Effect of material anisotropy on guided wave propagation and scattering in CFRP laminates

Carbon fiber laminates, consisting of highly anisotropic fiber-matrix ply-layers, are widely used in aerospace applications due to their good strength to weight ratio. However, poor interlaminar strength makes them prone to barely visible impact damage (BVID), significantly reducing the load bearing capacity of aircraft components. Guided ultrasonic waves have been widely used for structural health monitoring (SHM) of composite structures. Guided wave propagation and scattering at circular delaminations in a quasi-isotropic laminate was modelled using full three-dimensional (3D) Finite Element (FE) simulations in ABAQUS. Non-contact laser measurements were performed to obtain the scattered wavefield at a film insert delamination. The influence of ply layer anisotropy and incident wave direction were investigated both numerically and experimentally. Scattering directivity patterns were calculated using a baseline subtraction method and 2D scattering matrices were obtained for all incident wave directions. Circular magnets were used as a scattering target and numerical and measured scattering patterns were compared with those of the insert delamination. Strong directional dependency was observed for incident and scattered waves around both delamination and magnets, indicating energy focusing along the outer ply layers of the laminate. For the delamination a strong forward wave was observed, with low amplitude in other directions, whereas the magnet blocked forward transmission of the wave, demonstrating distinct scattering behavior. The anisotropic effects and different scattering patterns should be considered for guided wave sparse array SHM to ensure the robustness of imaging algorithms

Guided wave propagation and scattering at composite delaminations

Composite structures, consisting of highly anisotropic layers of polymer matrix reinforced with high strength carbon fibers, are widely used for aerospace applications due to their low weight and high strength. However, impact during aircraft operation can lead to barely visible and difficult to detect damage. Depending on impact severity, delaminations can occur that reduce the structural integrity and load carrying capacity. Efficient structural health monitoring (SHM) of composite panels can be achieved using guided ultrasonic waves propagating along the structure. Guided ultrasonic wave propagation and scattering at circular delaminations was modelled using full three-dimensional (3D) Finite Element (FE) simulations in ABAQUS. Individual ply layers were modelled using unidirectional composite material properties to accurately capture the anisotropy effects. The guided ultrasonic wave propagation and scattered field at an artificial delamination was measured using a noncontact laser interferometer and quantified. Good agreement between experiments and Finite Element predictions was found and the energy trapping on top of a shallow delamination was verified. The influence of delamination shape and depth was investigated from a FE parameter study. The sensitivity of guided waves for the detection of delaminations due to barely visible impact damage (BVID) in composite panels has been verified

Guided wave skew velocity correction in anisotropic laminates

Guided ultrasonic wave propagation in anisotropic structures results in directional dependency of velocity and wave skewing effects that can impact the accuracy of damage detection. Phase and group velocities of the A0 guided wave mode, propagating in a unidirectional carbon fiber reinforced laminate, were investigated experimentally and through finite element analysis. A correction for the significant offset in phase and group velocities due to wave skewing effects is illustrated for both point and short line sources, achieving good agreement with theoretical calculations assuming planar wave fronts. The influence of the line excitation length on velocity measurements is discussed

Guided Wave Energy Focusing and Steering in Composite Laminates

Lightweight, carbon fiber reinforced composites are often selected for aerospace components but are prone to barely visible impact damage, caused by low velocity impacts, during service. Guided-wave-based structural health monitoring (SHM) techniques can efficiently detect impact damage impact in composite structures. However, wave propagation is influenced by material anisotropy resulting in a number of effects. The phase and group velocity of propagating wave modes depend on the wave launching direction, with increased wave speeds in the high stiffness (fiber) directions. Wave energy tends to be focused along the fiber directions, resulting in beam steering or skewing away from the initial wave launching direction. These anisotropic effects, if unaccounted for, could lead to inaccurate localization of damage, and potential regions of the structure where guided waves cannot propagate with sufficient amplitude, reducing damage sensitivity. Wave propagation in an undamaged unidirectional carbon fiber reinforced polymer (CFRP) panel was investigated for the A0 mode for multiple wave launching directions. Finite Element (FE) modelling was carried out using homogenized anisotropic material properties to investigate the directional dependency of velocity. Point and line sources were modelled to investigate the influence of the excitation source on the guided wave evaluation and signal processing. Wave skewing behavior was visualized for the line source, and wave skew angles and beam spread angles were calculated for a range of propagation angles. Experimental non-contact guided wave measurements were obtained using a laser vibrometer. A PZT strip transducer was developed in order to measure wave skew angles. Experimental and numerical velocities and skew angles were compared with theoretical predictions and good agreement was observed

The bending strength of tablets with a breaking line—Comparison of the results of an elastic and a "brittle cracking" finite element model with experimental findings

