Ultrasonic Techniques for Damage Evaluation on polymer matrix Composite Laminates Prof. Claudio Scarponi Dipartimento di Ingegneria Aerospaziale e Astronautica, Universit

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Ultrasonic Techniques for Damage Evaluation on
polymer matrix Composite LaminatesProf. Claudio
ScarponiDipartimento di Ingegneria Aerospaziale
e Astronautica, Università degli Studi di Roma
La Sapienza-Via Eudossiana 18-00184 Roma. Tel.
06 44585313, e-mail claudio.scarponi_at_uniroma1.it
MADRID, Universidad Carlos III, 7 de Junio 2004
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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.

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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.
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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.

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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.

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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.

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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.

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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.

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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.

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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.

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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.

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NDT for surface defects

• Visual inspection
• Fluorescent penetrants
• Optical methods, using interferometric
  principles
• Eddy currents (for carbon fibers).

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NDT for internal defects

• X-ray radiografy
• Ultrasonics
• Thermografy
• Acoustic Emission.
• Our attention will be focused to the first two
  techniques, especially for Ultrasonics.

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Tipology of internal detectable defects
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100 mm


Material Fibre
reinforced laminates realized in vacuum bag
technique

V9
Woven roving E-glass vinylester 10 layers
?45 angle-ply
4 mm
100 mm
VX
Woven roving E-glass vinylester 5 layers ?45
angle-ply
3.5 mm
100 mm
100 mm
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J9

Woven roving juta/vinylester 10 layers
cross-ply
8 mm
100 mm
100 mm
JX
Woven roving juta/E-glass/vinylester 9 layers
(1,2,1,2,3)
100 mm
5 mm
100 mm
JB
Woven roving juta/E-glass/vinylester 9 layers
(2,2,1,2,2)
100 mm
5 mm
100 mm
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JV
Woven roving juta/E-glass/vinylester 14
layers (3,2,1,2,6)
5 mm
100 mm
100 mm
JA
Woven roving juta/ E-glass vinylester 14
layers (4,2,2,2,4)
100 mm
5 mm
100 mm
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Absorbed Energy/ Impact Energy
Contact Force/ Impact Energy
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X-Ray concept
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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.

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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.

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RISULTS
Glass 600 gr/m2
Glass 300 gr/m2
3.5 mm
4 mm
Impacted a 10 J
Impacted a 10 J
Impactor velocity
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JV Juta-Glass (300 gr/m2)
JX Juta-Glass (600 gr/m2)
5 mm
5 mm
Impacted a 20 J
Impacted a 20 J
Impactor velocity
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DELAMINAZIONE
V9 (glass 300 gr/m2) VX (glass 600 gr/m2)
JV (glass 300 gr/m2) JA (glass 300
gr/m2) JX (glass 600 gr/m2) JB (glass 600
gr/m2)
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CONFRONTO DELAMINAZIONE ENERGIA ASSORBITA
Glass fibers V9 VX
juta-glass JV, JA JB, JX
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quality controlfirst step
Devicesultrasonic reflection
system
USD 10 generatore segnale ultrasonoro-acquisitore
segnale di ritorno SONDA 5MHz emissione-ricezione
ANDSCAN posizione sonda rispetto sup. pezzo PC
con software per mappa 2D, calcolo area
delaminata
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A-scan representation of an internal defect
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B-scan representation of an internal defect
C-scan representation of an internal defect
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Ultrasonic test apparatus and USD 10 display
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Relative Depth Mode Scan map and USD-10 display
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Amplitude Mode Scan map and USD-10 display
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Signal equalization by DAC function
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• First scan
• The color is linked with attenuation
• Blue color means total attenuation (delaminations)

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MONITORING POST-IMPACT
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Effect of the signal attenuation

• The reduction of the acoustic pressure along the
  thickness (x-axes) can be expressed by the
  following relation PP0e-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.

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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.).

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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.

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