This characteristic value is defined as the energy per unit of surface needed to propagate a crack over a known distance. The result is a value that describes the crack prorogation sensitivity of a laminate.
During measurement, differentiation is made between the type of load application.
- Mode I: crack opening
- Mode II: in-plane shear
- Mode II: torsional loading transverse to the laminate plane
Measurement of the energy release rate in Mode I is used most often and is described in ISO, ASTM and EN standards, among others. There is currently no standard for Mode III.
ASTM describes a mixed mode method, which is a combination of Mode I and Mode II with variable mode proportions.
- Mode I energy release rates are usually measured with a DCB (Double Cantilever Beam) specimen.
- The specimen used here has a crack in the in-plane direction, for example one that is propogated by a plastic foil inserted during lamination.
- A load can be applied via adhered loading blocks, hinges, or special mounts that are clamped in the crevice.
- During the test, the crack opening is measured as the crosshead travel and corrected by the amount of specimen leg deformation. The energy introduced is calculated from the integral of the measured force via deformation. Crack propogation is visually recorded on the side of the specimen.
- The crack length can be manually calculated by the machine operator, who can view the marked side of the specimen using a magnifying glass. To achieve better measurement traceability, digital magnifying glasses are used, which are guided along the specimen as the crack progresses and record a video of the measurement.
- This video is synchronized frame for frame with the test curve, meaning the test can be verified after completion and corrected if necessary.
- The test sequences and evaluations differ depending on the standard used.
- The standards are as follows: ISO 15024, ASTM D5528, AITM 1-0005, AITM 1-0053, Boeing BSS 7273, CRAG method 600, NASA method RP 1092 ST-5, ESIS TC 4, prEN 6033 (withdrawn)
Mode II loading can be applied and measured in a flexure test and in a tensile and compression test with notched specimens.
Measurement of the Mode II energy release rate is standardized as a flexure test. Shear stresses arising in the flexure test are used to propagate the crack. The specimens are denoted with SENB (single end notch bending), but ENF (end notch flexure) is also term commonly used. Deflection is measured via the crosshead travel (with correction for stiffness) or using a displacement transducer centrally attached.
The crack initiation point is characterized by a maximum force. At the end of measurement, the specimen is cooled in liquid nitrogen and then completely broken to measure the fracture surfaces.