Abstract

This essay
introduces the typical damage modes, the principle of acoustic emission
detection, the characteristics of acoustic emission signals, the signal
analysis and processing techniques, and the advantages and disadvantages of
acoustic emission detection.

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

Contents
1.   
Introduction
1
2.   
Literature review
2
2.1 Acoustic emission detection
2
2.2 Acoustic emission signal generation
2
2.3 Acoustic emission signal characteristics
2
2.4 Acoustic emission signal analysis and processing technology
3
2.5 Carbon fibre composite materials testing
3
2.6 The advantages and disadvantages of acoustic emission detection

3
3.   
Conclusions
4
Reference
5
 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

1.Introduction

Carbon fibre
composite material refers to the use of carbon fibre reinforced
high-performance reinforced composite materials, as well as matrix is mostly
advanced resin-based. Its overall performance with the aluminum alloy, but the
stiffness and strength higher than the aluminum alloy (Mair, 2002).
Carbon fibre is mainly composed of a special type of carbon element. The carbon
content varies with different species, the general mass fraction of 90%. Carbon
fibre with the general characteristics of carbon materials, such as high
temperature, abrasion resistance, electrical conductivity, thermal conductivity
and corrosion resistance, but with the general carbon material differences are
that carbon fibre’s shape has a significant anisotropy, soft, can be processed
into kind of fabric and high strength along the fibre axis. In
the fields of density, stiffness, weight, fatigue properties and other
demanding requirements, as well as in demanding high temperature, chemical
stability of the occasion, carbon fibre composite materials are very
advantageous. Carbon fibre composite materials in the construction,
transportation, aerospace industry has been widely used
 (Huang and Zhao, 2017).
Although carbon fibre composites have been widely used as a new material, but
due to process instability, defects such as voids and inclusions cannot be
completely avoided in the production process, and their transverse bearing and
shear resistance are low. In the impact or fatigue Under the action of other
loads easily damaged until destroyed.

When the
parts of carbon fibre composite materials are assembled and connected with
other parts and components, a great deal of holes are inevitably processed, and
defects such as debonding of the composites are easily caused during the
processing of the holes. Zhang Yunfeng and other studies have found that the
direction of the fibre has a serious impact on the formation of drilling
defects; the greater the axial force, the more severe delamination defects, while
rapid increase in tear defects. Wang Hongfei and other defects in the carbon fibre
composite drilling holes were analyzed and classified, found that processing
will appear glitches and delamination defects.

Composite material
damage patterns varied. Basic damage patterns can be divided into two
categories, namely, in-plane and out-of-plane damage. In-plane damage includes
matrix cracking, debonding at the fibre interface, fibre buckling and fibre
breakage; and out-of-plane damage occurs in the laminate, exhibiting
delamination.

Figure. 1. Four types of
damage in fibre reinforced composites:

(a) matrix cracking; (b)
fibre buckling; (c) fibre breakage; (d) delamination.

(Wen, Xia and Choy,
2011)

 

In summary,
the carbon fibre reinforced composite material damage detection and real-time
monitoring is particularly important. In order to ensure the safe application
of composite materials, the detection and research of composite materials are
widely regarded by people. There are a variety of methods available for the
testing of carbon fibre composites, including acoustic emission testing,
ultrasonic testing, laser testing, penetration testing, optical fiber testing,
X-ray testing and magnetic particle testing. Various testing methods play a
role in their fields of application their own advantages, but also have their
own shortcomings. The following analysis of the acoustic emission detection of
carbon fibre composite materials in the application of the status quo.

 

2. Literature review

2.1 Acoustic
emission detection

Acoustic
emission, also known as stress wave emission, is a phenomenon in which
deformation or fracture or internal stress exceeds the yield limit and enters
the stage of irreversible plastic deformation when material or component is
subjected to external force and releases strain energy in the form of transient
elastic wave.

 

2.2
Acoustic emission signal generation

Materials or
structures that deform and break under stress can cause acoustic emission
signals. For composite materials, there are many types of damage and damage
that can produce acoustic emission signals such as commonly encountered fibre
breakage, cracking of the substrate, layered expansion and interface separation
damage are all sources of acoustic emission signals.

(1)
Plastic deformation

For
crystalline materials, the deformation or rupture can produce acoustic emission
signals, and plastic deformation is one of the important mechanisms to generate
acoustic emission signals. At the yield point, the count rate of acoustic
emission signal reaches the peak (Jemielniak, 2001).

