To understand the failure modes on composite lap joints using Acoustic Emission Testing

1. ACOUSTIC EMISSION
CHARACTERIZATION
OF FAILURE MODES
ON COMPOSITE
MATERIALS
Presented By
Dr. K.MOHAMED BAK
Assistant Professor
Department of Mechanical
Engineering
BSAR Crescent Institute of
Science and Technology
Chennai

2. • INTRODUCTION
• AIM AND OBJECTIVE
• NEED FOR THE STUDY (FLOW CHART)
• SPECIMEN PREPARATION
• EXPERIMENTAL WORK
• MECHANICAL CHARACTERIZATION OF
COMPOSITE MATERIALS
• ACOUSTIC EMISSION PROCEDURE
• AE APPLICATIONS
• AE CHARACTERIZATION OF SINGLE LAP
JOINTS IN FRP LAMINATE
• SUMMARY
• REFERENCE

3. COMPOSITES IN AEROSPACE
Boeing 787 Dreamliner Composite
Profile
Airbus A380 Composite Profile

4. Laminate Preparation

5. LAMINATION PROCEDURE
• A layer of Resin is applied with a brush on the gel coat which
has already gelled or cured.
• The cut glass fiber mat is placed over the resin
• By a stippling action using Resin wetted Brush; the resin is
squeezed to the top surface. Care should be taken to see that
the required glass-to-resin ratio is uniformly obtained.
• Using metal roller the layer is consolidated and the air bubbles
removed. It is important that all the air entrapped in the first
layer is removed. Otherwise, the air entrapped between gel
coat and first layer of mat will expand later causing blistering.
• After the first layer is laid up, subsequent layers are laid up in
a similar manner.
• The procedure has been repeated till the required thickness has
been built up.

6.  Overall aims of this work are to develop an
understanding of the mechanical behavior of
composite materials (FRP).
 Acoustic Emission (AE) response in adhesive-
bonded lap joints in Glass fiber reinforced plastics
(GFRP) and Basalt fiber reinforced plastics (BFRP)
laminates.
 Discuss how it is used as a basis to identify the AE
waveforms pertaining to the failure modes.
 To discriminate the failure modes on lap joints of glass-
fiber reinforced plastic (GFRP) and basalt-fiber reinforced
plastic (BFRP) laminates and the role of composite
structural design in the joint strength. 6

7.  To design structural components, a deep
understanding about material behavior and its
failure mechanisms is necessary.
 Characterization of failure modes in the case of
composite lap joints is a complex research
subject (Matsuzaki et al 2008).
 It is suggested that the application of
basalt/epoxy laminate as a strengthening material
is useful for joining of composite structural
members.
7

8. PREPARATION OF GLASS FIBER REINFORCED PLASTICS (GFRP) AND BASALT FIBER REINFORCED
PLASTICS (BFRP) LAMINATES USING HAND LAY UP PROCESS
MATERIAL SELECTION
E-GLASS FIBER BASALT FIBER
SINGLE LAP JOINTS
1. TENSILE TEST AS PER ASTM D638 STANDARD
2. FLEXURAL TEST SPECIMEN ASTM D790 STANDARD
3. IMPACT TEST SPECIMEN ASTM D5628 STANDARD
MECHANICAL CHARCTERIZATION –GFRP and BFRP
SUMMARY OF MECHANICAL
CHARCTERIZATION
1 STRESS-STRAIN DATA
2. FLEXURAL STRENGTH DATA
3. IMPACT ENERGY DATA
1. ADHESIVE JOINTS (BONDED)
AE TEST UNDER TENSILE LOADING
1 PARAMETRIC ANALYSIS OF AE DATA
2 FREQUENCY ANALYSIS OF AE DATA (FAST
FOURIER TRANSFORM)
IDENTIFICATION OF FAILURE MODES
SCANNING ELECTRON MICROSCOPY ANALYSIS
A

9. A
FINITE ELEMENT ANALYSIS (ANSYS)
PRE-PROCESSOR
SOLVER
POST PROCESSOR
9
STRESS Vs DISPLACEMENT VALUE
COMPARISION BETWEEN LAP JOINTS OF GFRP, BFRP
CONCLUSION
SUMMARY OF FAILURE MODES ON SINGLE
LAP JOINTS
B

