Ultrasonic Techniques for Damage Evaluation on polymer matrix

Composite LaminatesProf. Claudio Scarponi

Dipartimento di Ingegneria Aerospaziale e Astronautica, Università degli Studi di Roma “La Sapienza”-Via Eudossiana 18-00184 Roma.

Tel. 06 44585313, e-mail claudio.scarponi@uniroma1.it

MADRID, Universidad Carlos III, 7 de Junio 2004

Aims Aims

• To describe an original ultrasonic testing procedure for the detection of delaminations inside composite laminates and sandwich structures.

The work is divided in three steps :1) A general review of NDT techniques for

polymer matrix composite materials;2) The description of the instrumentation and

the original C-SCAN ultrasonic technique pointed out;

3) A review of the experimental data for different composite structures.

NDT philosophy

The purpose of Non Destructive Testing (NDT) is to inspect, qualify, and evaluate the quality of a structure without breaking, destroying, or otherwise significantly changing the structure. NDT methods for composite materials range from simple visual inspection and coin tapping to very sophisticated techniques.

Applications to Composite Materials

Many of these techniques were originally developed for detecting flaws in metals and now, with some modifications, are also used with fibers reinforced composite materials (FRCM). Metals are basically isotropic and homogeneous materials, whereas composites are nonisotropic and heterogeneous. Delaminations are peculiar defects of composites.

Quality of composite structures

• The quality is defined in terms of flaws or defects, both microscopic and macroscopic, owing the technological process or generated during the service life of the structure.

• The process-induced defects are created mainly at the time of moulding the laminate owing to lack of process control, inadequate raw material quality, improper tool design and human error. The nature of the process-induced defects depends on the particular process used for manufacturing.

• The service-related defects are caused by unintentional overloading, impact, fatigue etc. and environmental factors such as elevated temperatures and humid conditions.

Quality control tests 1• Quality control tests start with the incoming raw materials

(fibers, matrix, prepreg rolls, adhesive and core materials). • Once the laminate is moulded, several simple inspection

procedures can be implemented: measurements of important dimensions, part weight, density.

• Visual inspections can be used to detect blisters, surface porosity, sink marks, discoloration, warpage, etc.

• Lightly tapping the surface with a coin or a hammer can often give clues to blisters or large voids near the surface.

Quality control tests 2

• Obviously the internal flaws and delaminations remain undetected and can influence both short-term properties of the laminate, such as strength and modulus and long-term properties, such as moisture absorption and fatigue durability.

• The importance role of NDT is also the detection of the internal defects.

The process-induced defects commonly encountered 1

• 1. Contamination due to foreign particles, extraneous fibers, pieces of peel ply, not removed from the prepreg surface, etc.

• 2. Broken filaments due to scratches or cuts, drill breaking through the exit side of a hole in a hole drilling process.

• 3. Delaminations or separations of plies within the laminate, caused by poor consolidation in the molding operation or created during drilling a hole or machining a cutout in the cured laminate.

• 4. Resin-rich or fiber-starved areas, caused by non uniform resin distribution in the prepreg or non uniform flow during the molding process.

The process-induced defects commonly encountered 2

• 5. Resin-starved areas, which can be caused by uncontrolled resin bleed-out during vacuum bag molding or lack of resin flow through the dry fiber layers during RTM (resin transfer moulding), etc.

• 6. Fibers misalignment, which can be due to misoriented fibers in the prepreg, deviation from the preselected lay-up or filament winding pattern, or fibers washout due to excessive resin flow.

• 7. Undercure or variation in the degree of cure, which occur if proper temperature and/or time are not used in the molding process.

• 8. Fibers waviness or kinking, which can be due to improper tensioning during prepreg preparation, filament winding, and pultrusion.

The process-induced defects commonly encountered 3

• 9. Voids, which are formed by entrapped air between the prepreg layers or inside a filament wound structure, the presence of moisture, or an excessive amount of solvent used in making the prepreg and gases evolved during the curing reaction in the mold.

• 10. Knit lines, which occur in both compression molding and injection molding due to joining of two or more flow fronts.

