Compliant interfaces for wrinkle control and strain attenuation

Hung, Kuo-Kang

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https://hdl.handle.net/2142/113867 Copy

Description

Title

Compliant interfaces for wrinkle control and strain attenuation

Author(s)

Hung, Kuo-Kang

Issue Date

2021-12-01

Director of Research (if dissertation) or Advisor (if thesis)

Chasiotis, Ioannis

Doctoral Committee Chair(s)

Chasiotis, Ioannis

Committee Member(s)

• Chew, Huck Beng
• Lambros, John
• Nam, SungWoo

Department of Study

Aerospace Engineering

Discipline

Aerospace Engineering

Degree Granting Institution

University of Illinois at Urbana-Champaign

Degree Name

Ph.D.

Degree Level

Dissertation

Keyword(s)

• Wrinkle Control
• Glancing Angle Deposition
• Fragmentation
• Composite Laminate
• PDMS

Abstract

Microscale interfaces play an important role in the mechanics of material systems with dissimilar properties. In particular, compliant interfaces have been shown to provide resistance to crack propagation or accommodate the differential thermal expansion between dissimilar materials. In this dissertation research, compliant interfaces were taken advantage to address two problems in mechanics: (a) Control the details of surface wrinkling of soft substrates with hard coatings subjected to uniaxial compression, and (b) attenuate or completely eliminate the strain transfer from a load bearing substrate to an attached film. The first objective of this dissertation research was to investigate the geometry and material properties of a compliant interface that was employed to modify the wrinkle behavior of a material system comprised of a stiff film deposited onto a compliant substrate. In a conventional two-layer system, the wrinkle patterns, as described by their wavelength, amplitude and orientation are determined solely by the material properties and thickness of the two layers and the direction of the applied load. A compliant interface layer can provide the means for further control of wrinkling. Towards this goal, thin films were deposited via Glancing Angle Deposition (GLAD) between an elastomeric substrate of polydimethylsiloxane (PDMS) and a Cu thin film. The process parameters of GLAD were studied and calibrated to fabricate films comprised of uniform isotropic (nanosprings) and orthotropic (nanochevrons) Cu nanostructures with different geometrical parameters that controlled the effective in-plane compressive modulus of the GLAD films. The highly compliant Cu nanospring films served as novel means to reduce the physical length scale of wrinkle patterns, i.e. concurrently reduce the wrinkle wavelength and amplitude while maintaining the wrinkle amplitude-to-wavelength aspect ratio for a given applied strain. On the other hand, anisotropic Cu films comprised of nanochevrons with 0.5-µm pitch were shown to exhibit an anisotropy ratio of 9.3 which modified the wrinkle direction with respect to the principal stress direction by almost 10°. Thus, contrary to isotropic films in which the direction of applied stress exclusively dictates the wrinkle direction, this unique class of anisotropic films allowed modifying, for the first time, the wrinkle direction independently of the direction of the applied load. Thus, a compliant interface could be used quite effectively to control the geometrical details of wrinkle patterns without modifying the original material system or the type of loading. Finally, the mechanics of surface wrinkling was taken advantage to obtain for the first time the effective in-plane compressive modulus of GLAD films, as their discrete nature prevented such measurements in the past. The in-plane compressive modulus of GLAD Cu films comprised of nanosprings with 0.25-μm and 0.5-μm pitch was measured as 500±21 MPa and 830±20 MPa, respectively, which is more than two orders of magnitude smaller than the Young’s modulus of a solid Cu film (120 MPa). Similarly, the effective in-plane compressive modulus of nanochevron Cu films with 0.5-µm pitch was measured as 710±10 MPa and 75.1±2 MPa along the principal material axis and its normal, respectively. The second objective of this dissertation research focused on the attenuation or complete elimination of strain transfer from a load bearing substrate to an attached film via a compliant interface. Brittle films such as thin film photovoltaics (PV) that are integrated with load-bearing structures can fragment and suffer from performance degradation while subjected to strains larger than 0.3%. A properly designed interface can shield a brittle PV film from structural loads while also maintaining its functional performance. The design of effective (reaching 100% strain attenuation) and efficient (minimum layer thickness to attain 100% strain attenuation) interface layers depends on the dimensions and mechanical properties of the substrate, the PV film and the interface layer. An analytical elasticity model captured the coupled effects of shear modulus and thickness of a compliant interface on the attenuation of strain transferred from a Carbon Fiber Reinforced Polymer (CFRP) laminate to a bonded PV film. Based on the results of this model, a series of experiments were designed and carried out by using PDMS interface layers with effective shear stiffness values (shear modulus over thickness) in the range of 0.6-48 MPa/mm, leading to 36-100% strain attenuation. After accounting for manufacturing effects on the effective modulus of the PDMS interface layers, a very good agreement emerged between the experimental measurements and the model predictions. Importantly, the strain attenuation achieved by the interface layer preserved the original fill factor of the PV film until CFRP laminate failure at 1.8% strain, whereas, in the absence of a PDMS interface, the fill factor gradually decreased when the CFRP laminate strain exceeded 0.8%. Based on the experimentally validated analytical model, general strain attenuation maps were drawn to capture the coupled effects of the effective shear stiffness of an interface layer, and the Young’s modulus, thickness and length of an attached film.

Graduation Semester

2021-12

Type of Resource

Thesis

Permalink

http://hdl.handle.net/2142/113867

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Copyright 2021 Kuo-Kang Hung

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