Standard Test Method for Critical Mode I Interlaminar Strain Energy Release Rate (GIc) of Continuous Fiber-Reinforced Advanced Ceramics at Ambient Temperatures

Importancia y uso:

5.1 Interlaminar delamination growth can be a critical failure mode in laminated CMC structures. Knowledge of the resistance to interlaminar delamination growth of a laminated CMC is essential for material development and selection, and for CMC component design. (See (1-8)3 which give G_Ic values of 20 J/m² to 800 J/m² for different CMC and carbon-carbon composite systems at ambient temperatures.)

5.2 Conducting this test produces multiple values of G_Ic which are traditionally plotted against the delamination length at which that value was measured (see Fig. 2). The specific data of value to the test requestor will depend on the end use that motivated testing.

5.2.1 The first increment of growth, initiated from a pre-implanted insert or machined notch, is sometimes described as the non-precracked (NPC) toughness. NPC toughness may be of interest, as it can represent manufacturing or processing defects, such as foreign object debris in a laminate or an error during machining.

5.2.2 The next increment of growth, initiated from the sharp crack tip assumed to be present after the first increment, is sometimes defined as the precracked (PC) toughness. PC toughness may be of interest, as it is more representative of the resistance to delamination growth from a naturally occurring or damage-induced delamination.

5.2.3 The remaining increments of growth, collectively forming an R-curve, provide information on how G_Ic evolves as the delamination advances. In unidirectional tape laminates, the R-curve is often increasing due to bridging of nested fibers across the delamination plane, artificially increasing G_Ic. For 2-D woven laminates for which there is little interply nesting, the R-curve may be flat.

5.2.4 The increments of growth in which the R-curve is flat, and G_Ic has reached a steady state value defined as G_IR, may be of interest and may also useful in design and analysis.

5.3 This test method for measurement of G_Ic of CMC materials can serve the following purposes:

5.3.1 To establish quantitatively the effect of CMC material variables (fiber interface coatings, matrix structure and porosity, fiber architecture, processing and environmental variables, conditioning/exposure treatments, etc.) on G_Ic and the interlaminar crack growth and damage mechanisms of a particular CMC material;

5.3.2 To determine if a CMC material shows R-curve behavior where G_Ic changes with crack extension or reaches a stable value at a given amount of delamination growth. Fig. 2 shows R-curve behavior for a SiC-SiC composite (1);

5.3.3 To develop delamination failure criteria and design allowables for CMC damage tolerance, durability or reliability analyses, and life prediction;

Note 3: Test data can only reliably be used for this purpose if there is confidence that the test is yielding a material property and not a structural, geometry-dependent, property.

5.3.4 To compare quantitatively the relative values of G_Ic for different CMC materials with different constituents and material properties, reinforcement architectures, processing parameters, or environmental exposure conditions; and

5.3.5 To compare quantitatively the values of G_Ic obtained from different batches of a specific CMC material, to perform lot acceptance quality control, to use as a material screening criterion, or to assess batch variability.

Subcomité:

C28.07

Volúmen:

15.01

Número ICS:

81.060.30 (Advanced ceramics)

Palabras clave:

ceramic matrix composite; critical strain energy release rate; double cantilever beam; fracture resistance; fracture toughness; interlaminar delamination; mode I;