• 11. Missing plies, which can occur during hand lay-up due to miscounting the number of plies in the lay-up.

The process-induced defects commonly encountered 4

• 12. Ply gap and ply overlap, both of which can occur during hand lay-up, due to mistakes made in sizing, cutting, and placing the plies.

• 13. Blisters, which can occur in compression moulding due to air entrapment under the surface plies.

• 14. Unbonded areas or lack of adhesive in adhesively bonded joints.

• 15. Non uniform laminate thickness and non uniform bonded joint thickness.

NDT for surface defects• Visual inspection;

• Fluorescent penetrants;

• Optical methods, using interferometric principles;

• Eddy currents (for carbon fibers).

NDT for internal defects

• X-ray radiografy;

• Ultrasonics;

• Thermografy;

• Acoustic Emission.

Our attention will be focused to the first two techniques, especially for Ultrasonics.

Tipology of internal detectable defects

100 mm

Woven roving E-glass vinylester 10 layers 45 angle-ply

MaterialMaterial

Fibre reinforced laminates realized in vacuum bag technique

10

0 m

m

4

mm

Woven roving E-glass vinylester 5 layers 45 angle-ply

100 mm

10

0 m

m

3.5

mm

V9V9

VXVX

Woven roving juta/vinylester 10 layers cross-ply

100 mm1

00

mm

8 m

m

Woven roving juta/E-glass/vinylester 9 layers (1,2,1,2,3)

100 mm

10

0 m

m

5 m

m

JXJX

J9J9

JBJB

100 mm

10

0 m

m Woven roving juta/E-glass/vinylester 9 layers (2,2,1,2,2)

5 m

m

100 mm

10

0 m

m

Woven roving juta/E-glass/vinylester 14 layers (3,2,1,2,6)

5 m

m

100 mm

10

0 m

m

Woven roving juta/ E-glass vinylester 14 layers (4,2,2,2,4)

5 m

m

JVJV

JAJA

Absorbed Energy/ Absorbed Energy/ Impact EnergyImpact Energy

ENERGIA ASS./ ENERGIA DI IMPATTO tessuti di vetro (confronto)

0

2

4

6

8

10

12

14

16

0 5 10 15 20 25

Energia di impatto (J)

Ener

gia

asso

rbit

a (J

)

V9

VX

FORZA/ ENERGIA DI IMPATTO tessuti di vetro (confronto)

0

1

2

3

4

5

6

7

0 5 10 15 20 25

Energia di impatto (J)

Forz

a di

con

tatt

o (k

N)

V9

VX

Contact Force/Contact Force/Impact EnergyImpact Energy

X-Ray concept

X-ray technique 1• In radiographic techniques, one surface of the part is impinged with a burst of

electromagnetic radiation energy, most commonly from an X-ray tube. Part of the energy is absorbed by the constituents in the material as it passes through the thickness of the part. The transmitted energy is captured on a photographic film placed directly below the opposite surface. Defects or flaws in the material produce a variation in energy transmission that shows up as shadow images on the photographic film. Defects in polymer matrix composites that can be detected by radiography are resin-rich or resin-starved areas, non uniform fiber distribution, fibers misorientation (fibers buckling and knit lines), foreign particles, and voids.

• Cracks parallel to the radiation beam can also be detected by the radiographic methods.

• Planar defects normal to the radiation beam, such as delaminations or interlaminar cracks, are not detected by radiography for the all materials, unless a radio-opaque penetrant is first injected into these defect areas to improve the contrast. This technique is called penetrant-enhanced X-ray radiography (PEXR); however, its use requires a way for the penetrant to access the defect areas and is therefore limited to delaminations that are open to the surface.

X-ray technique 2

• Different imaging techniques, such as real-time display of the X-ray image on a fluorescent screen (fluoroscopy) and cross-sectional scanning (computer-aided tomography; CAT), have also been used with composite materials. CAT is particularly useful because it can form a three-dimensional image of the defect by taking X-ray images from a number of different angles.