(2)
Crack formation and expansion

The material
will undergo deformation and fracture under the stress, and the fracture
failure is just the result of crack formation and expansion. It is an important
acoustic emission source when the crack is formed and expanded.

Crack
formation, crack propagation and final fracture are three parts of the material
fracture process. All three phases release energy to form a strong acoustic
emission signal.

(3)
Generation of acoustic emission signals of fibre reinforced composites

Fibre-reinforced
composites are formed by the combination of matrix and fibres. The combination
of the matrix and the fibres enables the two completely different materials to
exert their respective excellent performances. The interlaced fibre lamination
makes the composite material as a whole, which causes a lot of acoustic
emission signals during stress destruction.

 

2.3
Acoustic emission signal characteristics

According to
the characteristics of acoustic emission signals, acoustic emission signals can
be classified, which can be divided into burst type and continuous type from
the waveform characteristics. Burst signals are waveform signals that are not
consecutive and separate in the time domain. For fibre-reinforced composites,
in the case of fibre breakage, crack propagation, and inclusions fragmentation,
a burst type acoustic emission signal is generated. For continuous acoustic
emission signals, it is mainly due to the frequency of acoustic emission in the
time domain to achieve the degree of inseparability and connected together.

 

2.4
Acoustic emission signal analysis and processing technology

Acoustic
emission detection refers to the use of professional instruments to be tested
under the external force deformation or rupture when issued by the acoustic
emission signal acquisition, and then use advanced acoustic emission signal
analysis and processing of the collected signal Analysis and processing,
through the analysis and processing technology to collect the signal analysis
and processing. Through the analysis and processing of acoustic emission
sources can be positioned to identify the type of acoustic emission source to
identify the severity of damage assessment and predict the future development
of damage and other functions. Currently, the commonly used acoustic emission
signal analysis and processing techniques are time domain analysis, frequency
domain analysis, parametric analysis, wavelet analysis and so on.

 

2.5
Carbon fibre composite materials testing

The research
on the acoustic emission characteristics of composites damage has become a
common method used to study the fracture mechanism of composites. At present,
the acoustic emission detection technology can not only detect and analyze the
load distribution of a single carbon fibre filament or a tow when the fracture
occurs, but also judge the quality of the carbon fibre filament or the carbon fibre
filament. At the same time, it is also possible to identify the type of damage
that the carbon fibre composite has in each stage of the damage process, such
as fibre breakage, crack initiation, crack formation and propagation, and matrix
delamination, etc.

 

2.6 The
advantages and disadvantages of acoustic emission detection

Acoustic
emission detection is an online dynamic detection method. Acoustic emission
detection can collect acoustic emission signals through professional
instruments. After analyzing and processing the signals, an online assessment
of the damage state of the tested object can be achieved. Acoustic emission
detection has the following advantages:

(1) Acoustic
emission detection belongs to passive detection. It detects the stress waves
released when the test object is damaged, while the signal source of
non-destructive testing methods such as ray detection or ultrasonic inspection
is some professional testing instruments. Therefore, Ray detection and
ultrasonic testing belong to the active detection.

(2) Acoustic
emission source can be achieved qualitative analysis, positioning and other
analysis.

(3) Since
acoustic emission detection detects and records the whole process of the
damage, acoustic emission detection can obtain complete acoustic emission
information about the occurrence and spread of the damage. By processing these
information by means of modern signal analysis and processing, not only can
Knowing the status quo of the damage to the test piece and predicting the
future trend of the damage can make it possible to assess the degree of damage
done, the useful life of the product and the structural integrity.

(4) Acoustic
emission technology detection area is very wide. As long as the surface of the
member to be detected coupling fixed enough sensors, it can detect components
from all directions of the defect acoustic emission signal, do not need to
frequently move the sensor position to do the scan operation.

(5) Acoustic
emission detection technology can be applied to almost all materials on-line
defect detection. At the same time the technology will not be influenced by the
shape and size of components, the application is very extensive.

(6) Since acoustic
emission detection does not require direct contact with the material to be
inspected, it is suitable for environments with strong radiation, flammable,
explosive, poisonous and extreme temperatures that other non-destructive
testing methods cannot achieve.

