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Experimental and numerical analysis of aluminum-aluminum bolted joints subject to an indentation process
Accepted Manuscript Experimental and Numerical Analysis of Aluminum-Aluminum Bolted Joints Subject to an Indentation Process S. Fragapane, A. Giallanza, L. Cannizzaro, A. Pasta, G. Marannano PII: DOI: Reference:
S0142-1123(15)00182-6 http://dx.doi.org/10.1016/j.ijfatigue.2015.05.023 JIJF 3615
To appear in:
International Journal of Fatigue
Received Date: Revised Date: Accepted Date:
27 January 2015 25 May 2015 26 May 2015
Please cite this article as: Fragapane, S., Giallanza, A., Cannizzaro, L., Pasta, A., Marannano, G., Experimental and Numerical Analysis of Aluminum-Aluminum Bolted Joints Subject to an Indentation Process, International Journal of Fatigue (2015), doi: http://dx.doi.org/10.1016/j.ijfatigue.2015.05.023
This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting proof before it is published in its final form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.
EXPERIMENTAL AND NUMERICAL ANALYSIS OF ALUMINUM-ALUMINUM BOLTED JOINTS SUBJECT TO AN INDENTATION PROCESS
S. Fragapane, A. Giallanza, L. Cannizzaro, A. Pasta, G. Marannano*
Department of Chemical, Management, Computer Science and Mechanical Engineering - University of Palermo *Corresponding author: [email protected]
Abstract The increasing interest of the industry (especially automotive, aviation and marine) in the fastener joints (riveted, bolted, etc..) between metallic materials, has re-opened the study on the possibility to improve the performance of the drilled structure using plastic deformation processes. Indentation process, performed before the drilling operation, creates circumferential compression stresses around the hole which increase significantly the mechanical performance of the drilled structures. In this paper, static and the fatigue performances of aluminum-aluminum (AW 6082-T6) single-lap bolted joints are studied. In particular, the study compares the mechanical strength of only drilled single-lap bolted joints (OD specimens) and single-lap bolted joints subject to an indentation process (IP specimens). In order to determine the cycles to failure and the corresponding Wöhler diagram, several fatigue tests are performed. The analyses allow to determine the mechanical performance and the failure mode of the analyzed joints. Several numerical analysis, conducted in ANSYS environment on three-dimensional models of the single-lap joint, are focused on the evaluation of the residual stress on the indented plate and, in particular, to compare the stress distribution on both type of analyzed joints.
Keywords: Single-lap joint, indentation process, residual stress, fatigue, finite element analysis.
1. Introduction In recent years there has been a continuous increase in the use of materials with good mechanical properties and low specific weight (such as aluminum alloys and fiber-reinforced composite), which are often joined in normal industrial applications.
Considered the importance of the fastener joints for mechanical constructions, many studies have been conducted in this area which are focused, in particular, on the analysis of the mechanical performance of riveted or bolted joints [1]. The studies fall into the following topics: •
effects of bearing pressure on the static and fatigue strength of the mechanical connection;
•
static and fatigue strength of the fasteners and adherents, in tension, shear, and combined stresses. Based on the results of these studies, many changes have been made in design procedures and design specifications; the riveted and bolted structures will be safer when designed on the basis of a scientific criterion and this provides an efficient and effective use of the materials to be joined. Esmaeili et al. [2] study the effects of clamping force on the fatigue life of 2024-T3 aluminum alloy double lap bolted joints. Numerical simulation and experimental results show that the fatigue life of double lap bolted joints are improved by increasing the clamping force due to compressive stresses which appeared around the hole. In [3], the effects of torque tightening on the fatigue strength of double-lap bolted joints are studied. Multiaxial fatigue analysis and experimental results reveal that the improvement in fatigue life can be related to compressive stresses around the hole caused by the bolt clamping force. Kim et al. [4] study the ultimate strength and failure mechanism of bolted joints in two different aluminum alloys. Experiments of single-lap joints with 6061 type alloys are performed and the results show that curling (out-of-plane deformation toward plate thickness direction) occurs in bolted joints with a long end distance. Moreover, the authors show that the curling influences the mechanical strength. Sharos et al. [5] perform an analytical model for strength prediction in multi-bolt composite joints at various loading rates. The method is validated against experimental data and excellent correlation is observed. Further studies carried out using the model suggest that slight variations in the energy absorption characteristics at each fastener hole in a multi-fastener joint can significantly alter the bolt-load distribution in the joint. Khashaba et al. [6] investigate the effects of tightening torque on the mechanical behavior of bolted joints obtained joining composite layers. The clearance between the bolt shank and hole is 0.1 mm. Authors show that the tightening torque does not significantly influence the failure mode. Dang Hoang et al [7] investigate the failure mode of a bolted single-lap joint from thin sheet of 6082 T6 aluminum alloy with several configurations of assemblies. The width and the distance between the hole center and the free edge of the substrate are investigate in order to verify the effect on the joint failure. The study shows that the tightening torque value has little effect on the failure mode of the bolted joint. In particular, the failure modes depend on the edge effects, on the number and disposition of bolts.
More recently, some authors use the finite element method in order to study performances of the mechanical joints. In [8], a three-dimensional solid finite element model of a composite-aluminum single-lap bolted joint with a countersunk titanium fastener is developed. The model includes progressive damage behavior of the composite and a plasticity model for the metals. The model is used to evaluate the local force-displacement responses of a number of single-lap joints installed in a hybrid composite-aluminum wing-like structure. Restivo et al. [9] perform a numerical–experimental study of a single-lap bolted joint involving a thick composite laminate and an aluminum plate. Experimental data, recorded by fiber-optic strain gauges that are embedded in the bearing plane of the composite plate, are compared with those obtained thought FE numerical analysis. The finite-element analysis correlates reasonably well with the experimental test and allows to provide general design considerations. All type of joints, either riveted or bolted, introduce significant concentrations of stresses near the holes, which inevitably influence the structural integrity of the joint itself. Given the relevance of the problem, several studies have been conducted in order to identify methods that can improve the fatigue life of these components, by delaying the nucleation and propagation of fatigue cracks. Among the methods mostly used in industrial processes, plastic deformation techniques allow to generate a circumferential compressive stresses field around the hole which significantly increase the fatigue strength of the drilled elements and, at the same time, to reduce the stress intensity factor [10-13]. In the paper, a study of the static and fatigue performances of single-lap aluminum-aluminum bolted joints is performed. Several experimental tests are conducted on specimens realized bolting two layer of AW 6082-T6 aluminum alloy with thickness of 3 mm and 5 mm. In particular, a bilateral indentation process [14] is performed on the adherent 3 mm thick, on opposing points of the plate where, subsequently, the hole will be realized. Indeed, the use of these methods allows to obtain a large area subject to compressive circumferential stress, and simultaneously, preserves the localized damage because the hole is realized after the indentation step. Several experimental tests are performed on bolted joints with only drilled adherents (OD specimens) and specimens subjected to an indentation process (IP specimens). Also, tests are subjected to both static and fatigue conditions. Several FE numerical models reproducing a bolted single-lap joint are developed using the ANSYS code [9]. Numerical analyses are focused on the evaluation of stress distribution on the adherent 3 mm thick, in the area surrounding the hole and at the end of the loading phase, on both type of analyzed joints.
