FIELD OF THE INVENTION
The present invention relates to a method of monitoring the start of the crack
generation on a punched hole during the hole expansion testing of sheet
metal.
BACKGROUND OF THE INVENTION
Hole expansion test (HET) is the method of determination of the stretch
frangibility of a sheet metal processing and applicable to steel sheet and is
useful to assess the suitability of the product for forming flanges. The method
of this test has been described in ISO/TS 16630:2003 (E). But during this
testing, the point of first crack generation needs to be decided precisely.
Several visual method and load drop method of the punch press were
generally applied to detect the cracking of the specimen. These methods are
insufficient to confirm the crack detection at its initial stage. Therefore several
trial needs to be done by testing several specimen to find out the closest
limiting value of the hole expansion.
During hole expansion test it is critical to know the point of crack initiation of
the metal sheet which is under increasing strain by the insertion of a conical
punch through a hole. The point of crack initiation is generally determined by
testing samples for hole expansion test repeatedly and examining the
sample visually. The load experienced by the punch of the hole expansion
tester is recorded and monitored to estimate the point of cracking. The
response of the punch load may have some delayed response to detect the
crack generation to stop the movement of the punch immediately after the
crack generation. The measurement of the load on the punch is shown on a
graph plotted (black line) by a Hole Expansion tester in Fig.1C.
Sometime, visual aids like high resolution cameras are being used to estimate
and measure the diameter of the expanding hole at the point of its first crack.

In a competitive industrial environment, maximizing the productivity with
ascertaining the quality performance of every batch of production, Hole
expansion test of multiple samples from the same batch will be a tedious and
time consuming process. Therefore, to find out the hole expansion test result
in one go is an important consideration for this invention. The use of AE
signal for determining the crack of the specimen during HET is found a viable
and accurate option. Though the use of AE n crack detection is not new, but
its application to detect the limiting hole expansion is not found in any
literature.
Therefore, the method of crack detection by AE technique can be used as an
effective alternative with the hole expansion testing machine, to reduce the
effort in hole expansion testing and to avoid the need of multiple sample
testing.
Problem to be solved: Rigorous trial and error method to determine the
limiting value of the hole expansion test for a metallic sheets.
OBJECTS OF THE INVENTION
It is therefore an object of the present invention to provide a hole expansion
testing (HET) system which obviates the drawbacks of detecting the limiting
position of the punch hitherto known prior art as detailed above.
Another object of the present invention is to propose a method which would
be simple to perform the HET and achieve consistent results.
A still another object of the present invention is to propose a method for
monitoring of crack generation during HET on the specimen without the need
to stop the machine for manual inspection of using some other means of
visual aids.

Yet another object of the present invention is to propose a method which is
tailored to function without human interaction for the judgement of the crack
initiation.
Another object of the present invention is to optimize the time of HET.
A still another object of the present invention is to propose a method which
allows integration of HET for batch by batch quality performance monitoring
of industrial products.
A further object of the present invention is to identify a location for adapting
the acoustic emission sensor on the specimen.
A still further object of the present invention is to propose a monitoring
system that is arranged to perform the method of invention which would be
easy to operate, inexpensive and reliable in operation, new material design,
designing of the component for using a specific material and material
research.
SUMMARY OF THE INVENTION
Accordingly the present invention provides an acoustic emission method of
determining the crack of the HET specimen which comprises using AE sensor
as an interface to capture the AE generated during expanding of a hole and
analyzing them by extracting AE characteristics like cumulative counts, RMS
as an indicator of the cracking.
The details of the process steps are explained below.
- generating high frequency AE through elastic and plastic interaction of
expanding hole leading to formation of crack of the specimen, by
pushing a conical punch using AE sensor placed on the specimen;

- processing the captured acoustic emission signal to extract acoustic
emission characteristics such as average amplitude (ASL), counts, hits
and RMS, variations of which characteristics causing cracking of
materials;
- depicting in a plot the ASL obtained and RMS of the acoustic emission
signal; and monitoring the level of acoustic emission generated at the
hole of the specimen, and quantify the initial condition of crack
formation on the specimen.
The present invention also defines an on-line HET system that is arranged to
perform the method of invention, the system comprising: an acoustic
emission sensor adapted to be mounted on the extended portion of the
specimen; signal processing means used to obtain the acoustic emission
characteristics. Devices used for capturing the AE signals are shown in Fig.3.
According to the invention, on-line acoustic signal has been accoutred and
analysed from the sample under hole expansion test to determine the start of
the crack generation. It has been found that, at the point of crack generation
the ASL (Average Signal Level in dB) is suddenly increased ( shown in Fig.6).
Seeing this change in acoustic emission level, suitable alarm has been raised
and stopped the hole expansion process. The result obtained from this
method is found accurate to and comparable to the conventional method of
repeated testing.
Solution: On-line checking of acoustic signals from one end of the sample
under the hole expansion testing to detect the high dB of acoustic signal at
the pint of first crack generation.
BRIEF DESCRIPTION OF THE ACCOMPANYING DRAWING
Fig.1a: Arrangement of the AE probe on the extended piece of specimen just
before HET.