The aim of this work was to ascertain the influence of the position of the breaking line of bevel-edged tablets in a three-point bending test. Two different brands of commercially available, flat-round, bevel-edged tablets with a single central breaking line were studied. Breaking line positions tested, relative to the upper loading roll, were 0°, 22.5°, 45°, 67.5° and 90°. The breaking line faced either up- or downwards during the test. The practical results were compared with FEM results simulating similar test configurations. Tablets failed mainly across the failure plane, resulting in two tablet halves. An exception to this was found for tablets where the breaking line faced down and was positioned at an angle of 22.5° relative to the loading plane. Here the crack followed the breaking line in the centre of the tablets and only diverged towards the loading plane position at the edges of the tablets. The breaking line facing upwards resulted in a significantly higher tensile strength of the tablets compared to it facing downwards. However, with one exception, the orientation of the breaking line relative to the loading plane appeared not to affect the tensile strength values. A fully elastic FEM model indicated that both the position of the breaking line relative to the loading plane and as to whether the breaking line faced up- or downwards during the bending test would result in considerably different failure loads during practical experiments. The results also suggested that regardless of the breaking line position, when it is facing down crack propagation should start at the outer edges propagating towards the midpoint of the discs until failure occurs. Failure should hence always result in equal tablet halves, whereby the failure plane should coincide with the loading plane. Neither predictions fully reflected the practical behaviour of the tablets. Using a brittle cracking FEM model significantly larger tensile stresses for tablets with the breaking line positioned downwards at 0° or 22.5° relative to the loading plane were still predicted, but the differences between model and experimental values was greatly reduced. The remaining differences are more likely due to the inadequacy of the equation available to calculate the experimental tensile strength values. This equation cannot account for the presence of a breaking line and overestimates the thickness of the loading plane by the depth of the breaking line when in 0° or 22.5° position. If the depth of the breaking line is taken into account, the model predictions and the experimental findings are comparable. Also, in the brittle cracking FEM simulations the predicted crack propagation patterns were similar to those found in the experiments, and the model stress distributions across the lower surfaces were much more homogeneous and streamlined parallel to the loading plane. The brittle cracking model hence reflected the practicalities of the bending test more closely. The findings suggested that with the breaking line facing down fracture should always start in the centre of a tablet at its lower surface, initiated by the breaking line. Due to simultaneous development of larger stresses along the y-axis the tablet should still break into two equal halves along the loading plane, unless the position of the breaking line relative to the loading plane was 22.5°. In this case the tablet would fail by a mixed process, whereby failure would occur mainly along the breaking line, but due to simultaneous crack formation at the lower surface close to the bevel edge parallel to the loading plane the final breaking pattern would deviate from the breaking line about half-way from its centre, as seen in the practical experiments

High-Frequency Guided Ultrasonic Waves to Monitor Corrosion Thickness Loss

Corrosion due to adverse environmental conditions can occur for a range of industrial structures, e.g., ships and offshore oil platforms. Pitting corrosion and generalized corrosion can lead to the reduction of the strength and thus degradation of the structural integrity. The nondestructive detection and monitoring of corrosion damage in difficult to access areas can be achieved using high frequency guided ultrasonic waves propagating along the structure. Using standard ultrasonic transducers with single sided access to the structure, the two fundamental Lamb wave modes were selectively generated simultaneously, penetrating through the complete thickness of the structure. The wave propagation and interference of the guided wave modes depends on the thickness of the structure. Numerical simulations were performed using a 2D Finite Difference Method (FDM) algorithm in order to visualize the guided wave propagation and energy transfer across the plate thickness. Laboratory experiments were conducted and the wall thickness reduced initially uniformly by milling of the steel structure. Further measurements were conducted using accelerated corrosion in salt water. From the measured signal change due to the wave mode interference, the wall thickness reduction was monitored and good agreement with theoretical predictions was achieved. Corrosion can lead to non-uniform thickness reduction and the influence of this on the propagation of the high frequency guided ultrasonic waves was investigated. The wave propagation in a steel specimen with varying thickness was measured experimentally and the influence on the wave propagation characteristics quantified

Guided wave scattering analysis around a circular delamination in a quasi-isotropic fiber-composite laminate

Carbon fiber reinforced composites are widely used in the aerospace industry, but barely visible impact damage can lead to delamination and compromise the structural integrity. The scattering of the fundamental anti-symmetric guided wave mode (A0 Lamb mode) at an artificial circular delamination in a quasi-isotropic laminate was investigated experimentally. A 5 cycle Hanning windowed wave pulse was used as the excitation signal for the experiments. Fast Fourier Transform was employed to identify the guided wave amplitude of the scattered field along various directions. The experimental wavefield was captured using a laser Doppler vibrometer. Experimental results are presented for the scattering pattern and scattering amplitude as a function of distance from the damage. The results of this study can help to improve delamination detection techniques using guided waves and to gain physical insights into the scattering of guided waves at a delamination