Constant Rate of Extension (CRE) tensile testing is a core mechanical test method used to determine the tensile strength, elongation, and modulus of textiles, geotextiles, plastics, elastomers, and many other materials. By pulling a specimen at a precisely controlled, constant extension speed until failure, this method provides reproducible data essential for quality control, product certification, and research. This article explains the testing principle, applicable international standards such as ISO 13934 and ASTM D5035, sample requirements, measured parameters, and typical industrial applications.
Technical Overview
Constant Rate of Extension (CRE) tensile testing is a universally recognized method for evaluating the tensile performance of materials. In a CRE system, one grip of the testing machine remains stationary while the driven grip moves away at a constant, precisely controlled velocity, extending the specimen at a fixed rate until it ruptures. The force is measured continuously by a calibrated load cell, and the corresponding elongation is recorded by the machine's displacement system or an extensometer, producing a complete force–extension curve.
CRE is distinguished from two older loading regimes: Constant Rate of Loading (CRL), in which the force increases at a uniform rate, and Constant Rate of Traverse (CRT), in which both grips move and the pull is applied through a pendulum mechanism. Modern accredited third-party laboratories almost exclusively use CRE equipment because it offers superior speed accuracy, better force measurement fidelity, and full compliance with current international standards. Because the extension rate directly influences the measured strength of viscoelastic materials such as polymers and textiles, strict control of the crosshead speed is critical for obtaining comparable and repeatable results.
Typical CRE instruments are universal testing machines (tensile testers) equipped with interchangeable load cells covering ranges from a few newtons (for fine yarns) to several hundred kilonewtons (for geotextiles and heavy webbing), pneumatic or mechanical grips, and software that calculates strength, elongation, modulus, and work of rupture automatically.
Testing Samples
Textiles and Fabrics
Woven, knitted, and nonwoven fabrics are the most common subjects of CRE testing. Specimens may be prepared as strip samples (full-width strips cut and frayed to a precise width) or grab samples (narrow specimens gripped in the center of wider fabric strips), tested in both warp and weft directions. Standard conditioning in a defined atmosphere is required before testing.
Geotextiles and Geosynthetics
Geotextiles, geogrids, and related civil-engineering materials are tested using wide-width specimens to account for their structure-dependent strength. These tests support design verification and conformity assessment for infrastructure projects such as roads, embankments, and landfills.
Plastic Films and Sheets
Packaging films, polymer sheets, and molded plastic test pieces are extended at constant rates to determine tensile strength, yield behavior, and elongation at break, following the general principles of ISO 527 and ASTM D638.
Rubber and Elastomeric Products
Rubber sheets, seals, hoses, and elastic bands are evaluated for tensile strength and ultimate elongation using dumbbell-shaped specimens cut with standard dies.
Webbing, Straps, Cords, and Yarns
Safety webbing, seat belts, lifting slings, ropes, and industrial yarns are tested to verify breaking strength and extension behavior, often as part of product certification programs.
Paper, Nonwovens, and Hygiene Materials
Tissue, filtration media, medical nonwovens, and disposable hygiene products are tested to ensure adequate tensile integrity during processing and end use.
Testing Items
CRE tensile testing generates a range of quantitative results, including:
- Maximum tensile force (breaking strength) expressed in newtons or kilonewtons
- Tensile strength per unit width (N/50 mm or kN/m) for fabric and geotextile samples
- Elongation at maximum force and elongation at break, expressed as a percentage of the initial gauge length
- Initial modulus and chord (secant) modulus derived from the force–extension curve
- Yield strength and yield elongation for plastic and film materials
- Work of rupture (toughness), representing the energy absorbed until failure
- Force at specified elongation values (for example, force at 5% or 10% extension)
- Seam strength and seam slippage behavior for joined or stitched fabric assemblies
Testing Methods
Test Principle
A conditioned specimen is mounted in the grips of a CRE tensile testing machine with a defined gauge length. The moving grip extends the specimen at a constant rate specified by the relevant standard, while the load cell records the force and the system records elongation until the specimen ruptures. The peak force, extension at key points, and curve shape are then reported.
