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Three-point and four-point bending tests were carried out on sandwich beams consisting of glass-fibre-reinforced plastic facings and a Divinycell H60 foam core. During the tests, final failure was invariably precipitated in the upper facing at the points of load introduction, whereas no other failure modes typically exhibited by a sandwich structure were observed. From the experimental results, the critical load causing final failure was found to be independent of the overall bending moment M and local force P l alone, in contrast with the hypotheses of the theoretical models presently available in the literature. On the contrary, for a given M / P l ratio adopted in the tests, the critical conditions were achieved more easily in the case of four-point than under three-point bending. A possible explanation of the observed phenomenon was given by calling into question the influence of the shear force, that may contribute to the sandwich local collapse. An empirical failure criterion, taking into account the bending moment, the local force and the shear force, was found to reasonably match the experimental results, although the physical meaning of one of the parameters appearing in the model formulation is matter of further research.

The results of orthogonal cutting tests on unidirectional carbon and glass fibre reinforced plastics are presented. The specimens were under shape of rectangular plates, circular disks and cylinders with different fibre architectures and a milling machine, a lathe machine and a five-axis high-speed vertical machining centre, were used for the experimental tests. The cutting speed was varied. During the tests, performed at low cutting speed, avoiding thermal effects, and high speed, to investigate about the effect of the cutting velocity on the cut quality, the fibre orientation respect to the cutting direction, the tool rake angle and the depth of cut were varied to investigate their influence on the phenomenon. A high speed steel tool in different geometries, was used. The mechanisms of chip formation and the cutting quality were investigated. A tentative to correlate the mechanisms of chip formation and cutting forces signals was done. Since the anisotropy, the mechanisms of chip formation consists of different failure modes occurring simultaneously and their identification, on the basis of the cutting force evolution, is very complex. Only in particular conditions, the features of cutting forces allow a precise identification of the chip development and detachment. The results indicated that the fibre orientation respect to the cutting direction determines the mechanisms of chip formation and influences the cutting quality. It was noted that for fibre orientation higher than 60°, the quality of the surface was revealed unacceptable. These conclusions were obtained independently of the particular shape of specimen tested and of the speed adopted. © 2017 The Authors. Published by Elsevier B.V.

In the present work, numerical, experimental and analytical results regarding impact events on composite structure are presented. The test case consists in a classic 24 plies CAI specimen (100×150 mm) subjected to 10 J impact. The work can be divided into two phases. The first phase is finalized to the definition of a procedure able to provide a robust numerical model, which can simulate accurately the structural response of composite plates subjected to impact events. At this phase, the numerical results are compared with analytical ones. In the second phase, both inter- and intra-lamina failure are considered. Regarding the inter-laminar failure, an experimental-numerical procedure is defined in order to set the right parameters related to cohesive behaviour. For both phases, trade-off analyses on the main numerical parameters are performed. All numerical results are compared with experimental ones in terms of both energy balance and damaged area. © 2016 The Authors.

Natural fibre composite have a potential to be widely applied in the alternative to a fibreglass composites in sustainable energy impact absorption structures. In this study, the behaviour of hemp fibre epoxy composites subjected to a low-velocity impact loading, using an instrumented falling weight impact equipment, is presented. Two types of hemp reinforcements were used: unidirectional and bi-directional fabric and three balanced laminates (0°/90°) different in thickness were studied: 4 plies, 8 plies, 12 plies. The results were compared. The laminates were fabricated by RIFT process using a common epoxy matrix. The reached volumetric fibre percentage V f was not too high. It assumed a value of 34% for the unidirectional reinforcement and an average value of 30% for the bidirectional one. Some laminates were impregnated by PLA resin at the aim to obtain a completely natural system. The impact behaviour was analysed carrying out tests impacting square specimens, 100 × 100 mm, with a hemispherical tup geometry with a diameter of 19.8 mm and a velocity of 4.0 m/s. Both penetration and indentation tests were performed at the aim to investigate about the damage start and evolution. The impact induced damage, characterised by an optical microscope, includes matrix cracking, delamination, fibre breakage and fibre pullout. An interesting compressed central zone of the laminate under the impact point was noted.

