Testing standard
Standard Test Method for Determination of the Mode II Interlaminar Fracture Toughness of Unidirectional Fiber-Reinforced Polymer Matrix Composites
Written and technically reviewed by Dak System Inc. engineeringLast reviewed
ASTM D7905 measures mode II interlaminar fracture toughness, G_IIc, using the end-notched flexure geometry: a unidirectional beam with a mid-plane starter delamination is loaded in three-point bending so the crack faces slide rather than open. It is limited to unidirectional carbon-fibre and glass-fibre laminates. The current edition is D7905/D7905M-19e1.
A unidirectional laminate beam with a thin non-adhesive insert at the mid-plane, forming a starter delamination at one end, is loaded in three-point bending with the delaminated end overhanging one support. This is the end-notched flexure, or ENF, geometry. Bending puts the two halves of the beam into relative sliding at the mid-plane, so the delamination is driven forward in shear rather than by being pulled open.
The force and the crosshead displacement are recorded, the delamination length is known from the specimen preparation and from a compliance calibration, and from those the critical strain energy release rate in mode II — G_IIc — is calculated.
Delamination is the characteristic failure of a laminated composite, and it happens in three modes: opening, sliding and tearing. Mode I, the opening mode, is measured by ASTM D5528; this method covers mode II, the in-plane sliding mode.
Mode II toughness is usually several times mode I toughness in the same material, and the ratio between them is one of the more revealing numbers a composites laboratory produces. It says how much of the resistance comes from the resin itself and how much from fibre bridging and other mechanisms that only operate when the crack faces separate.
The application is damage tolerance. Design allowables for a composite structure that must survive impact damage and continue to carry load are built on delamination resistance, and mode II governs the growth of a delamination under in-plane shear — the loading a stiffened panel sees. Aerospace, wind energy, automotive and marine structures all use these values.
The scope is limited to unidirectional carbon-fibre and glass-fibre reinforced laminates. The method notes it may prove useful for other types and classes of composite, with interferences noted.
Insert thickness is the variable that most often invalidates a mode II result, and it is decided in the layup room rather than in the laboratory.
Derived from the peak force, the beam compliance and the crack length
Reported in joules per square metre. The calculation is written into the method and depends on the calibration being valid.
Opening, in-plane sliding, tearing
Mode II toughness is usually several times mode I toughness
The ratio says how much resistance comes from the resin itself and how much from fibre bridging and other mechanisms that only operate when the crack faces separate.
The ENF geometry is three-point bending with the delaminated end overhanging one support. Roller radius and span are specified and enter the calculation, and the rollers must turn freely — a binding roller changes the fixture compliance between the calibration runs and the fracture run.
SpecificationsAn ENF specimen fails in the hundreds of newtons to low kilonewtons. A cell sized for the specimen is what resolves a peak that arrives quickly, and the peak is the result.
SpecificationsForces are modest — an ENF specimen fails in the hundreds of newtons to low kilonewtons — so a 5 to 10 kN frame with a load cell sized for the specimen is right, with force accuracy to ASTM E4.
What the test really needs is a stiff, well-aligned three-point bend fixture with rollers of the specified radius that turn freely. The compliance calibration procedure depends on the fixture behaving consistently between the calibration runs and the fracture run, so a fixture that beds in or a roller that binds corrupts the calibration and therefore the toughness.
Displacement has to be measured accurately and, ideally, on the specimen rather than from crosshead travel, because machine compliance enters the compliance calibration directly. Data must be logged continuously: the fracture event is fast and the peak defines the result.
An insert film that is too thick blunts the starter and inflates the first toughness value. Delamination that migrates out of the mid-plane invalidates the specimen entirely and is not always obvious afterwards. A worn or binding roller changes the fixture compliance between calibration and test. And crack length taken from an assumption rather than from the compliance calibration produces a G_IIc that is precise and inaccurate.
