Experimental investigation of fatigue
crack propagation behaviour on steel and
aluminium
Cite as: AIP Conference Proceedings 2317, 020039 (2021); https://doi.org/10.1063/5.0036163
Published Online: 05 February 2021
Chilaka Rishitha, C. Suresh, Sudheer, and Anji Reddy
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AIP Conference Proceedings 2317, 020039 (2021); https://doi.org/10.1063/5.0036163
© 2021 Author(s).
2317, 020039
Experimental Investigation of Fatigue Crack Propagation
Behaviour on Steel and Aluminium
Chilaka Rishithaa), C. Sureshb), Sudheerc and Anji Reddy
SIMCRASH Centre, School of Aeronautical Sciences Hindustan Institute of Technology and
Science, Chennai. India.
a)
Corresponding Author E-mail: chilakarishithareddy@gmail.com.
b)
csuresh@hindustanuniv.ac.in
Abstract: A structural member subjected to cyclic loading leads to a structural failure due to fatigue phenomenon, where
strength of the structure gets degrades by the damage initiation which eventually leads to failure. This paper is about the
prediction of crack growth rate experimentally and assessments of fatigue crack growth rates on steel and aluminium
plates. The experiments are conducted using MTS 100KN land mark fatigue test machine. In this work we analyse the
crack growth rate due to fatigue loading through the characterisation on the dissipative process of fatigue crack tip zone.
The crack growth tests are conducted on the steel and aluminium alloy for the stress ratio of 0.3 with the stress level of
60% of its ultimate stress. The properties of steel and aluminium alloys are estimated from the quasi static testing and
their average values are taken for the studies. Here we used extensometer to find the crack growth rate and also found
manually by measuring the crack length and respective time and their results are mutually agree with each other.
Key words: Fatigue, Crack Growth, cyclic loading, stress ratio, crack opening displacement, dissipated energy.
INTRODUCTION
Fatigue is the dynamic limited perpetual auxiliary change that happens in materials exposed to cyclic loads and
strains that may bring about breaks or crack after various fluctuations. Fatigue cracks are taken about by the
simultaneous activity of cyclic stress, ductile stress and plastic strain. The cyclic force begins the cracking, the
plastic stress produces crack progress spread. In low-cycle fatigue if the material has a predictable work-hardening
rate, the problems likewise might be over the static yield quality. Fatigue breaks start and spread in regions where
the strain is commonly exciting, in light of the fact that most designing materials contain distortions and along these
lines locales of stress loading that strengthen strain, most weariness splits start and develop from basic failures.
Under the activity of cyclic loading, a plastic zone creates at the imperfection tip. The crack extends under the
applied load through the material until complete crack outcomes.
Jie Wang et al [1] studied the crack closure effect and single peak overloading retardation factor for the material
subjected to fatigue loadingt. H.R. Wang et al [2] analyse the fatigue crack growth rate through the characterization
on the dissipative process of the fatigue crack tip zone. S. Rabbolinia et al [4] the propagation of short cracks are
investigated using the effect of crack closure. Crack closure was characterized based on digital image correlation. H.
Mayer et al [5] studied about the application of ultrasonic frequency loading to test mechanical properties of
materials at high frequency on high strength aluminium alloys. D. Kujawski [8] proposed a new parameter for short
and long crack growth rate correlation. H.Karrlson et al [10] investigates the structure when subjected to cyclic load
rather than static load that leads to castrophic failures. Explores the concept of using plastically dissipated energy as
3rd International Conference on “Advancements in Aeromechanical Materials for Manufacturing”
AIP Conf. Proc. 2317, 020039-1–020039-9; https://doi.org/10.1063/5.0036163
Published by AIP Publishing. 978-0-7354-4058-6/$30.00
020039-1
criterion of fatigue crack growth for a range of materials. P.J. Huffman et al [15] Fatigue crack initiation was
described using the empirical relation between the alternation stress or strain and number of cycles at the load, and
localized at the tip of crack zone. To design a fail-safe structure, it’s always required to have a complete research on
materials, especially the life of material after a small damage is always been a critical. Even though similar types of
various studies are available the focus at low frequency loading is countable. This motivates us to do an
experimental study on fatigue crack growth analysis on steel and aluminium alloys which are more widely used for
many engineering applications.
EXPERIMENTAL WORK
MTS 100 KN fatigue testing machine is with real time test control system is used for testing the specimens made
a per ASTM standard E647. The specimens required for testing are prepared using laser cutting for initial crack
opening displacement. Two types of specimens are prepared of which one will be fixed with extensometer to
measure the crack growth and other one the crack growth will be measured manually. The specimens are shown in
figure.1
FIGURE 1. Testing specimen after laser cutting with initial COD
The quasi static testing was carried out to find the tensile properties and other mechanical properties of steel and
aluminium which are used for the current testing. The test was carried out with the loading rate of 2mm/min
according to the tensile test standards. Three specimens are tested and their average values are taken as material
properties for the current study. The properties are shown in table.1
TABLE 1 Properties of Steel and Aluminium
Parameters
Steel
Aluminium
Young’s modulus
207.08 GPa
66 GPa
Shear modulus
80.0 GPa
Poisson’s ratio
0.3
0.33
7600 (Kg/m3)
2707 (kg/m3)
370 MPa
280 MPa
Density
Yield strength
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To determine the fatigue crack growth behaviour, tension-tension fatigue experiments were carried out at 60% of
the ultimate tensile strength of the test specimen with stress ratio of R=0.3 at constant amplitude loading and 5Hz
frequency.
