Chen, Zhi M.

Krueger, Ronald

Rinker, Martin

Typical damage modes in light honeycomb sandwich structures include face sheet/core disbonding and core fracture, both of which can pose a threat to the structural integrity of a component. These damage modes are of particular interest to aviation certification authorities since several in-service occurrences, such as rudder structural failure and other control surface malfunctions, have been attributed to face sheet/core disbonding. Extensive studies have shown that face sheet/core disbonding and core fracture can lead to damage propagation caused by internal pressure changes in the core. The increasing use of composite sandwich construction in aircraft applications makes it vitally important to understand the effect of ground-air-ground (GAG) cycles and conditions such as maneuver and gust loads on face sheet/core disbonding. The objective of the present study was to use a fracture mechanics based approach developed earlier to evaluate the loading at the disbond front caused by ground-air-ground pressurization and in-plane loading. A honeycomb sandwich panel containing a circular disbond at one face sheet/core interface was modeled with three-dimensional (3D) solid finite elements. The disbond was modeled as a discrete discontinuity and the strain energy release rate along the disbond front was computed using the Virtual Crack Closure Technique (VCCT). Special attention was paid to the pressure-deformation coupling which can decrease the pressure load within the disbonded sandwich section significantly when the structure is highly deformed. The commercial finite element analysis software, Abaqus/Standard, was used for the analyses. The recursive pressure-deformation coupling problem was solved by representing the entrapped air in the honeycomb cells as filled cavities in Abaqus/Standard. The results show that disbond size, face sheet thickness and core thickness are important parameters that determine crack tip loading at the disbond front. Further, the pressure-deformation coupling was found to have an important load decreasing effect [6]. In this paper, a detailed problem description is provided first. Second, the analysis methodology is presented. The fracture mechanics approach used is described and the specifics of the finite element model, including the fluid-filled cavities, are introduced. Third, the initial model verification and validation are discussed. Fourth, the findings from a closely related earlier study [6] are summarized. These findings provided the basis for the current investigation. Fifth, an aircraft ascent scenario from 0 to 12192 m (0 to 40000 ft) is considered and the resulting crack tip loading at the disbond front is determined. In-plane loading to simulate maneuvers and gust conditions are also considered. Sixth, the results are shown for a curved panel, which was used to simulate potential fuselage applications. Finally, a brief summary of observations is presented and recommendations for improvement are provided

NASA Technical Reports Server

Face Sheet/Core Disbond Growth in Honeycomb Sandwich Panels Subjected to Ground-Air-Ground Pressurization and In-Plane Loading

