Bolted connections involve sharp notched components that are therefore sensitive to fatigue loading. In internal combustion engines, the journal bearings of the crankshaft are supported by bearing caps which are bolted to the cylinder block. These threaded connections are a fatigue concern as cracking may be experienced on dynamometer tests of new engines. The critical element of this application however is not the steel studs rather the threaded bolt holes in the cast engine block. While the nominal stress concept is applicable to the fatigue design of studs, a local stress approach has to be adopted to assess the fatigue strength of threaded bolt holes. This paper addresses the fatigue design of this kind of threaded connection. Initially alternate methods of FE modeling and analysis of threaded joints by the local approach are critically examined. The parallel development of a simplified experimental test system involving a threaded hole in cast AlSi7 is described and used to generate baseline fatigue data under known loading conditions. The fatigue behavior of the experimental system is then analyzed on the basis of alternate FE models and post processing approaches. Finally, recommendations for the accurate and computationally efficient FE modeling and durability analysis of threaded connections in cast aluminum cylinder block are outlined. Speakers Marco Bersella, Engineer, TP Engineering srl

1. Enrica Riva – Il metodo degli elementi finiti nella progettazione meccanica
Sede Operativa Sede legale
Via Lanfranco N. 9 Borgo Regale N. 15
43126 Parma 43121 Parma
Tel./Fax 0521.774898 P.IVA 02399510342
E.mail: info@tpengineering.it
Stress and durability analysis of threaded
connections in a cast aluminum cylinder block
M. Bersella 1, M. Padovan1 , G. Baruffaldi 1, G. Nicoletto 1,2
1 TP Engineering S.r.l., Parma, Italy
2 Dept. of Industrial Engineering, University of Parma, Parma, Italy
email: marco.bersella@tpengineering.it
European Altair Technology Conference
Paris, 2015

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3. AGENDA
 Stud/threaded hole problem description
 Hybrid experimental/computational solution strategy
• Experimental tests on simplified threaded specimens
• Finite element models
• FEMFAT post-processing
 Transferring the experience to application
 Wrap up

4. MOTIVATION AND PROBLEM DESCRIPTION
CONTEXT
• High–performance gasoline V6 engine
development
• AlSi8 cast aluminum alloy
PROBLEM
• Early failures due to fatigue cracking of main
bearing wall of engine block during
dynamometer test
• Crack origin: multiple thread roots of stud
fixation holes
Failure location
Engine block
Stud
Bearing cap

5. HYBRID EXPERIMENTAL/COMPUTATIONAL STRATEGY
Simplified threaded configuration
for fatigue testing
Factors considered:
• different load levels
• different thread pitches
• different thread tapping
processes
1
FEM modeling of simplified
threaded configuration
• axial-symmetric model
to assess load stresses
• explicit model to predict
threading process
residual stresses
2
Base material (cast AlSi8)
characterization
using smooth specimens
extracted from castings.
• Tensile tests
• Fatigue tests
FEMFAT post-processing
FEM modeling
strategy validation
Transfer of FEM modeling strategy
to engine block assessment

6. SIMPLIFIED THREADED CONFIGURATION
• The experimental test system involved a single
threaded hole with mounted screw and was
developed to generate fatigue data under
controlled loading condition.
• Threaded hole specimens were directly extracted
from engine blocks.
• Specimen geometry was optimized with the aim of
obtaining fatigue crack initiation in the first
unloaded thread root.
• Different thread pitches and tapping processes
were examined.
Specimen
type #
Hole
thread
External
diameter
Tapping process
1 M12x1.75 18 mm CUT
2 M12x1.50 17 mm
ROLLED WITH
TAPPING PROCESS 1
3 M12x1.50 16 mm
ROLLED WITH
TAPPING PROCESS 2
4 M12x1.50 17 mm
ROLLED WITH
TAPPING PROCESS 3
1
2
3
4
Specimenshapes
Engine block
Extraction regions

