GE Bolt Mike III Operating Manual

Guide to Ultrasonic Inspection

of Fasteners

Part No. 021-002-175

Rev . B

50 Industrial Park Road Lewistown, PA 17044

Phone (866)243-2638 Fax (717) 242-2606 www.stresstel.com

Guide to Ultrasonic

Inspection of Fasteners

Important Notice

The following information must be read and understood by any user of a StressTel measurement instrument. Failure to follow these instructions can lead to errors in stress measurements or other test results. Decisions based on erroneous results can, in turn, lead to property damage, personal injury or death. StressT el assumes no responsibility for the improper or incorrect use of this instrument.

General Warnings

Proper use of ultrasonic test equipment requires three essential elements:

• Selection of the correct test equipment

• Knowledge of the specific “test application require-

ments”

• Training on the part of the instrument operator

This operating manual provides instruction in the basic set-up and operation of the StressTel BoltMike III measurement instrument. There are, however, additional factors which affect the use of ultrasonic test equipment. Specific information regarding these additional factors is beyond the scope of this manual. The operator should refer to textbooks on the subject of ultrasonic testing for more detailed information.

Operator Training

Read the information in this manual prior to use of a StressT el instrument. Failure to read and understand the following information could cause errors to occur during use of the instrument. Failure to follow these instructions can lead to error in stress measurement or other test results. Decisions based on erroneous results can, in turn, lead to property damage, personal injury or death.

Operators must receive adequate training before using ultrasonic test equipment. Operators must be trained in general ultrasonic testing procedures and in the set-up required before conducting a particular test. Operators must understand:

• Soundwave propagation theory

• Effects of the velocity at which sound moves

through the test material

More specific information about operator training, qualification, certification and test specifications is available from various technical societies, industry groups, and government agencies.

Testing Limitations

Information collected as a result of ultrasonic testing represents only the condition of test-piece material that is exposed to the sound beam. Operators must exercise great caution in making inferences about the test material not directly exposed to the instrument’s sound beam. When a less-then-complete inspection is to be performed, the operator must be shown the specific areas to inspect. Inferences about the condition of areas not inspected, based on data from evaluated areas, should only be attempted by personnel fully trained in applicable techniques of statistical analysis.

Sound beams reflect from the first interior surface encountered. Operators must take steps to ensure that the entire thickness of the test material is being examined.

Calibrating the instrument/transducer combination is particularly important when the test piece is being ultrasonically tested for the first time or in any case where the history of the test piece is unknown.

Transducer Selection

The transducer used in testing must be in good condition without noticeable wear of its contact surface. Badly worn transducers will have a reduced effective measuring range. The temperature of the material to be tested must be within the transducer’s temperature range. If the transducer shows any signs of wear it should be replaced.

• Soundwave propagation theory

• Effects of the velocity at which sound moves

through the test material

• Behavior of the sound wave

• Which areas are covered by the sound beam

More specific information about operator training, qualification, certification and test specifications is available from various technical societies, industry groups, and government agencies.

• Behavior of the sound wave

• Which areas are covered by the sound beam

Guide to Ultrasonic Inspection of Fasteners Page iii

Important Notice

Testing Limitations

Sound beams reflect from the first interior surface encountered. Operators must take steps to ensure that the entire thickness of the test material is being examined.

