September 2005

NASA/CR-2005-213911

Enhancements of Tow-Steering Design

Techniques: Design of Rectangular Panel

Under Combined Loads

Brian F. Tatting, and Shahriar Setoodeh

ADOPTECH, Inc., Blacksburg, Virginia

Zafer Gürdal

Virginia Polytechnic Institute and State University, Blacksburg, Virginia
The NASA STI Program Office . . . in Profile

Since its founding, NASA has been dedicated to the

advancement of aeronautics and space science. The

NASA Scientific and Technical Information (STI)

Program Office plays a key part in helping NASA

maintain this important role.

The NASA STI Program Office is operated by

Langley Research Center, the lead center for NASA’s

scientific and technical information. The NASA STI

Program Office provides access to the NASA STI

Database, the largest collection of aeronautical and

space science STI in the world. The Program Office is

also NASA’s institutional mechanism for

disseminating the results of its research and

development activities. These results are published by

NASA in the NASA STI Report Series, which

includes the following report types:

• TECHNICAL PUBLICATION. Reports of

completed research or a major significant phase

of research that present the results of NASA

programs and include extensive data or

theoretical analysis. Includes compilations of

significant scientific and technical data and

information deemed to be of continuing

reference value. NASA counterpart of peerreviewed formal professional papers, but having

less stringent limitations on manuscript length

and extent of graphic presentations.

• TECHNICAL MEMORANDUM. Scientific

and technical findings that are preliminary or of

specialized interest, e.g., quick release reports,

working papers, and bibliographies that contain

minimal annotation. Does not contain extensive

analysis.

• CONTRACTOR REPORT. Scientific and

technical findings by NASA-sponsored

contractors and grantees.

• CONFERENCE PUBLICATION. Collected

papers from scientific and technical

conferences, symposia, seminars, or other

meetings sponsored or co-sponsored by NASA.

• SPECIAL PUBLICATION. Scientific,

technical, or historical information from NASA

programs, projects, and missions, often

concerned with subjects having substantial

public interest.

• TECHNICAL TRANSLATION. Englishlanguage translations of foreign scientific and

technical material pertinent to NASA’s mission.

Specialized services that complement the STI

Program Office’s diverse offerings include creating

custom thesauri, building customized databases,

organizing and publishing research results ... even

providing videos.

For more information about the NASA STI Program

Office, see the following:

• Access the NASA STI Program Home Page at

http://www.sti.nasa.gov

• E-mail your question via the Internet to

help@sti.nasa.gov

• Fax your question to the NASA STI Help Desk

at (301) 621-0134

• Phone the NASA STI Help Desk at

(301) 621-0390

• Write to:

 NASA STI Help Desk

 NASA Center for AeroSpace Information

 7121 Standard Drive

 Hanover, MD 21076-1320
National Aeronautics and

Space Administration

Langley Research Center Prepared for Langley Research Center

Hampton, Virginia 23681-2199 under Contract NAS1-00135

September 2005

NASA/CR-2005-213911

Enhancements of Tow-Steering Design

Techniques: Design of Rectangular Panel

Under Combined Loads

Brian F. Tatting, and Shahriar Setoodeh

ADOPTECH, Inc., Blacksburg, Virginia

Zafer Gürdal

Virginia Polytechnic Institute and State University, Blacksburg, Virginia
Available from:

NASA Center for AeroSpace Information (CASI) National Technical Information Service (NTIS)

7121 Standard Drive 5285 Port Royal Road

Hanover, MD 21076-1320 Springfield, VA 22161-2171

(301) 621-0390 (703) 605-6000
1

Abstract

An extension to existing design tools that utilize tow-steering is presented which is used to

investigate the use of elastic tailoring for a flat panel with a central hole under combined loads of 

compression and shear. The elastic tailoring is characterized by tow-steering within individual

lamina as well as a novel approach based on selective reinforcement, which attempts to minimize 

compliance through the use of Cellular Automata design concepts. The selective reinforcement

designs lack any consideration of manufacturing constraints, so a new tow-steered path definition

was developed to translate the prototype selective reinforcement designs into manufacturable 

plies. The minimum weight design of a flat panel under combined loading was based on a model

provided by NASA-Langley personnel and analyzed by STAGS within the OLGA design

environment. Baseline designs using traditional straight fiber plies were generated, as well as

tow-steered designs which incorporated parallel, tow-drop, and overlap plies within the laminate.

These results indicated that the overlap method provided the best improvement with regards to

weight and performance as compared to traditional constant stiffness monocoque panels, though

the laminates did not measure up to similar designs from the literature using sandwich and isogrid

constructions. Further design studies were conducted using various numbers of the selective 

reinforcement plies at the core and outer surface of the laminate. None of these configurations

exhibited notable advantages with regard to weight or buckling performance. This was due to the 

fact that the minimization of the compliance tended to direct the major stresses toward the center

of the panel, which decreased the ability of the structure to withstand loads leading to instability.
2

1.0 Task Description 

The subject of this report concerns the enhancement of proven tow-steering design techniques for 

laminated composite structures1-4 through the implementation of novel weight-reduction methods based 

on a Cellular Automata (CA) design methodology. The structural design problem used as a basis of the 

study involves a flat rectangular panel under combined compression and shear loading, for which a test 

fixture and alternate designs were previously available.5

 Besides introducing the CA design concept, 

which resembles internal stiffening of the laminate and will be referred to as selective reinforcement, the 

results will also investigate designs using the standard tow-steering techniques developed in the earlier 

references. Comparisons of the optimized designs toward existing sandwich and isogrid panel 

constructions will be used to indicate if manufacture and testing of the tow-steered/selective 

reinforcement designs is warranted in the future. 

2.0 Problem Definition and Model Synthesis 

The problem under consideration consists of a flat panel with a 3-inch diameter central hole loaded by 

combined forces of compression and shear. A complete STAGS model detailing the structure under 

realistic test conditions was supplied to ADOPTECH by NASA-Langley personnel, and subsequently 

transformed into a slightly simpler model (in terms of edge conditions and additional elements required 

for accurate modeling) to facilitate the design techniques. This section describes the geometry of the 

problem and the associated simplifications to the model, while more detail of the model synthesis is 

supplied in the appendix. 

A three-dimensional representation of the finite element model is shown below in Figure 1 (without the 

central hole for simplicity). The model consists of a test panel (the regular grid in the center region) 

attached on each side to a picture-frame-type fixture that is connected by pins, which is attached to 

appropriate loading frames to achieve the combined loading stress state. During testing, the top edge of 

the upper frame is held fixed, while the bottom frame undergoes a vertical force from below and a 

horizontal force through an L-extension of the bottom frame (not shown here). This results in a horizontal 

force as well as a bending moment, which is represented in the STAGS model by the force couple shown. 

The test panel area is 24 inches wide and 28 inches high, with a grid size of one-half inch in both 

directions. The original model is assumed to be composed of aluminum (E = 10.0 Msi, ν = 0.33) with a 

X

Y

Top edge fixed Z

Bottom edge moves as one unit

9600 lb force

17,803 lb force

23,563 lb force

Figure 1: Geometry of Combined Loading Test Fixture and Specimen 
3

thickness of 0.125 inches. One element is removed at each corner of the test coupon to alleviate stress. 

Tabs are present on all four sides. The top and bottom tabs measure 23 inches wide (due to the 1/2 cut-out 

in each corner) by 2.94 inches high, while the sides are 27 inches high and 2.25 inches wide. The tabs are 

constructed with a layer of steel (E = 30.0 Msi, ν = 0.30, 0.19 inches thick) bonded to each side (though 

the tab material may change for composite materials, we can assume that one will be present and will use 

the steel tab assumption hereafter). The test panel and tabs are shown in Figure 2 below with the other 

components removed. The origin of the axes is in the bottom left corner of the test panel. Note that the 

meshing within the tabbed regions is quite erratic, which is due to the fact that earlier models possessed 

higher density meshes within the test panel region along with the fact that certain points within the tab 

had to be defined as nodes. These nodes are denoted by the colored dots in the figure, and represent the 

points of attachment between the tabbed regions and the fixture. The blue dots represent nodes in the tabs 

which are defined within the model as having identical displacements (horizontal u, vertical v, and out-ofplane w) as the corresponding point in the frame. The green dots are similarly defined, but are only 

constrained in the u and w displacement directions (the connections in the side fixtures are slotted to 

allow vertical movement). The red dots shown here represent the pinned connections between horizontal 

and vertical components of the fixture, and indicate that the displacements for each connected component 

are identical at that point (note that no connection to the test panel or tabbed areas exist for these red 

points, but are shown for completeness). 

Several stages of test model simplifications were used to simplify the model for design use. This mostly 

consisted of replacing the loading frame structure with equivalent boundary conditions and/or shell wall 

construction definitions so that the physics was still accurately modeled while minimizing the number of 

elements in the model. Implementation of these changes is catalogued in the appendix, and results in the 

model shown in Figure 3 for use within the design environment. 

