Mechanical loading-induced and residual stress measurements in transparent spinel ceramics using Digital Gradient Sensing

Cubas Pastor, Franco

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https://hdl.handle.net/2142/117711 Copy

Description

Title

Mechanical loading-induced and residual stress measurements in transparent spinel ceramics using Digital Gradient Sensing

Author(s)

Cubas Pastor, Franco

Issue Date

2022-09-09

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

Lambros, John

Department of Study

Aerospace Engineering

Discipline

Aerospace Engineering

Degree Granting Institution

University of Illinois at Urbana-Champaign

Degree Name

M.S.

Degree Level

Thesis

Keyword(s)

• DGS
• ceramics
• spinel
• ROBAX
• grains
• residual stresses
• optical method
• soda lime glass
• brittle materials
• transparent materials

Abstract

The use of transparent ceramics for multiple engineering applications is of interest because of the unqiue properties that ceramics possess such as considerably high hardness, wear, corrosion, and thermal resistance. In particular, applications such as windows and domes require transparency and thus require ceramics are able to support high loads, including impact or blast loads, and still remain intact and transparent. It is important to study the behavior of such materials in multiple configurations, so engineering design with transparent materials can be improved. Since ceramics are quite stiff and brittle compared to transparent polymers and glasses, it is often difficult to directly measure stress or strain quantities in ceramics as the-signal-to-noise can be low in many measurement techniques. A promising new optical technique is that of Digital Gradient Sensing (DGS) developed by Tippur and coworkers [1] to measure stress gradients on transparent materials. It is a non-contact optical technique able to resolve stresses in the material of interest. However, DGS so far has only been applied to considerably less stiff transparent materials such as soda lime glass (SLG) and various plastics. In addition, spinel is a granular material with relatively large grains (up to hundreds of microns) which will be noticeable in the field of view used throughout this research, whereas SLG and many polymers are amorphous. In this work we conduct extensive experimentation on Poly(methyl methacrylate) (PMMA), SLG, and ROBAX (a granular transparent ceramic) to apply DGS to these materials and then extend the study to spinel which possesses a greater challenge for DGS due to its high stiffness. In addition to applying DGS to situations involving mechanical loading, namely a point load on a thin transparent plate, we are also interested in developing a DGS-based method to measure residual stresses. It was shown here that DGS is capable of detecting residual stresses in a transparent object without the need of an undeformed state picture of the stressed object. By using as an undeformed image one with no sample (i.e., just air) and the appropriately removing the refractive index effect of the material, we can quantify any residual stresses as the remainder in the DGS measurements. Thus, DGS also shows great promise as a means of measuring residual stresses in transparent media.

Graduation Semester

2022-12

Type of Resource

Thesis

Copyright and License Information

Copyright 2022 Franco Cubas Pastor

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