Transducer Selection

Page iv Guide to Ultrasonic Inspection of Fasteners

Contents

Important Notice

Chapter 1: Ultrasonic Measurement of

Fasteners................................................................. 1

1.1 Important Concepts....................................... 1

1.1.1 Acoustic Velocity ................................. 1

1.1.2 The Use of Ultrasound ........................ 1

1.1.3 Initial Pulse and Multi-Echo

Measurement Modes .......................... 2

1.1.4 Time of Flight and Ultrasonic Length .. 2

1.1.5 Tensile Load........................................ 3

1.1.6 Stress .................................................. 4

1.1.7 Elongation ........................................... 4

1.1.8 Modulus of Elasticity (Eo) ................... 4

1.1.9 Stress Factor (K) ................................ 5

1.1.10 Temperature Coefficient (Cp) ............6

1.1.11 Calibration-Group Correction

Factors — Stress Ratio and Offset .... 6

1.1.12 Fastener Geometry ........................... 6

1.2 Principles of BoltMike Operation................... 7

1.3 Practical Limitations Of Ultrasonic

Measurement ................................................ 8

1.3.1 Material Compatible with Ultrasonic

Inspection............................................ 8

1.3.2 Significant Fastener Stretch ............... 8

1.3.3 Fastener End-Surface Configuration . 9

1.3.4 The Limitations of I.P. and M.E.

Measurement Modes .......................... 9

Chapter 4: Temperature Compensation .......... 17

4.1 Measuring Fastener Temperature .............. 1 7

4.2 Limits of Accurate Temperature

Measurement .............................................. 17

4.3 Adjusting the Temperature Coefficient ....... 1 8

Chapter 5: Selecting Phase ............................... 19

Chapter 6: Fastener Geometry.......................... 21

6.1 Approximate Length .................................... 21

6.2 Determining Effective Length...................... 21

6.3 Fastener Cross-Sectional Area .................. 2 4

Chapter 7: Material Constants .......................... 25

7.1 Standard Material Constants ...................... 25

7.2 Custom Material Constants......................... 2 5

7.3 Selecting a Material Constant..................... 25

7.4 Material Variations....................................... 26

Chapter 8: BoltMike Formulas........................... 27

Appendix: Tabular Data ....................................... 29

Chapter 2: Fastener Preparation ...................... 11

2.1 Fastener End-Surface Machining ............... 11

2.2 Methods Of Transducer Placement ............ 12

2.2.1 Practical Methods .............................12

2.2.2 Fixtures for Non-Magnetic Fasteners14

Chapter 3: Transducer Selection ...................... 15

3.1 General Acceptability .................................. 1 5

3.2 Transducer Frequency ............................... 15

3.3 Transducer Diameter .................................. 15

Purpose of Instrument and Transducer

Zeroing ........................................................ 15

Guide to Ultrasonic Inspection of Fasteners Page v

Important Notice

Page vi Guide to Ultrasonic Inspection of Fasteners

Chapter 1: Ultrasonic Measurement of Fasteners

When threaded fastening systems (comprised of a bolt or stud and a nut) are tightened, the threaded fastener is said to be tensioned. The tensioning force in the fastener (identified in the BoltMike as its load) is equal to the fastening system’s clamping force.

The BoltMike determines the load on a fastener by measuring the amount of time it takes for a sound wave to travel along a fastener’s length, before and after a tensioning force is applied to the fastener. The fastener material’s acoustic velocity, together with difference in the measured times, allows the instrument to calculate the change in fastener length under the tensile load. Provided the fastener’s dimensional and material properties are known, and the constants that represent the material properties are entered into the instrument, the BoltMike will calculate the load and stress present when the fastener is in its tensioned state.

1.1 Important Concepts

To best understand exactly how ultrasonic sound waves are used to determine loads, stress, and elongation of threaded-fasteners, it is necessary that you understand the concepts described in this section. Chapter 8 lists the actual formulas used by the BoltMike to calculate many of the quantities described below.

1.1.1 Acoustic Velocity

Applying a large electric pulse to a piezoelectric element in a transducer creates an ultrasonic shock wave. This type of shock wave, known as longitudinal wave, travels through a fastener at a speed equal to the fastener material’s acoustic velocity. A material’s acoustic velocity represents the speed with which sound moves through it. All materials have a representative acoustic velocity but true velocity can vary from one sample to another (of the same material type) and even throughout the material in a particular sample. It is important to realize that the actual acoustic velocity is not truly a constant. Instead, it varies between fasteners of like material, even when the fastener’s material composition is tightly controlled.

1.1.2 The Use of Ultrasound

The ultrasonic wave is transmitted from a transducer into the end of a fastener . When the ultrasonic wave encounters an abrupt change in density, such as the end of the fastener, most of the wave reflects. This reflection travels back the length of the fastener and back into the transducer. When the shock wave re-enters the piezoelectric element a small electrical signal is produced. This signal is represented on the BoltMike’s display panel by the triggering of a measurement gate. This signal is used by the BoltMike to indicate the returning wave. (Figure 1-1)

FIGURE 1-1—The BoltMike determines the length of a fastener by measuring how long it takes for sound to travel its length.