10. ASTM D 3039 standard tensile specimen -
GFRP ASTM D3039 standard tensile specimen - BFRP
10
60mm
180mm
ASTM D790 standards flexural test specimen (a) GFRP (b) BFRP
a b

11. Photographical images of single lap joints as per ASTM 5868 and 3528 Standards
(a) GFRP (b) BFRP

12. 12

13. (a) Tensile test setup (b) flexural test setup
13
Magnifying
Lens
(a) (b)

14. 14
CEAST Fractovis Drop Weight Impact Testing Machine

15. MECHANICAL CHARACTERIZATION OF
COMPOSITE LAMINATE

16. (a) (b)
(b) Fractured tensile test specimens
(a) GFRP (ASTM D 638) (b) BFRP (ASTM 3039)
16
(a) (b)
Stress-strain behavior of tensile test specimens: (a) GFRP (b) BFRP

17. Strength
Material
GFRP BFRP
Ultimate tensile strength
(MPa)
345 ± 10 355 ± 5
Flexural strength
(MPa)
195 ± 30 210 ± 20
 On the basis of the data presented in Table 4.1, the
mechanical behaviors of the two materials were found, and
the ultimate tensile strength and flexural strength were
obtained through the tensile and flexural mechanical tests.

18. FLEXURAL TEST RESULTS AT DIFFERENT
TEMPERATURE
18
Flexural Strength of GFRP and BFRP
laminate
Fractured specimens
(a) GFRP
(b) BFRP

19. IMPACT TEST RESULTS OF BASALT AND GLASS
AT DIFFERENT TEMPERATURE
19
Temperature (0c) Vs Impact Energy for GFRP and BFRP laminate

20. FAILURE MODES ON COMPOSITE LAMINATE
USING SEM
Adhesive (Matrix)
Failure
Fiber Failure
Fiber-Matrix
Debonding
Delamination
Failure

21. SUMMARY OF MECHANICAL CHARACTERIZATION
 Based on the test,no significant differences in tensile strengths
are found between basalt/epoxy and the glass/epoxy specimens.
 In contrast to GFRP, BFRP flexural strength reduction is very
minimal at higher temperatures.
 During impact tests at different temperatures, 28% more energy
was absorbed by BFRP laminates compared to GFRP laminates.
 This work confirms the applicability of basalt fiber as a fiber
reinforcing material in polymer composites.
21

22. Various NDT Techniques
• Visual inspection
• Acoustic Emission
• Ultrasonic testing
• Hydrostatic testing
• Radiographic testing
• Magnetic Particle inspection
• Eddy current testing
• Helium leak testing.

23. ACOUSTIC EMISSION PRINCIPLE
 Acoustic Emission refers to the generation of transient elastic waves during the
rapid release of energy from localized sources within a material when it is
stressed.
 Acoustic Emission Testing is extremely useful in evaluating, monitoring the
structural integrity of components.
23
Figure 1.11: Acoustic emission monitoring process

24. Why Acoustic Emission
 AE is a powerful non destructive technique for real time monitoring of
damage development in materials and structures which has been used
successfully for the identification of damage mechanisms in composite
structures under quasi static and dynamic-cycle loading.
 AE is expected to be a next-generation technique not only to monitor
conditions but also for the repair of damaged structures, combined with
an active-adaptive technique using the concept of smart materials.
 AE is considered to be a very promising technique, together with such
sensing techniques as optical fiber and shape memory alloy .
 AE can play a very important roll in monitoring, evaluating and repairing
structures

25. Advantages of Acoustic Emission Technique
 Online Testing- Real Time Monitoring.
 Rapid Inspection.
 External Stimulus not required (energy is
released from within the test object)
 Whole Structure can be tested with a few sensors
 Cost Effective.
 Permanent Recording of Test.
 Defect Location.

