Failure Analysis of a Composite Laminate in HyperWorks through “Ply Elimination” Procedure Ing. Daniele Di Sanzo Engineering - Rotor System Design [email protected]
Nov 28, 2014
Failure Analysis of a Composite Laminate
in HyperWorks through “Ply Elimination”
Procedure
Ing. Daniele Di Sanzo
Engineering - Rotor System Design
2
Failure Analysis of a Composite Laminate in HyperWorks
through “Ply Elimination” Procedure
SUMMARY
Introduction – Ply Elimination Procedure
Subroutine HM_PE.tcl
Presentation of the problem
Analysis and Results
Future developments and Conclusions
3
Composite Laminate Analysis (“Ply Elimination”) in HW
Introduction – Ply Elimination Procedure
Design assessment of a composite laminate
Failure Analysis: a negative MS (Margin of Safety) does not necessarily imply the failure
of the composite laminate.
In order to assess the final static ultimate strength, further investigation is needed.
“Ply Elimination” is a procedure suitable for this purpose. It allows in-depth
investigations into the resistance capabilities of composite laminates.
Obviously, a real test is needed to have the final validation and approval of the solution.
4
Composite Laminate Analysis (“Ply Elimination”) in HW
Introduction – Ply Elimination Procedure
What is Ply Elimination?
A negative Margin of Safety (MS) not always means a catastrophic failure of the
laminate.
The UD strength is much dominated by the fiber strength.
When MS negative is caused by transverse stress s2 overcoming the allowable, the
corresponding damage in the matrix can be simulated by degrading the material
properties in the transverse and shear directions.
Degrading the elastic properties of the areas with MS<0, the laminate stiffness
changes as well and thus stresses are re-distributed.
It is an iterative process, which ends with positive MS unless fibers break.
In case of fibers break, this actually means the final failure of the laminate
5
Composite Laminate Analysis (“Ply Elimination”) in HW
Introduction – Ply Elimination Procedure
Ply Elimination Logical Flow
The process can be summarized with the scheme below:
This is a simplified procedure, which does not consider the type of failure (matrix or fibers failure).
Further development will be discussed later.
MS > 0
MS < 0
Run analysis
Select elements
with MS<0
MS
calculation Apply “fake” material
(E2, G12 small)
End of analysis
Iterative Loop
End of analysis
6
Composite Laminate Analysis (“Ply Elimination”) in HW
Introduction – Ply Elimination Procedure
Ply Elimination Process in HyperWorks
Iterative Loop
Set A (Fake) Set B (Actual)
MS contour
HyperView HyperMesh
FE Software:
Radioss Linear
1. Create/update Actual Set (MS>0)
2. Create/update Fake Set (MS<0)
Re-run analysis
Assign the proper material
7
Composite Laminate Analysis (“Ply Elimination”) in HW
Introduction – Ply Elimination Procedure, Startup in HM
Modify the FE model as follows:
1) create fake material→ fake_mat
2) create element sets {el_actual_PLY# - el_fake_PLY#} for each ply
3) create ply sets {actual_PLY# - fake_PLY#} for each ply
4) define stack sequence (STACK)
STACK
el_actual_PLY# actual_PLY#
mat
el_fake_PLY# fake_PLY#
fake_mat
n plies
n plies
Ply Elimination Process: Startup in HyperMesh (i.e. “Step 0”)
Preliminary
operations
8
2) create element sets for each ply
el_actual_PLY#
identifies the original ply#
at step 0
el_fake_PLY#
empty set at step 0, gathers the
degraded elements
1) create degraded material fake_mat
degradation of transverse elastic properties: E2=G12=1
Composite Laminate Analysis (“Ply Elimination”) in HW
Introduction – Ply Elimination Procedure, Startup in HM
Stress
allowables
Ste
p 0
Xt Xc Yt Yc S
nu12 E1
9
3) create ply sets
- attribute properties and the corresponding element set
actual_PLY#
fake_PLY#
el_actual_PLY#
el_fake_PLY#
Empty elset
@ step 0
Composite Laminate Analysis (“Ply Elimination”) in HW
Introduction – Ply Elimination Procedure, Startup in HM
for each ply (actual)
for each ply (fake)
ID Ste
p 0
t
t
10
4) define stack sequence (from HM Solver Browser Tree)
…
:
:
PLY_301
PLY_302
.
:
PLY_313
.
:
…
…
:
:
actual_PLY_ 301
fake_PLY_ 301
actual_PLY_302
fake_PLY_ 302
.
:
actual_PLY_313
fake_PLY_ 313
.
:
…
n plies n + n plies
Composite Laminate Analysis (“Ply Elimination”) in HW
Introduction – Ply Elimination Procedure, Startup in HM
for the subset of plies, introduce the fake plies in couple with the actual ones and redefine
the stack sequence properly, as follows:
Ste
p 0
11
Then, Optimization Panel is used to define the equation for MS, as function
of ply stresses (s1, s2, s12), for both the actual plies and the fake ones.
