## Damping of Layered and Jointed Beams with Riveted Joints

Material type: TextLanguage: English Publisher: 2010Description: 196 pSubject(s): Engineering and Technology | Mechanical Engineering | Machine DesignOnline resources: Click here to access online Dissertation note: Thesis (Ph.D)- National Institute of Technology, Rourkela Summary: The present investigation highlights the eff ect of interfacial slip on the damping of layered cantilever beams jointed with rive ts undergoing free vibration. The inclusion of mechanical joints bears a strong influe nce in the overall system performance and behavior, particularly the damping level of the structures. In fact , the damping and its improvement in machines or structures are one of the biggest challenges to the practicing engineers. Usually, such struct ures inherently possess low structural damping necessitating the introduction of additional measures to improve their damping characteristics in order to control the harmful effects of vibration in normal operating conditions. Monolithic structures can be used as an alternative, but unfortunately these are very poor in dampi ng capacity and are not cost-effective. One of the techniques used in the present pr oblem for improving damping is fabricating these structures in layers by means of rivete d joints. The incorporation of such joints is the major source of energy dissipation th rough frictional effects associated with relative shear displacements at the interfaces of the various structural members. Most of the damping in built-up structures is thus attributed to micro-slip at the interfaces. The contribution of the micro-slip on the ove rall system damping is always significant in spite of its low magnitude. This thesis consists of two different parts: a theoretical analysis of the problem and an experimental work. The theoretical anal ysis proposes two di fferent methods to calculate damping: classical method and finite element method . The analyses are based on the assumptions of Euler-Bernoulli be am theory as the dimensions of test specimens satisfy the criterion of thin beam theory. In the first case, a continuous model is characterized by a partial differential equation with respect to spatial and time coordinates. An analytical exact soluti on is obtained for the above differential equation from which the dynamic characteri stics of the structure are represented accurately. In the latter case, the mode l is represented by one-dimensional beam elements with each element consisting of tw o nodes with two degrees of freedom, i.e. transverse displacement and rotation at each node. This model is approximate and characterized by stiffness and mass matric es from which natural frequencies and mode shapes are obtained by modal analysis . As the direct eval uation is not easy, an alternate energy approach has been us ed to derive the damping matrix. It is a general fact that the theoretically computed results will differ from the actual values due to the assumptions made in th e theoretical analyses. In view of this discrepancy in results, expe riments are conducted for different set of mild steel and aluminium specimens under different vibrating conditions. The logarithmic decrement technique has been used for measuring the damping from th e time history curve of the decaying signals recorded on the screen of digital storage oscilloscope. The experimental results are compared with the corresponding theoretical ones for establishing the authenticity of the theory developed. Finally, usef ul conclusions have been drawn from both the theoreti cal and experimental results. The damping characteristics in jointed structures are influenced by the intensity of pressure distribution, micro-slip and kinematic coefficient of friction at the interfaces and the effects of all these parameters on the mechanism of damping have been extensively studied. All the above vital parameters are largely influenced by the thickness ratio of the beam and thereby aff ect the damping capacity of the structures. In addition to this, number of layers, cantilever length and diameter of connecting rivet also play key roles on the damping capacity of the jointed structures quantitatively. The effects of all these para meters are studied vi vidly in the present investigation. It is established that the da mping capacity can be enhanced appreciably using larger cantilever length and rivet diamet ers as well as lower thickness ratio of the beams. Further improvement in damping is possible with the use of more number of layers compared to its equivalent solid one. This design concept of using layered structures with riveted joints can be effectiv ely utilized in trusse s and frames, aircraft and aerospace structures, bridges, machine members, robots and many other applications where higher damping is requiredItem type | Current location | Collection | Call number | Status | Date due | Barcode |
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Thesis (Ph.D/M.Tech R) | BP Central Library Thesis Section | Reference | Not for loan | T91 |

Thesis (Ph.D)- National Institute of Technology, Rourkela

The present investigation highlights the eff

ect of interfacial slip on the damping of

layered cantilever beams jointed with rive

ts undergoing free vibration. The inclusion

of mechanical joints bears a strong influe

nce in the overall system performance and

behavior, particularly the damping level of

the structures. In fact

, the damping and its

improvement in machines or structures

are one of the biggest challenges to the

practicing engineers. Usually, such struct

ures inherently possess low structural

damping necessitating the introduction of

additional measures to improve their

damping characteristics in order to control

the harmful effects of vibration in normal

operating conditions. Monolithic structures can be used as an alternative, but

unfortunately these are very poor in dampi

ng capacity and are not cost-effective. One

of the techniques used in the present pr

oblem for improving damping is fabricating

these structures in layers by means of rivete

d joints. The incorporation of such joints

is the major source of energy dissipation th

rough frictional effects associated with

relative shear displacements at the interfaces

of the various structural members. Most

of the damping in built-up structures is thus

attributed to micro-slip at the interfaces.

The contribution of the micro-slip on the ove

rall system damping is always significant

in spite of its low magnitude.

This thesis consists of two different parts: a theoretical analysis of the problem and an

experimental work. The theoretical anal

ysis proposes two di

fferent methods to

calculate damping:

classical method

and

finite element method

. The analyses are

based on the assumptions of Euler-Bernoulli be

am theory as the dimensions of test

specimens satisfy the criterion of thin beam

theory. In the first case, a continuous

model is characterized by a partial differential equation with respect to spatial and

time coordinates. An analytical exact soluti

on is obtained for the above differential

equation from which the dynamic characteri

stics of the structure are represented

accurately. In the latter case, the mode

l is represented by one-dimensional beam

elements with each element consisting of tw

o nodes with two degrees of freedom, i.e.

transverse displacement and rotation at

each node. This model is approximate and

characterized by stiffness and mass matric

es from which natural frequencies and

mode shapes are obtained by modal analysis

. As the direct eval

uation is not easy, an

alternate energy approach has been us

ed to derive the damping matrix.

It is a general fact that the theoretically computed results will differ from the actual

values due to the assumptions made in th

e theoretical analyses. In view of this

discrepancy in results, expe

riments are conducted for different set of mild steel and

aluminium specimens under different vibrating conditions. The logarithmic decrement

technique has been used for

measuring the damping from th

e time history curve of the

decaying signals recorded on the screen

of digital storage oscilloscope. The

experimental results are compared with

the corresponding theoretical ones for

establishing the authenticity of the theory

developed. Finally, usef

ul conclusions have

been drawn from both the theoreti

cal and experimental results.

The damping characteristics in jointed structures are influenced by the intensity of

pressure distribution, micro-slip and kinematic

coefficient of friction at the interfaces

and the effects of all these parameters on the mechanism of damping have been

extensively studied. All the

above vital parameters are

largely influenced by the

thickness ratio of the beam and thereby aff

ect the damping capacity of the structures.

In addition to this, number

of layers, cantilever length

and diameter of connecting

rivet also play key roles on the damping capacity of the jointed structures

quantitatively. The effects of all these para

meters are studied vi

vidly in the present

investigation. It is established that the da

mping capacity can be enhanced appreciably

using larger cantilever length and rivet diamet

ers as well as lower thickness ratio of

the beams. Further improvement in damping

is possible with the use of more number

of layers compared to its equivalent solid

one. This design concept of using layered

structures with riveted joints can be effectiv

ely utilized in trusse

s and frames, aircraft

and aerospace structures, bridges, machine members, robots and many other

applications where higher damping is required

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