26. Applications of AE
 Structural integrity evaluation
 Vessels testing (Hot or cryogenic, metallic and FRP
spheres)
 Nuclear components inspection (Valves, steam lines)
 Corrosion detection
 Pipeline testing, leak detection
 Railroads, Bridge monitoring
 Ageing aircraft evaluation
 Advanced material testing
 Rocket motor testing

27. Applications of AE

28. AE Inspection of Pressure Vessels

29. Why NDT ?

30. Figure 3.22: Wave velocity using pencil lead break test (www.ndt.in)

31. 31
Standard deviation values of wave velocity for different lap joints
ATTENUATION

32. Adhesive (Matrix) Failure
Fiber Tear Failure
Delamination
Delamination
Matrix Cracking
Fiber Failure
32

33. From parametric analysis, the literature (Santuli et al 2012) reveals that
there are three damage zones such as damage initiation, damage
accumulation and unstable damage growth on composite materials
during loading.
(a) Adhesive failure
(b) Light fiber tear failure
(c) Fiber tear failure
33

35. Characterization of Failure Modes on Adhesive-
Bonded Lap Joints
(a) Pure Resin Bonded Joints
Zone I-Adhesive failure

36. Single layer adhesive-bonded lap joints
Zone I-Damage
initiation
(Adhesive failure)
Zone II-Damage
accumulation
(Light fiber tear
failure)
Zone III-Damage
Growth (Fiber tear
failure)
Figure : Displacement vs AE count rate

37. Load Vs Displacement diagram of bonded,
riveted and hybrid single lap joints in glass
laminate
Failure modes observed on
single lap joints in GFRP
laminate
37
I III
II IV
Displacement and AE count rate Vs AE
Cumulative counts for Single lap joints
Single lap joints in Glass laminate

38. Displacement and AE count rate Vs AE
Cumulative counts for single lap joints
Single lap joints in basalt laminate
38
I III
II
Load Vs Displacement diagram of bonded,
riveted and hybrid single lap joints in Basalt
laminate
Failure modes observed on single
lap joints in BFRP laminate

39. Standard deviation values of ultimate load and
displacement for single lap joints in GFRP and BFRP
laminates (Table 4.6)
Material
Single Lap
Joints (SLJ)
Ultimate Load
(kN)
Displacement
(mm)
Glass/
epoxy
Bonded (B) 6 ± 1.5
1. 5 ± 0.25
Basalt/
epoxy
Bonded (B) 6 ± 2.0
1.5 ± 0.3
 The standard deviation values of displacement and ultimate load of
single lap joints in GFRP laminates showed good agreement with results
of single lap joints in BFRP laminates as shown in Table 4.6.

40. IDENTIFICATION OF FAILURE MODES USING AE
TOOL
1.PARAMETRIC ANALYSIS
1.AE cumulative Counts
2. Multi Scale Approach
i) FFT Analysis
2.FREQUENCYANALYSIS
3. SEM ANALYSIS
40

41.  Failure modes on lap joints in GFRP and BFRP are identified
using various AE parameters such as cumulative Counts,
counts rate, amplitude and duration.
 The multi-scale approach is used to identify failure modes
based on variation of amplitude and duration of the AE
events .
 As AE amplitude parameter suffers from different AE events
after preliminary investigations in the parametric analysis, the
failure modes are characterized mostly using frequency
analysis.
PARAMETRIC ANALYSIS
41

42. Pure Resin Bonded Joints
Peak frequency and Cumulative counts versus time (a) single lap joints
42
a

43. FAST FOURIER TRANSFORM (FFT)
The second approach is based on the extraction of
frequency content from AE waveform with
appropriate algorithms.
Frequency–based analysis of AE waveforms may be
used as a tool to investigate the overlapping nature of
the failure modes obtained from parametric analysis.
FFT analysis is performed on a number of randomly
chosen waveform pertaining to different frequency
ranges to identify the frequency content of each failure
mode.
29.11.13 43

44. Waveform of AE signal converted into FFT frequency plot
using MATLAB code (video)

45. SEM image representation of the adhesive failure mode in pure resin specimen
Typical AE time domain and FFT signal for individual failure mode on pure resin bonded lap
joint
45

46. (a) (b)
AE parametric analysis of (a) peak frequency and cumulative counts versus time
(b) Amplitude and duration versus time
AE PARAMETRIC ANALYSIS RESULTS
Delamination
Matrix
cracking
Fiber-
Matrix
debonding

47. SINGLE AND DOUBLE LAYER OF LAP JOINTS
The main problems from an analytical and experimental point of
view in AE monitoring are [Garg et al 1983]
1) how to distinguish between the different failures modes
2) how to assess the individual and associated failure modes
 A sudden increase in AE cumulative counts are observed due to the
recording of medium duration hits and high duration hits (Figures 4.24
and 4.25), which are attributed to light-fiber tear failure mode (Zone
II) and fiber tear failure (Zone III).

