Low-Tech - High Tech - and  Really High Tech  Non-Destructive Testing

(for the technophiles)

Percussion(Acoustic Emmission)

Since a sound wave consists of a repeating pattern of high pressure and low pressure regions moving through a medium, it is sometimes referred to as a pressure wave. If a detector, whether it is the human ear or a man-made instrument, is used to detect a sound wave, it would detect fluctuations in pressure as the sound wave impinges upon the detecting device. Since the fluctuations in pressure as detected by the detector occur at periodic and regular time intervals, a plot of pressure vs. time would appear as a sine curve on a graph.

For us in the boating industry, using a low tech (hammer) and high tech (ear) the surveyor is searching for delaminating and voids in fiberglass and other composites in places that he can't use a moisture meter.

Moisture Meters

How wet is too wet?  Previously, determining moisture levels has been a time-consuming and somewhat destructive process. A solution is the electric moisture meter-a cost-effective tool that can easily determine moisture levels and diagnose resulting problems. Electric moisture meters use known relationships between electrical properties and moisture levels in materials that are hygroscopic (able to absorb water from the environment). The moisture meters come in two varieties: conductance and dielectric

Conductance meters have two to four metal probes, or pins, that are physically inserted into the building material. They read the moisture content of the material through the electrical conductance detected between electrodes. Dielectric-type moisture meters are noninvasive-they do not physically puncture the material. They generate an electrical field through flat platens located on the bottom of the meter.. Moisture display options for this meter type are identical to the conductance-type meters: analog and digital.

Meter Limitations

Although the relationship between moisture content and electrical properties is well known, it is only predictable for a range of moisture values. Conductance-type meters are accurate for a wood moisture range of 7 to 30 percent moisture content and dielectric meters are accurate from 0 to 30 percent moisture levels.

Moisture meter readings for materials other than wood also useful in a qualitative capacity and should not be interpreted as actual moisture levels unless verified by a moisture detection method other than meter, usually cutting out a "sample" and having an analysis done at a lab. Other limiting factors associated with moisture meters include temperature, moisture distribution, material species or density, material thickness, and material coatings or treatments. Dielectric meter readings are particularly influenced by material density and often require correction or calibration. Dew or surface moisture even bottom paint on materials may give erroneously high readings.

Usefull tool to find out if, where, and how much water is in the hull, deck, stringers, etc. On a survey I run a moisture meter on above the waterline hull and deck surfaces.

Carbon Monoxide and Decibel Levels

Before we get onto the other test methods and procedures, I wanted to add two important tests I include in a survey. The U.S. Coast Guard has issued many alerts on Carbon Monoxide poisoning. Be it from leaky exhaust systems in the engine room, sitting on the swim step, or swimming around the generator exhaust port, etc. this stuff is deadly. From my own experience, I can tell you that a leaking exhaust water injector at the manifold gives you one hell of a headache. Luckily, no one off watch was sleeping close to the engine hatch and the outside air flow into the cabin was substantial. You can't see or smell it, and by the time you feel it, your life is at stake. A CO meter in a sleeping area is cheap insurance. A good, portable CO meter passed around the exhaust system can find a loose hose clamp, rotted metal fitting, worn out gasket, even a leaky hose. When opening the hatch on large engine rooms that's the first thing that I check. Here are the readings to look for,

As for Decibel levels and your boat, I grew up in the 60's (rock music), enlisted in the Marine Corps (gunfire), and worked as a mechanic and in the oil industry (machinery noise) and never used ear plugs until I was 45 years old. So now it's huh or what in some conversations. Marine engines are noisy; some soundproofing is minimal if at all installed! Running a sound meter in the engine room and into the cabin is a simple test to help you become aware of you and your family’s exposure that can cause permanent damage.

                                   Decible Exposure/Time Guidelines  

                               Accepted Standards by 29CFR1926.52,

                      OSHA, and the Center for Disease Control, 2002

Spectrochemical Analysis

The procedure analyzes a sample of engine and/or transmission oil for the presence of metallic elements. Every in-service lubricant will contain suspended microscopic particles of those elements because of its circulation and function within a mechanical system. Spectrochemical analysis identifies and measures these particles in parts per million (ppm) by weight. The analyzed elements are grouped into three main categories, and their measurement in PPM provides the keys for expert data analysts to determine the condition of equipment and provide specific maintenance recommendations. Those categories are: Wear Metals, Contaminants and Additives.

