EXPERIMENTAL TECHNIQUES IN STRUCTURAL ENGINEERING

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EXPERIMENTAL TECHNIQUES
PRESENTED BY:
R.KAVEEYA PREETHI
Department of Civil Engineering
23SE04
Strain Measurements

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OUTLINE OF PRESTENTATION
Strain Rosettes
Wheat Stone Bridge
Photo Elasitcity
Principle Application
Hydralic Jack And Pressure Gauges
Electronic Load Cells
Proving Rings

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Since for strain analysis in biaxial state of stress we should know strain in three directions and due to
drawbacks in a strain gauge, Strain rosettes came in to picture.
Strain rosette can be defined as the arrangement of strain gauges in three arbitrary directions.
These strain gauges are used to measure the normal strain in those three directions.
Depending on the arrangement of strain gauges,Classified as:
 Rectangular Strain Type,
 Delta Strain Type
 Star Strain Type
STRAIN ROSETTES

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 Strain rosettes use this method to measure pressure, force,
weight and tension.
 Strain rosettes attached to Wheatstone bridges can be used for
the measurement of tension, bending, and torsion.
 In biomedical applications, strain gages can be used for
determining forces in bones
 Slightly modified strain gauges can be used for muscle
contraction and blood pressure measurement.
Uses of Strain Rosettes:

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Applications of Strain Gauge Rosettes
The Strain Gauge Rosette sensors measure the strain on a material. It is made up of numerous
strain gauges arranged in a certain pattern, usually in the shape of a rosette. Strain gauge
rosettes have a wide range of applications in various industries, including:
 Civil Engineering: Strain gauge rosettes are used in the construction industry to measure
the strain on structures such as bridges, buildings, and dams. They are used to monitor
the structures’ performance and detect any potential issues.
 Aerospace: Strain gauge rosettes measure the strain on aircraft components such as
wings, fuselages, and engines. They are used to monitor the aircraft’s performance and
detect any potential issues.

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Contd.
 Automotive: Strain gauge rosettes measure the strain on car components such as frames,
suspension systems, and engines. They are used to monitor the car’s performance and detect any
potential issues.
 Biomedical Engineering: They are used in biomedical engineering to measure the strain on
bones and joints to study the human body’s biomechanical behaviour.
 Industrial: They are used in industrial applications such as monitoring the performance of
machinery and detecting any potential issues, such as cracks in rotating shafts.
These are some of the common applications of Strain gauge rosettes. Due to their ability to measure
strain in multiple directions, they are versatile tools that can be used in many fields.

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1.Multi-Axial Strain Measurement: Strain rosettes enable the measurement of strains in multiple directions simultaneously,
providing valuable information about the structural response to complex loading conditions. This capability is particularly useful in
analyzing structures subjected to biaxial or triaxial loading.
2.Local Strain Analysis: Strain rosettes allow engineers to obtain detailed information about the distribution of strains at specific
locations on a structure. This localized analysis helps identify stress concentrations, potential failure points, and areas requiring
reinforcement or modification.
3.Verification of Analytical Models: By comparing experimental strain data obtained from strain rosettes with theoretical predictions
from analytical models, engineers can validate and refine their structural analyses. This validation process enhances the accuracy of
structural models and improves the overall reliability of design calculations.
4.Dynamic Structural Monitoring: Some strain rosettes are designed for dynamic strain measurement, making them suitable for
structural health monitoring and vibration analysis applications. Continuous monitoring of dynamic strains helps identify structural
anomalies, fatigue-related issues, or changes in structural behavior over time.
5.Research and Development: Strain rosettes play a crucial role in research and development efforts aimed at optimizing
structural designs, materials, and construction techniques. By quantifying strain distributions under various conditions, engineers
can innovate and improve the performance of structural systems.
Merits:

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DEMERITS
 Installation Challenges: Proper installation of strain rosettes on structural components can be challenging, especially in inaccessible
or hazardous environments. Incorrect installation may lead to unreliable strain measurements and compromise the accuracy of
subsequent analyses.
 Temperature Sensitivity: Strain gauges, including those used in strain rosettes, are sensitive to temperature variations. In structural
engineering applications, temperature changes within the environment or the structure itself can introduce errors in strain measurements if
not adequately compensated for.
 Cost Considerations: High-quality strain rosettes and associated instrumentation can be expensive, particularly for large-scale
structural testing projects or long-term monitoring applications. Cost considerations may limit the widespread adoption of strain rosette
technology, especially in projects with budget constraints.
 Surface Accessibility: Strain rosettes measure strains on the surface of structural components, which may not always be accessible or
representative of internal stresses. In cases where internal strain distributions are of interest, alternative measurement techniques or
instrumentation may be required.
 Calibration and Maintenance: Regular calibration and maintenance of strain rosettes are essential to ensure their accuracy and
reliability over time. Calibrating strain gauges and verifying their performance can be time-consuming and may require specialized
equipment and expertise.

