Mechanical Joints
There are many problems and phenomena associated with joints present in mechanical structures. Some of these problems/phenomena include friction, damping, vibrations, loosening, wear, plastic deformation, temperature changes, fatigue, and fretting. Many of these problems are interrelated: vibrations cause friction which in turn causes wear, etc. Much research is being done in order to understand these problems/phenomena and to come up with solutions for them. Some of the solutions include lubrication, structural health monitoring, smart washers, bolt clamping, and higher rivet squeeze forces.
Friction is defined as a force that resists sliding and is proportional to the normal load [1]. In bolted joint the normal force is dependant on the bolt clamping force. Friction does not depend on the area of contact or the velocity of the contacting surfaces once motion has started [1]. Friction occurs during macro slip and micro slip. Friction dissipates energy in a joint by producing heat and wear. Ahmadian and Jalali found that friction was the main loss of energy in a bolted lap joint at moderate levels of excitation [2]. Friction can also be beneficial. It can increase the stiffness of the joint and reduce the shear stress on the bolt in a bolted joint [7].
Wear is damage to a surface that generally involves a progressive loss of material [1]. It is another problem in mechanical joints. There are several different types of wear including adhesive, abrasive, fatigue fretting, erosion, and corrosion Adhesive wear is loss of material by the rubbing of two surfaces together. Abrasive wear is the loss of material when hard particles or protrusions are forced to run against a solid surface. Fatigue and fretting both have to do with cyclic loading and oscillatory motion that may not always be visible to the naked eye. Fatigue and fretting both lead to cracking in a material. Erosion is loss of material when particles (usually at high velocity) bombard a surface. Corrosion involves a chemical reaction such as oxidation, where oxides form and, since they aren’t as hard as the original material, flake off.
Other problems in joints include damping, vibrations, loosening, plastic deformation, and temperature changes. Damping is when energy is lost in a joint. Damping represents the energy lost due to friction and depends the amplitude of vibrations in the joint [9]. This also depends on things like bolt clamping force and the roughness of the material. Vibrations are caused by dynamic loading on a joint. They lead to things such as damping, loosening, fatigue, and fretting. Loosening can be caused by tensile loading (due to Poisson’s ratio effect) [7]. In a study by Yanyao, Zhang, and Lee it was discovered that loosening (bolt clamping force reduction) ranged from ten to forty percent after 200 loading cycles [10]. This was found to be caused by localized cyclic plastic deformation [10]. Friction didn’t seem to have an effect on the early stages of loosening. Loosening is due more to shear loading and fatigue is due more to tensile loading [10]. Loosening is not significant if there is no slip between clamped parts. Loosening can also happen due to the clamping force of the bolt itself. Temperature changes can cause fatigue in a joint due expansion and contraction of the materials.
There are several ways to help to lessen the above mentioned effects or get rid of them all together. A few of these include lubrication, structural health monitoring, smart washers, bolt clamping, and higher rivet squeeze forces. Lubrication decreases the friction between the parts of a joint and helps to reduce wear. It can also help to maintain the stiffness of the joint while increasing the damping. Izumi et al. considered two ways to prevent loosening: the double nut and the spring washer. The double nut was found to be effective if tightened properly and the spring washer was found to be ineffective, in some cases even accelerating the loosening [8].
Structural health monitoring is meant to detect and assess damage in structures before it reaches a critical state and thus improve safety, reliability, and bring down maintenance costs. Mascarenas et al. studied an impedance based sensor that transmitted data wirelessly and could also be powered wirelessly [11]. The sensor would send out high-frequency ultrasonic pulses to sense changes in the electrical impedance of the structure [11]. The electrical impedance is related to the mechanical impedance (which depends on things such as mass, stiffness, and damping). Thus the sensor is able to tell if properties of the structure have changed, i.e. something might be wrong. Another example of the structural health monitoring is the smart washer considered by Okugawa and Tanaka [12]. It is a flat washer with piezoelectric material attached and monitors the natural frequency related to the bolt tightness [12].