Low-velocity impact tests were carried out on sandwich plates having CFRP facings and thin rubbery core. Two types of cores, differing in the material nature and thickness, were used. For comparison, similar tests were performed on the monolithic laminate. Various impact parameters, among which indentation, first failure energy, perforation energy, absorbed energy and maximum contact force, were analyzed, to highlight the effect of the core on the material response. The influence of the core on the macroscopic behaviour of the panels was quite limited, except in the elastic phase, where the lower stiffness of the sandwich configurations resulted in a higher energy at first failure. More relevant differences were found from the study of failure modes, carried out combining ultrasonic C-scan and a limited number of microscopic observations. In particular, in correspondence of the energy for barely visible impact damage, besides considerable facing-core debonding, both the facings of the sandwich structures exhibited fibre breakage at their back side.

Physics and mechanical tests were carried out on innovative sandwiches proposed for civil applications, at the aim to know their performances. The core was obtained by embedding short mineral fibre, in different percentages, in polyurethane matrix. The results of the experimental tests were compared to what obtained on sandwiches made without any reinforcement in the polyurethane core: better behaviours were found in terms of compression strength and elastic modulus, compression energy absorption capability, impact force and energy. Good results were observed in thermal conductivity too. A very large experimental campaign was carried out at the aim to characterize the innovative sandwiches under shear load too. It was not simple to obtain the pure shear strength since the very frequent local compression failures due to the not uniform load distribution under bending tests. So, more than one kind of skin, wood, aluminium and polymeric one, was tested: a glass fibre laminate, 1,5 mm in thickness, was revealed the most appropriate one giving the possibility to have the right load distribution to observe the classical shear failure. Better behaviour was noted also in this field even if it resulted very dependent on the reinforcement percentages.

Low-velocity impact tests were carried out on stitched carbon fibre-reinforced plastic laminates of various thicknesses, whose behaviour was studied with reference to the overall force–displacement curve, first failure load, penetration, indentation and damage extent. The results obtained were compared with similar data available for 2D laminates. Apparently, the presence of stitches did not affect substantially the material behaviour in terms of force–displacement curve, first failure load, and indentation. However, the stitched laminates exhibited a penetration energy about 30% lower than their 2D equivalent. Further, the advantage of stitching in terms of impact damage resistance was evident only for high thickness composites. The data generated suggest that the use of stitches could be unnecessary to hinder delamination in thin 2D laminates.

The scope of this paper was to establish a correlation between the damage occurring in a composite as a consequence of low-velocity impact and the energy dissipated during the impact phenomenon. To this aim, instrumented impact tests were carried out on glass fabric/epoxy laminates of three different thicknesses, using different energy levels. The irreversibly absorbed energy was obtained from the force–displacement curves provided by the impact machine. To assess damage progression as a function of impact energy, ply-by-ply delamination and fibre breakages revealed by destructive tests were measured. A previous model, based on energy balance considerations, was applied to interpret the experimental results, together with an original method of data reduction, allowing for the isolation of the maximum energy portion due to indentation and vibrational effects. From the results obtained, the contribution of fibre breakage and matrix damage to the irreversibly absorbed energy is comparable at low impact energies; with increasing initial energy levels, delamination becomes predominant in determining energy dissipation. However, the critical energy-release rate required to propagate delamination, as calculated from impact data, is considerably higher than the typical values deriving from Mode II delamination tests performed under laboratory conditions.

The design of advanced composite structures or components subjected to dynamic loadings requires a deep understanding of the damage and degradation mechanisms occurring within the composite material. The present paper deals with the numerical simulation of low-velocity impact tests on glass fabric/epoxy laminates through the LS-DYNA Finite Element (FE) code. Two laminates of different thickness were subjected to transverse impact at different energy levels and modeled by FE. Solid finite elements combined with orthotropic failure criteria were used to model the composite failure and stress based contact failure between plies were adopted to model the delamination mechanism. The final simulation results showed a good correlation with experimental data in terms of both force–displacement curves and material damage.

The empirical model recently found by the authors for the prediction of the indentation depth was verified for composite laminates different in matrix and fiber subjected to low velocity impact loads. The data obtained in a previous research on common glass and carbon fiber laminates made by commonly used epoxy matrix, were here compared to the results obtained on laminates made by glass fibers immersed in phenolic matrix and novel basalt fibers immersed in epoxy resin. The model takes into explicitly account the tup diameter and presents the advantage that only a single material constant to be experimentally determined is necessary for the prediction. The comparison showed that the value of the constant is different for the different material systems analyzed so that it can be assumed as an index for the indentation sensitivity. POLYM. COMPOS., 36:987–991, 2015. © 2015 Society of Plastics Engineers

An experimental activity conducted in order to assess the impact behavior at room and low temperature of carbon fibre in vinylester resin laminates used in the shipbuilding industry, was reported. The conditions which reproduce the impact of a hull at low temperature with a solid body suspended in the water was reproduced.A test equipment was designed and realized to reproduce the real material behaviour in water to obtain a load distribution on the entire surface of the specimen.The results were obtained impacting the laminates placed between the cilyndrical steel impactor and a bag containing water. A falling weight machine, equipped with an instrumented steel impactor and a thermal chamber, was adopted for the experimental tests.The impact behaviour in hostile environments was compared to the behaviour at room temperature and the data obtained under distributed load conditions were compared with the results from concentrated loads: a completely different behaviour was observed between the two different...

