Fracture strain determination at variable strain rates measures the ductility limit of metallic and polymeric materials under deformation speeds ranging from quasi-static conditions to dynamic impact loading. This testing service is essential for automotive crash simulation, aerospace impact design, high-speed metal forming, and material model calibration, and is performed in accredited third-party laboratories in accordance with ISO, ASTM, and GB standards.
Technical Overview
Fracture strain, also known as strain at fracture or strain at failure, is the engineering strain attained by a test piece at the instant of final separation during a tensile test. It is one of the most fundamental indicators of material ductility and defines the deformation limit used in forming simulations, crashworthiness analysis, and structural integrity assessment.
The mechanical response of most engineering materials is strain-rate dependent. Steels, aluminum alloys, magnesium alloys, polymers, and solder materials frequently exhibit increased strength but reduced ductility as the deformation speed rises. Because real loading events such as vehicle collisions, bird strike, metal stamping, and product drop impact occur at strain rates several orders of magnitude higher than conventional quasi-static testing, fracture strain must be characterized across a spectrum of strain rates rather than at a single rate.
Variable strain rate fracture strain determination covers a testing window typically spanning from 0.0001 per second to more than 1000 per second, using screw-driven or servo-static machines at low rates, servo-hydraulic high-speed systems at intermediate rates, and elastic-bar (split Hopkinson) systems at dynamic rates. The governing framework of methods includes ISO 6892-1, ASTM E8/E8M, and GB/T 228.1 for quasi-static tensile testing, ISO 26203-1 and ISO 26203-2 together with GB/T 30069.1 for high strain rate tensile testing of metals, and ISO 527-1 and ASTM D638 for polymeric materials, supplemented by ISO 8256 and ASTM D1822 for tensile impact behavior.
Testing Samples
Metallic Sheet and Strip Specimens
Flat proportional test pieces are prepared from cold-rolled, hot-rolled, galvanized, or press-hardened steel sheet as well as aluminum and magnesium alloy sheet. Specimens are machined, milled, or laser/waterjet blanked with gauge lengths of 50 mm or 80 mm and width-to-thickness ratios in accordance with ISO 6892-1 or ASTM E8/E8M. Edge quality is controlled because burrs and micro-notches artificially reduce the measured fracture strain.
Round Bar and Bulk Metallic Specimens
Turned round test pieces with threaded, shoulder, or wedge grips are used for forgings, castings, bars, extrusions, and additively manufactured components. Reduced sections with fillet radii conforming to standard proportional ratios ensure fracture occurs within the gauge length.
Polymer and Elastomer Specimens
Dumbbell specimens of types 1A and 1B per ISO 527-2, or Type I per ASTM D638, are injection molded or machined from plates. For tensile impact and high-speed loading, shorter specimens per ISO 8256 and ASTM D1822 are employed so that stress equilibration is achieved within the loading pulse duration.
Welded Joints and Coated Materials
Cross-weld tensile specimens, spot-weld coupons, and coated or clad materials can be evaluated to determine the weakest zone and its fracture strain as a function of strain rate, supporting weld procedure qualification and joint performance modeling.
Testing Items
The standard scope of variable strain rate fracture strain determination includes the following measured and derived quantities:
- Fracture strain at each specified strain rate (quasi-static through dynamic)
- Uniform elongation (Ag) and strain at maximum force
- Total elongation after fracture (A) and total extension at fracture (At)
- Percentage reduction of area (Z) for round specimens
- Yield strength, tensile strength, and flow curves as a function of strain rate
- Strain rate sensitivity exponent (m-value) and Cowper-Symonds or Johnson-Cook rate parameters for CAE input
- Post-necking strain and fracture behavior characterization
- Temperature-dependent fracture strain when combined with environmental chambers
Testing Methods
Quasi-static Reference Testing
Baseline fracture strain is established using universal testing machines operated under strain rate control per ISO 6892-1 Method A or ASTM E8/E8M, with deformation rates from 0.00007 per second to approximately 0.008 per second in the plastic range. Class 0.5 or class 1 extensometers attached until fracture, or dimensional measurement of the fractured gauge length, provide the reference ductility value.
