Investigation of the grid methods for accurate strain measurement Mechanical Engineering

92 Pages·2015·43.82 MB·Portuguese

by  Tiago Marques

Investigation of the grid methods for accurate strain
measurement

Tiago José Carvalho Rebelo Domingues Marques

Thesis to obtain the Master of Science Degree in
Mechanical Engineering

Supervisors: Prof. Pedro Alexandre Rodrigues Carvalho Rosa
Prof. Ivo Manuel Ferreira de Bragança

Examination Committee

Chairperson: Prof. Rui Manuel dos Santos Oliveira Baptista
Supervisor: Prof. Pedro Alexandre Rodrigues Carvalho Rosa
Member of the Committee: Prof. Alberto Eduardo Morão Cabral Ferro

November 2015

Dedicated to my family, friends and all the people that helped me be who I am today:
‘ If one does not fail at times, then one has not challenged himself ‘ – Ferdinand Porsche

Resumo

A aplicação do processo de estampagem no fabrico de componentes é muitas vezes limitada
pela enformabilidade máxima dos materiais. Essa enformabilidade pode ser caracterizada pela
resposta à fractura quando o material é submetido a uma determinada trajetória de deformação.
Recentemente, trabalhos de investigação neste domínio mostraram que esse limite de
enformabilidade pode ser ultrapassado com o aumento da taxa de deformação. Porém, têm surgido
algumas questões relativas à precisão das técnicas utilizadas para a quantificação dessas trajetórias
de enformabilidade, e em como esses desvios podem ocultar o histórico de deformação do processo.
O presente trabalho incide sobre as técnicas experimentais utilizadas na quantificação das
extensões no plano da chapa, observando separadamente (i) as técnicas de marcação de grelhas e
(ii) as técnicas ópticas de medição das grelhas. Relativamente às primeiras foram comparadas
tecnologias de marcação a laser, por transferência indireta de polímero e maquinagem
electroquímica, no que respeita à medição foram utilizadas técnicas de cromatografia confocal,
microscopia óptica e fotogrametria. Os ensaios foram realizados com provetes circulares de AA1050
através da ação electromagnética e por estampagem convencional. Os vários ensaios permitiram
avaliar o desempenho combinado entre as tecnologias de marcação e de medição de grelhas. Estes
resultados mostraram que a utilização combinada da marcação laser e da cromatografia confocal
permite obter uma medição das extensões no plano da chapa com maior precisão, validando-os
através da comparação dos valores calculados para a extensão na direção da espessura usando a
equação de incompressibilidade e os valores medidos para a mesma.

Palavras-chave: medição de extensões, marcação de grelhas, medição óptica, cromatografia
confocal, gravação laser

i
Abstract

The application of forming processes for the production of components is sometimes limited
by the maximum formability of the material. This formability can be characterized by the solicitation
upon fracture when the material is subdued into a forming path.
Recent investigation work in this domain shows that such forming limits can be overcome with
an increase of the forming rate. Although, some issues have arisen relatively to the accuracy of the
measurement technologies and techniques used to quantify those forming paths and how that can
affect the true history of the forming process.
The present work focuses on the experimental techniques used in strain quantification in the
metal plane, observing separately (i) the grid marking methods and (ii) the optical technologies for grid
measurement. Relatively to the first, laser marking, indirect transfer of polymer and electrochemical
etching marking methods were compared, and relatively to the latter, confocal chromatography, optical
microscopy and photogrammetry were the technologies used. The tests were performed using
AA1050 circular specimens on both electromagnetic and conventional forming processes. The
multiple tests allowed assessing the combined performance between the grid marking methods and
measurement techniques respectively. Those results lead to the conclusion that the combined use of
the laser grid marking method and the confocal chromatography microscope allowed to obtain better
and more accurate strain measurements in metal sheet and forming processes characterization,
results which were validated by comparison between the values for strain thickness computed using
the constant volume equation and the respective measure.

Keywords: strain measurement, grid marking, optical measurement, confocal chromatography, laser
marking
ii
Acknowledgments

To my supervisor, Prof. Pedro Rosa for his help throughout the thesis, by being always
available and ready to help in my work and for his guidance that led me in the right path to accomplish
this work. For his advices that enlightened me very much and gave me a different perspective on
some subjects.

To Prof. Ivo de Bragança and Eng. André Pereira for their help on the laser etching process of
the specimens at NOF.

Concerning the use of the laboratory of “Tecnologia Mecânica”, a big thanks to Prof. Carlos
Silva and Mr. Farinha for their guidance, helping me to find my way around the laboratory, use of the
instruments and machines.

