Plastic Tensile Fracture Strain and Deformation Testing: Standards, Methods, and Applications

Plastic tensile fracture strain and deformation testing measures how much a plastic material elongates and deforms before it breaks under uniaxial tensile loading, delivering essential data for material selection, quality control, and product design. Performed in accordance with internationally recognized standards such as ISO 527 and ASTM D638, this test quantifies tensile strength, stress and strain at yield, strain at break, and tensile modulus. This article explains the technical principles, specimen requirements, test procedures, data processing, and typical industrial applications of tensile deformation testing for plastic materials.

Technical Overview

Tensile fracture strain and deformation testing is one of the most fundamental mechanical characterization methods for plastic materials. A standardized specimen is gripped at both ends in a universal testing machine and stretched along its axis at a constant crosshead speed until it yields, deforms, and ultimately fractures. Throughout the test, the applied force and the resulting elongation are continuously recorded and converted into a stress-strain curve, which describes the complete deformation behavior of the material from initial elastic loading to final failure.

Two basic quantities are derived from the test. Tensile stress is calculated as the measured force divided by the original cross-sectional area of the specimen, expressed in megapascals (MPa). Tensile strain is calculated as the change in gauge length divided by the original gauge length, expressed as a percentage. The strain at break, also referred to as elongation at break, indicates the ductility of the material: brittle plastics such as general-purpose polystyrene may break at strains below 2%, while ductile materials such as polyethylene or polypropylene can elongate several hundred percent before fracture.

A typical stress-strain curve for a ductile plastic passes through several distinct deformation stages: an initial linear elastic region, a yield point where permanent plastic deformation begins, a necking and cold-drawing region in which deformation localizes, a strain-hardening region, and finally fracture. For brittle plastics, fracture occurs shortly after the elastic region with little or no plastic deformation. The test therefore provides a complete picture of both strength and deformability.

• ISO 527-1: Plastics — Determination of tensile properties — General principles
• ISO 527-2: Test conditions for moulding and extrusion plastics
• ISO 527-3: Test conditions for films and sheets
• ASTM D638: Standard Test Method for Tensile Properties of Plastics
• ASTM D882: Standard Test Method for Tensile Properties of Thin Plastic Sheeting
• GB/T 1040 series: Chinese national standard, technically aligned with ISO 527
• JIS K 7161 and JIS K 7162: Japanese standards corresponding to ISO 527-1 and ISO 527-2

Testing Samples

ISO Dumbbell Specimens for Rigid and Semi-Rigid Plastics

For moulding and extrusion plastics, ISO 527-2 defines the preferred dumbbell-shaped specimens. Specimen type 1A has an overall length of 170 mm, a gauge width of 10 mm, and a preferred thickness of 4 mm, and is normally injection moulded directly. Specimen type 1B has an overall length of 150 mm with the same gauge width and thickness, and is typically machined from sheets, compression-moulded plates, or finished products. Both types use a gauge length of 50 mm for strain measurement.

ASTM D638 Specimens

ASTM D638 defines five specimen types (Type I through Type V) for rigid and semi-rigid plastics. Type I is the preferred geometry for materials of 7 mm thickness or less, with an overall length of 165 mm and a gauge length of 50 mm. Type IV and the smaller Type V specimens are used when material availability is limited, for example when testing small moulded parts or thin sections. Specimens may be injection moulded, compression moulded, or machined from larger stock.

Film and Sheet Specimens

For plastic films and thin sheets up to approximately 1 mm in thickness, ISO 527-3 specifies several specimen geometries, including dumbbell types and rectangular strips, which are tested with specialized low-force grips. ASTM D882 covers the tensile testing of thin plastic sheeting using strip specimens, providing tensile strength, elongation, and modulus data specifically relevant to flexible packaging materials.

Specimens Cut from Finished Products

When the material must be evaluated in its final processed state, specimens can be machined from pipes, profiles, sheets, or moulded parts. Machining should follow ISO 2818 to ensure smooth, defect-free surfaces, because machining marks and edge notches act as stress concentrators that can significantly reduce the measured strain at break.

Testing Items

Depending on the selected standard and the customer's requirements, tensile fracture and deformation testing of plastics can report the following parameters:

• Tensile stress at break, the maximum stress sustained at the moment of fracture
• Tensile strain at break and nominal tensile strain at break, expressing the total deformation capacity of the material
• Tensile strength and tensile stress at yield, characterizing the load-bearing capability
• Tensile strain at yield, marking the transition from elastic to plastic deformation
• Tensile modulus (Young's modulus), determined from the slope of the stress-strain curve in the initial elastic region
• Secant modulus at a specified strain for materials without a clearly linear region
• Complete stress-strain curve data, including necking and strain-hardening behavior where applicable
• Statistical results, typically the mean value, standard deviation, and coefficient of variation of each parameter

Testing Methods

Specimen Preparation and Conditioning

Specimens are produced by injection moulding, compression moulding, or machining according to the relevant material specification. Before testing, specimens are conditioned in a standard laboratory atmosphere. ISO 291 specifies the commonly used 23/50 atmosphere of 23 °C ± 2 °C and 50% ± 10% relative humidity, while ASTM D618 Procedure A specifies 40 hours at 23 ± 2 °C and 50 ± 5% relative humidity for most moulding plastics. Proper conditioning is essential because temperature and moisture content measurably affect both stiffness and ductility.

