effect of surface contamination on composite bond

Effect of Surface Contamination on Composite Bond Integrity and Durability

Dwayne McDaniel, Vishal Musaramthota and Tomas Pribanic

Florida International University

Xiangyang Zhou University of Miami

Contact: [email protected] Ph: (305) 348-6554

2

Composite Bond Integrity/Long-Term Durability of Composite Bonds

•  Motivation and Key Issues –  Past research has focused on determining/understanding acceptable

performance criteria using the initial bond strength of composite bonded systems.

–  There is significant interest in assessing the durability of composite bonded joints and the how durability is effected by contamination.

•  Objective –  Develop a process to evaluate the durability of adhesively bonded

composite joints

–  Investigate undesirable bonding conditions by characterizing the initial performance at various contamination levels

–  Characterize the durability performance of the system using the same contamination levels

–  Support CMH-17 with the inclusion of content for bonded systems

3

Durability Assessment Procedure

Pristine Specimen

Contaminated Specimen

Surface Characterization

Initial Strength Characterization

DCB

Mechanical Fatigue

DCB

Environmental Exposure

DCB

Mechanical Fatigue &

Environmental Exposure

DCB

Assessment of Contamination Effects on Long Term Durability

4

Bonding System Materials

•  Material type and curing procedure for specimens: unidirectional carbon-epoxy system, film adhesive, secondary curing bonding and contaminants.

•  Materials utilized: §  Toray P 2362W-19U-304 T800 Unidirectional Prepreg System (350F cure) §  3M AF 555 Structural adhesive film (7.5x2 mills, 350F cure) §  Precision Fabric polyester peel ply 60001 §  Silicone Spray from CBS Aerosol & Paint, Inc §  Freekote 700-NC from Henkel Corporation

•  Specimen Conditioning:

§  Environmental Chamber : 50°C, 95% RH, for 8 weeks and 1.5 years §  Fatigue Loading: 3 point bending arrangement, 1 inch double amplitude,

2.6 million cycles

Fatigue Fixture

5

•  Manufactured using stainless steel materials

•  Center section slides on a ball bearing carriage

•  Designed to load up to four 11.5 in specimens with a deflection up to 2 inches DA

•  Current stainless steel pneumatic /hydraulic actuator is rated to 400 psi with a 1 inch bore diameter

•  Pneumatic controller can operate up to 2 Hz at 150 psi

The fatigue fixture can be placed in the environmental chamber to study the combined loading and environmental effects.

Environmental chamber with fatigue fixture Rendering of fatigue fixture

6

Stamped Approach

Bonding of CFRP

laminates with adhesives

CFRP laminate

stamp

container with contaminant

stamp

stamp

inked contaminant stamp

stamp

composite surface

composite surface patterned

contaminant on composite surface

contaminant in contact with the bonding

surface

Contamination Procedure

Undesirable bonding conditions will be used to evaluate how the specimen conditioning can effect durability. The contamination procedure aims to create weak bonds by placing a spatially ordered array of contaminated areas on the surface prior to bonding.

7 Optical Microscopy image of failed surfaces

Pristine and Contaminated Specimens �  FTIR data can be used to identify surface molecular bonds – identify contamination. �  Water contact angle - Determines the wetting characteristic behavior of various liquids on the

composite surface. �  AFM –can record the attraction/repulsion forces between the AFM probe and the surface. This

data is used to generate topography and force volume measurements to quantify changes in adhesion forces.

�  Optical Microscopy- Inspecting fracture surfaces, Line profile analysis.