X

Y

0 10 20 30

-5

0

5

10

15

20

25

30

35

Figure 2: Original Mesh of Laminate and Tabbed Regions (hole not shown) 
4

STAGS analysis produces the following results for this model. The numbers in parentheses represent the 

percent change from the original model for the important quantities, with the most relevant quantities 

being the eigenvalue and computation time: 

These results satisfy the goals of the model synthesis, providing a suitably accurate model with a 

significant decrease in computation time. More detail concerning the attributes of this model and the 

alterations from the original configuration (Figure 2) are provided in the appendix. 

3.0 Ply Design using Cellular Automata Solutions 

For the Cellular Automata (CA) design study, an equivalent model was constructed using the same 

dimensions and boundary conditions as the STAGS model (along with a 3-inch diameter central hole). 

Additionally, the mesh spacing within the tabbed regions was re-ordered with 1⁄2-inch square elements to 

facilitate definition within the Cellular Automata design technique. The CA design technique used the 

equivalent model to estimate the stiffness response of the structure subjected to axial compression and 

shear (the shear being applied as two different load cases in opposite directions). Topology methods were 

then used to reduce the weight of the structure (which was assumed to be constructed of a single ply) to a 

X

Y

0 10 20

0

5

10

15

20

25

30

Figure 3: Idealized Mesh of Laminate and Tabbed Regions 

STAGS results for ideal model:

u-displacement of top edge (x=12, y=28) = -0.000072 

u-displacement of bottom edge (x=12, y=0) = -0.022836 

Relative u-displacement = -0.022764 (2.6%)

v-displacement of top edge (x=12, y=28) = 0.0000302 

v-displacement of bottom edge (x=12, y=0) = 0.0061293 

v-displacement of bottom edge node (x=21.5, y=0) = 0.0060623 

Relative rotation of bottom edge = -7.053×10-6 radians 

Relative v-displacement = 0.0060991 (7.5%)

Eigenvalue of linear buckling solution = 0.71818 (2.0%)

Computation time (sec) = 49.8 (-70.8%)
5

specified volume fraction while minimizing the compliance of the structure. Note that no buckling 

estimations were used here; the designs were motivated by the topology design based on minimum 

compliance design. Further details of the topology design problem using this approach can be found in 

references 6 and 7. 

The problem of minimum compliance design was initially solved using fiber angles as continuous spatial 

design variables. Numerical experiments showed that substantial stiffness improvements can be gained 

only by re-orientation of the orthotropic material in an optimal manner. However, this formulation 

requires repeated transformation of the material properties using the classical trigonometric 

transformations. These transformations are not only expensive in terms of the computational costs, but 

also can cause convergence problems. A more rigorous formulation for the same compliance design 

problem is to use the lamination parameters as design variables instead of the fiber angles. The in-plane 

behavior of balanced symmetric laminates can be fully modeled using only two lamination parameters 

regardless of the actual number of layers. Moreover, the laminate extensional stiffness matrix is linear in 

terms of the lamination parameters. It can also be shown that compliance is convex in the lamination 

parameters space. These characteristics are very beneficial in the optimal design of laminated composites 

in the sense that they substantially improve the computational efficiency as well as the accuracy. 

However, transformation of these lamination parameter designs into manufacturable tow-steered plies is 

an intricate process that does not always lead to a viable solution. Furthermore, the results using the 

lamination parameters did not produce designs with lower compliance as compared to the method based 

on fiber orientation angles. Therefore, the design study implemented the fiber angle methodology and was 

limited to one-ply configurations, which could then later be transformed into viable candidates for 

stacking sequence design. 

During the design study, the CA design methodology was updated in several respects to facilitate better 

design results. As mentioned, one such improvement was to include multiple load cases (both positive and 

negative shear) within the analysis portion. This improvement also contained procedures to implement the 

combined in-plane bending/compression of the panel through a linear combination of displacement 

loading, which was required to attain the correct level of accuracy as compared to the original STAGS

model. Options considering the symmetry of the design were also considered, so that the user could 

decide if symmetry was required in the vertical or horizontal directions. Local balancing could be 

stipulated using the lamination parameters formulation, though the results are not shown here due to the 

inferiority of the designs using this constraint. 

3.1 Candidate Designs for Selective Reinforcement 

Using the available options within the CA design software, the following results were obtained for the 

problem under study. Note that the figures are for one ply only, and manufacturability has not been taken 

into consideration yet. The colors of the elements represent the fiber orientation angle, which is further 

displayed by the directional lines at the center of each element. The compliances and volume fractions for 

each design is given within the table. Designs in the first column were constrained to be symmetric in 

both the x- and y-directions, while for the second column the y-symmetry condition was removed. 
6

Compliance = 303.21 N·m Compliance = 331.43 N·m 

70% Volume Fraction 

Compliance = 441.52 N·m Compliance = 456.54 N·m 

50% Volume Fraction 
7

Compliance = 698.42 N·m Compliance = 612.92 N·m

30% Volume Fraction 

Compliance = 985.73 N·m Compliance = 766.66 N·m

20% Volume Fraction 
8

3.2 Manufacturing Concerns and Tow-Steering Definitions 

As opposed to typical tow-steered design techniques used by ADOPTECH’s OLGA software, the CA

design algorithm does not take manufacturability into account during the design process. This is quite 

evident in the results of the previous table, which indicate idealized ply orientation angles that would be 

difficult, if not impossible, to construct using real-world fabrication techniques. Therefore, the candidate 

designs were inspected by hand to determine which candidates were best suited for slight alterations that 

would enable manufacture. This task was completed prior to the next design step, so that only 

manufacturable designs are considered in the final design process. 

From the results of the previous table, it was decided that the designs with the higher volume fraction 

(70% and 50%) were too irregular for a suitable transformation to a manufacturable design. This was also 

true for the full coverage design (not shown), which resulted in patches of differing fiber orientation angle 

throughout the ply. Though some improvements with regard to manufacturability can be specified within 

the CA algorithm (such as limiting the orientation angles to discrete values), the resulting designs still 

contain these smaller patches which require significant tow-dropping within each ply. When implemented 

with an advanced tow-placement machine, the minimum cut length parameter associated with the 

cut/restart capability of the manufacturing process becomes a critical constraint which cannot always be 

satisfied by the idealized configurations. The minimum turning radius constraint also needs to be 

considered when variations in fiber orientation angle along a path were desired. Therefore, attention was 

focused on the remaining designs (30% and 20% volume fractions), which had definitive paths that were 

more conducive to tow-placement methods. 

3.2.1 Tow-steered definition based on Piecewise Arc formulation 

With regard to the 30% and 20% volume fraction CA designs, it is evident that the plies could be 

approximated by tow-steered paths of varying widths. Therefore, a new tow-steering path definition was 

formulated to reflect the variations that were required from the CA results and inserted into the catalog of 

tow-steering definitions already developed by ADOPTECH.1,3 The new path definition is dubbed 

“Piecewise Arc”, and is at the same level within the hierarchy as the “Linear Angle” and “Circular Arc” 

definitions referred to in the references. 

It is evident that the main mechanism for the 20% and 30% volume fraction designs is paths that tend to 

surround the central hole. Upon further inspection, various curvilinear paths could be constructed by hand 

to adequately simulate the desired ply coverage. However, it was easily seen that most of the 

constructions violated the curvature constraint for the tow-placement machine (here assumed to be a 

turning radius of 25 inches). Therefore, two options existed to model the given designs: to use multiple 

straight fiber paths with significant tow-dropping; or to use curvilinear plies with maximum allowable 

curvature that would approximate the desired configuration. The choice of curvilinear paths was 

motivated by the fact that (1) the straight fiber paths for the given designs might also have problems with 

the minimum cut length parameter; (2) the amount of tow-dropping required would lead to a slow 

manufacturing process for the ply; and (3) paths that run from one boundary of the structure to another 

constitute a more reliable structural configuration. 

Therefore, suitable curvilinear paths were considered for the modeling of these designs. Based upon 

earlier work, simple constructions based on constant curvature arcs were introduced. Like earlier 

definitions, these constructions used a handful of parameters to provide enable easy definition while still 

remaining general enough to produce a large variety of designs. The basic path, shown schematically in 

Figure 4 and hereby referred to as the piecewise arc reference path, assumes that a straight line is 

interrupted by four arcs of constant curvature that are intended to curve around the region of interest. The 

user-defined parameters are the characteristic distance d, which must be positive and represents the length 

of the opening (d = d0 + d1 in the figure), and two angles which represent spans of the arc. These angles 

must be between -90 and 90 degrees. The path consists of a straight line merging into a constant radius 
9

arc (as an option, the radius r of the arcs may be input instead of d), which opens concave upward or 

downward depending on the sign of T0. After the prescribed arc is completed, the path returns to the 

horizontal through an identical arc. At that point, another arc is begun, this time turning through T1

degrees and back again until the horizontal is again reached. The line then continues ad infinitum. 

