Crash Analysis And Design of Multi-Cell Octagonal S-Shape Members Under Axial And Oblique Impacts

International Refereed Journal of Engineering and Science (IRJES)
ISSN (Online) 2319-183X, (Print) 2319-1821
Volume 6, Issue 2 (February 2017), PP. 37-50
www.irjes.com 37 | Page
Crash Analysis And Design of Multi-Cell Octagonal S-Shape
Members Under Axial And Oblique Impacts
Sobhan Esmaeili-Marzdashti1
, Tamer A. Sebaey 2, *
, Sadjad Pirmohammad1
,
Sareh Esmaeili-Marzdashti3
1
(Department Of Mechanical Engineering, Faculty Of Engineering, University Of Mohaghegh Ardabili,
Ardabil,179, Iran.)
2
(Mechanical and Industrial Engineering Department, College Of Engineering, Qatar University, 2713,
Doha, Qatar.)
3
(Department Of Mechanical Engineering, Polytechnique Montreal, P.O. Box 6079, Station Downtown
Montreal, Quebec, Canada.)
*
Corresponding author email:sepaey@qu.edu.qa
Abstract : Crashworthiness of the structures used in automotive is widely being addressed as it is defining the
safety. The present article aims to investigate crushing performance under axial and oblique impact performed
under 10°, 20° and 30° for different cross-section configurations of S-shaped longitudinal members. Modeling
and numerical analysis are carried out using finite element code LS-DYNA. The model was validated using
experimental data. The complex proportional assessment (COPRAS) method is used to provide optimized
alternative design by considering two conflicting criteria, energy absorption (EA) and peak crushing force
(PCF). The results indicate that the structures (at which four blades connect the outer and the inner walls) with
inner reinforcing blades have a high crash performance than the others (without connecting blades). Besides, the
blades in multi-cell members, which connect the middle of edges (T4), lead to superior response in comparison
with the other ones (which connect corners). Finally, dimensions (the wall thickness and the distance of inner
and outer tubes) of T4 are optimized by design of experiment and response surface methodology techniques.
Keywords: Crashworthiness, Design of experiment, Impact, Multi-cell, Optimization
I. INTRODUCTION
To increase safety level in transportation systems for preventing loss of life and injuries in accidents,
there is a need for components that can absorb impact energy and prevent harm and damage in critical parts.
Energy absorbers are utilized to reduce the undesirable effects of accidents and dissipate the kinetic energy of
impact via plastic deformation. As an energy absorber in crashworthiness applications such as trains, cars, ships,
aircrafts and other high-volume industrial products, the thin walled structures have been widely used to ensure
crash safety due to their lightweight, low cost and high energy absorption [1,2]. The members encountered the
impact are expected to absorb maximum energy and possess minimum mass. So, due to having the high specific
strength and more energy absorption than the other materials, the use of aluminum has become more and more
predominant [3]. The early investigations in energy absorbers are focusing on steel columns for their low costs
and high ductility [4]. For better understanding the crashing behavior in aluminum columns, comprehensive
studies have been conducted using the numerical and experimental methods. In this regard, Alexander [5] first
presented an analytical solution for circular tubes to obtain mean crushing force under axial impact. Then, the
models were used by Wierzbicki and Abramowicz [6] to predict the crushing response of aluminum tubes under
dynamic and static loading. Abramowicz and Jones [7,8] and Langseth and Hopperstad [9] validated
experimentally the theoretical predictions. By comparing the results with the numerical simulations, Kim and
Wierzbicki [10] acquired the analytical solution for the crushing resistance of thin-walled S-shaped frames with
rectangular cross-sections. Khalkhai et al. [11] expressed a combined analytical and experimental research of
thin-walled S-shaped square tubes subjected to the quasi-static axial crushing. Optimization problem, about
energy absorption of the S-rail, has been solved by Han and Yamazaki [12].
Over the past few years there has been a growing interest in the use of multi-cell members as energy
absorption applications. Fang et al. [13] studied the crashing behavior of foam-filled bitubal circular tubes
subjected to axial loading to optimize them in crashworthiness. Chen and Wierzbicki [14] found that the multi-
cell members absorb greater specific energy than the conventional simple tubes. Ohkami et al. [15] and
Hidekazu Nishigaki et al. [16] studied the collapse behavior of S-shaped beams. They performed dynamic and
static collapsing tests. Zhang and Saigal [17] have also investigated the effects of different cross-section
reinforcement strategies during impacts. They concluded that applying the reinforcement in structures increases
the total energy absorption. Various arrangements of blades on the s-shaped tubes were studied by Hosseini-
Tehrani and Nikahd [18,19]. They focused on a remarkable arrangement of straight and sideling blades.

