Coating delamination tests 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, and
pull-off test.
Interlaminar fracture toughness testing Fracture toughness is a material property that describes resistance to fracture and delamination. It is denoted by critical
stress intensity factor K_c or critical
strain energy release rate G_c. For unidirectional fiber reinforced polymer
laminate composites, ASTM provides standards for determining
mode I fracture toughness G_{IC} and
mode II fracture toughness G_{IIC} of the interlaminar matrix. During the tests load P and displacement \delta is recorded for analysis to determine the strain energy release rate from the
compliance method. G in terms of compliance is given by{{Equation|1=G = \frac{P^2}{2B}\frac{dC}{da}|2=1}} where dC is the change in compliance C (ratio of \delta /P), B is the thickness of the specimen, and da is the change in crack length.
Mode I interlaminar fracture toughness ASTM D5528 specifies the use of the double cantilever beam (DCB) specimen geometry for determining mode I interlaminar fracture toughness. Multiple test architectures have been proposed for use in measuring interlaminar shear strength, including the short beam shear test, Iosipescu test, rail shear test, and asymmetrical four-point bending test. The goal of each of these tests is to maximize the ratio of
shear stress to
tensile stress exhibited in the sample, promoting failure via delamination of the fiber-matrix interface instead of through fiber
tension or
buckling. The
orthotropic symmetry of fiber composite materials makes a state of pure shear stress difficult to obtain in sample testing; thin cylindrical specimens can be used but are costly to manufacture. Sample geometries are thus chosen for ease of machining and optimization of the stress state when loaded. In addition to manufactured composites such as
glass fiber-reinforced polymers, interlaminar shear strength is an important property in natural materials such as wood. The long, thin shape of floorboards, for example, may promote deformation that leads to vibrations.
Asymmetric four-point bending Asymmetric four-point bending (AFPB) may be chosen to measure interlaminar shear strength over other procedures for a variety of reasons, including specimen machinability, test reproducibility, and equipment availability. For example, short-beam shear samples are constrained to a specific length-thickness ratio to prevent bending failure, and the shear stress distribution across the specimen is non-uniform, both of which contribute to a lack of reproducibility. Rectangular samples can be used with or without notches machined at the center; the addition of notches helps to control the position of the failure along the length of the sample, but improper or nonsymmetrical machining can result in the addition of undesired normal stresses which reduce the measured strength. The sample is then loaded in compression in its test fixture, with loading applied directly to the sample from 4 loading pins arranged in a parallelogram-like configuration. The load applied from the test fixture is transferred unevenly to the top two pins; the ratio of the inner pin load P and outer pin load Q is defined as the loading factor \lambda, such that :\frac{P}{Q} = \frac{S_2}{S_1} = \lambda, where S_1 and S_2 are the lengths from the inner pin to the applied point load and from the outer pin to the applied point load, respectively. The normal stress in the sample \sigma_{xx} is maximized at the locations of the inner pins, and is equivalent to :\sigma_{xx} = \frac{6(\lambda-1)FL}{(1+\lambda^2)bt}, where F is the total applied load on the sample, L is the sample length, b is the sample width (into the page as seen in a 2D free-body diagram), and t is the sample thickness. The shear stress \sigma_{xz} in the sample is maximized in between the inner span of the pins and is given by :\sigma_{xz} = \frac{3(1- \lambda)F}{2(1+ \lambda)bt}. The ratio of normal to shear stress in the sample C is given by :C = \frac{\sigma_{xx}}{\sigma_{xz}} = \frac{4L}{(1+\lambda)t}. This ratio is dependent both on the loading factor of the sample and its length-thickness ratio; both of these quantities are important in determining the mode of failure of the sample in testing. == References ==