Determination of Tensile Breaking Strength of Plastics: Standards, Methods and Applications

Tensile breaking strength determination is one of the most fundamental mechanical tests for plastics, providing critical data on how plastic materials behave under uniaxial stretching forces until fracture. This article explains the testing principles, specimen requirements, applicable standards such as ISO 527 and ASTM D638, calculation methods, and typical industrial applications of plastic tensile breaking strength testing.

Technical Overview

Tensile breaking strength determination measures the maximum engineering stress a plastic specimen can withstand when it is stretched uniaxially to the point of fracture. During the test, a standardized dumbbell-shaped or strip specimen is clamped in the grips of a universal testing machine and pulled at a constant crosshead speed while the force and extension are continuously recorded. The result, expressed in megapascals (MPa), together with the corresponding strain at break, characterizes the load-bearing capacity and ductility of the material.

This test is essential because tensile behavior reveals whether a plastic is brittle, ductile, or semi-ductile, and it directly influences material selection, part design, and quality control decisions. The most widely applied international and national standards include ISO 527-1 and ISO 527-2 (plastics in general), ISO 527-3 (films and sheets), ASTM D638 (rigid and semi-rigid plastics), ASTM D882 (thin plastic sheeting), and the GB/T 1040 series, which is technically aligned with ISO 527. Accredited third-party laboratories routinely perform this test to support product certification, supplier verification, and failure analysis.

Testing Samples

Injection-Molded Dumbbell Specimens

For moulding and extrusion plastics, the preferred specimen geometry is the dumbbell (dog-bone) shape defined in ISO 527-2 or ASTM D638. Common types include ISO specimens 1A and 1B, and ASTM Types I through V, selected according to the available material quantity and the required test speed. Injection-molded specimens are prepared under controlled melt temperature, mold temperature, and cycle time so that the test pieces are free of sink marks, flash, voids, and internal stress that could distort the results.

Machined Specimens from Finished Products

When the test must represent an actual product, specimens can be machined from sheets, pipes, profiles, laminates, or finished components. Machined dumbbells must be cut carefully, with milled or routed edges polished to remove notch-like machining marks, because surface defects act as stress concentrators and significantly lower the measured breaking strength.

Plastic Films and Sheets

Thin films below approximately 1 mm are tested as rectangular strips in accordance with ISO 527-3 or ASTM D882, using specimen types such as ISO Type 2 (200 mm × 20 mm strips) with wider grips and larger gauge lengths to accommodate the high elongation typical of film materials.

Testing Items

A complete tensile test campaign typically reports the following parameters derived from the force-extension curve:

• Tensile strength at break — the stress at the moment of specimen fracture.
• Tensile strength at yield — the stress at the first point where strain increases without a stress increase, relevant for ductile plastics.
• Tensile strength at maximum force — the maximum stress sustained during the test, particularly for materials that break after strain hardening.
• Elongation at break — the strain at fracture expressed as a percentage of the initial gauge length, an indicator of ductility.
• Tensile modulus (Young's modulus) — the slope of the stress-strain curve in the initial linear elastic region, measured with a contact or video extensometer.
• Secant modulus and nominal strain, when specified by product standards or customer requirements.

Testing Methods

Specimen Conditioning and Preparation

Prior to testing, specimens are conditioned in a standard atmosphere of 23 °C ± 2 °C and 50 % ± 10 % relative humidity for at least 24 hours (40 hours for certain hygroscopic materials) in accordance with ISO 291 or ASTM D618. The width, thickness, and gauge length of each specimen are measured at multiple points using a calibrated micrometer or optical measuring device, and the minimum cross-sectional area is used for stress calculation. At least five valid specimens are tested for each direction and condition, as required by both ISO 527-1 and ASTM D638.

Test Setup and Procedure

The specimen is mounted in self-tightening wedge-action or pneumatic grips with the gauge length aligned to the loading axis, and a small preload (typically ≤ 1 MPa) is applied to remove slack. The crosshead speed is selected from the standard schedule — for example 1, 2, 5, 10, 20, 50, 100, 200, or 500 mm/min — depending on specimen type and material rigidity; ISO 1A specimens are commonly tested at 50 mm/min for modulus and 1 mm/min for modulus determination where specified. An extensometer is attached to the gauge section when accurate modulus and strain-at-break values are required. The machine then extends the specimen at constant speed until fracture, while force and displacement data are acquired at sufficient sampling frequency to capture the yield point, maximum force, and break point.

Data Processing and Calculation

Stress is calculated by dividing the recorded force by the original minimum cross-sectional area of the specimen, and strain by dividing the extension by the initial gauge length. The tensile breaking strength is reported as the stress value at the break event, together with the statistical mean, standard deviation, and coefficient of variation of all valid specimens. Fractures occurring within the grips, at gauge marks, or outside the gauge length are classified as invalid and the corresponding results are discarded or flagged. The final test report documents the material identification, specimen type and preparation method, conditioning, test speed, temperature and humidity, individual and mean values, and the standard followed.

Applications

Tensile breaking strength data are used across nearly every industry that processes or specifies plastic materials:

• Quality control — verifying production consistency of injection-molded parts, extruded profiles, films, and fibers against internal specifications.
• Material selection and comparison — ranking engineering plastics such as polyamide, polycarbonate, POM, and PP compounds for load-bearing applications in automotive, electronics, and consumer goods.
• Research and development — evaluating the effect of fillers, reinforcements, plasticizers, nucleating agents, and processing parameters on mechanical performance.
• Recycled-content validation — comparing the retained strength of recycled resins and regrind against virgin material.
• Failure analysis — determining whether a fractured component met its specified tensile properties, supporting root-cause investigation.
• Regulatory and customer compliance — providing test reports from accredited third-party laboratories for product certification, incoming inspection, and export documentation.

FAQ

What is the difference between tensile strength at break and tensile strength at yield?

Yield strength is the stress at which a ductile plastic begins to deform plastically, while breaking strength is the stress recorded at the moment of fracture. For brittle materials the two values coincide or are very close; for ductile materials, the specimen may break at a stress lower than the yield strength after necking and cold drawing.

Which standard should be used: ISO 527 or ASTM D638?

The choice depends on the target market and specification. ISO 527 (and the equivalent GB/T 1040 series) is common in regions following ISO practice, while ASTM D638 dominates North American specifications. The two standards differ in specimen geometry, gauge length, and test speed, so results are not directly interchangeable and the governing standard must be agreed before testing.

What factors most strongly influence the test results?

Specimen preparation quality (weld lines, voids, edge notches), molding conditions and residual stress, specimen alignment in the grips, crosshead speed, temperature, humidity conditioning, and method of strain measurement all affect the measured breaking strength. Careful control of these variables is essential for reproducible data.

How many specimens are required?

Both ISO 527-1 and ASTM D638 require a minimum of five valid specimens per material and direction. For anisotropic materials such as fiber-reinforced plastics or oriented films, specimens should be cut and tested in both the machine and transverse directions.

Can the test be performed at elevated or sub-zero temperatures?

Yes. An environmental chamber allows testing across a defined temperature range to characterize how embrittlement or softening affects breaking strength, which is valuable for outdoor, automotive, and cryogenic applications.