The increased use of polymer-based composite materials such as CFRP requests an enhanced understanding of the materials’ behaviour under true operational conditions. A growing interest can also be seen in determining a possible damage tolerance potential of those materials hardly considered so far in the past. This paper looks into the aspect on how damage incubators in composite materials and structures including matrix fracture, delamination, fibre-matrix debonding, fibre fracture and any progression of those damage incubators can be detected by taking advantage of the material’s inherit mechanical properties. These properties considered in the paper will go beyond the classical way on how composite materials are inspected and characterised where measuring stiffness change as a function of progressing damage is state-of-the-art while looking at non-linearities resulting from non-stationary conditions such as observed with breathing cracks is the novelty to be reported. Such non-linear phenomena may be well observed when a material is subjected to high frequency vibrations such as acoustic fatigue resulting in very high cycle fatigue (VHCF).
To explain and understand the effect of the non-linear effects a 2D-FEM model using COMSOL for a three point bending test of a CFRP sample with the presence of delamination damage induced breathing under a fatigue load has been simulated and the frequency transforms of the time domain signals for various delamination sizes and locations has been studied. The numerical studies show that the presence of breathing cracks in the frequency domain exhibits higher harmonic peaks due to the opening/closing action of the delamination. Once the features of the frequency domain response as functions of damage size and location are formulated, numerical inversion can be applied to the experimental data to obtain the damage parameters such as the size and its location. The simulated results have been validated with experimental data obtained from VHCF testing on CFRP (Polyphenylenesulfide-PPS) specimens performed at the Institute of Materials Science and Engineering (WKK) at the University of Kaiserslautern. The validation has been complemented by analytical work based on computed tomography as well as high performance ultrasound measurements at the Federal Institute for Materials Research and Testing (BAM). These analyses provide basic information of the damage process ongoing during fatigue loading and thus make it possible to separate damage mechanisms and to significantly validate and refine the simulation model referenced.
Good agreement between the simulated, experimental and analytical results has demonstrated that the non-linearities determined are viable parameters to be considered for monitoring damages at small scales in rather complex and inhomogeneous materials such as a CFRP composite and where those damages may become relevant specifically with respect VHCF loading.