NDT methods for Composites

1. DIFFERENT NDT METHODS
FOR POLYMER AND
COMPOSITE MATERIALS
MOL-32246 INTRODUCTION TO NDT-TECHNIQUES
Jere Ylianunti
Wenxin Zhang
Madan Patnamsetty
Yaswanth Gowda

2. OUTLINE
• Basic information of polymers and composites
• Visual inspection
• Acoustic emission inspection
• Ultrasonic inspection
• Radiographic inspection
• Summary of NDT technology applied for polymers and composites

3. POLYMERS AND COMPOSITES
• Usual properties:
• Non-conductive
• Practically not magnetic
• Lighter and less dense than metals or ceramics
• Metallic and ceramic composites blur the boundaries

4. VISUAL INSPECTION
• First step in every inspection
• As usable for polymeric materials as for any other material
• Measuring devices, gauges, magnifying instruments, borescopes
• Resolution of bare eye: at best tenths of millimeter
• Optical microscopy: few hundreds of nanometers [1]

5. BORESCOPES
• For reaching small compartments
• Some magnification
• Rigid, flexible scopes
• videoscopes
A videoscope [2]

6. PENETRATING LIQUIDS
• A liquid sprayed on the surface of the component reveals surface anomalies
• Many chemicals used, chemical stability of the component has to be confirmed
• Penetrating liquid, developer liquid, cleaning agents
• The developer liquid increases the visibility of the penetrants
• Not usable for porous polymers (polyurethane foam, Styrofoam)
[3]

7. ACOUSTIC EMISSION INSPECTION
• Principle
Detect transient elastic waves that generated by the rapid
stress redistribution within a material
• Application
Structural health monitoring(SHM) , vessel testing, corrosion
detection, production quality control, aging aircraft evaluation,
advanced materials testing (composites, ceramics),etc.
• Advantages
High sensitivity, early and rapid detection of defects,
real time monitoring, cost reduction, defective area location, etc.
[4]

8. ACOUSTIC EMISSION FOR COMPOSITES
• Acoustic wave source:
1) matrix cracking; 2) fiber breakage;
3) matrix-fiber interface de-bonding; 4) delamination
• Unique advantages:
1) monitor whole structure; 2) remote inspection;
3) fast and easy to locate defect without scan over surface;
4) high sensitivity (possible to detect single fiber fracture );
5) effectiveness irrelevant with orientation of flaw;
• Limitations:
1) only detect flaws that are growing; 2) difficult to size flaws;
3) difficult to locate flaws in complex composites;
4) sensitivity greatly affected by ambient noise
[5]

9. CASE STUDY
[6]

10. ULTRA SONIC INSPECTION
• High frequency sound waves are introduced into material and they are reflected back from surface or
flaws.
• Frequencies done between 0.1 to 25 MHz frequencies
• Transducer transforms voltage pulse to ultrasonic pulse and transmit
• Also exits UT technique non-contacting sample. (LASER ULTRASONIC INSPECTION SYSTEM - LUIS)
developed by Ultra Spec.
f
plate
crack
0 2 4 6 8 10
initial
pulse
crack
echo
back surface
echo
Oscilloscope, or flaw
detector screen
[7]

11. ULTRA SONIC INSPECTION TO COMPOSITES
• Frequency ranged from 1-5 MHz due to increase in attenuation
• The 4 more popular techniques for composites are
o Pulse-echo
o Through transmission Ultra sound (TTU)
o Pitch catch
o Guided waves
• Different polymers can be tested are Laminated (Hand layup, Tape layup, Fibre placement, Resin transfer
moulding), Sandwich (Honeycomb core, Foam core), Three-dimensional (3D) performs and other forms
such as braided, stitched, or chopped fibre or tape, Bonds (Film or paste, Secondary or co-bond)
[8]

12. ULTRA SONIC INSPECTION TO POLYMER COMPOSITES
• A typical velocity of ultra sound across laminate is 2.8mm/μs and common ply thickness for aerospace
application is 0.1-0.35 mm.
• The values of attenuation and frequencies are subjected to specific materials (fibre, resin, ply type and lay type)
• UT attenuation is measured in a material consolidation
UT ATTENUATION = αx = ln(A0/A)
Where α = attenuation coefficient A0 = initial pressure amplitude A = Pressure amplitude after transmission X = transmitted distance
• Reflection co-efficient and Transmission co-efficient can be known by measuring Acoustic impedance
Acoustic impedance Z = ρν v = acoustic velocity and ρ = density
transmission co-efficient T = 2Z2/(Z1 + Z2) and Reflection co-efficient R = (Z2 – Z1)/ (Z2 + Z1)
• Considering acoustic impedance of polymer composite is 470,000 g /cm2-s and for air it is 40 g/cm2-s thus we get
Transmission co-efficient as 0.00017 and Refection co-efficient as 0.9998 thus the small value of Transmission co-
efficient and large value of Reflection co-efficient gives us clear idea of delamination of composites