Contaminants

Depending on the circumstances, many different substances can be classified as contaminants. Silicon, in the form of silicon dioxide (sand), is one of the most common contaminants monitored with spectrochemical analysis. Similarly, the presence of grease contaminating an oil system may be indicated by increases in aluminum or barium if the grease contains metallic soaps.

Although contamination is commonly associated with substances entering a components oil system from an outside source, wear metals themselves are also a form of contaminant.

Additives

Additives are chemical compounds added to oils, fuels, and coolants to impart specific beneficial properties to the finished products.

Additives enhance properties already present and reduce the rate at which undesirable changes take place in a fluid during service. Used extensively in marine propulsion systems worldwide by taking engine oil samples before oil changes.

Ultrasonics

Ultrasonic waves have a wide variety of applications over an extended range of intensity, with cutting, cleaning, and the destruction of tissue as one extreme and nondestructive testing (NDT) at the other end. The instruments are often called Ultrasonic Flaw Detector and Ultrasonic Thickness Gauges in respect to their main purpose of detection. The test equipment comprises test instrument, probes, cables and all devices connected to the instrument during an examination.

The method of measuring the thickness of a sample of known velocity, using the pulse echo method, measuring the displacement of the first back-wall echo from the start of the transmission peak (direct contact).. The typical measurement range of steel is between 0.3 mm and more than 100 mm. Accuracies as high as +/- 001mm are achievable in some applications. An ultrasonic technique for weld inspection should investigate discontinuities of welded joints are, e.g., nonfusion root, sidewall nonfusion, porosity, centerline cracking.

In the boating industry, used with steel and aluminum boats to measure hull thickness and welding flaws.  Some equipment can detect delamination in fiberglass, bonded wood, etc.

Thermographic Infrared Imaging

Thermography is used to detect thermal change and measures surface temperatures by using infrared still cameras. These “tools” see light that is in the heat spectrum. Images on the film record the temperature variations of the "skin", ranging from white for warm regions to black for cooler areas with black and white cameras, color cameras produce the image in reds and blues. The resulting images help determine whether differences are present.

Infrared imaging (thermography) is a non-contact optical method where an accurate two- dimensional mapping of steady or transient thermal effects is constructed from the measurement of infrared energy emitted by the target. Recent advances in infrared technology, specifically development of high-density imaging sensors have opened a new level of applications unreachable prior to the availability of this technology. Real-time infrared image acquisition and processing allows implementation of advanced thermographic test methods.

Used today in the small craft industry to accurately map out hull and deck core moisture, engine component wear, electrical system overheating, etc. Expensive? Not anymore, as a Level I thermographer, I charge my hourly consultant rate, portal to portal, and provide you visual proof of where the problems are in the detailed report and photos.

Aluminum hull from 90 feet, water in water tight collision bulkhead. Dry stack exhaust from 1 foot, 199 degree heat intrusion at thermal blanket leaks.

Ultraviolet Technology

Simple, safe, and reliable, the UV technique saves time and labor in many applications. It is even able to find critical indications that are not detectable by other methods. For example, a engine was burning six quarts of oil every 1,000 miles. No matter how hard the service people searched, they could not find the source of the oil loss with conventional detectors. There were no oil stains on the ground, no fouled plugs, and no telltale blue exhaust smoke. After adding a fluorescent tracer dye and running the engine for a few minutes, a quick scan of the system with high-intensity UV light pinpointed the exact source of the leak with a bright yellow-green glow. Once the UV system solved the mystery by making a small, steady drip obvious, this elusive problem was easily cured by tightening the bolts around the head gasket. This leak was not visible without UV because it was active only when the car was running; the oil burned off the instant it contacted the hot engine.

A UV lamp is used to locate suspect leaks, cracks, defects, traces, contamination, and spoilage. Some substances fluoresce naturally. Others need to be enhanced with an intensifying agent, while no fluorescent objects require a man-made, UV-tracer chemical. These chemicals, also called additives and tags, are available in a variety of colors and application-specific formulations. NDT to commercial and military specifications requires special care. These tests mandate that the ambient light be measured with a recently calibrated precision photometer and kept below established parameters. When UV light strikes one of these reactive substances, this energy, in the form of photons, causes each molecule to rotate violently. As the molecules slow down, called fluorescence, lets users detect otherwise invisible traces that indicate various quality defects, diseases, and contamination.  UV tracers, these chemical additives, tags, or materials make nonfluorescent substances glow under UV light.

