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Strain Gauges Were Used As An Instrument Engineering Essay

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Abstract

In our daily life we meet impact loading. There are many things happening according to impact loading theory. This report is about development a testing rig for impact loading of beams for laboratory use.

The aim of this project is to design a ready to manufacture product which can be used for the purpose of laboratory experiments and compare the real results from experiments with the theoretical results from calculations.

Main factor is to measure the instantaneous deflection in cantilever beam. So these purpose strain gauges were used as an instrument. For design the rig Solid Works software were used in the drawings.

Summary

This report is project goal at Greenwich University in United Kingdom. It is about development a testing rig for measuring deformation and stresses resulting from impact loading on beams for laboratory use.

The project began with the idea of affecting the impact loads on the standard steel beams and deformation and stress. The aim of this project is to development of a laboratory rig for measuring deformation and stresses resulting from impact loading. This can be used for the purpose of laboratory experiments and comperes the true results from the experiments with the theoretical results from the calculations.

The specimens to be used were 1.3 meter long 8 standard cantilever beams which i choose for finish this project. The initiation step in this project was to research theory behind the impact loading and deformations and stress according to the impact loading. The second step was design and dimensions a testing rig which is suitable for laboratory use. The height and the maximum weight were chosen according to laboratory use conditions, health and safety issues. The third step was to measure the falling height of the weight onto the test sample and measure the instantaneous maximum deflection of the beam. In this case, the approach of falling height was not very important but measuring the deflection of the beam with the highest possible approach was most important. A measurement system is used considering this factor.

In the project, Solid Works software was mainly used in the design, drawings and strength and stress analyses.

Table of Contents

Chapter 1

1. Introduction

1.1 backgrounds

This project research was made for Greenwich University School of engineering Mechanical Engineering technology Department for my final year project to development of a laboratory rig for measuring deformation and stresses resulting from impact loading.

One of the most important things in this project is to apply the theoretical background given in the engineering dynamics and material selection subject about impact loading and materials in reality. The main idea of this project is to development of a laboratory rig for measuring deformation and stresses resulting from impact loading testing rig which is suitable for laboratory conditions. Since the testing machine will be used by students, it should be as safe as possible and at the same time should be functional and easy to use.

The Task

The goal of this research is to design an experimental rig for measure the deformation in cantilever beam subject to impact loads. The picture below shows a sample design of the testing rig. Detailed drawings of all components and assemblies can be found in Appendix Drawings.

Figure 1 Sketch of testing machine

The following parts need to assemble

Support Blocks

Bottom Plate

Rope

Testing Beam

Falling Part

Rulers

The goal is to find solutions to the problems by providing answers to the following questions:

How shall a testing rig for impact loading be designed?

How can different loads apply to different sizes testing beams in different experiments?

How can the testing beam be fixed in a flexible way?

How can the movement of the plate be provided?

How can the beam deflection be measured?

Best alternatives for the problems above shall be chosen. And the testing machine will be designed ready to manufacture. Since this is a real project, manufacturing of the testing machine should also be affordable.

1.3 The Objectives

The objectives of this project are to development of a rig to measure the instantaneous maximum deflection resulting from impact loading testing machine for laboratory use. This report shall contain following chapters.

Research about the impact loading

Research about deflection and stress

Designing of the rig

Dimensioning of the rig

Calculating the minimum and maximum load that can be applied to the testing beam

Fixing of the testing beam. The testing beam should be steady, flexible and also this fixation should not block the free bending of the beam

Measuring the deflection of the beam and falling height of the load precisely

Designing the testing rig to be safe

1.4 Limitations and Assumptions

Some parts of this research are assumed and selected due to time limitation and the main purpose of the research. The main goal is to design and dimension the impact loading testing rig. For this purpose the focused fields are design and assembly. But since it is a real project it contains lots of other factors (welds etc). To stay in the main field these factors are assumed.

H beam assemble with the bottom plate using with bolts and nuts

For the design all the materials which is chosen ss 1035 steel.

