Abstract— In this lab, students learned how to use the Instron Universal Testing Machine to test adhesive materials. The two adhesives that were tested include: cyanoacrylate (abbreviate as CA) and epoxy. Using the data obtained in lab, students were responsible for finding the maximum shear stress/maximum shear strength of each adhesive and constructing a 95% confidence interval for the maximum shear stress of each specimen. Once complete, students compared the shear strength results obtained in lab with those of CA and epoxy adhesives from a credible reference.

Index Terms—Confidence interval, cyanoacrylate, epoxy, shear stress

I. INTRODUCTION

HIS lab was fairly quick and simple. Two double lap shear specimens (one for CA and one for epoxy) were

tested to failure using the Instron Universal testing machine. The test results as well as images of the failed adhesives were posted on canvas for the students to analyze. Images of the failed test specimens were analyzed using ImageJ software so that the areas of the adhered surfaces could be calculated by using the number pixels within the contact area. The area average was calculated for each section and recorded. From this, the shear stress and shear strength of each adhesive can be calculated.

T

Fig. 1. Double lap configuration

Fig. 2. Diagram of shear stress experienced by an adhesive in single lap

II.PROCEDURE

Specimen Testing The first step in this lab was to test the double lap specimens. The specimens were pre-prepared for testing prior to coming to lab so no preparation was necessary. The test were performed using the Instron Universal testing machine. The failed specimens were shown to the class and observations were made on how the specimen failed. The Instron data and images of the failed specimens were uploaded to canvas for students to analyze.

Area estimations using ImageJ Images of the failed surfaces for both specimens were imported into Image J in order to estimate the area of the contact surfaces. This was done by manually drawing around the contact area and seeing the resulting pixel area. A ruler was included in each image in order to accurately find the calibration constant (k) where k:

k=pd

d(1)

Where d is the measured length along the ruler and pd is the number of pixels along the measured length. Once the calibration constant is found, the contact area can be calculated incm2 using the following formula.

A=k2( pa∗pb)(2)

Where pa is the number pixels along length a and pb is the number of pixels measured along length b. The average area found by students in the section was then calculated and uploaded to canvas.

III. RESULTS

TABLE ISTATISTICAL DATA OF BOTH SPECIMENS

Using The Instron Universal Testing Machine to Test Adhesives

Ballingham, RylandSection 3236 3/16/2016

<Section####_Lab#> Double Click to Edit

Average maximum shear

stress (MPa)

Standard deviation of

maximum shear stress (MPa)

Number of samples

Cyanoacrylate 3.18 1.47 12

Epoxy 5.60 2.92 12

TABLE II95% CONFIDENCE INTERVAL OF MAX SHEAR STRESS

Cyanoacrylate(MPa)

High-strength epoxy(MPa)

Aluminum/Aluminum 3.18±0.83 5.60±1.65

IV. DISCUSSION

The above results were calculated using excel and the equations used can be found in the appendix. It appears that epoxy has a higher shear strength than cyanoacrylate. Some factors contributing to the shear strength of the specimen include: preparation of contact area, the adhesive used and configuration of the adhesive joint. The cleaner the contact area is prior to bonding, the larger the shear strength will be. This makes sense as a bonding surface with a lot of contaminates will have a smaller contact area between the adhesive and the metal due to these contaminants causing the shear stress to increase. The type of adhesive is going to affect the shear strength of the specimen, because adhesives have a wide range of material properties including shear strength. For this lab data, epoxy has the higher shear strength. Finally, the configuration of the adhesive joint is going to affect the shear strength of the specimen. For example, a specimen in a double lap configuration is going to have a larger shear strength than one in a single lap configuration. This is because the contact area is doubled for this configuration, causing the shear stress to decrease. The range of shear strength values for Cyanoacrylate is 6.9-13.8 MPa [2]. The average shear strength of cyanoacrylate from the data is 3.18 MPa. The range of shear strength values for epoxy is 10.3-27.6 MPa [2]. The average shear strength from the data is 5.60 MPa. The most likely reasons for the lab data showing a much smaller shear strength for both specimens is that the testing conditions/specimen preparation in a student lab are most likely not as good as in a professional testing setting. In a professional testing setting, specimens are likely prepared much more effectively and thoroughly to clear any contaminates from them. Also, more care is likely taken when applying the adhesives to the specimen in a professional testing setting, causing the applied load to be more uniformly over the bonding area thus increasing the shear strength of the specimen.

V. CONCLUSION

Adhesive joints are becoming more abundant in engineering applications. Adhesive joints offer a ton of advantages to traditional fastening methods. Adhesives remove stress concentrations, reduce weight/design complexity, improve

dissipation of vibrational energy and can allow dissimilar materials to be joined that otherwise couldn’t be using other methods [2]. In this lab, epoxy appears to have a higher shear strength than Cyanoacrylate.

APPENDIX

Uncertainty equations used

U A=(( 2d pa pb

pd2 )

2

(ud )2+( d2 pb

pd2 )

2

(u pa)2+( d2 pa

pd2 )

2

(upb )2−( 2 d pa pb

pd3 )

2

(upd )2)1/2

U P=√( ∂ P∂ m )

2

(Um )2+(∂ P∂ a )

2

(U a )2(4)

U τ=√( ∂ τ∂ P )

2

(UP )2+( ∂ τ∂ A )

2

(U A )2(5)

Statistic equations used for calculations in excel

σ f =√ 1N ∑

i=1

N

( x i− x )2(6)

x=∑ x i

N (7)

x i=μ f ± 1.96σ f

√N

(8)

TABLE III

1

1

<Section####_Lab#> Double Click to Edit

LAB DATA FOR CYANOACRYLATEArea

(cm2 ¿Max load

(kN)Maximum shear stress

(MPa)

7.40 2.90 3.926.14 2.26 3.685.53 1.80 3.26

5.88 1.18 2.00

7.24 1.06 1.47

7.43 2.17 2.92

13.42 1.96 1.46

12.79 3.29 2.58

12.22 1.82 1.49

6.27 2.97 4.73

4.22 2.00 4.74

6.37 3.77 5.91

TABLE IVLAB DATA FOR EPOXY

Area (cm2 ¿

Max load(kN)

Maximum shear stress(MPa)

5.80 4.66 8.04

4.26 2.37 5.55

5.77 2.93 5.07

4.76 4.98 10.46

10.90 8.91 8.18

6.08 5.51 9.09

9.40 1.16 1.23

10.59 2.47 2.33

9.25 2.30 2.49

5.54 2.88 5.20

6.58 2.17 3.29

5.68 3.55 6.26

TABLE VUNCERTAINTY VALUES

Parameter Calculated Uncertainty

U ( A)2 ±102 pixels2

U σ ±0.5 %

U P ±0.25 kN

U τ ±0.20 MPa

REFERENCES

[1] Ridgeway, S. “Lab 4 ImageJ Uncertainty,” EML 3301C Mechanics of Materials Lab – Spring 2016

[2] Shigley, Joseph Edward., and Charles R. Mischke. Mechanical Engineering Design. New York: McGraw-Hill, 1989. Print

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