2. Experimental configuration The analyzed joint is composed by two elements made of AW 6082-T6 aluminum alloy with a thickness of 3 mm and 5 mm. These sheets are obtained by rolling which give them particular characteristics in both longitudinal and transverse directions. The longitudinal direction (rolling direction) is considered for this study. Fig. 1 shows the geometric configuration of the single-lap joint used in the experimental tests. The geometry of the joint is chosen in order to promote the failure of the layer 3 mm thick. The minimum size of the overlapping area (Lmin) ensures the correct distance between the center of the hole to the free edge of the adherents (Eq. 1) [7]. This ensure a good resistance to shear-out failure. Lmin = 4·D
(1)
where D=5mm is the hole diameter. The coupling is realized by means of M5 grade 12.9 bolt with hex socket head UNI 5931 and steel washers. The bolt has a tensile strength of Rm=1300 MPa and an elastic limit of Re=1200 MPa. The high strength is selected in order to promote the failure in the adherents. The mechanical properties of the AW 6082-T6 aluminum alloy, shown in Table 1, are previously determined by tensile tests, carried out according to the ASTM E8-04. Static and fatigue tests are carried out on MTS 810 material testing machine (Fig. 2A) with a load cell of 100 kN. The specimens subjected to indentation process are characterized by spherical cavities on the two surfaces of the plate (see Fig. 2B). In a previous work [15], several numerical analyses have been performed with different values of indentation depth, spherical indenters radius and final hole diameter. The study shows that optimal results (in terms of circumferential residual stress values on the edge of the hole) are obtained with indentation depth corresponding to 60% of the thickness of the plate and final diameter of the hole equal to the value of the spherical cavity induced on the specimen. In this paper, in order to perform the indentation step, the same experimental parameters are used. In order to reduce the pile-up phenomena [16], a specialized jig (pressure foot tool) made of stainless steel is used (see Fig 3A). The pressure foot tool acts coaxially to the indenters and it is able to guarantee, by means of four M10 grade 8.8 bolts, a load of 10kN on each side of the plate. The system acts during the whole indentation process and reduces the out-of-plane displacement of the material near the field of action of the indenters. After the indentation process, the hole is realized in both the adherents. Moreover, in order to ensure the correct connection of the surfaces of the bolted joint, a countersink 0.3mm depth, necessary to receive the small crater formed in the indentation process, is created on the adherent 5mm thick (See Fig. 3B). In all the experimental tests, the bolts are tightened with a fixed preload of about 10Nm according to DIN 267 standard.
3. Static tests Static tests are conducted on MTS 810 material testing machine using, in accordance with ASTM D1002 standard, a crosshead speed of 2 mm/min. In the bolted joints, the load transmission mostly happens through coupled surface areas, near of the connection element, where compression and shear stresses are present. In order to transmit a part of the load through the friction between assembled adherents, a preload of about 8Nm is applied through a torque wrench. Such torque ensures the absence of local deformation and the possible damages on the specimen 3 mm thick. Three specimens are tested for each type of joint. Fig. 4 shows the static load as a function of the displacement for the only drilled bolted joints (OD specimens) and single-lap bolted joints subject to the indentation process (IP specimens). Table 2 shows the coefficients of variation for all static tests. From Fig.4 it is possible to observe that the trends are characterized by different behaviors. The first phase is characterized by the transition from the initial condition in which the load is transmitted exclusively by friction, due to the preload of the bolt, at the secondary condition in which part of the load is transmitted through shear stresses on the bolt, with bearing stress of the material at the hole edge. In the second stage the trends exhibit a quasi-linear behavior, followed by a successive elasto-plastic phase. Then it follows a final phase characterized by a load decrease, up to the complete failure of the joint. In particular, it is possible to observe that both the specimens (OD and IP specimens) exhibit in practice the same stiffness in the elastic and elasto-plastic phases, although the tensile strength are different (about 10 kN and 13.5 kN for OD and IP specimens, respectively). Fig. 4 shows that the indentation technique performed on the specimens determines an increase of the tensile strength of about 23%. The experimental tests show that the failure of the joint is caused by cleavage (characterized by a single shear surface) of the substrate 3 mm thick, followed by a significant localized damage (ovalization) of the adherent with 5mm of thickness (Fig. 5).
4. Fatigue tests Fatigue tests are performed in load control mode, at a frequency of 10 Hz and R-ratios (ratio of minimum/maximum load) equal to 0.1. The fatigue loads are chosen as a function of the maximum load determined during the static tests. In particular, a maximum applied load (related to the static strength of OD specimens) ranging between 90% and 60% of the
ultimate static load is chosen. Three specimens are used for each type of specimens and load value. In all tests, the failure occurs as a result of the crack propagation in the substrate with minor thickness. In all performed tests, only the number of cycles until failure is recorded. Fig. 6 shows the Wöhler curves for all fatigue tests. Here, it is preferred to adopt the log scale for X-axis because it leads to an approximately linear relation between the maximum load and the logN for a substantial range of Nvalues. Table 3 shows the coefficients of variation for all fatigue tests. The fatigue curves reported in Fig. 6 show that the fatigue behavior of the analyzed joints can be described by the following analytical function (Eq. 2):
Pmax=a+b·log(N)
(2)
The best fitting procedure of the experimental results, in the range 104 – 106 cycles, allows to evaluate the values of the characteristics parameters a and b for all the examined joints (Table 4). From Fig. 6 it can be observe that the indentation process, performed before the drilling phase, increases the fatigue life of the single-lap aluminum-aluminum joint. In fact, for higher value of applied load, specimens subjected to an indentation process show an increment of number of cycles of about 50%, compared to the only drilled specimens. For lower value of applied load, the comparison between responses of two types of specimens shows that the IP specimens reaches the failure condition for a number of cycles greater than 32%. Fig. 7 shows the fatigue damage of the indented adherent for maximum applied load equal to 90% (A) and 60% (B) of the static tensile strength. In particular, the analysis of the damaged surface shows that the failure of the joints is caused by net-tension of the substrate 3 mm thick. Moreover, the photographic observation of the fracture mode allows to assert that the indentation process may play an essential role in the failure of the joint. In fact, the crack path does not affect the hole and it is confined beyond the area of the specimen characterized by compressive circumferential residual stress.