The relevant equations defining the radius of the arcs and the width of the opening are given as: 

( )

( ) () ( ) ()

010 0 1 1

0 1

max 0 0 1 0 0

2 sin 2 sin

2 sin sin

2 1 cos sgn 2 cos cos sgn

dd d d r T d r T

d

r

T T

h r T T hr T T T

=+ = =

= +

= − ∆= −

 (1) 

Note that for T1 = -T0 (the most usual configuration), 0 rd T = / 4sin and ∆ = h 0 . Perfectly straight lines 

are stipulated with zero values for T0 and T1. 

To construct a complete ply course, a head width hw is supplied and parallel curves at a distance ±hw/2 

are created that represent the course edges. Additionally, a path mirrored about the x*-axis is constructed 

to complete the pair of paths (using negative values for the angle parameters). The vertical location of the 

mirrored path is determined by the value of the overlap parameter, so that the paths can be collinear or 

possess a gap between the horizontal lines. Where overlapping occurs, the choice of tow-dropping or 

thickness variation can be set by the user, and the locations of tow-drops are determined to maximize the 

cut length along a tow. A sample of a ply using these techniques combining the two mirrored paths is 

shown in Figure 5. 

y* 

x* ∆h

r

d0

T0

T1

d1

hmax

Figure 4: Piecewise Arc Reference Path 

y*

x* (1-overlap)hw/2

r

T0

T1

d

y*

x* (1-overlap)hw/2

r

T0

T1

d

Figure 5: Mirrored Paths using the Piecewise Arc Reference Path 
10

Lastly, the set of two courses can be positioned and rotated within the structure through the use of a global 

translation and rotation of the x*y*-axis, as defined for other path variations in earlier work (parameters 

{x0, y0} and φ). Algorithms exists that can extract the relevant fiber orientation information for a given 

point within the surface of the laminate. Balanced plies can also be included in the construction of the 

laminate, which automatically provide some symmetry and ensure that continuous paths are constructed 

equally in both directions. Some examples of this tow-steered ply definition are shown in Figure 6, where 

the origin of the stiffness variation is assumed to be in the center of the panel. 

Symmetric ply example

 Tow Drop method 

 Origin = {0, 0} 

φ = 30°

d = 25" 

T0 = 15°

T1 = -15°

overlap = 1 

hw = 2" 

Unsymmetric ply example

 Tow Drop method 

 Origin = {0, -3.35"} 

φ = 0°

d = 25" 

T0 = 15°

T1 = 0°

overlap = -2 

hw = 2" 

Interlocking straight path example

 Overlap method 

 Origin = {5", 2"} 

φ = 25°

T0 = T1 = 0°

overlap = -2 

hw = 2" 

Figure 6: Examples of Piecewise Arc Construction Method 
11

3.2.2 Tow-steered plies from candidate CA designs 

With the piecewise arc formulation in hand, parameters are chosen to best resemble the candidate designs 

found from the CA design study in Section 3.1. For these definitions, a minimum cut length of 5 inches 

and a minimum turning radius of 25 inches is assumed according to specifications from a typical towplacement machine. Vertically symmetric plies will also be formulated, where needed, to fully explore the 

possibilities for the selective reinforcement plies. 

Ply Set #1: 30% Straight

 (a) Overlap method 

 Origin = {0, 11"} 

φ = 67.5°

T0 = T1 = 0°

overlap = 1 

hw = 3" 

 (see Figure 7) 

 (b) Identical except for 

 Origin = {0, 0} 

overlap = -1.81 

 (vertically symmetric, similar to 

 Figure 8) 

Ply Set #2: 20% Straight

 (a) Overlap method 

 Origin = {0, 11"} 

φ = 66°

T0 = T1 = 0°

overlap = 1 

hw = 2.25" 

 (similar to Figure 7) 

 (b) Identical except for 

 Origin = {0, 0} 

overlap = -2.98 

 (see Figure 8) 

Ply Set #3: 30% Curvilinear

 (a) Overlap method 

 Origin = {0, 0} 

φ = 54.375°

d = 25.88" 

T0 = 15°

T1 = -15°

overlap = 1 

hw = 1.5" 

 (see Figure 9) 

 (b) Identical except uses Tow Drop method 

 (similar to Figure 10) 

Figure 7: Ply 1a 

Figure 8: Ply 2b 

Figure 9: Ply 3a 
12

Ply Set #4: 20% Curvilinear

 (a) Overlap method 

 Origin = {0, 0} 

φ = 54.375°

d = 25.88" 

T0 = 15°

T1 = -15°

overlap = 1 

hw = 1" 

 (similar to Figure 9) 

 (b) Identical except uses Tow Drop method 

 (see Figure 10) 

4.0 Design Study Formulation and Results 

The goal of the design study is to ascertain the weight savings potential of the tow-steering and selective 

reinforcement design schemes for the combined loading configuration. Due to the limitations of the 

STAGS8 finite element code (version 4), material failure constraints are not able to assessed when using 

the tow-steered ply definitions (through the WALL user-written subroutine that defines the stacking 

sequence). Therefore the design study will consider only the “linear’ buckling loads as the limiting load 

constraints. Though significant post-buckling deformation will occur before ultimate failure for this 

structure, the computation time associated with the complex nonlinear solutions are not warranted in this 

exploratory design phase. 

Published literature5

 associated with the test fixture used here has demonstrated that sandwich panels can 

withstand the designated loads while achieving weight densities as low as 0.49 lb/ft2

. Therefore the ideal 

goal of the design study is to find non-sandwich laminates that utilize tow-steering and remain 

competitive with these weight densities. Unfortunately, sandwich panels possess a tremendous advantage 

as compared to monolithic laminates when only buckling is considered (due to the high bending stiffness 

produced by the relatively thick sandwich core), so that reaching this goal is not realistic. Instead, it was 

decided to find the best configurations using these novel design techniques that do not use sandwich 

construction and compare the results to traditional straight fiber laminates. Future work could later 

integrate the tow-steering concepts within the face sheet if sandwich construction is warranted. 

4.1 Design Study Roadmap 

The approach used to investigate the design space afforded by the tow-steering and selective 

reinforcement options is built upon improving from baseline constant stiffness and unstiffened designs. 

The laminate will assumed to be constructed of multiple layers of IM7/8552 composite, with a ply 

thickness of 0.004 inches and a density of 98.5 lb/ft3

 (0.057 lb/in3). The laminate is assumed to be 

symmetric about the mid-plane, and laminate balance is achieved automatically by the laminate design 

software by using balanced two-ply stacks for all ply angle variables. The selective reinforcement plies 

can be situated either on the interior or the exterior of the laminate, but their number and configuration do 

not vary within the design process. That is, the type and number of selective reinforcement layers are 

chosen and then the rest of the laminate (using either traditional straight plies or tow-steered 

configurations) is designed around the base laminate. Within the OLGA design process, weight reduction 

is performed using a Genetic Algorithm developed specifically for stacking sequence optimization, which 

takes into account balancing and symmetry and strives to find the laminate with the smallest number of 

layers that satisfy the buckling load constraint. Ply angle variables are generally in increments of 15°, 

Figure 10: Ply 4b 
13

though for tow-steered plies some candidate plies are not considered due to violation of the 

manufacturing constraints (also calculated automatically by the design software). Results will be reported 

in terms of the stacking sequence for each laminate, the weight density for the structure, and the load 

factor for instability (a value less than one indicates that buckling is achieved, while larger values indicate 

that the structure buckles at a higher level than the design load). Design runs on a Windows-based 

computer require around six hours to complete, therefore some thought is required so as to minimize the 

number of runs while adequately investigating the design space. 

The “roadmap” to find the best design follows the diagram shown in Figure 11. First, a baseline design 

using only straight fiber plies within the laminate is determined. Secondly, tow-steered plies are 

introduced within the unstiffened laminate, using previous construction techniques and various stiffness 

variation directions. These results should indicate the tow-steered construction techniques and preferred 

directions that show the most promise. The next step investigates the influence of the eight selective 

reinforcement configurations (ply sets 1a-4b) inserted into the interior of the laminate and surrounded by 

constant stiffness plies. Note that locating the pseudo-stiffeners at the interior of the laminate is preferred 

to ensure a more robust external surface for the manufactured laminate. The selective reinforcement plies 

that provide the best increase in performance will then be used in conjunction with the tow-steered plies 

as the external laminate, to explore non-intuitive regions of the design space. At this point, there should be 

strong indications as to the best stiffener arrangement and tow-steered construction technique. The last 

step will be to determine the effect of increasing the number of plies within the internal stiffener and to 

investigate the possibility of locating the selective reinforcement plies on the outside of the laminate 

(even though this construction technique may lead to an inferior laminate construction). This strategy 

should provide a useful reflection of the available design space and point to the potential sources of 

performance gains. 

