PSI - Issue 38

Amaury Chabod et al. / Procedia Structural Integrity 38 (2022) 382–392 Amaury CHABOD / Structural Integrity Procedia 00 (2021) 000 – 000

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1. Introduction This paper is aimed at showing how to leverage information from strain gage measurements, and consequently to be able to perform fatigue analysis on full FE model, based on discrete measured strain time histories. Firstly, we will describe a way to define virtual strain gage analysis on FE model for a better test/CAE correlation. Secondly, we will focus on strain gage position and orientation optimization for a better applied loads representativity. Thirdly, we will present the load reconstruction process to get the load histories based on both strain gage measurements and independent unitary FE load cases. Finally, a fatigue damage analysis will be performed on a use case. It will show a manner to extrapolate stress level and life results from discrete points (strain gages) to a full component with previous load results, accessing life results in hot spots (large stress concentration areas). It will be done in the context of a dynamic transient analysis, where resonances are present, for which the material can be considered as linear elastic. 2. Virtual Strain Gages A virtual strain gage process makes the correlation between physical strain gages and FE model strain response at specific nodes easy. On one hand, the FE model describes the stress tensor response to unitary loads. On the other hand, full stress response is obtained thanks to a linear combination between known real-life loads and stress response from unitary loads. At specific FE nodes, where position and orientation match real life strain gages, the strain response is derived from Hooke's law. Using several unitary independent load cases with time steps input, the unitary strain response is obtained. It defines the strain matrix [A], which links p strain gages with q independent load cases. Introducing the load vector {L} applied for a given time step, the strain response output {S} can be directly obtained with this transfer function, using the principle of linear superposition (Munson and Mentley, 2015). In matrix form: { } = [ ]{ } (1) That process can be repeated for all the time steps of the known load history, as shown by equation (2) and described in Fig.1: { }( ) = [ ]{ }( ) (2) The component response is hence assessed from known inputs. Doing so allows to validate the FE model stress response, as regards material behavior and boundary conditions.

Fig. 1. Virtual Strain Gage Process

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