Enhancement of a new methodology based on the impulse excitation technique for the nondestructive determination of local material properties in composite laminates

A new approach for the nondestructive determination of the elastic properties of composite laminates is presented. The approach represents an improvement of a recently published experimental methodology based on the Impulse Excitation Technique, which allows nondestructively assessing local elastic properties of composite laminates by isolating a region of interest through a proper clamping system. Different measures of the first resonant frequency are obtained by rotating the clamping system with respect to the material orientation. Here, in order to increase the robustness of the inverse problem, which determines the elastic properties from the measured resonant frequencies, information related to the modal shape is retained by considering the effect of an additional concentrated mass on the first resonant frequency. According to the modal shape and the position of the mass, different values of the first resonant frequency are obtained. Here, two positions of the additional mass, i.e., two values of the resonant frequency in addition to the unloaded frequency value, are considered for each material orientation. A Rayleigh–Ritz formulation based on higher order theory is adopted to compute the first resonant frequency of the clamped plate with concentrated mass. The elastic properties are finally determined through an optimization problem that minimizes the discrepancy on the frequency reference values. The proposed approach is validated on several materials taken from the literature. Finally, advantages and possible limitations are discussed

Experimental and Numerical Investigation of a Lattice Structure for Energy Absorption: Application to the Design of an Automotive Crash Absorber

In this work, an experimental and numerical analysis of a lattice structure for energy absorption was carried out. The goal was to identify the most influencing parameters of the unit cell on the crushing performances of the structure, thus guiding the design of energy absorbers. Two full factorial plans of compression tests on cubic specimens of carbon nylon produced by fused deposition modeling (FDM) were performed. The factors were the beam diameter and the number of unit cells. In the first factorial plan, the specimen volume is constant and the dimensions of the unit cell are varied, while the second factorial plan assumes a constant size of the unit cell and the volume changes in accordance with their number. The results showed that the specific energy absorption increases with the diameter of the beam and decreases with the size of the unit cell. Based on these results, a crash absorber for the segment C vehicle was designed and compared with the standard component of the vehicle made of steel. In addition to a mass reduction of 25%, the improved crushing performances of the lattice structure are shown by the very smooth force-displacement curve with limited peaks and valleys

An innovative nondestructive technique for the local assessment of residual elastic properties in laminated composites

In this work, an innovative experimental methodology is presented for the assessment of damage severity in composites. The technique aims at determining the local variation of the elastic properties in the damaged region of a composite component. Based on the Impulse Excitation Technique (IET), the vibrational response of the inspected region is isolated by clamping its extremities through vacuum, thus allowing the assessment of local variations. Complementarily, a new analytical approach is derived for the assessment of the residual elastic properties of the damaged area from the measurement of the first resonant frequency. Validation of the proposed methodology is performed on two glass-fibre woven fabric composites, damaged by impact. The material properties of the damaged zone determined through the proposed technique are compared to the results of tensile tests performed on specimens cut from the impacted plates. In particular, the specimens are equipped with optic fibre in order to punctually measure the elastic parameters. Results show that the residual elastic properties assessed with the proposed technique are in very good agreement with those measured through the optic fibre, thus proving the effectiveness of the methodology

Single-lap joints of similar and dissimilar adherends bonded with a polyurethane adhesive used in the automotive industry

The mechanical performances of single-lap joints between similar and dissimilar adherends bonded with a bi-component polyurethane adhesive have been studied in the present work. The substrate materials include both carbon fibre reinforced composite material (CRFP) and painted metal substrates (PMS). The following substrate combinations were tested: CFRP/CFRP, PMS/PMS, and CFRP/PMS. Two adhesive overlaps, 12 mm and 24 mm, with a fixed thickness were studied to assess the mechanical behaviour of the adhesive joints. The experimental results have been used to construct a finite element model of the single lap joint tests. The objective is to determine the material cohesive properties, in particular the maximum shear stress and the corresponding energy release rate, of the adhesive layer for each retained combination of substrates. An optimization scheme based on transient nonlinear finite element analysis has been here considered, where cohesive parameters of the adhesive layer are handled as design variables. Material parameters are firstly identified for the 12 mm overlap, minimizing the discrepancy between the experimental and numerical force-displacement curves. Then, to validate the obtained properties, results of the 24 mm overlap single lap joint tests are used. The comparison between the experimental and numerical results shows a very good agreemen

Residual properties in damaged laminated composites through nondestructive testing: A review