4.2 Results and Discussion 

Baseline Case:

The baseline design case using straight fiber plies only resulted in a 36-ply laminate with stacking 

sequence [±45/±303/±155]s, possessing a weight density of 1.182 lb/ft2

 with a buckling load factor of 

1.104. As hinted earlier, this weight density is not comparable to the lightest weight sandwich panel from 

the literature, but serves as the base comparison for our novel laminate design techniques. 

Figure 11: Roadmap for Design Study Organization 

Straight fiber 

design 

Tow-steered

fiber design 

Straight fiber/

single layer 

stiffener design

Tow-steered/

single layer 

stiffener design

Straight fiber/

multiple layer

stiffener design

Select towsteered fiber/

multiple layer 

stiffener design 

2 

5 6 

3 4 

1 
14

Tow-steered designs with no selective reinforcement:

Six distinct tow-steered construction techniques were used for this portion of the study. Three different 

ply construction methods are used, namely Parallel, Tow Drop, and Overlap, with the major direction of 

variation being either horizontal or vertical for each case with the variation axis located at the center of 

the hole. The Circular Arc formulation (see reference 3) was used as the reference path. The characteristic 

distance d that is used to define the tow-steering variation was chosen as half of the width or height 

according to the major variation direction. Using the standard minimum turning radius of 25 inches and 

15° angle increments generated the following manufacturable designs for each case (as well as the 

constant stiffness plies for all cases, which are the only plies allowed on the laminate outer surface): 

 Parallel method 

Horizontal variation: 0<0|15>, 0<0|30>, 0<15|0>, 0<15|30>, 0<15|45>, 0<30|15>, 

0<30|45>, 0<30|60>, 0<45|15>, 0<45|30>, 0<45|60>, 0<45|75>, 0<60|30>, 

0<60|45>, 0<60|75>, 0<75|45>, 0<75|60> 

Vertical variation: 90<-75|-60>, 90<-75|-45>, 90<-75|-30>, 90<-60|-75>, 90<-60|-45>, 

90<-60|-30>, 90<-45|-75>, 90<-45|-60>, 90<-45|-30>, 90<-45|-15>, 90<-30|-75>, 

90<-30|-60>, 90<-30|-45>, 90<-30|-15>, 90<-30|0>, 90<-15|-45>, 90<-15|-30>, 

90<-15|0>, 90<-15|15>, 90<0|-30>, 90<0|-15> 

 Tow Drop and Overlap methods 

Horizontal variation: 0<0|15>, 0<0|30>, 0<15|0>, 0<15|30>, 0<15|45>, 0<15|-15>, 

0<30|0>, 0<30|15>, 0<30|45>, 0<30|60>, 0<30|75>, 0<45|15>, 0<45|30>, 

0<45|60>, 0<45|75>, 0<60|30>, 0<60|45>, 0<60|75>, 0<75|30>, 0<75|45>, 

0<75|60> 

Vertical variation: 90<-75|-60>, 90<-75|-45>, 90<-75|-30>, 90<-60|-75>, 90<-60|-45>, 

90<-60|-30>, 90<-60|-15>, 90<-45|-75>, 90<-45|-60>, 90<-45|-30>, 90<-45|-15>, 

90<-30|-75>, 90<-30|-60>, 90<-30|-45>, 90<-30|-15>, 90<-30|0>, 90<-15|-60>, 

90<-15|-45>, 90<-15|-30>, 75, 90<-15|0>, 90<-15|15>, 90<0|-30>, 90<0|-15> 

The associated weight densities and load factors for the six different tow-steered construction techniques 

are displayed in Figure 12. Note how the weight densities are the same for most of the construction Weight density (lbs/ft2

)

Load Factor

0

0.5

1

0

0.5

1

1.5

Baseline

Straight

Horz. Horz. Horz.

Parallel Tow Drop Overlap

Vert. Vert. Vert.

Figure 12: Design Results for Tow-Steered Plies with no Selective Reinforcement 
15

methods, which indicates that the same number of constant thickness plies are used for each design. For 

the overlap method, the horizontal variation actually converges to a constant stiffness design, while the 

vertical variation has only 32 plies as well as added weight due to the extra thickness within the overlap 

regions. The other significant data from the figure concerns the load factor as compared to the baseline 

design. For the Parallel method with a horizontal stiffness variation (stacking sequence 

[±30/0±<45|15>3/0±<15|0>/0±<45|15>/0±<0|30>/0±<30|15>/0±<0|30>]s) and the Tow Drop method with 

a vertical stiffness variation ([±30/90±<-75|-30>2/90±<-75|-45>2/±15/90±<-75|-60>3]s) the buckling loads 

are significantly higher than the baseline case, indicating better performance using these configurations. 

Similarly, though the Overlap method with a vertical stiffness variation has a lower load factor, the fact 

that the laminate is lighter in weight while still satisfying the constraints indicate the worthiness of the 

design. The layup for the best Overlap method design is [±45/90±<-60|-30>/±302/04/90±<-75|-60>/02]s. 

Straight fiber designs with single pair of selective reinforcement plies (interior of laminate)

The first test of the selective reinforcement plies uses a single pair (due to symmetry) of stiffeners at the 

mid-plane of the laminate. Each tow course has a thickness of 0.004 inches, though there do exist local 

thickness build-ups within overlaps of the courses (for all selective reinforcement plies except 3a and 4a 

since they utilize tow drops). The rest of the laminate is designed using the standard constant stiffness 

two-ply stacks in 15° angle increments. The results are shown in Figure 13, and indicate that the selective 

reinforcement plies (using a single pair at the interior of the laminate) offer no noticeable improvement 

over the baseline case. In fact, the introduction of the stiffening regions tends to degrade the performance 

of the structure, since the load factors are not improved while the weights are slightly increased. 

Tow-steered designs with single pair of selective reinforcement plies (interior of laminate)

Similar results to those in Figure 12 can be generated for each of the tow-steered ply construction 

techniques using a different selective reinforcement scheme. Relevant results for cases 1a, 3b, and 4b are 

shown in Figure 14-Figure 16, respectively. For all three cases, the stiffness variation that shows the most 

promise corresponds to the Overlap method with a vertical stiffness variation, while the 

Parallel/horizontal and Tow Drop/vertical variations exhibit slight increases in the buckling load factor. Weight density (lbs/ft2

)

Load Factor

0

0.5

1

0

0.5

1

1.5

Baseline

Unstiffened

b

Ply #1 Ply #2 Ply #3 Ply #4

a b a b a b a

Figure 13: Design Results for Straight Fiber Laminate with Single Pair of 

Selective Reinforcement Plies (at Interior of Laminate) 
16

However, for the most part weight improvement for all those except the Overlap/vertical variation does 

not exist. 

Weight density (lbs/ft2

)

Load Factor

0

0.5

1

0

0.5

1

1.5

Straight

Horz. Horz. Horz.

Parallel Tow Drop Overlap

Vert. Vert. Vert.

Figure 14: Design Results for Tow-Steered Laminates with Single Pair of 1a 

Selective Reinforcement Plies (at Interior of Laminate) 

Weight density (lbs/ft2

)

Load Factor

0

0.5

1

0

0.5

1

1.5

Straight

Horz. Horz. Horz.

Parallel Tow Drop Overlap

Vert. Vert. Vert.

Figure 15: Design Results for Tow-Steered Laminates with Single Pair of 3b 

Selective Reinforcement Plies (at Interior of Laminate) 
17

The results can also be viewed by choosing the best tow-steered construction technique and examining 

the effect of the selective reinforcement plies. This is displayed in Figure 17 below for the Overlap 

construction technique using a vertical variation of the stiffness. 

Note how this compares to Figure 13. Now the versatile overlap method generates noticeable differences 

between the different selective reinforcement plies. However, it should be noted that the best design in Weight density (lbs/ft2

)

Load Factor

0

0.5

1

0

0.5

1

1.5

Straight

Horz. Horz. Horz.

Parallel Tow Drop Overlap

Vert. Vert. Vert.

Figure 16: Design Design Results for Tow-Steered Laminates with Single Pair of 4b 

Selective Reinforcement Plies (at Interior of Laminate) 

Weight density (lbs/ft2

)

Load Factor

0

0.5

1

0

0.5

1

1.5

Unstiffened

b

Ply #1 Ply #2 Ply #3 Ply #4

a b a b a b a

Figure 17: Design Results for Overlap/Vertical Variation Laminate with 

Single Pair of Selective Reinforcement Plies (at Interior of Laminate) 
18

this group is for the unstiffened laminate, thereby indicating that the presence of the stiffening 

components still does not contribute to the performance of the structure. 