All other parts are weld together. Weld thickness in the design were assumed 10mm

8 different stranded testing beams are given below

Beam material

Cross section (mm)

Length(mm)

Young's modulus(Gpa)

Moment of inertia

Height

deflection

aluminium

50*50

40*40

steel

50*50

brass

Polyester fibre glass

Table 1 beam type

Chapter 2

2. Literature review

2.1 Impact Loading

A moving body striking a structure delivers a suddenly applied dynamic force that is called impact. It is also called shock, sudden or impulsive loading. In our real life many examples of the impact loading may be seen. Some of them are as follows:

Driving a nail

Razing of buildings with an impact ball

Automobile wheels dropping into potholes

Breaking up concrete with an air hammer

Automobile collisions

Dropping of cartoons by freight handlers

Impact loads may be divided into three categories:

Rapidly moving loads of essential constant, as produced by vehicle crossing a bridge

Suddenly applied loads, such as those in an explosion, or from combustion in an engine cylinder

Direct impact loads, as produced by a pile driver, drop forge or vehicle crash

Impact loads may be classified in some categories:

Compressive impact (Driving a nail)

Tensile impact (Starting a movement of a car which is pulling another car)

Torsional impact ( Jamming of a shaft for any reason)

Bending impact ( Falling of an object on a beam)

Combination of these listed above

2.2 Impact Loading in Daily Life

In our life daily we inevitably deal with many impact loading applications. Sometimes we either notice them or not. Impact loading applications in everyday life can sometimes be profitable and helpful (e.g. Driving a nail, pneumatic nailing tool, etc), but usually it is an undesired situation (collision of two automobiles, dropping a mass from a height onto something, load resulting from the blow of a hammer etc.)

http://www.musclecars.faketrix.com/content/crashes-pics/page-2/large/head-on-car-crash-two-vehicles-collide.jpg http://www.dreamstime.com/hammering-nail-thumb4065755.jpg

Picture 2 Collision of two auto mobile Picture 3 driving the nail

Diving board analyse is basic idea in my case. How much force has to give when diver jumps safely. Then only divers know how much deflection made from the diving board and return its original possession. Then divers can protect them self. As well as diving board designers also have to consider about the maximum deflection of the board.

http://craig.backfire.ca/img/diving-boards.png

Picture diving board (http://craig.backfire.ca/pages/autos/cutting-springs)

Here we can assume weight as divers weight (everybody different weight) and height as divers jumps vertical distance from the board.

In this case we can use the plastic material for designing. But plastic materials have relatively low modulus of elasticity as compared with steel or aluminium. This is a disadvantage when the structure requires high rigidity. Often to increase their rigidity reinforcing materials are added, as i mentioned in beam analysing calculations and beam chart (table ). Polyester is thermoplastic with an excellent balance of properties and can be low cost. It is widely used with glass to produce fiberglass components.it is also produced thermoplastic formulation. Polyester fiberglass has

Good impact strength

Good thermal stability

Light weight

Low cost

Good tensile strength

Flex and stress cracking

We can analyse with spring of our impact loading because all materials have some elasticity. Below picture shows there is x distance shows deflection when impact load is applied. Here we can assume F as a suddenly applied load (falling weight).

http://craig.backfire.ca/img/spring-equation.png

Picture Spring original stage and after load applied (http://craig.backfire.ca/pages/autos/cutting-springs)

2.3 Stress

2.4 Deflection

The energy balance approach is easy to extend to impact on the beam a fowling mass. In this case energy to be absorbed is the incoming kinetic energy with the additional work done by the weight acting through the beam deformation. The static deflection of the beam is under the weight. The quantity in the parentheses is the dynamic amplification the factor by which a load is amplified when suddenly imposed. Since the displacement and load are proportional the effective force carried by the beam during impact is the product of the dynamic amplification and the weight. Load suddenly applied from rest produces twice the stress and twice the displacement as the same load gradually applied.

