Elongation at break is one of the most critical mechanical property indicators for medical plastics, directly reflecting the ductility, flexibility, and fracture resistance of materials used in catheters, infusion sets, syringes, and sterile packaging. Determination of elongation at break for medical plastics is typically performed in accredited third-party laboratories in accordance with ISO 527, ASTM D638, and GB/T 1040 series standards, using universal testing machines equipped with calibrated extensometers. This article provides a comprehensive technical overview of the test principle, sample types, testing procedures, calculation methods, and typical applications in medical device quality control and regulatory submission.
Technical Overview
Elongation at break, also known as tensile strain at break, is defined as the percentage increase in gauge length of a plastic specimen at the moment of rupture relative to its initial gauge length. It quantifies the capacity of a material to deform plastically before fracturing and is therefore a key measure of ductility and toughness. For medical plastics, this property governs whether a device can withstand manufacturing stresses, clinical handling, kinking, bending, pressurization, and sterilization without catastrophic failure.
The determination is carried out by gripping a standardized specimen in a universal testing machine, extending it at a constant crosshead speed until rupture, and recording the increase in gauge length at break. The parameter is calculated as follows:
Elongation at break (%) = (Lb − L0) / L0 × 100, where L0 is the initial gauge length and Lb is the gauge length at rupture.
The principal standards governing this determination include:
- ISO 527-1 (Plastics — Determination of tensile properties — General principles) and ISO 527-2 (test conditions for moulding and extrusion plastics)
- ISO 527-3 (test conditions for films and sheets) for thin medical films
- ASTM D638 (Standard Test Method for Tensile Properties of Plastics), widely referenced in North American regulatory documentation
- ASTM D882 for tensile properties of thin plastic sheeting used in sterile barrier systems
- GB/T 1040 series, the Chinese national adoption of ISO 527, commonly required for NMPA registration dossiers
- JIS K 7161 and JIS K 7162, the Japanese industrial adoptions of the ISO 527 methodology
- ISO 37 and ASTM D412 for highly elastomeric medical materials such as silicone and thermoplastic polyurethane where applicable
Because test results are highly sensitive to specimen geometry, conditioning, and strain rate, reproducible determination requires strict adherence to the selected standard and calibration of all measuring equipment in accredited laboratories.
Testing Samples
Standard Dumbbell Specimens from Rigid Thermoplastics
For materials such as polypropylene, polycarbonate, ABS, HDPE, and cyclic olefin copolymer used in syringe barrels, luer connectors, and device housings, elongation at break is determined on injection-molded or machined dumbbell (dog-bone) specimens. ISO 527-2 type 1A and 1B specimens with a 50 mm gauge length are the most common formats, while ASTM D638 type I specimens serve the equivalent function in ASTM-based protocols. These specimens ensure a controlled stress concentration in the central parallel section so that fracture occurs within the gauge region.
Small-Format Specimens from Finished Devices
When the amount of available material is limited or when finished device behavior must be characterized, reduced-size specimens are used. These include ISO 527-2 type 5A and 5B small-format dumbbells and ASTM D638 type V specimens, which can be die-cut or precision-machined from molded components, extruded sheets, or recovered device sections. This approach is especially valuable for design verification and material qualification of miniature medical components.
Flexible Tubing and Catheter Segments
For extruded products such as PVC infusion tubing, polyurethane and PEBAX catheter shafts, and silicone drainage tubes, whole-tube tensile testing is often performed instead of cutting dumbbells. Tubular segments are gripped between appropriately sized fixtures or fitted over mandrels to prevent slippage and premature failure at the grips. The extension at break is determined from grip separation or from video extensometer measurements, providing data directly relevant to product standards for intravascular catheters and infusion equipment.
Medical Films and Sheets
Thin materials used in blood bags, IV solution containers, sterile barrier packaging, and blister trays are tested as rectangular strips in accordance with ISO 527-3 or ASTM D882. Specimen width, thickness measurement location, and the use of elongation-appropriate grip liners are critical for valid results, since films may elongate several hundred percent before rupture.
Testing Items
A tensile determination campaign for medical plastics typically reports the following properties, of which elongation at break is the primary focus of this article:
- Elongation at break (percent strain at rupture)
- Tensile strength at break
- Tensile stress and strain at yield
- Tensile modulus (Young's modulus, secant modulus where specified)
- Force at break for whole-tube and whole-device specimens
- Statistical summary: mean value, standard deviation, and coefficient of variation for each specimen set
For elastomeric materials tested to ISO 37 or ASTM D412, additional items such as tensile stress at given elongations may be reported to characterize the stiffness profile of catheter and tubing materials.
Testing Methods
Instrumentation and Equipment
The core instrument is a calibrated universal testing machine fitted with a load cell appropriate to the expected breaking force, commonly ranging from tens of newtons for films and catheters up to several kilonewtons for rigid molded plastics. Extension is measured either by a contact or video extensometer conforming to the accuracy requirements of ISO 9513, or, for highly extensible specimens where extensometer attachment is impractical, by calibrated crosshead displacement with the measurement method clearly documented. Pneumatic or wedge-action grips with compliant grip faces prevent specimen slippage and jaw breaks, both of which invalidate results.
