Wave Physics
The ID Creeping Wave 1 s produced by using a high angle longitudinal wave of approximately 70 degrees refraction. With the 70 degrees refracted longitudinal wave, an associated shearwave (direct shearwave) of about 30 degrees is also produced, which is called Collateral Echo 1 (CE-1). In addition, an OD Creeping Wave, at an angle slightly above the 70 degree L-wave, releases another shear wave (indirect shear wave) at approximately 31.5 degrees. This indirect shearwave strikes the ID surface and mode converts to a longitudinal wave that moves or "creeps" along the ID surface. This ID Creeping Wave is called Collateral Echo 2 (CE-2).

Figure 1.1 - IDCR Wave Physics

The CE-2 signal is a very short lived energy that travels along the ID surface to detect the base of an ID connected flaw. CE-2 has a very short Echo Dynamic (ED) pattern and is extremely sensitive to very shallow cracks. This is demonstrated by using a block of carbon steel material with very shallow notches. In order to prove this, the author has demonstrated that with a 4 MHz ID Creeping Wave transducer, a 0.005 inch deep EDM notch in a 1 inch thick carbon steel calibration block can be detected. The signal to noise ratio, in this case can be as much as 4 or 5 to 1.

The CE-1 signal is commonly called the 30-70-70 signal. The CE-1 signal is produced when the direct shear wave strikes the ID at a critical angle and mode converts to a 70 degree longitudinal wave. This mode converted L-wave strikes the face of a shallow to midwall crack and reflects a 70 degree L-wave. A broad Echo Dynamic (ED) movement is observed for the CE-1 signal as the flaw gets deeper.

The 70 degree longitudinal wave reflects from the upper tip of a very deep flaw. Depending upon the material type, transducer frequency, and manufacturer, the 70 degree L-wave signal is seen when the flaw approaches 40 to 50% or greater of the through wall thickness.

By observing the absence or presence of the 70 L, CE-1, and CE-2 signals, a qualitative estimate of the flaw depth can be obtained. Also, the echo dynamics, the amplitude of the CE-1 signal and the peaked 70 degree L-wave signal provides an indication of flaw depth for ID connected flaws. (See Figure 1.2). Figure 1.2 shows a sizing low chart that is used to bound the flaw depth and to show which complimentary sizing technique is used.

Ultrasonic Sizing Flow Chart
		Signal Presentation		Suspect Area		Sizing Method
	___		___	Suspect Very Deep Flaw	___	Refracted L-Wave and Bi-Modal	=>	Outsite Diameter ______ Outer 1/3T Zone
Calibrated ID Creeping Wave Method	___		___	Suspect Midwall to Deep Wall Flaw	___	Bi-Modal and Refracted L-Wave	=>	______
	_							Middle 1/3T Zone
	___		___	Suspect Shallow to Midwall > 15-20% Flaw	___	Tip Diffraction and Bi-Modal	=>	______
	___		___	Suspect Shallow Flaw <10-15%	___	Tip Diffraction	=>	Inner 1/3T Zone ______ Inside Diameter
Figure 1.2 - ID Creeping Wave Flow Chart

Transducers
Generally, any 70 degree refracted longitudinal wave transducer may be used. However, special transducers are designed to provide specific signal sound wave patterns unique to the creeping wave technique. These transducers are generally 2 and 4 MHz, and may be of integral or non-integral wedge design. Other refracted longitudinal waves at angles of 60 degrees refraction and higher produce similar wave physics for the CE-1 and CE-2 signals.

Calibration
See Technique 1 in Chapter 2.

Wave Physics
The Tip Diffraction Method employs the effect of sound energy striking the base of a crack or planar reflector which causes the tip of the crack to radiate sound energy. This sound energy radiates at the tip of the crack as a spherical wave or a cylindrical wave along the length of the crack.

(Note: These are not new sound modes, just specific sound patterns).

Figure 2.1 Tip Diffraction

Tip Diffraction uses diffracted or reflected sound energy radiating from the tip of the crack to accurately size the depth of the crack from the ID or the OD. Diffraction is the phenomenon whereby sound bends around the edge of the flaw. (See Figure 2.1).

Two techniques are used. One measures the time-of-flight (TOF) or sound path of the diffracted energy as it travels back to the transducer and is sometimes called the Pulse Arrival Time Technique (PATT), or Absolute Arrival Time Technique (AATT). The other technique measures the relative time travel or Delta (A) (TOF) or sound path difference between the tip diffracted signal and the corner trap or base signal and is sometimes called Satellite Pulse Observation Time Technique (SPOT) or Relative Arrival Time Technique (RATT). The technique acronyms were changed due to a new author of the technique, e.g., PATT/AATT, and SPOT/RATT are the same techniques. (See Figure 2.2).

The tip diffracted signal is generally a low amplitude signal. The signal-to-noise (S/N) ratios can be very low (2 to 1 S/N ratio) which makes it difficult to properly identify the tip signal. Typically, tip diffracted signals precede the base or corner trap signal. However, tip diffracted signals may be seen trailing the base signal. This is due to the spherical wave radiating from the tip of the flaw and reflecting off the ID surface and returning at a later time beyond the corner trap signal.

With low amplitude tip signals, radio frequency (RF) signal display may be helpful in identifying the tip signals. There is some consideration that a phase reversal is noted between the base and the tip signal. For example, the tip signal may have a positive excursion and the base may have a negative excursion. (See Figure 2.3).

