Determination of fracture strain at quasi-static strain rates is a fundamental mechanical test that quantifies the deformation capacity of metallic and polymeric materials before final failure under slow, controlled loading. Performed in accordance with international standards such as ISO 6892-1, ASTM E8/E8M and GB/T 228.1, this test provides critical ductility data for material qualification, structural design and failure analysis. This article explains the testing principle, sample types, measurement procedures and typical industrial applications of quasi-static fracture strain measurement.
Technical Overview
Quasi-static strain rate fracture strain determination measures the strain a material sustains at the moment of complete separation when loaded in tension, compression or bending at strain rates low enough that inertial and wave-propagation effects are negligible. In practice, quasi-static testing is performed on servo-controlled universal testing machines at strain rates typically ranging from 10-5 s-1 to 10-1 s-1, in contrast to dynamic or high strain rate testing performed on drop towers, flywheel machines or split Hopkinson pressure bars.
Fracture strain can be expressed in several complementary forms depending on the material and the engineering purpose: total elongation at fracture over a defined gauge length, uniform elongation at maximum force, local fracture strain measured over a reduced section or across a notch, and true fracture strain calculated from the reduction of area at the fracture surface. For smooth tensile specimens, the governing standards include ISO 6892-1, ASTM E8/E8M, EN ISO 6892-1, GB/T 228.1 and JIS Z 2241 for metallic materials, and ASTM D638 and ISO 527 for plastics. For notched and pre-cracked fracture mechanics specimens, fracture resistance is additionally characterized under ASTM E1820 and ISO 12135.
Because fracture strain is highly sensitive to strain localization, measurement technique and gauge length, the test requires calibrated force measurement per ISO 7500-1 or ASTM E4, calibrated strain measurement per ISO 9513 or ASTM E83, and careful specimen preparation. Modern laboratories increasingly employ non-contact video extensometry and digital image correlation (DIC) to capture the highly localized strain field at the necking region up to the instant of fracture.
Testing Samples
Metallic Sheet and Plate Specimens
Flat dog-bone specimens with rectangular cross sections are machined from thin sheet, strip or plate. Both proportional specimens (gauge length proportional to the square root of cross-sectional area, e.g., L0 = 5.65·√S0) and non-proportional specimens with fixed gauge lengths of 50 mm or 80 mm are used. These specimens are typical for automotive steels, aluminum alloys, pipeline steels and clad plates.
Round Bar and Forged Specimens
Cylindrical specimens turned from bars, forgings, castings and thick sections allow accurate determination of reduction of area and true fracture strain. Threaded-end, shouldered-end or wedge-grip geometries may be selected according to the material strength and the fixtures available.
Notched and Pre-Cracked Fracture Specimens
Single edge notch tension (SENT), single edge notch bend (SENB) and compact tension (CT) specimens with machined notches or fatigue pre-cracks are used to determine fracture strain and crack driving force for toughness-critical applications. Notch root radius, notch depth and crack length ratio are controlled to standard tolerances.
Welded Joint Specimens
Cross-weld tensile and notched specimens positioned with the weld centerline, fusion line or heat-affected zone located within the gauge section are used to evaluate fracture strain of girth welds and structural welds, particularly for strain-based design of pipelines and offshore structures.
Polymer and Composite Specimens
Dumbbell-shaped specimens per ASTM D638 (Type I–V) or ISO 527-2, including reinforced plastics and adhesively bonded joints, are tested to determine elongation at break and strain at fracture for polymer-based components.
Additively Manufactured and Thin-Foil Specimens
Specimens extracted from 3D-printed parts in different build orientations, as well as thin foils and electrode-type materials used in batteries and electronics, require miniature geometries and high-resolution non-contact strain measurement to resolve fracture strain reliably.
Testing Items
The principal measurands reported in quasi-static fracture strain determination include:
- Total engineering strain at fracture (percentage elongation after fracture) over a specified gauge length.
- Uniform elongation, defined as the plastic strain at maximum force prior to necking.
- Post-necking elongation and localized necking strain derived from DIC full-field measurement.
- True fracture strain, calculated as the natural logarithm of the initial-to-final cross-sectional area ratio (εf = ln(A0/Af)).
- Local fracture strain across notched sections and critical stretch-flange or bend regions.
- Reduction of area and fracture surface contraction measurements.
- Strain rate sensitivity of fracture strain, obtained by testing at multiple quasi-static rates (e.g., 10-4, 10-3 and 10-2 s-1).
- Environmental conditioning effects, including fracture strain after hydrogen pre-charging, aging, corrosion exposure or elevated and sub-zero temperature testing.
Testing Methods
Testing is conducted on calibrated universal testing machines fitted with an appropriate load cell, grips and strain measurement devices. The general procedure is as follows:
- Specimen preparation: dimensions are measured at multiple locations within the gauge section; gauge marks are applied; for DIC measurement, a random high-contrast speckle pattern is sprayed onto the gauge region; notched specimens are notch-machined or fatigue pre-cracked to the required geometry.
