View of FATIGUE LIFE OF BOLTS AND FASTENERS : A REVIEW STUDY

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ACCENT JOURNAL OF ECONOMICS ECOLOGY & ENGINEERING Peer Reviewed and Refereed Journal IMPACT FACTOR: 2.104 (ISSN NO. 2456-1037) Vol.03, Issue 09, Conference (IC-RASEM) Special Issue 01, September 2018 Available Online: www.ajeee.co.in/index.php/AJEEE

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FATIGUE LIFE OF BOLTS AND FASTENERS : A REVIEW STUDY Vishal Achwal

Research Scholar, Mechanical Engineering Department, Mewar University, Chittorgarh, Raj., India

Abstract - This review study contains references to conference proceedings, journals and theses/dissertations dealing with fatigue life of fastening and joining analyzed by experimental methods and finite element methods.

Key words : bolts, fasteners, FEM analysis , fatigue life , loosening 1. INTRODUCTION

The term fatigue damage is frequently used to describe the damage to a part that has been sustained as a result of loading. Such fatigue damage is nowadays equated to crack length. In some critical applications there is a requirement that bolts are periodically crack detected using dye penetrate or even by x-rays to ensure that there are no detectable cracks present. The purpose of this article is to provide some examples of ways in which the design and manufacture of bolts can affect their fatigue performance. Fatigue failure of nuts is practically unknown since fatigue cracks propagate as a result of a fluctuating tensile stress - nuts are under a compressive stress and so any pre-existing cracks tend not to propagate.[1] The literature on fatigue in bolts with threads rolled before and after heat treatment is contradictory. In many cases, axial fatigue tests were performed with stress ratio (R) equal to 0.1, which is not a desirable condition of preload, since the bolts of high strength are used with higher preload and, consequently, higher stress ratio. Under low stress ratio, a significant increase in the fatigue limit generally occurs. These beneficial results, usually, have been extrapolated to high stress ratio without verification. However, some researchers suggest that the benefits due to rolling the threads after the heat treatment will be significantly reduced for high stress ratios. This reduction can be associated with a relief of the residual stress due to high preload according to Stephens, et al [2]

2. LITERATURE REVIEW

In the year 1945, Goodier and Sweeny [3]

tested only a dynamically loaded bolted joint. In spite of their failure to obtain a complete self-loosening of threaded fasteners, they offered an explanation of partial loosening of threaded fasteners.

They pointed out that for axially loaded

joint, pulsating tension of a clamped bolted connection creates radial sliding motions between the thread flanks of the bolt and nut or the interface of the clamped bearing surfaces. The reasons for this are the contraction of the bolt according to Poisson's ratio and dilation of the nut walls caused by axial tension.

In the year 1964, Hongo [4] conducted some experiments on axial loading of nut and bolt assemblies. He varied the axial tensile force of a bolt and nut fastening having JIS M20 coarse screw threads 100 times in the range of 250 to 3000 kg in a reciprocal fashion. He examined whether there was any relative rotation between the bolt and nut by observing the oscillation of a beam of light projected on to a mirror pasted on the bolt. The results showed that the bolt did not continue to rotate in a direction that would loosen the nut. Goodier and Sweeny [3] in a similar way had reported detecting a relative rotation of 6.28×10⁻³radian by varying the axial tensile force of the bolt 100 times. Hongo [4] could not accept this conclusion of Goodier and Sweeny [3] that

―bolt and nut undergo relative rotation in the direction that would loosen the fastening as long as there is variation in the axial tensile force of the bolt‖ [3]. In the year 1966, Paland [5] tested various types of threaded fasteners for axial loading and gave the rule of loosening arithmetically and by measuring the tangential strain on the surface of the nut. He came to the conclusion that a loaded nut widens elastically in a radial direction at the area near the bearing surfaces and contracts in upper part. This very small amount of radial displacement by expansion of the nut would explain why Paland, in spite of heavy impact loading in an axial direction of the bolt, still needed a small external off-torque to turn the nut so as to loosen completely. In the year 1969, Junker [6] described the

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2 mechanism of loosening on the basis of friction between the flank surfaces.

According to him, the theory of mechanism of self-loosening of nut and bolt is based on the well-known law of physics that defines the effect of friction on two interacting solid bodies. As soon as the friction force between two solid bodies is overcome by an external force working in one direction, an additional movement in any other direction can be caused by the action of forces that can be essentially smaller than the friction force.

