Plastic Tensile Breaking Force Testing: Standards, Methods and Applications

Plastic tensile breaking force testing is a fundamental mechanical evaluation that determines the maximum force a plastic specimen can withstand before rupturing under uniaxial tension. Governed by internationally recognized methods such as ISO 527, ASTM D638, and GB/T 1040, this test delivers critical data on tensile strength, elongation at break, and product reliability for manufacturers, suppliers, and accredited third-party laboratories. This article explains the technical principles, sample types, test procedures, calculations, and industrial applications of plastic tensile breaking force testing in detail.

Technical Overview

Plastic tensile breaking force testing, also known as plastic tension-to-rupture testing, is a destructive mechanical test in which a plastic specimen is clamped between two grips and stretched at a constant rate until it fractures. The test records the maximum force, expressed in newtons (N) or kilonewtons (kN), required to break the specimen, along with the accompanying force-elongation curve. When this breaking force is normalized by the original cross-sectional area of the specimen, the result is the tensile strength at break, expressed in megapascals (MPa).

This distinction is important in practice. Breaking force is a direct, product-level measurement commonly specified for finished goods such as woven bags, films, straps, ropes, and seams, while tensile strength is a material property used to compare resins, compounds, and formulations. Together with elongation at break, yield behavior, and tensile modulus, the breaking force provides a comprehensive picture of how a plastic material or component will perform under real-world tensile loading.

Because tensile performance directly affects product safety, durability, and regulatory compliance, plastic tensile breaking force testing is one of the most frequently requested mechanical tests in incoming material inspection, quality control, product certification, and failure analysis programs conducted by accredited third-party laboratories.

Testing Samples

Standard Dumbbell (Dog-Bone) Specimens

Rigid and semi-rigid plastics are typically tested using injection-molded or machined dumbbell specimens. ISO 527-2 defines multipurpose specimens Type 1A (preferred for injection molding) and Type 1B (preferred for machining from sheets, laminates, or finished parts), both featuring a 50 mm gauge length. ASTM D638 defines specimen Types I through V, with Type I (50 mm gauge length, approximately 3.2 mm thick) being the most widely used in North America and Type V used when only limited material is available.

Film and Sheet Strip Specimens

Flexible plastic films and thin sheets up to approximately 1 mm in thickness are tested as rectangular strips in accordance with ISO 527-3 or ASTM D882. Typical strip widths range from 10 mm to 25.4 mm, with gauge lengths of 50 mm to 100 mm depending on elongation capability. This category includes packaging films, lidding films, agricultural films, barrier laminates, and protective membranes, where the breaking force per unit width is often the specified acceptance criterion.

Whole Product and Component Samples

Many plastic products are evaluated as complete functional units rather than cut specimens. Examples include plastic woven sacks and bags, flexible intermediate bulk container (FIBC) lifting loops and body panels, plastic strapping and banding, ropes, twines, monofilaments, nets, geogrid ribs, welded joints, heat seals, sewn seams, and snap-fit or mechanically fastened assemblies. Testing the finished component captures the influence of processing, orientation, and joint quality on breaking force.

Pipe, Profile, and Machined Section Samples

For plastic pipes, longitudinal strip or ring specimens cut from the pipe wall are tested in accordance with ISO 6259 or corresponding national standards. Sections machined from thick sheets, profiles, moldings, and oversized parts are prepared as reduced-section specimens so that the measured breaking force reflects the actual processed material rather than a separately molded plaque.

Testing Items

Depending on the specimen type, product specification, and referenced standard, a plastic tensile breaking force test program may include the following measured and calculated parameters:

• Breaking force (N or kN): the maximum tensile force sustained by the specimen at rupture
• Tensile strength at break (MPa): breaking force divided by the original minimum cross-sectional area
• Tensile strength at yield (MPa): stress at the yield point for ductile materials
• Elongation at break (%): increase in gauge length at rupture relative to the initial gauge length
• Elongation at yield (%): strain at the yield point
• Tensile modulus (MPa): slope of the stress-strain curve in the initial linear region
• Breaking strength per unit width (N/mm or N/50 mm): commonly specified for films, fabrics, and tapes
• Joint or seam efficiency (%): ratio of seam breaking force to parent material breaking force for welded, heat-sealed, or sewn assemblies

Testing Methods

Reference Standards

• ISO 527-1 and ISO 527-2: determination of tensile properties of plastics using multipurpose specimens
• ISO 527-3: tensile testing of films and sheets
• ASTM D638: tensile properties of plastics (dumbbell specimens)
• ASTM D882: tensile properties of thin plastic sheeting and film
• GB/T 1040.1, GB/T 1040.2, and GB/T 1040.3: Chinese national standards technically aligned with ISO 527
• GB/T 13022: tensile testing of plastic films
• ISO 6259: tensile properties of thermoplastics pipes
• JIS K 7161 and JIS K 7162: Japanese standards harmonized with ISO 527

Equipment and Calibration

Testing is performed on a universal testing machine operating at constant rate of extension, equipped with grips appropriate to the specimen type: wedge-action or pneumatic clamps for rigid dumbbells, rubber-faced or pneumatic film grips with optional slip-free liners for flexible films, and capstan or wrap grips for cords, ropes, and strapping. Force measurement systems are calibrated and verified in accordance with ISO 7500-1 or ASTM E4, typically to accuracy class 0.5 or 1. When tensile modulus is required, strain is measured with a contact or video extensometer meeting ISO 9513 or ASTM E83 requirements, since crosshead displacement alone overstates strain.