5. Numerical Analysis Several numerical analyses have been carried out in ANSYS APDL environment with explicit solver. All parts of the bolted joint are defined and separately meshed through parametric generation of volumes and elements. The ANSYS routines automatically generate the bolt, the nut, the washers, the indenters, the pressure foot tools and the aluminum plates.
In particular, in order to compare the stress distribution on both type of analyzed joints, the analyses are performed on OD (only drilled specimen) and IP (specimen subject to an indentation process) numerical model. In this latter case, the numerical simulation involves more complex steps: in particular, it is necessary to define the loading phase of the pressure foot tools and the displacement of the two indenters. Boundary and load conditions reproduce the experimental conditions. Only Bolt and washers are considered as a single solid. The applied load is equal to P=10kN. The indenters penetrate and move away from the plate according to appropriate time function. It is necessary to wait for residual stresses redistribution. During the loading phase, the 3mm aluminum plate is fixed at the free end. The load on the bolted joint is applied to the nodes of the top free surface of the aluminum plate 5mm thick. Three-dimensional models of the single-lap joint are discretized by means of 8-node SOLID164 elements, optimized for the 3-D modeling of solid structures. Node-to-surface (NTS) contact elements are used to define the contact between all parts of the numerical model. In order to reduce the computing time, the indenters and the pressure foot tools have been modeled as rigid bodies, whereas bolt, nut and aluminum adherents have been modeled as elasto-plastic with kinematic hardening behavior. Table 1 shows the parameters used in order to define the mechanical properties of aluminum alloy adherents. Special care is taken in the simulation of the preloading phase, carried out imposing relative displacement between bolt and nut. In more detail, the preload is distributed to the internal nodes of the nut and to the external nodes of the bolt in order to obtain the proper clamping load. The load simulation on the single lap joint subject to indentation process is divided in three different routines. The first routine involves the complete definition of the indentation process. The pressure foot tool provides restraint of the area surrounding the indenter and, while the indenter continues its penetration into the aluminum plate, provides an appropriate constant load of about 10kN on each side of the plate. The loading phase for the punching operation is defined in terms of imposed displacement of the indenters (in terms of displacement vs time curves). At this stage it is necessary to create all the parts needed to realize the numerical model: bolt, nut, washers, aluminum plates, etc. This is mandatory because the other routines require the definition of “full restart” analysis. In fact, a full restart is appropriate when many changes to the database are needed: in particular, the second and third procedures require to remove portions of the model and to apply different loading conditions. In the second procedure, the volumes corresponding to the hole are removed and the stress state arising from the first analysis is applied to the aluminum plate 3mm thick. In the third routine, the aluminum plate 3 mm thick
and the fastener elements (that in the previous analysis are moved from the field of action of the indenters) are repositioned by means of “vgen” sintax. A countersink 0.3mm depth, necessary to receive the small crater formed in the indentation process, is realized on the aluminum plate 5mm thick and on the washer (see Fig. 8A). Fig. 9 and Fig.10 show the circumferential stress distribution valuated along the line traced from the edge of the hole (point A in Fig. 8B) to hole edge (point B in Fig. 8B) and for different values of the thickness (Zcoordinate). The zero value of the Z-coordinate is related at the point positioned on the outer surface of the aluminum plate, while the value of the Z-coordinate equal to 3 mm corresponds to that positioned at the interface between the two plates. The directions along which the stresses are evaluated are parallel to the applied load (Ycoordinate). The graph in Fig. 9 is reported at the last step of the second procedure, after that the volume elements of the hole are removed. The figure shows that the indentation process allows the radial expansion of the hole beyond the material yield point and it produces a permanent compressive stress field in the neighborhood of the hole itself. Fig 10 shows the hoop stress distribution at the end of loading phase for IP specimen. Once the load reaches a well-defined level, the plates start to slide with respect to one another. The sliding continues until both plates are in contact with the bolt. As a result, the stress level on the edge of the hole decreases of about 35% when the bolt shank interacts with the inner surface of the hole. Fig 11 shows the stress field at the end of loading phase of the only drilled single-lap joint. Comparing the graphs of Fig. 10,11 it can be concluded that the indentation process performed on the aluminum plate may provide a significant delay on crack propagation. In fact, the plastic deformation process performed in an area characterized by the presence of high stress concentration, determines the onset of permanent compressive stress field that persists also when the hole is loaded by the bolt and the load is progressively transferred by the connection.
6. Conclusions In this paper, in order to evaluate the improvement of mechanical properties of drilled components, the mechanical behavior of aluminum-aluminum bolted single-lap joints subjected to an indentation process is studied.
Several experimental studies, conducted on only drilled single-lap bolted joints and on single-lap bolted joints subject to an indentation process reveal that these latter show an increase in terms of static and fatigue performances. More specifically, an increment of the tensile strength of about 23% is observed. The increment of number of fatigue cycles, for higher value of applied load, is about 50%. For lower value of applied load, the comparison between results of two types of specimens shows that the IP specimens reaches the failure condition for a number of cycles greater than 32%. Several numerical analysis are performed using ANSYS explicit solver. In particular, the study is focused on the evaluation of the residual stress on the indented plate and, in particular, on the comparison of the stress distribution on both type of analyzed joints. The analyses show that the indentation process, creating a permanent compressive stress field in the neighborhood of the hole, contribute to delay the crack nucleation. In conclusion, the study shows how this technique can determine a significant delay of the crack growth and, therefore, of the fatigue failure. Its use is therefore suitable for demanding applications, especially in aeronautical field.