Mechanical tests were carried out on a fibreglass/aluminium hybrid laminate, made of 0/90 S2-glass/epoxy laminae and Al 2024 T3 sheets, in order to find its tensile stress–tensile strain curve, residual tensile strain in correspondence of predetermined normal stress levels, and shear stress–shear strain curve. The basic layers were also suitably characterized, and a moderate anisotropy, reflecting the fabrication process, was found in the metal sheets. To theoretically predict the laminate response, classical lamination theory was modified to account for the inelastic behaviour of aluminium, which was substituted by an “equivalent” material governed by a simple constitutive law. Final failure conditions were calculated by assuming the maximum strain criterion and Tsai-Hill criterion for aluminium and fibreglass, respectively. Comparing the theoretical predictions with experimental results, it was concluded that the model adopted describes with sufficient accuracy the overall tensile stress–tensile strain curve, as well as the residual strains and the shear stress–shear strain response. The aluminium anisotropy influenced the tensile behaviour, markedly affecting the yield stress value, as also predicted by the analysis. Because of the complex phenomena occurring during final failure, the agreement between theory and data was only fair when the calculation of the tensile strength and failure strain was attempted. The material shear strength could not be determined, since the Iosipescu device used proven inadequate to result in material failure.

The effectiveness of a new empirical model, aiming to predict the indentation depth resulting in a composite laminate from a hemispherical tup impacting it at low velocity, is here proposed. With this simple model including only the diameter of the impactor and the ratio between the impact energy and the perforation one, a material-independent parameter characterising the indentation depth is identified. Several samples with different thicknesses, impacted by various impactor tips, are tested for estimating this parameter. To reach the above mentioned scopes, low velocity impact tests were carried out on two different composite systems with different stacking sequences, thicknesses and fiber volume fractions: (a) glass/epoxy prepreg; (b) graphite/epoxy prepreg. The samples were simply supported on steel plates or clamped and they were struck at the center by hemispherical steel noses having 16 and 19.8 mm diameters. After impact, indentation was measured according to EN 6038 standard. The CFRP indentation data were drawn from a database: about 200 test records, generated by various researchers were individuated. The advantages of the new model are that the effect of the tup diameter is explicitly accounted for. Furthermore, a single material constant has to be experimentally determined and it can be assumed as an index for the indentation sensitivity, on the basis of which different materials can be ranked. The constant was found similar for GFRP and CFRP laminates denoting independence of constraint conditions, laminate type or laminae orientation and stacking sequence. POLYM. COMPOS., 34:2061–2066, 2013. © 2013 Society of Plastics Engineers

Glass fibre-reinforced plastic beams were pre-fatigued in four-point bending up to selected portions of their fatigue life. The samples were then monotonically brought to failure while recording their acoustic emission response. The trend of the cumulative event counts was markedly affected by the specimen load history: for a given stress level, a higher activity was obtained from samples subjected to a higher number of cycles. A good empirical correlation was found between the material residual strength and the total event counts detected at maximum stress applied during pre-fatiguing cycles. Moreover, the correlation was improved when a previous model, relying on fracture mechanics concepts, was utilised. However, in both cases, a difficulty was experienced in evaluating the residual strength of the composite laminate after a small number of fatigue cycles. Under these conditions, the modulus decrease was seemingly more sensitive to the effects of fatigue than the acoustic emission response.

In the present research, basalt fibre reinforced plastic laminates were obtained through infusion technology. From the plates, square specimens 90x150 mm in side, were cut by a diamond saw and tested in low velocity impact loads. A simplified numerical procedure in LS-Dyna able to predict maximum force, impact energy, strain velocity, delamination extension and its location along the thickness was studied. The results of the simulations were validated by the experimental investigations and the agreement found was quite good.

