ME 473 SENIOR DESIGN 2:Biaxial Tension & Shear Testing Apparatus with

Controllable Loads

Group Members:Michael AraucoShahin Mannan

Israel Adejoro

Executive Summary

The objective of this project was to revise an existing in-plane biaxial tension and shear testing apparatus. This devise was designed and manufactured by a group of CCNYstudents in 2007. They were able to build a prototype which was capable of producing a tensile load of 500 lb while providing a rotational displacement up to 20 degrees.

The need for a high torque and low rpm input motor influenced there decision to use two high quality scissor jack power screw design to provide the driving force for the apparatus.

At the onset of receiving this project the prototype was not functional and experiencing a significant amount of bending at the grips. Along with the undesirable effects of bending testing became tedious due to the difficulty in applying the test specimen on the grippers. The controls were rather complex needing a program and matlab code to function. Therefore two of the main objectives was to redesign the grips as well as getting the apparatus functional.

Several options were taken into account in order to fulfill our objectives. A simple solution that did not affect the overall structure of the design was selected. The top jack was dropped approximately 1.5 inches. By doing so, the moment arm on the bottom grips was eliminated completely. The moment arm, on the top grips, was reduced by .75 inches. Along with this reduction both grips were redesigned with a thicker base and longer reach, in order to ease application of the test specimen. The top grips were reinforced with four screws securing it to the top jack more efficiently.

The stepper motors were replaced by DC motors allowing a simpler form of control to be designed. With the application of switches, breadboard, and potentiometer a control was created that could produce biaxial tension simultaneously and independently, as well as provide shearing.

An FEA was conducted on the top grips to verify our assumptions. Results displayed a tremendous reduction of stress around the area of bending. The new controls were tested and proved to work efficiently.

TABLE OF CONTENTS

1. INTRODUCTION…………………………………………….............................4

2. APPLICATION………………………………………………………………….5

3. 2007 PROTOTYPE………………………………………………………………7

4. OBJECTIVES……………………………………………………………………9

5. SOLUTION……………………………………………………………………..11

6. MANUFACTURING…………………………………………………………...18

7. CONCLUSION………………………………………………………………....25

8. FUTURE WORK……………………………………………………………….25

INTRODUCTION

The purpose of this apparatus is to provide biaxial tension and shearing on textile fabrics. The data gained from the test can be manipulated to estimate the Poisson’s ratio, shear modulus, and young’s modulus of various textiles. A textile is a flexible material consisting of natural or artificial fibres often known as thread or yarn. Textiles may be formed in various ways such as weaving, knitting, knotting, braiding, and lacing. For this reason fabrics have there own material property based on the form they were made.

By testing these textiles mechanical properties such as strength and stiffness may be computed. The biaxial load is essential for the measurement of the elastic modulus and a planar shear load for the measurement of the shear modulus.

In this case the modulus of elasticity describes the tendency of the fabric to deform along the axis where the force is being applied. When dealing with fabrics the Ewarp and Eweft are important in determining the properties of the textile. The warp is the set of lengthwise yarns in the axial direction through which the weft is woven. The weft is the yarn which is drawn under and over parallel warp yarns to create a fabric. The figure below shows how the weft is woven through the warp.

Figure 1.Warp and weft in weaving.

The shears modulus describes the fabrics tendency to deform as it experiences the torsional force by rotational displacement.

In a uniaxial test specimen the young’s modulus (E) may be calculated by dividing the tensile stress by the tensile strain.

E = tensile stress = F/Ao = FLo tensile strain ΔL/Lo ΔAoL

Where:

E = young’s modulusF = Force applied by the gripsAo = initial cross sectional area through which force is appliedΔL = Change in the length Lo = Initial length

The shear modulus may be obtained by applying the following equation:

G = E/2(1+v)

Where:

G = Shear modulusv = Poisson’s ratioE = Young’s modulus

APPLICATIONS

To broaden the significance of this apparatus it is important to note that textile fabrics have numerous applications in the aerospace, military, sports, and biomedical industries. Composite materials that are reinforced with textile woven or braided fabrics posses improved out-of-plane stiffness and strength compared to traditional type laminates.

