PRESENTER: Valentina Dolci

INTRODUCTION

Spacecraft are highly complex systems composed of various structural, hydraulic, propulsion, electronic and avionic elements. Such complex systems require extensive maintenance.

A widely recognized definition of Structural Health Monitoring states that Health Monitoring (as subsystem of the network of technologies and

processes known as Integrated Vehicle Health Management) is the process of nondestructively identifying the main characteristics related to the

fitness of space component (or system) as it operates.

The test will be simulated with the help of the Altair FE solver RADIOSS. Afterward there will be a post-processing

phase during which we will analyse displacements and contact forces.

We choose an analytical test from Abrate’s work, ‘Impact on composite structures’: Es. 3.22.

It deals with slow speed impacts causing BVID - Barely Visible Impact Damage.

PHASE ONE: ANALYTICAL COMPARISON

The FEA solver RADIOSS will be a really satisfying tool. The error in comparison with the analytical solution will be less than 7.5 %.

ES. 3.22-IMPACT ON RECTANGULAR COMPOSITE PLATE

;

E1 120 GPa

E2 7.9 GPa

ν12 0.3

G12 5.5 GPa

G23 5.5 GPa

G13 5.5 GPa

ρ 1580 Kg/m3

Steel sphere Diameter 12.7 [mm] Initial Velocity 30 [m/s]

Mass 8.537 [g]

FEM analysis CPT Analytical Results

Blue line: variation with time of the central node displacement of the plate Red line: variation with time of the sphere displacement

FEM ANALYSIS WITH THE FE SOLVER RADIOSS

POSTPROCESSING – CONTACT FORCE

FEM Analysis CPT Analytical Results

FEM ANALYSIS CPT * RELATIVE ERROR [%]

MAXIMUM DISPLACEMENT CENTRAL NODE [mm]

3.50 3.49 0.29

MAXIMUM CONTACT FORCE [N] 3750 3500 7.14

* Reference Solution: Classical Plate Theory

SUMMARIZING TABLE

- COPV of CFRP FILAMENT WINDING MATERIAL + Al 6061 LINER: Cylindrical coordinate system, need to reproduce the adhesion between liner and composite material P11_SHELL_SANDWICH

- Composite made of 11 layers, Stacking sequence: [ 904 / -12 / +12 / 902 / -12/ +12/ 90 ]: No symmetries!

M25_COMPSH

One of the more complex aspects of the numerical simulation is the modelling of the rupture behaviour of the composite material.

## Material Law No 25. COMPOSITE SHELL /MAT/COMPSH/1

This card is based on the TSAI-WU ELASTIC-PLASTIC MODEL and is used with composite shells with at least one orthotropic layer.

Material law COMPSH (25) describes orthotropic elasticity, has two plasticity models and brittle tensile failure.

The best description of our composite material is made by the LAW 25 - CRASURV FORMULATION.

ε t1 Tensile failure strain in direction 1

ε m1 Maximum strain in direction 1

ε f1 Total tensile failure in direction 1: when reached the element is

deleted

ε t2 Tensile failure strain in direction 2

ε m2 Maximum strain in direction 2

ε f2 Total tensile failure in direction 2: when reached the element is

deleted

The damage and failure behaviour is defined by the introduction of the following SIX INPUT PARAMETERS :

OUTLINE OF THE AUTOMATIC PROCEDURE

- Finally the damaged COPV is loaded again with the same pressure of step 1 to verify the REDUCED STRENGTH CAPABILITY of the structure.

- During the simulation the model of the tank will be subjected to a first PRESSURE ANALYSIS, to obtain a strength criterion of the structure.

- The stress state is recorded and we will proceed to the second step: the BARELY VISIBLE IMPACT DAMAGE.

