Guest Session

Time: 9:45 - 12:30

9:45 - 10:30

Optimization and stochastic analysis - towards robust design

C. Bucher (Vienna University of Technology)

The effect of stochastic uncertainties of structural parameters and/or environmental conditions frequently leads to imperfections which can significantly reduce the performance of a highly optimized design as obtained from traditional deterministic procedure. Robustness analysis takes this into account already during the optimization process thus leading to practically more useful optimized structural designs. The presentation will cover the basics concepts of combining optimization and stochastic analysis for the purpose of reducing sensitivities and increasing robustness. Several simple examples demonstrate the potentially achievable advantages as well as the inherent additional effort required.

10:30 - 11:00

A program view on POF modeling with uncertainties

R. Strunz (ASTRIUM GmbH - Space Transportation)

The program manager's preconception of probabilistic structural analyses is: "Do not need it because it is not only time consuming but also cost prohibitive!" It contradicts with the customer request of shortened development times and reduced development budgets. Historically, the design verification has been one of the drivers for cost overruns and schedule slippages which were mainly influenced by lengthy test-analyze-and-fix (TAAF) cycles. The breakup of these cycles or the reduction of a single cycle is paramount for future program successes.

The speech discusses how Physics-of-failure (POF) modeling in combination with Probabilistic Structure Analyses (PSA) can be used in a risk-informed satisficed decision making methodology that trades the three areas of concern cost, time, and demonstrated reliability. In that context, the POF modeling with uncertainties is used not only in an FMMEA but also in a Bayesian estimation based reliability-as-an-independent-variable (RAIV) strategy. Case studies highlight the impacts of test failures and use of knowledge transfer on the three areas of concern. By that means, the usefulness of POF with uncertainties is substantiated.

11:00 - 11:30

Continuum damage mechanics with ANSYS USERMAT: numerical implementation and application for life prediction of rocket combustors

W. Schwarz (ASTRIUM GmbH - Space Transportation)

A viscoplastic Chaboche material law is extended to account for material damage based on the effective stress concept. The damage evolution equations are formulated to be valid for failure under tensile loads as well as for low cycle fatigue. A second order tensorial damage state variable is employed in order to account for deformation induced anisotropy. The unified material-damage-model is discretized and numerically implemented in the finite element software ANSYS as a user defined material. It is applied for life prediction of the hot gas wall of a cryogenic rocket combustion chamber and the results are subsequently compared to data obtained during test campaigns. It is concluded that the continuum damage approach considerably improves the life prediction capabilities compared to classical, Coffin-Manson based, estimations.

11:30 - 12:00

Sequential LHS Design Strategy for Reliability Analysis and Robust Optimization

Eva Myšáková (Czech Technical University in Prague), Matěj Lepš (Czech Technical University in Prague)

Latin Hypercube Sampling Design Strategies (LHS) constitute an essential part of a simulation-based reliability analysis. Two main objectives are usually placed on the resulting designs-prescribed correlations and space-filling properties. The last decade has witnessed the development of several methods for both objectives.
In detail, our contribution presents a space-filling technique of sequential LHS generators. In comparison to standard procedures for generating uniform designs, sequential strategies offer the possibility of new samples inclusion into the existing design in case the initial set of designs is not sufficient. This is done by superimposing new set of designs. What is completely new of our methodology is the ability to solve non-constant boundaries that appear e.g. within the Asymptotic Sampling method. The idea is to repeatedly use as much already simulated samples as possible and concurrently minimize the number of additional samples needed to fulfil the LHS restrictions. The same technology can be used for Robust (Design) Optimization as will be shown in the final part of the contribution.

12:00 - 12:30

Analysis of the energy absorption of aluminium tubes for crash boxes

F.O. Riemelmoser (FH Kärnten, speaker), M. Kotnik (SZ Oprema Ravne), H. Lammer (Wood Competence Center W3C), V.K. Bheemineni (FH Kärnten), B. Käfer (FH Kärnten)

In the project UL4C the companies Fachhochschule Kärnten, SZ Oprema Ravne and Wood Competence Centre W3C develop a crash box for an electro car and solutions for their economic production are worked out. The crash box shall be designed as a hybrid material with aluminum EN AW 6060  and carbon fiber reinforced plastics (CFRP). The main challenge in this project is seen in a perfect adjustment of the aluminum and the CFRP part. There are several factors influencing the system behavior during the crash including:

  • the macro geometry such as height, length and thickness of the crash box the relative size of the geometry of the aluminum and the CFRP part
  • the heat treatment condition of the aluminum 6060
  • the orientation of the fibers in a multilayer system

In order to find the best crash box parameters to optimize the energy absorption per kg mass we conducted several tests on our crash sled. The results on aluminum tubes show that there are different failure mechanisms from Euler buckling to non-axisymmetric folding to axi-symmetric folding and non specific folding conditions. It is possible to plot diagrams with normalized geometrical parameters on the both axes such that regimes with different folding mechanism can be identified. It will be shown that these diagrams (we call them failure mechanism maps) depend on the geometry and to a lesser extent on the heat treatment conditions. In the presentation the failure mechanism map is explained and Finite Element simulations are shown in order to verify our theory. Also an outlook is given how the CFRP hybrid will influence the energy absorption capability.