Abstract: 3D body scanning has been used broadly including digital human modeling, simulation, ergonomic product design, and so forth. This research used template-registered faces of 336 Koreans in order to use them to design an oxy-gen mask that provides good fit to Korean faces. The finite element analysis method is applied onto the template-registered faces to predict the contact pres-sure of a mask design onto different faces. The average and variation of the esti-mated contact pressure values among all the Korean faces were analyzed for eval-uation of the appropriateness of a mask design for Koreans. The proposed meth-od can be usefully applied to find an optimal shape of wearable products for a specific target population.

1. 1
Wonsup Lee1, Jin-Gyun Kim2, Johan M. F. Molenbroek3,
Richard H. M. Goossens3, Hayoung Jung4, and Heecheon You4
1 School of Global Entrepreneurship and ICT, Handong Global University, South Korea
2 Department of Mechanical Engineering, Kyung Hee University, South Korea
3 Faculty of Industrial Design Engineering, Delft University of Technology, Netherlands
4 Department of Industrial and Management Engineering, Pohang University of Science and
Technology (POSTECH), South Korea

2. 2
Contents
 Introduction
• Background
• Objectives of the Study
 Template Model Registration
 Contact Pressure Estimation by
Finite Element Analysis
 Discussion
 Q & A

3. 3
Limitation of Previous Study
• Our previous study (Lee et al., 2018) proposed a virtual fit simulation
method to find the optimal shape of an oxygen mask (OM) design for
fighter plots
• A mask design drawn by CAD was virtually fit to each of 3D face images
and the distance between the mask and face was calculated
• Limitation: The material properties of the face and mask need to be
considered for more realistic virtual fit analysis and more optimized OM
design
3D Face Scanning Editing & Measurement Mask DesignVirtual Fit Simulation
Lee et al. (2018). Ergonomic evaluation of pilot oxygen mask designs. Applied Ergonomics, 67,
pp. 133-141.

4. 4
Contact Pressure Estimation
 A simple FE method is needed using a sufficient number of faces (say,
over 100) for contact pressure analysis
• A finite element analysis (FEA) approach using FEA software such as ANSYS,
LS-DYNA (Dai et al., 2011; Lei et al., 2012) was applied for realistic analysis
of contact pressure of a mask over a face
• Only one or a few face scans were applied to analyze the contact pressure
of a mask over the face
• A complex procedure and a significant amount of time (several hours) are
needed to manipulate 3D face and mask images using FEA software

5. 5
1. Applying a sufficient number of 3D face scans to designing an
optimal shape of mask
2. Application of finite element analysis (FEA) method to considering
material properties of the face and mask
3. Simplification of the face and mask images and FE model for easy
use of sufficient 3D face scans in FE analysis
Objectives of the Study
• A study on realistic virtual fit analysis with considering material properties
of the face and mask

6. 6
3D Face Scan Data
• Faces of Korean Air Force pilots and KAF Academy cadets (Lee et al., 2013)
• n = 336 (male: 278, female: 58)
small narrow medium narrow medium wide large wide

7. 7
Template Face Model
Characteristics of the template face model
• Average size
• Symmetry
• Unevenly distributed mesh
(more meshes around mask-fit area)
• Number of vertex = 1,340
• Number of faces = 2,624
• 24 landmarks on vertex points

8. 8
registration
Template Model
Template Registration
• Illustration of registration of the template face model to each 3D face
Template-registered image
…

9. 9
Application of FEA: Concept of FEA
• Each triangular mesh consisting the face image has
9 degrees of freedoms (DOFs)
= 3 vertex × 3 DOFs per vertex
• 3 DOFs mean possibilities of translation
in x, y, and z directions
• In total of 9 DOFs is describing the change of each mesh
• The change of mesh is described as a 9×9 stiffness matrix (called as K )
vertex 1
vertex 2
vertex 3
x, y, z translation of node 1
x, y, z translation of node 2
x, y, z translation of node 3
triangular
mesh
k11 k12 k13 k14 k15 k16 k17 k18 k19
k21 k22 k23 k24 k25 k26 k27 k28 k29
k31 k32 k33 k34 k35 k36 k37 k38 k39
k41 k42 k43 k44 k45 k46 k47 k48 k49
k51 k52 k53 k54 k55 k56 k57 k58 k59
k61 k62 k63 k64 k65 k66 k67 k68 k69
k71 k72 k73 k74 k75 k76 k77 k78 k79
K81 k82 k83 k84 k85 k86 k87 k88 k89
k91 k92 k93 k94 k95 k96 k97 k98 k99
Km =
ux1 uy1 uz1 ux2 uy2 uz2 ux3 uy3 uz3
ux1
uy1
uz1
ux2
uy2
uz2
ux3
uy3
uz3
where, m is the mesh number (1 ~ 2624)
k is component of the stiffness matrix
E is Young’s modulus, ν is Poisson ratio
𝑘 = 𝑓(𝐸, ν) ← defining the material property
of the face

10. 10
Application of FEA: Estimation of Force (f)
• Estimated force at each vertex consisting of the face is calculated as
• Contact pressure is assumed as a measure that is proportionally equivalent
to the force
𝑓1
𝑓2
𝑓3
⋮
=
𝑘11
𝑘21
𝑘31
⋮
𝑘12
𝑘22
𝑘32
⋮
⋯
⋯
⋯
⋱
𝑢1
𝑢2
𝑢3
⋮
f uK= ×
triangular
mesh
f3
f1
f2
where, K is an assembled stiffness matrix of all Km
(K is different by faces)
u is displacement vector of the vertex
vertex 3
vertex 1
vertex 2
u1
u2
u3
estimated pressure at each vertex
of the face (illustrated)

11. 11
Virtual Fitting with Different Designs of Mask
• The face-contacting part of the mask is drawn as a single curvature
• Then, the curvature was virtually fit to 3D faces
• The facial areas pushed by the curvature were simply calculated by f=Ku
using Matlab
Design A
Design B
Different mask designs (illustrated) Template-registered faces

12. 12
Virtual Fitting & FE Analysis (Video)
Front view Side view

13. 13
Results: Average Contact Pressure
• Average of the estimated contact pressure were analyzed by the vertex
point, then visualized by color
• Design B shows smaller and more equally-distributed contact pressure then
the design A
Mask design A
Mask design B

14. 14
Results: Variation of Contact Pressure
• Standard deviation of the estimated contact pressure were analyzed by the
vertex point, then visualized by color
• Design B shows less varied contact pressure then the design A
Mask design A
Mask design B

15. 15
Discussion & Further Study (1/2)
• This study developed a FEA-based virtual fit method for design and
evaluation of the pilot OM
• The template model registration was applied to 3D face scan database
 All the 3D faces have same mesh topology
 Average and variation of the estimated contact pressure could be
analyzed per each vertex point
 Further study: Design of the optimized shape of the mask based on
the information of average and variation of the contact pressure

16. 16
Discussion & Further Study (2/2)
• A simplified FE model was applied to estimate realistic interaction between
the faces and mask design
• Material property of the face and mask was considered
• Contact pressure of the mask over the face was estimated
• As a preceding research, this study limitedly applied the material properties
 Need more realistic material properties for more accurate estimation of
the contact pressure
 Need to apply different material properties by facial area (e.g., around
nasal bone, around cheek, around chin)
• The proposed method could be applied for optimal design of wearable
products

17. more questions to W.Lee@handong.edu (Wonsup Lee)