Low-velocity impact tests were performed on fibreglass–aluminium composites made of 2024 T3 sheets and S2-glass/epoxy prepreg layers, using an instrumented falling weight machine. For comparison purposes, similar tests were carried out on monolithic 2024 T3 sheets of equivalent thickness. In the tests, the impact speed, mass, and energy were varied, to ascertain the influence of these parameters on the material response. From the results obtained, the overall force–displacement curve only depends on the impact energy, rather than on the mass and speed separately. Further, the energy required for penetration is higher for monolithic aluminium than for the fibreglass–aluminium. However, the latter material seems to offer better performance than carbon fibre- and glass fibre-reinforced laminates in terms of penetration energy, damage resistance, and inspectability. The main failure modes of fibreglass–aluminium were assessed by both ultrasonic C-scan and chemical grinding of aluminium sheets. It was found that the energy required for first failure is very low, whereas the energy level resulting in first fibre failure is similar to that inducing first cracking in the 2024 T3 sheets. From the experimental data, simple empirical relationships were found for the calculation of maximum contact force, energy, and residual displacement as a function of the maximum displacement.

Acoustic Emission (AE) has long been recognized as a viable technique for real-time monitoring of metallic and composite structures, giving useful information not only on the presence of defects, but also on their criticity. Compared to other Non-Destructive Testing methods (NDTs) AE offers the advantage to locate defects in real time. The acoustic wave emitted by a source, related to a damage start or propagation, travels inside the material and it is received in different time by acoustic sensors arrayed on the surface. Knowing the different arrival times it is possible to locate the emission source position by the triangulation technique. For homogeneous material, this technique is easy to develop. However, the anisotropy nature of a composite material does not allow to have the same sound velocity in all the directions, so the development of mathematical models and algorithms for the implementation of the triangulation technique is difficult to set up. Since the above mentioned problem, this work proposes an alternative method to locate the point of an event on CFRP plates by analyzing the time of arrival of the ultrasonic signals received by three passive sensors attached to the plate surface in a known triangular configuration. The method is based on a simple concept: the comparison between the three measured arrival times and the predicted ones by analytical formulations. It was applied and validated through experimental tests carried out on a square unidirectional carbon fibre laminate. From the results, a good agreement between the predicted and the real position of the sound source was found.

Three-point bending tests in the elastic domain were carried out on sandwich specimens with the facings made of unidirectional T800H/3900-2 laminates, and the core of a soft and thin viscoelastic layer, varying the slenderness ratio. A closed-form formula available in the literature, developed for sandwich beams with thick facings, was used to predict the rigidities in flexure, adopting three different methods for the calculation of the shear rigidity. The best results were obtained by the classical sandwich theory, which provided reasonably good correlation with the experimental results, slightly overestimating the actual values. A finite element model consisting of three layers, made of standard CQUAD4 and CHEXA8 elements for facings and core, respectively, was developed, accurately predicting beam bending behavior. From the numerical results, the inability of classical sandwich theory to correctly estimate the bending rigidity is correlated with the through-the-thickness shear deformation of facings, not accounted for in the analysis.

In this work, the effectiveness of a new empirical model, aiming to predict the indentation depth resulting in a composite laminate from a hemispherical tup impacting it at low velocity, was verified. To reach the above mentioned scopes, low velocity impact tests were carried out on two different composite systems in different stacking sequences, thickness and fibre volume fraction: a) glass/epoxy prepreg; b) graphite/epoxy prepreg. The samples were simply supported on a steel plate and struck at the centre by a hemispherical steel nose having 16 mm and 19.8 mm diameter. After impact, indentation was measured according to EN 6038 standard. The CFRP indentation data were drawn from a database: about 200 test records, generated by various researchers were individuated. The advantages of the new model is that the effect of the tup diameter is explicitly accounted for. Furthermore, a single material constant has to be experimentally determined and it can be assumed as an index for the indentation sensitivity, on the basis of which different materials can be ranked. The constant was found similar for GFRP and CFRP laminates denoting independence of constraint conditions, laminate type or laminae orientation and stacking sequence.

Low-velocity impact tests were carried out on glass-fibre-reinforced plastic panels, varying the impact energy, laminate thickness, and tup diameter. A previous model, aimed at the prediction of indentation as a function of impact energy, was slightly modified, to establish a correlation of the dent depth with absorbed energy. Good agreement was found between the theoretical predictions and experimental results. Further, the possibility to resort to the absorbed energy from an indentation measurement, using a single curve valid whichever the laminate thickness and impactor diameter, was demonstrated. Analyzing previous experimental data, concerning carbon-fibre-reinforced plastics, the different behaviour of GFRP and CFRP in absorbing energy and undergoing indentation was revealed. New results about basalt fibre laminates based on a limited number of experimental tests were reported too.