Crash Analysis And Design Of Multi-Cell Octagonal S-Shape Members Under Axial …
Oblique loading can interpret all the situations in crashing events for pieces designed for axial loading.
The energy absorber system needs to endure an impact angle larger than 30º [20]. In order to carry out a
complete simulation, oblique loading should be taken into account. Tran et al. [21] implemented the analytical
modeling of dynamic oblique loading conditions, which were validated by the numerical results, by considering
an oblique impacting boundary conditions and exploring related effects. Elmarakbi et al. [22] carried out
numerical studies on energy absorption characteristics of simple and multi- walled S-shaped longitudinal
members with four kinds of cross-sections under axial quasi static loading. The multi-cell members are the tubes
with blades which connect the inner tubes with the outer ones. Their results reveal that the multi-walled
members with octagonal cross-sections for two types of materials, mild steel and aluminum are introduced as
optimized shapes which better react in crash procedure.
The current paper aims to investigate the specific energy absorption (SEA) and peak crushing force
(PCF) for multi-cell S-shaped longitudinal members by applying two kinds of blades to connect inner tubes and
outer ones in octagonal cross-sections. Then COPRAS method is used to choose the best configuration with the
high crashworthiness performance by considering two conflicting criteria, (SEA) and (PCF). In order to know
the behavior of changing geometric parameters in crushing problems and optimized their performance, response
surface models (RSM) and design of experiment (DOE) techniques are also considered.
II. SPECIMEN GEOMETRY AND MOLDING
The aim of this paper is to investigate the crushing behavior of different S-shape multi-wall structures
with octagonal cross sections and various types of blades for both axial and oblique impacts. The proposed
specimen configurations are shown in Fig. 1. Three different types of tubes are proposed; single-cell one single-
cell (Fig. 1a), two doubly-cells (Fig. 1b and 1e) and four multi-cell forms (Fig. 1c, d, f and g). Different cross-
sectional configurations with various layouts of the blades are considered for the comparative study. The
description of each configuration is summarized as:
T1: single tube;
T2: bitubal tube I;
T3: corner blades which connect the corners of the outer and inner walls,
T4: middle blades which link the middle of the edges of inner and outer tubes,
T5: bitubal tube II;
T6: blades directly coupling the middle of the outer walls with the inner wall corners.
T7: blades connecting the outer corners with the middle of inner walls.
The difference between bitubal type I and II is that the inner tube rotates 22.5o relative to the outer one
in type II. The mentioned sections possess the same perimeter of 534mm for outer tubes and the total length of
L'=1000 mm [22]. It is remarkable that, in order to maintain the same mass, different values are set to wall
thickness. The dimensional model is shown in Fig. 2.
Figure 1: Sectional configurations of multi-cell S-shape tubes with the same mass but different wall
thicknesses.

Figure 2: Dimensions of an S-shaped longitudinal member
Non-linear finite element code, LS-DYNA [23], is employed to analyze collapse mode and energy
absorption of thin-walled components with different sections under compression load. Specimens are modeled
using the quadrilateral full-integration (ELFORM 2) shell element with four nodes and with five integration
points through the thickness. In order to achieve an acceptable finite element model, several mesh convergence
analyses are accomplished.
The axial and oblique loads applied on the s-shaped tubes are modeled by the rigid wall planar moving
force available in LS-DYNA. In order to simulate the dynamic crushing condition, a 600 kg lumped mass with
an initial velocity of V=15 m/s strikes the end of the S-rail as shown in Fig. 3. The rear end of the S-shape tube
has fully clamped and in contrast, the front end, just in the direction of impact, is considered without any
constraint. The contact between the bodies during collapse is modeled as node to surface contact algorithm. For
both static and dynamic friction, a friction coefficient of 0.15 is adopted for all contacting surfaces [24]. The S-
shaped tubes material are considered as aluminum alloy AA6060. The material characteristics is measured using
tensile test as shown in Fig. 4. The values of the engineering constants are: Poisson’s ratio υ=0.3, yield strength
S_y=80 MPa, ultimate strength S_U=173 MPa, density ρ=2700 kg/m^3, and Young’s modulus E=68.2 GPa.
Figure 3: Boundary and loading conditions assumed in the finite element modeling
Figure 4: Stress-strain curve of aluminum AA6060

