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An approach to detecting discontinuities in carbon fiber-reinforced polymers, caused by impact loading followed by compression testing, was developed. An X-ray sensor-based installation was used, while some algorithms were developed to improve the quality of the obtained low-contrast radiographic images with negligible signal-to-noise ratios. For epoxy/AF (#1) composite subjected to a "high-velocity" steel-ball impact with subsequent compression loading, it was not possible to detect discontinuities since the orientation of the extended zone of interlayer delamination was perpendicular to the irradiation axis. After drop-weight impacts with subsequent compression loading of epoxy/CF (#2) and PEEK/CF (#3) composites, the main cracks were formed in their central parts. This area was reliably detected through the improved radiographic images being more contrasted compared to that for composite #3, for which the damaged area was similar in shape but smaller. The phase variation and congruency methods were employed to highlight low-contrast objects in the radiographic images. The phase variation procedure showed higher efficiency in detecting small objects, while phase congruency is preferable for highlighting large objects. To assess the degree of image improvement, several metrics were implemented. In the analysis of the model images, the most indicative was the PSNR parameter (with a S-N ratio greater than the unit), confirming an increase in image contrast and a decrease in noise level. The NIQE and PIQE parameters enabled the correct assessment of image quality even with the S-N ratio being less than a unit. [ABSTRACT FROM AUTHOR]

This study introduces a Radial Basis Function-Genetic Algorithm-Back Propagation-Importance Sampling (RBF-GA-BP-IS) algorithm for the multi-scale reliability analysis of Fiber-Reinforced Polymer (FRP) composite structures. The proposed method integrates the computationally powerful RBF neural network with GA, BP neural network and IS to efficiently calculate inner and outer optimization problems for reliability analysis with hybrid random and interval uncertainties. The investigation profoundly delves into incorporating both random and interval parameters in the reliability appraisal of FRP constructs, ensuring fluctuating parameters within designated boundaries are meticulously accounted for, thus augmenting analytic exactness. In application, the algorithm was subjected to diverse structural evaluations, including a seven-bar planar truss, an architectural space dome truss, and an intricate nonlinear truss bridge. Results demonstrate the algorithm's exceptional performance in terms of model invocation counts and accurate failure probability estimation. Specifically, within the seven-bar planar truss evaluation, the algorithm exhibited a deviation of 0.08% from the established failure probability benchmark. [ABSTRACT FROM AUTHOR]

The stress–strain characteristics and failure behavior of composites are strain rate dependent and affected by the fiber areal density. To elucidate the combined influence of areal density and strain rate on the strength of basalt fiber‐reinforced polymer composites (BFRP), an experimental study was conducted on BFRP laminates of two different fiber areal densities, that is, 380 GSM and 200 GSM, having distinct stacking sequence under three different loading rates. Failure modes, failure strength, and Weibull parameters were used to characterize the experimental outcomes. The experiment was carried out to investigate the mechanical responses and associated failure modes at strain rates ranging from quasi‐static 0.1 mm/min to a high strain rate of 10 mm/min. It has been demonstrated that there is a substantial correlation between the fiber areal density, loading rate, and stacking order of BFRP laminates and the increase in maximum flexural strength and interlaminar shear strength. For an increase in the fiber areal density from 200 GSM to 380 GSM flexural strength is increased by 18%–30%, while ILSS strength is increased by 30%–52%. Based on the finding, the asymmetric type‐2 laminate exhibits better properties than the symmetric and asymmetric type‐1 laminates due to the presence of more (0°/90°) laminae at the tensile side of the laminate. Inferring the mechanical characteristics of composite materials and their relationship to strain rate from experimental data required a statistical technique. The statistical analysis and experimental findings demonstrate that the shape parameter and linear coefficient are not reliant on the strain rate. [ABSTRACT FROM AUTHOR]

In the present work, a unified phase field-based cohesive zone model (PF-CZM) has been utilized to predict bulk fracture and most importantly interfacial delamination in carbon fiber-reinforced polymer composites (CFRP). The formulation is based on the classical phase-field approach of energy minimization, with a rational degradation function and a polynomial-type crack geometric function. Incorporation of those provides the PF theory to integrate various traction–separation behaviors by choosing optimal constitutive functions and makes the theory capable of predicting the quasi-brittle type of fracture. Since the interfacial delamination in composites occurs either due to adhesive failure or decohesion between stacked lamina, the PF-CZM model is used to predict such failure by exploiting the cohesive zone flavor of the model. Further, the model inherently incorporates the strength of the material without any additional calibration. Various standard simulations are performed considering CFRP geometry where delaminations due to adhesive failure or interface decohesion occur in a mode-I and mode-II fashion. The model is observed to predict mode-I and mode-II separation accurately. To test the accuracy of the model an experimental validation is also performed. In addition to the delamination, open hole tension (OHT) simulations of CFRP lamina for different fiber orientations are considered to validate the anisotropic crack growth predictions, and the results are compared with experiments. [ABSTRACT FROM AUTHOR]