Designs with multiple pairs of selective reinforcement plies

The results of the two previous sub-sections only used a single pair of selective reinforcement plies within 

the laminate. Design runs were also conducted to investigate the effect of increasing the number of 

stiffening layers for each configuration. Results for the straight fiber laminate designs using the 4b 

selective reinforcement case are shown in Figure 18 (other stiffening arrangements exhibited similar 

results). Note how both the weight density and load factor increase as more stiffening plies are added to 

the interior of the laminate. Surprisingly, the number of plies within the laminate (ignoring the stiffening 

plies) is 36 for all designs, even though the load factor approaches a level 30% above the required design 

load. Above 12 pairs, the selective reinforcement plies constitute 40% of the thickness within the stiffened 

regions, which would surely degrade the laminate during tow-steering construction. 

Similar studies were completed for the best tow-steered construction technique, namely the Overlap 

method with the vertical stiffness variation. The results were similar as to the straight fiber designs, 

though at the maximum value of 12 pairs the laminate design did drop down to 28 plies (from 32, the 

standard solution for this construction technique). However, it is interesting to note that due to the added 

volume of the internal stiffeners, the actual weight density for the 28-ply tow-steered laminate with 12 

pairs of selective reinforcement plies was higher than the best unstiffened tow-steered design (the leftmost 

data point in Figure 17). 

Lastly, several additional design runs were performed stipulating that the stiffening regions were located 

on the external surface of the laminate. Constructing the laminate in this way produces a much higher risk 

of a poorly made laminate, since the irregular outer surface may lead to complications during the tow 

placement process and cure, therefore complete design results were not rigorously sought. For the few 

runs that were performed, no relative improvement was observed. 

Pairs of selective reinforcement plies

Weight density (lbs/ft2

)

Load Factor

0 2 4 6 8 10 12 0

0.5

1

0

0.5

1

1.5

Baseline

Figure 18: Design Results for Straight Fiber Laminate with 

Multiple Pairs of Selective Reinforcement Ply Group 4b 
19

4.3 Summary of Results 

The results presented in the previous section lead to several points concerning the usefulness of towsteering and selective reinforcement for the problem under study. 

• The monolithic (non-sandwich) designs are not able to compete with the weight density of sandwich 

panels (from the literature). This is due to the fact that only linear buckling was assessed during the 

design process (due to limitations of the provided finite element program), which accentuates the 

usefulness of the sandwich construction technique. Future work should remedy the analysis method or 

attempt elastic tailoring within the face sheets of a sandwich laminate. 

• For the combined loading case presented here, weight improvements using tow-steered plies were only 

demonstrated by implementing the Overlap method with the variation in the vertical direction. For all 

other tow-steered construction techniques and directions (Parallel and Tow Drop method in both 

directions, Overlap method in the horizontal direction), there were only slight, if any, improvements in the 

buckling load factor as compared to the straight fiber design. 

• Inclusion of selective reinforcement plies for the panel with a central hole did not improve the 

performance of the structure under the given loading conditions. This is generally due to the fact that the 

stiffening arrangements tended to generate load paths toward the middle of the panel, which in earlier 

work has been shown to diminish the load carrying capability of a panel under compressive loads. In 

fairness, the selective reinforcement ply designs were formulated to minimize compliance and did not 

consider instability of any kind. It is believed that if the load directions were reversed (that is, applying 

tension as opposed to compression), then the stiffener configurations would alleviate stress concentrations 

around the central hole and help to reduce the potential of material failure. 

References 

1. Tatting, B. F., and Gürdal, Z., Design and Manufacture of Elastically Tailored Tow Placed Plates, 

NASA CR 2002-211919, August 2002. 

2. Jegley, D., Tatting, B. F., and Gürdal, Z., “Optimization of Elastically Tailored Tow Placed Plates 

with Holes,” Proceedings of the the 44th AIAA/ASME/ASCE/AHS/ASC Structures, Structural Dynamics 

and Materials (SDM) Conference, Norfolk VA, April 2003, AIAA Paper No. 1420. 

3. Tatting, B. F., and Gürdal, Z., Automated Finite Element Analysis of Elastically Tailored Plates, 

NASA CR 2003-212679, December 2003. 

4. Jegley, D., Tatting, B. F., and Gürdal, Z., “Tow-Steered Panels with Holes Subjected to Compression 

or Shear Loading,” Proceedings of the the 46th AIAA/ASME/ASCE/AHS/ASC Structures, Structural 

Dynamics and Materials (SDM) Conference, Austin TX, April 2005. 

5. Baker, D. J., Ambur, D. R., Fudge, J., and Kassapoglou, C., “Optimal Design and Damage Tolerance 

Verification of an Isogrid Structure for Helicopter Application,” Proceedings of the the 44th

AIAA/ASME/ASCE/AHS/ASC Structures, Structural Dynamics and Materials (SDM) Conference, 

Norfolk VA, April 2003. 

6. Setoodeh, S., Gürdal, Z., and Abdalla, M. M., “Simultaneous Topology and Curvilinear Fiber Path 

Design of Composite Layers using Cellular Automata,” Proceedings of the the 45th

AIAA/ASME/ASCE/AHS/ASC Structures, Structural Dynamics and Materials (SDM) Conference, Palm 

Springs CA, April 2004. 

7. Setoodeh, S., Gürdal, Z., Abdalla, M. M., and Tatting, B. F. “Design of Variable Stiffness Laminates 

for In-Plane Loading Using Lamination Parameters,” Proceedings of the the 46th

AIAA/ASME/ASCE/AHS/ASC Structures, Structural Dynamics and Materials (SDM) Conference, 

Austin TX, April 2005. 
20

8. Rankin, C. C., Brogan, F. A., Loden, W. A., and Cabiness, H. D., “STAGS Users Manual,” Lockheed 

Martin Missiles & Space Co., Inc., Report LMSC P032594, June 2000. 
21

Appendix 

This appendix is intended to illustrate the steps taken to produce a useful finite element model that (a) 

incorporates as closely as possible the test conditions of a panel under combined loading; and (b) is small 

enough to be used for elastic tailoring design techniques. In practice, the goal is to reduce the complexity 

of the STAGS model supplied by Don Baker of NASA-Langley Research Center to improve the 

efficiency of the analysis step within the design process. The first part of this document examines the 

details of the supplied STAGS model (referred to herein as the test case), and highlights the relevant 

portions that need to be correctly modeled. The second section presents differing levels of complexity of 

simpler models (referred to as ideal) that hope to capture the essence of the full solution but which are 

appreciably smaller in size (with regard to number of nodes, elements, and/or structural components). 

Once a suitable ideal model is discovered, the third section details the assumptions used to generate a 

constant grid-size model that can be used within the Cellular Automata design paradigm. 

STAGS Finite Element Model 

A three-dimensional representation of the finite element model is shown below. The model consists of a 

test panel (the regular grid in the center region) attached on each side to a picture-frame-type fixture that 

is 

connected by pins, which is attached to appropriate loading frames to achieve the combined loading stress 

state. During testing, the top edge of the upper frame is held fixed, while the bottom frame undergoes a 

vertical force from below and a horizontal force through an L-extension of the bottom frame (not shown 

here). This results in a horizontal force as well as a bending moment, which is represented in the STAGS 

model by the force couple shown. Relevant details for each component of the testing apparatus follow. 

X

Y

Top edge fixed Z

Bottom edge moves as one unit

9600 lb force

17,803 lb force

23,563 lb force
22

The test panel area is 24 inches wide and 28 inches high, with a grid size of one-half inch in both 

directions. The original model is assumed to be composed of aluminum (E = 10.0 Msi, ν = 0.33) with a 

thickness of 0.125 inches. One element is removed at each corner of the test coupon to alleviate stress. 

Tabs are present on all four sides. The top and bottom tabs measure 23 inches wide (due to the 1/2 cut-out 

in each corner) by 2.94 inches high, while the sides are 27 inches high and 2.25 inches wide. The tabs are 

constructed with a layer of steel (E = 30.0 Msi, ν = 0.30, 0.19 inches thick) bonded to each side (though 

the tab material may change for composite materials, we can assume that one will be present). The test 

panel and tabs are shown in the figure below with the other components removed. The origin of the axes 

is in the bottom left corner of the test panel. Note that the meshing within the tabbed regions is quite 

erratic, which is due to the fact that earlier models possessed higher density meshes within the test panel 

region along with the fact that certain points within the tab had to be defined as nodes. These nodes are 

denoted by the colored dots in the figure, and represent the points of attachment between the tabbed 

regions and the fixture. The blue dots represent nodes in the tabs which are defined within the model as 

having identical displacements (horizontal u, vertical v, and out-of-plane w) as the corresponding point in 

the frame. The green dots are similarly defined, but are only constrained in the u and w displacement 

directions (the connections in the side fixtures are slotted to allow vertical movement). The red dots 

shown here represent the pinned connections between horizontal and vertical components of the fixture, 

and indicate that the displacements for each connected component is identical at that point (note that no 

connection to the test panel or tabbed areas exist for these red points, but are shown for completeness). 

X

Y

0 10 20 30

-5

0

5

10

15

20

25

30

35
23

The fixture attached to the test panel is shown in the three-dimensional figure below (only the top fixture 

is shown, it is symmetric about the xy-plane. The test panel is situated below this structural component). 

