Delamination

ASTM provides standards for paint adhesion testing which provides qualitative measures for paints and coatings resistance to delamination from substrates. Tests include cross-cut test, scrape adhesion,[13] and pull-off test.[14]

Fracture toughness is a material property that describes resistance to fracture and delamination. It is denoted by critical stress intensity factor ${\displaystyle K_{c}}$ or critical strain energy release rate ${\displaystyle G_{c}}$.[15] For unidirectional fiber reinforced polymer laminate composites, ASTM provides standards for determining mode I fracture toughness ${\displaystyle G_{IC}}$ and mode II fracture toughness ${\displaystyle G_{IIC}}$ of the interlaminar matrix.[16][17] During the tests load ${\displaystyle P}$ and displacement ${\displaystyle \delta }$ is recorded for analysis to determine the strain energy release rate from the compliance method. ${\displaystyle G}$ in terms of compliance is given by

${\displaystyle G={\frac {P^{2}}{2B}}{\frac {dC}{da}}}$ (1)

where ${\displaystyle dC}$ is the change in compliance ${\displaystyle C}$ (ratio of ${\displaystyle \delta /P}$), ${\displaystyle B}$ is the thickness of the specimen, and ${\displaystyle da}$ is the change in crack length.

Schematic of deformed double cantilever beam specimen.

ASTM D5528 specifies the use of the double cantilever beam (DCB) specimen geometry for determining mode I interlaminar fracture toughness.[17] A double cantilever beam specimen is created by placing a non-stick film between reinforcement layers in the center of the beam before curing the polymer matrix to create an initial crack of length ${\displaystyle a_{0}}$. During the test the specimen is loaded in tension from the end of the initial crack side of the beam opening the crack. Using the compliance method, the critical strain energy release rate is given by

${\displaystyle G_{IC}={\frac {3P_{C}\delta _{C}}{2Ba}}}$ (2)

where ${\displaystyle P_{C}}$ and ${\displaystyle \delta _{C}}$ are the maximum load and displacement respectively by determining when the load deflection curve has become nonlinear with a line drawn from the origin with a 5% increase in compliance. Typically, equation 2 overestimates the fracture toughness because the two cantilever beams of the DCB specimen will have a finite rotation at the crack. The finite rotation can be corrected for by calculating ${\displaystyle G}$ with a slightly longer crack with length ${\displaystyle a+\Delta }$ giving

${\displaystyle G_{IC}={\frac {3P_{C}\delta _{C}}{2B(a+\Delta )}}}$ (3)

The crack length correction ${\displaystyle \Delta }$ can be calculated experimentally by plotting the least squares fit of the cube root of the compliance ${\displaystyle C^{1/3}}$ vs. crack length ${\displaystyle a}$. The correction ${\displaystyle \Delta }$ is the absolute value of the x intercept. Fracture toughness can also be corrected with the compliance calibration method where ${\displaystyle G_{IC}}$ given by

${\displaystyle G_{IC}={\frac {nP_{C}\delta _{C}}{2Ba}}}$ (4)

where ${\displaystyle n}$ is the slope of the least squares fit of ${\displaystyle log(C)}$ vs. ${\displaystyle log(a)}$.

Schematic of edge notch flexure test.

Mode II interlaminar fracture toughness can be determined by an edge notch flexure test specified by ASTM D7905.[16] The specimen is prepared in a similar manner as the DCB specimen introducing an initial crack with length ${\displaystyle a_{0}}$ before curing the polymer matrix. If the test is performed with the initial crack (non-precracked method) the candidate fracture toughness ${\displaystyle G_{Q}}$ is given by

${\displaystyle G_{Q}={\frac {3mP_{max}^{2}a_{0}^{2}}{2B}}}$

where ${\displaystyle B}$ is the thickness of the specimen and ${\displaystyle P_{max}}$ is the max load and ${\displaystyle m}$is a fitting parameter. ${\displaystyle m}$ is determined by experimental results with a least squares fit of compliance ${\displaystyle C}$ vs. the crack length cubed ${\displaystyle a^{3}}$ with the form of

${\displaystyle C=A+ma^{3}}$.

The candidate fracture toughness ${\displaystyle G_{Q}}$ equals the mode II fracture toughness ${\displaystyle G_{IIC}}$ if strain energy release rate falls within certain percentage of ${\displaystyle G_{Q}}$ at different crack lengths specified by ASTM.