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WHEAT STONE BRIDGE
A Wheatstone bridge is a precision instrument used to measure an unknown electrical
resistance. It's widely used in scientific and industrial applications due to its high
accuracy and reliability.
Basic Components of Wheatstone Bridge:
 Resistors
 Galvanometer
 Battery

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CONTD
Resistor:Four resistors are crucial to the operation of a Wheatstone bridge.
They provide the necessary paths for current flow.
Galvanometer:A sensitive instrument that detects small currents flowing
through the bridge. It serves as the output device
Battery:Provides the necessary voltage to drive the current through the
bridge's circuit.

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WORKING PRINCPLE OF WHEATSTONE BRIDGE
Power Supply: A voltage source is connected across the two opposite corners of the bridge circuit.
Unknown Resistor (R_x): The resistor whose value we want to measure is connected between one pair of adjacent corners.
Ratio Arms (R_1 and R_2): Two known resistors (often called ratio arms) are connected in series between the other pair of
adjacent corners.
Variable Resistor (R_v): A variable resistor (often called a rheostat or potentiometer) is connected across one of the ratio arms.
This resistor allows for fine adjustment of the bridge balance.

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Advantages:
 High Precision: The Wheatstone bridge offers high precision in resistance measurements, allowing
for accurate determination of unknown resistance values.
 Noise Rejection: It provides inherent noise rejection, making it suitable for measuring small changes
in resistance even in noisy environments.
 Versatility: The bridge can be adapted for various measurement tasks by replacing the unknown resistor with other
sensors or transducers, such as strain gauges or temperature sensors.
 Simple Design: The Wheatstone bridge circuit has a relatively simple design, making it easy to implement and
troubleshoot.

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Disadvantages
Sensitivity to Environmental Factors: The accuracy of the Wheatstone bridge can be
affected by various environmental factors such as temperature changes, stray magnetic
fields, and mechanical vibrations. These factors can introduce errors in the measurements.
Power Dependency: In some configurations, the Wheatstone bridge requires a power source
to operate. This dependency on an external power supply can be a limitation in certain
applications, especially in situations where power availability is limited or unreliable.
Calibration and Adjustment: Achieving accurate measurements with a Wheatstone bridge
often requires careful calibration and adjustment. This process can be time-consuming and
may require specialized equipment or expertise.

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APPLICATION
 Resistance Measurement: The primary application of the Wheatstone bridge is for measuring unknown resistances
precisely, which finds use in various fields such as electronics, physics, and engineering.
 Strain Gauge Measurements: In structural engineering and materials testing, strain gauges are often used to measure
mechanical strain. The Wheatstone bridge is employed to measure the small changes in resistance caused by strain in the
gauge.
 Temperature Sensing: Resistance thermometers (RTDs) and thermistors, which exhibit changes in resistance with
temperature, can be incorporated into Wheatstone bridge configurations for accurate temperature sensing applications.
 Pressure Sensing: Some pressure sensors operate on the principle of resistance change with pressure. These sensors can
be integrated into Wheatstone bridge circuits for precise pressure measurements.
 Moisture Sensing: Moisture sensors based on changes in resistance in response to moisture levels can be connected to
Wheatstone bridges for moisture detection in soil, air, or other substances.
 Strain Measurement in Load Cells: Load cells, used for measuring force or weight, often employ strain gauges in
Wheatstone bridge configurations to accurately measure the strain induced by the applied load.

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PHOTO ELASTICITY
 Photo-elasticity is an experimental technique for stress and strain analysis that is
particularly useful for members having complicated geometry, complicated loading
conditions, or both. The photo-elastic method is based upon a unique property of
some transparent materials, in particular, some plastics.
 Photoelasticity is a nondestructive, whole-field, graphic stress-analysis technique
based on an optomechanical property called birefringence, possessed by many
transparent polymers.

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CONTD…
 Combined with other optical elements and illuminated
with an ordinary light source, a loaded photoelastic
specimen (or photo elastic coating applied to an ordinary
specimen) exhibits fringe patterns that are related to the
difference between the principal stresses in a plane
normal to the light propagation direction.
 The method is used primarily for analyzing
twodimensional plane problems, which is the emphasis
in these notes.
 A method called stress freezing allows the method to be
extended to three dimensional problems.
 Photoelastic coatings are used to analyze surface stresses
in bodies of complex geometry

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Advantages:
Photoelasticity, as used for two dimensional plane problems.
 Provides reliable full-field values of the difference between the principal normal
stresses in the plane of the model.
 Provides uniquely the value of the nonvanishing principal normal stress along
the perimeter(s) of the model, where stresses are generally the largest.
 Furnishes full-field values of the principal stress directions (sometimes called
stress trajectories).
 Is adaptable to both static and dynamic investigations .
 Requires only a modest investment in equipment and materials for ordinary
work .
 Is fairly simple to use