Figure 2.2 Time of Flight (TOF) Technique Figure 2.2 A (Delta) Time of Flight (TOF) Technique	Figure 2.3 RF Signal Display
Other considerations include: 1. - Multiple faceted cracks like stress corrosion cracking (SCC) and thermal fatigue, cracks, may produce multiple tip diffracted signals.
2. - For OD connected cracks, the tip diffracted technique can size flaws using the full vee-path technique.
3. - Midwall cracks (not connected at the ID or OD) may be sized with the Tip Diffraction method. Special calibration blocks are necessary

Transducers
Generally, 45 degree 5 MHz transducers are used for Tip Diffraction Methods. Frequency selection is determined by the type of material (e.g., carbon steel or stainless steel), thickness of component, and grain structure of the material.

Some important considerations for transducer selection include: refracted angle, wavelength, beam spread, mode of sound propagation, detectability, sensitivity, and resolution. These parameters are controlled by incidence angle, refracted angle, diameter, frequency, and damping.

For thin materials (less than 0.500 inches) and very shallow cracks (generally less than 5 to 10%), highly dampened (1 and 1/2 cycles, pulse length) search units with high frequencies (greater than 5 MHz) may be required to improve resolution and sensitivity.

Calibration
See Technique 2 in Chapter 3.

Wave Physics
The Bi-Modal Method is defined as follows: three main signals (or pulse train) are observed: Pulse 1 is the reflected longitudinal wave from the flaw tip, Pulse 2 is the direct shear wave which mode converts to a longitudinal wave and then reflects a longitudinal wave from the face of the flaw (similar to CE-1 with the ID creeping wave technique), Pulse 3 is an indirect shear wave which mode converts to an ID creeping wave at the ID surface of the component (similar to the CE-2 signal with the ID Creeping wave technique). (See Figures 3.1 A and 3.1 B).

Note: A fourth signal is sometimes seen between Pulses 1 and 2. It is called Pulse 1*. Generally, the 1* signal is observed during calibration on notches. (See Figure 3.1B).

Figure 3.1A	Figure 3.1B
Bi-Modal Method	Pulse 1 - Direct L-Wave from tip. Lt meeting Lr at the crack tip.
	Pulse 2 - Mode converted. St meeting mode-converted Lr at the crack base.
	Pulse 3 - ID Creeping Wave. St meeting Sr at the crack base.
	Additional Pulse

Figure 3.2 A Technique 1, (TOF)

Figure 3.2 B Technique 2, () (TOF)

The two basic sizing techniques for the Bi-Modal Method are:

1. Time of Flight (TOF) of the peaked Pulse 1 signal set to specific screen ranges. (See Figure 3.2A). Also called Tau or M-AATT
2. Delta () Time of Flight (TOF) of the screen division separation of the Pulse 1 and Pulse 2 signals. (See Figure 3.2B). Also, called Sigma, or M-RATT.

The Technique 1 is Time-of-Flight (TOF) measurement of reflected longitudinal wave from the tip of the crack. Depth measurements are recorded directly from the CRT screen.

Technique 2 is the measurement of the separation of the Pulse 1 and Pulse 2 signals. This measurement is read directly from the screen in divisions.

Transducers
The Bi-Modal Method uses a special dual element search unit, either side-by-side or tandem orientation, which produce shear and longitudinal waves. Tandem designs tend to be more predominant.

It is the opinion of the author that the tandem (transmit in front, receive in back) design is more effective for sizing the midwall flaws since the reflected sound energy is directed more to the trailing crystal. However, this is greatly affected by flaw depth, material type, component thickness, and refracted angles, etc. When sizing on pipe components the tandem designs are less affected by pipe curvature thin the side-by-side designs.

These search units are very specific to the manufacturer's recommendations. Generally, the frequency is approximately 3 MHz. Refracted angles for the transmit and receive crystals vary with manufacturer and depth of focus. Typically, the refracted angles are between 50 degrees and 70 degrees for the L-wave, which is dependent upon whether the transmitter is in the front or the back.

Calibration
See Technique 3 Chapter 4

Wave Physics
The Refracted Longitudinal (RL) Wave Method uses a dual element, high angle refracted longitudinal wave search unit to detect the reflected sound path travel or Time of Flight (TOF), from the tip of a deep crack. Shear waves may be used if the coarse grain structure of the base material or the weld metal does not impede the transmission of the sound energy.

Figure 4.1 - 1/10 inch depth increments Side Drilled Holes (SDH) Figure 4.2

Using side drilled holes at specific known depths, a depth calibration is established on the CRT (See Figure 4.1). As an alternative, a sound path calibration may be used.

In this way, the RL method effectively measures the remaining ligament of good material between the SDH and the scanning surface. The true crack depth is derived from subtracting the total thickness from the measured remaining ligament.

A limitation of this method is the associated shear wave that is observed during calibration and examination. This shearwave, if not properly identified, may cause confusion from mode conversion which may produce false signals.

The Refracted L-Wave Method produces essentially the same wave physics as the ID Creeping Wave Method. The refracted angles and mode conversion angles change, but basically produce a high angle Longitudinal wave, a mode converted S-Wave signal (CE-1), and an ID Creeping Wave (CE-2) signal.

Refracted L-wave signals are displayed for each SDH at specific screen divisions. Also, during the calibration, the CE-1 and CE-2 signals are delayed to the right of the CRT out of the area of interest. (See Figure 4.2).

Transducers
The RL Method uses various refracted angles of longitudinal waves to examine the outer portion of the material. Refracted angles of 45 degrees, 60 degrees, 70 degrees, and OD Creeping Waves of about 80 to 85 degrees refraction are used. Frequency selection, element size, and depth of focus determine the zone or depth of examination.

Generally, 2 MHz and 4 MHz transducers are used. For coarse grain stainless steels, the 4 MHz may reflect sound energies from the weld interface or at grain boundaries, thus producing misleading flaw depth estimates.

Refracted shear waves may be used in some applications. This is greatly controlled by the weld or base material grain structure and is acceptable for carbon steel base materials.

Calibration
See Technique 4 in Chapter 5.