He tested cap screws, spring washers and free spinning locking screws with respect to its anti-loosening characteristics. Since transversely loaded joints tend more to self-loosening, the test procedure suggested imitates these actual conditions. The first attempt was made with a device consisting of two parts clamped together by the specimen, with load cell and displacement pickup to record transverse load, preload and displacement. He also reported that maximum values of vibration energy (transverse force x displacement) were significantly different for various locking elements. Sase et al. [7,8] tested the effectiveness of screw threads, spring washers, nylon inserted nuts, double nuts and eccentric nuts of few sizes to resist loosening. Test results showed that the popularly known anti-loosening fasteners did not possess much resistance to loosening. In the year 1998, Sase et al. [8]

introduced and evaluated the Step Lock Bolt (SLB) with regard to its anti- loosening performance using a displacement based loosening device.

They found the presence of desirable anti- loosening characteristics of SLB. The displacement and turning angle of the bolts and the nuts were examined in loosening tests. Following the experimental procedure and conclusions drawn by Sase et al. [7], a testing rig was designed and fabricated by a group lead by Das and co-workers [9,10], where a constant vibrating force of constant frequency and amplitude is applied at the right angle of the bolt axis. In this set-up, several tests were carried out with BSW and metric bolts of different materials, sizes with various types of nut and washer arrangements to conclude that nyloc nuts give substantial resistance to loosening compared to other fasteners.

The AISI 4135 and SCM 435H steels presented chemical compositions closed to each other and in accordance with its respective standards.

Metallographic analysis revealed that both wires have ferritic matrix with approximate grain size. All groups, with heat treatment (quenching and tempering), presented tempered martensite microstructure differing only by the well defined rolling texture in the boundary of the root fillets of the group where threads were rolled after quenching and tempering at 550°C. Besides, AISI 4135 steel presented lower strength and higher ductility than SCM 435H steel.

Based on the tensile properties of the bolts, as expected, the higher the tempering temperature, the lower the strength levels. Furthermore, bolts with threads rolled after heat treatment presented higher strength than those heat treated at the same tempering temperature and thread rolled before heat treatment. These levels were also higher than those from bolts tempered at a lower temperature. Moreover, wires made of AISI 4135 steel had hardness levels lower than those from the SCM 435H steel, while bolts tempered with higher temperatures presented lower hardness.

However, bolts with thread rolled after heat treatment presented higher hardness than those tempered at the same temperature (or with a little lower temperatures) with thread rolled before heat treatment. The results of axial fatigue tests in a neutral environment for bolts with thread rolled before heat treatment revealed that variations in tempering temperatures (490, 520, and 550°C) of the steels had little influence on the fatigue limit with constant preload (minimum stress) of 700 MPa and load ratio of about 0.8. On the other hand, fatigue tests (with load ratio of about 0.8) for bolts with thread rolled after heat treatment presented an increase of about 9% in the fatigue limit compared with those with thread rolled before heat treatment. This increase can be explained due to the introduction of compressive residual stresses on the surface of the bolt, and by the alignment of the grains in the vicinity of the root of the thread in the rolling direction, which difficults the nucleation and propagation of the fatigue cracks. Previous research on simplified approaches to connection modeling has

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3 focused on finite element modeling of bolted connections with validation through experiment, although much of this research has focused on non-Navy applications. This research falls into a variety of categories including simplified connection models for applications ranging from progressive collapse to pipe structure behavior and to plate structure behavior. In the field of progressive collapse modeling, work has focused on developing simplified models, or macromodels, of steel and concrete beam- column connections using combinations of beam and discrete spring elements [1–

6]. These macromodels were used to examine the progressive collapse resistance of a variety of 2D frame [1–3]

and 3D building structures [4–6]. Prior research on simplified modeling of joints in pipe structures was carried out by Luan et al. [7]. They developed a simplified nonlinear model with bilinear springs to model the bolted flange joints in cylindrical pipe structures. Previous research has been conducted on simplified modeling of bolted connections in generic or plate structures [3–

10,14,15]. Kwon et al. [8] modeled bolt behavior using a detailed model and a selection of simplified ―practical‖ models for both static loading and modal analysis experiments. Kim et al. [9] modeled bolt behavior using four different approaches:

 solid bolt model,

 coupled bolt model,

 spider bolt model,

 no-bolt model.

Their detailed solid-element based model with contact and their simplified coupled bolt model, which used a single beam element with degree-of-freedom coupling between its nodes and the solid element nodes of the plates on their outermost surfaces, produced the most accurate results as compared to experimental results [9] for a static loading experiment of a simple lap joint. Shi et al. [10]

investigated a bolted joint where a thin plate was connected by a single bolt at each end to a thicker plate that was fixed to a main frame. They developed twelve simplified models, which were used to simulate a drop test where the middle of the thin plate was hit by an impactor.The best results using a simplified approach were produced using deformable shell elements to model the bolt-nut assembly, where the bending stiffness of the

cylindrical shell had been set equal to that of the bolt shank and contact between the bolt shank and the plates was included. A variety of work has carried out regarding detailed finite element modeling of bolted connections with validation through experiment.