- Equipment verification: force and displacement systems are verified per ISO 7500-1/ASTM E4 and ISO 9513/ASTM E83; the extensometer class (e.g., Class 0.5 or Class 1) is selected according to the required accuracy.
- Mounting and alignment: the specimen is centered and gripped with verified alignment to suppress bending strains; a small preload removes slack and seating effects.
- Strain rate control: tests are run under constant crosshead speed or closed-loop strain rate control (Method A of ISO 6892-1), with the target rate documented in the report; typical quasi-static rates lie between 0.00007 s-1 and 0.008 s-1 depending on the standard and product specification.
- Data acquisition: force, displacement, and extension signals, together with video or DIC strain fields, are recorded continuously until complete specimen separation.
- Data reduction: engineering stress–strain curves are computed; elongation at fracture is measured either by fitting the broken halves together and measuring final gauge length, or directly from extensometer/DIC data; true fracture strain is obtained from post-test measurement of the minimum cross section at the fracture plane; for notched specimens, the local strain history over the notch ligament is extracted from DIC data at the instant of crack initiation.
- Replication and reporting: a minimum of three valid replicates is customary; results are reported with specimen identification, orientation, heat treatment condition, gauge length, strain rate, temperature, and individual and mean fracture strain values.
Quality control measures include validation of fracture position within the gauge length, exclusion of tests failing outside the reduced section, fractographic examination to confirm the failure mode, and documented measurement uncertainty covering dimensional metrology, strain measurement class and rate accuracy.
Applications
Quasi-static fracture strain data underpin a wide range of engineering decisions across industries:
- Pipeline engineering: strain-based design and girth weld qualification require fracture strain of pipe steel and welds to confirm deformation capacity during installation by reeling and laying, and in service under ground movement.
- Automotive and transportation: fracture strain at quasi-static rates provides baseline ductility input for crash simulation material cards and for selecting advanced high-strength steels and aluminum alloys in forming and joining development.
- Aerospace and energy: qualification of structural alloys, forgings and castings for pressure vessels, turbines and offshore structures uses fracture strain as a key ductility acceptance criterion.
- Additive manufacturing: build orientation and process parameter qualification rely on fracture strain comparisons to detect porosity, lack-of-fusion and anisotropy effects.
- Welding procedure qualification: cross-weld fracture strain demonstrates that weld metal and heat-affected zone ductility matches parent material requirements.
- Corrosion and hydrogen research: degradation of fracture strain after hydrogen charging or environmental exposure is a sensitive indicator of embrittlement susceptibility.
- Polymer and battery industries: elongation at break of plastics, adhesives, separators and electrode foils supports durability assessment, quality control and safety evaluation of components and assemblies.
- Failure analysis: measured fracture strain of service-exposed material is compared with baseline data to determine whether failure involved excessive deformation, material degradation or pre-existing defects.
FAQ
What strain rate range is considered quasi-static?
Quasi-static testing generally covers strain rates from about 10-5 s-1 up to approximately 1 s-1, within which inertial effects are negligible and the specimen deforms essentially uniformly under machine control. Most structural test standards specify working rates between 0.00007 s-1 and 0.008 s-1 for determination of tensile properties.
How does fracture strain differ from elongation at break?
Elongation at break is the average engineering strain over a defined gauge length and includes contributions of both uniform deformation and necking distributed over that length. Fracture strain, especially local or true fracture strain, characterizes the deformation actually accumulated at the fracture location and is therefore independent of gauge length when measured by DIC or reduction of area.
Which standards govern this test?
Metallic materials are tested per ISO 6892-1, ASTM E8/E8M, EN ISO 6892-1, GB/T 228.1 and JIS Z 2241; plastics per ASTM D638 and ISO 527; and fracture mechanics testing of notched specimens per ASTM E1820 and ISO 12135. Accredited third-party laboratories select the applicable standard based on product specification and jurisdiction.
Why is non-contact strain measurement preferred for fracture strain?
Contact extensometers may slip or fall off when the specimen necks and fractures abruptly. Video extensometers and DIC systems record the full strain field continuously up to the instant of separation, allowing accurate determination of local strain at fracture even for thin foils, notched specimens and brittle materials.
Does gauge length affect the reported result?
Yes. For ductile metals that neck significantly, measured elongation decreases as gauge length increases because post-necking strain is averaged over a longer span. Comparisons between data sets are only valid when gauge lengths and specimen geometry are equivalent, which is why proportional specimens or explicitly stated fixed gauge lengths are used.
How many specimens should be tested?
At least three valid replicate specimens per condition is the common minimum, with additional replicates when scatter is high, when statistical design values are required, or when the material is heterogeneous such as thick-section welds and castings.