REFERENCES [1] Bickford JH. Introduction to the Design and Behavior of Bolted. London: Joints CRC Press; 2008. [2] Esmaeili F, Zehsaz M, Chakherlou TN, Hasanifrd S. Experimental and Numerical Study of the Fatigue Strength of Double Lap Bolted Joints and the Effect of Torque Tightening on the Fatigue Life of Jointed Plates. Transactions of the Indian institute of metals 2014; 67:581-588. [3] Esmaeili F, Chakherlou TN, Zehsaz M. Prediction of fatigue life in aircraft double lap bolted joints using several multiaxial fatigue criteria. Material & Design 2014;59:430-438. [4] Kim TS, Cho YH. Investigation on ultimate strength and failure mechanism of bolted joints in two different aluminum alloys. Material & Design 2014;58:74-88. [5] Sharos PA, Egan B, McCarthy CT. An analytical model for strength prediction in multi-bolt composite joints at various loading rates. Composite Structures 2014;116:300-310. [6] Khashaba UA, Sallam HEM, Al-Shorbagy AE, Seif MA. Effect of washer size and tightening torque on the performance of bolted joints in composite structures. Composite Structures 2006;73: 310-317. [7] Dang Hoang T, Herbelot C, Imad A. On failure mode analysis in a bolted single lap joint under tensionshearing. Engineering Failure Analysis 2012; 24:9-25.
[8] Kapidžić Z, Nilsson L, Ansell H. Finite element modeling of mechanically fastened composite-aluminum joints in aircraft structures. Composite Structures 2014;109:198-210. [9] Restivo G, Marannano G, Isaicu GA. Three-dimensional strain analysis of single-lap bolted joints in thick composites using fibre-optic gauges and the finite-element method. The Journal of Strain Analysis for Engineering Design 2010;DOI: 10.1243/03093247JSA599. [10] Gopalakrishna HD, Narasimha Murthy HN, Krishna M, Vinod MS, Suresh AV. Cold expansion of holes and resulting fatigue life enhancement and residual stresses in Al 2024 T3 alloy–An experimental study. Engineering Failure Analysis 2010;17(2):361-368. [11] Marannano G, Virzì Mariotti G, D’Acquisto L, Restivo G, Gianaris N. Effect of Cold Working and ring indentation
on
fatigue
life
of
aluminum
alloy
specimens.
Experimental Techniques
2013;DOI:
10.1111/ext.12018. [12] Lapalme M, Hoseini M, Bocher P, Colle AR, Levesque M. Realistic cold expansion finite element model and experimental validations for aluminum alloys. Experimental Mechanics 2014;54(5):841-855. [13] Maximov JT, Duncheva GV, Ganev N, Bakalova TN. The benefit from an adequate finite element simulation of the cold hole expansion process. Engineering Failure Analysis 2009;16(1):503-511. [14] Easterbrook ET, Landy MA. Evaluation of StressWave cold working (SWCW) process on high-strength aluminum alloys for aerospace. StressWave Inc 2009. [15] Marannano G, Pasta A, Parrinello F, Giallanza A. Effect of the indentation process on fatigue life of drilled specimens. Journal of Mechanical Science and Technology 2015 [accepted for publication]. [16] Taljat B, Pharr GM. Development of pile-up during spherical indentation of elastic–plastic solids. Int J Solids Struct 2004;41:3891–3904.
CAPTIONS Figure 1: Geometrical configuration of the single-lap joint Figure 2: (A) Single-lap bolted joint test; (B) dimple shape on the specimen after indentation phase Figure 3: (A) Schematic section of the specialized jig used during the indentation process; (B) schematic section of the bolted joint Figure 4: Load-displacement curve of bolted joints, with and without indentation process Figure 5: Static failure of the joint. (A) Aluminum adherent with thickness of 3mm; (B) Aluminum adherent with thickness of 5mm Figure 6: Wöhler’s curves for all the experimental tests Figure 7: Fatigue failure of the indented adherents for maximum applied load equal to 90% (A) and 60% (B) of the static tensile strength Figure 8: (A) Section of the numerical model; (B) coordinate system used in the numerical simulations Figure 9: Residual stress on the adherent 3 mm thick after that the volume elements of the hole are removed Figure 10: Stress distribution on the adherent 3 mm thick at the end of loading phase on IP specimens Figure 11: Stress distribution on the adherent 3 mm thick at the end of loading phase on OD specimens
Table 1: Constitutive parameter of AW 6082–T6 aluminum alloy Density [kg/mm3] Young Modulus [MPa]
2.7·10-6 69000
Poisson Modulus
0.33
Proof Stress 0.2% [MPa]
260
Tensile Strength (MPa)
310
Shear Strength (MPa)
210
Elongation A5 (%)
11
Hardness Vickers (HV)
100
Tangent modulus [MPa]
20
Hardening parameter
0.5
Table 2: Coefficients of variation for all static tests Specimens OD
IP
Stiffness of the joint
OD
IP
Final failure of the joint
Average value
6.059 kN/mm
7.246 kN/mm
10.143 kN
13.497 kN
Standard deviation
0.717 kN/mm
1.419 kN/mm
0.489 kN
0.199 kN
Coefficient of variation
0.118
0.196
0.048
0.015
Table 3: Coefficients of variation for all fatigue tests Specimens OD
IP
Standard Average value of
deviation of
Average
Standard
value of the
deviation of
number of
the number
Coefficient of Maximum applied load [kN]
Coefficient of
the number of
the number
cycles to failure
of cycles to
cycles to
of cycles to
failure
failure
failure
variation
variation
6.1
233513.33
32734.19
0.14
345204.67
41814.84
0.12
7.1
105576.00
7775.95
0.07
153026.00
21013.65
0.14
8.1
42909.67
3254.78
0.08
74649.00
7727.71
0.10
9.1
17740.67
2065.03
0.12
36377.33
3896.39
0.11
Table 4: Fatigue constants Specimens Fatigue constants
OD
IP
a
20.344
22.921
b
-1.149
-1.321
Fig. 1
Fig. 2
Fig. 3
Fig. 4
Fig. 5
Fig. 6
Fig. 7
Fig. 8
Fig. 9
Fig. 10
Fig. 11
•
mechanical performances of aluminum-aluminum single-lap bolted joints are studied
•
An indentation process creates a residual stress field around the hole
•
several static and fatigue tests are conducted
•
several numerical analysis with explicit solver are performed
•
The indentation process increases the fatigue life of bolted joints
S0142-1123(15)00182-6 http://dx.doi.org/10.1016/j.ijfatigue.2015.05.023 JIJF 3615
To appear in:
International Journal of Fatigue
Received Date: Revised Date: Accepted Date:
27 January 2015 25 May 2015 26 May 2015
Please cite this article as: Fragapane, S., Giallanza, A., Cannizzaro, L., Pasta, A., Marannano, G., Experimental and Numerical Analysis of Aluminum-Aluminum Bolted Joints Subject to an Indentation Process, International Journal of Fatigue (2015), doi: http://dx.doi.org/10.1016/j.ijfatigue.2015.05.023
This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting proof before it is published in its final form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.