Remember the assumptions

The beam stiffness is the same for static and dynamic loading.

The beam mass is ignored.

Deformation occurs without energy loss. Energy exchanges between kinetic energy of the mass and strain energy of the beam.

Deflection depending on

Force applied

Object geometry

Material properties

The appropriate method of measuring deflection and stress I was analysed different types of instruments they have some disadvantages so finally I was choose strain gauge as measuring instruments. There are many types of sensors available when we select the sensors have to consider about environmental factors, economic factors and sensor characteristics there are

Temperature range

Humidity effect

Corrosion

Size

Power consumption

Cost availability

Life time

Sensitive

Rage

Error

accuracy

According to I am going to analyse three types of sensors.

Linear variable displacement transducer

Dial gauge

Strain gauge

2.4.1 Liner variable displacement transducer

Liner variable displacement transducer used to measure very small displacements in a seismometer that measures movements in the earth's crust due to earthquakes. It consists of a middle primary coil and two outer secondary coils. The magnetic core moves freely without touching bobbins, and at the null (zero) position, it extends halfway into each secondary coil.

Advantages

Infinite resolution

High cycle life

High accuracy

Good in hostile environment

Disadvantage

Can't measure the stress

Require high frequency

Very complicated

2.4.2 Dial gauge

Dial gauge is the instrument used to accurately measure a small distance. The measurements result is displayed in a magnificent way by means of dial. It is available in digital model and analogue model. This used to check the variation in tolerance during the inspection process of machined part, measure the deflection of the beam.

Advantage

More accurate

Low cost

Disadvantage

Can't measure the stress

Not accurate unless mounted to a base

2.4.3 Strain gauge

To measure the instantaneous deflection in this project were used strain gauge. It is an instrument can measure the deflection as well as stress. This has more advantages and dis advantages.

Advantage

It can use in steel

Cost effective

Non corrosive

Sensitive

Small and light

Wide range

Disadvantage

Have to be careful when installing

Expensive processing equipment

Temperature can be effect on sensor

Relatively new technology

More detail about this strain gauge described in chapter 3

2.5 Cantilever Beam deflection

In this research I have chosen cantilever beam as an experimental beam. Below picture shows how the beam will deflect according to impact loading.

When we done impact calculation we made some assumptions there are

The mass of the materials is ignored

The damping of the specimen and friction are neglected

The stiffness of the specimen is same both static and dynamic load

Picture 2 cantilever beam deflection

http://www.advancepipeliner.com/Resources/Others/Beams/Beam_Deflection_Formulae.pdf

Where;

p- Load

L- Length of the beam

h- Height dropped the load

- Maximum deflection of the beam

Equations

description

symbol

unite

Static deflection

mm

Maximum deflection

mm

Height

h

mm

length

L

mm

Moment of inertia

I

mass

m

kg

Table description of symbol

Weight is dropped from a height h onto the free end of a cantilever steel beam. According to that there is a small deflection ᵟx

Here maximum deflection caused by impact load.

Maximum deflection is depending on static deflection and impact factor. Maximum deflection will be product of static deflection and impact factor.

The equation of the static deflection shown below

……………………………………………………………….. [1]

Where;

P- Load

L- Length of the beam

E- Young's modulus

I - Moment of inertia (where; b-width d-depth)

The equation of impact factor shown below

………………………………………………………………… [2]

Where;

K= impact factor

h- Weight dropped from height

- Static deflection

The maximum deflection of the cantilever beam is

..................................................................................................... [3]

Minimum static deflection

……………………………………………………………………….. [4]

Where;

M - Weight

C- Area of the beam ( b*L where b- width L-Legnth)

Moment of inertia

Maximum stress

………………………………………………………………………. [5]

Where;

K- Impact factor

Above equations are used for calculate the instantaneous maximum deflections and maximum stress resulting from the impact loading.

Calculation

3 Design of the testing rig

The intention of this research is to design an impact loading testing rig. Like all other design processes, this design has many criteria, assumptions and limitations.

