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METHOD AND EQUIPMENT FOR MEASURING THE LINEAR PLASTIC STRAIN OF MATERIALS

 

The method is used for the precision measurement of the permanent strain (measurement accuracy higher than ± 0.01mm) on short distances. The method measures the distance between two points attached to the tested surface (approx. 25mm). The measured points are either attached by welding using a special welding device and/or, if not possible, glued using cement. The changes in the length base are transferred by an impression of a special test piece with a negligible thermal expansion coefficient propelled by a simple spring-loaded device and then measured with a measuring microscope in laboratory conditions. This technology is an extremely precise, fast and cheap measurement method applicable even under very high temperatures. Segmental application is also possible.

Principle

Fig. 1   Special portable welding kit for the welding of two tips on the surface of the structure. When the condenser is charged, the device can be operated without power supply from the grid.

 


The method for measuring permanent strain includes the determination of change in the original distance between two measured points on the material surface in accordance with the fundamental theoretical relationship. The first two impressions are made on a portable impression piece and they correspond with the two measurement points before the presumed strain; the other two impressions are made after the  material was loaded. The difference between the first two and the second two impressions is compared and the material strain is calculated. The impression piece is made from a ceramic material with a very low thermal expansion coefficient which permits us to disregard the changes in the temperature of the impression piece at the time of the impression and at the time of its measurement for most applications. The device uses the permanent length base between two extremely hard tips of sintered carbides with tetrahedron shape, which represent the measured points on the material. The tips are molded into a steel bed and attached to the surface by welding or gluing. The portable impression ceramic piece with a square cross-section, with a thin coat of a special material with zero retentivity on one of its sides, makes it possible to make impressions of the tips and to measure the distance and compare the distances between the tips before and after strain.

The measurement can be carried out as frequently as required to determine when and if the structure has exceeded its safety limit.

 

Description of the application set

 

  • Two tips in a steel bed
  • Impression piece
  • Hammer to make impact of the measuring piece on the scale
  • Fig.3    Detail view of the tip molded in a steel bed.


    Measuring microscope to determine the length differences between the tip impressions with the possibility to take a picture of the piece with tip impressions.
  • Software to evaluate the impression distances.

 

Fig.2 Close view of weld connecting the pin to the surface of material to be measured and metallographic sample of weld


The shape and the material of the steel bed depend on the method of application and in particular on whether or not it is possible to weld it onto the structure. A detailed view of the polished section of the welded steel bed with the top on a stainless-steel structure is shown in Fig. 2. A special welding kit (Fig. 1) can weld the tip with a steel bed onto most conductive materials, except for pure aluminum (Fig. 5 and Fig. 7). Both tips are permanently attached to the measured surface, so they can be used for measuring as frequently as needed.

Fig. 5   Surface of the test material with welded tips.

 

Fig. 4   Hammer with impression piece. There is a trigger of the spring mechanism in the left.

 


The impression piece (Fig. 6) is made from a ceramic material with an extremely high thermal resistance (similar to those used for home safety fuses) with a special material on one side of the surface (impact surface). This surface is used for perfectly accurate impressions of the tips. It is possible to measure the tip positions on the ceramic piece using a measuring microscope or by taking a photograph of the piece with the tip impressions (Fig. 6) with a sufficient resolution and evaluating any movement using software specially developed for this application. The piece as well as the impression photograph can be easily stored to keep the obtained data for record. In case of long-term monitoring it is possible to create a strain curve of the monitored element in the given place at the given time.

The hammer (Fig. 4) is a simple, easily adjustable hand-operated tool that holds the impression piece so the user could obtain two tip impressions. The hammer has a button which, when pressed, unleashes the impression piece to the front (driven by the compressed spring), thus creating a sufficient impact to make the required tip impressions. The hammer can  easily be mounted on the extension part to make sure that distant places can be reached. The whole process is very quick and it is only necessary to align the pin holding the impression piece with the tips and press the trigger.  If needed the impression piece can also be guided toward the tips by  special fittings. It is possible to measure the distances between the tips using a measuring microscope

      distance between the two tips l

 

Fig. 6 Impression piece with a detailed view of tip impressions

 

l1

l0

?l

 

l0    distance of attached tips 

before loading

 

l1   distance of tips after loading

 

?l   strain

 

?     relative strain in per cent

 

 with an aiming cross and a traversing device with a sufficient degree of accuracy. The operator puts the piece with the tip impressions under the microscope, locates the position of the impression of the first tip and records its position. After that he slides the piece using a traverse movement device so as to locate the impression of the other tip. The difference between the positions of the first and the second impressions is then used to calculate their distance and to compare it with the distance before critical load. It is also possible to measure the distance between the tip impressions using a photograph of the impressions taken under  predefined conditions. This method allows faster and more accurate measurement, minimizing the requirements on the operator. The measurement accuracy is higher than ± 0.01mm and can be even increased depending on the specific conditions.

 

Fig. 7   Tips welded onto several types of conductive materials.  1- steel, 

2- copper, 3- aluminium alloy, 4 – brass, 5 – stainless steel

3

5

2

4

1


The method and the device for measuring plastic strain provides a cheap, fast and efficient way to monitor and determine the strain of structures even in  very extreme conditions (high temperature, under water, etc).

 

 

 


The Director of the United States

Patent and Trademark Office

Has receiveda n applicanofno r a patentJ br a

newa ndu sefuilw ention.T het itle andd esuipion

of thei nvent[oanr ee ncloseTd.h er equirementso

f law haveb eenc ompliedw ith, and it

has been detemined that a patent on the inventions

hallb eg rantedu ndert hel av,

Thereforeth, is

United States Patent

Grantsto thep erson(sh) avixtgtit le to thi; patent

the right to excludeo thersfr om makingu, sing,

oft\'ering;f or sale, or selling the iwention

rhroughotuhte U nitedS tareosl Americao r [mporting

the int)entionin to the UnttedS tateso f

Americafo r the tem setf orlh below,s ubject

to thep aymenot f maintenanfceee sa sp rovided

l:y law.

If this applicationw asJ iled prior to June 8,

1995, the term of this patent is the longer of

seventeeyne arsf rom the dateo f grant of this

patento r fiuentyy earsf rom the earliestf fictive

U.Sf.i ling dateo f the applications, ubject

to anys tatutorye xtes ion.

If this applicationw as.f iledo n or aJterJ une8 ,

1995t,h et em of thisp atentt s twenty earsf\' c,m

the U.Sf.i ling date,s ubject0 ary)s tatutorye xtensionI.

f the applicationc ontainsa specific

relerencero an earlierJ iled opplicariono r applicationsu

nder3 5 U.S.C1. 20,1 21o r 365(c),

the tem of thep atenti s twentyle arsJ i\'1mt he

dateo n whicht hee (nliesat pplicationw asfi led,

subjectto ary statutotye xtensions.

 

 

 

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