Static tests were carried out on circular carbon fibre reinforced plastic plates of various thicknesses, which where loaded at the center by a hemispherical steel indentor. The tests were stopped at predetermined values of the displacement, and the indentation after the loading stage was evaluated. Tests were also performed up to complete penetration, using different indentor diameters. The data generated were compared with similar results previously obtained under low-velocity impact conditions, and treated according to models developed for the dynamic case. Apparently, both the penetration energy and indentation were not affected by the loading speed. The new experimental results showed that an empirical law previously proposed by the authors, aiming to predict the indentation as a function of the absorbed energy, is inaccurate when high energy levels are imparted to the material. A new empirical law, more efficient than the earlier, was assessed. The data collected indicate that the constants appearing in the new model are negligibly influenced by the laminate type and thickness, and constraint conditions adopted.

Infrared thermography (IRT) was used to gain information, in real time and without destroying the material, about damage initiation and propagation in GFRP specimens, 2.90 mm thick, subjected to low velocity impact. A modified Charpy pendulum was used since it allowed for positioning of the infrared camera and viewing on-line the specimen surface opposite to the impact. Then, it was possible to record the impact phenomenon. The pendulum was modified to allow impacts on rectangular flat specimens, 130 x 100 mm, by a cylindrical steel impactor with a hemispherical nose. The specimens were loaded at different energy values by varying the height of the Charpy arm at the aim to investigate about the evolution of delamination and of other failure modes. The recorded thermal images were analysed by the ThermalCAM® ResearcherTM software. It was possible to investigate the different in plane damages by comparing the thermal mapping to the fixed energy level. Other important pieces of information were obtained from the temperature spectrum, which displayed in some cases a maximum peak in correspondence of fibre breakage. Each specimen was also inspected with lock-in thermography (LT); results are presented as phase images. Some specimens were also microscopically observed at the aim to validate the results.

Laser marking tests were carried out on AISI 304 steel, using a Q-switched diode pumped Nd:YAG laser. The aim was to determine the correlation occurring between working parameters (i.e. pulse frequency, beam scanning speed, and current intensity) and resulting mark visibility. The latter was evaluated as the contrast index measured from digital images of the marks. To characterize mark features, its width and roughness were estimated, and analyses employing optical and scanning electron microscopy coupled with energy dispersive X-ray technique were carried out. From the experimental results, both surface roughness and oxidation increased as a function of frequency, resulting in an improvement in contrast, up to a characteristic value, decreasing afterwards. Besides, the maximum visibility value was beneficially affected by lower scanning speeds and lower current intensities. An empirical model was built, and from it the best processing conditions for optimum mark visibility, taking into account the operating constraints of the laser system used, were drawn.

Acoustic emission source location in a unidirectional carbon-fibre-reinforced plastic plate was attempted employing Artificial Neural Network (ANN) technology. The acoustic emission events were produced by a lead break, and the response wave received by piezoelectric sensors, type VS150-M resonant at 150 kHz. The waves were detected by a Vallen AMSY4 eight-channel instrumentation. The time of arrival, determined through the conventional threshold crossing technique, was used to measure the dependence of wave velocity on fibre orientation. A simple empirical formula, relying on classical lamination and suggested by wave propagation theory, was able to accurately model the experimental trend. Based on the formula, virtual training and testing data sets were generated for the case of a plate monitored by three transducers, and adopted to select two potentially effective ANN architectures. For final validation, experimental tests were carried out, positioning the source at predetermined points evenly distributed within the plate area. A very satisfactory correlation was found between the actual source locations and the ANN predictions.

Pencil lead break tests were carried out on symmetric 0/90 laminates, and the results in terms of extensional velocity and amplitude were analysed. The aim was to investigate about the influence of the stacking sequence and direction on the acoustic response of the materials. The wave velocity was measured from the difference in arrival time of the signal between two successive Acoustic Emission sensors, and predicted by lamination theory in the different test conditions analysed. A general agreement was found between theory and experiments, even if the differences found in correspondence of some orientation confirmed previous results, indicating an influence of layer thickness on the wave velocity. The data were used to verify the possibility to correctly locate the acoustic source through the location analysis, yielding satisfactory results. A simple, exponential law was able to predict the dependence of signal amplitude on the distance from the source. Indeed, attenuation was scarcely affected by fibre orientation and lay-up. This suggests that a method based on attenuation, rather than arrival time, could be useful for source location in composite laminates.