EXPERIMENTAL AND NUMERICAL ANALYSIS OF ALUMINUM-ALUMINUM BOLTED JOINTS SUBJECT TO AN INDENTATION PROCESS
S. Fragapane, A. Giallanza, L. Cannizzaro, A. Pasta, G. Marannano*
Department of Chemical, Management, Computer Science and Mechanical Engineering - University of Palermo *Corresponding author: [email protected]
Abstract The increasing interest of the industry (especially automotive, aviation and marine) in the fastener joints (riveted, bolted, etc..) between metallic materials, has re-opened the study on the possibility to improve the performance of the drilled structure using plastic deformation processes. Indentation process, performed before the drilling operation, creates circumferential compression stresses around the hole which increase significantly the mechanical performance of the drilled structures. In this paper, static and the fatigue performances of aluminum-aluminum (AW 6082-T6) single-lap bolted joints are studied. In particular, the study compares the mechanical strength of only drilled single-lap bolted joints (OD specimens) and single-lap bolted joints subject to an indentation process (IP specimens). In order to determine the cycles to failure and the corresponding Wöhler diagram, several fatigue tests are performed. The analyses allow to determine the mechanical performance and the failure mode of the analyzed joints. Several numerical analysis, conducted in ANSYS environment on three-dimensional models of the single-lap joint, are focused on the evaluation of the residual stress on the indented plate and, in particular, to compare the stress distribution on both type of analyzed joints.
Keywords: Single-lap joint, indentation process, residual stress, fatigue, finite element analysis.
1. Introduction In recent years there has been a continuous increase in the use of materials with good mechanical properties and low specific weight (such as aluminum alloys and fiber-reinforced composite), which are often joined in normal industrial applications.
Considered the importance of the fastener joints for mechanical constructions, many studies have been conducted in this area which are focused, in particular, on the analysis of the mechanical performance of riveted or bolted joints [1]. The studies fall into the following topics: •
effects of bearing pressure on the static and fatigue strength of the mechanical connection;
•
static and fatigue strength of the fasteners and adherents, in tension, shear, and combined stresses. Based on the results of these studies, many changes have been made in design procedures and design specifications; the riveted and bolted structures will be safer when designed on the basis of a scientific criterion and this provides an efficient and effective use of the materials to be joined. Esmaeili et al. [2] study the effects of clamping force on the fatigue life of 2024-T3 aluminum alloy double lap bolted joints. Numerical simulation and experimental results show that the fatigue life of double lap bolted joints are improved by increasing the clamping force due to compressive stresses which appeared around the hole. In [3], the effects of torque tightening on the fatigue strength of double-lap bolted joints are studied. Multiaxial fatigue analysis and experimental results reveal that the improvement in fatigue life can be related to compressive stresses around the hole caused by the bolt clamping force. Kim et al. [4] study the ultimate strength and failure mechanism of bolted joints in two different aluminum alloys. Experiments of single-lap joints with 6061 type alloys are performed and the results show that curling (out-of-plane deformation toward plate thickness direction) occurs in bolted joints with a long end distance. Moreover, the authors show that the curling influences the mechanical strength. Sharos et al. [5] perform an analytical model for strength prediction in multi-bolt composite joints at various loading rates. The method is validated against experimental data and excellent correlation is observed. Further studies carried out using the model suggest that slight variations in the energy absorption characteristics at each fastener hole in a multi-fastener joint can significantly alter the bolt-load distribution in the joint. Khashaba et al. [6] investigate the effects of tightening torque on the mechanical behavior of bolted joints obtained joining composite layers. The clearance between the bolt shank and hole is 0.1 mm. Authors show that the tightening torque does not significantly influence the failure mode. Dang Hoang et al [7] investigate the failure mode of a bolted single-lap joint from thin sheet of 6082 T6 aluminum alloy with several configurations of assemblies. The width and the distance between the hole center and the free edge of the substrate are investigate in order to verify the effect on the joint failure. The study shows that the tightening torque value has little effect on the failure mode of the bolted joint. In particular, the failure modes depend on the edge effects, on the number and disposition of bolts.
More recently, some authors use the finite element method in order to study performances of the mechanical joints. In [8], a three-dimensional solid finite element model of a composite-aluminum single-lap bolted joint with a countersunk titanium fastener is developed. The model includes progressive damage behavior of the composite and a plasticity model for the metals. The model is used to evaluate the local force-displacement responses of a number of single-lap joints installed in a hybrid composite-aluminum wing-like structure. Restivo et al. [9] perform a numerical–experimental study of a single-lap bolted joint involving a thick composite laminate and an aluminum plate. Experimental data, recorded by fiber-optic strain gauges that are embedded in the bearing plane of the composite plate, are compared with those obtained thought FE numerical analysis. The finite-element analysis correlates reasonably well with the experimental test and allows to provide general design considerations. All type of joints, either riveted or bolted, introduce significant concentrations of stresses near the holes, which inevitably influence the structural integrity of the joint itself. Given the relevance of the problem, several studies have been conducted in order to identify methods that can improve the fatigue life of these components, by delaying the nucleation and propagation of fatigue cracks. Among the methods mostly used in industrial processes, plastic deformation techniques allow to generate a circumferential compressive stresses field around the hole which significantly increase the fatigue strength of the drilled elements and, at the same time, to reduce the stress intensity factor [10-13]. In the paper, a study of the static and fatigue performances of single-lap aluminum-aluminum bolted joints is performed. Several experimental tests are conducted on specimens realized bolting two layer of AW 6082-T6 aluminum alloy with thickness of 3 mm and 5 mm. In particular, a bilateral indentation process [14] is performed on the adherent 3 mm thick, on opposing points of the plate where, subsequently, the hole will be realized. Indeed, the use of these methods allows to obtain a large area subject to compressive circumferential stress, and simultaneously, preserves the localized damage because the hole is realized after the indentation step. Several experimental tests are performed on bolted joints with only drilled adherents (OD specimens) and specimens subjected to an indentation process (IP specimens). Also, tests are subjected to both static and fatigue conditions. Several FE numerical models reproducing a bolted single-lap joint are developed using the ANSYS code [9]. Numerical analyses are focused on the evaluation of stress distribution on the adherent 3 mm thick, in the area surrounding the hole and at the end of the loading phase, on both type of analyzed joints.