7. EXPERIMENTAL DETAILS
• Pulsating cyclic force F is applied to
the test configuration (MTS 810
hydraulic system).
• The stair-case method (at least 10
specimens) were used to determine
the fatigue strength at 2e6 cycles.
• Failures generally initiated at the root
of the thread and propagated to
fracture.
• Material defects (pores) were not
found at initiation points.
Test configuration
Steel screw
Threaded hole in
cast Al/Si specimen
Fatigue cycle
F
F

8. TEST RESULTS
Specimen type
#
Hole thread Tapping process
Fatigue strength
at 2 10^6 cycles
1 M12x1.75 CUT 34 MPa
2 M12x1.50
ROLLED WITH TAPPING
PROCESS 1
44 MPa
(+ 30%)
3 M12x1.50
ROLLED WITH TAPPING
PROCESS 2
42 MPa
(+ 24%)
4 M12x1.50
ROLLED WITH TAPPING
PROCESS 3
30 MPa
(- 12 %)
• Fatigue strengths are associated to the nominal stress calculated at the thread
root cross-section.
• The thread forming method is found to influence the fatigue strength.

9. FE MODEL DEVELOPMENT OF TEST CONFIGURATION
Detailed FE modeling is necessary to assess local stresses in the threads:
• 1st order and 2nd order axial-symmetric quad elements compared
• Finer mesh at critical thread roots to determine peak stress and stress gradient
• Contact between threads flanks
• Boundary conditions on the model to simulate specimen clamping
Mesh size
on critical root:
0.04 mm
Steel
AlSi8Cu3Fe
Max traction force
Full scale at AlSi8
yield stress
only localized
plasticity
Linear elastic behavior of thread material is assumed.

10. ENGINE BLOCK MATERIAL CHARACTERIZATION
Specimens for material characterization were
extracted from the block main bearing wall.
Types of test on base material:
o Tensile test
• #3 specimens
• Geometry according to ASTM A 370-09
• MTS 810 test system.
o High cycle fatigue test
• #10 specimens
• AMSLER vibrophore
• Stair-case procedure.
Smooth specimen geomery
Cast poreInitiation
Porosity did not affect the static
properties.
Crack initiation in the smooth fatigue
specimens generally started at cast
pores.

11. MATERIAL PROPERTIES OF ALSI8 (A380)
The large difference between experimental and literature data highlights the
influence of the casting process on the material properties.
Briefly, the material from engine block castings has:
• Lower strength, considering both UTS and fatigue limit.
• Lower ductility, elongation < 1%.
A380 extracted from
production casting
A380
separately die cast
Value at 50% S.P. Std dev. s Reference value (^)
Yield strength
(0.2% strain)
150.6 MPa 11.1 MPa 159 Mpa
Ultimate tensile
strength
188.8 MPa 18.2 MPa 324 MPa
Elongation at
rupture
0.74 % 0.17 % 3.50 %
Fatigue limit
in push-pull
55.0 MPa 7.0 MPa 138 MPa
(^) literature

12. FEMFAT POSTPROCESSING
Material data:
• Static strength according to
experimental tensile tests
• Alternating strength from push-
pull experimental data
• Gradient sensitivity on default
value (FKM) for cast aluminum
alloy
FEM loads for each specimen
geometry:
• upper stress
• lower stress
FEM loading, for each geometry, is
derived from staircase experimental
results on threaded specimens.
Influence factors:
• Gradient and support effect
• Mean stress influence
• PLAST stress re-arrangement
Parameters:
• Endurance analysis with R = const
overload line
• Scaled normal stress in critical
plane

13. 0,85
0,90
0,95
1,00
1,05
1,10
1,15
CUT ROLLED #1 ROLLED #2 ROLLED #3
SF(50%surv.prob.)
FEMFAT SF at critical thread root
1st order elements 2nd order elements
FEMFAT RESULTS – THREAD CUTTING
M12x1.5 CUT specimen
SF_A = 1.004
on critical
thread root • Safety factors SF according to FEMFAT are very
close (within ± 10% range) to experimental data
for all specimens.
• 1st order elements reach the best accuracy and...
How to include the influence of the plastic forming process of the threads on
the fatigue predictions?