Other solutions include increases bolt clamp forces and rivet squeeze forces. Chakherlou, Abazadeh, and Vogwell discovered that fracture strength of a bolted joint increases as the bolt clamping force is increased [7]. Finite element analysis showed that for a hole with a crack on the edge and a given tensile load, increased clamping force reduces the stress intensity factor at the crack tip due to the normal stress and friction (which acts to oppose the loading) [7]. They also found that the reduction of the stress intensity factor was greater at the surface than at the mid-plane [7]. Rans, Alderliesten, and Straznicky considered the effects of rivet squeeze forces using universal and countersunk rivets [13]. They found that fatigue life increased with increasing rivet squeeze force for both universal and countersunk rivets. They found that joints with countersunk rivets were more sensitive to squeeze forces and that the residual stresses produced by these squeeze forces help to stop crack propagation around the hole [13].
Some Examples to Mechanical Joints
Here is a list of examples to
structures containing mechanical joints:
Joints
in the Human Body- Ball and Socket (Hip)
- Hinge (Elbow)
- Gliding (Vertebrae)
- Fixed (Skull)
- Woodwork
- Legos/ K’nex
- Tools
- Aircraft/Spacecraft/Cars
- Rivets in Wings, etc.
Movies/Action
Figures- Terminator
Transformers
- Gas Turbines
References
1. Affatato, S., M. Spinelli, M. Zavalloni, C. Mazzega-Fabbro, and M. Viceconti. "Tribology and total hip joint replacement: Current concepts in mechanical simulation." Medical Engineering & Physics 30.10 (2008): 1305-317.
2. Ahmadian, Hamid, and Hassan Jalali. "Identification of bolted lap joints parameters in assembled structures." Mechanical Systems and Signal Processing 21.2 (2007): 1041-050.
3. Barut, A., and E. Madenci. "Analysis of bolted-bonded composite single-lap joints under combined in-plane and transverse loading." Composite Structures 88.4 (2009): 579-94.
4. Brandon, John. "AN, MS hardware: rivets, bolts & locking devices." Builders guide to aircraft materials, 2006. http://www.auf.asn.au/scratchbuilder/hardware.html>.
5. Celic, Damjan, and Miha Boltezar. "Identification of the dynamic properties of joints using frequency-response functions." Journal of Sound and Vibration 317.1-2 (2008): 158-74.
6. Celic, Damjan, and Miha Boltezar. "The influence of the coordinate reduction on the identification of the joint dynamic properties." Mechanical Systems and Signal Processing 23.4 (2009): 1260-271.
7. Chakherlou, T. N., B. Abazadeh, and J. Vogwell. "The effect of bolt clamping force on the fracture strength and the stress intensity factor of a plate containing a fastener hole with edge cracks." Engineering Failure Analysis 16.1 (2009): 242-53.
8. Izumi, Satoshi, Takashi Yokoyama, Masatake Kimura, and Shinsuke Sakai. "Loosening-resistance evaluation of double-nut tightening method and spring washer by three-dimensional finite element analysis." Engineering Failure Analysis 16.5 (2009): 1510-519.
9. Jalali, Hassan, Hamid Ahmadian, and John E. Mottershead. "Identification of nonlinear bolted lap-joint parameters by force-state mapping." International Journal of Solids and Structures 44.25-26 (2007): 8087-105.
10. Jiang, Yanyao, Ming Zhang, and Chu-Hwa Lee. "A Study of Early Stage Self-Loosening of Bolted Joints." Journal of Mechanical Design 125.3 (2003): 518-27.
11. Mascarenas, David L., Michael D. Todd,
12. Okugawa, Masayuki, and Tohru Tanaka. "Effect on Detection Sensitivity for Smart Washer Configuration, and Ambient Temperature Characteristics on Bolted Joint." Proc. SPIE 6532 (2007).
13. Rans, C.D., R.C. Alderliesten, and P.V. Straznicky. "Assessing the effects of riveting induced residual stresses on fatigue crack behaviour in lap joints by means of fractography." International Journal of Fatigue 31.2 (2009): 300-08.