In various military areas there is an increasing need for lightweight materials to be used as portable tents and shelters. In the medical industry inflatable structures made of woven fabrics are used as emergency rooms when responding to disaster sites. Another example can be noted from the aerospace industry where airbags made from vectron fabric were used to design landing gear for spacecrafts.

Figure 2. Textile fabrics used as portable shelters and tents.

Figure 3. Airbag made from vectron fabric.

2007 PROTOTYPE

The objective of the 2007 prototype was to apply an in-plane biaxial tension and shearing device with controllable loads on textile fabrics. The need for high torque at low rotational speeds inspired the decision to use high quality scissor jacks as the driving force for the apparatus.

The prototype consists of two scissor jacks placed parallel to each other. Both jacks expand and contract through the rotation of a power screw. A set of grips were placed between the scissor jacks connected at the vertices. This arrangement provides the biaxial tension on the fabric. Each power screw is connected to a stepper motor which applies the appropriate torque.

The bottom jack lays on a circular wooden disk which is connected to gear system that provides a rotational displacement of 20 degrees.

Figure 4. 2007 prototype

Figure 6. Side view of 2007 prototype. Figure 7.

Design Specifications:

The apparatus must be capable of applying and maintaining a maximum biaxial tension of 500 lb.

While applying biaxial loading, the apparatus must be capable applying a shear loading up to 20° degrees in the clockwise direction.

Must be capable of applying an in-plane biaxial and shear load, both simultaneously and independently.

Must be capable of applying loads without exhibiting damage to any of its components.

Stepper motors for scissor jacksStepper motor for circular disk

Top Grips

OBJECTIVES

1. Getting the apparatus functional

The project was issued in the sense of modifying an existent model, which in this case is a biaxial tensile device. Working with a preexisting model can sometimes prove to be more difficult than starting out fresh with individually thought out ideas. This is because one is limited to a certain degree of freedom when it comes to altering an existing design. Going from the aforementioned, the objectives for this project include steps which will allow the apparatus to be functional and also to improve on testability factors. Functionality is referring to the apparatus’s ability to perform its task appropriately. In this case it is to induce a tensional load on to a textile fabric by stretching it in biaxial directions. This stress is induced on to the textile fabric by the expansion of the scissor jacks. Correspondingly, the driving force which expands the scissor jacks are two stepper motors. These motors however, run by utilizing drivers and software in connection with a computer. See (Figure).

Even though, initially the project came with motors and drivers, the Matlab code which will direct the stepper motors was not available. Basically a protocol system between the driver and the computer was missing. The Matlab code was finally obtained later in the project phase but unfortunately there were missing parts within the code, which corrupted the program. Due to this reason, alternative objectives were established to revive the system to its functional state.

2. Modify grips

The objectives in terms of testability factors included the overall improvement of the system design. During previous testing, when the apparatus was functional, it was noticed that a severe bending moment was created during the application phase. More particularly, the improper design of the grips caused it to bend upwards during the tensional pull of the textile fabric (figure).

Figure 8.

Analyzing from the images above, it can be noticed that the bending moment occurs do to the shape of the grips. It can be generalized, that the shape of the grips from the frontal view is an L shape. One end connects to vice with two screws while the other end is used to lock in the textile fabric. It is the height of the base of the grips, which acts as the moment arm causing this unwanted bending moment. This results in both the top and bottom grips which can lead to unwanted failure of the apparatus during testing. When a force of 500 pounds is used to create tension, this bending moment created can surely play a negative role. Therefore it is one of the primary objectives to redesign and manufacture grips which will reduce the bending moment.

In summary, the objectives are to get the apparatus fully functional, Improve testability by reducing bending moments created in the grips and also to improve other features such as the esthetics of the system as a whole. When the biaxial tension device is fully operational, it should be able to induce significant tension on to the textile fabric without any component of the design failing. Also to obtain results from testing strain gages are to be mounted on the top and lateral sides of the grip. By doing so, an examiner will be able to calculate the young modulus of the test specimen.

SOLUTION

Objective #1: Simplifying controls

In many fields, there is great uncertainty as to whether a new design will actually do what is desired. New designs often have unexpected problems. A prototype is often used as part of the product design process to allow engineers and designers the ability to explore design alternatives, test theories and confirm performance prior to starting production of a new product. Engineers use their experience to tailor the prototype according to the specific unknowns still present in the intended design. For example, some prototypes are used to confirm and verify consumer interest in a proposed design where as other prototypes will attempt to verify the performance or suitability of a specific design approach.