Specimen Conditioning and Preparation

Specimens are conditioned in a standard laboratory atmosphere of 23 ± 2 °C and 50 ± 10% relative humidity in accordance with ISO 291, or 23 ± 2 °C and 50 ± 5% relative humidity for at least 40 hours in accordance with ASTM D618 Procedure A. Hygroscopic materials such as polyamide may require additional conditioning, for example following ISO 1110, because moisture content strongly influences breaking force and elongation. Specimen dimensions, particularly width and thickness within the gauge section, are measured at multiple points using instruments with suitable resolution, and the minimum cross-sectional area is used in stress calculations. Edges of machined specimens must be smooth and free of nicks, scratches, and stress raisers that would produce premature failure.

Test Procedure

• Verify specimen dimensions and calculate the original cross-sectional area or record the specimen width for film strips
• Mount the specimen in the grips with careful axial alignment to eliminate bending stresses, applying a small preload if specified
• Attach the extensometer to the gauge section when accurate strain, yield strain, or modulus data are required
• Select the crosshead speed prescribed by the standard and the material specification, typically within the range of 1 mm/min to 500 mm/min depending on specimen type and material stiffness
• Start the test and continuously record the force-elongation or stress-strain curve until the specimen ruptures completely
• Record the breaking force, maximum force, yield point, and elongation values directly from the curve and data acquisition system
• Repeat the test on a minimum of five valid specimens; specimens that fail in the grip faces (jaw breaks) are normally rejected and retested unless the failure behavior itself is being evaluated
• Examine and document the fracture location and failure mode as part of the test record

Calculation and Reporting

The tensile strength at break is calculated as the breaking force divided by the original cross-sectional area of the specimen. Elongation at break is calculated as the increase in gauge length at rupture divided by the initial gauge length, expressed as a percentage. For film and tape products, breaking force or breaking strength is frequently reported per unit width. When tensile modulus is required, ISO 527-1 specifies the chord modulus determined between strain points of 0.05% and 0.25%. The final test report typically includes the individual and mean values, standard deviation, specimen type and dimensions, conditioning details, test speed, grip separation, failure mode and location, and the complete identification of the standard applied, providing traceable evidence for quality records and certification purposes.

Applications

Plastic tensile breaking force testing supports quality assurance and product development across virtually every industry that uses polymeric materials:

• Packaging: verification of breaking force and seal strength for flexible films, bags, pouches, woven sacks, and shipping containers
• Construction and infrastructure: qualification of geomembranes, geogrids, drainage nets, plastic pipes, and profiles used in civil engineering projects
• Automotive and transportation: validation of interior trim, clips, ducts, fuel system components, and composite parts against specification limits
• Electrical and electronics: evaluation of insulation materials, cable sheaths, connectors, and structural housings
• Medical and pharmaceutical: mechanical characterization of tubing, device components, and sterile barrier materials
• Consumer goods: incoming resin inspection, production quality control, and supplier qualification for household and sporting products
• Research and development: comparison of formulations, fillers, recycled content, and processing conditions, as well as studies of degradation after heat aging, UV exposure, or chemical contact
• Regulatory compliance: generation of objective test evidence for product certification, tender submissions, and acceptance by buyers, supported where required by testing at accredited third-party laboratories

FAQ

What is the difference between breaking force and tensile strength?

Breaking force is the absolute force, in newtons, at which the specimen ruptures, and it depends on the specimen geometry. Tensile strength is the breaking force divided by the original cross-sectional area, expressed in MPa, making it a geometry-independent material property. Product specifications for films, bags, straps, and ropes usually cite breaking force, while material datasheets cite tensile strength.

Which standard should be selected for my plastic product?

For molded or machined rigid plastics, ISO 527-2 or ASTM D638 is appropriate. For films and thin sheets, ISO 527-3, ASTM D882, or GB/T 13022 applies. For thermoplastic pipes, ISO 6259 is used. Many product-specific standards, such as those for woven sacks or geosynthetics, reference these base methods and add their own sampling and acceptance criteria, so the governing product standard should always be confirmed first.

How many specimens are required for a valid test?

Both ISO 527 and ASTM D638 require a minimum of five valid specimens per condition. Additional specimens are tested when results show high scatter, when several directions of anisotropic material are evaluated, or when different conditioning states are compared.

What happens if the specimen breaks inside the grips?

A failure at or in the grip faces, known as a jaw break, is generally considered invalid because grip-induced stress concentration lowers the measured breaking force. The result is discarded, the gripping method is adjusted, and a replacement specimen is tested. If jaw breaks persist, the specimen design, grip pressure, or grip lining should be reviewed.

Can finished plastic products be tested without cutting specimens?

Yes. Whole components such as bag bodies, lifting loops, straps, seals, and welded joints are frequently tested in their final configuration to assess real performance, including the effect of seams and processing. This type of testing is often more meaningful for acceptance purposes than tests on cut specimens alone.

What factors most strongly influence breaking force results?

Key influences include material orientation from processing, moisture conditioning for hygroscopic polymers, specimen preparation quality, test speed, temperature, grip alignment, and dimensional measurement accuracy. Consistent control of these variables is essential for reproducible results, which is why standardized conditioning and calibrated equipment are mandatory elements of every test.