Gap and overlap defects are formed during automated fiber placement (AFP) layup due to stochastic variations in tow position and geometry. There is utility in predicting the size and frequency of these defects since they affect the manufacturing rate and structural performance. Two methods which reflect the measured process variability were implemented to simulate realistic tow geometry. A Monte Carlo (MC) simulation used independent random sampling from the distributions of the sources of variation. A Markov Chain Monte Carlo (MCMC) simulation used the Random Walk Metropolis algorithm which samples these distributions while considering the neighborhood of past samples thus better preserving spatial continuity. Simulated tows were assembled into virtual laminae. Tow-level validation using spatial-frequency Fast Fourier Transform (FFT) analysis demonstrated that the MCMC method reproduces the dominant wavelengths and spectral characteristics of experimentally measured tow waviness, whereas the MC method introduces unrealistic high frequency variations. Lamina-level comparisons showed that while both methods predict similar gap and overlap area percentages, only the MCMC method accurately captures defect length distributions. The proposed framework provides realistic AFP layup geometries for use in mesoscale modeling and for informing process improvement efforts.

Delamination is a critical mode of failure that occurs between plies in a composite laminate. The cohesive element, developed based on the cohesive zone model, is widely used for modelling delamination. However, standard cohesive elements suffer from a well-known limit on the mesh density—the element size must be much smaller than the cohesive zone size. This work extends the line of research on structural cohesive elements onto 3D mixed-mode problems. A new triangular Kirchhoff–Love shell element is developed for orthotropic materials to model the plies. A new structural cohesive element, conforming to the shell elements of the plies, is developed to model the interface delamination. The proposed method is verified and validated on the classical benchmark problems of Mode I, Mode II, and mixed-mode delamination of unidirectional laminates, a recent unidirectional benchmark problem with curved delamination front, as well as the single-leg bending problem of a multi-directional laminate, significantly increasing the range and complexity of applicable problems as compared to the previous works. All the results show that the element size in the proposed models can be ten times larger than that in the standard cohesive element models, with more than 90% reduction in CPU time, while retaining prediction accuracy. This would then allow more effective and efficient modelling of delamination in composites without worrying about the cohesive zone limit on the mesh density.

A simulation methodology for assessing the damage in thick fabric Carbon Fibre Reinforced Polymer (CFRP) composite laminates under low- and high-velocity impacts is presented. It encompasses steps for calibration, verification, and validation of the elastic and fracture material properties as well as determination of model parameters for the numerical simulations. Damage is modelled using a discrete fracture approach with cohesive interface elements that capture individual cracks occurring in and between plies. For computational efficiency, the method is implemented in a two-dimensional (2D) axi-symmetric model. Results from double-cantilever beam, end-notched flexure, and quasi-static indentation experiments align well with numerical simulations and serve to calibrate and verify the implementation of the discrete fracture approach. The methodology is extended to dynamic impact analysis to predict damage mechanisms, force–displacement histories, and is validated using test results. This methodology combines meaningful insight in the failure mechanisms with a manageable computational effort, achieving a factor 50 improvement compared to a benchmark. A parametric analysis summarised in failure maps relates damage mechanisms to impact energy, mass, and laminate thickness. The proposed methodology strikes a balance between computational efficiency and accuracy, making it a valuable tool for optimum design and certification of thick CFRP composite laminates under impact.

Strain energy release rate calculations for various cases of delaminations emanating from matrix cracks are developed and used to predict the onset of delaminations and their growth size as a function of applied tension and shear loads in composite laminates. The method determines the matrix crack spacing, the delamination onset load, the delamination size at onset and, through the use of a newly proposed delamination resistance curve, the size of delaminations as they grow under load. The method can be applied to any symmetric laminate. Comparisons to test results in the literature for a variety of layups and materials shows very good agreement with the exception of cases where significant edge delaminations appear before delaminations caused by matrix cracks.

Improvements in current design approaches require further studies of the damage interaction effects of composite materials subjected to repeated out-of-plane concentrated loads. To that end, a combined simulation and experimental investigation on composite laminate under repeated indentations is reported. The repeated indentations consist of seven identical peak-force indentations that are separately applied to the centre of the laminate. The results show that delaminations grow in all seven indentations, which can be interpreted as a continuous degradation of the effective delamination growth threshold with each subsequent indentation. More specifically, the second indentation effective delamination growth threshold is 62.4 MPa, which is about 19 % lower compared to the first one (77.2 MPa). Subsequently, the delamination growth threshold degraded approximately linearly with indentation. This effective delamination growth threshold reduction can be associated with the occurrence and evolution of the crack-rich zone preceding the delamination front.

The behavior of delaminations and fiber breakage resulting from three-point bending test is a major concern in the study of composites. This research focused on analyzing fiber breakage and delaminations in advanced placed ply composite laminates through a series of tests. In order to compare simulation results with the experimental data, the cohesive zone model was implemented for analyzing the damage in the model. The results of the simulation were validated using experimental results. The predicted damage initiation load exhibited an approximate 20% deviation from the actual test loads and the discrepancy stems are analyzed.

An approach to determine the specific energy dissipated during cyclic loading of metal alloys with load ratios between 0 and 1 is developed. The dissipated energy per cycle is determined by using a limiting procedure to obtain the area enclosed between successive loading and unloading curves and can be used for low-, medium-, and high-cycle fatigue. Comparisons with published data from two aluminum, one titanium, and one steel alloy show that the predictions capture the test results very well. A universal curve relating cycles to failure to two nondimensional parameters is derived. It is shown, in some limiting cases, that the method leads to power law equations with the parameters in these equations determined directly from the complete stress–strain curve of the material with no need to fit experimental data. Furthermore, if certain conditions are satisfied, the method is consistent with the existence of an endurance limit and Miner's rule.

