Radial in-plane digital speckle pattern interferometer combined with instrumented indentation Matias R. Viotti n , Armando Albertazzi Jr., Danilo Bonomo, Filipe Fontana Laboratório de Metrologia e Automatização, Universidade Federal de Santa Catarina, CEP 88040-970, Florianópolis, SC, Brazil article info Article history: Received 11 September 2014 Received in revised form 14 January 2015 Accepted 2 March 2015 Keywords: DSPI Radial in-plane sensitivity Instrumented ball indentation FEM abstract This paper presents a modular device based on digital speckle pattern interferometry (DSPI) which is combined with instrumented indentation. The interferometric module uses a diffractive optical element that confers radial in-plane sensitivity enabling the measurement of whole displacement field generated by the shallow indentation print on the surface of the material under testing. The indentation module uses a piezoelectric loading cell and an inductive transducer to simultaneously measure the loading applied on the ball indenter tip as well as its penetration on the material under testing. A mechanical/ hydraulic scheme was developed to achieve a high loading capability with a compact indentation module, suitably sized with the interferometric module. A finite element simulation was carried out for a generic low carbon steel material without residual stresses and under a tensile external loading of 25%, 50% and 75% of its yielding stress. In the same way, a steel bar was experimentally indented by using the compact indenter module and the radial in-plane displacements around the indentation were measured with the measurement module. Good agreement was found between the simulated and measured displacement fields. In addition, the influence of the tensile load on the measured displacement fields was clearly observed by the measurement module. & 2015 Elsevier Ltd. All rights reserved. 1. Introduction Frequently, the word indentation is related to hardness testing used for the characterization and quality control of materials [1]. For all traditional tests, a static force is applied on a specified tip shape and tip material generating a shallow impression on the surface of the material under testing. This impression evaluates the material resis- tance to plastic deformation. There are several hardness tests which can mainly be grouped in two categories: (a) macroindentation tests and (b) microindentation or nanoindentation tests. The former one is usually only related to hardness measurements. On the other hand, the application field of indentation testing has been broadening by the latter category throughout the development of instrumented indentation. Instrumented indentation has the capability to apply a speci- fied force or displacement history [1] and to monitor them in order to obtain the loading cycle and build the force versus depth plot. Instrumented indentation systems have been used to study, for instance, dislocation behavior in metals [2–4], fracture beha- vior in ceramics [5,6], time dependent behavior of soft materials [7–9] and polymers [10–14], and mechanical behavior of thin films [15,16]. Furthermore, some applications have used instrumented indentation for residual stress measurements [17,18]. For all previous cases, testing machines are relatively bulky to be joined to a complementary device such as a speckle inter- ferometer to measure displacements around the indentation print. Moreover, they are usually not portable and the test specimen must be carried out to the testing machine facility. In order to overcome these practical cons, Refs. [19,20] presented a modular and portable device composed by: (a) a digital speckle pattern interferometer with radial in-plane sensitivity, (b) an indenter module with impact force application and (c) a universal base. Thus, the portable device was positioned and rigidly clamped on the surface of the specimen under investigation by means of the universal base. The measurement and indenter modules allow the introduction of the indentation and the measurement of the displacement field around the indentation print. Authors used this displacement field to evaluate the related residual stress field present in the region covered by the indentation depth. The device presented in Refs. [19,20] showed some advantages and disadvantages. As pros, it can be cited that the measurement is performed without the evaluation of the projected area of the indentation print. Thus, problems generated by a clear interpreta- tion of indentation results associated with sink-in and pile-up are Contents lists available at ScienceDirect journal homepage: www.elsevier.com/locate/optlaseng Optics and Lasers in Engineering http://dx.doi.org/10.1016/j.optlaseng.2015.03.002 0143-8166/& 2015 Elsevier Ltd. All rights reserved. n Corresponding author. Tel.: þ55 48 32392037; fax: þ55 48 32392039. E-mail address: matiasviotti@gmail.com (M.R. Viotti). Please cite this article as: Viotti MR, et al. Radial in-plane digital speckle pattern interferometer combined with instrumented indentation. Opt Laser Eng (2015), http://dx.doi.org/10.1016/j.optlaseng.2015.03.002i Optics and Lasers in Engineering ∎ (∎∎∎∎) ∎∎∎–∎∎∎