widely applied standards for film tensile testing are ASTM D882 (Standard Test Method for Tensile Properties of Thin Plastic Sheeting), ISO 527-3 (Plastics — Determination of tensile properties — Part 3: Test conditions for films and sheets), and GB/T 13022 (the Chinese national standard for plastic film tensile properties, technically aligned with ISO 527-3). JIS K 7127 is the corresponding Japanese standard. For rigid plastic sheets thicker than approximately 1 mm, testing shifts to ASTM D638 or ISO 527-2 using dumbbell specimens.

Sample Preparation

Films are tested as straight rectangular strips. Under ISO 527-3, the common specimen is 10 to 25 mm wide, at least 150 mm long, and tested with a 100 mm gauge length. Specimens must be cut with a sharp die cutter or razor strip cutter so that the edges are smooth and free of nicks, since even minor edge defects cause premature failure and invalid results. Width is measured with calibrated calipers or a measuring microscope, and thickness is measured with a dead-weight micrometer per ISO 4593 at a minimum of five points, because a one-micrometer thickness error directly biases the calculated stress. Specimens are conditioned in a standard laboratory atmosphere of 23 ± 2 °C and 50 ± 5 percent relative humidity for at least 24 hours, with extended conditioning for hygroscopic materials such as PA and EVOH.

Instrument Configuration

Select the smallest load cell that keeps the expected breaking force comfortably within its verified range; typical film testing uses 100 N or 200 N cells, while stiff BOPET laminates may require 500 N. Pneumatic side-action grips with rubber inserts provide uniform, repeatable clamping pressure and prevent jaw breaks on most films. For slippery or extremely extensible films such as LLDPE and cling film, capstan grips that wrap the specimen around a low-friction roller eliminate slippage entirely. Because contact extensometers can damage or stiffen thin films, elongation is normally derived from crosshead displacement, while modulus measurement is best performed with a non-contact video extensometer.

Test Procedure

At least five specimens are tested in each direction, since orientation makes film properties strongly anisotropic. The procedure is as follows:

• Verify the specimen dimensions and record the average thickness and width
• Set the initial grip separation to the value required by the standard, typically 100 mm for ISO strips or 50 to 100 mm under ASTM D882 depending on expected elongation
• Mount the specimen squarely without twist or slack and apply a small preload to remove curvature
• Run the test at the specified crosshead speed: ISO 527-3 commonly uses 100 or 200 mm/min, while ASTM D882 assigns speeds ranging from 5 to 500 mm/min according to the expected elongation at break
• Record the force-extension curve until specimen failure, flagging any break that occurs at or within the grip jaws as invalid
• Repeat for all specimens in both MD and TD directions

Data Processing

The software calculates each parameter automatically: tensile strength equals maximum force divided by original width times thickness; elongation at break equals extension at break divided by initial gauge length times 100 percent; and modulus equals the slope of the stress-strain curve over the defined strain interval. Results are reported as the mean with standard deviation for each direction, and any specimen that breaks at the grips or exhibits slippage is discarded and replaced so that the statistical set is complete.

Applications

Film tensile testing on universal testing machines supports quality control, product development, and regulatory compliance across many industries:

• Food and beverage packaging: incoming film inspection, orientation process control, and verification of laminates for form-fill-seal operations
• Pharmaceutical packaging: mechanical qualification of blister films, cold-form foil laminates, and high-barrier structures
• Lithium battery manufacturing: MD and TD tensile strength evaluation of separators to guarantee electrode-winding and assembly safety
• Agriculture: strength and elongation verification of greenhouse, mulch, and silage films against service life requirements
• Electronics and displays: mechanical characterization of optical films, protective films, and flexible substrates
• Materials research: development of biodegradable PLA and PBAT films, nanocomposite films, and recycled-content structures
• Third-party laboratories: certification testing and supplier conformity assessment against ASTM, ISO, and GB standards

FAQ

What capacity of universal testing machine is suitable for film testing?

A frame of 100 N to 5 kN capacity is generally sufficient, provided it is fitted with a small-capacity load cell matching the film type. Soft packaging films usually break below 200 N, so a 100 N or 200 N cell gives the best measurement resolution and accuracy.

Which grips should be used for thin and slippery films?

Pneumatic grips with rubber-faced jaws suit most films because they deliver constant, adjustable clamping force. For highly extensible or slippery materials such as LLDPE stretch film and cling film, capstan grips are recommended because they clamp by wrapping rather than pinching, eliminating slippage and jaw breaks.

What are the main differences between ASTM D882 and ISO 527-3?

Both standards test rectangular strips, but they differ in specimen dimensions, grip separation, crosshead speed selection, and modulus definition. ISO 527-3 uses a 100 mm gauge with typical speeds of 100 or 200 mm/min and defines modulus between 0.05 and 0.25 percent strain, while ASTM D882 selects speed from 5 to 500 mm/min according to elongation class and reports modulus as a secant value. Always state the governing standard in the test report.

Why do specimens keep breaking at the grips, and how can this be fixed?

Jaw breaks are usually caused by excessive grip pressure, hard jaw faces, or edge damage from poor cutting. Reduce the pneumatic pressure, use rubber or line-contact inserts, switch to capstan grips, recut specimens with a sharp die, and mount the specimen with a slight slack so that the gauge section, not the clamped ends, carries the initial load.

Can the same universal testing machine test both films and rigid plastics?

Yes. By exchanging grips and load cells, one frame can perform film strip tests to ASTM D882 or ISO 527-3 and rigid plastic dumbbell tests to ASTM D638 or ISO 527-2. This flexibility makes a mid-range UTM a cost-effective investment for general-purpose materials laboratories.