Fracture strain testing at different strain rates determines how materials deform and fail when loaded at velocities ranging from quasi-static to dynamic conditions. This article explains the principles, sample types, measured parameters, standard test methods, and industrial applications of strain-rate-dependent tensile fracture strain evaluation in accordance with ISO, ASTM, and GB standards.
Technical Overview
Fracture strain, often expressed as elongation at fracture or percentage total extension at fracture, is a fundamental measure of material ductility obtained during tensile testing. In practice, ductility is not a fixed material constant: it varies with the rate at which strain is applied. Many metallic materials exhibit positive strain rate sensitivity, meaning their flow stress rises as strain rate increases, while their capacity for plastic deformation before fracture may decrease. Conversely, some polymers and specific alloy classes show increased ductility at intermediate rates due to thermal softening or viscoelastic effects.
Strain-rate-dependent tensile testing quantifies this behavior by performing a series of tensile tests at controlled, systematically varied strain rates, typically spanning quasi-static conditions of approximately 10-5 to 10-2 s-1 up to dynamic conditions exceeding 1 s-1. The resulting fracture strain data are essential inputs for material cards in finite element simulation, crash and impact analysis, and forming process design. Testing is performed in accredited third-party laboratories using universal testing machines, high-speed servo-hydraulic systems, and advanced optical strain measurement systems, following standards such as ISO 6892-1, ASTM E8/E8M, GB/T 228.1, ISO 26203, and ISO 527 series for polymeric materials.
Testing Samples
Metallic Flat Sheet Specimens
Flat rectangular specimens are the most common sample form for sheet metals used in automotive body panels, appliance housings, and structural profiles. Proportional specimens with gauge lengths of 50 mm or 80 mm (per ISO 6892-1) or 50 mm rectangular specimens (per ASTM E8/E8M and GB/T 228.1) are machined with parallel-sided gauge sections. Specimens are typically cut in multiple orientations relative to the rolling direction to capture anisotropic ductility behavior.
Round Bar Specimens
Cylindrical specimens with diameters of 12.5 mm, 10 mm, 8 mm, or smaller proportional geometries are used for wrought bars, forgings, castings, and fastener stock. Round specimens permit accurate measurement of both elongation at fracture and reduction of area after fracture, and they are preferred when through-thickness ductility of thick sections must be characterized.
Wire, Tube, and Foil Specimens
Wires are tested in full-section form with long gauge lengths, guided by ISO 6892-1 Annex provisions or ASTM E8 practices. Tubes may be tested as full sections or with machined longitudinal strips, while thin foils below 0.15 mm are tested according to ASTM E345 with specialized handling to avoid pre-test damage.
Polymer and Composite Specimens
Dumbbell-shaped specimens such as ISO 527-1 type 1A/1B or ASTM D638 type I geometries are used for plastics and fiber-reinforced polymers, where strain rate sensitivity is particularly pronounced due to viscoelastic deformation mechanisms.
Specimen Preparation Requirements
- Gauge sections must be free of machining marks, burrs, and residual stresses that could induce premature fracture.
- Gauge length marks or speckle patterns are applied for extensometer or digital image correlation (DIC) strain tracking.
- Specimen dimensions are verified before testing, since gauge length errors directly bias fracture strain results.
- Minimum recommended quantities are three valid specimens per material condition per strain rate level.
Testing Items
Testing programs at multiple strain rates typically report the following parameters for each rate condition:
- Fracture strain (elongation at fracture, A), measured by extensometer tracking to failure or by reassembly of fractured halves.
- Uniform elongation (Ag) and percentage total extension at maximum force (Agt), which separate uniform deformation from diffuse necking.
- Local fracture strain within the necked region, determined by DIC or by measuring minimum cross-section dimensions after fracture.
- Reduction of area (Z) at fracture for round specimens.
- Yield strength, tensile strength, and strain hardening exponent (n) at each strain rate, providing context for the ductility response.
- Strain rate sensitivity exponent (m), calculated from flow stress comparison across tested rates.
- True stress–true strain curves to fracture, used directly as input for constitutive material models.
Testing Methods
Quasi-Static Strain Rate Control
For conventional rates, ISO 6892-1 and GB/T 228.1 define Method A using closed-loop strain rate control (recommended rates such as 0.00007 s-1 near yield and 0.0067 s-1 in the plastic range) and Method B using estimated crosshead separation rates. ASTM E8/E8M provides equivalent rate options based on stress rate and crosshead speed. Each specimen is tested at a designated strain rate, and the full rate series is repeated across identical specimens to construct the rate dependence curve.
