ia! Bureau fl( Standards NBS TECHNICAL NOTE

480

Misalinement Detector for

Axial Loading Fatigue Machines

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U.S. DEPARTMENT OF COMMERCE

National Bureau of Standards

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UNITED STATES DEPARTMENT OF COMMERCE

Maurice H. Stans, Secretary

NATIONAL BUREAU OF STANDARDS • A. V. Astin, Director

NBS TECHNICAL NOTE 480

ISSUED APRIL 1969

Misalinement Detector for

Axial Loading Fatigue Machines

Donald C. Robinson

Engineering Mechanics Section

Mechanics Division

Institute for Basic Standards

National Bureau of Standards

Washington, D.C. 20234

NBS Technical Notes are designed to supplement the Bureau's regular publications program. They provide a means for making available scientific data that are of transient or limited interest. Technical Notes may be

listed or referred to in the open literature.

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Washington, D.C, 20402 - Price 25 cents
Contents

Page

Abstract ------------------------ 1

1. Introduction -------------------- 1

2. Misalinement Detector Design ------------ 2

3. Installation -------------------- 3

4. Measurement Procedure ---------------- 3

5. Sensitivity --------------------- 4

6. Results and Discussion --------------- 4

7. Conclusions --------------------- 5

8. References --------------------- 6

II
MISALINEMENT DETECTOR FOR AXIAL LOADING FATIGUE MACHINES

by Donald C. Robinson

ABSTRACT

A strain gage device for measuring changes

in alinement between the test fixtures of axial

loading fatigue machines is described. Variations in fixture orientation are determined by

measurement of the bending induced in a compliant

beam clamped in parallel mounts attached to the

test fixtures. Changes in bending with applied

tension loads are an indication of alterations

in the alinement between the load transmitting

members of a machine. An approximately linear

relationship was found to exist between changes

in bending determined by the detector and the

fatigue life of high strength fasteners for various levels of misalinement induced in a fatigue

machine

.

Key words: Axial loading, fatigue machine,

misalinement, test fixtures.

1. Introduction

In order for a fatigue testing machine to give significant results,

it must produce the desired stress distribution within the specimen,*

Fatigue lives of high strength metal fasteners, for example, have shown

large variations in tension- tension tests due to the presence of undesired bending loads* » For the quality control of such products, it

is necessary to detect undesired stresses due to misalinement of the

testing machine in order to determine when variations in the test

results are due to an inferior lot rather than due to the machine „ This

note describes the development of a device for detecting such misalinement o

Various methods have been used by other investigators for

minimizing misalinement in a fatigue machine. Some require precision

machining of specimens and the machine test f ixturesLl J**,, Other

Personal Communications with the Engineering Staff of the Precision

Fastener Division, Standard Pressed Steel Company, Jenkintown,

L^ Pennsylvania.

Numbers in brackets indicate the literature references at the end

of this paper „
alining procedures have been developed in which a misalinement indicating device is installed in a machine in place of, or in series with, a specimen . This technique allows the possibility of a change in

alinement when the device is removed. The device described in this

report is able to detect misalinement of the load transmitting members

of an axial- loading machine for the range of loads applied during a tens ion- tens ion test while a specimen remains in place. Changes in the

alinement of the machine are indicated by bending induced in the

detector which is attached to the machine test fixtures.

2. Misalinement Detector Design

A schematic of the misalinement detector is shown in Figure 1.

Its principal components are a beam, a flexure, and fittings for

attaching the detector to fatigue test fixtures. The beam was con- structed from an 11 in. (279.4 mm) x 1 in. (25.4 mm) x 3/16 in„ (4.8

mm) aluminum bar. The center portion of this member was reduced to

approximately 0.625 x 0.016 in. (15.9 x 0.4 mm). A four-arm, temperature compensated bridge circuit constructed of foil-type, resistance

strain gages was bonded to the reduced section. It was connected to measure bending and to cancel axial strain components.

The purpose of the flexure was to transmit the effects of bending

due to misalined test fixtures to the gaged section of the beam. It was required that the flexure have a low resistance to axial motion and

also provide for a connection to the test fixture surfaces having high

stiffness in the horizontal direction. A leaf flexure, whose basic

operating principle is given by Jones and YounglSjj was employed. A

cross section through the flexure is shown in Figure 1. The flexure

leaves were constructed from 0o005 in. (0.13 mm) brass shims arranged

to provide parallel motion of a rectangular element having a 0.281

in. (7.1 mm ) surface area which was in contact with the surface of

the beam. Polytetrafluoroethylene tape was applied to each of these

surfaces and to the end of the beam to minimize the friction due to

sliding.