Before running the FE models, it is required to validate them by comparing their results with
experimental ones. For this purpose, experiments are carried out on circular tubes using the drop hammer test
machine. Experimental dynamic tests are performed on circular tubes of 60 mm diameter, 150 mm length and
2.1 mm wall thickness. The drop-weight test performed using hammers mass of 110 kg and impact velocity of
13 m/s. The experimentally obtained load–displacement responses are compared with the numerical predictions.
Hammer mass crushed the circular tubes 90 mm. Fig. 5 compares the crushing force versus displacement and
the final deformation patterns between FE simulation and experimental tests of circle tubes. It can be noted that
the FE simulation is in good agreement with the experiment in terms of force–displacement curve and final
deformed profile of circle tube (Fig. 5). Therefore, the FE modeling established in this paper is considered
sufficiently accurate for further comparative analyses.
Figure 5: Numerical and experimental results for the circular tubes
III. CRASHWORTHINESS ASSESSMENT
Structural crashworthiness indicator under oblique loading
In crashworthiness problems, the energy absorption capacity of multi-cell S-shaped longitudinal
members is computed by specific energy absorption (SEA), which is acquired by dividing energy absorption
capability (EA) by the total mass of structures. Therefore, by increasing the EA and decreasing weight of
members, the SEA as beneficial indicator goes up. Energy absorption (EA) and specific energy absorption
(SEA) are determined as:
0
( )EA F x dx

  (1)
EA
SEA
m
 (2)
where EA is the absorbed energy, δ is the effective stroke length, F(x) presents the instantaneous
crushing force in the axial and oblique direction, and m is the total mass of the member. High specific (SEA)
capability and a low peak crushing force (PCF) should be adopted to prevent severe damage and injury during a
collapse.
For such a multi criteria decision making (MCDM) process, the complex proportional assessment
(COPRAS) is used as the ranking method [25,26]. This method introduces the best decision by considering both
the ideal and the ideal-worst solutions of alternative designs under the presence of mutually conflicting criteria
[27-31]. The procedure is described in certain details as follows:
a) Select the available set of the most important criteria which depicts the alternatives.
b) Generate the initial decision- matrix, X.
11 12 1
21 22 2
1 2
...
...
[ ]
... ... ... ...
...
n
n
ij mn
m m mn
x x x
x x x
X X
x x x
 
 
  
 
 
 
(3)

where Xij is the performance value of ith
alternative design with respect to jth
criterion, m is the number
of comparative alternative design and n is the number of criteria.
c) Normalize the decision matrix ( ) in order to obtain the dimensionless values of different criteria and
compare them as:
1
[ ]mn m
i
ij
ij
ij
R
X
X
r

 

(4)
d) Identify the weight of each criterion (Wj) by using individual weightage method [32,33]. The indicators
considered herein are the specific energy absorption (SEA) and the peak crushing force (PCF). SEA is
assumed more significant than PCF. So, the weight for SEA is assigned as 0.1875 and for PCF is 0.0625 by
considering four angle strokes.
e) Determine the weighted normalized decision matrix, D
D=[ ]mn ij jij
Wry  
(5)
Where is the normalized performance value of ith criterion with respect to jth alternative and is
the weight of ith criterion. The sum of dimensionless weighted normalized values of each criterion is
always equal to the weight for that criterion:
1
n
ij j
i
y W


(6)
f) Sums of the weighted normalized values are computed for both the beneficial attributes and non-beneficial
attributes. Due to considering more weight to the SEA as an ideal criterion in comparison with PCF as a
non-ideal criterion, the value of Eq. 9 becomes greater than Eq.10.
1
n
i ij
j
S y 

  (7)
1
n
i ij
j
S y 

 
(8)
where and are the weighted normalized values of the beneficial and non-beneficial criteria,
respectively. The better alternative design is obtained by the greater value of and the lower value of .
Also, the sums of and of the alternatives are always respectively equal to the sums of weights for the
beneficial and non-beneficial criteria as expressed by:
1 1 1
m m n
i ij
i i j
S S y  
  
  
(9)
1 1 1
m m n
i ij
i i j
S S y  
  
  
(10)
g) Determine the relative significance or priorities for each candidate alternative ( ).
min
1
min
1
( 1,2,..., )
( / )
n
i
i
i i n
i i
i
S S
Q S i n
S S S
 


 

  


(11)
where is the minimum value of
h) Determine the quantitative utility in a complete ranking of the alternatives (U).