The development of damage tolerance strategies in the design of composite structures constitutes a major challenge for the widespread application of composite materials. Damage tolerance approaches require a proper combination of material behavior description and nondestructive techniques. In contrast to metals, strength degradation approaches, i.e., the residual strength in pres-ence of cracks, are not straightforwardly enforceable in composites. The nonhomogeneous nature of such materials gives rise to several failure mechanisms and, therefore, the definition of an ulti-mate load carrying capacity is ambiguous. Nondestructive techniques are thus increasingly re-quired, where the damage severity is quantified not only in terms of damage extension, but also in terms of material response of the damaged region. Based on different approaches, many nonde-structive techniques have been proposed in the literature, which are able to provide a quantitative description of the material state. In the present paper, a review of such nondestructive techniques for laminated composites is presented. The main objective is to analyze the damage indexes related to each method and to point out their significance with respect to the residual mechanical perfor-mances, as a result of the working principle of each retained technique. A possible guide for future research on this subject is thus outlined

Statistical estimation of fatigue design curves from datasets involving failures from defects

In the present paper, two methodologies for the estimation of the design curves of datasets with failures originating from defects are proposed. With the first methodology, the Likelihood Ratio Confidence Bound of a specific quantile P-S-N curve is considered. The second method is based on the bootstrap approach, with a large number of datasets simulated starting from the stress life and the defect size distributions estimated from the experimental data. The two approaches have been validated on literature datasets covering also the Very High Cycle Fatigue (VHCF) life region, proving their effectiveness

Statistical models for estimating the fatigue life, the stress–life relation, and the P-S–N curves of metallic materials in Very High Cycle Fatigue: A review

The research on the Very High Cycle Fatigue (VHCF) response of materials is fundamental to guarantee a safe design of structural components. Researchers develop models for the fatigue life in VHCF, aiming at assessing the stress–life relation and, accordingly, the probabilistic S–N (P-S–N) curves. In the paper, the models for the stress–life relation in VHCF are comprehensively reviewed. The models are classified according to the approach followed for defining the stress–life dependency, that is, power law, probabilistic, fracture mechanics, or Paris law-based approach. The number of failure modes that can be modeled, the statistical distribution for the fatigue life, and the characteristics of the estimated P-S–N curves are also reviewed by analyzing the fitting capability of experimental datasets for each model. This review is supposed to highlight the strengths and weaknesses of the currently available models and guide the future research

Residual elastic response in damaged woven laminates through local Impulse Excitation Technique

In this work, the assessment of the residual elastic response in damaged woven laminates is addressed through an innovative nondestructive technique. Based on the Impulse Excitation Technique (IET), the goal is to determine the local variation of the elastic properties through local vibrational tests. In particular, by non-destructively clamping its extremities, the vibrational response of the region of interest can be isolated and a vibrational mode can be excited, which depends only on the material properties of the inspected region. In presence of damage, the local degradation of the elastic response can be then correlated to the decrement of the first resonant frequency. The technique is applied to glass-fibre/epoxy laminates damaged by impact and its sensitivity to the size of the inspected region is investigated through three clamping devices of different dimensions. For validation, tensile tests are performed on specimens cut from the impacted plates, where the axial deformation is punctually measured through optic fibre glued along the specimen axis. Results show that the residual elastic properties assessed with the proposed technique are in very good agreement with those measured through the optic fibre, thus proving the effectiveness of the methodology

Adhesive Thickness and Ageing Effects on the Mechanical Behaviour of Similar and Dissimilar Single Lap Joints Used in the Automotive Industry

The effects of the adhesive thickness and overlap of a polyurethane adhesive have been studied by using different substrate configurations. Single lap joint (SLJ) specimens have been tested with homologous substrates, carbon fibre-reinforced plastics and painted metal substrates. Furthermore, a configuration with dissimilar substrates has been included in the experimental campaign. Both types of these adhesive and substrates are used in the automotive industry. The bonding procedure has been carried out without a surface treatment in order to quantify the shear strength and stiffness when surface treatments are not used on the substrates, reproducing typical mass production conditions. Three different ageing cycles have been used to evaluate the effects on SLJ specimens. A finite element model that uses cohesive modelling has been built and optimised to assess the differences between the different adopted SLJ configurations

Experimental and numerical characterization of adhesive joints with composite substrates by means of the Arcan test

An experimental and numerical investigation on adhesive joints subjected to three different loading conditions (pure shear, combined shear-tensile load and pure tensile) has been carried out through the Arcan test. The aim of this work is to investigate the effect of compliant substrates on the mechanical performances of the adhesive. Differently from traditional Arcan tests, where highly rigid substrates are used, U-shaped specimens made of carbon prepreg, final thickness 1 mm and 2 mm, have been bonded with a stiff epoxy and polyurethane adhesives. Nonlinear finite element analyses were performed to assess the stress state in the adhesives obtained by the experimental tests and showed a good correlation with the experimental results. In particular, the shear and tensile tests with the thicker substrates were used to determine the properties of the adhesives, while the 45◦ loading case combined with the thinner substrate results were used for validation. Results show that the compliance of the substrates significantly affects the fracture propagation of both adhesives in Mode I, as well as the maximum load of the epoxy-based joint for all the loading conditions. The results suggest that the Arcan test is suitable for the complete characterization of this type of adhesive joints