Rony Das_Sampe Presentation_ June 5 2012

STRESS RELAXATION BEHAVIOR OF CARBON
FIBER-EPOXY PREPREG COMPOSITES

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DURING AND AFTER CURE

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Rony Das

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MS in Mechanical Engineering, May 2012
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Supervisor: Dr. Bob Minaie
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Mechanical Engineering Department
Wichita State University, Wichita, KS
R

Wichita SAMPE chapter meeting, June 5, 2012

Outline of the Presentation

 Motivation

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 Objectives
 General Background
 Methodology

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 Results

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 Conclusions
 References N
O
R

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Introduction
Motivation

 Motivation
Composites are materials of high specific strength and stiffness as

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

well as the low CTE
 One significant setback to the advancement of composite materials

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is the poor dimensional stability caused by process-induced
stresses

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 Thermosetting composites (TSCs) have a tendency to release
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these stresses as a function of time until it reaches a steady state
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 Therefore, the knowledge of stress relaxation is important
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Introduction
Objectives

 Objectives
Obtain better understanding of stress relaxation behavior during

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
and after cure of TSCs
 Compare the performance between different prepreg systems

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 Study factors that could potentially affect the relaxation behavior of
TSCs

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Study stress relaxation behavior of cured composites using time-
 N
temperature superposition principle and determine their life cycle
O
R

4

General Background
Factors Affecting Stress Formation during Processing

 Resin Shrinkage
30% of total process-induced stress can be caused by the resin shrinkage

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

 It can be significantly different in three principal directions of the laminate

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Y
N
O
R

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General Background

 Process Cycle
Process history of composite materials varies with the cure cycle

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

 Heating/cooling rate and cure temperature dominates the CTE mismatch
between resin and fiber

D
Y
N
O
R

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General Background

 Stacking Sequence
Fiber orientation mismatch introduces inter-ply CTE mismatch between the

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

adjacent plies which creates more residual stress during processing

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Y
N
O
R

Reference: [1]
7

General Background

 Tool-Part Interaction
CTE in the fiber direction of TSCs ≈ -0.5 μm/m while CTE of aluminum

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

≈ 24 μm/m

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Y
N
O
R

Reference: [2]

Reference: [1]
8

General Background
Concept of Stress Relaxation

 Stress Relaxation
 At the time of stress relaxation, polymer molecules rotate and unwind due

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to the applied strain. By maintaining the stress level same, rearrangement
of molecules continues and reduces the stress as a function of time

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Y
N
Relaxation behavior of polymers is expressed by the stress relaxation
O

modulus:  (t )
E (t ) 
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o
Where E(t) is the relaxation modulus, σ(t) is the time-dependent stress, εo
is the applied strain

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Methodology
Materials

 Materials
 IM7/977-2 unidirectional tape (UD)

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 IM7/977-2 plain weave fabric (PW)

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Cure Profile:

Y

Schematics of (a) unidirectional (UD) prepreg and
 Manufacturer’s recommended cure cycle
N (b) plain weave (PW) prepregs [2]
O
Cure Procedure UD/PW Prepreg
R

Ramp rate 0.6 - 2.8 oC/min

Cure temperature 177 oC

Cure time 3 hours

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Methodology
Experimental Methods and Equipment

 Study of cure kinetics:
 Q2000 DSC (TA Instruments New Castle, DE)

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 Specimen weight: 10-15 mg

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 Study of stress relaxation:
 Q800 DMA (TA Instruments New Castle, DE)

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 Specimen size: 40mm × (1.8-4)mm × (0.25-0.60)mm
 Loading conditions: Static or Dynamic
N
O
R

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Methodology
Stress Relaxation during Cure

 Experiments were performed with a static load of 0.01 N
Considered different factors that could potentially affect the relaxation behavior

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
of laminates

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Y
N
O
R

3

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Methodology
Stress Relaxation of Cured Laminates

 Time-Temperature Superposition (TTS) Principle: Mechanical behavior of
polymers at high temperature is equivalent to the behavior of the material in

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longer time intervals (i.e., low frequency), and vice versa.