FACE SHEET/CORE DISBOND GROWTH IN 
HONEYCOMB SANDWICH PANELS SUBJECTED 
TO GROUND-AIR-GROUND PRESSURIZATION 
AND IN-PLANE LOADING  
Zhi M. Chen 
University of California at San Diego, La Jolla, USA 
Ronald Krueger  
National Institute of Aerospace, Hampton, VA, USA 
Martin Rinker 
Rolls-Royce Deutschland Ltd & Co KG, Dahlewitz, Germany 
The Aerospace Corporation, El Segundo, CA, June 25, 2015                                                               
Work was funded by NASA Langley Research Center under contract number NNL09AA00A  
https://ntrs.nasa.gov/search.jsp?R=20160007045 2019-08-31T02:49:52+00:00Z
OVERVIEW 
•  Background 
•  Objective 
•  Detailed Problem Description 
•  Analysis Methodology 
–  Fracture Mechanics Approach 
–  Finite Element Modelling 
•  Initial Model Verification and Validation 
•  Analysis Results 
–  Analysis of a Flat Panel Under Internal Pressure, In-Plane and Combined 
Loading 
–  Analysis of a Curved Panel 
•  Summary 
•  Concluding Remarks 
BACKGROUND 
•  Problem 
•  In-service component failures associated with disbonding in unvented 
honeycomb core sandwich 
•  Degradation due to disbonding affects operational safety  
•  Failures may discourage use of composites in ‘future’ vehicles 
•  Methods for assessing propensity of sandwich structures to disbonding 
not fully matured, accepted and documented 
•  Methods development is currently being discussed within the Disbond/
Delamination Task Group in CMH-17  
Aviation Marine Space 
OBJECTIVE 
•  Identify, describe and address the phenomenon associated with face 
sheet/core disbonding  
•  Increase the knowledge on the subject and the awareness of 
consequences 
•  Develop a methodology to assess face sheet/core disbonding in 
honeycomb sandwich components similar to delamination in 
composite laminates 
–  Develop standard test methods for characterizing face sheet/core 
disbonding in sandwich components 
–  Develop a fracture mechanics based methodology to assess face sheet/
core disbonding in sandwich components 
–  Develop models and analysis tools for face sheet/core disbonding in 
sandwich components subjected to ground-air-ground cycles and/or in-
plane loading 
–  Evaluate the developed test methods and analysis tools using 
honeycomb sandwich panel tests 
DETAILED PROBLEM  
DESCRIPTION 
•  Pressure difference between the in- and 
outside of unvented sandwich structures  
•  Caused by alternating ambient pressure and 
temperature 
•  Results in significant deformations and core 
volume increase 
•  Volume increase results in pressure decrease 
based on the ideal gas law 
    p V = n R T  
•  Initial disbonds between face sheets and 
core  
–  increase the peeling effect and  
–  decrease the structural reliability significantly 
•  For an accurate structural analysis, a 
coupled pressure-deformation problem 
needs to be solved 
•  Initial configuration at ground 
elevation 
•  Deformed configuration at 
cruising altitude 
disbond at face sheet/
core interface
face sheet
honeycomb 
core
intact intact
cavity created by bulging of disbonded 
section
intact intact
ANALYSIS METHODOLOGY 
Fracture Mechanics Approach 
•  Two steps are required to identify, 
describe and address face sheet/
core disbonding  
–  Test standard development in ASTM 
committee D30 (WK 47682) 
o  Characterize the properties of the face 
sheet/core interface[14] 
o  Measure fracture toughness Gc  
–  Analysis Development 
o  Compute the energy release rate along 
the disbond front  
o  Use the Virtual Crack Closure Technique 
(VCCT) based on the results obtained 
from a finite element analysis  
•  Propagation is predicted to occur 
once the computed value exceeds 
the measured fracture toughness 
[14] reference to publication cited in conference proceedings 
ANALYSIS METHODOLOGY 
Finite Element Modelling – 1/4 
•  A quarter section of a flat panel was 
modelled 
–  Circular disbond radius: 152.4 mm (6”) 
–  Square section modelled: 304.8 mm (12”)  
–  Abaqus/Standard® was used (C3D20) 
o  Boundary conditions applied at symmetry 
planes 
o  Surface contact used between top face 
sheet and core in the disbonded section  
•  Sandwich properties based on 
previous results 
-  Thin face sheet: 0.772 mm (0.03”) 
o  CYCOM 5320PW plain weave fabric 
o  [45/0/90/-45] quasi-isotropic layup 
