Tensile Yield Strength Testing of Packaging Materials

Tensile yield strength testing is a core mechanical evaluation that determines the stress at which a packaging material transitions from elastic deformation to permanent plastic deformation. This test is critical for verifying the performance of plastic films, laminates, sheets, and molded packaging components used in food, pharmaceutical, and industrial applications. This article covers the testing principles, applicable standards, sample types, test procedures, and practical applications of tensile yield strength testing for packaging materials.

Technical Overview

Tensile yield strength is the stress level at which a packaging material stops deforming elastically and begins to deform plastically, meaning the deformation becomes permanent and non-recoverable. In a typical tensile test, a specimen is gripped at both ends and pulled at a controlled speed until it yields or breaks, while force and extension data are recorded to construct a stress-strain curve. For many ductile packaging polymers such as polyethylene (PE) and polypropylene (PP), the yield point appears as a distinct peak or plateau on the curve, followed by strain softening and drawing before final fracture.

Yield strength is one of the most important design and quality parameters for packaging because it defines the practical load limit of the material. During converting operations such as printing, lamination, bag making, and form-fill-seal, films are subjected to significant web tension, and a material that yields prematurely may stretch unevenly, lose dimensional stability, or develop weak seals. Similarly, thermoformed trays and rigid containers must resist yielding under stacking, filling, and distribution loads to maintain package integrity. Where a material does not exhibit a clear yield peak, the yield strength is conventionally determined using an offset method, most commonly the stress at 0.2% plastic strain.

Testing Samples

Flexible Plastic Films

Single-layer and coextruded films such as LDPE, LLDPE, CPP, BOPP, BOPET, and BOPA with thicknesses typically below 0.25 mm are tested as rectangular strip specimens in accordance with ISO 527-3 or ASTM D882. Because films are anisotropic, specimens are cut in both the machine direction (MD) and transverse direction (TD) to characterize directional differences in yield behavior.

Multilayer Laminates and Barrier Structures

Composite structures such as PET/PE, PA/PE, PET/AL/PE, and paper/film laminates used for pouches, sachets, and blister packaging are tested as full laminates. The measured yield response reflects the combined contribution of all layers, which is the performance that matters in the final package rather than that of any individual layer.

Rigid and Semi-Rigid Sheets

PVC, PET, PS, PP, and PLA sheets with thicknesses from approximately 0.25 mm to several millimeters, used for thermoformed trays, clamshells, and blisters, are tested using dumbbell-shaped specimens per ISO 527-2 or ASTM D638. Specimens may be machined, die-cut, or punched from the sheet in defined orientations.

Molded Packaging Components

Injection-molded or compression-molded items such as closures, caps, pails, drums, and rigid trays can be evaluated using specimens cut from the part itself or from companion test plaques molded under representative conditions, providing yield data that reflects the actual processed state of the material.

Testing Items

A tensile yield strength test program for packaging materials typically reports the following parameters:

• Tensile stress at yield (MPa), calculated as the force at the yield point divided by the original cross-sectional area of the specimen
• Tensile stress at break (MPa), representing the stress at the moment of specimen rupture
• Elongation at yield (%), the strain corresponding to the yield point relative to the initial gauge length
• Elongation at break (%), the total strain achieved at fracture
• Tensile modulus (MPa), the slope of the initial linear region of the stress-strain curve; for films, a 0.1% to 1% or 1% secant modulus is often specified
• Upper and lower yield stress, where applicable for materials that show a distinct yield peak followed by stress drop

Testing Methods

Applicable Standards

• ISO 527-1 and ISO 527-2: determination of tensile properties of plastics, including molding and sheet materials
• ISO 527-3: tensile properties of films and sheets less than 1 mm thick
• ASTM D882: tensile properties of thin plastic sheeting (film)
• ASTM D638: tensile properties of plastics using dumbbell specimens
• GB/T 1040 series and GB/T 13022: Chinese national standards aligned with ISO 527 for plastics and plastic films
• JIS K 7127: Japanese industrial standard for tensile properties of plastic film and sheeting

Specimen Preparation and Conditioning

Film specimens are cut with a sharp die or razor-equipped cutter to widths of 10 mm to 25 mm and lengths sufficient to accommodate the grips and gauge length, with edges verified free of nicks or defects that could cause premature failure. Rigid materials use standardized dumbbell geometries such as ISO 527 1A/1B or ASTM D638 Type I. Thickness and width are measured at several points along the gauge section to determine the minimum cross-sectional area. Unless otherwise specified, specimens are conditioned at 23 ± 2 °C and 50 ± 5% relative humidity for a minimum of 40 hours in accordance with ISO 291 or ASTM D618, since temperature and moisture strongly influence polymer yield behavior.