Force curve on the peel ply imprint

peek obtained FTIR mapping

Surface Characterization Methods

8


8

Contact angle measurements 3 Fluids - DI water, diiodomethane and ethylene glycol

Higher contact angles for contaminated specimens Ester (C-O stretch) peak resulting from Polyester peel ply

CH3 asymmetrical resulting from prepreg (Toray)

Characteristic Silicone peaks identified in Aerosol but no Silicone in Freekote

Fourier Transformed Infrared Spectroscopy

9

Previous Results

Fracture Toughness Testing- DCB

Specimen

Non Contaminated

Contaminated

Baseline 9.29 mils 12.99 mils

Environmentally Exposed 14.72 mils 13.86 mils

Fatigue 15.47 mils 13.50 mils

Combined Env Exposed+ Fatigued

11.96 mils 12.71 mils

Fracture Toughness

Bondline Thickness

10

Mode of Failures

11

Contaminated

Coh

esiv

e fa

ilure

Adhesive

failure

Non Contaminated

Cohesive failure

Spectral Mapping

Line Profiles

Fractography- Line Profile Analysis

12

Surface Roughness

Rz (µm) 1) 5.83 µm ± 0.9

2) 4.38 µm ± 1.1

3) 4.66 µm ± 0.9

4) 2.88 µm ± 1.2

5) 2.58 µm ± 1.4

Rz (µm) 1) 3.17 µm ± 0.8

2) 4.46 µm ± 1.0

3) 4.62 µm ± 0.3

4) 5.08 µm ± 0.1

5) 3.02 µm ± 0.9

Avg - 4.19 µm ± 1.1

13

Quantification of Modes of Failure

The modes of failure are quantified for correlation with bond strength

Baseline Environmentally exposed Fatigue in air

-cohesive -adhesive/interlaminar

14

Contaminated Bonded Area (mm2) @8.271 pixel/mm

Non Bonded area (mm2) @8.271 pixel/

mm

Total Area (mm2) @8.271 pixel/mm Cohesive ratio Interlaminar/

Adhesive ratio

Sample 1 4161.6 ± 32.8 2033.2 ± 31.1 6194.9 ± 29.4 67.18 32.82 Sample 3 3620.5 ± 22.2 2193.5 ± 26.5 5814.1 ± 30.7 62.27 37.73 Baseline Sample 4 2444.9 ± 19.2 3623.2 ± 22.5 6068.2 ± 25.7 40.29 59.71 Sample 5 2682.5 ± 19.0 3156.5 ± 23.9 5839.1 ± 28.7 45.94 54.06 Sample 1 2730.2 ± 19.8 3405.6 ± 25.0 6135.8 ± 30.1 44.50 55.50 Sample 2 3752.2 ± 19.9 1940.5 ± 24.3 5692.8 ± 28.6 65.91 34.09 Env exposed Sample 3 4197.0 ± 18.1 2480.8 ± 23.2 6677.9 ± 28.2 62.85 37.15 Sample 4 1806.5 ± 17.9 5041.9 ± 24.2 6848.5 ± 30.6 26.38 73.62 Sample 1 4546.7 ± 23.2 2150.3 ± 26.7 6697.1 ± 30.2 67.89 32.11 Fatigue in air Sample 2 4175.9 ± 16.5 2552.5 ± 20.4 6728.4 ± 24.2 62.06 37.94 Sample 3 4029.3 ± 20.4 2881.1 ± 24.4 6910.5± 28.3 58.31 41.69 Sample 4 4044.9 ± 16.8 2788.2 ± 21.7 6833.2 ± 26.6 59.20 40.80

Cohesive ratio = bonded area of specimen/ total area of specimen

Interlaminar/adhesive ratio = non-bonded area of specimen/ total area of specimen

15

Additional Testing and Characterization

•  Additional specimens were created to increase the number of specimens for the contaminated and pristine set

•  A stamp with a larger contamination area was used to attempt to increase reduction in bond strength

•  Baseline, small stamp contamination and large stamp contamination were evaluated

•  Bondline thickness measurements were taken and gravimetric analysis was conducted

•  Fracture toughness (DCB testing) was measured for each set and modes of failure were evaluated.

•  AFM, water contact angle and FTIR is to be conducted

•  Area analysis of modes of failure for contaminated specimens is to be conducted.

•  Specimens manufactured with the large stamp will be aged in the environmental chamber and fatigue fixture.