Intermediate and High Strain Rate Testing
For rates from roughly 0.1 s-1 to several hundred s-1, high-speed servo-hydraulic testing machines with slack-type acceleration mechanisms are used, following the guidance of ISO 26203-1 for sheet metals. For extreme dynamic conditions above approximately 500 s-1, split Hopkinson tension bar systems generate reliable stress–strain data, although fracture strain measurement requires high-speed imaging. Temperature chambers may be combined with rate variation when service conditions demand coupled evaluation.
Strain and Fracture Measurement Techniques
- Class 1 clip-on or automatic extensometers per ISO 9513 or ASTM E83 for gauge length extension up to specimen removal before fracture.
- Non-contact video extensometers that track gauge marks through complete fracture, capturing the full fracture strain without interruption.
- Digital image correlation systems providing full-field strain maps, enabling local fracture strain at the neck and reliable measurements at high rates when paired with high-speed cameras.
- Post-fracture reassembly method: fractured specimen halves are fitted together and the final gauge length is measured to compute elongation at fracture, as specified in ISO 6892-1 and ASTM E8/E8M.
Data Validity and Reporting
Valid tests require fracture within the gauge length, stable rate control during yielding and hardening, and corrected strain signals accounting for machine compliance at higher rates. Reports include specimen geometry and orientation, test temperature, the actual strain rate achieved in each segment, force–extension curves, fracture strain values with statistical scatter, and comparative plots of fracture strain versus strain rate. All load frames, load cells, and extensometers must hold current calibration traceable to national standards.
Applications
- Automotive crashworthiness: fracture strain data at dynamic rates feed crash simulation material cards for body-in-white components, crash boxes, and B-pillar reinforcements.
- Sheet metal forming: strain rate dependent ductility defines safe process windows for stamping, deep drawing, and roll forming of high-strength steels and aluminum alloys.
- Aerospace engineering: dynamic tensile ductility supports impact, bird strike, and foreign object damage assessments for airframe and engine materials.
- Energy and pipeline sectors: rate-sensitive fracture characterization of linepipe steels and weldments informs strain-based design in seismic and ground movement regions.
- Additive manufacturing qualification: as-built and post-processed parts are screened for rate-dependent ductility that may differ from wrought equivalents.
- Consumer electronics and packaging: polymers and thin foils are evaluated across loading speeds representative of drop events and high-speed converting operations.
- Constitutive model calibration: fracture strain versus strain rate data populate Johnson-Cook, Cowper-Symonds, and other rate-dependent damage models used in finite element analysis.
FAQ
What is the difference between fracture strain and elongation at fracture?
Fracture strain is the true strain accumulated at the location where the specimen ultimately separates, often measured locally by DIC or cross-section reduction. Elongation at fracture is the engineering strain averaged over the original gauge length at the moment of fracture. Both quantify ductility, but fracture strain better represents the material limit state used in damage models.
Why does fracture strain change with strain rate?
Increasing strain rate raises flow stress in rate-sensitive materials, restricts time-dependent deformation mechanisms, and can shift the balance between strain hardening and thermal softening. Many metals therefore become stronger but less ductile at higher rates, while some polymers show rate-dependent embrittlement or, at other rates, enhanced ductility.
Which standards govern this type of testing?
ISO 6892-1 and GB/T 228.1 cover room-temperature metallic tensile testing with defined strain rate control; ASTM E8/E8M is the equivalent American practice; ISO 26203 addresses high strain rate testing of sheet metals; and ISO 527 or ASTM D638 apply to plastics. Elevated temperature variants include ISO 6892-2 and ASTM E21.
How are fracture strains measured reliably at high strain rates?
Non-contact optical methods are used: video extensometers and DIC systems synchronized with high-speed cameras track gauge marks and full-field strain up to fracture, eliminating the influence of machine compliance and allowing accurate local strain measurement during rapid deformation.
How many specimens and strain rate levels are recommended?
A typical program uses at least three valid specimens at each of four to six strain rate levels spanning the service range of interest, for example 0.0007, 0.007, 0.1, 1, 100, and 500 s-1, with additional conditions added when the rate sensitivity curve shows strong curvature or when simulation accuracy requirements demand denser sampling.