When I design the rig i have to conceder about following factors

Safety reliability

Cost effectiveness

User friendly

Dimensions of the all machine

Tolerances

Total mass which produce impact loading

Cost

Many components were designed for the testing rig. These components are; falling part, columns, supporting block fitting and connecting plate with them sub-designs. These components are described below.

The accurate dimensions and detailed drawings were not given in this chapter. All the related drawings and tables can be found in Appendix Drawings and Appendix Data Sheets

3.1 Bottom Plate

3.2 Support Blocks

3.3 Rope

3.4 Testing Beam

3.5 Falling Part

3.6 Rulers

3.7 strain gauge

The strain in a material is determined by measuring small displacements that occur when the beam is subjected to an impact load. Bonded metallic strain gages are the most common method used to measure deflection. The electrical resistance of these gages changes when deformed due to impact load. Gauge length is the most important factor to consider when selecting a strain gage because the gauge averages the measured strain over this length. Metallic foil strain gauge (SGD-6/120-LY1130) is shown in Figure.

http://www.sensorland.com/Images/HowPic44-Strain%20Gauge.gif

Picture strain gauge (http://www.circuitstoday.com/strain-gauge)

The below picture is shows arrangement of beam deflection measuring methods. The beam deflection will display on strain gauge indicator. Below beam testing different way of produce the impact load but this will be a basic idea of our testing rig can measure the deflection.

http://www.kostic.niu.edu/Strain-gage.gif

Picture Measuring deflection strain gauge (http://www.kostic.niu.edu/strain_gages.html )

In bonding strain gauges elements to a beam surface, it is important that the gauges experience the same strain as the object. With an adhesive material inserted between the sensors and the beam surface, the installation is sensitive to creep due to degradation of the bond, temperature influences, and hysteresis caused by thermo elastic strain. Because many glues and epoxy resins are prone to creep, it is important to use resins designed specifically for strain gauges.

A bonded metallic strain gauge will not provide accurate measurements if not properly mounted to the test specimen. The adhesive between gauge and specimen is responsible for transferring the strains produced in the specimen to the gauge. It is crucial that the specimen be prepared properly to ensure the adhesive performs its mechanical function.

Strain gages will not perform correctly unless they are properly adhered to a specimen that has received proper surface preparation. To ensure this important phase of the laboratory is completed correctly, a manual entitled, Student Manual for Strain Gage Technology, prepared by the manufacturer of the strain gages, Vishay Measurements Group, Inc., will be provided.

The most important factor is determining strain gauge locations. There are three ways to determine the locations.

Experience

Finite element analysis prediction

Theoretical prediction

Procedure to install the strain gauge

The below picture shows how can place the strain gaugehttp://www.straingage.com/strain_gages/images/straingage.gif

Picture placing the strain gauge on the beam

(http://www.straingage.com/strain_gages/what_strain.html)

Surface Preparation

Cleaning Surface:

Remove grease and oil on the specimen surface by solvent, e.g., Alcohol, Acetone, or some other degreasing agent

Abrading Surface:

Use silicon-carbide paper to sand away uneven surface, paint, or rust and smooth the gauging area.

Marking Layout Lines:

Use a clean rule and a fine pencil (2H or harder) or ball-point pen to draw the layout lines, usually a dash-cross, a cross skip the targeting strain gage area, for alignment.

Conditioning:

Re-clean the gaging area

Neutralizing:

This is an optional step. A proper neutralizer will provide the right pH level at the specimen surface for better bonding with adhesive.

Gauge bonding

Use proper catalyst and adhesive.

Handling and Preparation of Gage -handle gages with tweezers and grip gage at corner, away from gage grid area and solder tabs

Gage Transfer- cellophane tape anchored at one end to box

Apply Catalyst on the gauge surface

Apply adhesive on the part surface

Immediately place thumb over the gauge and apply firm and steady pressure on the gauge for at least one minute

Tape Removal

Lead wire Attachment

Stripping Lead wire-Cut the lead wires to the desired length. Strip off 2 - 3 cm (1 in) insulation for attachment.