RISULTSRISULTS

3.5

mm

4 m

m

Glass 600 gr/m2 Glass 300 gr/m2

Impacted a 10 J Impacted a 10 JImpactor velocity

5 m

m

JX Juta-Glass (600 gr/m2)

Impacted a 20 J20 J

5 m

m

JV Juta-Glass (300 gr/m2)

Impacted a 20 J20 J

Impactor velocity

DELAMINAZIONE tessuti di vetro (confronto)

0

100

200

300

400

500

600

700

0 5 10 15 20 25

Energia di impatto (J)

Are

a del

amin

ata

(mm

2 )

V9

VX

DELAMINAZIONE tessuti misti (confronto)

0

100

200

300

400

500

600

700

0 5 10 15 20 25

Energia di impatto (J)

Are

a del

amin

ata

(mm

2 )

JX

JB

JV

JA

V9 (glass 300 gr/m2)VX (glass 600 gr/m2)

DELAMINAZIONE

JV (glass 300 gr/m2)JA (glass 300 gr/m2)JX (glass 600 gr/m2)JB (glass 600 gr/m2)

CONFRONTO DELAMINAZIONE ENERGIA ASSORBITACONFRONTO DELAMINAZIONE ENERGIA ASSORBITA

DELAMINAZIONE tessuti misti (confronto)

0

100

200

300

400

500

600

700

0 2 4 6 8 10 12 14 16 18

Energia assorbita(J)

Are

a del

amin

ata

(mm

2 )

JX

JB

JV

JA

DELAMINAZIONE tessuti di vetro (confronto)

0

100

200

300

400

500

600

700

0 2 4 6 8 10 12 14 16

Energia assorbita (J)

Are

a del

amin

ata

(mm2 )

V9

VX

Glass fibers :V9VX

juta-glass :JVJV, , JAJAJBJB, , JXJX

quality controlquality controlfirst step

Devicesultrasonic reflection system: USD 10USD 10 generatore

segnale ultrasonoro-acquisitore segnale di ritornoSONDA 5MHzSONDA 5MHz emissione-ricezioneANDSCAN ANDSCAN posizione sonda rispetto sup. pezzoPCPC con software per : mappa 2D, calcolo area delaminata

A-scan representation of an internal defect

B-scan representation of an internal defect

C-scan representation of an internal defect

USD-10 PC

AndScan

Ultrasonic test apparatus and USD 10 display

Relative Depth Mode: Scan map and USD-10 display

Amplitude Mode: Scan map and USD-10 display

Signal equalization by DAC function

First scan First scan

The color is linked with attenuation: Blue color means total attenuation

(delaminations)

MONITORING POST-IMPACTMONITORING POST-IMPACT

Effect of the signal attenuation

• The reduction of the acoustic pressure along the thickness (x-axes) can be expressed by the following relation: P=P0e-ax

• The parameter a is the attenuation and depends on the probe frequency and on the energy absorption and diffusion, due to the material discontinuities.

• As much is high the frequency, as little is the minimum dimention of the detected defects, as better is the resolution, as bigger is the attenuation, as lower is the detectable thickness.

The acoustic pressure attenuation factor

• The signal reduction is due to the transmission-reflection phenomena at the plies interfaces, encountered by the sound wave during the reverberation path;

• the losses are related to the materials acoustic impedances and can be evaluated in form of dB/cm for each material;

• The losses depends on both imperfections (voids, resin distribution etc.) and discontinuities (fiber/matrix; fibers form, interfaces between layers etc.).

The choice of the probe frequency

• The probe frequency is a very important item; from such a value depends the sensitivity and the resolution of the sensor;

• As high is the frequency, as high is the attenuation, as little is the detectable thickness for the same material;

• As worst is the fabrication processo, as high will be the attenuation;

• A wrong choice of the probe frequency can give underevaluated values for the internal defects;

• If a high value of attenuation is expected, a good solution could be the reduction of the probe frequency.

Carbon fiber/epoxy composite aircraft skin and frame with integrated fiber-optic sensor used as delamination detectors.