Acoustic
emission testing has wide range of applications, which can predict and evaluate
the development trend of damage, and can realize the dynamic monitoring of
damage and many other advantages. This method is gradually accepted by the
various walks of life and widely used. However, acoustic emission detection in
practical applications also has some limitations:

(1) Acoustic
emission detection cannot detect the static defect because acoustic emission
detection records stress wave release when the object is damaged.

(2) In the
composite material testing, due to irreversible damage to the composite
material, if the material itself has been damaged under the test, then the
applied load does not exceed the required load of the existing damage,
Therefore, in order to obtain accurate acoustic emission detection results, it
is necessary to understand the current status of material damage before
performing acoustic emission testing. At this time, other damage-free materials
such as ultrasonic testing or radiation testing are needed Detection method.

(3) Due to
the particularity of acoustic emission detection, the detected signal results
are often disturbed by noise. Therefore, if the filtering process is not
performed on the obtained signal by using the correct filtering method, the
analysis result will be greatly affected.

(4) Only
the detection of acoustic emission signals can not determine the type and
extent of damage to materials or components, and a number of signal analysis
methods are also needed to assist with the assessment.

 

3. Conclusions

Acoustic
emission detection technology compared with other conventional non-destructive
testing methods, has the following advantages: (1) Acoustic
emission technology detection area is very wide. As long as the surface of the
member to be detected coupling fixed enough sensors, it can detect components
from all directions of the defect acoustic emission signal, do not need to
frequently move the sensor position to do the scan operation. (2) Acoustic
emission detection technology can be applied to almost all materials on-line
defect detection. At the same time the technology will not be influenced by the
shape and size of components, the application is very extensive. (3) Since
acoustic emission detection detects and records the whole process of the
damage, acoustic emission detection can obtain complete acoustic emission
information about the occurrence and spread of the damage. By processing these
information by means of modern signal analysis and processing, not only can
Knowing the status quo of the damage to the test piece and predicting the
future trend of the damage can make it possible to assess the degree of damage
done, the useful life of the product and the structural integrity. (4) Since acoustic
emission detection does not require direct contact with the material to be
inspected, it is suitable for environments with strong radiation, flammable,
explosive, poisonous and extreme temperatures that other non-destructive
testing methods cannot achieve.

In practical
applications, often need to use a variety of non-destructive technology in
order to achieve the most comprehensive test results. Acoustic emission
detection is suitable for detecting dynamic crack growth, crack initiation and
crack growth; ultrasonic testing and X-ray testing are suitable for detecting
defects such as cracks, delamination, inclusions, pores and slag inclusions in
materials or components, that laser detection is only suitable for detecting
near-surface defects; for minor changes in materials or components, defects
such as splice quality and cracks in plywood honeycomb structures can be
detected using laser testing; for magnetic materials, magnetic particle testing
can be used to detect defects such as cracks, folds, interlayers, and slag
inclusions on the surface or near the surface of the material or component; optical
fibre detection is suitable for detecting defects in the pump body, castings,
boilers, pressure vessels and pipe surfaces as well as defects such as weld
quality and fatigue cracks; for non-porous metal and non-metallic materials,
penetration testing can be used to detect defects such as cracking, folding and
loosening, and at the same time, the location, size and shape of defects can be
obtained.

 

References

Mair,
R. (2002). Advanced composite structures research in Australia. Composite Structures,
57(1-4), pp.3-10.

 

Huang,
X. and Zhao, S. (2017). Damage tolerance characterization of carbon fibre
composites at a component level: A thermoset carbon fibre composite. Journal of
Composite Materials, 52(1), pp.37-46.

 

Zhang,
Y., Luo, R., Zhang, J. and Xiang, Q. (2011). The reinforcing mechanism of
carbon fibre in composite adhesive for bonding carbon/carbon composites. Journal
of Materials Processing Technology, 211(2), pp.167-173.

 

Wang,
H., Li, H., Lu, L., Xie, Y. and Xiao, Y. (2014). Delamination Analysis in
Drilling Carbon Fibre-Reinforced Composites. Applied Mechanics and Materials,
697, pp.62-66.

 

Jemielniak,
K. (2001). Some aspects of acoustic emission signal pre-processing. Journal of
Materials Processing Technology, 109(3), pp.242-247.

 

Wen,
J., Xia, Z. and Choy, F. (2011). Damage detection of carbon fibre reinforced
polymer composites via electrical resistance measurement. Composites Part B:
Engineering, 42(1), pp.77-86.