48. SINGLE LAYER SPECIMEN (GLOBAL METHOD)
Adhesive
Failure
Light Fiber
Tear Failure
Adhesive Failure
Light Fiber Tear
Failure
Fiber Tear Failure
Fiber Tear
Failure
frequency and Cumulative counts versus time and location
(a) single lap joints (b) Double lap joints
48
20mm

49. DOUBLE (TWO) LAYER SPECIMEN
Adhesive Failure
Light Fiber Tear
Failure
Fiber Tear
Failure
Peak frequency and Cumulative counts versus time and Location
(a) Single lap joints (b) Double lap joints
49

50. Adhesive
Failure
Light Fiber Tear
Failure
Fiber Tear
Failure
Adhesive Failure
Light Fiber Tear
Failure
Glass FR Laminate Basalt FR Laminate
Single Lap Bonded Joints
Figure 4.36 and 4.72 :Peak frequency and Cumulative counts versus time (a) GFRP(b) BFRP
50

51. Light Fiber Tear Failure
Adhesive Failure
Peak frequency Vs Location for bonded joints
Glass FR Laminate Basalt FR Laminate
Adhesive Failure
Fiber Tear Failure
Light Fiber Tear Failure

52. Failure modes on single Layer Specimen using FFT and SEM images 52
FFT ANALYSIS RESULTS

53. Peak frequency Vs
location for lap joints
After careful examination of the AE waveforms, it is found that there
are three different ranges of characteristic frequencies involved
(Ramirez-Jimenez et al 2004).
 A different AE behavior was noticed where AE counts are detected at
the initial stage of loading. A large number of AE waveforms in the time
domain (events) are recorded during tensile test and the fiber tear failure
was observed at the final stage loading.
Adhesive Failure
Overlap Region
Light Fiber Tear Failure
Fiber Tear Failure

54. Adhesive Failure
(Crack Initiation)
Low Amplitude
Low Duration
Light Fiber Tear failure
Moderate Amplitude
Moderate Duration
Low Frequency
Medium Frequency
54
COMPARATIVE OF THE FAILURE MODES USING AE DATA

55. Delamination
High Amplitude
High Duration
High Frequency
Fiber Tear Failure
High Amplitude
High Duration
High Frequency

56. SUMMARY OF FAILURE MODES USING AE DATA
 The discrimination of failure modes from AE signal
parametric analysis to separate the failure modes was
largely unsuccessful due to the overlapping nature of the
parametric ranges for different failure modes.
 As AE amplitude parameter suffers from different AE
sources after preliminary investigations in the parametric
analysis, the failure modes are characterized mostly using
frequency analysis.
 AE Signals and their characteristics representing different
failure modes are identified using AE Parametric analysis.
 AE parametric results verified with Fast Fourier Transform
(FFT) analysis.
 SEM was performed on the failure surfaces to characterize

57. FINITE ELEMENT ANALYSIS OF COMPOSITE
LAP JOINTS

58. S.No PROPERTY VALUE
Glass/Epoxy
Laminate
Basalt/Epoxy
Laminate
1 E1
16.450 GPa 15.350GPa
2 E2
5.517 GPa 9.030GPa
3 E3
5.517 GPa 9.030GPa
4 ν 12
0.1168 0.193
5 ν 23
0.241 0.0787
6 ν 31
0.241 0.0787
7 G12
6.561GPa 3.310GPA
8 G23
6.651GPa 2.450GPa
9 G31
6.651GPa 2.450GPa
10 Coefficient
of
Friction
0.2 0.37
S.No PROPERTY VALUE
1 E 2.8 GPa
2 ν 0.4
Properties of Glass/Epoxy Composite and Basalt/Epoxy
Composite
Table 5.1 and 5.2 : Properties of
Epoxy resin
58
• Layered 46, a 3-D brick
element.
• SOLID-45, an 8-node
brick element.