The present paper aims to study different laser drilling/cutting parameters including laser intensity, cutting speed, and gas pressure in order to achieve minimum surface roughness (Ra), heat-affected zone (HAZ), taper angle (TA), and maximum tensile strength (TS) of the laser-drilled glass fiber-reinforced plastic (GFRP) laminate. Full factorial design of experiment method and analysis of variance were adopted to investigate the effect of each parameter on the responses, and in addition, the surface roughness and tensile strength were compared with that of conventional drilling. The morphology of the laser-drilled holes was studied through the scanning electron microscopy (SEM). It was found that optimum laser drilling parameters can yield higher tensile strength and lower surface roughness in drilled GFRP laminate compared to the conventional drilling. Moreover, multi-response optimization was carried out on the results in order to obtain maximum tensile strength, minimum HAZ, minimum surface roughness, and minimum taper angle. The results revealed that the laser intensity, cutting speed, and assist gas pressure should be set at 2.04 W/cm2, 8 mm/s, and 4 bar, respectively, to obtain higher tensile strength and surface integrity. The results drawn from this study can be applied to improve the surface quality and mechanical properties of laser drilling of FRP materials especially in aircraft part manufacturing. [ABSTRACT FROM AUTHOR]

In the present work, a phase-field fracture model is developed to create an experimentally validated, physically motivated, and computationally tractable framework to predict the orientation-dependent complex fracture response of the unidirectional fiber-reinforced polymer matrix composites (PMC). The damage mechanism in PMC is many-fold, such as matrix micro-cracking, fiber breakage or pullout, fiber debonding, and delamination. The progressive damage and failure in the composite can be attributed to the competition of all these different damage mechanisms individually or in concert, depending on fiber volume fraction and orientation of the plies. Moreover, other important factors at the constituent level, such as viscoelasticity or plasticity of the matrix, can significantly influence the constitutive behavior of the composite. In this work, a homogenized, coupled thermo-mechanical constitutive model is developed considering the viscoelasticity of the polymer matrix for the fiber-reinforced PMCs. Subsequently, a phase-field fracture model is adopted to capture the essential damage mechanisms as matrix cracking and fiber pullout/breakage for unidirectional composite lamina. The phase-field variable considers an anisotropic tensorial term in its gradient to account for the orientation-dependent fracture of fiber-reinforced PMC. The model is numerically implemented by writing an ABAQUS user-element subroutine (UEL). The model can predict the direction-dependent damage propagation and the final load-deformation response at fracture in commercially acquired unidirectional glass-fiber-reinforced epoxy composites at different fiber orientations in reasonable agreement with the experiments. Based on the decoupling of the driving energies the model can also identify the dominant damage mechanism among the two such as fiber breakage or matrix cracking, for a given fiber orientation. Simulations are also conducted to validate the model predictions for carbon-fiber reinforced epoxy lamina with available experimental data in the literature. • A phase-field fracture model is developed for unidirectional fiber composites. • The model is based on a homogenized constitutive description of the PMC. • Incorporates viscoelastic behavior of polymer matrix. • Considers the anisotropy in crack propagation influences by the fiber orientation. • Experimentally validated and computationally tractable in a finite element framework. [ABSTRACT FROM AUTHOR]

Abstract: Power transmission poles are subjected to dynamic cantilever bending due to wind gusts and cable unilateral failure, or may also be subjected to vehicle impacts. In this paper, transient dynamic analysis of tapered fiber-reinforced polymer (FRP) composite transmission poles with circular thin-walled cross-section subjected to dynamic cable tension and vehicle impacts is investigated by combination of tapered beam finite element and precise time integration method. It is assumed that the material behavior is linearly elastic and the laminate of the cross-section of the wall is symmetric or antisymmetric angle-ply. The effect of fiber type and orientation, the pole geometry, and the concentrated mass at the pole tip are evaluated by performing the dynamic analysis of FRP poles under step, triangular and sine pulses. There is a good agreement between the results of the present method and those obtained from the poles modeled by ANSYS commercial finite element software and existed literatures. Also, there is a significant shorter run-time in the present method. It is concluded that beyond 10 layers for the laminate with constant thickness of the wall, the pole tip deflection does not change. [Copyright &y& Elsevier]

To investigate the mechanical degradation of the shear properties of glass fiber-reinforced polymer (GFRP) laminates in bridge decks under hygrothermal aging effects, short-beam shear tests were performed following the ASTM test standard (ASTM D790-10A). Based on the coupled hygro-mechanical finite element (FE) analysis method, an inverse parameter identification approach based on short-beam shear tests was developed and then employed to determine the environment-dependent interlaminar shear modulus of GFRP laminates. Subsequently, the shear strength and modulus of dry (0% Mt/M∞), moisture unsaturated (30% Mt/M∞ and 50% Mt/M∞), and moisture saturated (100% Mt/M∞) specimens at test temperatures of both 20 °C and 40 °C were compared. One cycle of the moisture absorption–desorption process was also investigated to address how the moisture-induced residual damage degrades the shear properties of GFRP laminates. The results revealed that the shear strength and modulus of moisture-saturated GFRP laminates decreased significantly, and the elevated testing temperature (40 °C) aggravated moisture-induced mechanical degradation. Moreover, an unrecoverable loss of shear properties for the GFRP laminates enduring one cycle of the moisture absorption–desorption process was evident. [ABSTRACT FROM AUTHOR]