M. Marchetti

Elasto-plastic behavior of thermoplastic composite laminates under cyclic loading

An experimental and numerical study of the elasto-plastic behavior of thermoplastic matrix composite laminates under static and cyclic loads is presented. Off-axis and angle ply specimens cut from laminates of poly(ether ether ketone) (PEEK) reinforced with continuous carbon fibers have been tested under cyclic sinusoidal tensile loads and the hysteresis loops have been monitored. A micro mechanical model, which includes a parabolic criteria based on the plastic behavior of the matrix, has been adopted to study the composite non-linear behavior and a correlation between plastic deformation and a strong rise of damping and temperature at high stresses is outlined. Good agreement is shown between theory and experimental results. The mathematical mdoel presented here can be used to predict the visco-elastic-plastic response of the material at high stresses and its influence in the fatigue damage.

Microdamage effects on the overall response of long fibre/metal-matrix composites

Abstract Metal-matrix composites represent a new generation of material in which the mechanical properties of metals and the effectiveness of composite structure are matched together. Experimental work performed on SiC long fibre/Ti composites revealed that two distinct slopes can be identified on the overall stress/strain curve before the appearance of hardening effects. A microscopical investigation provided evidence of distributed fibre debonding after the application of load. Further consideration on the nature of the contact zone between fibre and matrix revealed that the interface is a very weak zone, in which the material properties are degraded by the action between SiC and reactive Ti matrix. The adhesion between fibre and matrix is mainly due to the gripping effect of the matrix, resulting from cooling after material manufacture. To develop a micromechanical model able to predict the overall response of a simple unidirectional laminate, a finite element investigation was carried out on the matrix zone surrounding a fibre. The existence of a periodic fibre array was assumed and a unit cell chosen as representative of the average behaviour of the array. The stress/strain field at the interface was evaluated, including the forming process of the material in the analysis. Numerical results were compared with experimental data. Damage computation was performed using a continuum damage mechanics formulation, to evaluate the interaction between the matrix plastic damage and the interface damage phenomena in the unit cell.

On closed form solution for the elastic stress field around holes in orthotropic composite plates under in-plane stress conditions

Abstract The present paper presents analytical work performed care of the Aerospace Department of the University of Rome ‘La Sapienza’. Using the classical Airys function solution method the expression of the stress-strain field, in a composite material, has been determined for different simple hole geometries in a closed form. The results have been compared with several extensive finite element calculations that confirm the accuracy of the theoretical solutions found.

A computational procedure to calculate stress-strain field around simple shape holes in composite laminates

Abstract The problem of stress intensification around holes has been discussed and completely solved in the case of isotropic homogeneous materials by many authors. The problem becomes more complex in the case of orthotropic materials and composite laminates. This subject has been approached by numerical methods that are very time consuming, even in simple cases. A solution in closed form has been developed by the authors for simple hole geometries, such as circular, elliptical holes and sharp cracks. Here a brief summary of the theoretical approach is given. A solving procedure in the symbolical language used by Mathematica® is provided. This program allows one to determine the complete stress-strain field in any point of the laminate by just inferring the material properties. Some examples are discussed and results in terms of stress and strain trends are also presented.

Experimental verification and theoretical simulation of fracture behaviours of composite materials

Abstract The paper presents an experimental and analytical study of composite laminates subjected to cyclic loads. Experimental results indicate that transverse crack formation jeopardizes the load carrying capability of the laminates by decreasing the elastic moduli of the material even though the number of transverse cracks seems to saturate at each load amplitude with the number of cycles. Experimental results are compared with theoretical predictions. A microscopic model has been developed to predict the micro-behaviour of the composite through the formation and coalescence of voids related to the stress concentration in the neighbourhood of fibres and to the stress singularity due to an edge macro-crack. In addition, a 3D FE analysis has been developed in order to evaluate the elastic stress fields within a multi-layer body which provides the actual boundary stresses for micro-damage analyses of the considered microelement.