Sample Preparation and Conditioning
Specimens are cut parallel to the machine (warp) and cross (weft) directions, avoiding creases, folds, and damaged areas. Unless otherwise specified, samples are conditioned in a standard atmosphere for textiles and tested under the same conditions. Dimensional parameters such as specimen width, thickness, and gauge length are measured before the test because they directly affect reported strength values.
Strip Method versus Grab Method
The strip method grips the full width of the specimen and measures the contribution of all yarns across the width, providing a fundamental measure of fabric tensile performance. The grab method grips only the central portion of a wider strip and better reflects the strength of fabric as it behaves in service, particularly for woven constructions where yarn movement affects results. Both methods are defined in ISO 13934 (Parts 1 and 2), ASTM D5035 and ASTM D5034, and the corresponding GB/T 3923 standards.
Referenced Standards
- ISO 13934-1 / GB/T 3923.1 — Textiles: tensile properties of fabrics, strip method
- ISO 13934-2 / GB/T 3923.2 — Textiles: tensile properties of fabrics, grab method
- ASTM D5034 and ASTM D5035 — Breaking strength and elongation of textiles by grab and strip procedures
- ISO 10319 / ASTM D4595 — Wide-width tensile testing of geotextiles
- ISO 527 series and ASTM D638 — Tensile properties of plastics
- ASTM D412 — Tensile properties of vulcanized rubber and thermoplastic elastomers
- ISO 9073-3 — Tensile strength of nonwovens using the strip method
Instrumentation and Verification
The testing machine must be verified for force accuracy, speed accuracy, and grip alignment according to the requirements of the applicable standards and the general calibration practices referenced in ISO 7500-1. Proper grip selection (pneumatic, capstan, or wedge-type) prevents specimen slippage and jaw breaks, both of which invalidate results. At least the minimum number of valid specimens specified by the standard is tested, and the mean, minimum, maximum, and coefficient of variation of the key results are calculated and reported.
Applications
CRE tensile testing supports a wide spectrum of industrial and regulatory needs:
- Incoming material inspection and outgoing quality control in textile mills, dyeing and finishing plants, and nonwoven production lines
- Certification of geotextiles used in road construction, railway projects, landfill liners, and erosion-control systems
- Verification of safety-critical products such as seat belts, harness webbing, lifting slings, and personal protective equipment
- Performance evaluation of packaging films, laminates, and shrink films for the food and consumer goods industries
- Automotive and aerospace interior material qualification, including trimmed fabrics and coated textiles
- Medical textile assessment for surgical gowns, drapes, wound dressings, and hygienic disposables
- Research and development support for new fibers, fabric constructions, coatings, and polymer formulations
- Compliance evidence for regulatory submissions, supplier approvals, and buyer specifications
FAQ
What does "constant rate of extension" mean? It means the testing machine increases the distance between the grips at a fixed, uniform speed throughout the test, regardless of how the force on the specimen changes. This ensures that every specimen is stretched under identical, reproducible conditions.
How does CRE differ from CRT testing? In CRE machines only one grip moves at a constant speed and force is measured electronically, while CRT machines pull through a pendulum lever system in which the applied force rate varies with specimen behavior. CRE delivers higher accuracy and is required by most current international standards.
What test speeds are typically used? For apparel-weight fabrics under ISO 13934-1, a common speed is 20 mm/min with a 100 mm gauge length (or 100 mm/min with a 200 mm gauge length for technical textiles), while geotextiles are commonly tested at 20 mm/min elongation rate on 200 mm gauge lengths. The exact speed is always dictated by the governing standard.
How many specimens are tested? Most textile standards require a minimum of five valid specimens per direction, and geotextile or high-variability materials may require ten or more. Specimens that slip in the jaws or break at the grip edge are discarded and replaced.
Why is sample conditioning important? Textile and polymeric materials are sensitive to temperature and humidity, which alter their moisture content and mechanical response. Conditioning in a standard atmosphere ensures that results are comparable between laboratories and across production batches.
How are the results reported? A typical report includes the testing standard, specimen dimensions, gauge length, test speed, conditioning details, individual and mean values of maximum force and elongation, statistical variation, and a description of the failure mode, all issued under the scope of an accredited third-party laboratory where certification is required.