Surface Testing

 
Contoured Surface Testing
 
 
 
 
 
 
 
 
Wear Testing-
Provided your specimen is flat, a simple test for evaluating it's abrasion or wear resistance is
 
 
Contoured Surface Testing
 
Measure abrasion resistance and other material properties of finished products of any size or shape -
flat, concave or convex.
 
Taber Linear Abraser uses a free floating head to follow the contours of every sample, 
permitting testing of finished products. With virtually no limit on sample size or shape, 
the Linear Abraser is ideal for testing plastics, automotive components, painted parts, 
printed graphics, optical products, rubber, leather, textiles and for use in testing laboratories
 
Ultrasonic testing tasks
 
Is there a primary classification of tasks assigned to the ultrasonic operator?
If we limit ourselves to testing objects for possible material flaws then the
classification is as follows:
 

  1. Detection of reflectors
  2. Location of reflectors
  3. Evaluation of reflectors
  4. Diagnosis of reflectors (reflector type, orientation, etc.)
 
Instead of using the word "reflector", the ultrasonic operator very often uses the term 
"discontinuity"
This is defined as being an "irregularity in the test object which is suspected as being a flaw". 
In reality, only after location, evaluation and diagnosis has been made, can it be determined 
whether or not there is a flaw which effects the purpose of the test object. The term "discontinuity" 
is therefore always used as long as it is not certain whether it concerns a flaw which means a 
non-permissible irregularity.
 
Detection of discontinuities
 
The essential "tool" for the ultrasonic operator is the probe, Figs. 1a + 1b. The piezoelectric element,
excited by an extremely short electrical discharge, transmits an ultrasonic pulse. The same element
on the other hand generates an electrical signal when it receives an ultrasonic signal thus causing
it to oscillate. The probe is coupled to the surface of the test object with a liquid or coupling paste
so that the sound waves from the probe are able to be transmitted into the test object.
 
Asphalt, Bitumen and Slurry testing Equipment
 
 

Fig. 1b Angle-beam probe (section)
 
 

Fig. 1b Angle-beam probe (section)
 
The operator then scans the test object, i.e. he moves the probe evenly to and fro across the
surface. In doing this, he observes an instrument display for any signals caused by
reflections from internal discontinuities, Fig. 2.
 
 

Fig. 1b Angle-beam probe (section)
 
 

Fig. 1b Angle-beam probe (section)
 
 
Every probe has a certain directivity, i.e. the ultrasonic waves only cover a certain section of the test object. The area effective for the ultrasonic test is called the "sound beam" which is characteristic for the applied probe and material in which sound waves propagate. A sound beam can be roughly divided into a convergent (focusing) area, the near-field, and a divergent (spreading) part, the far field, Fig. 3. The length N of the near-field (near-field length) and the divergence angle is dependent on the diameter of the element, its frequency and the sound velocity of the material to be tested. The center beam is termed the acoustic axis.
 
 
 
The shape of the sound beam plays an important part in the selection of a probe for solving a test problem. It is often sufficient to draw the acoustic axis in order to show what the solution to a test task looks like. A volumetric discontinuity (hollow space, foreign material) reflects the sound waves in different directions, Figs. 4a + 4b.
 

Fig. 4a Volumetric discontinuity - straight-beam probe

Fig. 4b Volumetric discontinuity - angle-beam probe
 
The portion of sound wave which comes back to the probe after being reflected by the discontinuity
is mainly dependent on the direction of the sound wave; i.e. it does not matter whether scanning is made with a straight-beam probe or an angle-beam probe or whether it is carried out from different surfaces on the test object, Fig. 5. If the received portion of the reflected sound wave from the probe
is sufficient then the detection of the existing volumetric discontinuity is not critical, this means that
the operator is able to detect it by scanning from different directions. A plane (two-dimensional) discontinuity (e.g. material separation, crack) reflects the ultrasonic waves mostly in a certain direction, Fig. 6.
 