13. COMPARATIVE STUDY OF PULSE ECHO AND TTU IN GLASS/EPOXY COMPOSITES
Parameter Unit E-glass Epoxy resin
Density g/cm3 2.58 1.13
Tensile strength MPa 3500 65.4
Elastic modulus GPa 75 3.1
no:
Glass content
[Wt.%]
Average wave velocity [m/s]
Pulse-echo TTU
1 31 2461 2656
2 37.2 2580 2676
3 56.8 2949 2808
4 57.3 2963 2866
5 65.2 3045 2920
Properties of constituent materials Determined properties of investigated glass/epoxy specimens
Ultrasonic wave velocity Vs Fibre content [9]

14. RADIOGRAPHIC INSPECTION TO COMPOSITES
• X –Ray and neutron radiation are main sources.
• Materials (parts of specimen) can be easily discriminated by the image formed
on the film.
High Electrical Potential
Electrons
-+
X-ray Generator
or Radioactive
Source Creates
Radiation
Exposure Recording Device
Radiation
Penetrate
the Sample
X-ray
radiography
Conventional
Conventional radiography used to
detect voids,
Enhanced
Enhanced X-ray radiography (Specially
formulated liquids are used to enhance
the contrast of radiographic images)
Detects Delamination and Cracks, Fibre
volume fraction and fibre alignment
• The advantage over ultrasonic testing is that more thicknesses can be
inspected and better resolution images can be produced.
[4]

15. RADIOGRAPHY TESTING OF POLYMER COMPOSITES
• The general equation of attenuation of radiation beam is
I(E) = I0(E) e -μ(E)x
Where I (E) = transmitted beam intensity as a function of energy E
I0 (E) = Initial X-ray beam spectrum intensity,
μ (E) = material linear attenuation co-efficient , x = thickness
• Then we get an equation
d (I)/I = -μ dx
• Comparing the equation to plot it means the intensity % change
is related to ply thickness.
• Detects Planar defect like Delamination and knows volumetric
features for Sandwich structures.
[5]

16. CASE STUDY FOR INSPECTION OF POSSIBLE DEFECTS IN THE
COMPOSITE STRUCTURE OF HELICOPTER ROTOR BLADES
• MI-24 type helicopter, Dimensions are 10 m * 0.7 m,
Weighs 110 Kg
• A biological shield is made for larger sections
• Voltage = 150 kV and Current = 3mA
• Radiographed by Neutron radiation , Wet
measurement by Neutron radiation and X-ray
radiography
• Most defects are Cavities, Holes, Cracks (near
interfaces)
• Defects not visible to X-ray and NR, but visible for Wet
NR, this is due to Hydrogen – Neutron attenuation
• If water is present prior comparison is too hard
• Resin rich and poor areas is identified by X-ray method
• The important study is about the defects in Adhesive
filling and water percolation at sealing interfaces
Water percolation into the honeycomb structure near the
stiffener: (a) dry NR; (b) wet NR
C) Resin rich area in the honeycomb structure
[10]

17. NDT METHOD APPLICATIONS FOR COMPOSITES
NDT Method Porosity
Laminate
Delamination
and Disbond
Skin-to-
Core
Disbond
Cracks
Surface
Damage
Water
Ingress
Wrinkles
Visual R R
Tap test L R
Ultrasound TTU R R R R
Ultrasound PE R R L R
Bond testers R R L
Radiography L L R R L
Thermography L R R R
Shearography L R R L L
Electromagnetic L L
Acoustic emission R R
R = Recommended, L = Limited applicability
[8]

18. REFERENCES:
[1]:ASM Handbooks Online, Vol. 17, Nondestructive evaluation and quality control, Inspection equipment and techniques, Visual
inspection, available at: http://products.asminternational.org/hbk/index.jsp, accessed on 16.2.2015
[2]:Picture, available at: http://www.toolstop.co.uk/sealey-vs8196-video-borescope-8.5mm-probe-p58066, accessed on 25.2.2015
[3]:Picture, available at: http://www.nde.com/pt.htm, accessed on 25.2.2015
[4]: Picture, available at: http://www.mistrasgroup.com/products/technologies/acousticemission.aspx, 03.03.2015
[5]:Picture, available at: http://www.muravin.com/downloads/Muravin%20-
%20Acoustic%20Emission%20Science%20and%20Technology.pdf, 03.03.2015
[6]: S.E.Mechraoul, et, al. “Reliability of damage mechanism localisation by acoustic emission on glass/epoxy composite material plate”,
Composite structure, vol. 94, pp.1483-1494, 2012.
[7]:Picture https://www.nde-ed.org/GeneralResources/IntroToNDT/GenIntroNDT.htm
[8]:Picture R. H. Bossi and V. Giurgiutiu, Polymer Composites in the Aerospace Industry. Elsvier,2015, pp. 413–448
[9]: Picture and data: available at :G. Wróbel and S. Pawlak, “A comparison study of the pulse-echo and through-transmission ultrasonics
in glass / epoxy composites,” J. Achiev. Mater. Manuf. Eng., vol. 22, no. 2, pp. 51–54, 2007
[10]: Picture and data :M. Balaskó, I. Veres, G. Molnár, Z. Balaskó, and E. Sváb, “Composite structure of helicopter rotor blades studied by
neutron- and X-ray radiography,” Phys. B Condens. Matter, vol. 350, no. 1–3, pp. 107–109, Jul. 2004.