The coloring of each element denotes the shell wall property assigned to it within the model definition. 

Property IDs 5, 6, 8, and 9 (shades of green), which make up the bulk of the vertical and horizontal frame 

components within the plane of the test panel, are made of steel with a thickness of 1/2 inches. The red 

areas within the side frames (IDs 15 and 16) are made of thicker steel (1 inch), as are the yellow regions 

within the top and bottom frames. Finally, the red areas within the top and bottom frame (the region in the 

bottom area is hidden, but it is identical to the top frame) have a thickness of 1 1/2 inches, also steel. The 

frames are connected to each other at the corners (corresponding to the red dots shown in the figure of the 

test panel) through stipulation that the displacements for the corresponding nodes in each frame are the 

same. However, for the connection of the bottom frame with the two side frames, the v-displacement is 

again allowed free due to the slotting of the connection point. To summarize, this (upper) frame is made 

of four steel L-beams, which range in thickness from 1/2 to 1 1/2 inches. A similar lower frame exists, 

symmetric about the xy-plane. Dimensions of the frame are not significant, as long as they are wide 

enough to cover the tabbed areas (for reference, the top and bottom frames are 3 inches high, while the 

side frames are 2.25 inches wide). The upper and lower frames are connected to each other by bolts/pins 

through the tabbed areas that they envelop, and each vertical/horizontal component is also connected to 

each other through pins at the corners of each frame element. Note that these frames are quite rigid, 

especially in the areas with the extra thickness. 

Lastly, the loading frame shown in the first figure is a steel structure that is loaded by external forces. 

Note that the loading frame components are not perfectly rigid, but are constructed of thick steel plates 

that inhibit most of the deformation (relative to the deformation of the test panel). The top of the 

uppermost loading frame is defined by zero displacements, while the bottom loading frame is constrained 

by constant v-displacement along the lower face and constant u-displacement along the right face (as well 

as zero w-displacement everywhere. This effectively limits the rotation of the load frame even when a 

bending moment is applied to it. 

Y X

Z

PropID

16

15

14

13

12

11

10

9

8

7

6

5

4

3

2

1
24

The bottom loading frame is 5.5 inches high. Thus the center of this structural component is 19.75 inches 

from the center of the test panel where the horizontal shear force is applied [19.75 inches = 28/2 (half the 

test panel height) + 3 inches (the attachment frame/tabbed area) + 5.5/2 (half the loading frame 

dimension)]. Thus the bending moment at the center of the load frame for a 5760 lb force applied at the 

center of the test panel is 113,760 in-lbs, which has been separated into a resultant force-couple and 

applied at the top and bottom edges of the loading frame. 

Buckling Solution (Case: test) 

For the applied loads, the in-plane and linear buckling solution for the test case: 

u-displacement of top edge (x=12, y=28) = -0.000247 

u-displacement of bottom edge (x=12, y=0) = -0.023611 

Relative u-displacement = -0.023364 

v-displacement of top edge (x=12, y=28) = 0.0001084 

v-displacement of bottom edge (x=12, y=0) = 0.0067016 

Relative v-displacement = 0.0065932 

v-displacement of bottom edge node (x=21.5, y=0) = 0.0064069 

Relative rotation of bottom edge = -3.102×10-5 radians 

 Eigenvalue of linear buckling solution = 0.73299 

Suitable models should provide reasonable agreement with the quantities in bold, as well as exhibiting 

similar trends with respect to the stress variation and deformation patterns. 

Reduced Models 

In an effort to diminish the size of the models, an effort was made to concentrate on the constraints of the 

test panel component and remove as many other structural components as possible. In general, it was 

assumed that the relatively rigid steels tended to move as rigid bodies, and by using suitable edge 

constraints for the test panel an analogous response could be produced. This section, therefore, starts with 

a model with the broadest assumptions, analyzes it and notes the discrepancies with the test case, and tries 

to improve the correlation between the two by adding another detail of the complex model. Results and 

discussion of each successive model follows. 

Constant v-displacement on bottom edge (Case: idealV) 

The most basic case modeled only the test panel component, without the tabbed areas but including the 

notched corners. Identical grid spacing was used (1/2 inch square), and the model was constrained in the 

following manner: (a) displacement w = 0 on all edges, along with rotations about the x– and y-axes, 

which approximates a clamped condition; (b) u and v displacements are zero along the top edge, which 

fixes the structure in space; (c) bottom edge has all nodes with identical u and v displacements; (d) side 

edges have constraint on u-displacement to vary linearly along the edge, from zero at the top to the 

unknown u-displacement of the bottom edge at the bottom. The loading was applied using a horizontal 

force of –2760 lbs and a vertical force of 9600 lbs at the center of the bottom edge. A figure showing the 

mesh with the deformation magnified 200 times is shown on the next page. 
25

The solution data for this case idealV: 

u-displacement of top edge (x=12, y=28) = 0.0 

u-displacement of bottom edge (x=12, y=0) = -0.024306 

Relative u-displacement = -0.024306 

v-displacement of top edge (x=12, y=28) = 0.0 

v-displacement of bottom edge (x=12, y=0) = 0.0080245 

Relative v-displacement = 0.0080245 

v-displacement of bottom edge node (x=21.5, y=0) = 0.0080245 

 Relative rotation of bottom edge = 0.0 radians 

 Eigenvalue of linear buckling solution = 0.49044 

The constraints applied to this structure basically assume infinite stiffness of the loading frame and no 

allowance for rotation of the bottom edge. Though the u-displacement solution is relatively comparable to 

case test, both the v-displacement and the eigenvalue indicate that the model is not adequate for our 

needs. Investigation of the stress variation indicates that the loading here is symmetric about the vertical 

center of the panel and does not show any skewness with respect to the shear loading, which is evident in 

the test case solution. 

Rotation allowed on bottom edge (Case: idealM) 

To alleviate one of the problems of the earlier model, the constraints along the bottom edge were changed 

to allow for rotation of the edge. Instead of identical v-displacements, vertical motion along the bottom 

edge was constrained to vary linearly with respect to two unconstrained v-displacements: the one at the 

center and at the rightmost point along the bottom edge. A bending moment was also introduced using a 

force-couple combination. For the bottom edge, the resultant moment for a horizontal force applied at the 

center of the panel is calculated as [5760 lbs × 14 inches = 80,640 in-lbs], which is broken into a vertical 

downward force of 7012 lbs applied at the center of the bottom edge and an upward force at the rightmost 

edge, 11.5 inches away [80,640/11.5 = 7012]. At the center of the bottom edge, a vertical force of 9600 
26

lbs is already being applied, so the resultant force applied there is 2588 lbs. The rest of the constraints are 

identical to the ones applied for the previous case. 

The solution data for this case idealM: 

u-displacement of top edge (x=12, y=28) = 0.0 

u-displacement of bottom edge (x=12, y=0) = -0.024306 

Relative u-displacement = -0.024306 

v-displacement of top edge (x=12, y=28) = 0.0 

v-displacement of bottom edge (x=12, y=0) = 0.0080245 

Relative v-displacement = 0.0080245 

v-displacement of bottom edge node (x=21.5, y=0) = 0.0080242 

 Relative rotation of bottom edge = -3.158×10-8 radians 

 Eigenvalue of linear buckling solution = 0.49044 

In summary, hardly any change from the previous case. The rotation was small enough to be almost 

indiscernible and had no apparent affect on the buckling load. The large discrepancy in the vdisplacement was still a major concern, and indicated that the test case possessed more stiffness than the 

idealized model that was being used. 

Inclusion of tabbed regions (Case: idealT) 

To improve the stiffness of the model, the tabbed regions were added to the model with no other changes 

to the previous constraints (from case idealM). It was reasoned that since the v-displacement along the 

vertical edges was unconstrained, the material within the tabbed regions played a major role in the 

stiffness of the panel. Therefore, elements were added to the model that corresponded exactly to the 

dimensions and shell wall construction of the test case model. That is, the top and bottom tabs were 2.94 

inches high and constructed of a 0.125 inch aluminum layer sandwiched by two steel layers of 0.19 

inches, while the side tabs were 2.25 inches wide and constructed with the same layup. It should also be 

noted that each tab region used four elements across it, which were not necessarily regularly spaced. The 

top/bottom were spaced with intervals {0.75, 1.5, 2.25, 2.94} inches with regular spacing in the horizontal 

direction, while the elements within the side tabs were located at {0.5, 1.25, 1.75, 2.25} and regular 

spacing in the vertical direction. These choices were made to conform to the correct dimensions and to 

serve as useful points for the connection point constraints further down the evolution of the model 

(discussed in detail later). 

For the new nodes within the tabbed regions, the only applied constraint was to enforce zero wdisplacement for all points. Because of the presence of this constraint and the added material within the 

tab shell walls, the constraints on the rotations about the x and y axes that were stipulated for all edges (in 

case idealV) were removed. 