 

Fig. 1b Angle-beam probe (section)
 
 

Fig. 1b Angle-beam probe (section)
 
If the reflected portion of the sound wave is not received by the probe then it is unlikely that the discontinuity will be detected. The possibilities of detection only increase when the plane
discontinuity is hit vertically by the sound beam. This applies to discontinuities which are
isolated within the test object.
 
Improved Rotary Platform Abraser

Taber Abrasers are durable, precision-built instruments designed to perform accelerated wear tests on a variety of specimens. These include solid materials, painted, lacquered, electro-plated surfaces, plastic-coated materials, textiles, metals, leather, rubber and linoleum.
Materials are subjected to the wear action of two abrasive wheels at a known load. This wear action results when the abrasive wheels are rotated in opposite directions by a turntable on which the specimen material is mounted. The abrading wheels travel on the material about a horizontal axis displaced tangentially from the axis of the test material which results in a sliding action. An exclusive feature of the Taber Abraser is an “X” pattern of abrasion, produced by the rotary rub-wear action of the wheels. The wear pattern formed is that of two intersecting areas or a slightly curved herringbone confi guration from the outside to the center, and from the center to the outside of the specimen. An area of 30 square centimeters is subjected to test and a complete circle on the material surface is abraded at all angles of grain or weave.
 
Wear Testing Paints, Inks & Chemical Coatings
Wear Testing Rubber, Plastics & Films
 
Wear Testing Stone, Concrete & Glass
Wear Testing Building & Electrical Materials
Wear Testing Metal Products & Coatings
 
 

Fig. 1b Angle-beam probe (section)
 
With plane discontinuities which are open to the surface of the test object, e.g. 
a crack running vertically from the surface into the test object, a vertical scan of 
the crack does not always produce the required success. In this case wave 
overlapping occurs (interferences) due to sound wave reflection on the side 
wall of the test object which seems as if the sound wave bends away from the 
corresponding side wall, Fig. 7. In such cases, the probability of crack detection
is very good if the angle reflection effect is used, Fig. 8a. At the 90° edge, between 
the crack and the surface of the test object, the sound waves are reflected back 
within themselves due to a double reflection, Fig. 8b. Use of the angle reflection 
effect is often even possible when a plane discontinuity, which is vertical to the 
surface, does not extend to the surface and under the condition that the sound 
wave reflections at the discontinuity and the surface are received by the probe, Fig. 9.
 
 
 
Fig. 8b Angle reflection effect
Fig. 9 Plane, vertical reflector near the surface
 
 
 
Often in thick-walled test objects, in which there are vertical discontinuities, this condition cannot be fulfilled so that the reflected sound waves from the discontinuity and the surface of the test object do not return to the probe. In this case, a second probe is used for receiving the reflected portions of sound thus enabling detection of the discontinuity.
With this type of testing, the Tandem Technique, one probe is used as a transmitter, and the other probe is used as the receiver. Both probes are moved over the surface of the test object and are spaced apart at a fixed distance. Scanning is made for vertically positioned discontinuities at different depths of the test object, depending on the probe spacing, Figs. 10a, 10b and 10c.
Although, with angle scanning in thin test objects, there is a possibility that plane discontinuities cannot be vertically hit, Fig. 11 a, the detection sensitivity is much better, especially by suitable selection of the scanning angle and the test frequency so that the user favours the single probe test as opposed to the more complicated tandem method. This is normally the case when testing welds up to a thickness of about 30 mm.
Of course the possibility of detecting discontinuities which are not vertically hit is reduced. However, this deficiency is often compensated by an additional test with another angle of incidence, Fig. 11 b, or by using a probe with a lower frequency, Fig. 11 c. A typical procedure can be found in the corresponding specifications (test instructions) for weld testing.
 

Fig. 11a 70° scanning: unfavourable angle


 


Fig. 11b 45° scanning: favourable angle
 
 
 
 
 
 
 
 
 
 
 
 
 
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E-mail: info@eltrap.co.il