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Bonded joints used in wing sections and frames of aircraft structures are mostly exposed to cyclic loadings instead of static ones during their services. Bending types of dynamic loadings are mostly encountered. In this study, the fatigue response of a single lap joint (SLJ) exposed to bending loading was studied with the developed advanced finite-element (FE) model. The cohesive zone model describing the behaviour of the adhesive layer used the damage mechanism, where static and fatigue damages were linked to each other; i.e., the total damage was accumulated because of material deterioration and cyclic plastic separation. This enabled us to predict the fatigue characteristics including the finite fatigue life, crack propagation rate using Paris law. The model was implemented via a user-defined UMAT subroutine offered in ABAQUS-Standard. The numerical model was validated by experiments available in the literature. The fatigue performance of an SLJ subjected to bending loading was investigated for different lap joint configurations. A smaller bending load, a thicker adherend or a longer overlap length (OL) led to enhanced fatigue life. For instance, the fatigue life was observed to increase up to 50 times for a 66% increase in OL. [ABSTRACT FROM AUTHOR]

Bonded joints used in wing sections and frames of aircraft structures are mostly exposed to cyclic loadings instead of static ones during their services. Bending types of dynamic loadings are mostly encountered. In this study, the fatigue response of a single lap joint (SLJ) exposed to bending loading was studied with the developed advanced finite-element (FE) model. The cohesive zone model describing the behaviour of the adhesive layer used the damage mechanism, where static and fatigue damages were linked to each other; i.e., the total damage was accumulated because of material deterioration and cyclic plastic separation. This enabled us to predict the fatigue characteristics including the finite fatigue life, crack propagation rate using Paris law. The model was implemented via a user-defined UMAT subroutine offered in ABAQUS-Standard. The numerical model was validated by experiments available in the literature. The fatigue performance of an SLJ subjected to bending loading was investigated for different lap joint configurations. A smaller bending load, a thicker adherend or a longer overlap length (OL) led to enhanced fatigue life. For instance, the fatigue life was observed to increase up to 50 times for a 66% increase in OL.

The aerospace industry is widely employing strain-life methodologies for structural fatigue predictions. Under spectrum loading, overloads significantly affect the fatigue, therefore it is very important to accurately account for the cyclic transient deformation phenomena. Describing these phenomena requires advanced plasticity models that involve a set of material parameters. Even for the well-known Chaboche model, there is lack of understanding of each parameter's sensitivity in strain-life fatigue calculation among engineering practitioners. A parameter optimisation technique using a multi-objective genetic algorithm is applied for the Chaboche model parameters by employing varying strain and stress controlled uniaxial data from tests on Aluminium Alloy 7075-T6. The parameters obtained from each of the optimisations and for various workflows, are then used in strain-life fatigue calculations with a Defence Science and Technology Group in-house software. Fatigue life predictions for P-3C aircraft load spectra are compared against experimental lives obtained from the Masing model to ascertain the parameters offering the most accurate results. The optimum uniaxial material dataset for strain-life predictions employing the Chaboche model is determined. The results of this study offer new insights of the model parameters' function and their sensitivity in fatigue predictions. [ABSTRACT FROM AUTHOR]

For the detection of deep-lying flaws in aircraft structures, an eddy current system in conjunction with a planar SQUID gradiometer is being developed. The need for a mobile system imposes additional requirements on cooling regarding mobility, operation independent of spatial orientation, and handling. We present results on the operation of HTS SQUIDs with the commercial Joule-Thomson-cryocooler KC 100 ("cryotiger") by APD and report on cooler characteristics and system performance. Magnetometers and gradiometers of both rf and dc type have been integrated with the cryocooler. Noise spectra both in unshielded and shielded environments have been shown to be orientation independent. Compared to stationary operation, the system sensitivity is reduced by additional noise generated by the movement. With an improved integration scheme, this motion-related noise is nearly eliminated. The system was equipped with a differential eddy current excitation and a synchronized digital lock-in detection. Two-dimensional scans are presented that illustrate the system's capability to find fatigue cracks and corrosion pits hidden below several layers of aluminum. [ABSTRACT FROM PUBLISHER]