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Chapter 1: Ultrasonic Measurement of Fasteners

1.1.3 Initial Pulse and Multi-Echo Measurement Modes

The BoltMike III can be operated in one of two ultrasonic measurement modes: initial pulse (I.P.) and multi-echo (M.E.). In I.P . mode, as illustrated in Figure 1-2A, a sound pulse is sent through the fastener. The BoltMike’s triggering gate is positioned (based on the userinputted value of the fastener’s approximate length) to detect this sound pulse’s first returning echo. The BoltMike measures the time duration between transmitting and receiving the sound pulse, and uses this value as the basis for its calculations.

In M.E. measurement mode, a sound pulse is again transmitted into the fastener. This time, however, the BoltMike utilizes two triggering gates. These gates are positioned so that the first returning echo triggers the first gate, and the second returning echo triggers the second gate. The gates are again positioned based on the user-in-

putted value of the fastener’s approximate length. In this mode the BoltMike measures the time duration between triggering of the two gates by two consecutive echoes. It is critical, however, that similar features on the two consecutive packets be used to trigger the gates.

An advantage of operating in M.E. mode is that all measurements are taken between the first and second returning echoes. This means that variations in transducerto-fastener coupling (caused, for instance, by varying couplant thickness) and instrument zeroing are factored out of the BoltMike’s measurement. This is shown in Figure 1-2B.

1.1.4 Time of Flight and Ultrasonic Length

The elapsed time between transmitting and receiving the shock wave is known as the sound-path duration. Of course, as shown in Figure 1-1, the sound-path duration actually represents the elapsed time taken by the

FIGURE 1-2—In Initial Pulse (I.P.) mode, the BoltMike measures the time to the first gate triggering. In Multi-Echo mode the time between two consecutive gate crossings is measured.

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Chapter 1: Ultrasonic Measurement of Fasteners

wave to travel the length of the fastener two times. This duration is divided by two to find the time of flight (TOF), which represents the time it takes for the shock wave to travel once down the length of the fastener. The BoltMike then determines the

ultrasonic length

by first correcting the measured TOF for any changes in temperature, and then multiplying by the fastener’s acoustic velocity . Acoustic velocity is represented in the BoltMike with the variable V and is determined by the fastener’s material type). Further corrections (as described below) are then made to this ultrasonic length to determine a measured physical length.

Because the actual acoustic velocity is not truly a constant, the uncorrected ultrasonic length is not exactly the same as the physically measured length. Even if two identical fasteners’ physical lengths are very tightly controlled, the measured time of flight through each fastener may vary by as much as one percent. Because of

this variability, the

change

in measured time of flight (recorded before and after each fastener is tensioned) must be used to accurately determine the tensile stress in a fastener. As you will learn shortly, acoustic velocity also varies with factors other than material type including stress (sections 1.1.9) and temperature (section 1.1.10). For this reason the BoltMike incorporates logic to compensate for these effects on ultrasonic length.

1.1.5 Tensile Load

As you may be aware, when the nut in a threaded fastening system is tightened, the clamping force the fastening system (nut and bolt or stud) places on the joint is equal to the tensile load placed on the fastener. This effect is shown in Figure 1-3. The BoltMike calculates Load (L) by first determining tensile stress (as described below), then multiplying by the fastener’s cross-sectional area.

FIGURE 1-3—As the threaded fastening system is tightened, tensile loads are applied to the bolt or stud and elongation occurs.

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Chapter 1: Ultrasonic Measurement of Fasteners

1.1.6 Stress

Stress occurs when load is applied to a fastener. When a tensile load (like the one shown in Figure 1-3) is applied to a fastener, the tensile stress is equal to the tensile load divided by the fastener’s average cross-sectional area (see the Appendix for average cross-sectional areas). The BoltMike calculates tensile stress in units of pounds per square inch (psi) or mega Pascal (MPa). This calculation is performed using the change in ultrasonic length, the effective length, acoustic velocity (described in section 1.1.1), the material’s stress factor (a property that is described below), and stress compensation parameters known as Stress Ratio and Stress Offset. These are instrument correction parameters that are described in section 1.1.11.