7
1
100%i
i
i
i
Q
U
Q

 

(12)
where defines the maximum relative significance which is the best option for the alternative
selection decision.
IV. NUMERICAL RESULTS AND DISCUSSIONS
In this study, the influences of the load angle and importance of the geometric parameters on the S-
shaped tubes responses (SST) under four stroke angle impact loading (0º, 10º, 20º, 30º) are investigated.
Numerical analyses are carried out to compare the crush behavior of the seven types of SST (S-shaped tubes). It
can be noted that the deflection is specifically defined as the vertical displacement, δ, of the rigid wall with
respect to various load angle and initial velocity of 15 m/s. The effective stroke length is taken as 0.4 L, while L
is the total length of the energy absorbing structure. The dynamic force-displacement curves of the proposed
SST subjected to the four crushing angles are plotted separately in Fig. 6 and Fig. 7. Each rising and falling in
load-deflection curves obviously implies the existence of a folding in the structures. By increasing load angle θ
from 0º to 30º, the arrival time to the peak increases. Although they are small, differences between the
crashworthiness responses can be seen on the plots.
Figure 6: Crushing force vs. displacement curve for specimens without blades; (a) , (b) , (c)
and (d)
Due to the presence of the inside end parts offset (D), less energy is absorbed by the tubes which leads
the deformation mode to change from axial progressive folding dominant to global bending one. In this case,
the impact force reaches the highest value and then drops sharply. The observed load-deflection characteristics
of the SST can be further understood by looking at the final stage deformation modes of some of the tubes at
typical load angles, Fig. 8.

Figure 7: Crushing force vs. displacement curve for specimens with blades; (a) , (b) , (c)
and (d)
Fig 9a and 9b show the crashworthiness indicators (EA and PCF) calculated from the numerical results,
for all the members with same mass and under multiple load studied. As it is evident in Figs. 9b, by increasing
loading angle from θ=0° to θ=30°, the PCF decreases. It is noteworthy that bitubal S-shaped tubes with blades
connecting the middle sides of inner and outer tubes (T4) has more PCF than bitubal structures with blades
connecting the corners of inner and outer ones (T3). Besides, according to the rotational effect of inner tube
respect to the outer one on PCF value, multi-cell S-shaped tubes introduced by T7 achieve less PCF than multi-
cell S-shaped ones named T6. It is clear in Fig. 9b that the higher PCF value belongs to S-shaped tube T4 in
stroke angle of θ=0° and 10° and T1 and T6 in crushing angle of θ=20° and 30°. In general, multi-cell forms of
SST absorb more energy than single and double-cell structures with the same mass and under multiple load
conditions. Similar result was noticed by Wu et al. [34]. The authors of that paper concluded that the higher the
number of cells, the more efficient energy absorption characteristics. The comparison between various structures
shows that the amount of this criterion in bitubal structures with interaction between middle sides by using
blades (T4) is more than the interaction between the corner sides of inner and outer tubes by utilizing blades
(T3). On the other hand, rotating inner tubes with respect to the outer ones in bitubal structures has an
undesirable effect on absorbing energy. Consequently, T7 absorb less energy than T6 under axial and oblique
dynamic loading. It can be expressed that the number of corners and the types of blades play a significant role
for energy absorbers in crash problems. Fig. 9 illustrates that, for all loading angle, the highest energy absorbing
indicators belong to T4 configuration.