Manufacturing variations in the AFP process are one of the causes of gaps and overlaps. These manufacturing variations can be due to robot inaccuracy, tape lateral movement on the roller, tape width variation or tape compaction. These manufacturing variations result in incorrect position or incorrect geometry of the laid tape. An experimental setup was built to measure and investigate the causes of these manufacturing variations and their relative contributions to gap and overlap defects. This setup consisted of an instrumented AFP head. A laser tracker measured the achieved trajectory of the AFP head. A camera measured the lateral movement of the tape on the roller. Laser line scanners measured tape width before and after layup. Experimental results show that the 99^th percentile absolute deviations for each of the four measured sources vary from 0.119 mm to 0.534 mm as compared to the specified tape width of 6.35 mm. Among all the measured sources of variations, lateral movement of the tape on the compaction roller was the biggest contributor to gaps and overlaps.

Fiber-reinforced composite materials are widely used in the aviation, civil, and shipbuilding industries. Especially the latter two industries are typically dealing with thicker composites. At the same time, in these industries the need for structural health monitoring, to assess degradation and failure, is becoming more prevalent. Acoustic emission (AE) measurement and analysis for damage source localization and characterization can be a useful method for the assessment of structural integrity for these structures. In the case of composite panels, acoustic emissions can propagate in the form of elastic guided waves. The location of the AE source exposes regions in a structure that are subject to degradation. Typical acoustic emission source localization methods assume that the recorded AE signals consist of a single dominant fundamental wave mode. However, with thicker composites, the acoustic emissions may propagate in a multitude of modes. This will complicate the signal processing operations for accurate source localization. This research assesses experimentally how guided wave multimodality influences acoustic emission localization. An acoustic emission source is excited in a thick glass fiber-reinforced plastic (GFRP) panel. Measurements from this excitation are first assessed for their content of higher modes. Source localization is carried out based on dispersion compensation through time-distance domain migration. Different possibilities and combinations of wave modes are considered. The localization error is assessed for each option. The results highlight the added complexity of multimodality and show how the inclusion of multiple modes into the procedure can improve the accuracy of source localization.

Marine propellers made of fibre-reinforced composites have demonstrated the potential to outperform metallic propellers in terms of efficiency and under-water noise radiation. For full realisation of this potential in a tailored design process with realistic constraints, accurate information on the hydrodynamic loads acting on composite marine propellers and the structural integrity is of key importance. It is conceptualised that this information can be acquired without disturbing propeller hydrodynamics using a network of piezoelectric sensors embedded inside the blade. In this paper, feasibility of this concept has been investigated numerically and experimentally. Hydrodynamic loads on a composite propeller obtained from numerical simulations were used to assess the sensitivity of piezoelectric sensors in measuring the dynamic strain field due to the blade deformation. Subsequently, 25 small-scale carbon-epoxy composite samples were manufactured with embedded piezoelectric wafer sensors of different sizes, and subjected to non-destructive and destructive loading scenarios. Feasibility of measuring strains at different frequency ranges and damage-induced acoustic emissions was quantitatively assessed from these experiments. Furthermore, the influence of the embedded sensors on the ultimate strength and toughness of the specimens was investigated. It was found that at least 92% of the studied propeller blade would have dynamic strains measurable up to the first four harmonics by the considered piezoelectric sensors. In a four-point bending setup, it was additionally demonstrated that the embedded piezoelectric sensor captured damage-induced acoustic emissions up to specimen failure with an average signal to noise ratio of 17 dB. The results indicate that embedded piezoelectric sensor networks can have the capability to measure both low-frequency dynamic strains in composite marine propeller blades and damage-related acoustic emissions.

Impact experiments of thick fabric carbon/epoxy laminate specimens, with small thickness ratio, are conducted at distinct energy levels and thicknesses to characterise the damage process. These specimens and loading conditions are representative of a new generation of critical structural components in aviation, such as wing spars, landing gear beams and fittings, that are increasingly being made entirely from composites. The tests address the need to better understand the damage process for specimens with a small thickness ratio since existing experimental impact data for large thickness ratio (thin laminates) may not be directly applicable. Two energy levels, two different fabric layups and two impact methods (drop-weight and gas-cannon) were used. Data from high-speed cameras were processed in a novel way, providing the force during impact. C-scans and micrographs were used to characterise damage. The results show that specimens with a thickness ratio of 5 (20 mm thick) experience more bending compared to specimens with a ratio 2.5 (40 mm thick). For gas-cannon impacts, this results in a higher delaminated area. The drop-weight impacts show almost no differences in damage size for the thickness range analysed. The influence of layup on the global impact response is negligible, but locally it can result in significant variations in dent depth. The dent depth scales linearly with the impact energy and the delaminated area linearly with the impact velocity. There is no clear correlation between the compression-after-impact failure mechanisms and the residual strength. Impact damage, at the current energy levels, showed a minimal reduction of residual strength.

The recording and processing of acoustic emissions can be used to identify and localise damage mechanisms occurring in engineering structures. In plate-like structures, acoustic emissions propagate through the structure as guided waves. With a measurement location away from the source location, dispersion effects in the guided wave distort the acoustic emission signal. The distortion of the original signal hampers identification of damage mechanisms. This research describes and assesses a method to reconstruct the original acoustic emission signal using dispersion compensation. Simulations and experiments are performed involving thick glass-fibre reinforced plastic laminates. The signal reconstruction on the simulated data gives a reasonable representation of the simulated signal at the location of interest. In the experimental case, similarity slightly degrades. Deviation in arrival time between original measurement and reconstruction is attributed to a possible discrepancy in material properties in reality versus the properties used in the reconstruction.