Equipment and Instrumentation

Testing is performed on a calibrated universal testing machine whose force measurement system complies with ISO 7500-1 or ASTM E4. Suitable grips, such as mechanical wedge grips or pneumatic grips, hold the specimen without slippage or premature failure at the shoulders. Strain is measured with a contacting or non-contacting extensometer meeting ISO 9513 or ASTM E83, which is mandatory for accurate modulus determination and for valid strain-at-break values of low-ductility materials. Specimen width and thickness are measured with a calibrated micrometer or thickness gauge at several points within the gauge length, and the minimum cross-sectional area is used for stress calculation.

Test Procedure

• Measure and record the specimen dimensions and calculate the original cross-sectional area.
• Mount the specimen in the grips with careful axial alignment to avoid bending stresses.
• Set the gauge length and attach the extensometer, or establish the initial grip separation for nominal strain measurement.
• Apply a small preload to remove slack from the load train.
• Select the test speed specified by the standard or material specification, for example 1 mm/min for modulus determination under ISO 527, common speeds of 5, 20, or 50 mm/min for yield and break properties, and typically 5 mm/min for ASTM D638 Type I specimens.
• Start the test and continuously record force and extension until the specimen fractures completely.
• Test at least five valid specimens per condition, as required by ISO 527-1 and ASTM D638.
• Examine each fracture location and apply the validity criteria of the standard, discarding or annotating specimens that fail in or near the grips.

Data Processing and Calculation

Tensile stress is computed as the force divided by the original cross-sectional area, and tensile strain as the extension divided by the original gauge length, multiplied by 100 to obtain a percentage. The strain at break is the strain recorded at the moment of fracture. If the specimen necks or fractures outside the extensometer gauge length, or if the extensometer must be removed before break, the nominal strain at break based on grip separation is reported instead and clearly identified as such. Under ISO 527-1, the tensile modulus is determined as a secant slope between strains of 0.05% and 0.25%. The final report presents individual and mean values, the test speed, specimen type, conditioning details, and all deviations from the standard.

Influencing Factors

Several factors must be controlled to obtain reproducible strain-at-break data. Higher test speeds generally increase strength and reduce measured elongation, especially for viscoelastic polymers. Elevated temperatures increase ductility, while low temperatures promote brittle fracture. Surface defects, sink marks, voids, moisture in the resin, residual moulding stresses, and poorly machined edges all reduce fracture strain. Consistent specimen preparation, alignment, and conditioning are therefore critical for meaningful comparisons between materials or production batches.

Applications

Tensile fracture and deformation testing is used throughout the plastics value chain, from resin producers and compounders to converters, OEM manufacturers, and accredited third-party laboratories. Typical applications include:

• Quality control and incoming inspection of resins, compounds, films, and moulded parts against material data sheets
• Material qualification and comparison during new product development and material substitution studies
• Generation of design data and input parameters for finite element analysis and structural simulation
• Verification of the effect of processing conditions, additives, fillers, fibers, and regrind content on mechanical performance
• Long-term property assessment, including aging, heat exposure, UV weathering, and chemical compatibility studies that track the retention of elongation at break
• Failure analysis of broken components and root-cause investigation of embrittlement
• Compliance demonstration for regulatory and customer specifications across industries

Sectors that rely heavily on this test include automotive, packaging, consumer electronics, medical devices, wire and cable, construction products, appliances, toys, and aerospace components, wherever the deformation capacity and fracture behavior of plastics determine product safety and reliability.

FAQ

What is the difference between strain at break and elongation at break?

The two terms describe the same property. Strain at break is expressed as a dimensionless ratio or a percentage of the original gauge length, while percent elongation at break is the value multiplied by 100. Both quantify how far the material stretches before fracturing.

What is nominal strain at break?

Nominal strain at break is calculated from the relative movement of the grips or from the initial distance between extensometer contact points rather than from strain measured within a defined gauge length. It is reported when the specimen necks beyond the measured range, when fracture occurs outside the gauge length, or when a class D modulus measurement using grip separation is used, and it should always be clearly labeled as nominal.

Are ISO 527 and ASTM D638 results directly comparable?

No. Although both standards measure the same properties, they differ in specimen geometry, gauge length, preferred test speeds, and modulus calculation. These differences can produce systematically different values, particularly for strain at break and modulus, so data should only be compared when generated under the same standard and the same conditions.

How many specimens should be tested?

Both ISO 527-1 and ASTM D638 require a minimum of five valid specimens for each condition. More specimens may be needed for materials with high scatter, and results should always be reported with their statistical spread.

Why is an extensometer required?

Crosshead displacement includes system compliance, grip settling, and specimen deformation outside the gauge length, which distorts strain measurements, especially at small strains. A calibrated extensometer is required for accurate modulus values and for reliable strain-at-break data of low-ductility plastics.

Can the test be performed at extreme temperatures?

Yes. Equipped with an environmental chamber, an accredited laboratory can conduct tensile testing at elevated temperatures, sub-zero temperatures, or after defined conditioning, which is valuable for automotive, outdoor, and cold-chain applications where deformation behavior changes significantly with temperature.

What does a low strain at break indicate?

A low strain at break, for example below 5%, indicates brittle behavior, which may be intrinsic to the material or caused by degradation, contamination, excessive filler loading, poor processing, physical aging, or stress-concentrating defects. Comparing fracture strain before and after exposure or processing is a sensitive indicator of material embrittlement.