Failure Analysis of a Composite Laminate in HyperWorks through “Ply Elimination” Procedure

Failure Analysis of a Composite Laminate

in HyperWorks through “Ply Elimination”

Procedure

Ing. Daniele Di Sanzo

Engineering - Rotor System Design

[email protected]

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


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



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



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



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


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



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



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



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.



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.


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:


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


Problem Definition

Composite Skin Tip

15


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.


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


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


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


Skin Tip – Ply Elimination, Fiberglass plies


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


Skin Tip – Ply Elimination, Graphite plies



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


Skin Tip – Ply Elimination, Graphite plies


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


Skin Tip – Ply Elimination, results


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




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



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



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



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

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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

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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



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

Technology

Failure Analysis of a Composite Laminate in HyperWorks through “Ply Elimination” Procedure