Composite Laminate Analysis (“Ply Elimination”) in HW
Introduction – Ply Elimination Procedure, Startup in HM
Failure Criterion: TSAI-WU
This allows to have MS among the outputs of the analysis in OptiStruct.
Thus, MS pattern is available in HyperView for post-processing.
The model is now ready for ply elimination iterative loop, performed by subroutine
HM_PE.tcl.
Ste
p 0
F11
F22 F66
F12
F1
F2
SF
12
The subroutine HM_PE.tcl, programmed in Tcl-Language, has been developed
as an accessory tool to be run in HM (from command window) for an automatic
application of the ply elimination loop.
Composite Laminate Analysis (“Ply Elimination”) in HW
The subroutine HM_PE.tcl
#######################################################################
# SUBROUTINE HM_PE.tcl: MAIN INPUT DATA #
#######################################################################
set MODEL_ref "MyModel.fem" # original fem model (filename)
set actual_matID 20 # ID original material (actual)
set fake_matID 21 # ID degraded material (fake)
set SUBCASE_ID 7 # ID load condition
set plyID_list "25 26 27 28 29 30 31 32" # List of plies (ID) for PE process
set stepID k # current step of ply elimination
#######################################################################
At each step (k), HM_PE.tcl retrieves the analysis results from FE output file MyModel_step(k).out
and generates the new FE input file MyModel_step(k+1).fem, with the update of actual plies and
fake plies.
k = 0:1:K
13
The working flow proceeds as following:
Composite Laminate Analysis (“Ply Elimination”) in HW
The subroutine HM_PE.tcl
and so on
Model_step0.fem
Ste
p 0
HM_PE.tcl
Model_step0.out
Run
Analysis Iterate until:
o element sets have not
been updated anymore
o the overall laminate fails
Model_step1.fem
Ste
p 1
HM_PE.tcl
Model_step1.out
Run
Analysis
Model_step2.fem
Warning Message in HM
14
Problem: Design optimization of a helicopter MR blade composite tip
Composite Laminate Analysis (“Ply Elimination”) in HW
Problem Definition
Composite Skin Tip
15
Composite Laminate Analysis (“Ply Elimination”) in HW
Problem Definition
Considering a skin tip made up of U/D plies, two different composite materials
have been investigated:
Which of them is the most reliable solution? The evaluation of MS (Tsai-Wu),
and the PE procedure as well, will help us to find out the best choice.
Glass fibers or Graphite fibers?
16
The evaluation has been carried out with the most critical load condition.
Composite Laminate Analysis (“Ply Elimination”) in HW
Skin Tip – Ply Elimination, MS Contour
Upper skin tip,
Ply 306 @ +45°
PE Step 0
MS CONTOUR PLOT
MS < 0
Note that, without stress evaluation, you are not able to say if MS<0 is due to an
overstress in fiber direction or in transverse/shear direction.
actual_PLY306 fake_PLY306
MS ≥ 0
0.0
P
O
S
NEG
17
Composite Laminate Analysis (“Ply Elimination”) in HW
Skin Tip – Ply Elimination, Fiberglass plies
PLY ELIMINATION: Upper skin tip, Fiberglass plies (Ply 303 @ 0°)
actu
al_
PLY
303
Step 0 Step 1 Step 2
Step 3 Step 4 Step 5
0.0
P
O
S
NEG
0.0
P
O
S
NEG
0.0
P
O
S
NEG
0.0
P
O
S
NEG
0.0
P
O
S
NEG
0.0
P
O
S
NEG
18
Composite Laminate Analysis (“Ply Elimination”) in HW
Skin Tip – Ply Elimination, Fiberglass plies
Step 10 Step 15 Step 20
Step 25 Step 30
PE convergence has been reached.
Updates of the model after step 20
are not significant.
PLY ELIMINATION: Upper skin tip, Fiberglass plies (Ply 303 @ 0°)
actu
al_
PLY
303
0.0
P
O
S
NEG
0.0
P
O
S
NEG
0.0
P
O
S
NEG
0.0
P
O
S
NEG
0.0
P
O
S
NEG
19
Composite Laminate Analysis (“Ply Elimination”) in HW
Skin Tip – Ply Elimination, Graphite plies
Step 0 Step 1 Step 2
Step 3 Step 4 Step 5
PLY ELIMINATION: Upper skin tip, Graphite plies (Ply 303 @ 0°)
actu
al_
PLY
303
0.0
P
O
S
NEG
0.0
P
O
S
NEG
0.0
P
O
S
NEG
0.0
P
O
S
NEG
0.0
P
O
S
NEG
0.0
P
O
S
NEG
20
Composite Laminate Analysis (“Ply Elimination”) in HW
Skin Tip – Ply Elimination, Graphite plies
Step 8 Step 10 Step 13
PE convergence has been reached
for both configurations.
Faster convergence for Graphite skin
tip (reached at step 13)
PLY ELIMINATION: Upper skin tip, Graphite plies (Ply 303 @ 0°)
actu
al_
PLY
303
The degraded area is smaller than
that for the fiberglass solution.
Is graphite really to be
preferred to fiberglass?