14. THREAD FORMING PROCESS SIMULATION
• Thread rolling is a process based on
plastic deformation of the material.
• High residual stresses in the thread root
are expected.
• Explicit FE analysis can simulate the
rolling process and predict local stresses
using a two step procedure.
Single thread generation
• The elastic-plastic material model was
obtained experimentally.
• Different rolling tool profiles and
deformation depths were examined, to
reproduce possible rolling processes.
Residual stresses at thread root
The FE results confirm the presence of a
compression stress field (- 200/-300 MPa)
near the thread root after the rolling
process.

15. THREAD FORMING PROCESS EFFECT USING FEMFAT
Thread forming simulation
provide FEMFAT with
a constant stress field.
Influence factors:
• Gradient and support effect
• Mean stress influence
• PLAST stress re-arrangement
• Constant stress
Parameters:
• Endurance analysis with R = const
overload line
• Scaled normal stress in critical
plane

16. FEMFAT RESULTS – THREAD FORMING
0,85
0,90
0,95
1,00
1,05
1,10
1,15
ROLLED #1 ROLLED #2 ROLLED #3
SF(50%surv.prob.)
FEMFAT SF at critical thread root
W/O rolling influence With rolling influence
• FEMFAT is influenced by the thread forming process simulation.
• ROLLED #1 and ROLLED #3 process simulations result in an increase in safety
factor.
• ROLLED #2 process simulation decrease (slightly) the safety factor.
Fatigue life assessment considering residual stresses is not conservative.
Why?

17. APPLICABILITY OF THREAD ROLLING
Plastic strain, axial direction
FULL DEPTH FORMING
53% plastic
strain!
Real threads after
FULL DEPTH FORMING
Explicit FE analysis exceeds the maximum
elongation of the material at the thread root.
Microscopic view of actual full depth rolled
thread confirm excessive plastic deformation
and geometrical distortion.
• FEMFAT postprocessing overestimate life prediction because does not consider
possible local micro-fractures due to ductility exhaustion.
• Full-depth thread rolling is unsuitable for hole threading in cast Al/Si alloys.
ISOMETRICPROFILE

18. TRANSFERING THE EXPERIENCE TO APPLICATION
3D mesh
Threaded hole
submodel
Detail of engine block model
Local modification of threaded hole
Different threaded hole geometries and forming
process of the main bearing wall/stud connection
can be compared using FEMFAT and the modeling
strategy proposed here.
An optimized 3D model of the local threaded
hole (in violet) can be defined by comparison
with the experimentally-calibrated axisymmetric
model developed in this study.

19. WRAP UP
• The experimental fatigue data obtained on simplified threaded configurations
was useful to understand the underlying phenomena affecting the performance
of AlSi8Cu3Fe threaded holes.
• Axisymmetric FE modeling with a fine mesh, elastic analysis and subsequent
FEMFAT post-processing show very good correlation with experimental data
obtained with different threaded configurations.
• An explicit FEM model of the thread rolling process was developed. Calculated
residual stresses were inserted in FEMFAT to determine new fatigue strengths.
The accuracy of the prediction was not improved possibly because actual micro-
fracture may occur during the thread rolling process in this material.
• The experimental/computational activity reported here provided guidance and
support for and effective FE modeling approach of stud connections in cast
aluminum engine blocks.

20. Enrica Riva – Il metodo degli elementi finiti nella progettazione meccanica
Sede Operativa Sede legale
Via Lanfranco N. 9 Borgo Regale N. 15
43121 Parma 43121 Parma
Tel./Fax 0521.774898 P.IVA 02399510342
E.mail: info@tpengineering.it
Stress and durability analysis of threaded
connections in a cast aluminum cylinder block
M. Bersella 1, M. Padovan1 , G. Baruffaldi 1, G. Nicoletto 1,2
1 TP Engineering S.r.l., Parma, Italy
2 Dept. of Industrial Engineering, University of Parma, Parma, Italy
email: marco.bersella@tpengineering.it
European Altair Technology Conference
Paris, 2015