Intermediate and High-Rate Servo-Hydraulic Testing
Servo-hydraulic high-speed testing machines achieve strain rates from about 0.1 per second to 100 per second in conformance with ISO 26203-2. Piston velocities from fractions of a meter per second up to approximately 20 meters per second are applied, and load is measured by strain-gauged load cells or instrumented bars to avoid inertial force contamination. Wave propagation effects on force measurement are corrected according to one-dimensional elastic wave theory.
Elastic-Bar (Split Hopkinson) Tensile Testing
For strain rates from approximately 200 per second to 5000 per second, split Hopkinson tensile bar systems conforming to ISO 26203-1 and GB/T 30069.1 are used. The specimen is loaded between two instrumented elastic bars, incident and transmitted stress waves are recorded with strain gauges, and stress, strain, and strain rate histories are reconstructed using one-dimensional wave analysis. This method provides valid fracture strain data under impact-level loading rates.
Full-Field Strain Measurement
Non-contact digital image correlation (DIC) combined with high-speed cameras operating up to one million frames per second captures full-field strain evolution, localizes necking, and identifies the true strain at fracture initiation. DIC is particularly valuable beyond the uniform elongation stage where conventional extensometers only provide average strain.
Data Evaluation and Reporting
Test reports document the strain rate applied in the plastic range, specimen geometry, machine and measurement system class, strain measurement method, fracture strain values at each rate, complete flow curves, and derived rate-sensitivity parameters. Traceability to calibrated force and strain measurement chains per ISO 7500-1 and ISO 9513, or their ASTM E4 and ASTM E83 equivalents, is maintained for all measurements.
Applications
Variable strain rate fracture strain data underpin material cards for crash and impact simulation in finite element codes such as LS-DYNA, PAM-CRASH, and ABAQUS, where failure strain as a function of strain rate directly controls element deletion and rupture prediction.
- Automotive body-in-white development: mild, high-strength, and press-hardened steels plus aluminum sheet for crashworthiness validation
- Aerospace structures: impact tolerance of aluminum, titanium, and composite-to-metal joints for bird strike and foreign object damage assessment
- Sheet metal forming: high-speed stamping, electromagnetic forming, and blanking limit characterization
- Energy and pipeline industries: strain-rate-dependent toughness of line pipe steels for arrestor and fracture control design
- Electronics reliability: solder alloys and encapsulants subjected to board-level drop and shock loading
- Additive manufacturing qualification: rate-dependent ductility verification of laser and electron beam melted alloys
FAQ
What is the difference between fracture strain and elongation after fracture?
Elongation after fracture is conventionally determined by reassembling the broken specimen and measuring the final gauge length, whereas fracture strain obtained from extensometer or DIC records represents the strain at the moment of separation. At high strain rates, non-contact optical methods are preferred because manual reassembly cannot be performed reliably.
Why must strain rate be controlled and reported?
Plastic flow stress and ductility of most metals and polymers vary significantly with strain rate. ISO 6892-1 defines strain rate classes for quasi-static testing, and ISO 26203-1/-2 define procedures for high-rate testing, precisely because results are only comparable when the applied strain rate in the parallel length is known and documented.
Which strain rate range suits which test system?
Screw-driven and servo-static machines cover roughly 0.0001 to 0.01 per second, servo-hydraulic high-speed frames cover 0.1 to 100 per second, and split Hopkinson bar systems cover approximately 200 to 5000 per second. Accredited third-party laboratories select the system based on the target rate and specimen size.
How is strain measured accurately at high strain rates?
Contact extensometers are limited by inertia and risk of damage, so high-rate tests rely on strain gauges bonded to the specimen or elastic bars, laser extensometers, and DIC with high-speed cameras. These techniques provide local strain histories up to and including fracture.
How many specimens and rates are required for a rate-dependent material model?
A typical characterization program includes at least three valid specimens at each of four to six logarithmically spaced strain rates, for example 0.001, 0.1, 10, 100, and 1000 per second, allowing reliable fitting of Johnson-Cook, Cowper-Symonds, or piecewise rate-dependent failure models.
Which standards govern this testing?
The primary references are ISO 6892-1, ASTM E8/E8M, and GB/T 228.1 for quasi-static tensile testing; ISO 26203-1, ISO 26203-2, and GB/T 30069.1 for high strain rate tensile testing of metallic materials; and ISO 527-1, ASTM D638, ISO 8256, and ASTM D1822 for plastics under conventional and high-speed loading.