The flexure was used to clamp the upper end of the beam to a steel

fitting, and the lower end of the beam was bolted to another fitting*

Both fittings were clamped to the circumference of the test fixtures by means of an adjustable metal clamp. In order to minimize the misalinement between the pair of fittings themselves, they were fabricated from

one piece of material and cut along a plane perpendicular to the mounting surfaces during the last machining operation. Although these

fittings were designed for cylindrical test fixtures, fittings for

attaching to other fixtures are feasible.

In order to protect the compliant gaged section of the beam from

being damaged during assembly, cover plates which attach to the beam

by screws completely enclosed the reduction section. It was necessary

to loosen these plates when the detector was being used in order for
bending of the beam due to misalinement between its mounting surfaces

to be effectively transmitted to the strain gages.

In order to obtain an absolute determination of misalinement, the

test fixtures could be compared to a precision alined surface external

to a testing machine. Due to the beam configuration and gage arrangementj it is difficult to prevent the beam from transmitting undesired

torsional strains to the compliant gaged section which would be

misinterpreted as due to bending. This device measures bending in

only one plane. In this investigation, care was taken to aline this

plane with the plane of bending

»

3 o Installation

To minimize any misalinement between the fittings which attach

the beam to the machine test fixtures, a uniform steel fitting alinement bar, having the same nominal size as the gaged beam, is first

clamped to the fittings. The adjustable clamps which fasten the

fittings to the test fixtures are then tightened, the alinement bar

is removed and the gaged beam is installed,.

One end of the beam is fastened to the upper fitting by means of

the flexure, and its other end is fastened to the lower fitting by

screws, as shown in Figure 1. To insure that the bearing surface of

the flexure is in uniform contact with the beam, the upper end of the

beam is first connected, the screws attaching the plates covering the

gaged section are loosened and the lower end of the member is then

tightened.

4. Measurement Procedure

The bending determined by the detector, which is an indication

of the bending transmitted between the fatigue test fixtures to the

specimen, is measured using a strain indicator. The procedure for

estimating the change in alinement between test fixtures over the

range of loads applied during a tens ion- tens ion fatigue test is as follows

:

1. Apply a static tension load to a specimen corresponding to the mean cyclic load.

2. Balance the strain gage bridge.

3. Reduce the load to a value corresponding to the

minimum cyclic load, rebalance the bridge, and re- cord the indicated strain.

4. Remove the misalinement detector before beginning

a fatigue test.
5. Sensitivity

A calibration of the misalinement detector which was used to

estimate its sensitivity was determined by recording the bending at

its gaged section induced by various size shims placed under one of

the attachment fittings while the detector was clamped to a uniform

cylindrical surface. The calibration curve shown in Figure 2 was determined by placing shims under one fitting located approximately

5.0 in. (127 mm) from the gaged section while the other end of the

beam was clamped „ From these data, it is estimated that the detector

is capable of determining bending less than one percent of the axial

strain experienced by a 1/4 in. (6 mm) diameter specimen when subjected to a tension load. When the detector is attached to the test

fixtures of a machine, its sensitivity is also influenced by the

rigidity of the machine.

6. Results and Discussion

In order to verify that the bending determined by the detector

was representative of the bending experienced by a specimen, a strain

gage bridge circuit for measuring bending was bonded to the surface

of a 1/4 in„ (6 mm) diameter bolt a A comparison of the bending in a bolt with the bending determined by the misalinement detector, shown

in Figure 3, was found by attaching the detector to 3 in. (76 mm) diameter test fixtures installed in a 5,000 lbf (22,000 N) capacity,

hydraulic, axial loading fatigue machine. The data plotted in Figure

3 shows two regions, represented by dashed and solid lines. The

portion of the curves which are dashed indicates a region where the

detector measured bending before the bolt was tightly seated in the

test fixtures. After the fastener was seated, the region represented

by solid lines, the coefficient of correlation for the two sets of

data was 0.95. This indicates that the bending experienced by

the two correlate linearly.

Another investigation was made in which misalinement was induced

in a fatigue machine with beveled washers „ The washers, whose flat

surfaces were ground non-parallel, were placed under the bolt heads

of 1/4 in. (6 mm) diameter steel fasteners. A 24,000 lbf (107,000 N)

capacity, electromechanical, axial" loading machine was used. The test

fixtures were 2 1/2 in. (64 mm) in diameter.

