Ultrasonic Thickness Measurements

Techniques: Ultrasonic Thickness Measurements

Version date: 16 October 2016

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Introduction

An ultrasonic thickness (UT) gauge can be used to measure thickness of metal and many other solid materials. In maritime heritage work this instrument can be used to measure the thickness of metal remaining in iron and ship hull structure and plating.

A UT gauge sends short pulses of very high frequency sound waves from a hand-held probe in contact with the material and measures the time taken for each sound pulse to travel through the material, reflect off the back wall of the material then return to the probe. If the speed of sound in the material being tested is known then it is possible to calculate the thickness of the material by multiplying the speed of sound by half the total travel time.

Fig 1: The Cygnus DIVE II underwater ultrasonic thickness gauge

This method has a number of advantages:

It does not affect the material being tested
A direct measurement of thickness is made
It provides instantaneous results
Minimal processing of the measurements is required
Minimal expertise is required to collect the measurements
Access is needed to only one side of the material
Minimal preparation is required
The method is non-hazardous to the operator
The equipment is portable and easy to use

Some disadvantages of this method include:

The ultrasound does not penetrate concretion so the surface of the metal must be cleaned
Tight coupling is needed between the probe and the metal
Materials that have rough surfaces, are exceptionally thin or not homogeneous are difficult to measure
Cast iron and other coarse grained materials are difficult to inspect due to low sound transmission and high signal noise

The Corrosion of Iron and Steel

Iron and steel corrode in seawater forming a hard scale or concretion layer on the metal surface. The concretion is formed from iron corrosion products, the remains of marine organisms plus seabed material if the object is close to the bottom. The composition of the concretion can vary considerably as can its hardness and its ability to adhere to the underlying metal. The concretion forms a barrier over the metal which slows the rate of decay by reducing the amount of dissolved oxygen reaching the metal surface. The concretion forms a protective layer over the bare metal and removal of this layer may increase localised corrosion at that point. So any method used to measure hull plate thickness should ideally include minimal disturbance to this concretion layer and should include steps to mitigate for the disturbance if it has to occur. The underlying metal slowly corrodes and is replaced by layers of corrosion products; the corrosion is uneven across the metal plate which gives the metal a rough or pitted surface. Over time the corroding metal plate can become so thin that holes form within the plate leaving a lace-like web of metal held together by corrosion products. Finally, all of the metal corrodes away leaving just the layers of structurally weak corrosion products. At some point in the decay process the structure may become so structurally weak that it cannot support its own weight or any forces acting on it such as the effects of tidal currents and storms. At this point the structure will collapse, shedding some of the outer concretion layers in the process and exposing what remains of the bare metal to seawater, further accelerating the corrosion [1].

Fig 2: Using the UT gauge underwater

Fig 3: Using the UT gauge on a foreshore wreck

The Cygnus DIVE II Ultrasonic Thickness Gauge

Ultrasonic hull thickness measurements were undertaken as part of the A7 Project, the archaeological investigation of HM submarine A7 lost in Whitsand Bay, Plymouth, in 1914. The instrument chosen for this project was the DIVE Mk2 underwater ultrasonic digital thickness gauge made by Cygnus Instruments Ltd. (Fig. 1). This instrument is robust, small and lightweight so can be worn on the diver’s wrist. The gauge is automatic in operation so there are no controls to adjust before making a measurement, this is a big advantage when time on site is short and also makes the instrument very easy to use. The gauge can be fitted with different types of probe so we undertook experiments to determine the effectiveness of each type for heritage work. The display shown to the operator is easily understood and provides essential feedback about the quality of the ultrasonic signals being received and the measurements that have been made.

An upgrade to the basic gauge is the ability to store up to 5000 thickness measurements in its internal memory, arranged in groups selected by the operator. The measurements are automatically logged when a stable measurement is made so the operation is hands-free. Once the measurements have been made and the gauge is back on the surface the logged measurement data can be uploaded in to a computer for display and annotation using the Cygnus Instruments CygLink software supplied with the gauge (Fig. 6). Once any annotations have been added the CygLink software can be used to produce a PDF report on the measurements or the measurements can be exported to a spreadsheet.

