36158 [Compatibility Mode]

11th International Fatigue Congress
Melbourne, Australia
2014
Fatigue crack growth rate in laser-welded web core sandwich panels – fatigue
crack propagation in welded base metal
Anghel Cernescu
Pauli Lehto
Heikki Remes
Jani Romanoff
Department of Applied Mechanics
Aalto University of Finland
OUTLINE
1. Introduction
2. Material and method
3. Results
4. Conclusions
1. INTRODUCTION
• The shipbuilding industry was pioneer in adopting firstly pure laser and hybrid laser welding
as also the first simple sandwich panels.
• The web-core sandwich panel is an all-metal
structure fabricated as two face plates stiffened by
one directional system of web plates.
•In-plane and out-of-plane fatigue loadings combined with residual stresses resulted from the
welding process can cause initiation of cracks that would propagate leading to early failure of
the whole structure.
2. MATERIAL AND METHOD
2.1. Material characterization
•The face plate material was analyzed in terms of chemical composition, microstructure and
mechanical properties.
•The material analysis indicated an ferrite low-alloy steel with the following mechanical
properties:
Component
Material
Face plate
Low-alloy
steel
Young’s
modulus [MPa]
191351.18
The microstructure of face plate material
Yield Strength
[MPa]
175.27
Ultimate Tensile
Strength [MPa]
287.53
The microstructure of laser welded material
2.2. Fatigue crack growth rate tests
• 10 samples were cropped from one of the face plate of the web core sandwich panel.
• 8 samples were used for fatigue crack growth rate tests and 2 samples (LTW-4 and LTW5) were used to determine the residual stress intensity factor, Kres
50±0.4
Ф 10±0.25
8
48
±0.4
2.5
22
t = 2 mm
10
40
The samples cut from web core sandwich panel
The CT specimen according to ASTM 647
standard
• The fatigue crack growth rate tests were carried out at room temperature and in laboratory
air conditions, according to ASTM E 647 while crack length was monitored by the
compliance method.
•The fatigue crack growth rate curve was determined for the base metal in longitudinal and
transverse directions, welded metal and HAZ, for a cyclic loading with sinusoidal wave form
and two stress ratios, 0.1 and respectively 0.5.
The fatigue crack growth rate tests on CT samples
2.3. Welding residual stresses
• The cut compliance method (CCM) was used to determine the residual stress state through
the thickness of the sample and respectively for determination of the residual stress intensity
factor, Kres.
E′ dε
K rs =
Z (a ) da
a
−6.694 
2.532 
W
Z (a) = −
1− e

1.5 
(W − a) 