Tinning Lead wires: Coat the non-insulated parts of the lead wires with solder.

Tinning Gage: Place the solder on the copper tabs of the gauge.

Attaching Lead wire: Position the non-insulated conductors directly on top of the solder pillow. Press the solder pencil on the conductor and push it into the solder pillow

Removing Rosin: Use solvent to clean the gaging area. Remove the tape attached on the lead wires.

Anchoring Lead wire: Secured the lead wires to the specimen (when possible) by a durable tape

Quarter bridge strain gauge circuit

http://www.sensorland.com/Images/SG-002.gif

Picture strain gauge connected into Wheatstone bridge

(http://www.circuitstoday.com/strain-gauge)

The strain gauge is connected into a Wheatstone bridge circuit with a combination of four a single gauge (quarter bridge). In the quarter circuits, the bridge is completed with precision resistors. Typically, the rheostat arm of the bridge (R2 in the diagram) is set at a value equal to the strain gauge resistance with no force applied. The two ratio arms of the bridge (R1 and R3) are set equal to each other. Thus, with no force applied to the strain gauge, the bridge will be symmetrically balanced and the voltmeter will indicate zero volts, representing zero force on the strain gauge.

4 test procedure

5 Material selections

Stress and Strain are linearly related to each other by Hooke's law. Young's Modulus is a stiffness constant that relates stress and strain, and is a property of the material. The curve shows stress versus strain for different types of materials. Each material has a linear region called the elastic region. The slope of those lines is determined by Young's Modules. As stress increases, the material enters a plastic region, which means that the material will deform and no longer return to its original shape completely when stress is removed. The curves end abruptly when the material breaks. Note that the ceramic material breaks before entering the plastic region, and steel has a higher Young's Modulus than aluminium.

SSCURVE

Graph stress vs strain

Above graph shows clearer about young's modulus about material

In rig design main factor is using materials. So have to consider about the materials selection. The performance of a technical component is limited by the properties of the material of which it is made, and by the shapes to which this material can be formed. Under some circumstances a material can be selected satisfactorily by specifying ranges for individual properties. An example is the young's modulus indices are governed by the design objectives. Component shape is also an important consideration. Beams are lighter than solid ones for the same bending stiffness and beams may be better still. Information about section shape can be included in the performance index to enable simultaneous selection of material and shape. Most of the rig design ductile materials are using.

http://www-g.eng.cam.ac.uk/125/now/mfs/tutorial/non_IE/metals.jpg

Graph Young's modulus vs density graph

Above graph shows young's modulus vs density we can see the ductile materials shows higher young's modulus and good density too. So I have chosen steel material for my development of testing rig. And I also compare with same group materials such as aluminium and brass for more research for the beam design. And further I also compare with one of the composite material to beam design.

Graph strength vs cost http://www-g.eng.cam.ac.uk/125/now/mfs/tutorial/non_IE/strength_generics.jpg

Above graph shows strength compare with cost. Cost factor also one of the major factor when we design or development. So that I choose this graph to described about material selection.

5.1 beam materials

For the beam design I have choose steel as a materials because it's have higher young's modulus. To further I also compare with more materials such as aluminium, brass and composite material which we can use in our testing procedure.

Reason for choosing steel as a material

Advantages

High strength per unite of weight (smaller weight of structure)

More durability

Ability to be rolled into various size and shape

Very high young's modulus

Uniformity

Ductility

Possible reuse and recyclable

Steel will not crack, shrink, splinter, creep, split, warp, or swell.

Steel studs are non-flammable, and won't add fuel to a fire.

User family

Disadvantage

High maintenance cost

Fatigue

Brittle fracture

Advantages for steel compare with aluminium

Low cost

Low co2 emission

5.2 testing rig materials

The same materials using for design the rig.

6 Assembling process

6.1 welding

6.2 bolts and nut

7 simulations express

8 Garnt chart


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