1.1.7 Elongation

As a tensile load is applied, a fastener stretches in the same way a spring would. The amount of stretch, known

elongation

, is proportional to the tensile load as long as the load is within the fastener’s working range (which means at loads that are less than the fastener’s yield

strength – a term we’ll describe shortly). Using the effective length, the material’s modulus of elasticity, and the calculated value for corrected stress the BoltMike calculates elongation. (Figure 1-3)

1.1.8 Modulus of Elasticity (Eo)

When a fastener is loaded with a tensile force, its length increases. As long as the loading does not approach the fastener’s

yield strength

(defined as the loading point beyond which any change in material shape is not completely reversible), the relationship between the tensile stress and elongation is linear. By this we mean that if the stress level increases by a factor of two, the amount of elongation also increases by a factor of two. For load levels in the fastener’s elastic region (meaning that the loads are less than the yield strength of the fastener), the relationship between stress and elongation is described by a material constant known as the

elasticity

. The variable Eo in the BoltMike represents the

modulus of

modulus of elasticity . The concepts of tensile stress, elongation, modulus of elasticity, and yield strength are illustrated in Figure 1-4.

FIGURE 1-4—This graph shows the relationship between tensile stress and elongation in a fastener. The material’s modulus of elasticity equals the slope of the straight portion of this curve (this area is known as the material’s elastic region). The point at the top of the curve, where it is no longer linear, represents the material’s yield strength. Note that the graph actually plots stress verses strain. Strain is simply the amount of elongation, divided by the original length of the stressed section.

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Chapter 1: Ultrasonic Measurement of Fasteners

1.1.9 Stress Factor (K)

The velocity at which a longitudinal wave moves through an object is affected by stress. When a fastener is stretched there are two influences on its ultrasonic length (as determined by multiplying the sound wave’s time of flight by the constant value of acoustic velocity). First, the length of material through which the sound must travel increases. Also, the fastener’s actual acoustic velocity decreases as stress increases. In other words, even when the stretching effect on the fastener’s physical length is ignored, tensile stress leads to an increase in the fastener’s ultrasonic length. In the BoltMike, a material constant known as the

Stress Factor (K)

compensates for the effect stress has on the fastener’s actual acoustic velocity.

A great deal of confusion surrounds this effect. Consider the example shown in Figure 1-5 as you read the following description. In Figure 1-5A, no load is applied to the fastener when the reference ultrasonic length (UL1) is recorded. In Figure 1-5B, a load is applied and a new ultrasonic length (UL2) is recorded. Note that Figure 1-5A and B also identify the physical length when unloaded (Physical Length 1) and loaded (Physical Length 2). The actual physical elongation of the fastener equals Physical Length 1 – Physical Length 2. The difference between the ultrasonic lengths (UL1 and UL2) is about three times the actual physical elongation of the fastener.

FIGURE 1-5—Applied tensile stress affects the ultrasonic (measured) length of a fastener in two ways. First, it stretches the fastener, thus increasing the actual length. Second, tensile stress reduces the fastener’s acoustic velocity, further increasing its ultrasonic length. In the BoltMike, the material constant K (stress factor) is used to compensate for the effect of tensile stress on acoustic velocity.

Guide to Ultrasonic Inspection of Fasteners Page 5

Chapter 1: Ultrasonic Measurement of Fasteners

It is important to note that in order to change the acoustic velocity, stress must be applied in the same direction traveled by the ultrasonic shock wave. Thus shear and torsional stress have no effect on the acoustic velocity when measured along the fastener’s length.