2. Experimental configuration The analyzed joint is composed by two elements made of AW 6082-T6 aluminum alloy with a thickness of 3 mm and 5 mm. These sheets are obtained by rolling which give them particular characteristics in both longitudinal and transverse directions. The longitudinal direction (rolling direction) is considered for this study. Fig. 1 shows the geometric configuration of the single-lap joint used in the experimental tests. The geometry of the joint is chosen in order to promote the failure of the layer 3 mm thick. The minimum size of the overlapping area (Lmin) ensures the correct distance between the center of the hole to the free edge of the adherents (Eq. 1) [7]. This ensure a good resistance to shear-out failure. Lmin = 4·D
(1)
where D=5mm is the hole diameter. The coupling is realized by means of M5 grade 12.9 bolt with hex socket head UNI 5931 and steel washers. The bolt has a tensile strength of Rm=1300 MPa and an elastic limit of Re=1200 MPa. The high strength is selected in order to promote the failure in the adherents. The mechanical properties of the AW 6082-T6 aluminum alloy, shown in Table 1, are previously determined by tensile tests, carried out according to the ASTM E8-04. Static and fatigue tests are carried out on MTS 810 material testing machine (Fig. 2A) with a load cell of 100 kN. The specimens subjected to indentation process are characterized by spherical cavities on the two surfaces of the plate (see Fig. 2B). In a previous work [15], several numerical analyses have been performed with different values of indentation depth, spherical indenters radius and final hole diameter. The study shows that optimal results (in terms of circumferential residual stress values on the edge of the hole) are obtained with indentation depth corresponding to 60% of the thickness of the plate and final diameter of the hole equal to the value of the spherical cavity induced on the specimen. In this paper, in order to perform the indentation step, the same experimental parameters are used. In order to reduce the pile-up phenomena [16], a specialized jig (pressure foot tool) made of stainless steel is used (see Fig 3A). The pressure foot tool acts coaxially to the indenters and it is able to guarantee, by means of four M10 grade 8.8 bolts, a load of 10kN on each side of the plate. The system acts during the whole indentation process and reduces the out-of-plane displacement of the material near the field of action of the indenters. After the indentation process, the hole is realized in both the adherents. Moreover, in order to ensure the correct connection of the surfaces of the bolted joint, a countersink 0.3mm depth, necessary to receive the small crater formed in the indentation process, is created on the adherent 5mm thick (See Fig. 3B). In all the experimental tests, the bolts are tightened with a fixed preload of about 10Nm according to DIN 267 standard.
3. Static tests Static tests are conducted on MTS 810 material testing machine using, in accordance with ASTM D1002 standard, a crosshead speed of 2 mm/min. In the bolted joints, the load transmission mostly happens through coupled surface areas, near of the connection element, where compression and shear stresses are present. In order to transmit a part of the load through the friction between assembled adherents, a preload of about 8Nm is applied through a torque wrench. Such torque ensures the absence of local deformation and the possible damages on the specimen 3 mm thick. Three specimens are tested for each type of joint. Fig. 4 shows the static load as a function of the displacement for the only drilled bolted joints (OD specimens) and single-lap bolted joints subject to the indentation process (IP specimens). Table 2 shows the coefficients of variation for all static tests. From Fig.4 it is possible to observe that the trends are characterized by different behaviors. The first phase is characterized by the transition from the initial condition in which the load is transmitted exclusively by friction, due to the preload of the bolt, at the secondary condition in which part of the load is transmitted through shear stresses on the bolt, with bearing stress of the material at the hole edge. In the second stage the trends exhibit a quasi-linear behavior, followed by a successive elasto-plastic phase. Then it follows a final phase characterized by a load decrease, up to the complete failure of the joint. In particular, it is possible to observe that both the specimens (OD and IP specimens) exhibit in practice the same stiffness in the elastic and elasto-plastic phases, although the tensile strength are different (about 10 kN and 13.5 kN for OD and IP specimens, respectively). Fig. 4 shows that the indentation technique performed on the specimens determines an increase of the tensile strength of about 23%. The experimental tests show that the failure of the joint is caused by cleavage (characterized by a single shear surface) of the substrate 3 mm thick, followed by a significant localized damage (ovalization) of the adherent with 5mm of thickness (Fig. 5).
4. Fatigue tests Fatigue tests are performed in load control mode, at a frequency of 10 Hz and R-ratios (ratio of minimum/maximum load) equal to 0.1. The fatigue loads are chosen as a function of the maximum load determined during the static tests. In particular, a maximum applied load (related to the static strength of OD specimens) ranging between 90% and 60% of the
ultimate static load is chosen. Three specimens are used for each type of specimens and load value. In all tests, the failure occurs as a result of the crack propagation in the substrate with minor thickness. In all performed tests, only the number of cycles until failure is recorded. Fig. 6 shows the Wöhler curves for all fatigue tests. Here, it is preferred to adopt the log scale for X-axis because it leads to an approximately linear relation between the maximum load and the logN for a substantial range of Nvalues. Table 3 shows the coefficients of variation for all fatigue tests. The fatigue curves reported in Fig. 6 show that the fatigue behavior of the analyzed joints can be described by the following analytical function (Eq. 2):
Pmax=a+b·log(N)
(2)
The best fitting procedure of the experimental results, in the range 104 – 106 cycles, allows to evaluate the values of the characteristics parameters a and b for all the examined joints (Table 4). From Fig. 6 it can be observe that the indentation process, performed before the drilling phase, increases the fatigue life of the single-lap aluminum-aluminum joint. In fact, for higher value of applied load, specimens subjected to an indentation process show an increment of number of cycles of about 50%, compared to the only drilled specimens. For lower value of applied load, the comparison between responses of two types of specimens shows that the IP specimens reaches the failure condition for a number of cycles greater than 32%. Fig. 7 shows the fatigue damage of the indented adherent for maximum applied load equal to 90% (A) and 60% (B) of the static tensile strength. In particular, the analysis of the damaged surface shows that the failure of the joints is caused by net-tension of the substrate 3 mm thick. Moreover, the photographic observation of the fracture mode allows to assert that the indentation process may play an essential role in the failure of the joint. In fact, the crack path does not affect the hole and it is confined beyond the area of the specimen characterized by compressive circumferential residual stress.