Specimen Preparation and Conditioning
Specimens are prepared by injection molding, die cutting, or precision machining according to the requirements of the governing standard, with burrs, notches, and visible defects rejected before testing. Unless otherwise specified by the material or product standard, specimens are conditioned in a standard laboratory atmosphere of 23 °C ± 2 °C and 50% ± 10% relative humidity in accordance with ISO 291 or ASTM D618, typically for a minimum of 40 hours. Specimen dimensions, particularly thickness and width within the gauge section, are measured at multiple points using calibrated instruments with resolution appropriate to the specimen size.
Test Procedure
The general procedure is as follows:
- Verify instrument calibration status and select a load cell such that the anticipated breaking force falls within its accurate working range
- Set the initial gauge length and grip separation specified for the specimen type
- Mount the specimen symmetrically with the gauge axis aligned to the loading axis
- Attach the extensometer or activate the video measurement system
- Apply the specified constant test speed; typical practice uses approximately 1 mm/min for modulus determination followed by 50 mm/min for the determination of break properties of rigid specimens, while flexible films and elastomeric tubing are tested at higher rates, commonly up to 500 mm/min, as specified by the standard
- Extend the specimen to rupture, recording force and extension continuously
- Reject results from specimens that fail in the grips or outside the gauge region, and test additional specimens to obtain the required number of valid results
ISO 527 and ASTM D638 both require a minimum of five valid specimens per condition unless otherwise justified, and most medical device protocols increase this number to support statistical evaluation.
Calculation and Expression of Results
Elongation at break is calculated from the extension measured between the initial gauge length and the moment of rupture, expressed as a percentage. Reported values are typically the arithmetic mean of all valid specimens, accompanied by the standard deviation. For small specimen populations or data sets that are not normally distributed, the median and range may be reported in accordance with the statistical conventions of the applied standard. Test reports must state the standard used, specimen type and dimensions, conditioning regime, test speed, measurement method for extension, number of specimens, and individual as well as mean results, since all of these factors materially influence comparability between laboratories.
Result Interpretation and Acceptance Criteria
Elongation at break values range from a few percent for highly filled or highly crystalline rigid plastics to well over 500% for plasticized PVC and thermoplastic polyurethane. Acceptance criteria are established by the device manufacturer based on risk analysis, historical material data, and functional requirements of the device; a significant reduction in elongation at break relative to the baseline material specification is a widely recognized early indicator of polymer degradation, embrittlement, or process deviation.
Applications
Determination of elongation at break supports numerous activities across the medical device lifecycle:
- Incoming material quality control for polymers used in catheters, tubing, syringes, connectors, and housings, verifying supplier consistency against material specifications
- Design verification of extrusion and molding processes, confirming that processing parameters have not degraded ductility
- Sterilization validation and dose auditing, since gamma irradiation, electron beam, ethylene oxide, and steam sterilization can induce chain scission or crosslinking that measurably reduces elongation at break; testing is performed before and after exposure alongside sterilization standards such as ISO 11137
- Shelf-life and aging studies, including real-time and accelerated aging programs, in which elongation at break serves as a sensitive endpoint for polymer stability of both devices and sterile barrier systems
- Regulatory submissions, including technical documentation supporting device registrations in multiple jurisdictions, where test data generated under ISO 527, ASTM D638, or GB/T 1040 protocols must be reported with full traceability
- Failure analysis of returned or complaint devices, comparing retained samples against production controls to identify degradation mechanisms
Because elongation at break responds strongly to molecular weight changes, plasticizer loss, residual stress, and thermal history, it is one of the most informative single measurements available for monitoring the mechanical integrity of medical plastics.
FAQ
What is elongation at break and why is it important for medical plastics?
Elongation at break is the percentage increase in length a plastic specimen undergoes before rupturing in tension. For medical plastics it indicates whether a material is sufficiently ductile to resist cracking during device manufacture, clinical use, and sterilization, and it serves as an early warning indicator of polymer degradation in aging and sterilization studies.
Which standards are used for the determination of elongation at break of medical plastics?
The principal standards are ISO 527-1 and ISO 527-2 for molded and extruded plastics, ISO 527-3 and ASTM D882 for films and sheets, ASTM D638 for plastics under ASTM protocols, GB/T 1040 series for Chinese regulatory purposes, and JIS K 7161 and JIS K 7162 for Japanese industrial testing. Elastomeric materials may be tested to ISO 37 or ASTM D412 where appropriate.
Can finished catheters and tubing be tested instead of molded dumbbell specimens?
Yes. Whole-tube tensile testing of catheter and tubing segments is common practice and often more representative of device performance. The extension at break is measured by grip separation or video extensometry, and specialized fixtures or mandrels are used to prevent grip-induced failure. The chosen method must be documented in the test report.
How does sterilization affect elongation at break?
High-energy sterilization such as gamma or electron beam irradiation can cause chain scission in polymers such as polypropylene, reducing elongation at break significantly. Ethylene oxide and steam cycles may also affect plasticized or hydrolysis-sensitive materials. Comparing elongation at break before and after sterilization within a validated dose or cycle range is a standard element of sterilization process qualification.
How many specimens are required for a valid determination?
Both ISO 527 and ASTM D638 require at least five valid specimens per test condition. Specimens that break in the grips or outside the gauge length are discarded and replaced. Medical device test protocols frequently specify larger sample sizes to enable robust statistical comparison against acceptance criteria.
What test speed should be used?
The test speed is dictated by the applied standard and specimen type. Rigid specimens are commonly tested at approximately 50 mm/min after modulus measurement at a slower rate, while flexible films and elastomeric tubing are tested at higher speeds, frequently up to 500 mm/min. Because strain rate strongly influences the measured elongation, the speed used must always be reported along with the results.