The Influence of Adhesion and Sub-Newton Pull-Off Forces on the Release of Objects in Outer Space

[+] Author and Article Information
M. Benedetti1

Department of Materials Engineering and Industrial Technologies, University of Trento, 38050 Trento, Italymatteo.benedetti@ing.unitn.it

D. Bortoluzzi, M. Da Lio

Department of Mechanical and Structural Engineering, University of Trento, 38050 Trento, Italy

V. Fontanari

Department of Materials Engineering and Industrial Technologies, University of Trento, 38050 Trento, Italy


Corresponding author.

J. Tribol 128(4), 828-840 (Apr 26, 2006) (13 pages) doi:10.1115/1.2345407 History: Received September 29, 2005; Revised April 26, 2006

The theoretical background and the numerical modeling results of a ground-based verification activity of a critical space mission phase affected by adhesion issues are presented. Tribological models are first reviewed with an emphasis on the contact forces assessment and their relationship to the geometrical, material, and mechanical properties of the contacting metal bodies. An approach based on a finite element analysis of the contact, accounting for the adhesion forces, is then proposed for studying the contact behavior of smooth surfaces in vacuum. Some solutions aimed at reducing adhesion pull-off forces are discussed. Special emphasis is placed on the role of surface roughness in reducing adhesion. To this purpose, a fractal surface theory is used to estimate interaction forces. The obtained results are applied to discuss the role of adhesion on the release of a test mass under zero gravity as well as to suggest an appropriate detachment procedure that finds a specific application in a scientific space mission.

Copyright © 2006 by American Society of Mechanical Engineers
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Figure 10

Detail of the finite element model of the (a) test mass and (b) plunger near the contact region. It is assumed that the plunger engage a pyramidal recess of the test mass

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Figure 11

Variation of contact radius with load obtained by FEM for the test mass—plunger contact

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Figure 1

General scheme of a caging device for in-orbit launch of objects. The object, here called test mass, is pressed by the upper plunger against the lower stoppers during launch. The decaging of the object is obtained by pushing the test mass on the upper stoppers using the lower auxiliary plunger and by retracting the main upper plunger.

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Figure 2

Variation of adhesive force T with joining load L for clean gold specimens measured by Gane (the picture is taken from Ref. 5)

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Figure 3

The JKR theory. (a) contact pressure p(r) is made up of two terms: Hertz pressure p1(r) and adhesion traction pa(r). (b) Variation of the contact radius with load according to Eq. 4; both for adhesion and adhesionless (Hertz) contacts.

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Figure 4

(a) Force-distance curves for different gold-coated surfaces of radius R=0.02 or 0.0093m measured at decreasing approach-separation rates. (b) The JKR plots giving the first loading-unloading paths for the contact radius a versus applied load F for a number of different coalesced gold surfaces of initially undeformed radii R at different approach (loading) and separation (unloading) rates. Both graphs have been obtained by Alcantar and are taken from Ref. 20.

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Figure 5

Effect of random roughness on adhesion (pull-off force P) between nominally flat surfaces having N asperities each of radius R and standard deviation of height σ. pc=JKR pull-off force (=1.5πwR) and δc=JKR pull-off displacement=34(π2w2R∕E2)1∕3 for each asperity. The graph is taken from Ref. 15.

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Figure 6

(a) Overview of the finite element model used to validate the numerical computation of adhesive forces acting on the test mass. The half-plane-sphere contact has been analyzed, assuming axial symmetry. (b) Detail of the finite element model near the contact region.

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Figure 7

A comparison between the theoretical JKR plot [described by Eq. 3] and the FEM result for the model shown in Fig. 6

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Figure 8

A comparison of analytical and numerical results for the distribution of contact stresses due to Hertzian repulsion and adhesive attraction over the contact radius for different values of the interference δ

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Figure 9

(a) Overview of the finite element model used for the numerical computation of adhesive forces acting between the test mass and the caging mechanism. Because of the symmetry, only a quarter of the system has been modeled. Boundary conditions are indicated by gray symbols.

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Figure 14

Simulated 2D fractal rough surface profiles of the microplunger (D=1.4, γ=1.5, L=0.2mm)

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Figure 15

Effect of fractal roughness G on van der Waals, electrostatic and deformation force versus the average surface separation distance for a silica microplunger and a gold test mass. (a) G=4.2×10−12m; (b) G=8×10−12m.

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Figure 16

(a) Effect of roughness asperities separation on attractive van der Waals and electrostatic forces versus the average surface separation distance. It should be emphasized that, at small separation distances, i.e., before roughness asperities separation, also repulsive (negative) deformation forces are active. (b) Variation of the attractive electrostatic force with the average surface separation distance.

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Figure 12

(a) Schematic illustration of an auxiliary microplunger for the final test mass release. (b) Detachment of the plunger from the test mass actuated by the microplunger followed by its separation from the test mass itself.

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Figure 13

(a) Equivalent contact model of two rough surfaces. (b) Electrostatic contact model of metallic and dielectric materials



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