13 cm

6.5 cm

7.5 cm

7 cm

Dia- 3 mm

Dia- 1 mm

16

Gravimetric Analysis and Bondline Thickness

Baseline Contaminated small stamp

Contaminated large stamp

14.68 mils 9.75 mils 12.81 mils

Bondline Thickness Averages

Gravimetric Analysis

242.6

238.62

242.64

239.83

236

237

238

239

240

241

242

243

Small stamp Big stamp

Wei

ght,

mg

Before After

Before After Gain %

Small stamp 242.6 242.64 0.017

Big stamp 238.62 239.83 0.507

17


4.48 4.13

3.33

14.68

9.75

12.81

0.00

2.00

4.00

6.00

8.00

10.00

12.00

14.00

16.00

0.00

1.00

2.00

3.00

4.00

5.00

Baseline Small Stamp Big Stamp

Avg.

Bon

dlin

e (m

ils)

Avg

G1C

(in-

lb/in

2 )

18


19

Load-Displacement Curves

20

Weight Measurements

Moisture Uptake Analysis

– determine saturation limit at 50°C, 95% RH

0

0.2

0.4

0.6

0.8

1

1.2

Moi

stur

e U

ptak

e (%

)

Time (in weeks)

Bonded with adhesive (4 specimen) Bonded without adhesive (4 specimen)

After 632 days,

Moisture uptake by laminate: 0.848 %

Moisture uptake by adhesive itself: 0.104 %

21

Fracture Toughness Testing - DCB

4.22 4.24

2.53

2.85

2.44

0

1

2

3

4

5

6

0 days- pristine 60 days 632 days

G1C

(in-

lb/in

2 )

Moisture Exposure Timeline

Bonded-AF555 NonBonded-Laminate only

CMH-17 Support

•  Previous CMH-17 meeting held at FIU (Miami) in August 2014 and the upcoming meeting will be hosted by Univ. of Utah in March 2015.

•  Currently, CMH -17 is seeking to increase/improve the content for bonded systems in Rev H. A number of working groups are involved which include Guidelines, M&P and Supportability.

Working Group Responsibilities: Guidelines - generic guidance information used for design, certification and production of composite parts. They also provide leadership and recommendations on scope responsibilities and direction of the Handbook. Materials and Process - provide descriptions and case studies of material types and process options for characterization and fabrication of PMC materials. Supportability – provide guidelines needed for post production support – inspection, repair, design, facilities, maintenance and disposal.

CMH-17 Support

•  Guidelines – bonding effort includes •  Vol. 3 Ch. 10 Design and Analysis of Bonded Joints

•  Modifications to outline in Rev G •  Configuration specific analysis of joints •  Incorporation of Fracture Analysis of Bonded Joints (pointer)

and Durability of Bonded Joints to 10.5 Certification Issues •  Additional of metal bond information – not for Ch.10 but possibly a

new chapter (or new volume) – Volunteers???? •  Vol. 3 Ch. 5 Materials and Process – The Effects of Variability on

Composites •  Incorporate bond process content to sections 8-10 (Process Control)

CMH-17 Support

•  Supportability – develops content for Vol. 3 Ch13 Defects, Damage and Inspection Vol. 3 Ch14 Supportability, Maintenance and Repair

•  After discussions with Supportability chair (Rakow) – content would be very useful for non-destructive inspection tools for bonded systems.

•  Ch13.2 1. Non-destructive Inspection – visual, tap testing, ultrasonics, radiography, shearography, etc.

•  Content is aligned with detection in laminates •  Little information on detection of bond failures – bond testers

•  Potential contributions to support and add content to bonded repairs (Ch14.7.4)

25

•  A contamination procedure has been developed to evaluate the effect of contamination on adhesively bonded joints.

•  Durability assessment is conducted by conditioning of specimens using a 3-point bending fixture for mechanical fatiguing in air and in environmental chamber.

•  Two different stamps were utilized to create a range of contamination levels on the bonded surface.

