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									Pull-off Tests and Analyses of Composite Skin and
Frame T-Joint
J. LI




ABSTRACT

   Twenty pull-off specimens were tested to identify the mechanism and associated crucial factors that
control the failure mode of frame to skin joint using angle clips stitched to the skin. The pull-off
specimens were tested under both simply supported and clamped conditions. Four different support
spans were used in either simply supported or clamped conditions. As expected, the support
conditions and span lengths are greatly complicating failure analysis and pull-off allowable
development based on pull-off load. As the test data indicate that the critical pull-off load reduces as
the support span increases, and the critical pull-off loads are higher under clamped condition as
compared with simply supported condition when the support span is the same. These behaviors
greatly increase the cost of allowable development based on pull-off load and increase the likelihood
of error in applying allowable defined in terms of pull-off load.

On the other hand, the dependence of the critical bending moment on support span and support type
is less significant. Which shows that the crack initiation is controlled by the maximum bending
moment at the critical location. The mechanism behind the joint failure initiation is the maximum
tension strain caused by bending. Allowable developed in terms of maximum critical bending moment
is independent of the support condition and span length. The allowable bending moment developed
from either clamped condition or simply supported condition becomes equivalent. Enormous savings
of allowable development costs and greatly increased accuracy in analysis can be achieved by
capturing the right crucial factor that governs the actual failure mode.

Keywords: Composite angle clip joint, pull-off test, out-of-plane failure mode, delamination


INTRODUCTION

   With the increased emphasis on reducing the cost of manufacturing composite structures,
secondary bonding or co-curing is an attractive option to eliminate the need for mechanically
fastening sub-assemblies. Many composite components in aerospace structures consist of flat or
curved panels with co-cured frames and stiffeners. Out-of-plane loading such as internal pressure in a
composite fuselage or out-of-plane deformations in a compression loaded post-buckled panel may
cause the frame or stiffener to debond from the panel [1]. The out-of-plane failure mode have been
studied extensively using stiffener pull-off tests [1-12].
_____________
Jian Li, The Boeing Company, M530-B229, 5000 E. McDowell Road, Mesa, AZ 85215
   Pull-off tests results of co-cured joints were analyzed to understand the failure mode and strength
of attaching frames and bulkheads to the fuselage skin using angle clips [12]. The angle clip joint is
similar to blade stiffeners studied in Ref. 7. Ref. 12 indicated potential misrepresentation of pull-off
strength when the clamps of the pull-off fixture get close to either the vertical flanges of the angle clip
or the horizontal flange tip of the angle clip. The clamps pressed on the angle clip flanges introduced
unrealistic constraints to the delamination failure mode in the noodle between the clip flanges and the
skin that inflated the magnitude of the pull-off load at ultimate failure.
   Ref. 12 and 13 also indicated that the bending moment at the delamination front drives the failure
initiation and growth from the corner region. To validate the analysis [13] and obtain the critical
bending moment that characterizes the strength of the joint, twenty pull-off specimens were tested
with four different support spans either under clamped conditions or simply supported conditions. In
this paper, the results from these tests and their interrogations are presented.

PULL-OFF TESTS

   The pull-off specimens were cut with diamond-tipped tool from a simple panel made of carbon
fiber fabric and infused with the SI-ZG-5A resin system using the VARTM process [12]. The fabrics
were intermediate modulus AS4 with 3000 denier in the plain weave (PW) and 6000 denier in the 5-
harness satin (5HS) plies. The symmetric skin lay-up was (±45PW, 0/905HS, ±45PW, 0/90PW)s. The
angle clips were made of four plies of the same plain weave AS4 carbon fabric, all of which were
laid-up in ±45° orientation. The angle clips were stitched to the skin before infusing.

Pull-off Specimen Preparation and Test Setup

   Twenty 2-inch wide and 14-inch long specimens were cut from frame B (Figure 1) and milled to
final finish. A schematic drawing of the top view of the simple panel and the nominal dimension of
the pull-off specimen is shown in Figure 1. Each specimen was measured at three locations for width,
three locations for thickness and one location for length as shown in Figure 2. Where W1, W2 and
W3 were width measurements taken from the skin and flange regions. Thickness T1 and T2 measured
the combined thickness of the skin and flange, while thickness measurement T3 represented the skin
thickness. The length of the specimen was also measured and denoted as L on Figure 2. The results of
these measurements are given in Tables 1 and 2 for simply supported (roller) specimens and clamped
specimens, respectively. Also provided in the Tables are the test rates associated with these
specimens when tested.




                                    Frame B
             14.0


                          2
Figure 1 Simple panel and pull-off specimen dimensions (in).
                                      W1                W2         W3




                                                              Adhesive Noodle

                                         T1                        T2
                 T3




                                                        L

Figure 2 Specimen dimension measurement locations.

