G. Caprino

Influence of material thickness on the response of carbon-fabric/epoxy panels to low velocity impact

Low velocity impact tests were carried out on carbon-fabric/epoxy laminates of different thicknesses, by means of a hemispherical impactor. The force and absorbed energy at the point of delamination initiation, the maximum force and related energy, and penetration energy were evaluated. From the experimental results, all these quantities, except the energy for delamination initiation, followed the same trend, increasing to the power of approximately 1.5 with increasing plate thickness. For what concerns the force at delamination initiation, it is shown that its trend agrees with the assumption of a Hertzian contact law, coupled with the hypothesis that only the shear stress is responsible for delamination. It is also demonstrated that the force/displacement curves recorded for the different thicknesses sensibly superpose with each other when the forces are scaled to the power 1.5 and the displacements are held unchanged. This explains the observed dependence of the maximum force, energy at maximum force, and penetration energy on the thickness. Finally, the energy at delamination initiation is calculated by an analytical model, assuming that the total energy is shared in two parts, one of which is stored in flexure, and the other in the material volume close to the contact zone.

Flexural fatigue behaviour of random continuous-fibre-reinforced thermoplastic composites

Abstract 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.

Impact and post-impact behavior of foam core sandwich structures

Abstract 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.

On the penetration energy for fibre-reinforced plastics under low-velocity impact conditions

This paper deals with the prediction of the penetration energy for fibre-reinforced plastics subjected to low-velocity impact. Some results available in the literature, allowing evaluation of the main parameters affecting the energy-absorbing capacity of a composite laminate, are reviewed first. It is shown that, for a given fibre type, the penetration energy is substantially influenced by the total fibre volume and tup diameter, whereas other factors, such as resin type and content, fibre architecture, stacking sequence and orientations, play a secondary role in the phenomenon. An empirical power law equation recently proposed by the authors, from which the penetration energy can be evaluated, is then assessed on the basis of experimental data previously published. The results indicate that the exponent of the power law is probably independent of the material considered, being practically the same for carbon- and glass-fibre-reinforced plastics, and even for an isotropic material such as polycarbonate, prone to extensive plastic yielding before final failure. The formula proposed, useful for in-plane isotropic and moderately anisotropic composites, can also permit the comparison of impact data generated under different impact conditions.

The significance of indentation in the inspection of carbon fibre-reinforced plastic panels damaged by low-velocity impact

Low-velocity impact tests were carried out at different energy levels on three types of T400/934 angle-ply laminates, by means of an instrumented drop-weight apparatus. After impact, the indentation depth and the residual tensile strength were measured as a function of impact energy. From the results obtained and experimental data available in the literature, an indentation law, allowing for the prediction of the impact energy from the depth of indentation, was developed. This indentation law appears to have a quite general applicability, being scarcely affected by fibre type and orientation or matrix type. A previous formula, modelling the residual tensile strength decay as a function of impact energy, was proved to be capable of correctly predicting the residual strength results for all the laminates tested. Combining the indentation model and the residual strength model, a closed-form model, explicitly correlating the residual strength and the indentation depth, was obtained. The theoretical predictions were in very good agreement with the experimental results. It is shown that the new model assumes a simple analytical form for a given laminate, permitting material characterisation by a minimum amount of test data.

A Note on Specially Orthotropic Laminates

A particular class of composite laminates is presented, where coupling ef fects are rigorously zero.

Long-term behaviour of PEI and PEI-based composites subjected to physical aging

Abstract Glass-forming materials, including thermosetting and thermoplastic resins commonly used in polymer-matrix composites for high-performance applications, undergo structural relaxation when they are cooled through the glass transition region owing to the glassy non-equilibrium state. The process of moving towards equilibrium consists essentially of a densification of the matter and follows complex paths deriving from its inherent non-linear and non-exponential character. The structural relaxation is observable in a laboratory time-scale at temperatures below but close to T g , and is related to the durability of polymeric materials because it is characterized by changes of structure-sensitive properties until equilibrium is approached. Polymer-based composites suffer the same shortcoming even if the properties are fiber-dominated. In this paper sub- T g annealing studies have been carried out with both plain PEI and its carbon-fiber composites in order to observe the kinetics of structural relaxation. The experimental technique used is differential scanning calorimetry (DSC), which measures the enthalpy recovery during the structural relaxation. The investigation regarding the plain resin consisted of aging treatments at T g −10°C, T g −20°C and T g −30°C, for different annealing times ranging between 0 and 168 h. The experimental data were used to calculate the parameters of a long-term predictive model for the enthalpy relaxation based on the Narayanaswamy approach. The structural relaxation of the in situ resin (i.e. the resin constrained into the fibers lattice) was also investigated at T g −20°C taking into account the marked decrease of the glass transition temperature resulting from the overall manufacturing process. The comparison between the plain and the in situ matrix was done on the basis of the same degree of undercooling (i.e. the same distance from T g ). It was found that the composite aged faster than the plain matrix. The macroscopic effects of structural relaxation on PEI based composites were analyzed by fatigue tests in four-point bending geometry at different level of stress ratio R (the ratio of the minimum to the maximum stress). The aged samples showed a higher characteristic strength, which resulted in a correspondingly higher fatigue life compared to the as manufactured materials.

Composite materials response under low-velocity impact

Abstract A simple model, based on energy considerations, has been tested to predict the maximum contact force during a low-velocity impact between an impactor and a composite plate. Three different composites, i.e. glass cloth-polyester, carbon cloth-polyester and nylon cloth-polyester, were examined. The results reported here, obtained using an instrumented apparatus, show that the total energy applied during the impact is the governing parameter of the phenomenon, rather than the impactor speed or mass. All the composites under evaluation did not show any variation of elastic modulus with impact velocity. Moreover, the dynamic behaviour of carbon-polyester and nylon-polyester composites can be predicted by simple static tests, because of their insensitivity to rate-dependent phenomena; for these materials a simulation of impact tests by static tests is therefore suggested. Glass-polyester composites did show a rate-dependent behaviour, by an increase in strength of about 70% with respect to the static case; a small number of dynamic tests is, however, sufficient to characterise their behaviour under impact conditions.

Elastic behaviour of composite structures under low velocity impact

Abstract The results of a study on the behaviour of glass cloth/polyester panels under low velocity impact are reported. The experimental load/time curve in the elastic region showed good agreement with that predicted by a simple analytical model, based on energy considerations, using elastic properties obtained by static tests. The structural rigidity of the panels was not significantly affected by premature local shear damage, due to the concentration of load, even after first fibre failure. The strength of the material was found to be strongly rate-dependent; as a consequence the response of the structure in the post-elastic region cannot be predicted from the results of static tests.

Residual strength evaluation of impacted GRP laminates with acoustic emission monitoring

Abstract 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.