I dedicate this thesis to my parents, family and closest friends.
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Table of Contents
Resumo ................................................................................................................................................. i
Abstract .............................................................................................................................................. ii
Acknowledgments ......................................................................................................................... iii
List of Tables ..................................................................................................................................... v
List of Figures ................................................................................................................................... vi
List of Abbreviations & Symbols ................................................................................................ ix
1. Introduction .............................................................................................................................. 1
2. Grid Marking Methods ........................................................................................................... 2
2.1 Electrochemical etching method ............................................................................................... 2
2.2 Screen printing method (Serigraphy) ...................................................................................... 4
2.3 Indirect transfer printing method ............................................................................................ 5
2.4 Photochemical etching method ................................................................................................. 7
2.5 Laser etching method .................................................................................................................... 8
2.6 Comparison between methods ................................................................................................. 10
3. Measurement Techniques ................................................................................................. 12
3.1 Optical microscope ....................................................................................................................... 12
3.2 Confocal chromatography microscope .................................................................................. 15
3.3 Photogrammetry ........................................................................................................................... 19
3.4 Comparison between measurement technologies and their limitations .................. 22
4. Materials and Experimental Procedures ...................................................................... 24
4.1 Materials properties and specimen preparation ............................................................... 24
4.2 Experimental apparatus ............................................................................................................. 24
4.2.1 Optical microscope .................................................................................................................................. 24
4.2.2 Confocal chromatography microscope ........................................................................................... 25
4.2.3 Photogrammetry equipment and software .................................................................................. 27
4.2.4 EM machine and tool ............................................................................................................................... 29
4.2.5 Conventional stamping machine and tool ..................................................................................... 30
4.3 Experimental procedures .......................................................................................................... 31
4.3.1 Etching processes ..................................................................................................................................... 31
4.3.2 Grid marking measurement ................................................................................................................. 33
4.4 Plan of experiments ..................................................................................................................... 34
5. Results and Discussion ....................................................................................................... 35
5.1 Comparison between grid marking methods measurements ....................................... 35
5.2 Measurement technologies accuracy and their limitations ........................................... 41
5.3 Comparison of EMF and simulated conventional processes .......................................... 43
5.3.1 Strain distribution .................................................................................................................................... 44
5.3.2 Thickness strain value dispersion ..................................................................................................... 47
6. Conclusions and Future Work .......................................................................................... 50
7. References ............................................................................................................................... 51
Appendix ........................................................................................................................................... A

iv
List of Tables
Table 1 - Evaluation of advantages of each etching method (based on Table 1 from [6]) ................... 11
Table 2 - Typical effective wavelengths and diffraction limits [61] ........................................................ 17
Table 3 - Evaluation of advantages of measurement technologies (Based on [6,57,60,74,78,81]) ....... 23
Table 4 - STIL initial – optical head technical specifications [93] ......................................................... 26
Table 5 - Flat spiral coil parameters ..................................................................................................... 29
Table 6 - Measurements of points of interest using three different technologies before and after
forming process [µm] - NA = not applicable .................................................................................. 40
Table 7 - Etching methods average line thickness before and after forming processes [µm] .............. 41

v
List of Figures
Figure 1 – Multiple types of applicable grids – (a) Square, (b) Butted circle, (c) Single circle, (d)
Speckle pattern, (e) Check board, (f) Solid square, (g) Solid/wide square [11] .............................. 2
Figure 2 – (a) Principles of electrochemical sheet marking [10], (b) Example of stencil pattern ............ 3
Figure 3 - (a) Screen printing process [20], (b) Steps of printing [21] .................................................... 4
Figure 4 - (a) Film release paper [28], (b) Sublimation paper [29], (c) Melt transfer paper [30], (d) Wet
transfer paper [31] ........................................................................................................................... 6
Figure 5 - (a) Photochemical etching flow chart [33], (b) Photochemical engraving machine [34] ......... 7
Figure 6 - Photochemical etching process [36] ...................................................................................... 8
Figure 7 - (a) First ruby laser [40], (b) Laser principle [41] ..................................................................... 8
Figure 8 - Difference between flashlight and laser wavelengths [42] ..................................................... 9
Figure 9 - (a) Nd:YAG laser diagram [43], (b) CO2 laser diagram [44], (c) Laser engraving [45], (d)
Laser etching with paint [46] ........................................................................................................... 9
Figure 10 - (a) Raster and vector etching laser parameters [48], (b) Typical laser etching/cutting
machine [49] .................................................................................................................................. 10
Figure 11 - (a) Aperture in microscope lenses, (b) Aperture for photographic lenses .......................... 13
Figure 12 - Schematic of compound microscope ................................................................................. 14
Figure 13 - (a) Typical stereo microscope [59], (b) Schematic of a stereo microscope ....................... 14
Figure 14 - Typical confocal chromatography microscope and data setup [62] ................................... 15
Figure 15 - (a) LSCM schematic [63], (b) WLI schematic [64] ............................................................ 15
Figure 16 - (a) LSCM and WLI data acquisition differences [61], (b) Schematic of spinning disk
implementation in confocal microscopes [65] ............................................................................... 16
Figure 17 - (a) CMM table [68], (b) Typical confocal probe [69,70] ....................................................... 17
Figure 18 - (a) Leica Microsystems confocal microscope [71], (b) Nikon NEXIV confocal microscope
[72] ................................................................................................................................................ 18
Figure 19 - Common industrial applications of confocal chromatic sensors [74] ................................. 18
Figure 20 - (a) CCS Schematic [76], (b) CSI Schematic [77] ............................................................... 19
Figure 21 - Technological gap in photogrammetry [78] ........................................................................ 19
Figure 22 - Definition of field of view [82] ............................................................................................ 21
Figure 23 - (a) Definition of triangulation [81], (b) Accuracy pyramid [81], (c) Typical scale bars [83] . 21
Figure 24 - Applications of photogrammetry: (a) Aerial [85], (b) Police investigation [86], (c)
Archaeology [87], (d) Industrial and engineering [88] ................................................................... 22
Figure 25 - (a) Typical photogrammetry setup [89], (b) Multiple camera photogrammetry setup [90] .. 23
Figure 26 - (a) Hobersal oven, (b) Oven temperature controller .......................................................... 24
Figure 27 - (a) Mitutoyo toolmakers microscope, model 176-901-1A, (b) Mitutoyo digimatic micrometre
head, series 164 ............................................................................................................................ 25
Figure 28 - Perspective view of CMM machine and relevant component description .......................... 26
Figure 29 – Different views of the CMM and STIL head, (a) real representation, (b) CAD
representation front view, (c) CAD representation side view ........................................................ 27
vi