Figure 8: Deformation modes of multi-cell tubes under different loading angles
The COPRAS method is applied on the LS-DYNA results of multi-cell members investigated in this
paper to rank them in terms of crashworthiness capability. The COPRAS aims to choose the best energy
absorber in terms of the high crash performance. In order to diminish the damage received to any structure
which is in direct relationship with high amount of EA as a beneficial attribute and low amount of PCF as a non-
beneficial attribute, COPRAS calculation is reassumed for those mentioned criteria (totally 8 indicator) in four
crushing angles. Due to assuming EA as a major indicator than PCF in this essay, the weights of 0.1875 is
assigned to EA as and 0.0625 is given to PCF in four angle stroke. According to COPRAS steps, the decision-
matrix, weighted normalized decision matrix is generated in Table 1, 2. According to Table 3, the final ranking
of s-shaped tubes (i.e. T1-T7) is obtained as 5-6-3-1-7-2-4. This means that the best choice of these seven
alternative sections with same mass and under multiple loads is T4.
Consequently, by increasing the number of corners (N) in S-shaped tubes and also by a rotating the
inner tubes by 22.50º, with respect to the outer tube, structural crashworthiness performs better. The number of
corners for T1 through T7 is 8, 16, 16, 16, 24, 20, and 20, respectively. Among mentioned tubes, T4, which
possesses 24 corners and parallel tubes, has the best crashworthiness capability. Therefore, by utilizing
COPRAS method, the rank of 100% is assigned to it. Moreover, by comparing the performance of Type 2 and
Type 5 with the same corners, it is deduced that Type 2 with parallel tubes has higher crashworthiness than the
other. In addition, the comparison of the results between T1, T2, and T5 shows that the crushing performance of
bitubal structures without blades are less than simple tubes. Consequently, it can be expressed that blade plays a
significant role in crushing performance.
T1 T2 T3 T4 T5 T6 T7
EA/kJ
0
1
2
3
4 0
0
10
0
20
0
300
(a)

T1 T2 T3 T4 T5 T6 T7
PCF/kN
0
10
20
30
40
50
00
100
200
300
(b)
Figure 9. Performance comparisons under different loading angles: (a) EA, (b) PCF.
Table 1. Decision matrix.
Table 2. Normalized decision matrix.
Angle
Criteria EA PCF EA PCF EA PCF EA PCF
T1 0.1338 0.1392 0.1362 0.1464 0.1437 0.1531 0.1407 0.1477
T2 0.1416 0.1442 0.1343 0.1336 0.1250 0.1294 0.1291 0.1334
T3 0.1407 0.1359 0.1429 0.1374 0.1489 0.1414 0.1482 0.1445
T4 0.1551 0.1546 0.1555 0.1559 0.1638 0.1477 0.1581 0.1466
T5 0.1352 0.1383 0.1334 0.1302 0.1208 0.1325 0.1258 0.1408
T6 0.1497 0.1477 0.1526 0.1499 0.1571 0.1459 0.1535 0.1542
T7 0.1435 0.1397 0.1447 0.1461 0.1403 0.1498 0.1440 0.1324
Table 3. Results of COPRAS method
Angle
Criteria EA PCF EA PCF EA PCF EA PCF
T1 2.915 40.06 2.745 38.78 2.205 36.41 1.875 28.05
T2 3.085 41.49 2.705 35.39 1.917 30.75 1.72 25.3
T3 3.065 39.12 2.88 36.4 2.285 33.59 1.975 27.38
T4 3.378 44.51 3.132 41.31 2.513 35.09 2.112 27.81
T5 2.945 39.78 2.688 34.48 1.853 31.49 1.676 26.7
T6 3.26 42.5 3.075 39.72 2.41 34.67 2.045 29.24
T7 3.125 40.19 2.915 38.7 2.153 35.6 1.918 25.12