For this project, the final design of another group will serve as the prototype for all the modifications to come. Something that is redesigned requires a different process than something that is designed for the first time. A redesign often includes an evaluation of the existent design and the findings of the redesign needs are often the ones that drive the redesign process. For any engineering design there should be specific orders of process to follow. For this project the following design process will be utilized:

At this point steps 1 and 2 have already been accomplished and it’s now time to brainstorm and generate ideas for possible solutions to the design problem. Primarily, the focus was to make the stepper motors run because this will essentially make the apparatus functional. It would be unnecessary to come up with a complete new system to drive the scissor jacks; therefore a motor based system was the way to go. However, since the integration of stepper motors became an intricate task, it was decided to replace it with DC motors. This was the most efficient possibility because the precision of a stepper motor was not required since it will be turning a threaded rod. A DC motor with sufficient torque will suffice for the operation. In particular a DC motor with low rpm and high torque was desirable for the operation. The following is the image and specs of the DC motor which will be integrated as a design solution.

12 VDC Gearhead DC Motor 13.6 RPM

Torque: 14 lb-in

Approx. 180:1 Ratio

12 Volt ~ 13.6 RPM ~ 0.11 Amp

14 Volt ~ 16.5 RPM ~ 0.125 Amp

24 Volt ~ 26 RPM ~ 0.19 Amp

Dims: 

Motor length: 4.5" 

Mounting Flange" 1.5" x 1.5"

Low Speed Shaft: 0.312" x 1" long

High Speed Shaft: 0.156" x 0.5" long

The solution to getting the apparatus functional does not end simply by replacing the stepper motors to dc motors. Three dc motors are now in place of the stepper motors however it will take much more effort to integrate it to the previous system. Two DC motors will be used to expand and contract the scissor jacks and one dc motor will be used to create a shearing tension. The motor used to create shearing will be placed below the disc, which serves as a base for most of the primary components of the system. The shearing is created by using a gear attached to the motor which will run along a circular teeth track that is mounted with the base of the disc. The disc does not need to rotate more than 20 degrees for application purposes. In order to mount the motor below the disc a special type of mount was manufactured.

The first problem for the other two dc motors was related to the shaft size of the dc motors. In order to insert it to the component which attaches to the threaded rods along with the scissor jacks, the diameter of the shaft for the new motors had to be reduced. Also new mounting brackets, which house the motors, will have to be manufactured. Finally, when the motors are in place and secured, an electrical system will be assembled to control the motors. Usually the integration of dc motors is fairly easy because it powers directly from a voltage source. However, for this project the motors have a bit more functionality than just to turn on and off. It is desirable to the control the motor simultaneously and also independently. This is accomplished by creating a simple circuit with the use of a bread board, and electrical components such as resistors, diodes, terminals, and a potentiometer. All the wires were connected to the terminals and two switches are used to control the spinning direction of the motors. The potentiometer is used to control voltage flow so the motors can spin fast or slow or as desired for application purposes. Once all the wiring was complete it was important to set up a user friendly interface for all the controls to be displayed on. This control dashboard can now be used to manipulate the motors with ease. Below is a schematic of the circuit which was made to serve as a control system for the motors.

To the tackle the objectives relating to the grips, it was unavoidable to manufacture the grips from scratch. This is because the problem resulted from the geometry of the grips itself. It is important to take note that the biaxial stretch has to be performed in the same plane of reference, which all four grips must share. Due to this reason the initial designers created an extended base for the grips so the top and bottom grips align as equally as possible in the same plane. However they failed to realize that the extended base would serve as a moment arm during a tensional pull of the grips.

Objective #2: Modify grips

Solution #1:

Idealy we wanted to solve for the bending by avoiding any tampering of the structure. Therefore solution #1 was to redesign the bottom grips by manufacturing it in a T shape geometry. Figure #8 shows how the bottom grip is connected to the jack by a rectangular aluminum piece which is attached by two screws. In this new design the grip and the

rectangular aluminum attachment would be fabricated as one piece. See the figure below. The T shape form adds extra material on the back of the grip acting as a reinforcement counter acting the bending produced by the tension. The top grips would be redesigned by adding a rib preventing the moment occurring in the counter clockwise direction.