Three modes, three documents, and one screening test that is often mistaken for a fourth.
| Mode I (D5528) | Mode II (D7905) | Mixed mode (D6671) | Short beam (D2344) | |
|---|---|---|---|---|
| Crack faces | Pulled open | Slide over each other | Both | No pre-crack |
| Specimen | Double cantilever beam | End-notched flexure beam | Mixed-mode bending | A short flexure coupon |
| Result | G_Ic, J/m² | G_IIc, J/m² | G_c at a mode ratio | Apparent shear strength, MPa |
| Is it a fracture test | Yes | Yes | Yes | No — a quality screen |
| Typical magnitude | Lowest | Several times G_Ic | Between the two | Not comparable |
ASTM D2344 short-beam strength is a quality-control screen and not an interlaminar fracture measurement, however often the two are quoted side by side. It gives an apparent strength on an uncracked coupon and carries no strain energy release rate.
ASTM D7905, published as D7905/D7905M, determines the mode II interlaminar fracture toughness of unidirectional fibre-reinforced polymer matrix composites using the end-notched flexure test. A beam with a starter delamination at its mid-plane is loaded in three-point bending, driving the crack forward in shear, and G_IIc is computed from the peak force and a compliance calibration. The current edition is D7905/D7905M-19e1.
The two describe how a crack is loaded. Mode I is the opening mode, where the crack faces are pulled directly apart — measured by ASTM D5528 with a double cantilever beam. Mode II is the in-plane sliding mode, where the faces slide over one another. Mode II toughness is typically several times the mode I value in the same material, because sliding mobilises friction and matrix shear that opening does not.
Because it sets the sharpness of the starter crack. The insert is a thin non-adhesive film laid at the mid-plane during layup, and if it is too thick the resulting crack tip is blunt rather than sharp. A blunt tip needs more energy to start moving, so the first measured toughness comes out high — and it is the first value that is usually quoted. The limit is specified for that reason and it is a layup-room control, not a laboratory one.
It relates the beam compliance, which is displacement per unit force, to the crack length. Running the specimen at several known crack lengths before the fracture run builds that relationship, so the crack length during the fracture event can be inferred rather than watched. It also means the fixture has to behave identically between the calibration and the test: a roller that binds or a fixture that beds in changes the compliance and moves the answer without anything appearing wrong.
Because the analysis assumes the delamination stays in the mid-plane of a beam whose two halves are identical and isotropic in the plane of loading. Cross-plied or woven laminates deflect the crack out of that plane and introduce mode mixing, so the geometry no longer isolates mode II. The standard notes the method may prove useful for other types and classes of composite, with interferences noted — which is an invitation to proceed with care, not a general permission.
Damage tolerance. Design allowables for composite structures that must survive impact damage and keep carrying load are built on delamination resistance, and mode II governs how a delamination grows under in-plane shear — the loading a stiffened aerospace panel, a wind turbine blade shell or an automotive floor sees. Material selection between competing resin systems is also settled on these numbers more often than on tensile strength.
A small frame with a stiff, well-aligned three-point bend fixture. Forces are in the hundreds of newtons to low kilonewtons, so a 5 to 10 kN frame with a load cell sized for the specimen and force accuracy to ASTM E4 is right. What matters more than the frame is fixture consistency and accurate displacement measurement, because machine compliance enters the compliance calibration directly and therefore enters G_IIc.
Dak verifies against whichever standard the method names, and where a class applies our frames sit a class tighter than it asks.
| The method asks for | Dak supplies | |
|---|---|---|
| Capacity | Low — an ENF specimen fails in the hundreds of newtons to low kN | Load cells from 1 kg to 60 ton on the Series 7200, and 0.5 to 100 kN on the Series 9000 |
| Force accuracy | ASTM E4 | Verified to ASTM E4, and to ISO 7500-1 Class 0.5 |
| Gripping | A stiff three-point bend fixture with free-turning rollers of the specified radius | Our bend fixtures, built to the specimen |
| Environment | Moisture conditioning to the state the test plan requires, and reported; mode II toughness is matrix-dominated | 3009 series chambers, −150 °C to +400 °C — temperature only |
This page describes the method as practised. The governing text is the current edition from the issuing body. Tell us what you are testing and we will answer with the machine, the fixture and a quotation.