McCarthy et al. [11] andC.T.McCarthy and M. A. McCarthy [12] presented results on single-lap, single-bolt composite joints with titanium bolts. Base model development and validation against experimental strain gauge data from the joint surface and experimental joint stiffness data is discussed in [11], and C.

T. McCarthy and M. A. McCarthy [12]

examined the effects of bolt clearance.

The theory proposed by Sakai [11] that describes the mechanism of self-loosening due to rotational loading is introduced here referring to the setting shown in Fig.

1.

The upper and lower jigs are tightened by a bolt and nut pair to develop bolt tension Fb and a torque T is applied to the jigs in the hydraulic testing machine. In this setting, the torques that are required to cause the loosening and tightening slip on the thread surface and the slip on the bearing surface are expressed as Tsl, Tst, and Tw, respectively;

where ls, lw, d2, dw, P, and a1 are the coefficients of friction on the thread and the bearing surfaces, pitch diameter of the thread, equivalent diameter of the bearing surface, thread pitch, and thread half angle, respectively. In order for the bolt to keep rotating in the loosening direction, it is necessary that the thread surface undergoes a complete slip while

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4 torque is applied in the loosening direction and the bearing surface undergoes a complete slip while torque is applied in the tightening direction. That is, the condition below needs to hold;

The rotation angle on the thread surface during loading in the loosening direction corresponds to the loosening rotation angle. The relative rotation angle hcr needs to be applied to the jigs in order for the bolt to initiate rotation in the loosening direction in addition to the conditions expressed in Eq. (4). Loosening rotation does not occur if the relative rotation angle of the jigs is less than hcr because it causes only the torsion of the bolt axis and does not cause a complete slip on the thread surface. hcr can be expressed as the sum of the tightening torque left in the bolt axis Tw and the loosening torque on the thread surface Tsl,

where l and Ip are the grip length and the polar moment of inertia of the bolt axis, respectively.[13]

3. CONCLUSION

The review study of bolts and fasteners shows that there is a lot of work done in this area , and there is a scope of fatigue analysis of the bolted joints by FEM analysis in different loading condition for different types of conditions and parameters which can be done and analyzed to prevent the loosening of the bolts and fasteners in theses condition.

REFERENCES

1. H. Fuji, N. Sase, SLB Concept for screw fastening and its anti-loosening performance, Souvenir, All India Manufacturing Technology and Research Conference, December 21–23 1998, pp. 25–

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2. L. da Vinci, Cobex Madrid I Vol. I, a facsimile ed.McGraw Hill Book Company Limited, UK, 1974.

3. J.N. Goodier, R.J. Sweeny, Loosening by vibration of threaded fastenings, Mechanical Engineering (December-1945) 798–802.

4. K. Hongo, Loosening of bolt and nut fastening, Transaction of Japan Society Mechanical Engineering 30 (1964) 215.

5. E.G. Paland, Investigation of the locking features of dynamically loaded bolted connections, Dissertation TH, Hannover, 1966.

6. G.H. Junker, New criteria for self-loosening of fasteners under vibration, SAE International Automotive Engineering Congress, 1969, pp. 934–939, Paper no.

690055.

7. N. Sase, S. Koga, K. Nishioka, H. Fujii, Evaluation of anti-loosening nuts for screw fasteners, Journals of Materials Processing Technology 56 (1996) 321–332.

8. N. Sase, K. Nishioka, S. Koga, H. Fujii, An anti-loosening screw fastener innovation and its evaluation, Journal of Materials Processing Technology 77 (1998) 209–215.

9. P. Sarkar, P. Mallick, A. Bhattacharya, S.

Das, Enhancing anti-loosening characteristics of threaded fasteners—need of the threaded components under vibratory condition, Proceedings of the National Seminar on Strategies and Techniques for Maintenance of Equipments and Machinery — Recent Trends, Kolkata, India, 2004, pp. II 18–II 22.

10. S.K. Saha, S. Srimani, S. Hajra, A.

Bhattacharya, S. Das, On the anti- loosening property of different fasteners, Proceedings of 13th National Conference on Machines and Mechanisms (NaCoMM), IISc Bangalore, India, 2007, pp. 229–232.

11. Sakai T. Investigation of bolt loosening mechanisms: 2nd report, on the center bolts of twisted joints. Bull JSME 1978;21(159):1391–4.

12. Fujioka Y, Sakai T. Rotating loosening mechanism of a nut connecting a rotary disk under rotating-bending force. J Mech Des 2005;127:1191–7.

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Three-dimensional finite element analysis of tightening and loosening mechanism of threaded fastener. Eng Fail Anal 2005;12:604–15.

14. Fujioka Y. Behavior and mechanisms of bolt self-loosening under transverse load due to vibrations of a washer along an arc.

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