The solution data for this case idealT: 

u-displacement of top edge (x=12, y=28) = 0.0 

u-displacement of bottom edge (x=12, y=0) = -0.021781 

Relative u-displacement = -0.021781 

v-displacement of top edge (x=12, y=28) = 0.0 

v-displacement of bottom edge (x=12, y=0) = 0.0055136 
27

Relative v-displacement = 0.0055136 

v-displacement of bottom edge node (x=21.5, y=0) = 0.0055136 

 Relative rotation of bottom edge = 0.0 radians 

 Eigenvalue of linear buckling solution = 0.67506 

As expected, the presence of the tabs increased the stiffness, thereby increasing the buckling load value 

and decreasing the vertical displacement of the bottom edge. It also decreased the rotation of the bottom 

edge to insignificant magnitude. However, the presence of the tabbed regions also introduced some 

skewness to the shear stress resultant, which followed the basic trend of the test case solution. Therefore it 

was believed that the inclusion of the more accurate edge conditions was a significant improvement. A 

check was also needed on the edge rotation, since some rotation of the lower edge was expected. 

Transformation to edge rotation constraint (Case: idealW) 

To check the validity of the straight edge constraints, a different approach was used to model the rotation 

of the bottom edge. Note that in case idealM, the linear variation of the bottom edge was constrained in 

terms of two vertical displacements, one in the center and one at the edge. To verify that this method was 

introduced the constraint and the loading as expected, the straight edge constraint was altered so that the 

displacement of each point along the bottom edge was dependent on the relative translation and rotation 

of the point at the center of the bottom edge. Thus, the u-displacement constraint stayed the same, while 

the v-displacement constraint became: 

0 0 vx v x () [] =+ω (2) 

where the zero subscript denotes the point at the middle of the bottom edge. Besides this new constraint, 

the loading was changed by introducing a corresponding moment about the middle edge point. Thus the 

loading consisted of a vertical force of 9600 lbs, a horizontal force of –5760 lbs, and a bending moment 

about the z-axis of 80,640 in-lbs. 

The solution data for this case idealW: 

u-displacement of top edge (x=12, y=28) = 0.0 

u-displacement of bottom edge (x=12, y=0) = -0.021781 

Relative u-displacement = -0.021781 

v-displacement of top edge (x=12, y=28) = 0.0 

v-displacement of bottom edge (x=12, y=0) = 0.0055136 

Relative v-displacement = 0.0055136 

v-displacement of bottom edge node (x=21.5, y=0) = 0.0055137 

 Relative rotation of bottom edge = -1.044×10-8 radians 

 Eigenvalue of linear buckling solution = 0.67506 

These results indicate that either method of constraint/loading works equally well (displacement 

constraints with pure force loading versus rotation constraint with force/moment loading), which bodes 

well for future solutions. 
28

Turning our attention back to the results with regard to the deformation of the test case, both the 

translation displacements of the bottom edge are smaller than the expected value. This implies that the 

stiffness of our ideal model still does not match up exactly with the more complex model. Thus it was 

reasoned that a more accurate modeling of the load introduction through the top and bottom tabs is 

required, since the constraints presently applied to the edges of the test panel region seem quite 

prohibitive. Additionally, examination of the vertical displacements along the entire bottom edge for 

cases test and idealW reveal a startling disparity, as shown in the figure below: 

Besides the disparity of magnitude, the graph reveals that the straight edge constraint assumed at the 

edges does not hold true for the test case near the extremities of the edges. This is due to the fact that the 

connection points to the test panel (shown by the colored dots in the figure on page 2) do not extend to the 

ends of the edges, but stop 2.5 inches short of it for each fixture. This results in the sharp variation of the 

displacements as shown above, and will also greatly affect the stiffness response of the panel. 

Load application through fixture connections (Case: idealF) 

To account for the non-uniform loading through the connection points within the tabbed regions, a more 

complex constraint system was introduced. The system is based on the assumption that the connection 

points move as a rigid body within each frame member. The reference point for the translation and 

rotation of the bottom edge is shifted to the middle point within the tabbed area, and the loading is applied 

there as well (note that due to the new location, the applied moment is now 84,960 in-lbs). The following 

figure and associated equations describe the method of constraint, where the color coding of the remarks 

and equations correspond to the colored dots in the figure. 

X

v-displacement along bottom edge

0 4 8 12 16 20 24 0

0.002

0.004

0.006

0.008

Case: test

Case: idealW
29

All points within tabbed areas: 

w = 0 (3) 

Points within top frame: 

u v = = 0 (4)

Frame connection points at top: 

u v = = 0 (5)

Points within bottom frame: 

uu xx vv yy =−− ω =+− ω 0 00 0 00 [] [] (6)

Frame connection points (hinges) at bottom: 

0 0 [0.75] unconstrained hinge hinge uu v =− ω = (7)

Points within vertical frame members: 

[ ] 28

hinge

hinge

u

u yy =− − (8)

To summarize, the connection points within the top tabbed region are set to be zero. This assumes a rigid 

frame that is not allowed to move. Since the top connection points are within the top frame, they too are 

constrained to be zero. For the bottom frame, the horizontal and vertical displacements are assumed move 

as a rigid body about Point 0, where the rotation is now also included in the u-displacement constraint. 

Calculations for the connection points (referred to as hinges for they are the pinned hinge between the 

bottom and the side frames) are calculated in Eq. (7), though it should be remembered that the test case 

model does not enforce any constraint in the vertical direction for this hinge. Since the u-displacement for 

Point 0 
30

the top and bottom hinges are now both known (or expressed in terms of the displacements at Point 0), 

the linear equation for the u-displacements within the side frames follows. 

The solution data for this case idealF: 

u-displacement of top edge (x=12, y=28) = -0.000006 

u-displacement of bottom edge (x=12, y=0) = -0.022912 

Relative u-displacement = -0.022908 

v-displacement of top edge (x=12, y=28) = 0.0000250 

v-displacement of bottom edge (x=12, y=0) = 0.0057571 

Relative v-displacement = 0.0057321 

v-displacement of bottom edge node (x=21.5, y=0) = 0.0055994 

 Relative rotation of bottom edge = -1.660×10-5 radians 

 Eigenvalue of linear buckling solution = 0.65810 

 

Unfortunately, application through the connection points seemed to make the agreement worse, at least 

on the surface. However, comparison of the results between this case and the test case revealed that 

though the magnitudes of the numbers were not comparable (especially the buckling eigenvalues), the 

variations of the field quantities were following the same basic trends. Therefore, only a few more 

adjustments were required. To gain insight into the motivation behind the last improvement, plots of the 

Nx stress resultant are shown for the test panel and tabbed regions for both the test case and idealF. The 

color of each element represents the level of magnitude of the stress resultant. 

Nx

100

75

50

25

0

-25

-50

-75

-100

-125

-150

-175

-200

-225

-250

-275

-300

-325

-350

-375

-400

-425

-450

-475

-500

Case: test Case: idealF

The surprising result indicated by these results is that the test case actually exhibits a positive value of the 

Nx stress resultant in center of the panel. Except for this discrepancy, the other resultants for the two cases 

look reasonably similar, yet the disparity highlighted here would lead directly to the differences in 

eigenvalues for the two cases. Therefore, more attention was focused on the side edges of the panel. The 

ideal cases have been assuming straight edges along the length, but closer inspection revealed that the test
31

case did not follow this rule and exhibited some deformation along the length of the vertical tabbed 

regions. 

A plot of the u-displacement as a function of y for the right edge of the connection line (a vertical line 

within the right-side tabbed regions that would go through the green points closest to the center of the 

panel ) is shown on the next page for both cases. As one would expect, the linear constraint applied to the 

connection points for the idealF case maintains a relatively straight line during deformation (some slight 

nonlinearity is present near the ends, outside of the connection point region). However, the test case 

reveals that significant nonlinearity is present in the middle of the region as well. In physical terms, the 

vertical compressive load applied to the test panel introduces expansion in the horizontal direction due to 

the Poisson effect. However, the side frames resist this expansion. For the idealF case, the side frames 

(and therefore the connection points within the tabbed area) are assumed to be rigid with respect to 

horizontal deformation, therefore this results in a compressive Nx stress resultant as evidenced by the Nx

plots shown above (the plot on the right). In actuality, the side frames are not rigid but experience elastic 

deformation due to the expansion of the test panel. This allows for more expansion of the test panel and 

results in a tensile field in the horizontal direction for the test case (see the plot on the left in the figure 

above). Therefore, for our ideal models to match the test conditions, some relaxation of the rigid side 

constraints are required. 

