Claudio Scarponi

Impact Testing on Composites Laminates and Sandwich Panels

The purpose of this work was the evaluation of the behavior of fiber-reinforced composites and sandwich panels for aeronautical applications under impact. Experimental tests were performed on several specimen configurations, based on different quasi-isotropic lay-up and materials such CFRP, CFRTP and a syntactic foam as the core of sandwich panels. Considering the high specific mechanics characteristics of such a foam and its cocurability with the facesheet material, symmetric and non-symmetric sandwich configurations have been tested. The study has pointed out the advantages for aeronautic constructions of different materials and the influence of the core position along the panel thickness. The impact tests were performed with a falling weight machine, which allowed the most important dynamic and kinetic parameters, such as the contact force, impactor velocity and displacement and perforation energy to be measured. Delamination areas of each specimen were also measured using N.D.I. reflection ultrasonic techniques. In order to determine the residual strength, Compression After Impact tests were performed on specimens that have an indentation of approximately 0.3-0.4 mm. A numerical simulation was performed using a transient dynamic Finite Element Analysis. The main goal of this analysis was to define the controlling factors for high velocity impact simulation, such as the increase of geometric non-linearity and the delamination effects. Moreover, the delamination areas and positions were determined taking in account the different boundary conditions. The study have pointed out the behavior of thermoset, thermoplastic and unsymmetric sandwich panels.

Ultrasonic Detection of Delaminations on Composite Materials

An accurate N.D.I. methodology can give a good knowledge of the internal damage status. The main purpose of the present paper is to describe the testing methodology and to discuss the testing procedures that have been performed in order to evaluate the position and the dimensions of the delaminations that occur in different composite materials under various load conditions. For the delaminations detection inside CFRP laminates and sandwich panels subjected to impact and fatigue loads, an ultrasonic apparatus, in form of reflection, has been utilized. This apparatus, suitable for in situ inspections, has been successfully utilized in our laboratory. The analysis of the experimental results shows the capability of the system to be adopted for any application where the presence of delaminations is expected.

The Importance of the Temperature and Lateral Pressure on the CFRP Joints Strength under Uniaxial Loading

The purpose of the present paper is to provide the assessment and evaluation results of the temperature and lateral pressure influence on the CFRP pin loaded joints strength while they are subjected to tension. The subject study is composed of the following two main parts: 1. the Experimental Results, which are obtained through the application of four (4) different lateral pressure conditions (none, low, medium, and high), and range of temperatures (room, + 60°C, + 80°C, -55°C, -100°C, and -150°C) 2. the Theoretical Results, which are obtained through the use of 2D finite element analysis In addition, the study provides an in depth description of the employed test methodology and a comparison of the numerical and experimental final results.

Impact behavior of natural fiber composite laminates

Abstract: The behavior of natural fiber-reinforced composites under low-velocity impact is described, with particular emphasis on the phenomenon of impact, the sequence of damage, and the testing methods and instruments utilized. The ultrasonic nondestructive inspection (NDI) technique for the delamination evaluation of composite materials is illustrated. For each of these, both the ‘ classic’ composites and the natural fiber composites are discussed. There are many images and tables, which show the behaviors and mechanical characteristics of the particular composite materials considered.

The influence of interlaminar stresses on fatigue behaviour of notched composite laminates

The influence of the interlaminar stresses on the fatigue behaviour of composite laminates is studied. The present paper is comprised of experimental results and comparison with theoretical results on the delamination onset and propagation. Numerical results are obtained by a specialized computer code which has shown the points where the high intensity of the interlaminar stress tensor components can produce a delamination. Several tension-tension fatigue tests have been performed on angle-ply and cross-ply centrally holed composite specimens which present the major sensitivity to the interlaminar stresses in order to verify the onset and the propagation of delaminations in the same points revealed by the static analysis. For the identification of the presence of delaminations an ultrasonic NDI equipment in the form of reflection has been utilized; this system provides a 3D positioning of the delaminations inside the laminates. The experimental results are in good agreement with the theoretical previsions for both the stacking sequences.

Delaminations Onset and Propagation for CFRP Angle Ply Laminates under Uniaxial Fatigue Loads

Delamination is a failure mode particular to composite materials and it is introduced by the interlaminar components of the stress tensor. Under static uniaxial loading, a 3D stress state occurs at the free edges of a composite structural member whereas the values of the interlaminar stress components can become critical and determine the static failure of such member. The purpose of this paper is to assess and determine the influence of the interlaminar stresses on the failure of unnotched angle ply CFRP laminate specimens subjected to tension-tension uniaxial fatigue loading. This study is composed of two parts, namely: (1) a numerical analysis to determine the 3D stress state in the composite laminate under static loading with the purpose to identify the points where the interlaminar stresses are critical and delaminations are expected; (2) an experimental analysis, based on tension static and fatigue T-T tests, to evaluate if delaminations occur at the same points as those identified during the numerical analysis and if their propagation produces the final fatigue failure at an external maximum load equal to 50% of the ultimate static failure load.