M10/T1000 Resin-Fiber composite

Thanks to experimental characterizations we provide the solver with the following mechanical properties: 𝑬𝟏𝟏 188 [GPa]

𝑬𝟐𝟐 9 [GPa]

𝑬𝟑𝟑 9 [GPa]

𝑮𝟏𝟐 4.3 [GPa]

𝑮𝟐𝟑 4.3 [GPa]

𝑮𝟑𝟏 4.3 [GPa]

ν𝟏𝟐 0.3

ρ 1100 [Kg/m3]

PROPERTIES OF THE COMPOSITE MATERIAL

The deformations of the composite inserted in the solver through the material law 25_COMPSH are:

ε f1 3e-3

ε m1 2.7e-3

ε t1 2,4e-3

ε f2 2e-3

ε m2 1.9e-3

ε t2 1.8e-3

Caused to the fact that we have to model a composite obtained by filament winding, we have to impose different cylindrical coordinate systems to the two segments of the tank.

DEFINITIVE FEM MODEL

The cylinder is 500 [mm] height, with a radius of 416 [mm].

The impact occurs at 0° to avoid the complication of modelling a filament wound dome.

STEP 1) PRESSURE ANALYSIS Pressure Load 50 [MPa] The pressure load is imposed by a curve

In the postprocessing phase we will have to control the absence of unacceptable oscillations on the variation of the internal energy with time.

σxxmax = 387 [MPa]

Medium Tensional State = 200 [MPa]

RESULTS

CHECK OF THE CORRECT APPLICATION OF THE PRESSURE LOAD

The variation of the internal energy with time is regular, showing that the load is applied in an appropriate lapse of time during the explicit analysis.

STEP 2) BVID

Energetic Level: 4.56 [J] Sphere Mass: 7.7 [Kg] Sphere Diameter: 12.7 [mm] Impact Position: 0°

TENSIONAL STATE CAUSED BY THE IMPACT

Initial Velocity of the Sphere: 1.088 [m/s]

Maximum stress σxxmax = 412 [MPa]

- The delamination depth obtained on the experimental results is 1.5 [mm]. Considering that each composite layer has a thickness of 0.2 [mm], we can say that the damage touches 7 plies and a half.

- The impact area is 4 [mm]

FAILED LAYERS

- The post-processing software HyperView is able to show damage’s depth providing the number of the impacted layers.

The numerical simulation overstate depth and area of the damage. In fact we can see that it concern with the 8th ply. Because each finite element has an edge of 2 [mm], the impact area is

overstated too.

STEP 3) PRESSURE ANALYSIS OF THE DAMAGED STRUCTURE

A pressure load of 50 [MPa] is applied to the damaged tank.

The strength capability of the tank is reduced. The COPV is no more able to resist to the applied load after the impact.

CONCLUSIONS

The FEM simulation is able - To show the damage occurred to the structure after a Barely Visible Impact, - To provide the stress state and an overstate of the geometrical parameters of the

damage (dimensions and depth), - To verify the reduction of the mechanical properties of the material.

FORESEEN IMPROVEMENTS

To avoid the actual overstate of the damage with respect to the experimental results making the mesh thickness more consistent and refining the description of the material.

To make a better simulation of the failure behaviour of the composite giving

more information about the amount of plastic work absorbed before the rupture.

[1] R.M. Jones, Mechanics of Composite Materials, McGraw-Hill, Washington, D.C., 1975.

[2] Hashin,Zvi, The Elastic Moduli of Heterogeneous Materials, J. Appl. Mech.,

March, 1962. [3] Hashin,Zvi e B. Walter Rosen, The Elastic Moduli of Fiber-Reinforced

Materials, J. Appl. Mech., June, 1964. [4] Tsai, Stephen Structural Behaviour of Composite Materials, The Netherlands,

1953. [5] Abrate Impact on Composite Structures, Cambridge University Press, 1998.

[6] Thimoshenko Zur Frage Nach der wirkung eines Stosses auf einen Balken,

ZAMP 62, 1913. [7] RADIOSS THEORY MANUAL, Large Displacement Finite Element Analysis

PART2 10.0 version, january 2009

BIBLIOGRAPHY

THANK YOU FOR YOUR ATTENTION!

SPECIAL THANKS to Ing. Gerlando Augello Ing. Giulio Turinetti

CONTACTS: valentina.dolci@polito.it valentinadolci@hotmail.com