At this point, a choice had to be made. To most correctly model the elasticity of the vertical tabbed 

regions, attachments to the framing components (shown in the figure on page 3) should be modeled by 

constraining them to move with the nodes of the frames, which would have to be added to the model. This 

would closely follow the technique employed for the test case model, but would involve added many 

nodes and elements (which would reduce the all-important analysis time). Alternatively, the present 

idealF model could be slightly altered to allow for some relaxation of the vertical tabbed regions. 

u displacement along right edge connection line

Y

-0.02 -0.015 -0.01 -0.005 0 0

4

8

12

16

20

24

28

Case: test

Case: idealF
32

Relaxation of linear constraints within side tabs (Case: idealR) 

Closer examination of the test case deformation revealed that though the displacements along the edge 

looked nonlinear, the points within the thicker steel regions of the frame (as indicated by the red areas for 

the right frame in the figure on page 3) followed a linear variation more closely than the rest of the frame. 

Therefore, an ad hoc relaxation of the linear constraint could be accomplished by removing the 

constraints for the connection points outside of the thicker region of the frame. Note that this assumption 

is not exact, since the movement of these points do possess a correspondence to the deformation of the 

frame. Nevertheless, the constraints for the u-displacement for the vertical tabs were limited to the top and 

bottom five points on each side, corresponding to the thicker region of steel for the frame (which is 1 inch 

thick as opposed to 1/2 inch thick for the rest of the frame). 

The solution data for this case idealR: 

u-displacement of top edge (x=12, y=28) = -0.000055 

u-displacement of bottom edge (x=12, y=0) = -0.022918 

Relative u-displacement = -0.022863 

v-displacement of top edge (x=12, y=28) = 0.0000238 

v-displacement of bottom edge (x=12, y=0) = 0.0060548 

Relative v-displacement = 0.0060310 

v-displacement of bottom edge node (x=21.5, y=0) = 0.0058934 

 Relative rotation of bottom edge = -1.700×10-5 radians 

 Eigenvalue of linear buckling solution = 0.72216 

 

The resulting displacements along the vertical connection line are shown below:. As expected, the 

relaxation of the rigid frame results in much better agreement between the two cases, though the regions 

within the constrained thick frame region are still imperfectly modeled. However, the excellent agreement 

between the idealR case and the test model for the displacements and eigenvalue estimate indicate that 

this case is adequate for design purposes. 

u displacement along right edge connection line

Y

-0.02 -0.015 -0.01 -0.005 0 0

4

8

12

16

20

24

28

Case: test

Case: idealR
33

Model Definition for Cellular Automata Design (Case: idealCA) 

Lastly, some slight modifications to the best ideal model are required for easy implementation of the 

Cellular Automata design paradigm. The two major modifications include: (a) regular grid spacing of all 

elements; and (b) enforcement of constraints through displacements only. 

The first of these modifications is satisfied by changes the dimensions of the elements within the tabbed 

regions of the model. For the top/bottom region, the vertical spacing of the elements is altered from four 

elements aligned at {0.75, 1.5, 2.25, 2.94} inches to six elements located at {0. 5, 1.0, 1.5, 2.0, 2.5, 3.0}, 

which satisfies the 1/2 grid size of the model. Along with the location change, the connection points are 

moved from their previous location of 0.75 and 1.5 to 1.0 and 1.5. Along the length of the horizontal tabs, 

the points are located at {2.5, 4, 5.5, 7, 10, 12, 14, 17, 18.5, 20, 21.5} for the line closest to the panel 

center and {2.5, 4.5, 7.5, 10.5, 13.5, 16.5, 19.5, 21.5} for the exterior line. For the vertical tabs, the 

spacing changes from {0.5, 1.25, 1.75, 2.25} to {0.5, 1, 1.5, 2}, resulting in a smaller tab size on the 

sides. The locations of the points within the thick region of the frame are {2.5, 4, 5.5, 22.5, 24, 25.5} for 

the line closest to the panel center and {2.5, 4.5, 23.5, 25.5} for the outside line. 

Changing the straight edge constraints back to displacement equations results in the following alterations 

to the constraint equations: 

Points within bottom frame: 

[ ]( ) 21.5 0

0 0 9.5

x

o

v v

uu vv xx = − = =+− (9)

Points within vertical frame members: 

0 [ ] 28 hinge

u

u yy =− − (10)

Implementation of these changes leads to the following results for case idealCA: 

u-displacement of top edge (x=12, y=28) = -0.000072 

u-displacement of bottom edge (x=12, y=0) = -0.022836 

Relative u-displacement = -0.022764 

v-displacement of top edge (x=12, y=28) = 0.0000302 

v-displacement of bottom edge (x=12, y=0) = 0.0061293 

Relative v-displacement = 0.0060991 

v-displacement of bottom edge node (x=21.5, y=0) = 0.0060623 

 Relative rotation of bottom edge = -7.053×10-6 radians 

 Eigenvalue of linear buckling solution = 0.71818 

The deformed shape for the two cases are shown on the next page, magnified 200 times. 
34

Case: test

Case: idealCA
REPORT DOCUMENTATION PAGE Form Approved

OMB No. 0704-0188

2. REPORT TYPE 

Contractor Report

 4. TITLE AND SUBTITLE

Enhancements of Tow-Steering Design Techniques: Design of 

Rectangular Panel Under Combined Loads

5a. CONTRACT NUMBER

NAS1-00135

 6. AUTHOR(S)

Tatting, Brian F.; Setoodeh, Shahriar; and Gürdal, Zafer

 7. PERFORMING ORGANIZATION NAME(S) AND ADDRESS(ES)

NASA Langley Research Center

Hampton, VA 23681-2199

 9. SPONSORING/MONITORING AGENCY NAME(S) AND ADDRESS(ES)

National Aeronautics and Space Administration

Washington, DC 20546-0001

 8. PERFORMING ORGANIZATION

 REPORT NUMBER

10. SPONSOR/MONITOR'S ACRONYM(S)

NASA

13. SUPPLEMENTARY NOTES

Zafer Gürdal: Departments of Engineering Science and Mechanics, and Aerospace and Ocean Engineering, Virginia 

Polytechnic Institute and State University, Blacksburg, Virginia, currently, Chair, Delft University of Technology, 2629 HS 

Delft, The Netherlands. Technical Monitor: Dawn C. Jegley. An electronic version can be found at http://ntrs.nasa.gov

12. DISTRIBUTION/AVAILABILITY STATEMENT

Unclassified - Unlimited

Subject Category 39

Availability: NASA CASI (301) 621-0390

19a. NAME OF RESPONSIBLE PERSON

STI Help Desk (email: help@sti.nasa.gov)

14. ABSTRACT

An extension to existing design tools that utilize tow-steering is used to investigate the use of elastic tailoring for a flat panel 

with a central hole under combined loads of compression and shear. The tailoring is characterized by tow-steering within 

individual lamina and Cellular Automata design concepts based on selective reinforcement. Designs using traditional straight 

fiber plies, and tow-steered designs using parallel, tow-drop, and overlap plies within the laminate were generated. These 

results indicated that the overlap produces the method most structurally efficient designs compared to traditional constant 

stiffness monocoque panels, though the laminates were not as efficient as sandwich and isogrid designs. Further design studies 

were conducted using selective reinforcement plies at the core and outer surface of the laminate, however, the minimization of 

the compliance directs the major stresses toward the center of the panel, which decreased the ability of the structure to 

withstand loads leading to instability.

15. SUBJECT TERMS

Tow-steering; Curvilinear fibers; Composite structures; Selective reinforcement; Optimization; Combined loading;

Graphite-epoxy

18. NUMBER

 OF 

 PAGES

39

19b. TELEPHONE NUMBER (Include area code)

(301) 621-0390

a. REPORT

U

c. THIS PAGE

U

b. ABSTRACT

U

17. LIMITATION OF 

 ABSTRACT

UU

Prescribed by ANSI Std. Z39.18

Standard Form 298 (Rev. 8-98)

3. DATES COVERED (From - To)

5b. GRANT NUMBER

5c. PROGRAM ELEMENT NUMBER

5d. PROJECT NUMBER

5e. TASK NUMBER

5f. WORK UNIT NUMBER

23-064-30-34

11. SPONSOR/MONITOR'S REPORT

 NUMBER(S)

NASA/CR-2005-213911

16. SECURITY CLASSIFICATION OF:

The public reporting burden for this collection of information is estimated to average 1 hour per response, including the time for reviewing instructions, searching existing data sources, 

gathering and maintaining the data needed, and completing and reviewing the collection of information. Send comments regarding this burden estimate or any other aspect of this 

collection of information, including suggestions for reducing this burden, to Department of Defense, Washington Headquarters Services, Directorate for Information Operations and 

Reports (0704-0188), 1215 Jefferson Davis Highway, Suite 1204, Arlington, VA 22202-4302. Respondents should be aware that notwithstanding any other provision of law, no person 

shall be subject to any penalty for failing to comply with a collection of information if it does not display a currently valid OMB control number.

PLEASE DO NOT RETURN YOUR FORM TO THE ABOVE ADDRESS.

1. REPORT DATE (DD-MM-YYYY)

01- 09 - 2005