1.1.10 Temperature Coefficient (Cp)

The temperature of a fastener affects its physical length. As the temperature of a fastener increases, its physical length increases. In addition, as a fastener’s temperature increases the amount of time it takes for sound to travel through the fastener also increases. In other words, when a fastener is subjected to increased temperature, its acoustic velocity decreases and, therefore, its ultrasonic length increases. In fact, temperature’s affect on ultrasonic length is even greater than its affect on physical length. The thermal expansion of the fastener and the ultrasonic velocity change with temperature are two separate effects. However, for the purpose of the BoltMike they are compensated for with a single combined factor known as the

Temperature Coefficient (Cp)

The Bolt Mike relies on a temperature compensation system to normalize the measured time of flight (TOF) and thus correct for temperature-caused changes in its physical and ultrasonic length. The compensation system normalizes the TOF to the value expected at 72 degrees Fahrenheit (22 degrees C) before attempting to calculate the fastener’s stress, load, and elongation. This compensation greatly improves accuracy when the temperature has changed during tightening.

1.1.11 Calibration-Group Correction Factors —

Stress Ratio and Offset

The accuracy of the BoltMike’s stress, load, and elongation calculations depends on many factors. Two major influences on the accuracy of these calculations are the material-property constants inputted and the fastener’s geometric characteristics.

While the material-property constants (including elasticity, acoustic velocity, and stress factor) are considered to be standard values, actual material properties vary widely. This variation is even found among fasteners produced in the same manufacturer’s lot. The BoltMike’s accuracy depends partly on the difference between the fastener’s actual material properties and those properties represented by the standard material constants. Similarly, variations in fastening systems’ physical characteristics affect the accuracy of load and elongation calculations.

When BoltMike III users desire to calculate load, elongation, stress, or TOF (time of flight) values with a higher degree of accuracy, they generally choose to create calibration groups. During the process of creating a calibration group, the BoltMike uses inputted values of actual tensile load, as well as its own measured load data, to calculate two correction factors: Stress Ratio and Stress Offset. These correction factors are used to convert the BoltMike’s raw stress value into a corrected stress, as shown in Chapter 8 of this guide.

The BoltMike uses one of two methods to determine these correction factors. The first method, called a regression correlation, uses a linear regression technique to determine the stress factor and offset. (Figure 1-6) The stress factor is actually the slope of a line that represents the relationship between actual and calculated load. The stress offset represents the Y intercept of the actual verses calculated load line. This value can be thought of as the level to which actual load can increase before the BoltMike can measure an observable load.

The second method used to determine correction factors is known as vector correlation. With this approach the BoltMike calculates only a stress ratio. The value of the stress offset is set to zero. (Figure 1-6)

When creating a calibration group, the user must decide which correction method to use. This decision should be based on the application. If accuracy over a wide range of loads (including low-level loads) is desirable, the vector correction is usually preferred. If the highest level of accuracy at a single target load is desired, the regression method is best.

Why are two methods required? Often the relationship between actual and measured stress is non-linear, especially at the low end of the curve (as shown in Figure 1-6). This can be caused by a skin effect. When a small amount of load is applied to a fastener, most of the stress is in the surface layers, not evenly distributed across the cross-section. Since the longitudinal wave travels predominantly down the center of the fastener, less of the actual stress is observed.

1.1.12 Fastener Geometry

Several geometrical characteristics of fasteners affect the ultrasonic measurement of load, stress, and elongation. While these characteristics are described in great detail in Chapter 6 and the Appendix, Figure 1-7 briefly illustrates them.

Page 6 Guide to Ultrasonic Inspection of Fasteners

+ 32 hidden pages

GE Bolt Mike III Operating Manual

Specifications and Main Features

Frequently Asked Questions

User Manual

Chapter 1: Ultrasonic Measurement of Fasteners

1.1 Important Concepts

1.1.1 Acoustic Velocity

1.1.2 The Use of Ultrasound

1.1.3 Initial Pulse and Multi-Echo Measurement Modes

1.1.4 Time of Flight and Ultrasonic Length

1.1.5 Tensile Load

1.1.6 Stress

1.1.7 Elongation

1.1.8 Modulus of Elasticity (Eo)

1.1.9 Stress Factor (K)

1.1.10 Temperature Coefficient (Cp)

1.1.12 Fastener Geometry