C. Braccesi et al. 2010. Int. J. Vehicle Structures & Systems, 2(3-4), 127-138 International Journal of Vehicle Structures & Systems Available online at www.ijvss.maftree.org ISSN: 0975-3060 (Print), 0975-3540 (Online) doi: 10.4273/ijvss.2.3-4.06 © 2010. MechAero Foundation for Technical Research & Education Excellence 127 Synthesis of Equivalent Load Conditions for Military Vehicles Claudio Braccesi a , Filippo Cianetti a,b and Luca Silvioni a a Dipartimento di Ingegneria Industriale, Università degli Studi di Perugia, Via Duranti 1 – 06125, Perugia, Italy. b Corresponding Author, Email: cianfi@unipg.it ABSTRACT: The aim of this paper was the identification of the load conditions to consider during the design and assessment of military vehicles. This meant identifying test tracks and their corresponding ground speeds and track lengths, and to define their combinations that are useful to simulate the load spectrum representing the system/component life in an accelerated way. The result of this activity was the development of a formulation for the synthesis of a single power spectrum density function (PSD) representing multiple load conditions expressed by a set of PSD functions. This function is equivalent in terms of damage to the given PSD functions set. Another important result was the definition of a virtual test ring, equivalent to those defined in standards for equipment and supplies, but useful for the virtual check of the military vehicle integrity. This new formulation and the proposed test ring intend to supply tools that would be very useful in the early phases of vehicle design and assessment. In order to verify these results, numerical and experimental analyses of a generic military vehicle were undertaken. Results demonstrated the validity of the load synthesis formulation and the load combination that are useful for the assessment of military vehicle durability. KEYWORDS: Fatigue, Accelerated durability tests, Military vehicles, Multibody simulation CITATION: C. Braccesi, F. Cianetti, and L. Silvioni. 2010. Synthesis of equivalent load conditions for military vehicles, Int. J. Vehicle Structures & Systems, 2(3-4), 127-138. ACRONYMS AND NOMENCLATURE: PSD Power Spectral Density. FEM Finite Element Model. a Intercept of the fatigue strength curve with the axis of ordinates. b Slope of the fatigue strength curve. f Frequency value. n Number of applied cycles. σ a Amplitude of the applied stress cycle. D Fatigue damage. EN Number of elements of the FEM. G Acceleration PSD function. G σ Stress PSD function. M Number of tracks/grounds. M 2 i, j Second order spectral moment of the PSD function. M 4 i, j Fourth order spectral moment of the PSD function. N Number of cycles which defines the fatigue strength of the material/component. N* Mean value of fatigue cycles number on all the elements of the FEM. P Number of output acceleration measures for each of the M tracks/grounds and for each of R vehicles. R Number of tested vehicles. S PSD matrix of the stress tensor. S f Amplitude of the stress cycle which defines the fatigue strength of the material/component. T Test or travel duration. 1. Introduction There are few standards available to regulate the assessment of military vehicles, equipment and supplies (materiel). Amongst the existing standards, MIL-STD- 810F is the most representative [1] to provide the guidelines for laboratory (“General Vibration”) or field tests (“Large assembly transportation”) to evaluate military vehicle durability by considering the life cycle environment profile. However, there are no standards exist to indicate loads, load conditions and load spectra to be considered in the design and assessment of military vehicles. Therefore, the designer of the military vehicle is always faced with choosing a method - “numerical or experimental assessment” to verify the design choice. Clearly, the numerical assessment is economically advantageous and can be easily integrated into the design process ([2] - [7]). The experimental assessment, besides being more expensive, is more time consuming and heavily affects the time required to develop the design. Nevertheless, it is necessary to conduct an experimental test of the materiel at the end of the design iter. During the experimental assessment phase, a methodological choice to test either “in the laboratory or in the field must be made”. However, such tests do not resolve the doubts of the vehicle manufacturer regarding the reliability of the full vehicle and therefore its hypothetical life. This un-certainity is higher if the experimental testing has been done in a laboratory where the object being tested is exclusively the materiel.