0.0
P
O
S
NEG
0.0
P
O
S
NEG
0.0
P
O
S
NEG
21
Composite Laminate Analysis (“Ply Elimination”) in HW
Skin Tip – Ply Elimination, results
actual_PLY303 fake_PLY303
Gra
ph
ite
F
ibe
rgla
ss
MS < 0
MS > 0
Up
pe
r S
kin
Tip
Fiber
Failure
0.0
P
O
S
NEG
0.0
P
O
S
NEG
0.0
P
O
S
NEG
0.0
P
O
S
NEG
22
Composite Laminate Analysis (“Ply Elimination”) in HW
Skin Tip – Ply Elimination, results
actual_PLY303 fake_PLY303
Gra
ph
ite
F
ibe
rgla
ss
MS < 0
MS > 0
Lo
we
r S
kin
Tip
Fiber
Failure
0.0
P
O
S
NEG
0.0
P
O
S
NEG
0.0
P
O
S
NEG
0.0
P
O
S
NEG
23
Composite Laminate Analysis (“Ply Elimination”) in HW
Skin Tip – Ply Elimination, results
Ply Elimination has shown that graphite skin tip can be subjected to fiber failure.
Is it really a local phenomenon or does it propagate?
In response to this question, a further degradation of the material can be
included in the procedure. A new iterative loop is performed.
Fake_mat
Fail_mat
Actual_mat
Actual_PLY E2=G12=1
E1=E2=G12=1
Fake_PLY
Fail_PLY
If the new ply set Fail_PLY becomes larger and larger as the iteration proceeds,
it means that fiber failure propagates: catastrophic failure of the laminate occurs.
NEW !
24
Composite Laminate Analysis (“Ply Elimination”) in HW
Skin Tip – Ply Elimination, results
PLY ELIMINATION (extended): Lower skin tip, Graphite plies
actual_PLY303 fake_PLY303 fail_PLY303
Ste
p 2
.1
Ste
p 2
.2
Ste
p 2
.0
0.0
P
O
S
NEG
0.0
P
O
S
NEG
0.0
P
O
S
NEG
0.0
P
O
S
NEG
0.0
P
O
S
NEG
0.0
P
O
S
NEG
0.0
P
O
S
NEG
0.0
P
O
S
NEG
0.0
P
O
S
NEG
25
Composite Laminate Analysis (“Ply Elimination”) in HW
Skin Tip – Ply Elimination, results
actual_PLY303 fake_PLY303 fail_PLY303
Ste
p 2
.5
The failed area (MS<0, due to s1 overcoming the allowable) is bound to expand.
It is confirmed that catastrophic failure occurs.
Thus, the graphite skin tip is not acceptable.
PLY ELIMINATION (extended): Lower skin tip, Graphite plies
Note that: actual_PLY + fake_PLY + fail_PLY = PLY (the real ply)
0.0
P
O
S
NEG
0.0
P
O
S
NEG
0.0
P
O
S
NEG
26
Composite Laminate Analysis (“Ply Elimination”) in HW
Conclusions: further developments
The current procedure, just as discussed, is conservative.
The degradation to fail_mat is not direct, but passes through the intermediate
fake_mat (i.e. matrix failure), even though s1 overcoming the allowable has
occurred immediately (that is fiber failure).
As development, the routine degrades the areas with MS<0 depending on the
type of failure.
MS<0
fake_PLY fail_PLY
MS<0
MS<0
s1>X
Fiber failure
s2>Y | t12>S (s1<X)
Matrix failure
Real PLY
actual_PLY
Iterative loop
s1>X
Fiber failure
Fake_mat: E2=G12=1
Fail_mat: E1=1 + E2=G12=1
27
Composite Laminate Analysis (“Ply Elimination”) in HW
Conclusions: further developments
TSAI-WU failure criterion is not capable of distinguishing matrix failure from
fiber failure. This limitation can be overcome by including the max stress
criterion. The procedure shall be modified as follows:
evaluation of the stresses (post-processing of file .h3d, in addition to .out)
comparison of the stresses with the allowables:
s1>X: fiber failure
s2>Y | t12>S (s1<X): matrix failure
degradation of the elements with MS<0 according to the type of failure.
and so on
Model_step(k+1).fem
Model_step(k).fem
Ste
p k
HM_PE.tcl
Model_step(k).out
Run
Analysis
Model_step(k).h3d
MS
sij
28
Composite Laminate Analysis (“Ply Elimination”) in HW
Conclusions: further developments
The subroutine HM_PE.tcl has been modified accordingly. The main benefits are:
faster convergence/divergence of the loop: fewer steps are needed
distinction of fiber failure from matrix failure, from the very beginning of the procedure
if fiber failure occurs, it is found out earlier (thanks to the “parallel” degradation)
Remember that:
matrix failure is acceptable
fiber failure may be catastrophic
In conclusion, the implementation of this subroutine has allowed to automatize
Ply Elimination in HW. Thanks to this tool, a more in-depth investigation of the
structure is possible. It is easier to understand where a reinforcement is needed,
thus helping to re-design the structure properly.
The results with the modified subroutine are not here presented