The mechanical response of PP foam in a wide range of strain rate has been studied. The linear and non linear viscoelastic behaviour has been analized. The material has been tested in creep and dinamico-mechanical experiments and the correlation between strain rate effects and viscoelastic properties of the foam has been obtained using viscoelasticity theory and separating strain and time effect. A scheme for the prevision of stress strain curve at any strain rate has been developed introducing a strain rate dependent scaling factor.

A recent two-parameter fatigue model, aimed at the predictionof the fatigue behaviour of composite materials, was applied to fatigueresults available in the literature, concerning graphite/epoxy laminatessubjected to tension-compression loadings. It was found that the model,successfully applied in the case of tension-tension andcompression-compression fatigue, also works when the sign of stress isreversed during fatigue. However, the constants appearing in theproposed formula must be evaluated anew when the loading mode changesfrom pure tension to tension-compression. The probability of failure infatigue is predicted with reasonable accuracy by the model, which isvery conservative in calculating the residual strength after a givennumber of cycles.

Glass Fibre Reinforced Plastic (GFRP) pipes, (90/0/90°)n and (0/55°)n (n = 1-3) in lay up, were subjected to hydrostatic tests in a Hammel Sciteq P400 machine, by which an internal pressure was applied through water inlet. During mechanical characterisation tests, carried out at the aim to compare the performances of the different stacking sequence, the specimens were continuously monitored by two Acoustic Emission (AE) sensors. The scope of AE analysis was twofold: a) to verify the usefulness of typical AE parameters in identifying the different failure modes occurring during loading; b) to attempt an effective localisation of the damage, rendered difficult by the acoustic anisotropy of the material. Good results were obtained by the theoretical formulation about the prediction of the behaviour of the different stacking sequence, in a good agreement with the AE system revealing a good solution for the on line monitoring of the pipes performances.

Long-term orthogonal cutting tests were carried out on sheet moulding compound, using high speed steel tools with 0° rake and 5° relief angle and a constant depth of cut, in order to verify the effect of tool wear on cutting forces. The tests were periodically interrupted, and the variation of cutting forces as a function of depth of cut at various degrees of wear was measured. It was observed that the thrust force is much more sensitive to wear than the principal one. The tool wear features suggested negligible sliding of the chip against the tool face, and intense friction along the tool flank, in agreement with previous results concerning the machining of unidirectional carbon-fibre-reinforced plastics. Accordingly, a force model previously proposed for unidirectional composites was applied to the cutting data. It was found that the increase in the principal force with wear is attributable to the increase in the thrust force generated at the tool flank, whereas the force arising at the tool face is substantially unaffected by tool wear. This allowed for the definition of a unit cutting force independent of both the depth of cut and the tool wear.

Static and fatigue tests were carried out in four-point bending on three random Glass Fibre Reinforced Plastics (GFRP), different in resin type (thermosetting and thermoplastic), resin content, and fibre form. The experimental results were evaluated in the light of a recent two-parameter model, allowing for a statistical analysis of the fatigue data. It is shown that, despite the different nature of the materials considered, the theoretical model accurately predicts the fatigue response, permitting the calculation of the scatter in fatigue life from the scatter in static strength. A physical interpretation is given for the two parameters appearing in the model, correlated with the material sensitivity to stress amplitude and number of cycles, respectively. From the values of the two parameters, evaluated for the GFRPs tested, a common trend is observed: the higher the material sensitivity to stress amplitude, the lower its sensitivity to number of cycles. This implies that comparing different materials on...

Static and fatigue tests have been performed in four-point bending on a random continuous-fibre-reinforced polypropylene. The data obtained were used to assess a closed-form formula, previously presented, aimed at the prediction of the fatigue response of a composite material, and explicitly accounting for the effect of the stress ratio. Comparing the experimental results with similar data available in the literature for thermoset-based composites, it was shown that the presence of an inherently ductile, thermoplastic matrix does not sensibly affect the fatigue sensitivity of the material, probably because of the constraint action of the reinforcement. From the fatigue model, a statistical model was developed, assuming a distribution of the static strength according to a two-parameter Weibull distribution. The data were in excellent agreement with theoretical predictions, indicating that a fatigue characterisation for probabilistic design can be achieved by a minimum of experimental tests.

The acoustic emission activity generated in monotonic four-point bending tests by pre-fatigued glass fibre-reinforced plastic specimens was treated by a conventional interpolation method, with the aim to predict the material residual strength. From the numerical analysis, a three-constant empirical model was derived, by which the residual strength was predicted as a function of the applied stress and the associated cumulative number of acoustic emission events. The results obtained were compared with those discussed in a previous work, where the same scope was pursued by the application of artificial neural networks. The response of the two methods was practically the same, in terms of both mean values and scatter. It was concluded that the classical interpolation may be preferable in the case examined, because the artificial intelligence approach required considerably longer times in the selection of the best network configuration.