Table 1 Dimensions and test rates for simply supported pull-off specimens
                  Roller                Width (in)                          Thickness (in)           Length (in) Test Rate
   Specimen ID
                 Span (in)    W1      W2         W3      Average     T1          T2           T3         L       (in./min)
       101           4       2.003   2.002      2.005     2.003     0.147       0.136        0.090      14.1        0.1
       102           4       1.999   2.000      2.000     2.000     0.145       0.139        0.087      14.1        0.1
       121           6       2.000   2.002      2.000     2.001     0.145       0.138        0.087      14.1        0.1
       122           6       2.000   2.000      2.001     2.000     0.139       0.142        0.086      14.1        0.1
       111           8       2.000   1.999      2.000     2.000     0.150       0.141        0.089      14.1        0.1
       112           8       2.000   1.999      2.001     2.000     0.149       0.139        0.090      14.1        0.4
       131          10       2.000   2.001      2.001     2.000     0.134       0.134        0.087      14.1        0.8
       132          10       2.000   1.998      1.998     1.999     0.135       0.134        0.090      14.1        0.7


Table 2 Dimensions and test rates for clamped pull-off specimens

                  Clamp                 Width (in)                          Thickness (in)           Length (in) Test Rate
   Specimen ID
                 Span (in)    W1      W2        W3       Average     T1          T2           T3         L       (in./min)
       151           4       1.999   1.999     1.999      1.999     0.141       0.130        0.088      14.1        0.02
       152           4       1.992   1.992     1.995      1.993     0.133       0.132        0.089      14.1        0.05
       153           4       1.997   1.998     2.001      1.999     0.133       0.137        0.087      14.1        0.05
       161           6       1.999   1.999     1.998      1.999     0.136       0.131        0.087      14.1        0.09
       162           6       2.001   2.001     2.000      2.001     0.146       0.131        0.089      14.1        0.09
       163           6       1.999   1.998     1.998      1.998     0.146       0.129        0.086      14.1        0.09
       171           8       1.998   1.997     1.997      1.997     0.135       0.138        0.090      14.1        0.18
       172           8       1.995   1.997     1.996      1.996     0.145       0.136        0.087      14.1        0.16
       173           8       1.993   1.995     1.998      1.995     0.139       0.139        0.087      14.1        0.16
       181          10       1.939   1.939     1.942      1.940     0.141       0.131        0.083      14.1        0.32
       182          10       1.991   1.992     1.990      1.991     0.133       0.149        0.092      14.1        0.26
       183          10       1.509   1.510     1.508      1.509     0.131       0.145        0.085      13.9        0.20
   The simply supported and clamped pull-off test set-ups are schematically shown in Figures 3 and 4,
respectively.



                                                 Span




                                                          P

Figure 3 Simply support pull-off specimen test set-up.


                                                   Span



                                               Base Plate




                                                              P


Figure 4 Clamped support pull-off specimen test set-up.

Measured Test Results

   All specimens were tested at ambient conditions. The laboratory temperature during testing was
between 76º-80º F. The relative humidity level in the laboratory was between 44-48%. The test was
discontinued when the load dropped 20% or more from the peek load. The loading rate of the static
test could vary from specimen to specimen so that failure would occur within 5 to 10 minutes.
Simply Supported Test Results

  A total of eight specimens were tested under simply supported conditions where two specimens
were tested for each of four different spans. Specimens 101 and 102 were tested with a four-inch
span. The load versus displacement behavior for specimens 101 and 102 are shown in Figure 5. Test
records indicate that audible cracking sounds were first heard at 100 lb and 145 lb for specimens 101
and 102, respectively. Notable load drops at these levels can be observed from the load-displacement
curves shown in Figure 5. To be specific, the indicated first notable load drop shown in Figure 5 is
101 lb for specimen 101 and 148 lb for specimen 102. Visual observations of cracks were detected at
higher load levels (170 lb for specimen 101 and 176 lb for specimen 102).




              300
                                           4-inch span simply supported

              250
                            Specimen 102 first drop

              200
 Load (lb.)




              150


              100                             Specimen 101 first drop


              50


               0
                    0            0.1                0.2              0.3           0.4          0.5                0.6


                                                          Displacement (in.)

Figure 5 Load-displacement curves for 4-inch span simply supported.