59. Figure 5. 3: Analysis Result of Single Lap Bonded Joint
(a) GFRP (b) BFRP
Crack initiation at
Maximum stress
Crack initiation at
Maximum stress
(a) (b)
FEA Results for Single Lap Bonded Joints of GFRP and
BFRP Laminates

60. From FEA results, it is observed that the maximum
value of stresses occurred near both ends of the
adhesive region.
Based on the stress distributions in the lap joints,
FEA was used to predict the initial failure and crack
initiate at the edge of the overlap region.
Maximum displacement value of 1.46 mm and 1.55
mm are obtained for single lap bonded joints of
GFRP laminates and BFRP laminates.
The use of bonded lap joints of basalt/epoxy
specimens provide better results for Von Mises stress
which are maximum than bonded lap joints of
glass/epoxy specimens.
FEA results are compared with experimental results
and the error percentage was calculated to be 2%.

61. SUMMARY
 The concept of composite materials is explained along with
aspects related to glass fiber, basalt fiber and matrix
materials.
 The overview of composite lap joints namely adhesive-
bonded joints are discussed along with their merits and
demerits.
 The discrimination of failure modes from AE signal
parametric analysis to separate the failure modes was
largely unsuccessful due to the overlapping nature of the
parametric ranges for different failure modes.
61

62.  Frequency–based analysis of AE waveforms may be used
as a tool to investigate the overlapping nature of the failure
modes obtained from parametric analysis.
 From the FEA results, it is concluded that the single lap
joints of BFRP laminates reached their ultimate load
capacity before final failure when tensile load was applied.
 It is suggested that the application of basalt/epoxy laminate
as a strengthening material is useful for joining of
composite structural members.
 This is due to the higher interfacial strength of basalt
fiber/epoxy specimens.
 It was also observed that basalt fiber/epoxy might be used
as a replacement for glass fiber/epoxy, especially in the
area of composite bonded lap joints.

63. It is observed that the Young's modulus of the
GFRPs is equivalent to Young's modulus of the
BFRPs (Liu et al 2006).
Hence, the same frequency ranges are identified for
lap joints of GFRP and BFRP laminates using AE
monitoring.
Based on this research finding, the coupon-level
studies carried out in the laboratory might be
extended to real-size structures.

64. REFERENCE
1.ASTM D 5868 – 01 Standard practice for lap shear adhesion
for fiber reinforced plastic (FRP) bonding. West Conshohocken,
PA, United states.
2.ASTM E976, ASTM Standard, 1994. American society of
nondestructive testing, www.ndt.net
3. ASTM D 5573., 1999. Standard practices for classifying
failure modes in fiber-reinforced-plastic (FRP) joints, Annual
book of ASTM standards, Vol.15.06.
4. ASTM E1781/E1781M–13 ‘Standard practice for secondary
calibration of acoustic emission sensors’, ASTM International,
West Conshohocken, PA,USA.
5. Adam, L, Bonhomme, E, Chirol, C & Proust, A 2012,
‘Benefits of acoustic emission for the testing of aerospace
composite assemblies’, 30th European conference on acoustic
emission testing, pp. 9-20, www.ndt.net/ewgae-icae12. 64

65. 6. Bohse, J 2000, ‘Acoustic emission characteristics of micro-
failure processes in polymer blends and composites’,
Composites Science and Technology, vol. 60, pp. 1213-1226.
7. Brunner, AJ, Tannert, T & Vallee, T 2010, ‘Waveform
analysis of acoustic emission monitoring of tensile tests on
welded wood joints’, Journal of Acoustic Emission, vol. 28,
pp.59-67.
8. Camanho, PP & Matthews 1997, ‘Stress analysis and strength
prediction of mechanically fastened joints in FRP. A review’,
Composites Part A, vol. 28, pp. 529-547.
9. Czigany, T 2006, ‘Special manufacturing and characteristics
of basalt fiber reinforced hybrid polypropylene composites
Mechanical properties and acoustic emission study’, Composites
Science and Technology, vol.66, pp.3210-3220.
10. EN 13477-2013 ‘Non-destructive testing- acoustic emission
- equipment characterisation - Part 2: verification of operating
characteristic’, DIN standard.

66. 66