Theoretical forecasting and experimental validation of damage tolerance and accumulation in glass/epoxy laminates

Damage and damage accumulation mechanisms under cyclic loads were studied in orthotropic cross-ply composite laminates realized in S-glass fibers with epoxy resin. Five different basic mechanisms of damage were observed and evaluated making use of x-ray and dye-penetrant nondestructive techniques. They are: transverse matrix cracks, fiber failure, fiber debonding, intralaminar delamination and random disperse longitudinal cracks. The effects of these different damage mechanisms are revealed through the reduction of the mechanical properties of the composite material. A comprehensive experimental analysis of GFRP and CFRP, with two different lay-ups for each one, was made. Variations of the elastic moduli in two different directions and Poissons ratio were measured as a function of the number of cycles, for different stress levels, under alternate loads. The basic relations, developed by Talreja et al., which permit the prediction of the reduction of the elastic moduli for a given damage state, were directly related to the number of cycles N and to the stress levels employed in the fatigue tests. Tests to evaluate an eventual frequency effect on damage accumulation processes in composite material were made. Cumulative damage tests were also conducted to analyse material response under different and alternate load spectra. A damage index based on residual normalized strain energy ratio, which can describe a global damage state, is also proposed.

Evaluation of the built-in stresses and residual distortions on cured composites for space antenna reflectors applications

Abstract Manufacturing and thermal distortion RMS is an important dimensional parameter which can be correlated with the antenna performances. For this reason it is used to characterize their allowed dimensional stability in order to guarantee the mission requirements. The antenna reflectors on board of space platforms are generally manufactured with composite materials. The RMS of these structures is very tightly connected with the technologies like curing cycles, kind of materials, lay up tipology, mould configuration etc. The maximum effort is produced to minimize this parameter due to the manufacturing, while the prediction methods able to correlate the residual distortions versus the applied technology can be very useful to optimize the hardware performances.

Cleavage fracture prediction and KIc assessment of a nuclear pressure vessel carbon steel using local approach criteria

Abstract This paper presents a theoretical and experimental work carried out by the Aerospace Department of the University of Rome and ENEA-DISP on local approach. The main goal was to apply the local approach criteria to cleavage fracture prediction in a structural material. The material considered was a low alloy steel used in the nuclear industry, type 22NiMoCr37, of German production provided by KWU in the framework of an international round robin on local approach to fracture in steels, promoted by the European Structural Integrity Society (ESIS). Twelve tension tests on round notched bars were performed at low temperature (−90°C) in order to get the experimental data needed to calculate the theoretical Weibull stress and exponent and infer the probability of failure by a finite element analysis. Results have been applied to a three point bend specimen of the same material to predict cleavage fracture.

Cumulative Damage Characterization in Glass Fibre Reinforced Plastic by Acoustic Emission under Monotonic and Fatigue Loading

The paper presents a study of changes in mechanical properties of a composite material, glass/epoxy laminate [904, 0]s, and the application of the acoustic emission technique to detect the moment at which cracks start to appear in the material. Monotonic tensile load steps were applied and cracks counted to correlate the number of cracks and the Young’s modulus reduction to the load achieved during each step. Also a fatigue test was performed to check the cumulative damage introduced after different load amplitudes at a frequency if 1 Hz. Results were consistent with those obtained in a previous study by the authors on a similar material. Acoustic emission was used to monitor the formation of cracks in the laminate. The occurrence of cracking seems to wash out low amplitude peaks and, therefore, can be detected using an acoustic emission technique

Design and manufacturing criteria for high precision composite antenna reflectors. Prediction of the residual distortions after the manufacturing process

The application of composite materials to the design and manufacture of radar antenna reflectors is considered in this paper. The peculiar requirements of this kind of structure are described, particularly as regards the high precision of the reflecting surface. The general and specific characteristics of the composites are high-lighted, along with their advantages and disadvantages. The ‘tailorability’ of the material is indicated as the most impressive feature, involving a high impact on the design methods. Typical manufacturing technologies for the antenna reflector are briefly reviewed, in order to underline the aspects which affect the achievement of the required structure characteristics. The analysis/design stage is then taken into account, with a description of the calculation strategies and techniques. The necessity for a wide experimental support to the analytical tools is also shown, along with an overview of the typical activities and methodologies in this field. The subject of the surface accuracy is then exposed, highlighting the design requirements, the causes of the residual distortions after manufacture, and the ways to evaluate them at the design stage. A method, useful in the starting phases of the project, for the estimation of a global index of surface accuracy, is extensively reported.