Fig.1b: Arrangement of the AE probe on the extended piece of specimen just
after cracking of the specimen at the punch hole.
Fig.1c: Conventional method of predicting the crack formation during hole
expansion test from the plot of load experience by the punch.
Fig.2: A typical bust of AE signal observed during cracking of the specimen
Fig.3: Devices used for capturing the AE signals
Fig.4: ASL obtained and RMS of the acoustic emission signal is depicted in a
plot during HET.
Fig.5: Specimen on the left showing one crack which was detected from the
sudden increase in the ASL and RMS value of the AE signals. Specimen on the
right showing multiple cracks formed during HET trials due to the overrun of
the conical punch and leads to a wrong value of the HET limit.
Fig.6 : Indication of the start point of cracking by analysing Acoustic signals
from the specimen during hole expansion testing.
DETAILS DESCRIPTION OF THE INVENTION
In the present invention described herein, Acoustic emission (AE) signals are
measured. AE is commonly defined as transient elastic waves within a
material caused by the release of stress energy stored in the elastic and
plastic stress field produced by the damage created by the movement of a
conical punch.. AE are associated with a wide range of processes covering
from micro processes caused by the movement of solid structure fragments
to macro processes related to failure of assemblies. It is known to have a
quite broad frequency spectrum ranging from tens of hertz to tens of
megahertz. Several process like material deformation, phase transformations,
fracture or frictional processes are associated with significant acoustic

emissions. In the present inventive method, AE associated with cracking
processes during HET are analysed.
Referring to Fig.1 the preferred embodiment of the present investigation is
illustrated as a system for monitoring cracking of the specimen under HET.
The arrangement of the conical punch, top and bottom blank holder
(specimen holder) along with the specimen has been shown. The size of the
specimen is made little longer than the minimum size of (90 mm x 90mm) as
specified by the standard. The extended portion of the specimen was used to
place the AE probe.
To ensure proper transfer of AE stress waves to the probe it is important to
maintain a good mechanical contact between the sensor surface and the
specimen surface on which it is mounted. There are several ways to achieve
good mechanical contact between the surfaces. One way is to use a magnetic
clamp to hold the sensor on the surface and an adjustable screw is used to
apply pressure on the sensor head to ensure proper mechanical contact. In
addition to maintaining a good contact between the AE sensor and die nut
surfaces, the transmission of AE waves to the sensor can be enhanced by
using a couplant material (grease, oil etc) between the sensor and die nut
surface. The couplant material is primarily used to remove any air from the
interface introduced due to the microstructure of the contacting surfaces.
The AE sensor engaged here has a wide operating frequency range of 100
KHz - 1 MHz. An advantage of using such wide frequency range is that the
unwanted noise and vibration signal from other machines are filtered out. It
has been found out that AE waves with high energy are emitted in this range
due to the cracking of the specimen. The probe captures the AE signals
generated by the formation crack due to the movement of the conical punch

and provides electrical signal as a response to the generated acoustic
emissions.
The response of the probe whose strength is very small is fed to the USB AE
node to enhance the signal strength, band pass filtered within the range of
100 KHz-1 MHz and further processed through analog to digital converter
(ADC converter) after which the signal is stored in the laptop for analyzing.
According to the present invention, to monitor the generation of crack, AE
emitted at the punch hole is captured and analyzed by extracting AE
characteristics by using threshold - based signal processing. In such a signal
process the threshold level is chosen such that the background noise and
unwanted noise from the other sources are below the threshold level and
acoustic emission generated by cracking exceeds the threshold level. The
most commonly measured AE characteristics are events, signal duration, RMS
and ASL of the signal. RMS and ASL value of the signal are proportional to
the energy of signal. The signal duration and number of threshold crossings
will define the energy content of the signal.
The laptop screen provides a visual display of a recorded trace representation
of characteristics of AE. During HET, as conical punch progress with time and
when the cracking starts at inner diameter of the expanding hole it shows
significant deviation in the characteristic of AE thus providing an indication of
the crack.
The following examples are given by way of illustration of the working of the
invention in actual practice.
Example:
The proposed method of using acoustic emission for monitoring the HET
process is applied at the press forming laboratory of R & D, TATA STEEL. The