-  Thick core: 76.5 mm (3.0”) 
o  Hexcel HRH-10® honeycomb 
o  NOMEX® paper with 48 kg/m3 (3.0 lb/ft3) 
density and 3.175 mm (1/8”) cell size 
o  Modelled as an orthotropic, homogeneous 
continuum   
cavity for 
intact 
sandwich
cavity for 
disbonded 
sandwich
disbond front
xy
z
intact sandwich section
lower face sheet
upper face 
sheet
honeycomb core
xy
z
ANALYSIS METHODOLOGY 
Finite Element Modelling – 2/4 
•  Pressure deformation coupling was 
simulated using fluid filled cavities 
–  Abaqus/Standard® feature enabled the 
definition of fluid-filled cavities enclosed 
by structural elements  
–  The ideal gas law is solved within each 
increment until equilibrium is found 
–  The volume of the fluid cavities was 
assumed to be equal to that of the 
entire sandwich core  
–  Two separate cavities were defined 
o  One cavity was used to simulate the 
intact part 
o  The other cavity included only the 
disbonded section  
o  The disbonded cavity extended by one 
cell size, 3.175 mm (1/8”), ahead of the 
disbond front  
disbond front
cavity for intact 
sandwich
cavity for disbonded 
sandwich
x
y
ϕ
cavity for intact 
sandwich
cavity for disbonded 
sandwich
disbond front
x
y
z
R = 3 m
ANALYSIS METHODOLOGY 
Finite Element Modelling – 3/4 
xy
z applied end 
displacement to simulate 
0.2% in-plane strain
•  Model of a flat panel with in-plane 
loading 
–  Study the effect of in-plane service 
load on a flat control surface 
–  In-plane displacement applied to the 
model to simulate a 0.2% (2000 µε) 
strain condition during a flight 
maneuver 
–  A compressive strain condition was 
chosen since it was believed that it 
would aggravate the condition 
•  Model of a curved panel 
-  Honeycomb sandwich constructions 
may be used for cylindrical fuselage 
structures 
-  A 3 m radius (wide body airliner) was 
chosen for this study  
ANALYSIS METHODOLOGY 
Finite Element Modelling – 4/4 
•  Internal pressurization of the 
disbond 
–  Commercial jetliner ascent scenario 
was considered from 0 to 12192 m (0 
to 40000 ft).  
–  The pressure and temperature values 
were taken from the International 
Standard Atmosphere ISO 2533 
–  The temperature in the core was 
defined to be equal to the ambient 
temperature  
–  Pressure and volume inside the 
cavities were calculated during the 
analysis  
•  Additional load conditions 
–  0.2% (2000 µε) strain condition only 
–  Combination of GAG and 0.2%    
(2000 µε) strain 
•  Decrease of temperature and 
pressure with increasing altitude 
0
220
230
240
250
260
270
280
290
0
0.02
0.04
0.06
0.08
0.1
0.12
0 2000 4000 6000 8000 10000 12000 14000
T [K]
p [MPa]
T, K p, MPa
altitude h, m
12,200 m
(40,000 ft)
0.0188 MPa
216.65 K
ANALYSIS RESULTS 
Flat panel under internal pressure loading – 1/3 
•  Initial study[6] 
–  Variation of 
o  Face sheet thickness, number of 
plies  
o  Disbond radius 
o  Core density: 29 kg/m3, 48 kg/m3, 
80 kg/m3 (1.8 lb/ft3, 3.0 lb/ft3, 
5.0 lb/ft3) 
o  Core thickness: 12.5 mm, 
25.4 mm, 50.8 mm, 76.5 mm  
(0.5” – 3.0”) 
–  Results 
o  Variation of core density does not 
have a significant effect on 
computed GT 
o  Large disbond radius and thin 
face sheets result in maximum GT 
•  Current study 
-  Dimensions based on results 
from initial study 
GT, J/m2 
disbond 
radius, mm number of face 
sheet plies 
•  Averaged GT along crack front 
3.275 mm (1/8”) cell size, 48 kg/m3 (3.0 lb/ft3) core density 
0
1000 
500 
ANALYSIS RESULTS 
Flat panel under internal pressure loading – 2/3 
•  Conditions 
–  12,192 m altitude (40,000 ft) 
o  External pressure p=0.0188 MPa 
o  External temperature T= 216.65 K  
•  Verification for using a FE 
model of a quarter section of 
the panel 
-  Analysis using a full model of the 
panel with circular disbond 
-  Analysis using a model of a 
quarter panel with boundary 
conditions 
-  Excellent agreement of computed 
GT along the front for the currently 
used quasi-isotropic layup 
-  Deviation, however, for other 
layups that violate the symmetry 
conditions of the model 
•  Distribution of energy release 
rate along the disbond front 
ANALYSIS RESULTS 
Flat panel under internal pressure loading – 3/3 
•  Conditions 
–  12,192 m altitude (40,000 ft) 
o  External pressure p=0.0188 MPa 
o  External temperature T= 216.65 K  
•  Result 
•  Max GT observed at ϕ=45° 
 