The values of bending induced by washers with various angles at

tension loads of 2,000 lbf (8,900 N), 1,000 lbf (4,450 N) and 200 lbf

(890 N) are given in Table 1. The results of fatigue tests on the same bolts are given in Table 2. These fasteners have an assembly

minimum tensile strength of 260,000 lbf/in. 2

(1, 790 MN/m2

)

, and were known to be particularly sensitive to bending loads when subjected

to tension-tension fatigue tests. The curves in Figure 4 give the

comparison of the data from Table 1 for a tension load of 200 lbf

(890 N) and the average fatigue life of the bolts. The bending was measured at 200 lbf (890 N) to determine whether the test fixture
alinement changed significantly below the minimum load applied during

the fatigue tests. The data did not indicate any large changes in

fixture alinement for the two loads within the range of the fatigue

test and the alinement for the load below the test range. The data

for a tension load of 200 lbf (890 N) was used for comparison with

the average fatigue life of the bolts since the larger scatter in

the measurements at 2,000 lbf (8,900 N) and 1,000 lbf (4,450 N)

suggests that the orientation of the larger washers below the bolt

heads was more critical at the larger loads and this orientation was not closely controlled. The questionable data are identified in

Table 1.

The values for the fatigue life of the 1/4 in. (6 mm) diameter

bolts and the bending determined by the misalinement detector when

the fixtures were alined (0 degree washer) were most affected by the

smallest angularity induced by the beveled washers, 0.18 degree, with

relatively smaller changes being observed for larger angularities up

to 1.1 degrees. The magnitude of the coefficient of correlation!- 3 J between the bending determined at 200 lbf (890 N) and the fatigue life

for various washers was approximately 0.9; in Figure 5 is shown a direct comparison of the bending at this load and the average fatigue

life of the bolts.

7. Conclusions

The device described in this report utilizes bending induced in a compliant beam to detect changes in the alLnement of the test fixtures

of an axial loading fatigue machine for a range of loads applied during

a tens ion- tens ion test. The changes in bending with applied load are an indication of alterations in the alLnement of the load transmitting

members, which should be kept to a minimum to prevent excessive bending loads from being applied to a specimen.

Based on the experimental measurements, the relationship between

the bending determined by the detector due to misalined test fixtures

and bending experienced by a specimen installed in the fixtures was linear. An approximately linear relationship was found to exist between the bending determined by the detector and the fatigue life of

high strength bolts for various levels of misalinement induced in a fatigue machine

.

Mr. R. Snyder assisted in the construction of the misalinement

detector. Mr. T. Baumgartner, Engineering Manager of the Precision

Fastener Division, Standard Pressed Steel Company, Jenkintown, Pennsylvania, arranged for tests to be conducted on fatigue equipment at

the Jenkintown plant.
The project which led to this report was conducted under the

sponsorship and with the financial assistance of the Naval Air Systems

Command, Department of the Navy.

8. References

1. Weibull, W.j Fatigue Testing and Analysis of Results, Pergamon

Press (1961).

2. Jones, R. V. and Young, I. R., Some Parasitic Deflexions in

Parallel Spring Movements, J. Sci. Instr., 3_3, p. 11-15 (Jan.

1956).

3. Li, J. C. R., Statistical Inference, Vol. I, p. 301, Edwards

Brothers, Inc., (1964).
Table 1 - Strain Indicated by the Misalinement Detector Due To

Beveled Washers Installed in an Electromechanical

Testing Machine ( a )

Applied tension loads

Angle of 2000 lbf 1000 lbf 200 lbf

beveled washer (8900 N) (4450 N) (890 N)

deg in./in» in. /in. in. /in.

0.18

0,41

0.62

0.93

1,08

(a) These values represent changes in bending found at a load

of 2500 lbf (11,120 N) and the indicated load.

Questionable data due to orientation of beveled washers.

58 x 10"•6 69 x 10-6 85 x 10-6

144 160 166

176 175 177

149 O) 162(b) 176

228

(

b) 223

(

b) 180

238 (b) 242< b > 215
Table 2 - Effect of Angularity of Bolt Head Seat on the Fatigue

Life of 1/4 inch (6 mm) Diameter Fasteners. Tension- , v Tension Load Cycles of 505 to 5050 lbf (2250 to 22,500 N) W

Angle of

beveled washer

Cycles

to failure

Average

fatigue life

Average reduction

in fatigue life

deg cycles percent

126,

78 :

,000

,000

102,000

0.,18 53,

39.

85,

73,

,000

,000

,000

,000

62,500 39

0,,41 66,

34 : 28,

43

.

,000

,000

,000

,000

42,750 58

0<,62 45,

46

.

36.

42,

,000

,000

,000

,000

42,250 59

0,,93 56,

42 :

,000

,000

49,000 52

1,,08 44

.

32 :

,000

,000

38,000 63

(a.)Axial stress levels corresponding to this loading were 10,300 to

103,000 lbf/ in. 2

(71 to 710MN/m2

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