AE sensor was mounted on the specimen surface to measure the AE activity.
The AE data was acquired continuously during the HET. The AE signal was
analyzed to determine the acoustic emission characteristics and monitored
continuously to determine the variation of characteristics behaviour
associated with the movement of the conical punch. The plot representation
of ASL and RMS value of AE characteristics revealed a sudden increase in
while cracking of the sample as shown in Fig. 4.
The punch movement was stopped immediately and the specimen was taken
out for verification of the crack. After repeating the process with many
samples it has been ensured that the stat of the crack has been detected just
at the right time.
ADVANTAGES OF PRESENT INVENTION
This method helps in determining the hole expansion test result accurately
and without repeating the test with several samples.
Therefore, this can be quickly applied to test and develop a product suitable
to stretch flanging application. Also, we may adopt the hole expansion test
for regular testing for a batch of production to anticipate suitability of the
material to the desired applications.
The most advantageous attribute of the method is that the acoustic emission
(AE) signals are measured and analyzed in a frequency range of 100 KHz-
1MHz and the AE signal in this frequency range are associated with the
cracking which means that the unwanted noise and vibration signal from
other sources like hydraulic noise are filtered out. Therefore, the AE system
as described is suitable for detection of the crack at its start.
Other advantages of this system are:
(i) Can be used to test at production line.
(ii) Easy to mount the AE probe on the sample.

(iii) Arrangement of automatic stop control of the punch movement at
the onset of cracking.
(iv) Automatic measurement of the Limiting Hole expansion diameter
from the position of the conical punch at the onset of cracking.
(v) Automatic calibration of the system with standard sample.
(vi) AE data can be stored for future analysis and performance of the
sheet.
(vii) More no of tests can be conducted to capture the variation within a
batch of metal sheet produced.
(viii) Segregate the non-performable portion of the metal sheets
produced from different batches and within a batch when some
process variation is observed.
(ix) Grading of the sheet metal product for the requirement of different
stretch frangibility.
(x) Reverse design of a component by immediately knowing the hole
expansion limit of the material to be used.

WE CLAIM:
1. An acoustic emission method of monitoring Hole expansion Test to
determine the start of cracking of a specimen, the method comprising
the steps of :-
- measuring high frequency acoustic emissions generated as a
consequence of cracking of the specimen during the advancement of a
conical punch and generating a measurable voltage signal as a
response ;
conditioning the measured signal and extracting the AE characteristics; and
monitoring the AE characteristics for the occurrence cracking of the
specimen.
2. The method as claimed in claim 1, wherein said conditioning means
band pass filtering the amplified signal in the range of 100 KHz -1
MHz.
3. The method as claimed in claim 1, wherein said AE characteristics
include Root Mean Square (RMS) and Average Signal Level (ASL).
4. The method as claimed in claim 1, wherein said monitoring AE
characteristics include providing a visual display of behaviour of AE
characteristics.
5. An apparatus for measuring hole expansion at the onset of crack
formation of a sheet metal, includes
A wide band acoustic sensor for measuring the acoustic emission and
generating an electrical signal as a response, an USB Node as an pre-

amplifier to enhance the signal strength with inbuilt band pass filter
and ADC (Analog to Digital converter) to digitize the signal for analysis
in the Laptop with the help of AE win software.
6. The apparatus as claimed in claim 5, wherein said measuring AE using
sensor that is mounted on the specimen under testing.
7. The apparatus as claimed in claim 5, wherein said band pass filter
means a means used to neglect the unwanted noise from other
machines.
8. The apparatus as claimed in claim 5, wherein said analysis means
extracting the AE characteristics and provide a visual display of
variations in the characteristics.
9. The apparatus as claimed in claim 5, wherein said analysis means
extracting the AE characteristics and provide a visual display using AE
win software.
10. A method of monitoring the start of the crack generation on a
punched hole during the hole expansion testing of sheet metal, the
method comprising the steps of :-
- generating high frequency AE through elastic and plastic interaction of
expanding hole leading to formation of crack of the specimen, by
pushing a conical punch using AE sensor placed on the specimen;
- processing the captured acoustic emission signal to extract acoustic
emission characteristics such as average amplitude (ASL), counts, hits
and RMS, variations of which characteristics causing cracking of
materials;

- depicting in a plot the ASL obtained and RMS of the acoustic emission
signal; and monitoring the level of acoustic emission generated at the
hole of the specimen, and quantify the initial condition of crack
formation on the specimen.