•  Surface characterization (Contact Angle and FTIR) showed a different surface chemistry on contaminated specimens when compared to pristine specimens and validates its surface chemistry.

•  Adhesion/Cohesion failure mode patterns were observed with the Freekote contamination. The larger stamp yielding larger adhesion failure areas.

•  Line Profile analysis and area analysis of the failure surface are used to quantify the areas of contamination.

•  GIC vaules from the recent results appear more consistent but the correlation with contamination area and surface characterization methods still needs to be evaluated.

•  The asymptotic increase in the moisture uptake curve was monitored for 78 weeks, determining the environmental effect on adhesive itself.

Conclusions/Summary

26


Future Work:

•  Characterize contaminated surfaces prior to bonding via AFM

•  Repeat aging process using larger stamp size

•  Draw correlations between surface characteristics and mechanical strength.

Benefit to Aviation:

•  Better understanding of durability assessment for adhesively bonded composite joints.

•  Assisting in the development of bonding quality assurance procedures.

27


Questions?

28

Rubber Stamp

3 in X 3 in equally spaced dots

(New Times Roman, font size 12)

Should include bigger stamp

29

Specimen Design

Max Stress (psi) Avg. Life (Cycles)

4500 1.58 x 104

4000 5.28 x 104

3500 4.75 x 105

3000 2.67 x 106

2200 1.03 x 107 + (No failure)

# Plies Thickness (inches)

Force required by piston (lb)

Stress at Surface (ksi)

16 0.120 240 225 18 0.135 270 200 20 0.150 300 180 22 0.165 330 164 24 0.180 360 150

Selected laminate configuration: •  Specimen dimensions: 11.5 in long x 1 in wide •  20 ply unidirectional laminate (0.15 thick + adhesive)

30

•  Specimens were tested to deflections of 0.25 in, 0.5 in, and 0.75 in

• 80% of tested specimens to 0.75 in of deflection fractured

•  Toray800 material ultimate strength is 212x103 psi > 1 in specimen deflection

• Specimens have an effective length of 8 in

• Surface effect of 1 inch diameter rollers lowered Su to 129x103 psi -> 0.6 in deflection

•  Conditioned specimens will be deflected to 0.5 inches (2,300 psi of transverse shear).

Surface Stresses

31

Advantages

-  Apply uniform shear stress at bondline

-  Simple to set up – potential to enclose in an environmental chamber

-  Can use DCB (ASTM 5528) or wedge specimens (ASTM 3762)

Disadvantages

-  Specimen geometry needs to be adjusted to to limit fatigue in adherend/adhesive

- Need to consider surface stress effects resulting from contact points

Fatigue Loading Procedure

Durability evaluation using double cantilever beam test

Loading

at constant

displacement

DCB specimens are conditioned by mechanically fatiguing and/or exposure to an accelerated aging environment. A fatigue structure was manufactured that loads the specimens in three point bending.

32

Previous Results

Specimen Non Contaminated

Baseline 9.29 mils

Environmentally Exposed

15.72 mils

Fatigue 17.47 mils

Combined Env Exposed+ Fatigued

11.96 mils

Bondline thickness

A – pristine B - environmentally exposed C – fatigued D - environmental exposed and fatigued

Contaminated Specimens (Aerosol)

E –mesh controlled F - stamp controlled

33

Failure Mode of Contaminated Specimens

DCB Specimens contaminated with Freekote 700-NC

Next Step

Durability Analysis on contaminated specimen

Previous Results

34


•  FIU and UM has conducted research utilizing Atomic Force Microscopy and epoxy-modified probes tip to characterize surfaces prior to bonding.

•  AFM can record the attraction/repulsion forces between the AFM probe and the surface.

•  AFM data is used to generate topography and force volume measurements to quantify changes in adhesion forces. Deflection image_2µ

SEM image

Need to know about the inclusion of this slide

35

Load-Displacement - Initial

effect of surface contamination on composite bond

Documents