   The pull-off loads associated with initial audible cracking, load drop and visual crack for all simply
supported specimens are tabled in Table 3. The pull-off load at initial audible cracking was at the
same load level as the initial load drop with the exceptions of specimen 122 and 112. This
coincidence is significant in confirming the failure initiation by using the first load drop or deviation
from linearity from the load-displacement curve. Also provided in Table 3 are the ultimate pull-off
loads recorded from tests. Where the ultimate failure is defined as a 20% or more load drop from the
peak load. The sequence of events is first audible sound, initial load drop, visual crack and ultimate
failure.
Table 3 Initial audible, load drop, visual crack and ultimate for simply supported specimens

          Span (in.)                   4                         6                  8                  10
          Specimen ID          101           102          121        122    111          112    131         132
          Audible (lb.)        100           145           95         78    65.7          57     42          42
          Load drop (lb.)     100.6         148.2         95.7       93.5   65.7         64.9   42.4        42.5
          Visual (lb.)         170           176          123         99     81           81     50          43
          Utimate (lb.)        214           260          149        143     91           92     60          64
              500
                                                     4-inch span clamped
              450                                                                              152
              400

              350

              300
 Load (lb.)




                                    151
              250

              200

              150                            153

              100

              50

               0
                    0           0.05          0.1            0.15            0.2         0.25           0.3            0.35
                                                            Displacement (in.)

Figure 6 Load-displacement curves for 4-inch span clamped.

Clamped Test Results

   There were twelve specimens tested under clamped conditions with three specimens tested at each
span of 4, 6, 8 and 10 inches. The first audible, load drop, visual crack and ultimate failure loads are
summarized in Table 4 for the clamped specimens. Reasonable agreement can be seen between the
first audible crack and first load drop from the load-displacement curve for most of the specimens.
Typical load-displacement curve can be seen from Figures 6 for clamped span of 4 inches. The first
load drops are more identifiable in clamping spans of 4, 6 and 8 inches. It is much harder to identify
the initial load drops in those 10-inch span specimens due to their strong nonlinear load-displacement
behaviors.
Table 4 First audible, load drop, visual crack and ultimate failure for clamped specimens
    Span (in.)                      4                          6                         8                      10
    Specimen ID          151       152     153       161      162     163       171     172      173    181     182       183
    Audible (lb.)        198       189     165       137      143     147       109     119      118     60      72        67
    Load drop (lb.)     206.5     206.8   170.2     137.7    159.8   147.8     130.3   120.7    118.4   87.7   130.4      71.7
    Visual (lb.)         297       291     310       200      280     243       164     170      170    140     160       136
    Utimate (lb.)        460       433     406       307      322     329       235     314      276    210     276       184


Trends in Pull-off Test Results

   The simply supported critical pull-off loads associated with each of the four failure definitions
(first audible cracking sound, initial load drop, first visual crack observed and ultimate failure) are
shown in Figure 7. The critical pull-off load follows the trend of decreasing magnitude with
increasing support span irrespective with the type of failure definition. The first audible crack and
initial load drop are close to one another and representative of crack initiation. The pull-off load for
initial visual crack is less important as the crack initiation is likely to occur at the center away from
both edges of the pull-off specimen [12]. The stitching seems to increase the ultimate failure load and
maximum deflection after crack initiation. If the joint is designed by crack initiation, the stitching is a
welcome feature for damage tolerance reserves. Whether the joint can be designed based on the
ultimate pull-off load should depend on the application and be carefully reviewed.
   Similar to Figure 7, critical pull-off loads for clamped specimens are plotted in Figure 8. The trends
are similar with the results of simply supported specimens. The critical pull-off loads are higher for
specimens under clamped conditions than for those under simply supported conditions for the same
span as seen from Figure 9. Since the difference in pull-off load varies significantly with the support
conditions, allowable strength of a joint should not be based on pull-off load. A parameter controlling
the failure mode that is independent of the span length and support condition is crucial to the design
of this type of joints. If such a parameter can be identified, the joint strength allowable could be
efficiently developed based on this parameter for design and analysis.

                                                               Critical Pull-off Loads
                                           300
                                                                             Audible (lb.)
                                           250                               Load drop (lb.)
                                                                             Visual (lb.)
          Pull-off Loads (lb.)




                                           200                               Utimate (lb.)


                                           150


                                           100


                                            50


                                             0
                                                 3   4   5               6             7               8       9        10
                                                                Simply Supported Span (in.)



Figure 7 Critical pull-off loads by four failure definitions for simply supported specimens.

                                                             Critical Pull-off Loads
                                           500
                                                                                   Audible (lb.)
                                           450                                     Load drop (lb.)
                                           400                                     Visual (lb.)
                                                                                   Utimate (lb.)
                    Pull-off Loads (lb.)




                                           350

                                           300

                                           250
                                           200

                                           150

                                           100

                                            50

                                             0
                                                 3   4   5           6             7               8       9       10
                                                                 Clamped Span (in.)