Equipment and Procedure

Testing is performed on a calibrated universal testing machine fitted with an appropriate load cell and grips; pneumatic side-action grips with plain or rubber-faced faces are common for films, while wedge or capstan grips may be used for higher-strength laminates and rigid specimens. Extension is measured either by crosshead displacement, by a contact extensometer, or by a non-contact video extensometer, with direct strain measurement preferred for accurate modulus determination. A typical procedure includes:

• Mounting the specimen axially in the grips with a defined gauge length, commonly 50 mm to 100 mm for film strips
• Selecting the crosshead speed specified by the standard, for example 100 to 500 mm/min for films per ASTM D882 depending on elongation, or 1 to 500 mm/min per ISO 527-3; rigid dumbbells are commonly tested at 50 mm/min
• Running the test to yield and, where required, to break, continuously recording force and extension
• Identifying the yield point on the stress-strain curve and computing yield strength as the force at yield divided by the original minimum cross-sectional area
• Reporting results as the mean of at least five valid specimens, separately for machine and transverse directions where applicable

Where no distinct yield peak exists, the yield strength is taken as the stress at the intersection of the curve with a line drawn parallel to the initial linear slope and offset by a defined strain, conventionally 0.2%.

Applications

Tensile yield strength data for packaging materials support a wide range of engineering and quality decisions:

• Incoming inspection and supplier qualification for films, laminates, and sheets against purchase specifications
• Process window definition for printing, laminating, slitting, and form-fill-seal operations where web tension must stay safely below the yield point
• Formability assessment for thermoformed trays, blisters, and containers
• Structural design of pouches, bags, and rigid packages to resist stacking, handling, and distribution loads
• Comparative material selection studies, such as evaluating alternative resins, recycled content, or downgauging options
• Failure analysis of packages that stretched, distorted, or lost seal integrity in service
• Regulatory and customer documentation, often generated by accredited third-party laboratories to demonstrate compliance with recognized standards

FAQ

What is the difference between yield strength and tensile strength?

Yield strength is the stress at the onset of permanent plastic deformation, while tensile strength is the maximum stress the specimen sustains during the test. In ductile packaging films, yield strength is usually lower than tensile strength at break because many polymers strain-harden after yielding; for design purposes, yield strength is often the more relevant limit since the package must not permanently deform in use.

Which standard should be used for thin plastic films?

Thin plastic sheeting below approximately 0.25 mm is normally tested with strip specimens per ASTM D882 or ISO 527-3, while thicker sheets and molded materials use dumbbell specimens per ASTM D638 or ISO 527-2. The choice of standard should match the material form and the requirements of the customer or end specification.

Why are machine direction and transverse direction results different?

Films are molecularly oriented during manufacturing, so their mechanical properties are directional. Biaxially oriented films such as BOPP and BOPET often show significantly higher yield strength and lower elongation in both MD and TD compared with the cast or blown counterparts, so both directions must be tested and reported separately.

How is the yield point determined if the material shows no clear peak?

When the stress-strain curve rises smoothly to break without a distinct peak, the offset method is applied. A line parallel to the initial elastic slope is offset by a specified strain, typically 0.2%, and the stress at its intersection with the curve is reported as the offset yield strength.

How many specimens are required for a valid test?

Most standards require a minimum of five valid specimens per direction and material condition. Specimens that fail in the grips, slip, or break outside the gauge section due to edge defects are discarded and replaced, and outliers are treated according to the statistical rules of the applicable standard.

What environmental factors affect yield strength results?

Temperature, humidity, and strain rate all influence polymer yield behavior. Higher temperatures and humidity generally reduce yield strength, while higher test speeds tend to increase the measured yield stress. Standard conditioning at 23 °C and 50% relative humidity and the specified crosshead speeds are therefore essential for comparable, reproducible results.