5. Numerical Analysis Several numerical analyses have been carried out in ANSYS APDL environment with explicit solver. All parts of the bolted joint are defined and separately meshed through parametric generation of volumes and elements. The ANSYS routines automatically generate the bolt, the nut, the washers, the indenters, the pressure foot tools and the aluminum plates.
In particular, in order to compare the stress distribution on both type of analyzed joints, the analyses are performed on OD (only drilled specimen) and IP (specimen subject to an indentation process) numerical model. In this latter case, the numerical simulation involves more complex steps: in particular, it is necessary to define the loading phase of the pressure foot tools and the displacement of the two indenters. Boundary and load conditions reproduce the experimental conditions. Only Bolt and washers are considered as a single solid. The applied load is equal to P=10kN. The indenters penetrate and move away from the plate according to appropriate time function. It is necessary to wait for residual stresses redistribution. During the loading phase, the 3mm aluminum plate is fixed at the free end. The load on the bolted joint is applied to the nodes of the top free surface of the aluminum plate 5mm thick. Three-dimensional models of the single-lap joint are discretized by means of 8-node SOLID164 elements, optimized for the 3-D modeling of solid structures. Node-to-surface (NTS) contact elements are used to define the contact between all parts of the numerical model. In order to reduce the computing time, the indenters and the pressure foot tools have been modeled as rigid bodies, whereas bolt, nut and aluminum adherents have been modeled as elasto-plastic with kinematic hardening behavior. Table 1 shows the parameters used in order to define the mechanical properties of aluminum alloy adherents. Special care is taken in the simulation of the preloading phase, carried out imposing relative displacement between bolt and nut. In more detail, the preload is distributed to the internal nodes of the nut and to the external nodes of the bolt in order to obtain the proper clamping load. The load simulation on the single lap joint subject to indentation process is divided in three different routines. The first routine involves the complete definition of the indentation process. The pressure foot tool provides restraint of the area surrounding the indenter and, while the indenter continues its penetration into the aluminum plate, provides an appropriate constant load of about 10kN on each side of the plate. The loading phase for the punching operation is defined in terms of imposed displacement of the indenters (in terms of displacement vs time curves). At this stage it is necessary to create all the parts needed to realize the numerical model: bolt, nut, washers, aluminum plates, etc. This is mandatory because the other routines require the definition of “full restart” analysis. In fact, a full restart is appropriate when many changes to the database are needed: in particular, the second and third procedures require to remove portions of the model and to apply different loading conditions. In the second procedure, the volumes corresponding to the hole are removed and the stress state arising from the first analysis is applied to the aluminum plate 3mm thick. In the third routine, the aluminum plate 3 mm thick
and the fastener elements (that in the previous analysis are moved from the field of action of the indenters) are repositioned by means of “vgen” sintax. A countersink 0.3mm depth, necessary to receive the small crater formed in the indentation process, is realized on the aluminum plate 5mm thick and on the washer (see Fig. 8A). Fig. 9 and Fig.10 show the circumferential stress distribution valuated along the line traced from the edge of the hole (point A in Fig. 8B) to hole edge (point B in Fig. 8B) and for different values of the thickness (Zcoordinate). The zero value of the Z-coordinate is related at the point positioned on the outer surface of the aluminum plate, while the value of the Z-coordinate equal to 3 mm corresponds to that positioned at the interface between the two plates. The directions along which the stresses are evaluated are parallel to the applied load (Ycoordinate). The graph in Fig. 9 is reported at the last step of the second procedure, after that the volume elements of the hole are removed. The figure shows that the indentation process allows the radial expansion of the hole beyond the material yield point and it produces a permanent compressive stress field in the neighborhood of the hole itself. Fig 10 shows the hoop stress distribution at the end of loading phase for IP specimen. Once the load reaches a well-defined level, the plates start to slide with respect to one another. The sliding continues until both plates are in contact with the bolt. As a result, the stress level on the edge of the hole decreases of about 35% when the bolt shank interacts with the inner surface of the hole. Fig 11 shows the stress field at the end of loading phase of the only drilled single-lap joint. Comparing the graphs of Fig. 10,11 it can be concluded that the indentation process performed on the aluminum plate may provide a significant delay on crack propagation. In fact, the plastic deformation process performed in an area characterized by the presence of high stress concentration, determines the onset of permanent compressive stress field that persists also when the hole is loaded by the bolt and the load is progressively transferred by the connection.
6. Conclusions In this paper, in order to evaluate the improvement of mechanical properties of drilled components, the mechanical behavior of aluminum-aluminum bolted single-lap joints subjected to an indentation process is studied.
Several experimental studies, conducted on only drilled single-lap bolted joints and on single-lap bolted joints subject to an indentation process reveal that these latter show an increase in terms of static and fatigue performances. More specifically, an increment of the tensile strength of about 23% is observed. The increment of number of fatigue cycles, for higher value of applied load, is about 50%. For lower value of applied load, the comparison between results of two types of specimens shows that the IP specimens reaches the failure condition for a number of cycles greater than 32%. Several numerical analysis are performed using ANSYS explicit solver. In particular, the study is focused on the evaluation of the residual stress on the indented plate and, in particular, on the comparison of the stress distribution on both type of analyzed joints. The analyses show that the indentation process, creating a permanent compressive stress field in the neighborhood of the hole, contribute to delay the crack nucleation. In conclusion, the study shows how this technique can determine a significant delay of the crack growth and, therefore, of the fatigue failure. Its use is therefore suitable for demanding applications, especially in aeronautical field.
