PSI - Issue 12

Lorenzo Bergonzi et al. / Procedia Structural Integrity 12 (2018) 392–403 Lorenzo Bergonzi / Structural Integrity Procedia 00 (2018) 000 – 000

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The purpose of this work is to identify an alternative specimen geometry for tensile tests, with particular attention to those carried out on plastic materials, and of the relative fixtures allowing for self-alignment of the specimen with load direction during the test. Specimen and fixtures geometry have been finely tuned via FEM (Finite Element Method) and subsequently they have been validated through an experimental campaign with different materials and crosshead speeds on a universal testing machine.

Nomenclature F m Ultimate load force R m Ultimate tensile strength E Elastic Modulus A s % Elongation at yielding s M Crosshead displacement

2. Specimen shape determination Different shapes of the specimen shoulders are taken into consideration, identifying for each advantages and disadvantages. At the end of process of concepts generation and selection, it was found that a “drop” shape is the one that allows to better implement aspects such as self-alignment of the specimen, ease of fabrication and ease of test set up. The first concept, as can be seen in Figure 2, has the same reduced section of the Type I specimen described in ASTM D638 - Standard Test Method for Tensile Properties of Plastics ASTM International, (1999), that is 57 mm long and 13 mm wide. This geometry was chosen as a reference because the machine is initially designed to operate mainly with plastic materials obtained by tooling as well as injection molding or AM. Round shoulders transfer the load from the fixture to the specimen by means of shape constraint with fixtures seat, allowing self-alignment. In the center of the shoulders, a hole is introduced to mount a lid on the fixture with the purpose of positioning the specimen inside the fixture seating but without transmitting loads. A nominal clearance of 0.3 mm between specimen and fixture seat is provided to facilitate insertion and extraction of the specimen.

Figure 2. First specimen shape concept, showing reduced section dimensions and gage length.

Specimen round heads have a radius of 15 mm to maintain compactness and the thickness is set to be 3 mm. The radius of the transition region between shoulders and the reduced section is identified iteratively by numerical analysis, in order to allow a stress distribution within the gage length as close as possible to that of standard specimen. Starting from a FEM model of standard ASTM D638 Type I specimen, the stress state was determined at the root of the transition between shoulder and the reduced section and at the middle of the gage length. Subsequently, specimens with equivalent geometry and different transition radii between shoulders and rectilinear section were analyzed. In this phase of the work, the main purpose is a direct comparison between the two geometries (standard and non-standard): displacements and imposed material do not necessarily reflect the test conditions, but are identical between the two models. In this way it is possible to identify the trend of the stress distribution as a function of the variation of the connecting radius only, maintaining the remaining boundary conditions, thus separating the study of the equivalent connecting radius from that of the fixturing system, treated in Sect. 3.