Ea  1 1  t
Arrhenius equation: log aT      Reduced time:  
2.303R  T Tg 

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  aT

Y
N
O
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13 Reference: [3]

Results
Thermal Analysis (I)

 Most of the crosslinking reactions take place during isothermal cure
Dynamic scans reveal the incompleteness of crosslinking reactions after

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
isothermal cure

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Y
N
O
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Results
Thermal Analysis (II)

 Degree of cure (α(t)) of the specimen was determined from the ratio of total
heat of reaction at time t (H(t)) to the ultimate heat of reaction (HU)

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H (t )
 (t ) 
HU

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Y
N
O
R

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Results

 Cure Dependent Stress Relaxation (UD)
Stress relaxation behavior of UD prepreg in the fiber direction

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

 After reaching plateau value, relaxation modulus starts to increase again
 This transition occurs at the cure state of α ≈ 0.45-0.5

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Y
N
O
R

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Results

 Determination of Gelation Time (UD)
20 minutes from the onset of dwell stage was identified as the time of

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

gelation for the manufacturer’s recommended cure cycle (MRCC)

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Y
N
O
20 min
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Results

 Fiber Directional Mismatch (UD)
Mismatch angles were varied from 0 to 90 degrees with an increment of 15

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

degrees
 PRM varied significantly with the change of ply orientations

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Y
N
O
R

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Results

 Stacking Sequence
A noticeable variation in stress relaxation performance of the specimens was

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
observed among four individual cases

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Y
N
O
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Results

 Three In-plane Directions of UD Laminates
Relaxation modulus is different in three in-plane directions of UD laminates.

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

 UD laminate has the ability to release higher residual stress in 45 degree direction
than 0 and 90 degree directions.

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Y
N
O
R

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Results

 UD vs. PW Stress Relaxation Modulus
Waviness of fiber tows makes PW material much deformable than UD

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

material, regardless of their fiber orientations
 Lower stress relaxation modulus of PW prepregs states that the PW
material will produce less stress than UD prepregs

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Y
N
O
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Results

 LVR and Tg Test
 Specimens were prepared at four cure stages of the material

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Y
N
O
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Results

 Time-Temperature Superposition (UD)
 Experiments were conducted in the DMA utilizing TTS principle to obtain raw data

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and construct stress relaxation master curves

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Y
N
O
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Results

 Master Curve (UD)
 Results indicate that the temperature can significantly affect the off-axis directional

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performance of polymer composites

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Y
N
O
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Results

 Fiber Directions and DOC (α) (UD)

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D
Y
N
O
R

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Results

 UD vs. PW Laminate
PW laminate shows very insignificant stress relaxation behavior in 0 and 90 degree

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
fiber directions
 PW laminate shows superior performance than UD laminates in three in-plane
directions (0, 45, and 90 degree)

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Y
N
O
R

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Conclusions
 Conclusions
 SR behavior of TSCs is strongly dependent on cure state of materials

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 Fiber directional mismatch between two adjacent plies and stacking
sequence significantly affects the SR behavior
Among 0, 45, and 90 degree fiber directions, the UD laminate has the

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
lowest relaxation modulus in 45 degree direction.
PW material will produce less process-induced stress than UD material.

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

 PW material demonstrates better long term mechanical behavior than UD
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material
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References

1. Roozbehjavan, Tavakol, Koushyar, Das, Ahmed, Minaie, Experimental Study of
Effect of Different Parameters on Distortion of Composite Panels, SAMPE 2011 –

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Long Beach, CA – May 23-26, 2011
2. Graham Twigg, Anoush Poursartip, , Göran Fernlund. Tool–part interaction in
composites processing. Part I: experimental investigation and analytical model,

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Composites Part A, Volume 35, Number 1, January 2004 , pp. 121-133(13)
3. Shaw MT, MacKnight WJ. Introduction to Polymer Viscoelasticity. 3rd Eddition ed.

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Hoboken, New Jersey. : John Wiley and Sons., 2005 Aug 1, 2005
N
O
R

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THANK YOU

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Questions and Comments?

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Rony Das_Sampe Presentation_ June 5 2012

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