REFERENCES [1] Bickford JH. Introduction to the Design and Behavior of Bolted. London: Joints CRC Press; 2008. [2] Esmaeili F, Zehsaz M, Chakherlou TN, Hasanifrd S. Experimental and Numerical Study of the Fatigue Strength of Double Lap Bolted Joints and the Effect of Torque Tightening on the Fatigue Life of Jointed Plates. Transactions of the Indian institute of metals 2014; 67:581-588. [3] Esmaeili F, Chakherlou TN, Zehsaz M. Prediction of fatigue life in aircraft double lap bolted joints using several multiaxial fatigue criteria. Material & Design 2014;59:430-438. [4] Kim TS, Cho YH. Investigation on ultimate strength and failure mechanism of bolted joints in two different aluminum alloys. Material & Design 2014;58:74-88. [5] Sharos PA, Egan B, McCarthy CT. An analytical model for strength prediction in multi-bolt composite joints at various loading rates. Composite Structures 2014;116:300-310. [6] Khashaba UA, Sallam HEM, Al-Shorbagy AE, Seif MA. Effect of washer size and tightening torque on the performance of bolted joints in composite structures. Composite Structures 2006;73: 310-317. [7] Dang Hoang T, Herbelot C, Imad A. On failure mode analysis in a bolted single lap joint under tensionshearing. Engineering Failure Analysis 2012; 24:9-25.
[8] Kapidžić Z, Nilsson L, Ansell H. Finite element modeling of mechanically fastened composite-aluminum joints in aircraft structures. Composite Structures 2014;109:198-210. [9] Restivo G, Marannano G, Isaicu GA. Three-dimensional strain analysis of single-lap bolted joints in thick composites using fibre-optic gauges and the finite-element method. The Journal of Strain Analysis for Engineering Design 2010;DOI: 10.1243/03093247JSA599. [10] Gopalakrishna HD, Narasimha Murthy HN, Krishna M, Vinod MS, Suresh AV. Cold expansion of holes and resulting fatigue life enhancement and residual stresses in Al 2024 T3 alloy–An experimental study. Engineering Failure Analysis 2010;17(2):361-368. [11] Marannano G, Virzì Mariotti G, D’Acquisto L, Restivo G, Gianaris N. Effect of Cold Working and ring indentation
on
fatigue
life
of
aluminum
alloy
specimens.
Experimental Techniques
2013;DOI:
10.1111/ext.12018. [12] Lapalme M, Hoseini M, Bocher P, Colle AR, Levesque M. Realistic cold expansion finite element model and experimental validations for aluminum alloys. Experimental Mechanics 2014;54(5):841-855. [13] Maximov JT, Duncheva GV, Ganev N, Bakalova TN. The benefit from an adequate finite element simulation of the cold hole expansion process. Engineering Failure Analysis 2009;16(1):503-511. [14] Easterbrook ET, Landy MA. Evaluation of StressWave cold working (SWCW) process on high-strength aluminum alloys for aerospace. StressWave Inc 2009. [15] Marannano G, Pasta A, Parrinello F, Giallanza A. Effect of the indentation process on fatigue life of drilled specimens. Journal of Mechanical Science and Technology 2015 [accepted for publication]. [16] Taljat B, Pharr GM. Development of pile-up during spherical indentation of elastic–plastic solids. Int J Solids Struct 2004;41:3891–3904.
CAPTIONS Figure 1: Geometrical configuration of the single-lap joint Figure 2: (A) Single-lap bolted joint test; (B) dimple shape on the specimen after indentation phase Figure 3: (A) Schematic section of the specialized jig used during the indentation process; (B) schematic section of the bolted joint Figure 4: Load-displacement curve of bolted joints, with and without indentation process Figure 5: Static failure of the joint. (A) Aluminum adherent with thickness of 3mm; (B) Aluminum adherent with thickness of 5mm Figure 6: Wöhler’s curves for all the experimental tests Figure 7: Fatigue failure of the indented adherents for maximum applied load equal to 90% (A) and 60% (B) of the static tensile strength Figure 8: (A) Section of the numerical model; (B) coordinate system used in the numerical simulations Figure 9: Residual stress on the adherent 3 mm thick after that the volume elements of the hole are removed Figure 10: Stress distribution on the adherent 3 mm thick at the end of loading phase on IP specimens Figure 11: Stress distribution on the adherent 3 mm thick at the end of loading phase on OD specimens
Table 1: Constitutive parameter of AW 6082–T6 aluminum alloy Density [kg/mm3] Young Modulus [MPa]
2.7·10-6 69000
Poisson Modulus
0.33
Proof Stress 0.2% [MPa]
260
Tensile Strength (MPa)
310
Shear Strength (MPa)
210
Elongation A5 (%)
11
Hardness Vickers (HV)
100
Tangent modulus [MPa]
20
Hardening parameter
0.5
Table 2: Coefficients of variation for all static tests Specimens OD
IP
Stiffness of the joint
OD
IP
Final failure of the joint
Average value
6.059 kN/mm
7.246 kN/mm
10.143 kN
13.497 kN
Standard deviation
0.717 kN/mm
1.419 kN/mm
0.489 kN
0.199 kN
Coefficient of variation
0.118
0.196
0.048
0.015
Table 3: Coefficients of variation for all fatigue tests Specimens OD
IP
Standard Average value of
deviation of
Average
Standard
value of the
deviation of
number of
the number
Coefficient of Maximum applied load [kN]
Coefficient of
the number of
the number
cycles to failure
of cycles to
cycles to
of cycles to
failure
failure
failure
variation
variation
6.1
233513.33
32734.19
0.14
345204.67
41814.84
0.12
7.1
105576.00
7775.95
0.07
153026.00
21013.65
0.14
8.1
42909.67
3254.78
0.08
74649.00
7727.71
0.10
9.1
17740.67
2065.03
0.12
36377.33
3896.39
0.11
Table 4: Fatigue constants Specimens Fatigue constants
OD
IP
a
20.344
22.921
b
-1.149
-1.321
Fig. 1
Fig. 2
Fig. 3
Fig. 4
Fig. 5
Fig. 6
Fig. 7
Fig. 8
Fig. 9
Fig. 10
Fig. 11
•
mechanical performances of aluminum-aluminum single-lap bolted joints are studied
•
An indentation process creates a residual stress field around the hole
•
several static and fatigue tests are conducted
•
several numerical analysis with explicit solver are performed
•
The indentation process increases the fatigue life of bolted joints