ISO 15024 Fibre-Reinforced Plastic Composites - Determination of Mode I Interlaminar Fracture Toughness (Gic) for Unidirectional Reinforced Materials

The ISO 15024 standard, “Iso 15024 Fiber-reinforced plastic composites - Determination of mode I interlaminar fracture toughness (Gic) for unidirectional reinforced materials”, developed by the International Organization for Standardization (ISO) , describes a method for determining the mode I interlaminar fracture toughness (critical energy release rate) of unidirectional fiber-reinforced plastic composites using a double cantilever beam specimen.

For the purposes of this standard, the following terms and definitions apply:

• Mode I interlaminar fracture toughness, the critical energy release rate Gic, expresses the resistance to delamination crack initiation and propagation in unidirectional fiber-reinforced polymer matrix composite laminates under mode I opening load. It is measured in joules per square meter.
• Mode I crack opening refers to the crack opening mode due to a load applied perpendicular to the delamination plane using a double cantilever beam specimen. The double cantilever beam specimen is shown in the figure given in the standard.
• The NL point represents the point of deviation from linearity in the load-displacement trace. The NL point is shown in the figure given in the standard.
• VIS point refers to the starting point of delamination determined by visual observation at the edge of the specimen, marked on the load-displacement trace. VIS point is shown in the figure given in the standard.
• The 5 percent / MAX point refers to the point that occurs first between when loading the sample (shown in the figure provided in the standard):
  • Compliance increase point of 0 percent (C5 percent) from the starting value (C5)
  • Maximum load point
• PROP points refer to the points of discrete delamination length increments beyond the insertion or incipient crack tip marked on the load-displacement trace. Points where the crack is arrested are excluded. PROP points are shown in the figure given in the standard.
• The delamination resistance curve represents the cross plot of Gic for the initial and subsequent propagation values ​​of mode I crack opening depending on the delamination length. The delamination resistance curve is shown in the figure given in the standard.

In testing, a mode I double cantilever beam specimen is used to determine the critical energy release rate or interlaminar fracture toughness Gic of fiber-reinforced plastic composites. The figure given in the standard shows three different loading arrangements for the specimen:

• Sample loading using load blocks
• Sample loading using piano hinges
• Sample loading using attachment hinges

The test method is limited to zero-degree unidirectional stratifications only.

Data reduction yields the initial and subsequent spread values ​​of Gic for the mode I opening fracture toughness. The delamination resistance curve is constructed by plotting Gic on the ordinate as a function of the delamination length plotted on the abscissa.

The purpose of this testing method is to determine the initiation values ​​for the composite material being tested. Fiber bridging is observed in a double cantilever beam test and may not be representative of the composite material being tested. Fiber bridging is generally considered the main reason for the observed shape of the rising delamination resistance curve before reaching an approximately constant Gic value, usually for long delamination lengths. A crack-opening load is applied to the double cantilever beam specimen perpendicular to the delamination plane via load blocks or piano hinges under constant rate displacement control. The double cantilever beam specimen contains a thin, non-adhesive initiation film embedded in the mid-plane, which is used to simulate the initial delamination. The specimen is pre-cracked by unloading and then reloading the double cantilever beam specimen immediately after the first increase in delamination growth from the insert. The onset of stable delamination growth is monitored, and delamination initiation and propagation readings are recorded. The delamination resistance curve is plotted with both insert and mode I pre-crack initiation values ​​and pre-crack propagation. Under certain foreseen conditions, an alternative wedge pre-cracking procedure may be used but is not recommended.

The machine used in the tests must meet the following requirements: test speed, fixture, load and displacement measurements, and recorder.

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