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DISADVANTAGES
 Requires that a model of the actual part be made (unless photoelastic coatings
are used)
 Requires rather tedious calculations in order to separate the values of principal
stresses at a general interior point
 Can require expensive equipment for precise analysis of large components
 Is very tedious and time-consuming for three-dimensional work

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Applications:
 Photoelasticity is widely used in engineering and materials science to study
stress distribution in mechanical components, model structural behavior, and
validate finite element analysis (FEA) simulations. It provides valuable
insights into the stress concentration, load transfer, and failure mechanisms in
various materials and structure

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HYDRAULIC JACK AND PRESSURE GAUGES
 Introduction
 basic Principle
 Working rule
 Construction
 function

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INTRODUCTION
 Fluid system is defined as the device in which power is transmitted
with the help of fluid which may be liquid or gas under pressure.
 Most of devices are based on principle of fluid static and fluid
kinematics.
 Hydraulic jack if one type of fluid system

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The basic principle of hydraulic jack may
explained by the help of fig. a be Consider a
ram and plunger, operating in two cylinders
of different diameters, which are
interconnected at the bottom, through a
chamber, which is filled with some liquid. It
is based on Pascal's law.
Basic Principle

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Working rule
 This is a cross-section of a simple hydraulic jack.
 When lever is working , water is contained in
casing is forced out of into the portion of the
inverted cylinder above ram. Due to pressure, the
upward force acts on the inverted cylinder.
 The cylinder therefore moves up and lifts load
placed on the top of casing.
 There is screw arrangement to take water back
from cylinder to the casting and thus lower the
load when required.

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Construction
 It consist of cylinder which is inverted and can move downward on ram which
is fixed with frame of jack as shown in fig.
 Also consist one type of lever and it is connected with reciprocating pump.

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Working Rule:
When lever is working , water is contained in casing is forced out of into the
portion of the inverted cylinder above ram. Due to pressure, the upward force
acts on the inverted cylinder. The cylinder therefore moves up and lifts load
placed on the top of casing. There is screw arrangement to take water back from
cylinder to the casting and thus lower the load when required.

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FUNCTION
Hydraulic jack may be employed for the following jobs:
 Metal press work (to press sheet metal to any required shape).
 Drawing and pushing rods.
 Bending and straightening any metal piece.
 Packing press.
 For lifting heavy load.

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PRESSURE GAUGES
 Mechanical Gauges are the best instruments to measure High Fluid
Pressure in hydraulics engineering, where Tube Gauge cannot be
conveniently used.
 The principle on which all Gauges work is almost same.

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TYPES OF PRESSURE GAUGESNNNNNNN
 Bourdon Tube Pressure Gauges
 Diaphragm Pressure Gauge
 Bellows Pressure Gauge
 Dead Weight Pressure Gauge

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Bourdon Tube Pressure Gauges
 Invented by E. Bourdon.
 Pressure responsive element is made up of
bronze & steel in tube from.
 Outer end is closed and free to move.
 Inner end is connected to the point where
pressure is to be measured.
 Due to increase in pressure elliptical shape
converts into circular, and this movement is
measured thorough link & pinion.

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Diaphragm Pressure Gauge
• Diaphragm Pressure Gauge Pressure
responsive element is an elastic steel
corrugated diaphragm.
• Elastic deformation of diaphragm under
pressure is transmitted to a pointer by
similar arrangement as in first case.

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 Bellows Pressure Gauge Pressure
responsive element is made of metallic
tube having deep circumferential
corrugation.
 During increase or decrease of pressure,
elastic element expand & contrasts that
reads on pointer.
Bellows Pressure Gauge

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Dead Weight Pressure Gauge
 It is a balancing device.
 It consist of a Piston and a Cylinder of
known area and connected to a Fluid
by a Tube.
 By changing the Weight on the Piston,
the Pressure on the Fluid is calculated
and marked on the Pointer.

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Advantages:
 Simple and rugged construction
 Good for low to moderate pressures
 Available for gauge, differential and absolute pressure measurements
 Moderate cost
Limitations:
 Greater hysteresis and zero shift problems
 Unsuitable for transient measurements due to a longer relative motion and mass
 Needs spring for accurate characterization
 Requires compensation for ambient temperature changes.

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Electronic Load Cells Load Cells
• Measurement of Weight is an act Performed by our Ancestors and still it is
Performed in our day to day Life.
• Technology also had Taken a Leap in these Years.

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LOAD CELL
 When we hear about Load Cell One thing that comes into mind is Weight
Measurement and the Pointer scale we used in our Earlier Times.
 But now the Question is "What is a Load Cell" ? Load Cell is a Device or
an Instrument which Converts the Quantity of Force Applied over it to the
Electrical Signal Form.
 There are Many Types of Load Cells Available in Market these Days.

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TYPES OF LOAD CELLS

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ROLE OF STRAIN GAUGE
 Strain Gauge acts as the Transducer in the Strain Gauge Load Cell.
 We can also Say that the Strain Gauge is the Heart of this Load Cell.

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