Figure 8 Critical pull-off loads by four failure definitions for clamped specimens.
                                                       Crack Initiation Pull-ff Loads
                               250


                                                                               Clamp supports
                               200
                                                                               Roller supports
   Pull-off Loads (lb.)




                                                                          Clamped = -15.95x + 252.32
                               150



                               100


                                                 Simply = -13.758x + 177.99
                               50



                                0
                                     3   4         5            6          7              8        9   10
                                                              Support Span (in.)


Figure 9 Crack initiation loads for clamped and simply supported conditions.

                                             Maximum Bending Moments at Crack Initiation
                               100

                                90
                                                   Clamped = 1.9049x + 54.226
                                80
   Moment/Width (lb-in./in.)




                                70

                                60

                                50                     Simply = -1.6454x + 74.393

                                40

                                30                           Clamp supports
                                20                           Roller supports

                                10

                                 0
                                     3   4         5           6          7               8        9   10
                                                             Support Span (in.)


Figure 10 Trend of maximum bending moments at crack initiation.

Critical Bending Moment

   The crack initiation pull-off load decreases linearly as the support span of the pull-off specimen
increases as shown in Figure 9 for either clamped or simply supported conditions. The bending
moments at mid span at crack initiation defined by the first load drop are shown in Figure 10 for
clamped and simply supported conditions. The calculation of the mid-span bending moment for
simply supported pull-off specimen is

                                               M simply       P L
                                                          =                                           (1)
                                                 W            W 4

Where P is the pull-off load and L is the span length. The bending moment is normalized by the
specimen width, W. The bending moment at the mid-span for clamped condition is evaluated from

                                   M clamped      P L   f 
                                                      
                                                                     s
                                                              1 − D11
                                                                           
                                                                           
                                                ≈     1 +               
                                                                                                      (2)
                                      W           W 8   L 
                                                                     sf
                                                                   D11     
                                                                           

Where f is the length between the tips of the horizontal angle clip flanges (or, approximately from the
                                                                      S
middles of the taper regions if the flanges taper down at the tips) D11 is the skin bending stiffness and
  sf
D11 is the bending stiffness of the composite skin and flange.
   The dependence of the critical bending moment on support span and support type is insignificant as
seen from Figure 10. The results shown in Figure 10 indicate that the initiation of crack is controlled
by the maximum bending moment at the critical location. Figure 10 also shows the advantage of
developing pull-off strength in terms of critical bending moment by eliminating the span length
effect. The allowable bending moment developed from either clamped condition or simply support
condition becomes less important.
   There are eight simply supported data points with an average of 62.9 in.-lb/in and a standard
deviation of 9.4 in.-lb/in. There are twelve clamped data points with an average of 67.6 in.-lb/in and a
standard deviation of 8.8 in.-lb/in. The average of the twenty data points is 65.7 in.-lb/in with a
standard deviation of 9.1 in.-lb./in. The averages and standard deviations are very close among the
three different sample methods. If the numbers of data points are equal, equivalent allowable could be
developed from any of the sample methods. The test data validated that the crucial factor controlling
the joint corner failure initiation is the bending moment and showed that the pull-off test under simply
supported condition and clamped condition are similar in failure mechanism and equivalent in
allowable development. The allowable bending moment is potentially significant for unitized
composite structures since it serves as a crucial link between the joint failure model and global
nonlinear analysis that constitutes the high fidelity failure analysis methodology outlined in Ref. 13.

CONCLUSION

   Tests and analyses of stitched pull-off specimens show that crack initiation is controlled by the
maximum bending moment at the critical location. The mechanism behind the joint failure initiation
is the maximum tension strain caused by bending. Allowable developed in terms of maximum critical
bending moment is independent of the support condition and span length. The allowable bending
moment developed from either clamped condition or simply supported condition becomes equivalent.
Enormous savings of allowable development costs and greatly increased accuracy in analysis can be
achieved by capturing the right crucial factor governing the actual failure model.
   On the other hand, the support conditions and span lengths greatly complicate failure analysis and
allowable based on pull-off load. As the test data indicate the critical pull-off load reduces as the
support span increases, and the critical pull-off loads are higher under clamped condition than under
simply supported condition for the same support spans. The dependence of pull-off load on support
span and support conditions greatly increases the cost of allowable development based on pull-off
load and the likelihood of error in design when this allowable is applied.

ACKNOWLEDGEMENTS

This work was supported with shared funding by the U. S. Rotorcraft industry and government under
the RITA/NASA Cooperative Agreement Contract No. NCC2-9019, under WBS No. 01-B-03-7.2-P2
for Structural Joining Technologies. The contributions of Integrated Technologies Corporation in
specimen preparations and testing are greatly appreciated.

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