Artificial neural networks (ANNs) were used to predict the residual strength of glass fibre-reinforced plastic beams pre-fatigued in flexure up to different portions of their fatigue life. To this aim, the acoustic emission signals recorded during the tests for the measurement of the residual strength, and the associated applied stress, were provided as input. An optimisation of the network configuration was carried out, using the root-mean-square error calculated in the training stage as the optimisation parameter. The predictive accuracy of the optimised ANN, consisting of two nodes in the input layer, four nodes in the hidden layer, and a single node in the output layer, was tested by the “leave-k-out” method. From the results obtained, ANN provided quite reliable predictions when the applied load was sufficiently far from the failure load, performing better than a previous theoretical model, relying on fracture mechanics concepts. Therefore, ANN was shown to be a valid tool in the non-destructive evaluation of composite materials employed in fatigue-sensitive applications.

The results of orthogonal cutting tests carried out on unidirectional glass fiber reinforced plastic composites, using HSS tools, are presented and discussed. During the tests, performed on a milling machine at very low cutting speed to avoid thermal effects, the cutting speed was held constant and parallel to the fiber direction. Three parameters, namely the tool rake angle {alpha}, the tool relief angle {gamma}, and the depth of cut t, were varied. According to the experimental results, the horizontal force per unit width, F{sub hu}, undergoes a dramatic decrease, never verified for metals, with increasing {alpha}. Besides, F{sub hu} is only negligibly affected by the relief angle, and linearly increases with t. Similarly to metals, an effect of the depth of cut on the specific energy (size effect) is found also for composites. However, the presented results indicate that the size effect can be analytically modeled in a simple way in the case of composites. The vertical force per unit width, F{sub vu}, exhibits a marked reduction when the relief angle is increased. F{sub vu} is also very sensitive to the rake angle: the lower {alpha}, the higher is F{sub vu}. It is shown that this behavior probably reflects amore » strong influence of the rake angle on the forces developing at the flank. A linear dependence of the vertical force on the depth of cut is also demonstrated. Finally, the experimental data are utilized to obtain empirical formulae, allowing an approximate evaluation of cutting forces.« less

Orthogonal cutting tests were carried out on unidirectional glass fibre-reinforced plastic composites, holding the cutting direction parallel to the fibre direction. The tools were made of high speed steel, with rake angle α = 0°; two relief angles γ, namely 7° and 15°, were adopted. A very low cutting speed was utilized, in order to avoid thermal effects on both the tool and the work material. The experimental results show that, under the selected operating conditions, the tool wear essentially consists of a very rapid rounding of the tool nose (nose wear). The face wear evolution is practically independent of the relief angle, whereas the latter affects the flank wear rate, which is slightly lower for the higher γ value. Both the horizontal and the vertical cutting forces exhibit notable increases with tool wear. The force data are interpreted in the light of a previously presented model, aiming to predict cutting forces as a function of the cutting parameters. It is shown that the observed increase in the horizontal force can be simply attributed to the increase in the vertical force at the tool flank, whereas the chip-tool interaction forces occuring at the tool face seem to be unaffected by wear phenomena. Finally, a strict correlation is found between the flank wear and the recorded vertical force variations.

This paper deals with tensile testing of GRP laminates containing impact damage generated with different impact energies. Acoustic emission (AE) detection and analysis was used to attempt the prediction of residual tensile strength after impact (RTSAI). A modified version of a previous model for residual strength evaluation of center-hole composite laminates is proposed. Its rationale is the introduction of a non-dimensional stress to make the composite laminate AE response independent of damage geometry. The experimental results show that AE activity is strictly correlated to RTSAI rather than to impact energy. Experimental agreement with the model is verified for both impacted and center-hole composite samples.

A previously proposed micromechanical formula, aiming to predict the vaporization energy Q[sub v] of composite materials as a function of fiber and matrix properties and fiber volume ratio, was assessed. The experimental data, obtained on glass fiber reinforced plastic panels with different fiber contents cut by a medium power CO[sub 2] cw laser, were treated according to a procedure previously suggested, in order to evaluate Q[sub v]. An excellent agreement was found between experimental and theoretical Q[sub v] values. Theory was then used to predict the response to laser cutting of a composite material with a fiber content varying along the thickness. The theoretical predictions indicated that, in this case, the interpretation of the experimental results may be misleading, bringing to errors in the evaluation of the material thermal properties, or in the prediction of the kerf depth. Some experimental data were obtained, confirming the theoretical findings.