0.0
0.1
0.2
0.3
0.4
0.5
0.6
0 2000 4000 6000 8000 10000 12000
G
T
,
kJ/m2
altitude h, m
•  Conditions 
–  0 m - 12,192 m altitude 
–  Sea level to cruising altitude 
•  Results for max GT at ϕ=45°  
-  GT  increases monotonically with 
increasing altitude 
 
ANALYSIS RESULTS 
Flat panel under in-plane and combined loading 
•  Conditions 
–  12,192 m altitude (40,000 ft) 
o  External pressure p=0.0188 MPa 
o  External temperature T= 216.65 K 
-  0.2% (2000 µε) applied in-plane 
strain to simulate service loads on 
a flat control surface  
-  Combined internal pressure + 0.2% 
(2000 µε) in-plane strain 
•  Results 
-  Out of plane deformation of the 
disbonded section changes 
-  Leads to a change in the GT 
distribution 
-  In-plane strain aggravates the 
condition 
-  Due to non-linearity superposition 
of the results is not possible  
•  Distribution of energy release 
rate along the disbond front 
xy
z
ANALYSIS RESULTS 
Analysis of a curved panel 
•  Conditions 
–  12,192 m altitude (40,000 ft) 
o  External pressure p=0.0188 MPa 
o  External temperature T= 216.65 K 
-  Flat panel 
-  Curved panel with 3 m radius 
•  Results 
-  Symmetry of the GT distribution is 
lost for the curved panel 
-  Locally and on average the 
computed GT is higher than the 
result obtained from the flat panel 
-  Result is unexpected 
-  In-plane strain may further 
aggravate the condition 
-  Additional analyses with different 
radii and more refined mesh 
should be preformed before a 
definite statement is made 
•  Distribution of energy release 
rate along the disbond front 
x
y
z
R = 3 m
SUMMARY 
•  A sandwich panel containing a circular disbond at the face sheet/core 
interface was studied.  
•  A fracture mechanics approach was used.  
•  The pressure-deformation coupling was a focus of the analysis.  
•  Special fluid-filled cavities were used to model the entrapped air.  
•  Sandwich panels with large disbonds, thin face sheets, and thick cores are 
most critical.  
•  Computed averaged energy release rate values increased almost linearly 
with increasing altitude. 
•  The presence of the in-plane compressive strain aggravated the condition 
along the crack front. 
•  Due to the non-linearity of the problem, the results for combined load cases 
cannot simply be obtained by superposition of the individual load cases. 
•  For a curved panel with 3 m radius, the computed energy release rate 
values were higher than the values computed for a flat panel. 
CONCLUDING REMARKS 
•  Overall, the finite element analysis with fluid cavities appears to perform 
well and is capable of capturing the pressure-deformation coupling in the 
disbonded section of the panel. 
•  Based on the current preliminary results, however, it is recommended that 
additional validation studies be performed to compare. 
–  The computed local deformation field of the disbonded face sheet with 
far field measurements  
–  The computed pressure inside the cavity with measured values.  
•  Additionally, analyses of curved panels with different radii should be 
performed before a definite statement about the effect of panel curvature 
on the crack tip loading is made. 
•  Methods development will continue within the Disbond/Delamination Task 
Group in CMH-17  
ACKNOWLEDGEMENTS 
The analyses were performed at the Durability, 
Damage Tolerance and Reliability Branch at NASA 
Langley Research Center, Hampton, Virginia, USA 
while Zhi Chen was a participant in the Langley 
Aerospace Research Student Scholars (LARSS) 
program. Ronald Krueger (NIA) was supported under 
contract NNL09AA00A and Martin Rinker was a 
visiting scientist at the National Institute of 
Aerospace (NIA). 
FACE SHEET/CORE DISBOND GROWTH IN 
HONEYCOMB SANDWICH PANELS SUBJECTED 
TO GROUND-AIR-GROUND PRESSURIZATION 
AND IN-PLANE LOADING  
Zhi M. Chen 
University of California at San Diego, La Jolla, USA 
Ronald Krueger  
National Institute of Aerospace, Hampton, VA, USA 
Martin Rinker 
Rolls-Royce Deutschland Ltd & Co KG, Dahlewitz, Germany 
The Aerospace Corporation, El Segundo, CA, June 25, 2015                                                               
Work was funded by NASA Langley Research Center under contract number NNL09AA00A  
BACKUP SLIDES 
INITIAL MODEL VERIFICATION  
AND VALIDATION – 1/2 
•  X-33 cryogenic fuel tank 
–  NASA sandwich disbond 
investigation[3] 
o  Square delamination 
o  Panel pressurized by a 
compressor  
o  Defined load, no pressure-
deformation coupling 
o  Calculations were performed 
using surface loads 
–  Current analysis approach[6] 
o  Same dimensions as NASA 
publication  
o  Pressure application with Abaqus 
fluid elements 
o  VCCT calculation using post-
processing routine  
–  Result comparison 
o  Good correlation between GT values 
calculated using different models 
GT, 
 J/m2 
disbond length, mm 
p= 552 kPa 
p= 552 kPa[3] 
p= 827 kPa 
p= 827 kPa[3] 
 
INITIAL MODEL VERIFICATION  
AND VALIDATION – 2/2 
•  Sandwich panel with disbond  
–  Airbus test in vacuum chamber[4] 
o  Panel with 350 mm disbond 
o  Pressure-deformation coupling needs to 
be considered 
o  Pressure in disbonded core section was 
measured during test 
o  FE analysis was performed calculating 
pressure-deformation coupling 
iteratively 
–  Current analysis approach 
o  Same dimensions as Airbus panel 
o  Pressure pressure-deformation 
coupling solved with Abaqus fluid 
elements 
–  Result comparison 
o  Pressure-deformation coupling is 
correctly solved via Abaqus Fluid 
Cavity Simulation 
o  Pressure in core: 
o  Airbus test:  0.0582 MPa 
o  Airbus analysis: 0.0577 MPa 
o  Current analysis: 0.0571 Mpa 
 
•  Additional validation studies 
should be performed to compare 
test results and analysis 
–  Compare deformation field  
–  Compare pressure inside the cavity 


Face Sheet/Core Disbond Growth in Honeycomb Sandwich Panels Subjected to Ground-Air-Ground Pressurization and In-Plane Loading

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