Solution #2:

A better solution for the reduction of bending moment was sought out by realizing that a new plane of operation can by created for the grips.

Figure 8.

By carefully analyzing the scenario, it was noticed that the scissor jacks are connected to an aluminum base which was situated about 4 wooden blocks. The height of wooden blocks determined the plane of operation for the top grips. So by eliminating the wooden blocks it was possible to lower the top scissor jacks along with the grips to a new plane. This in return allowed the reduction of the height for the base of the grips, which was the cause of the bending moment.

Figure 9.

By creating a new plane of operation bending moment was completely eliminated for the bottom grips and significantly reduced for the top grips as seen from the figure above. It was necessary to validate all assumptions and therefore the Finite Element Analysis was conducted for both the old and new grips.

The following are the results of the FEA conducted on the old and new grips respectively for the top grips. No FEA was done on the bottom grips since the moment arm was eliminated completely.

Old Grip:

Figure 10. FEA on old grips

New Grip:

Figure 11. FEA on new top grips.

The FEA was conducted using CosmosWorks, which is a built in FEA manager in Solidworks. The appropriate boundary conditions and restraints were applied and Cosmosworks does the analysis. A restraint, represented by the green arrows, was placed at the back of the vice and a tensile force of 500 lb, represented by the pink arrows, was placed at tip of the grip. Referring back to the results it can be seen that maximum stresses, represented by red, are occurring in the junction between the old grips and the vice. Comparing it to the new grips, there are no red zones at all found in the new design. Comparing the maximum von mises stresses between the old grip and the new grip, it can be seen that the maximum stress occurring was reduced by a whole magnitude.

MANUFACTURING

Most of the manufacturing works for this project were completed using large sized machinery except for the integration of the strain gauges on the fabric grips. The fabrication process ranged from micro-soldering to macro cutting and drilling, which compose of about 90 % of the manufacturing process. Re-fabricating major parts of the biaxial and shearing machine such the fabric grips, motor mounts, and controller dashboard while other parts include provisions and replacements of the items such as the project box, dc motors and strain gauges.

Grip Fabrication

The four grips on the biaxial and shearing machine were fabricated from Aluminum 6061 because it is easier to machine and capable to withstand more than the specified load for this project. Manufacturing the grips began with outlining the Aluminum stock to trace line where the bit of the machines will cut or drill shown in figure a. Figure b and c illustrate some of the fabrication phase (1st and 2nd) in action.

Figure a: Aluminum 6061 outlined for machining. Figure b: 1st phase of grip fabrication

Figure c: 2nd phase of grip fabrication (part cut from 1st phase turned over for further cut).

Figure d: Final grip fabricated consisting of two pairs of top and bottom grips

Old and New Grip Comparison

The prominent difference between the old and new grips is not only their differing ability to minimize bending effect when machine is actively in use as shown by the finite element analysis (FEA) but their distinctive features fostering easier fabric set-up to the fabric grips.

Base

Cover

The new fabric grips are made such that the screws, which tighten the cover to the base, can be maneuvered from the top. Meanwhile, screw maneuvering on the old grips is from the bottom. Screw maneuvering from the bottom is difficult because of the small space (volume capacity barely allow a hand to fit-in) between the head of the screw and the table that supports the machine. Placing the screws on the top eradicates the problem of space shortage.

Moreover, the new grips were made relatively longer than the old ones for easier reach when they are integrated into the biaxial and shearing machine. The old grips were too short that they were barely visible in the machine assembly when viewed from the top. This shortness of the old grips made the top jack hinder easy access to the tightening screws, which tighten the covers to the bases, posing as an obstacle to the rotational movement of the spanner, which aides screw tightening. However, increasing the length of the new grips is also equivalent to increasing the length of the middle rectangular sections of grips, which protrudes the grips and make them visible from the top and reachable.

Figure e: Old grips inside machine assemble and the inconveniences encountered. Old grips are barely protruding and bottom screw positioning is too close to wood platform.

Screw position butfrom the bottom

Top jack too close to tightening screw position, which causes hindrances.

Figure f: New grips in machine assemble clearly visible from the top and the eradication all hindrance problems.