Procedure

The procedure for making UT measurements is given below:

Just before diving, 20mm diameter balls of epoxy putty to be used for filling holes in the concretion were mixed together then placed in a plastic bag
On site, the surface of the metal to measure was prepared by removing the surface concretion with a chisel to leave a clean but pitted surface with an area large enough to admit the UT probe
The UT gauge was set to record the next set of measurements in a new logging group
The probe was placed on the cleaned area and a minimum of three measurements were made and logged at each location
Finally the cavity made in the concretion was filled with epoxy putty

Probe Type and Measurement Method

A series of experiments were undertaken by the A7 Project team prior to and during fieldwork to determine the most reliable way to record plate thickness measurements. Significantly, few successful measurements were made on corroded and pitted metal using the single crystal probe most often used previously for heritage work. The experiments showed that the best results would be obtained using the 5 MHz twin crystal probe (Cygnus Type T5B, yellow band) as it uses a different method for measuring plate thickness, see Experiments in Ultrasonic Hull Thickness Measurements on the A7 Project.

Fig 4: Removing a small area of concretion from the hull of HM submarine A7

Fig 5: Concretion removed showing the rough and pitted surface of the metal underneath

Surface Preparation

Measurements can only be made with the concretion completely removed from the underlying metal (Fig. 4) as the ultrasonic signal from the probe will not penetrate even the thinnest layer of concretion. The layered nature of the corrosion layer or the interface between base metal and corrosion layer are such that they do not allow the transmission of ultrasonic signals at the frequencies used by the UT gauge.

After removal of the concretion the underlying metal tends to be pitted and uneven (Fig. 5) so it may be this that effectively limits the effectiveness of any UT gauge. The UT probe needs to be coupled to the base metal on a flat and smooth surface so that the probe does not move and so there is no gap between probe and metal to cause acoustic scattering and reverberation. Even the smallest variation in the surface of the bare metal will hinder the probe from being replaced in exactly the same position and orientation when taking the same measurement twice, so the variation in repeat measurements may be more to do with surface roughness than actual variation in metal thickness. On rough metal surfaces there is a limit to how precisely we can measure the thickness of the metal.

The hole left after the concretion has been removed has to be repaired to avoid accelerated corrosion at that point. Milliput epoxy can be used for sealing and stabilising holes in concretion.

Fig 6: Cygnus Cyglink software showing a thickness measurement as a time series plot

Measurement Quality

The ability to make a thickness measurement lessens as material degrades. On poorer quality samples it is important to make an assessment of the quality of repeat measurements before attempting to measure variation within a small area or across an entire wreck. The single crystal multiple echo method works on good to average quality metal samples. On poorer quality samples it is harder to make a measurement but the measurements are reliable. The twin crystal single echo method will produce results on poorer quality metal samples but it is essential that a minimum of three measurements per point are made when using the twin crystal probe so quality and measurement error can be assessed. On poor quality materials the ability to inspect logged traces using the CygLink software is essential for post-dive analysis of measurement quality (Fig. 6).

Recommendations

The Cygnus DIVE Mk. 2 gauge is a suitable instrument to use for thickness testing for UCH work
Remove all concretion before making any UT measurements
The simple hammer and chisel method worked best for removing concretion
Use the single crystal probe on good to average quality metal
Use the twin crystal probe on average to poor quality metal
Make a minimum of three repeat measurements at each sample point
Use the logging capability of the Cygnus DIVE and inspect the measurement plots in post processing
Calculate the variation in measurements at each sample point and use the value when calculating changes in thickness over time or across a site
Use Milliput epoxy for repairing holes in concretion

[1] For a detailed discussion about iron corrosion see Pearson, 1987, Conservation of Marine Archaeological Objects, p212

Equipment List

Ultrasonic thickness gauge - an instrument for measuring the thickness of metal and other solid materials
Milliput epoxy putty - used to fill in the holes in concretion