The paper deals with the design of a composite arm, to be employed in the architecture of industrial, measuring robots. Based on the real working conditions, the arm is reduced to a cantilever beam, shaped as a thin walled tube, supporting a mass at the free end. An optimization study is carried out, aiming to determine the laminate properties capable of providing the maximum fundamental frequency (FF) in bending. The optimization relies on an approximate solution, obtained by modifying the Rayleigh energy method to account for the shear effect. It results in a closed-form formula, explicitly correlating FF with composite elastic properties. Two classes of laminates, namely (± θ) s and ( 0 ± 45 n ) s , are examined in order to assess their ability to fulfil the requirement of a maximum FF value. Moreover, the advantages and drawbacks of composites compared to metals are highlighted for the application under concern. Finally, the validity of the proposed solution is demonstrated by comparison with the results of a finite element analysis.

Bari, Italia

The thermal and mechanical behaviour of three types of pre-coated sands for selective laser sintering was characterised. First, the possibility of predicting the temperature evolution in the material with time under assigned heating conditions was verified. Then, a model aiming to calculate the degree of cure of the resin for a fixed heating history was assessed. Finally, hardness tests were performed on the sands, in order to measure their mechanical properties as a function of the thermal history. A strict correlation was found between the hardness data and the predicted degree of cure of the resin.

An extensive study of the behaviour of composite laminates subjected to dynamic loads was carried out by the authors many years in order to understand the complex mechanisms of damage initiation and propagation under low velocity impact loads. A review of the main results is hereafter presented.

The cutting forces arising in machining fibre-reinforced plastics are considered. The mechanisms of chip formation and material damage in the orthogonal cutting of unidirectional composites are recalled; the trends of cutting forces as functions of the operating parameters, as highlighted by the experimental results available, are illustrated; a number of schemes for interpreting the cutting forces, taking into account or disregarding the tool flank action, are presented; and a number of analytical and numerical models, put forward in the literature, are critically reviewed. The topic of forces developing in drilling and milling, two of the most important machining operations in composite industry, is then examined. Here, certain tendencies identifying the main parameters affecting the cutting forces are discussed, along with models that aim to predict the cutting forces depending on tool geometry.

This paper deals with the evaluation of residual tensile strength of composite laminates containing artificial defects, consisting of impact damages of different severity and implanted holes of various diameters. Sensor fusion of acoustic emission and load data was carried out through artificial neural networks, to obtain a reliable prediction of residual tensile strength as early as possible in the loading history. The results show that neural network processing offers an effective method for the monitoring of composite specimens based on acoustic emission detection and analysis.

Residual strength prediction through acoustic emission (AE) detection and analysis is attempted for tensile loading of GFR composite laminates containing center holes. By utilizing a previously developed two-parameter fracture mechanics model, the mechanical behavior of circular center hole composite specimens was interpreted. The AE response of the specimens was shown to be controlled by stress intensity factor K, rather than stress. In order to establish a quantitative basis for AE analysis aimed at predicting the fracture behavior of center hole composite laminates, a description of the experimental AE response through a mathematical model was considered necessary. An analytical expression of exponential form was assumed and experimentally verified. The excellent agreement between theory and experiment quantitatively confirmed the validity of the model. Through AE detection and analysis, it is therefore possible to assess hole criticality in composite laminates by direct prediction of their residual strength under tensile loading.

Low velocity, instrumented impact tests were carried out on sandwich panels made of glass fiber reinforced plastic facings and polyvinylchloride foam core. Three different core densities and three core thicknesses were examined. A damage parameter D was defined to account for the fiber damage size as evaluated through visual inspection. The experimental results demonstrate that D depends only on impact energy, whereas it is substantially independent of core density and thickness. An explanation for this behavior is formulated on the basis of recorded impact history. The results of tensile tests, performed to assess the residual strength after impact of the facing material, were analyzed using a previous model, modified according to the findings of an experimental work carried out by Cantwell and Morton. A correlation is found between the strength loss determined by impact damage and artificially implanted circular holes. Accordingly, a procedure is presented to allow for an accurate prediction of residual strength after impact as a function of kinetic energy. Finally, it is shown that residual strength can be reasonably predicted on the basis of damage parameter D, although in some cases this can result in a nonconservative estimate of the composite load-carrying capability.