Motor Mount Fabrication

The Motor mounts were manufactured from Aluminum 6061 plates. The two Aluminum plates were bent at 1/3 of their lengths. Holes were made on the bent ends to provide spots for screw passage to tighten motor to the mounts, and other straight ends to tighten to wood for support.

D.C Motor

Three DC motors were purchased to replace the former AC motors, whose connection and application was more complex. The AC motor installation required software such as mat lab, Nidaq, and other server software. Though the AC motors were able to accomplish the force application, which is required by specifications, via applying corresponding torque to the transducer, the programs that runs it pops up error messages frequently more than expect. These continual errors encountered made AC motors usage unreliable for this application. On other hand, DC motors usage is very simple and requires no software for its application.

Hindrances eradicated totally. A good measure of distance between head of screw and top jack.

Figure g: Bottom motor mount and DC motor. Figure h: Top motor mount and DC motor

Figure i: Shearing DC motor and mount

Controller Dashboard and Project Box

The controller dashboard and project box improves aesthetics of the circuitry and complex network of wires running across terminals. The controller dashboard sorted the buttons that controls all the moving parts, and made identifying them easier and possible. On the dashboard are also labels to show the function of each buttons. Moreover, the project helps package the complex network of wire, making it portable and appear sophisticated.

Figure j.Dashboard and project box improve aesthetics of controlling unit and circuitry of the machine.

Resistance-type Strain Gauges Installation

Old strain gauges are replaced with new ones to erase accuracy uncertainties with data generated. Resistance-type strain gauges generate voltage readings that can be converted to strain readings by the strain indicator. The surface areas on the fabric grips, where the strain gauges are to be placed, are polished with different cleansing chemicals to get rid of dirt and leave the surface clear for pure chemical bonding between the adhesive

Top jack motor Button

Bottom Jack motor Button

Shearing motor button

CW and CCW button

Simultaneous and Independent button Potentiometer

Project Box

(epoxy) and metal surface. The thin film of epoxy spread on the metal surface glues the strain gauges to the metal. After the epoxy cures then two wires are soldered to the terminals of the strain gauges for power supply and data generation. Two strain gauges are placed each on two of the fabric grips, one on top and the other on the side.

Figure k. Strain gauges applied to grips.

TESTING

Figure l:

After the new grips were attached and DC motors connected to the controls, we were ready to conduct a test. The strain indicators were giving valid data which was measured in volts. This verified that the strain gauges were applied properly. The new controls proved to work efficiently as it was able to provide a biaxial load both simultaneously

Strain gauges

and independently, while producing shearing. In summary the following objectives were met:

To get apparatus functional

Modify grips by reducing bending and ease application of specimen.

CONCLUSION

Ultimately, the new designed and fabricated parts, in addition to new components installed in the machine will optimize the usage of the machine, making it more reliable, portable, safe, and aesthetically presentable a device for customer’s consumption. Major problems such as bending effects on the grips, old worn-out motor mounts, exhausting AC motor application, old prone to defect strain gauges, poor organizing of controlled units and disperse circuitry are totally checked and remedy with the new modifications made to the machine.

The new top grips have been made longer at the base meanwhile the base of the bottom grips have made flat and flushed with other surface of the grip to reduce bending. Moreover, new fabricated motor mount have replace the old ones to achieve proper interception and alignment of the motor.

Furthermore, new DC motors have replace former stepper motors to ease application by reducing time to fix and run software programs that is required to set it to function. DC motor application in the machine also increases the scope of users that are suitable to use them. Fabrication of the dashboard to hold the control buttons and functioning labels solidly defines the control unit which makes the machine easy to use. Introduction of the project box to house the wire network increases the portability and safe working condition of the machine.

Finally, testing the machine assembly re-iterated a successful design project as the machine works to meet all the specified design requirements as demonstrated in the testing portion of the report.

FUTURE WORK

Even though all objectives were met there were some improvements that could be done.

The DC motor for the shearing seemed to have a relatively high rpm as well as an insufficient amount of torque. Replacing this motor would be recommended.

Appendix A

Engineering Drawing of Manufacture Parts

Figure 1M. Engineering drawing of top grip.

Figure 3M. Engineering drawing of bottom grip.

Figure 2M. Engineering drawing of motor mount.