G. Pappalettera

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.

Analysis of the effects of strain measurement errors on residual stresses measured by incremental hole-drilling method

Among the many available methods for measuring residual stresses, the hole-drilling method is surely one of the most popular methods. It is a semidestructive method consisting in drilling a very small hole into the specimen. This produces residual stress relaxation around the hole that causes strains also to change. Strain on the surface can be measured by strain gages or by optical techniques like electronic speckle pattern interferometry. Holes can be drilled at different depths, so that a stress profile can be determined using the stress–strain relationships. However, due to the mathematics of the problem, small measurement errors on the strain can imply big errors in the calculated stress. Accuracy of the final results is strongly dependent on the measurement procedure and on the algorithms adopted to calculate stresses. In this study, the influence of strain measurement errors is evaluated with respect to different measurement procedures. Two kinds of algorithms, namely, integral and power series, are compared also with respect to the use of two different commercial systems both implementing the cited methods. Moreover, the effects of a proper choice of depth increments and also of the number of steps are investigated. Effects of implementation of regularization algorithm are discussed as well as the influence of the drilling depth. Finally, the ability of the analyzed methods in discriminating two different strain signal sources is investigated.

Preliminary Study on Residual Stress in FDM Parts

The Fused Deposition Modelling (FDM) is nowadays one of the most widespread techniques for 3D object rapid prototyping. In recent years, the FDM evolved from rapid prototyping technique towards a rapid manufacturing method, changing the main purpose in producing finished components ready for use. However, as the parts are built as a layer-by-layer deposition of a feedstock wire, the FDM technique shows, during the building process, distortion and de-layering problems. This issue influences the shape and the final dimensions of the parts or it can prevent the finalization of the objects due to unsticking problems from the bed. Several techniques can be employed in order to obtain parts of correct shape and dimensions. Many of these, such as depositing glue on the bed, aim to constrain the object. As a consequence, the FDM parts could show residual strain and residual stress that could influence their mechanical behaviour. The aim of this work is to measure, by ESPI technique, the displacements around a hole drilled into the material. This can be considered as a preliminary indication of the level of residual stress inside the FDM parts.

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.

Design of a Double-Illumination ESPI System for the Measurement of Very Slow Motions

Necessity appears in some contexts to measure velocity of objects moving at very low speed. In this work a double illumination speckle interferometer was used in order to determine the velocity of a plate moved by a PZT stage at velocity from 7 to 21 nm/s. When the time history for each pixel intensity is recorded oscillations in the signal appear due to phase variation connected to the movement of the object. By measuring the frequency of the oscillations of the autocorrelation function of the signal intensity and by knowing the geometrical and physical parameters of the set-up it is possible to calculate the velocity of the objects. Results show that the set up adopted in this work is able to measure very slow motion down to 7 nm/s with small percentage error and rather quickly.