Operating Manual

BOLT STRESS MEASUREMENT USING ULTRASONICS

This document presents a brief description of fastener elongation measurement using ultrasonics. For more details on ul-

trasonic testing in general, ULTRASONIC TESTING OF MATERIALS, by Josef and Herbert Krautkramer, 4th Edition 1990,

(IBSN 0-387-51231-4), is highly recommended.

1. ULTRASONIC PROPERTIES

The BoltMike III measures the time of ight of a shock wave as it travels through a fastener. This type of shock wave is

called a longitudinal wave. A shock wave is created when an electrical pulse is applied to a piezoelectric element inside the

transducer. The frequency of the shock wave is controlled by the thickness of the piezoelectric element. The frequencies

useful for fastener measurements range from 1 to 10 megahertz. This range of ultrasound will not travel through air. A

dense liquid substance, usually glycerin or oil, must be used to couple the ultrasound from the transducer into the fastener.

When the ultrasonic wave encounters an abrupt change in density, such as the end of the fastener, most of the wave re-

ects. This reection travels back through the length of the fastener, through the layer of couplant and back into the

transducer. When the shock wave enters the piezoelectric element, a small electrical signal is produced. This signal is am-

plied by the BoltMike III and used to stop the timing counter.

Ultrasound travels in a fastener at a constant speed determined by many material factors, such as density, temperature

and stress. The velocity may be found by dividing twice the physical length of the fastener by the transit time. It is impor-

tant to realize that the sound velocity varies from sample to sample even when the sample materials composition is tightly

controlled. Therefore, the ultrasonic reference length is not exactly the same as the physically measured length. Even if the

length of a fastener is very tightly controlled, the ultrasonic length may vary by as much as one percent. For accurate mea-

surements of elongation, the change in ultrasonic length can be used. This requires a before and after measurement of the

ultrasonic length for each fastener.

The BoltMike III measures an ultrasonic reference length by measuring the time from the launching of the ultrasonic wave

to the reception of the echo from the end of the fastener. This time is divided by 2 and then multiplied by the sound

velocity to produce the length. In order to obtain the required resolution, multiple samples are averaged. Sufcient time

must be allowed for the ultrasound to diminish before ring another pulse into the fastener to obtain a stable reading.

2. EFFECTS OF STRESS

When a fastener is placed under stress, its length changes. The greater the stress the greater the length change. In the

elastic region, below the yield stress of the bolt, this relationship is linear and described by Hooke´s law. The modulus

of elasticity is the constant describing the ratio of stress to strain for a given material.

The velocity of sound in a material is also effected by stress. As a fastener is stretched, velocity of ultrasound through the

fastener decreases. This makes the fasteners ultrasonic length longer than the physical length change due to stress. A

great deal of confusion surrounds this effect. If a reference length is recorded on a faster with no applied load, then a load

is applied and a new reference length is taken, the difference between the two reference lengths is about three times the

physical elongation of the fastener. In the BoltMike III, a constant known as the Stress Factor (K) compensates for the

change in ultrasonic velocity due to stress.

It is important to note that in order to change the sound velocity; stress must be applied in the same direction as the travel

of the ultrasound. Thus stress due to shear loading or torsional stress due to tightening does not effect the sound velocity

along the length of the fastener.

PAGE 1
BOLT STRESS MEASUREMENT USING ULTRASONICS This document presents a brief description of fastener elongation measurement using ultrasonics. For more details on ultrasonic testing in general, ULTRASONIC TESTING OF MATERIALS, by Josef and Herbert Krautkramer, 4th Edition 1990, (IBSN 0-387-51231-4), is highly recommended. 1. ULTRASONIC PROPERTIES The BoltMike III measures the time of flight of a shock wave as it travels through a fastener. This type of shock wave is called a longitudinal wave.
PAGE 2
3. EFFECTS OF TEMPERATURE The temperature of a fastener effects its physical length and the velocity the ultrasound travels. As the temperature of a fastener increases, its ultrasonic length increases at a rate greater than the physical length changes. The BoltMike III temperature compensation corrects the ultrasonic length of a fastener to normalize it to 72 degrees Fahrenheit. Therefore, a fastener will always measure the same length at all temperatures if properly compensated.
PAGE 3
Transducer Selection Basic to the operation of the BoltMike III is the ultrasonic transducer. There are a variety of transducers available, and there is overlap in their applications. This can make choosing the “best” transducer for the job difficult. Transducers vary in center frequency, diameter, and damping. There is no “rule of thumb” for selection. For an “easy” fastener, a large variety of transducers will measure with great results.