Review Questions Flashcards
A test specimen used as the basis for calibrating test equipment or as a comparison when evaluating test results is called a what?
Reference standard
The ratio of response/amplitude from signals of interest to response/amplitude of indications that contain no useful info for the test conducted are called what?
Signal to noise ratio
When conducting an EC test, using a differential coil arrangement that compares an external reference standard with the test specimen, the system should be nulled or balanced with what?
A reference standard in one coil and an acceptable specimen in the other
The impedance of an EC test coil will increase if what?
The test frequency increases
What would normally be considered the best fill factor when testing straight tubing with an encircling/feed through coil?
.95 (95%)
When a magnetic metal part is placed in an EC test coil, the impedance of the coil will be changed by which property in the part?
Conductivity, permeability and dimensions
An eddy current is a circulating electrical current induced in a conducting article by what?
An alternating magnetic field
When testing tubing with an inside diameter coil, most of the eddy currents will be where ?
Flowing around the inside diameter of tubing
An eddy currents test coil’s opposition to flow of alternating current is called what?
Impedance
An increase in impedance of EC test coil will do what to current flow in coil?
Decrease in current flow through test coil
What 3 things will affect the impedance of an encircling EC test probe?
- Conductivity
- Permeability
- Fill factor
Eddy current test coils that are wound to form a wide coil would normally be used to detect what?
Changes in conductivity
Eddy current test coils that are wound to form a narrow coil would normally be used to detect what?
Small surface defects
DC saturation coils are usually used when testing what type of material?
Steel
A single absolute coil arrangement can be used in what (3)?
- Probe
- Encircling
- ID coils
The primary purpose of the secondary winding in an eddy current coil is to what?
Detect changes in eddy current flow
When attempting to determine the exact point of a discontinuity, the test system should include what?
A probe coil
It’s possible to sort various non-magnetic alloys of metal by means of eddy current testing when there’s what?
A unique range of conductivity values for each alloy
What’s an example of a product that would be most applicable to a test using an ID coil?
Bolt hole
What would be a method to reduce irrelevant indications when EC testing ferromagnetic materials?
Use a DC saturation coil to magnetically saturate the test specimen
Eddy currents are circulating currents induced in a conducting material by what?
Varying magnetic fields
In order to generate measurable eddy currents in a test specimen, what must the specimen be?
A conductor
All ferromagnetic materials that have been magnetically saturated to suppress permeability variations, may retain a certain amount of magnetization which is referred to as what?
Residual magnetism
An AC current flowing in a conductor will set up a magnetic field where?
Around the conductor
When conducting an eddy current test on tubing, the magnitude of the indication caused by a discontinuity is dependant on what 3 things?
Depth, width and length of discontinuity
A coil’s magnetic field may be viewed as a distribution of lines of flux around the coil. The number of lines in a unit area is called what?
Flux density
How do eddy currents travel?
In closed paths
What is the condition of balance in a device or system which results in zero output?
Null balance
What is a unit of flux density?
Gauss
What is the region around the magnet called that will attract other pieces or iron or steel to it?
Magnetic field
Which eddy current arrangement does not make a comparison between 2 sections of a specimen?
Absolute coil
A length of tubing with a notch running end to end, having uniform width & depth, when tested with an encircling self referencing differential coil will produce what type of indication?
No indication
Spinning, probe type eddy current instruments are most useful in detection of what?
The smallest surface blemishes
One principle advantage of a spinning probe eddy current instrument is what?
The ability to find the circumferential position of defects
At any given instant during a test, a spinning probe will be inspecting what area?
The area defined by the size of coil
What causes a phase shift in an eddy current coil?
Change in ratio of inductive reactance to resistance
When eddy current testing a non ferrous specimen, a discontinuity will do what to conductivity of specimen?
Decrease the effective conductivity of specimen
Most basic eddy current systems monitor changes in voltage due to changes in what?
Impedance
An increase in impedance of an eddy current test coil will cause a what in test coil?
Decrease in current flow through test coil
The ideal fill-factor percentage of an ID bobbin probe in tube inspections would be what?
80% to 95%
When material temperature increases, what happens to the eddy currents?
They get weaker
Probe wobble is a form of what?
Lift off
According to Ohm’s Law, in any electrical circuit, the voltage divided by the circuit impedance is equal to what?
Current flow
Who’s law states that when an alternating voltage induces an EMF, the induced EMF in opposition to the change causing it?
Lenz’s Law
What is a device that stores electrical energy within a specified volume?
Capacitor
In a fixed magnetic field, how would the compass needle arrange itself in relation to the lines of force?
Parallel
How can direct current be used to induce eddy currents?
If the test piece or conductor is moved cylindrically or
1 mA is how many amps?
0.001 A
A secondary field is generated by the test object and how is it comparable to the primary field?
Opposite the primary field but much smaller
When a non ferromagnetic part is placed in the test coil, what happens to the coil’s voltage?
Decreases
Eddy currents generated in a test object flow where?
The same plane as the coil is wound
A spinning probe would most likely be what type of coil?
Probe coil
How is an absolute coil measurement made?
Without reference to or a direct comparison with a standard
When coils in a differential arrangement are affected simultaneously with the same test object variables, what will the output signal read?
0 or near 0
A hybrid coil is composed of two or more coils. What are the coils like?
Maybe of widely different dimensions
Inductance is comparable to what?
Inertia
What is the unit for inductance?
Henry
What type of crack detector may use the uncalibrated meter approach?
Single frequency crack detector
