C. Barile

Mechanical Characterization of SLM Specimens with Speckle Interferometry and Numerical Optimization

This paper describes the process of mechanical characterization of specimens built via Selective Laser Melting (SLM). An hybrid approach based on the combination of phase-shifting electronic speckle pattern interferometry (PS-ESPI) and finite element analysis is utilized. Three-point-bending experimental tests are carried out. The difference between displacement values measured with ESPI and their counterpart predicted by FEM analysis is minimized in order to find the values of the unknown elastic constants.

Drilling Speed Effects on Accuracy of HD Residual Stress Measurements

The effect, on the residual stress measurement accuracy, of the drilling speed of the end-mill during the hole-drilling measurements was evaluated in Ti6Al4V. In spite of the well-known consideration that the highest achievable speed should be used during hole drilling, very few experimental works exist analyzing the effects of using lower velocities. Hole-drilling experiments were performed in this study by measuring the released strain by electronic speckle pattern interferometry. A known stress state was generated by loading the sample in a four point bending frame up to 50 % of the yield strength. Drilling speed ranging in 5,000 ÷ 50,000 rpm was investigated by using an electronically controlled mill. The expected stress field, evaluated by a numerical model in ANSYS®, was compared with the measured one at different drilling speeds.

Hybrid Characterization of Laminated Wood with ESPI and Optimization Methods

In the last decades wood has assumed an increasing relevance in building engineering both due to new technologies in wood production and manufacturing, such as computer-aided optimization of log sawing, automated lumber grading etc., and to the development of the green building. Fibrous structure of the wood makes it an orthotropic material so that it becomes important to perform a measurement, which allows to fully characterize the orthotropic properties of the specimen. Hybrid numerical-experimental techniques can be used in order to accurately determine the Young modulus and the Poisson coefficient of the material.

Hybrid thermography and acoustic emission testing of fatigue crack propagation in Aluminum Samples

In this paper the crack propagation process was monitored by two different experimental techniques. Acoustic emissions sensors were placed on the sample in order to monitor the evolution of the acoustical events during the test; at the same time the change in temperature was monitored by thermography. Test were run on aluminum samples (Al 5068). Specimens were previously cracked, by cutting notches having known sizes and geometry. Successively, X-Ray diffractometry analysis were performed in order to establish the given initial stress state of each sample. Specimen were then subjected to mechanical tests. During these tests the crack propagation was continuously monitored and recorded by both techniques. Data obtained, in terms of number of hits, amplitude signals and maps’ temperature, were critically compared in order to assess the capability of each technique in following the evolution of the damage process.

Remarks on Residual Stress Measurement by Hole-Drilling and Electronic Speckle Pattern Interferometry

Hole drilling is the most widespread method for measuring residual stress. It is based on the principle that drilling a hole in the material causes a local stress relaxation; the initial residual stress can be calculated by measuring strain in correspondence with each drill depth. Recently optical techniques were introduced to measure strain; in this case, the accuracy of the final results depends, among other factors, on the proper choice of the area of analysis. Deformations are in fact analyzed within an annulus determined by two parameters: the internal and the external radius. In this paper, the influence of the choice of the area of analysis was analysed. A known stress field was introduced on a Ti grade 5 sample and then the stress was measured in correspondence with different values of the internal and the external radius of analysis; results were finally compared with the expected theoretical value.

Acoustic Emission Analysis of Aluminum Specimen Subjected to Laser Annealing

This paper deals with the study of acoustic emissions (AE) from aluminum Al5068 specimen during local annealing process by a laser diode source. The heating cycle, obtained by irradiating the surface of the specimen, causes a change in the local stress state which can be studied by X-Ray diffractometry; at the same time acoustic emissions can be recorded as a consequence of residual stress relieving.

Considerations on the choice of experimental parameters in residual stress measurements by hole-drilling and ESPI

Residual stresses occur in many manufactured structures and components. Great number of investigations have been carried out to study this phenomenon. Over the years, different techniques have been developed to measure residual stresses; nowadays the combination of Hole Drilling method (HD) with Electronic Speckle Pattern Interferometry (ESPI) has encountered great interest. The use of a high sensitivity optical technique instead of the strain gage rosette has the advantage to provide full field information without any contact with the sample by consequently reducing the cost and the time required for the measurement. The accuracy of the measurement, however, is influenced by the proper choice of several parameters: geometrical, analysis and experimental. In this paper, in particular, the effects of some of those parameters are investigated: misknowledgment in illumination and detection angles, the influence of the relative angle between the sensitivity vector of the system and the principal stress directions, the extension of the area of analysis and the adopted drilling rotation speed. In conclusion indications are provided to the scope of optimizing the measurement process together with the identification of the major sources of errors that can arise during the measuring and the analysis stages.

Hybridization Effect of Sisal/Glass/Epoxy/Filler Based Woven Fabric Reinforced Composites

Development of the Polymer based Composites from both natural and synthetic fibers is a sustainable alternative material for some engineering fields like automotive and aerospace. This work is aimed to incorporate the sisal and E-glass fabrics with the epoxy matrix and by adding silicon carbide filler to the sisal fabrics. Five different composite laminates were prepared by hand layup combined with vacuum bagging method as per laminate sequences. The physical and mechanical properties of composite laminates were evaluated according to ASTM. Results show that incorporation of E-glass and silicon carbide filler can reduce the voids and enhance the physical properties. As the amount of E-glass fibers slightly grows, tensile properties of composites grow. Effect of filler can enhance the flexural properties. Failure of composites mainly occurs due to the poor interfacial bonding between fabrics and matrix, fabrics pull out and fracture occurs in fabrics or matrix when load is applied.

Overview of the Effects of Process Parameters on the Accuracy in Residual Stress Measurements by Using HD and ESPI

Several researches have been recently carried out about the feasibility of adopting optical methods, such as Electronic Speckle Pattern Interferometry (ESPI), in combination with hole-drilling. Using this kind of approach instead of adopting strain gage rosette, as usual, can be advantageous in view of the fact that costs and time of the measurement can be reduced. However it should be taken into account that several parameters involved in the whole measurement process can significantly determine the final accuracy of the measurement. In this paper an overview of the effects of some of those parameters is presented: errors on the knowledge of some of geometrical paramters, direction of the sensitivity vector of the system with respect to the principal stress direction, drilling rotation speed.

Investigation of kinetic triplets for thermal degradation of thermally cured vinyl ester resin systems and lifetime predictions

The thermal degradation of thermally cured vinyl ester resin systems is studied for different heating rates. The kinetic triplets, the activation energy, pre-exponential factor and the reaction model f(α) for the different reaction extent of conversions (α) are estimated using advanced isoconversional methods. Although the thermal degradation curves show the degradation occurs as a single stage, the kinetic parameters suggest the otherwise. The activation energy remains constant for α = 0.3–0.575 but varies during the initial and final stages of conversion. Similarly, the pre-exponential factor shows considerable variation between the lower and higher reaction extent (α) values. This shows the complexity in the reaction. The probable reaction mechanism that the degradation follows has been explained. The complexity of the thermal degradation and the changes in reaction model f(α) over different reaction extent has been related. The appropriate working temperature for different thermal lifetime of the cured vinyl ester resin system for the failure of conversion α = 0.2 has been predicted under the nitrogen atmosphere.