The location
of strain
gage
The
WEDM
cutting
Laser
welded
seam
The welded
CT
specimen
The cutting technique used in CC method
3. RESULTS
3.1. Fatigue crack growth in base metal
• For base metal, the tests carried out have shown effect of stress ratio on fatigue crack growth
rate.
The da/dN=f(ΔKapp) curves for fatigue crack growth in longitudinal and transverse direction (BM-L – Base
Material–Longitudinal direction; BM-T – Base Material-Transverse direction)
3.2. Fatigue crack growth in HAZ
The da/dN=f(ΔKapp) curves for crack propagation in HAZ (HAZ-L – Heat Affected Zone-Longitudinal
direction)
• There is a decrease in fatigue crack growth rate in HAZ compared to base metal
Location of crack
propagation
BM-L
(Longitudinal)
BM-T
(Transverse)
HAZ-L
(Longitudinal)
R-ratio
C
m
0.1
0.5
0.1
0.5
0.1
0.5
1∙10-10
3∙10-10
2∙10-10
4∙10-10
9∙10-11
5∙10-8
4.37
4.169
4.282
4.052
4.361
2.293
• There is a decrease in fatigue crack growth rate in HAZ compared to base metal
Location of crack
propagation
BM-L
(Longitudinal)
BM-T
(Transverse)
HAZ-L
(Longitudinal)
R-ratio
C
m
0.1
0.5
0.1
0.5
0.1
0.5
1∙10-10
3∙10-10
2∙10-10
4∙10-10
9∙10-11
5∙10-8
4.37
4.169
4.282
4.052
4.361
2.293
• There is a decrease in fatigue crack growth rate in HAZ compared to base metal
Location of crack
propagation
BM-L
(Longitudinal)
BM-T
(Transverse)
HAZ-L
(Longitudinal)
R-ratio
C
m
0.1
0.5
0.1
0.5
0.1
0.5
1∙10-10
3∙10-10
2∙10-10
4∙10-10
9∙10-11
5∙10-8
4.37
4.169
4.282
4.052
4.361
2.293
• There is a decrease in fatigue crack growth rate in HAZ compared to base metal
Location of crack
propagation
BM-L
(Longitudinal)
BM-T
(Transverse)
HAZ-L
(Longitudinal)
R-ratio
C
m
0.1
0.5
0.1
0.5
0.1
0.5
1∙10-10
3∙10-10
2∙10-10
4∙10-10
9∙10-11
5∙10-8
4.37
4.169
4.282
4.052
4.361
2.293
3.3. Fatigue crack growth in welded metal
• The results show an high stress ratio effect on the fatigue crack growth rate for crack
propagation in the welded metal.
The da/dN=f(ΔKapp) curves for fatigue crack propagation in welded metal (WM-L – Welded
Material-Longitudinal direction)
• Comparing the results obtained for fatigue crack growth in welded metal with those
obtained for the base metal, can be observed a noticeable apparent improvement of the
fatigue crack propagation resistance in welded metal.
• The apparent improvement is more obvious for stress ratio 0.1.
The da/dN=f(ΔKapp) curves for crack propagation in welded metal and longitudinal direction of base metal
The fatigue crack growth rate curves in base metal and welded metal
• Based on the fatigue crack growth data in welded metal, corresponding to stress ratio 0.1, it
was made an assessment of the residual stress intensity factor during crack propagation by
applying the inverse method and closure approach.
f =
S0
= A0 + A1 R + A2 R 2 + A3 R 3
S max
for R ≥ 0
A0 = 0.32565, A1 = 0.0819, A2 = 0.8592 and A3 = -0.26679 – were determined based on calibration of
closure function on fatigue crack growth rate function of ΔKeff , for crack propagation in base metal
• For fatigue crack growth in welded material it has been determined the function:
U=
∆K eff
∆K app
2
3

 K min + K res 
 K min + K res  
K min + K res
1 −  A0 + A1
+ A2 
 + A3 
 
K
+
K
K
+
K
K
+
K

max
res
max
res
max
res



 
1 − ftr

=
U=
K + K res
1 − Rtr
1 − min
K max + K res
Rtr =
K min + K res
K max + K res
• The negative values of Kres have a tendency to close the crack due to compressive residual
stresses, while the positive values keep the crack open and the tensile residual stress does
not affect the stress intensity factor range.
The effect of residual stress intensity factor on fatigue crack growth rate in the welded metal (sample LTW-2)
• The analytical and experimental results shows similar distributions with high residual
stress intensity factor near the notch and respectively at the beginning of crack propagation
which decreases with increasing the crack length.
The distributions of residual stress intensity factor: LTW-4
and LTW-5 – experimental values determined by CC
method; LTW-2 – analytical values determined by inverse
method
3.4. Fatigue fracture mechanism
• Analyzing the fracture surface of the samples from the base metal, it has been determined
the fracture mechanism that led to fatigue crack propagation.
• The fracture surface shows multiple microcracks and detachments oriented along the
direction of crack propagation.
The fracture surface of CT specimen at 10x magnification
• The fracture surface appearance shown in Figures 13-14 was observed in all samples
tested from base metal and HAZ and it was established that the detachments on the
direction of crack propagation represent intergranular and respectively intragranular
separations.
The fracture surface of CT specimen at 12000x
magnification for crack propagation in base metal
The fracture surface of CT specimen at 10000x
magnification for crack propagation in base metal
• In the case of fatigue crack propagation in welded metal is preserved the initiation of
microcracks and the fracture appearance is more fragile.
Cleavage fracture in the welded metal given by an
overloading effect due to positive residual stresses.
4. CONCLUSION
• In this paper is conducted an experimental study to analyze the fatigue crack growth rate in
the face plate of a laser welded web core sandwich panel.
• The fatigue crack growth tests carried out in the base metal, HAZ and weld metal showed a
significant effect of the stress ratio on fatigue crack growth rate in the threshold domain
which decreases as the stress intensity factor range increase to the stable domain of the crack
propagation.
• It has been shown that the negative values of the residual stress intensity factor and
respectively residual stresses have an effect of closure the crack while the positive values
have an effect of opening the crack.
• The appearance of the fracture surfaces revealed the fatigue fracture mechanism during
crack propagation which is preceded by the occurrence of microcracks.
THANK YOU FOR YOUR ATTENTION!

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