PSI - Issue 39

Riccardo Caivano et al. / Procedia Structural Integrity 39 (2022) 81–88 Author name / Structural Integrity Procedia 00 (2019) 000–000 3 the defect population can be estimated a-priori using a statistical distribution model. Murakami demonstrated in [32] that the defect population follows the Largest Extreme Value Distribution (LEVD). Therefore, the probability √ to find a defect with size √ is expressed in Eq.(2): √ �√ � = − −� √ − √ √ � (2) where √ and √ are the location and the scale parameters of the supposed defect distribution which can be estimated experimentally within a defined test value, named reference volume 0 . Reversely, it is possible to use Eq.(2) to calculate the defect size √ given a certain probability. Combining Eq.(1) and Eq.(2), it is possible to estimate a priori , the fatigue limit in presence of defects as expressed in Eq.(3). ̄ = 1 ∙ ( + 120) � √ + √ ∙ �− ( − ( )) + � 0 ��� 1 6 ∙ � 1 − 2 � 0 . 226+ ∙10 −4 (3) The term is the component volume which is needed to properly scale up the inspected volume with respect to 0 . Anyway, Hypermesh provides the possibility to limit only the maximum first principal stress ̄ , in the TO problem setup, and not the alternate one ̄ as presented in TopFat [33]. Therefore, to adapt the TopFat procedure to the Hypermesh software, Eq.(4) is employed to evaluate the limit in place of Eq.(3) [34]. ̄ , = 2 ̄ 1 − (4) 3. Results In this section, the TopFat procedure is extended to Hypermesh commercial software and a real application case from the aerospace industry is shown. Fig.1 reports the Leonardo Spa company's original component together with the related boundary conditions. It consists of a bracket whose purpose is to connect the hatboxes to the structural beams inside the aircraft fuselage. The original geometry is achieved employing traditional methodologies, i.e. milled from a semifinished product, in aluminium T7050. The bracket is linked to the fuselage with 12 rivets through the tiny holes reported in Fig. 1, named accordingly fixed holes . The hatboxes are connected to the bracket using two pins that fit in the two bushings, named 9gLug and Clevis in Fig. 1. During the flight manoeuvres, the aircraft undergoes several accelerations and, therefore, the weight of the hatboxes burden the components, applying consistent loads. To evaluate the most critical loads and the related bracket quasi-static safety, the highest accelerations that the aircraft can bear are considered. For more, the hatboxes are considered fulfilled with passenger goods, to simulate the worst condition. This analysis is carried out internally to Leonardo Spa company considering the full aircraft model and it is here omitted for brevity reasons. Under these severe conditions, it is possible to evaluate the pin loads transferred to the bracket with respect to the reference system called u-v-w reported in Fig. 1. In the present paper, the bracket only is modelled, and the rivets connections are simulated locking all the degrees of freedom of the internal hole surfaces whereas the pins are substituted by rigid elements (RBE2 in Hypermesh) to transfer the single point loads presented in Fig. 1 to the internal surfaces of the bushings. The main purpose of the bracket re-design is to reduce the component weight since it is a crucial prerequisite for aerospace parts, affecting fuel consumption remarkably. Leonardo Spa fixed a desirable target mass reduction of about 2% with respect to the original bracket in Fig. 1 while guaranteeing the structural safety of the structure. Furthermore, Leonardo Spa addressed as desirable technology for the new bracket production the Electron Beam Melting (EBM) additive process by Arcam company, with Ti6Al4V powder. The re design is therefore carried out using the TopFat procedure within the Hypermesh environment considering these Leonardo Spa prescriptions and guidelines. 83