On boats used for detecting leaks, in engines, transmissions, hydraulic steering and windlass systems, reefers and air conditioning, sanitation systems, any system that has a liquid and sometimes for external air and water leaks!

 Eddy Current Testing Basics  

Eddy current inspection is one of several NDT methods that use the principal of “electromagnetism” as the basis for conducting examinations.

Eddy currents are created through a process called electromagnetic induction. When alternating current is applied to the conductor, such as copper wire, a magnetic field develops in and around the conductor. Eddy currents are induced electrical currents that flow in a circular path. They get their name from “eddies” that are formed when a liquid or gas flows in a circular path around obstacles when conditions are right.

In the proper circumstances, eddy currents can be used for:

Crack Detection, Material Thickness Measurements, Coating Thickness Measurements,  Conductivity Measurements for Material Identification, Heat Damage Detection, Case Depth Determination, and Heat Treatment Monitoring.

Eddy current probes are used for crack detection, corrosion inspection, or conductivity testing. These types of instruments contain a simple bridge circuit, which compares a balancing load to that measured on the test specimen. If any changes in the test specimen occur which deviate from normal you will see a movement on the instruments meter. 

Hardness Testing

Hardness is not an intrinsic material property dictated by precise definitions in terms of fundamental units of mass, length and time. A hardness property value is the result of a defined measurement procedure.
With regard to materials, hardness has always been a subject of much discussion among technical people, resulting in a wide range of definitions. Hardness properties include such varied attributes as resistance to abrasives, resistance to plastic deformation, high modulus of elasticity, high yield point, and high strength, absence of elastic damping, brittleness or lack of ductility.

To a metallurgist, hardness is a material’s resistance to penetration. In general, an indenter is pressed into the surface of the material to be tested under a specific load for a definite time interval, and a measurement is made of the size or depth of the indentation. Hardness is not a fundamental property of a material, but a response to a particular test method. Basically hardness values are arbitrary, and there are no absolute standards for hardness. It has no quantitative value, except in terms of a given load applied in a specific, reproducible manner and with a specified indenter shape.

Barcol hardness is a method that a hardness value obtained by measuring the resistance to penetration of a sharp steel point under a spring load. The instrument, called the Barcol impressor, gives a direct reading on a 0 to 100 scale. The hardness value is often used as a measure of the degree of cure of a plastic.

For boats, hardness testing can be valuable after exposure to fire, severe bilstering, etc. to ensure the integrity of construction panels. US Coast Guard's NVIC 8-87 publication for passenger boat building states that at a Barcol hardness of 40 for resin cure, the boat hull or panels can be removed from the mold. (Gives you an idea of how a Barcol test is used)

Shearography and  Digital Holography

Shearography and holography are laser based interferometry methods that are capable of imaging minute surface deformations due to substrate discontinuities.. While most NDT methods require point by point scanning, these methods are non-contacting and provide a full field video image of flaws in real time

Shearography is a variation of laser technology ESPI (holography), specifically designed for NDT applications.  Shearography is an optical video strain gauge and an appropriately applied stress is used to locate strain concentrations caused by internal defects.  As an example, a composite helicopter blade can be inspected in production with vacuum excitation, while it can be rapidly inspected in the field with thermal excitation from a heat source, such as a heat gun or even a hairdryer.  However, the inspection of composites by shearography does not stop at the detection of defects; once a damaged area has been repaired, laser shearography is able to qualify the integrity of the repair.

Holography, along with other coherent optical techniques, has already been proved to be an effective technique in non-destructive testing of composites. One of the difficulties of holographic non-destructive testing is finding the stressing technique able to produce singularities in the strain state at the surface of the object under test. Holographic interferometry, has already proved to be a technique of great potential in non-destructive testing of mechanical components and materials, especially composites.It is already a useful tool in detecting local anomalies such as flows, delaminations, voids, inclusions, imperfect gluing, broken glass or carbon fibers, and many others. The principle of holographic NDT is very general: in the presence of local anomalies (defects) under the surface, the state of strain at the surface produced by a uniform object stressing will be locally modified.

 Are these two test methods being used in the boating industry? You spend 60 million for an America's Cup Challenger or 100 + million on a mega yacht, or taxpayer dollars for the U.S. Navy, sure! Sooner or later these NDT systems will filter down to be used on that nice 30 footer you will be interested in buying, just not this year. 

Contact me at,

Phone: 808-375-8260

Email: Bob@BoatSurveysHawaii.com

Robert J. "Bob" Dupuis

Marine Surveyor/Consultant