V. RESPONSE SURFACE MODELS
According to Section 4, the bitubal structure with four connecting blades in octagonal tubes named
Type 4 is introduced as the best alternative sections by considering COPRAS method. In this section, response
surface methodology (RSM) in terms of design of experiments (DOE) is used to know the behavior of changing
geometric parameters in crushing problems and optimized their performance [35, 36]. Fig. 10 displays the
flowchart of implementing response surface methodology for optimization problem.
Figure 10: Flow chart showing the steps of creating the RS models.
In the first step of optimization, the design points are determined by the design of experiment (DOE)
techniques such as CCD (central composite design) in the design-expert software. The wall thickness (t) and the
length of the blades (L) are selected to be the independent variables. The ranges and step size of the two design
variables are summarized in Table 4. Based on the optimal tube size in previous section, the dimensions are
chosen in such a way to cover the typical range of tube sizes generally used in crashworthiness applications,
such as in automotive body [37]. Table 5 shows the design matrix predictions. Structures with dimensions of the
design points are then analyzed in LS-DYNA to find the crush indicators (SEA1-4 and PCF1-4) under multiple
loading angles (0°, 10º, 20°, and 30º).
Table 4 Range and step size of design variables and impact angle of Type 4 S-shaped tubes.
Table 5. The design matrix
Iteration t (mm) L(mm) SEA1 PCF1 SEA2 PCF2 SEA3 PCF3 SEA4 PCF4
1 2.5 35 0.672 75.65 0.58 60.86 0.484 54.97 0.415 48.41
2 2.5 45 0.847 79.36 0.78 66.87 0.681 60.56 0.634 53.56
3 2 45 0.696 60.92 0.66 52.13 0.628 46.41 0.622 41.78
4 2 55 0.881 69.83 0.82 59.67 0.731 54.09 0.674 45.68
5 2 45 0.696 60.92 0.66 52.13 0.628 46.41 0.622 41.78
6 1.5 55 0.815 52.38 0.81 46.21 0.671 38.39 0.638 33.42
7 1.5 45 0.662 48.42 0.58 44.62 0.542 34.52 0.508 27.82
8 2 45 0.696 60.92 0.66 52.13 0.628 46.41 0.622 40.78
9 2 35 0.624 56.78 0.58 49.13 0.571 43.35 0.548 39.45
10 2 45 0.696 60.92 0.66 52.13 0.628 46.41 0.622 41.78
11 2 45 0.696 60.92 0.66 52.13 0.628 46.41 0.622 41.78
12 1.5 35 0.521 38.52 0.43 37.05 0.395 26.12 0.381 20.75
13 2.5 55 1.031 90.48 0.95 84.51 0.881 54.36 0.733 66.42

Fig. 11 and Fig. 12 show the 3-D diagrams of SEA and PCF at different geometrical and loading
conditions. They illustrate the influence of geometrical changes on crashworthiness performance in various
impact angles. For all impact loading conditions, by increasing t and L the amount of SEA1-4 and PCF1-4
increased. Conclusion, enlarging t and L have a negative effect on PCF and in contrast a positive effect on SEA
in the current study for the s-shaped members of Type 4.
Figure 11: Variation of SEA1-4 with L& t for the impact angles; (a) , (b) , (c) , and (d)
RSM is employed to predict the crashworthiness indices of SEA and PCF of the T4 tubes under axial
and oblique impact loading (0°, 10º, 20°, and 30º). These indices are then to be maximized or minimized for
optimization. The desirability objective function vs design variables L and t is shown in Figs. 13. It is evident in
these figures that the optimal points meet the same geometrical parameters properties, which are as follows:
L=55 mm, and t=1.5 mm. In general, increasing the thickness of the tube and increasing the blade length show
the way to optimum design with the limits of the current study introduced in Table 4.
VI. CONCLUSIONS
In this paper the crashworthiness characteristics of single, doubly and multi-cell S-shaped tubes with
octagonal cross-section (totally seven structures) and with the presence of five blades to connect inner and outer
tubes in bitubal structures have been investigated. The loading angles ranges from 0º to 30º. Moreover, non-
linear finite element code LS-DYNA is employed to establish the model. The model is validated by
experimental results for the simple extrusion circular tubes under axial dynamic loading.

Figure 12: Variation of PCF1-4 with L& t for the impact angles; (a) , (b)
, (c) , and (d)
Figure 13: Ideal surface of desirability for the design variables
Due to presence of internal end parts offset, all the s-shape tubes deform as global bending multiple
impact angle, leading to the decrease in SEA and PCF as two conflicting criteria in this paper. The complex
proportional assessment method (COPRAS) is chosen for the multi-criteria decision making process (MCDM)
by considering beneficial and non-beneficial criteria. It is extracted that the bitubal S-shaped tubes with mid-
edge blades connector (namely T4) has got high crashworthiness. On the other hand, the number of corners (N)
in section shape and the rotation of inner tube in bitubal members play a significant role to improve the crushing
performance. Within the scope of the present study, more corners are counted in bitubal members with the
blades connecting mid-edge leads to absorb more energy than members with the blades connecting corner-edge.
Design of experiment (DOE) method and response surface methodology (RSM) are considered for all impact
angles to describe the effects of geometrical parameters changes on the T4. The results shows that increasing the

tube blade length and decreasing the tube wall-thickness resulted in the optimum design, as per the desirability
objective function.
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