Rade Vignjevic

Cost effective honeycomb and multi-layer insulation debris shields for unmanned spacecraft

Ways to improve the tolerance of unmanned spacecraft to hypervelocity impact are presented. Two new honeycomb and multi-layer insulation (MLI) shields were defined: (1) double honeycomb, and (2) enhanced or toughened MLI (with additional Kevlar 310 and/or Betacloth layers). Following hypervelocity impact testing, a new ballistic limit threshold was defined, based on rear facesheet perforation and witness plate damage characteristics. At 12 km/s, the ballistic limit of single honeycomb was 0.58 mm (aluminium sphere), rising to 0.91 mm for double honeycomb, 1.00 mm for double honeycomb with MLI and 1.17 mm for double honeycomb with toughened MLI. A damage equation, based on the modified Cour-Palais equation with ESA constants, was compared with the data and found to be conservative. The impact angle exponent was increased in order to reduce the equation under-prediction for the oblique incidence data. An equivalent rear wall thickness was defined in order to distinguish between shield types above 7 km/s. The spacecraft survivability analysis showed that the double honeycomb and toughened MLI significantly reduced the number of perforating particles over the baseline single honeycomb design. The mass increase of these shields is approximately 1.2 kg/m2 for double honeycomb and 0.8 kg/m2 for toughened MLI.

A penalty approach for contact in smoothed particle hydrodynamics

Summary This paper describes the development and testing of a contact algorithm for SPH. The treatment of contact boundary conditions in SPH has not been adequately addressed, and the development of the normalised smoothing function approach has highlighted the need for correct treatment of boundary conditions. A particle to particle contact algorithm was developed for 2D. The penalty formulation was used to enforce the contact condition, and several equations for the penalty force calculation were considered. The contact algorithm was tested for one and two dimensional problems for the velocity range between 0.2 and 4.0 km/s to determine the best penalty force equation and the best approach for applying the contact force. The tests showed that the zero-energy mode problem in the SPH method had to be addressed, as contact excited a zero-energy mode that caused non-physical motion of particles. The test results were compared to the DYNA3D results for the same problems. Background to boundary condition treatment in SPH is given in Section 1. In Section 2 the development of a contact algorithm for SPH in 1 and 2D is described. Testing of the contact algorithm in 1 and 2D is described in Section 3. Conclusions are presented in Section 4.

Development of lagrangian hydrocode modelling for debris impact damage prediction

Abstract Work on improving the Lawrence Livermore DYNA3D code to allow it to accurately model hypervelocity impact is presented. The unmodified version of DYNA3D was tested, results showed that improvements were required in three main areas: material modelling, the element erosion criterion, the ability to model debris cloud formed by a thin plate impact. The SESAME Equation of State has been implemented to improve the material modelling. To improve the element erosion criterion two different criterion have been implemented and tested. The first is based on total element deformation and shows improved results for semi-infinite and thin plate targets. The second is intended to delete an element when it becomes numerically inaccurate. Initial results show the limitation of the Lagrangian finite element approach and that further improvement is required.

Finite Element Modelling of Failure of a Multi-Material Target due to High Velocity Space Debris Impacts

Lagrangian finite element methods have been used extensively in the past to study the non-linear transient behaviour of materials, ranging from crash tests of cars to simulating bird strikes on planes. However, as this type of space discretisation does not allow for motion of the material through the mesh when modelling extremely large deformations, the mesh becomes highly distorted. This paper describes some limitations and applicability of this type of analysis for high velocity impacts. A method for dealing with this problem by the erosion of elements is proposed, where the main driver is the definition of element failure strains. Results were compared with empirical perforation results and were found to be in good agreement. The results were then used to simulate high velocity impacts upon a multi-layered aluminium target in order to predict a ballistic limit curve. LS-DYNA3D was used as the FE solver for all simulations. Meshes were generated using Truegrid.

Experimental observations of an 8 m/s drop test of a metallic helicopter underfloor structure onto a hard surface: Part 1

Abstract This is the first part of a two-part paper that describes the experimental observations for two similar sections of floor that were dropped onto both hard and water surfaces at 8 m/s, as a part of one experimental campaign. The current paper provides an assessment of a simple box-beam underfloor structure typically found in metallic helicopters and provides an overview of the failure modes and the collapse mechanism observed when dropped onto a hard surface. All findings are supported by quantitative measurements and extensive photographic evidence. The current paper identifies two limitations with the existing design, which are based upon the observations of the failure modes for different frame types and the performance of the intersection joints. In order to increase the level of crashworthiness currently offered, significant frame and joint redesign is required in order to provide a more progressive collapse. The simple buckling modes currently observed should be avoided, as the existing stroke is not fully utilized in the event of a crash, resulting in an inefficient structure. The current paper also discusses the sensitivity to impact angle, as slight variations from a normal impact may result in a detrimental response.

LOCALIZATION AND DAMAGE INDUCED SOFTENING USING FINITE ELEMENT AND SMOOTH PARTICLE HYDRODYNAMIC METHODS

The main aim of this work is investigation of localization problem in strain softening materials and regularization techniques, which will reduce and possibly remove mesh dependency of the numerical results and balance the effects of heterogeneous microstructure on local continua while keeping the boundary value problem of softening (damaged) continua well-posed. Finite Element Method (FEM) and Smooth Particle Hydrodynamic (SPH) combined with a local continuum damage model (CDM) were used for analysis of a dynamic stress wave propagation problem, which was analytically solved in (Bažant and Belytschko 1985). The analytical solution was compared to the numerical results, obtained by using a stable, Total-Lagrange form of SPH (Vignjevic et al. 2006, Vignjevic et al. 2009), and two material models implemented in the FEM based on: 1) classic CDM; and 2) equivalent damage force. The numerical results demonstrate that the size of the damaged zone is controlled by element size in classic FEM and the smoothing length in the SPH, which suggests that the SPH method is inherently non-local method and that the smoothing length should be linked to the material characteristic length scale in solid mechanics simulations.