RESULTS
In the figure.2 shows the effect of fluctuation of stress intensity factor (Δk) under constant amplitude load testing
on Compact tension specimen. The test was done on three mutually same configured specimen and the average
values are taken for the justifying results. The results from the graph are observed that Δk increases expressively
after reaching mode Ⅱ when compared to before cases and also it clearly shows that Δk increases exponentially with
increasing number of cycles and even increase rapidly when reaching mode Ⅱ region.
70
Delta-k(MPa-m0.5)
60
50
40
30
20
10
0
0
50000 100000 150000 200000 250000 300000 350000 400000
Cycles(N)
FIGURE 2. Stress intensity factor (Δk) vs number of cycles (N) curve
The figure.3 plot between stress intensity factor (Δk) and crack length also have similar properties as figure 2, it
also increases exponentially with the increase of crack length, but when compared between mode Ⅰ and Ⅱ the rate is
rapid. In mode Ⅱ region it is increasing but not as fast as number of cycle’s case.
020039-3
70
Delta-K(MPa-m0.5)
60
50
40
30
20
10
0
0
10
20
30
40
50
60
70
a(mm)
FIGURE 3. Stress intensity factor (ΔK) vs crack length (a) curve
A typical logarithmic graph between da/dn and stress intensity factor (Δk) is shown in figure.4 Crack
propagation rate always increases with the value of Δk this happens because the more valve of Δk signifies more
driving force to propagate a crack. Actually, in fatigue crack growth short crack behaviour is not well explained by
long crack in da/dn vs Δk curve. Generally short crack starts growing sooner than long crack with comparable stress
intensity range and reduces down as they get longer crack.
The fatigue crack growth propagation was tested under constant amplitude loading and stress ratio of 0.3. The plot
drawn between crack length vs number of cycles as shown in figure.5 and figure.6 says that crack length also
increases
1.20E-05
da/dn(m/cycles)
1.00E-05
8.00E-06
6.00E-06
4.00E-06
2.00E-06
0.00E+00
0
10
20
30
40
50
60
70
Delta-k(MPa-m0.5)
FIGURE 4. Log da/dn vs log Δk curve
exponentially with increase of number of cycles and form FIGURE.4 it is observed slow fatigue crack
propagation and it is due to large plastic zone developed near crack tip region during the test time so that more
number of cycles required to overcome this region.
020039-4
80
70
a(mm)
60
50
40
steel
30
20
10
0
100000
200000
300000
400000
500000
cycles(N)
a(mm)
FIGURE 5. Crack length vs number of cycles
90
80
70
60
50
40
30
20
10
aluminium
0
100000
200000
300000
400000
500000
cycles(N)
FIGURE 6. Crack length vs number of cycles
Manual Method
The curve plotted between crack length and the number of cycles using manual method is shown in figure.7
which consists of average values taken between three mutually configured CT specimens. This curve clearly says
that crack length erratically rises up after a long crack length
020039-5
70
60
a(mm)
50
40
steel
30
20
10
0
100000
200000
300000
400000
500000
cycles(N)
FIGURE 7. Crack length vs number of cycles
80
70
a(mm)
60
50
40
aluminium
30
20
10
0
100000
200000
300000
400000
500000
cycles(N)
FIGURE 8. Crack length vs number of cycles
Comparison Between Clip Gauge And Manual Method
The plot of crack growth verses number of cycles between clip gauge and manual method are been compared in
the figure.9 and figure.10. The comparison of two methods as mentioned can be done with either Pari’s law curve or
by means of a-N curve, here we obtained Pari’s law. Besides two methods a numerical confirmation also performed.
Both methods have an excellent correlation, mainly at initial and last part of the test there is only a small deviation
among them, but overall, the results obtained are same with respect to cyclic loading
020039-6
80
70
60
a(mm)
50
40
clip gauge
30
manual
20
10
0
0
100000
200000
300000
400000
500000
cycles(N)
FIGURE 9. Crack length vs number of cycles for steel
80
70
a(mm)
60
50
40
clip
gauge
30
20
10
0
100000
200000
300000
400000
500000
cycles(N)
FIGURE 10. Crack length vs number of cycles for aluminium
020039-7
CONCLUSION
The experimental method for predicting fatigue crack growth rate under cyclic loading has been established
using two different methods called Clip gauge method and Manual method. In clip gauge method an extensometer is
used find the crack growth rate. In manual method the crack length and instantaneous time are taken manually to
find the crack growth rate. It’s observed that the results from both the experimental methods are similar to each
other. The stress intensity factor Δk increases exponentially with increasing number of cycles and even increase
rapidly when approaches mode Ⅱ failure. In fatigue crack growth short crack behaviour is not well explained by
long crack in da/dn vs Δk curve. Generally short crack starts growing sooner than long crack with comparable stress
intensity range and reduces down as they get longer crack. The fatigue crack growth propagation was tested under
constant amplitude loading and stress ratio of 0.3. The plot between crack length vs number of cycles shows that
crack length also increases exponentially with increase of number of cycles. The numerical results show a similar
crack propagation as that in the experimental results. Further planned to extend this study for different stress ratios
and also by varying the frequency keeping the stress ratio constant.
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