FI104761B - Procedures for determining extensions - Google Patents

Description

104761

METHOD FOR DETERMINING STRENGTHS

The present invention relates to a method for determining the elongations and changes thereof in the surface of bodies such as metal containers, whereby a brittle material, so called, is applied to the surface of the body. a stretching coating, for example a tension varnish, while the body is in a resting state, and then subjected to a stress, preferably at least equivalent to the use situation, whereby, when the stretching threshold elongation is exceeded, cracks are formed. placement of ribbons, for more accurate measurement or determination of elongation.

One area of technology where stress measurement is important is the transport industry, in which in the following, we will use tanker tanks as an example, i.e., a hard-to-size, shell-shaped shell structure. For a long time, a solution has been sought for sizing, which is complicated by deformation errors, but it has not been successful. Fatigue performance studies, and especially experimental measurements to determine the strengths and structures of the material required, are expensive and time consuming. When examining welded shells 20, such as tanks, it is particularly difficult and impossible to securely attach the stretch tabs to suitable locations. Finding the right v direction of the strip, and especially the position, is difficult. The so-called. the tension ceases.

25 Tension lacquer or other so-called. brittle coatings have traditionally been used to find the most stressed point in the following way: thin layers of lacquer, approx. 0.05 mm thick are applied to the surface of the piece. Apply varnish by spraying and allow • l to air dry. A reflective coating can also be used under the varnish. As the component is loaded, cracks appear when the threshold elongation is exceeded.

Typically, the lacquer has a nominal threshold elongation of 500 pm / m. Cracking occurs first where the tensile stress is highest, which quickly indicates areas of stress concentration. Furthermore, the cracks occur at right angles to the direction of the maximum tensile stress. This identifies the locations of the critical areas where the strain gauges should be placed for accurate measurement.

Although the position and orientation of the strip is basically correct by means of the varnish, the point 5 is often very problematic due to the actual multiaxial and yet often multistage design of the fatigue load. In a welded structure, the greatest stresses (elongation or tension) usually occur at the welding edge at the very edge of the weld, to which the strip cannot be attached. At their most dangerous, they are perpendicular to the weld.

10 In a stressed metal surface, such as a container, the strip cannot be positioned, for example, between the critical cradle and the container. In this case, the measurement has to be made, for example, from the inside of the container. This again results in difficult protection measures and measurements have to be made to achieve the correct load with water fill. In addition, results on the "wrong" side are far more uncertain. On the other hand, some growing cracks can sometimes be "seen" from one side of the sheet as densified cracks in the stretch coating. section. For example, the torque of the container structure may be used as a load.

20 Due to structural discontinuity, the strain gauge cannot always be attached to the desired location. For example, in welded container structures, the strip will need to be slightly fixed to the area of the flat plate prior to welding. In this case, the increase in tension caused by the discontinuity of the weld and any edge error, such as a notch, must be taken into account in other ways. In practice, it must be possible to extrapolate reliably the value of the elongation / strain to the worst-case location of the welding fault caused by the welding error described above. This - * can be done to a certain degree analytically by methods of strength theory. Once the parameters describing the geometry of the weld are known, the design can also be made according to the SAE fatigue design method.

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In the welded tank structure, the interruption caused by discontinuity is usually not reliably and simply quantifiable at the design stage. In simple cases, if the shell thickness of the shell is sufficient and there is no deformation, the elemental method (FEM) may be used for the calculation at design stage 5, but the procedure is also e.g. due to the vagueness of the welds (statistical treatment) quite laborious. The increase in tension when approaching a weld is often difficult to control. In thin-walled tanks (shell structures), secondary bending stresses caused by deformations prevent reliable FE modeling. The situation is difficult for all tanks if / when a cross weld occurs at the same location. 10 A typical example is when the joint of the shell of the container meets the joints of the shell end. The situation becomes even more difficult if the bracket, the lead-through, the change in the material strength, etc., fall into the same place. Practice has shown that fatigue damage hits the same discontinuity, often even at the same point. Real dimensioning cannot be done with a simplified theoretical model. In practice, the situation easily becomes very complex and the element methods are then expensive, uncertain or, due to the complexity of the situation, not even usable. In this case, experimental design has to be resorted to. Sizing can be done with a prototype or with test pieces.

20 If it appears necessary and profitable, the study can be carried out quite reliably with several stretch tabs glued to the same area. In this case, it is worthwhile to use the so-called multi-directional measurement. rusettiliuskoja. The elongation of the area inside the strips on a flat plate can be predicted very reliably, for example, by a linear model. Tank shape errors exacerbate 25 situations. Also, the increase in stress caused by weld geometry without the effect of a weld, edge error, can be predicted linearly by two or three strips glued at a suitable distance. This is now called 'so-called. hot spot excitement. There are three ways of determining hot spot stress in the scientific literature: calculation by the element method, multiplying the nominal stress by any known stress concentration factor 4 104761, and experimental stress measurement by means of strain gauges.

The multi-strain strip method with its uncertainty factors may be practically accurate enough to perform the study, but it is laborious and therefore expensive. In some large measurements, there may be up to a hundred stretch tabs. When analyzing the measurement results later, one should often know the tension in the vicinity of the strips. In practice, this is usually solved by gluing a new sheet of the same strip. , make wiring and make the necessary measurements. This requires a great deal of work, especially if the instrumentation has already been disassembled, as has often happened in practice.

The object of the invention is to provide a method which is simpler and sufficiently reliable. The method according to the invention is characterized in that by means of the cracks in the stretch coating, the value indicated by at least one stretching strip is extrapolated in a desired direction to a desired point or locations, for example, a fatigue bar. Also, for example, at the bottom of the weld edge, ie the maximum local elongation, which is usually calculated using the Neuber method, for example. The elongation coating can be very thin, even less than 0.01 mm.

An embodiment of the invention is characterized by measuring the number of fracture coatings of a stretchable structure over a length of structure under investigation, making the same measurement of, for example, a thinner structure, resulting in more cracks occurring over the length under investigation, determining corresponding elongations / extensions.

25

Another embodiment of the invention is characterized in that a strain gauge is measured at a selected crack density and the behavior measured from at least one strain gauge, such as stress variation, is extrapolated directly to the desired location and direction by means of an elongation coating.

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The basic idea of the invention is that in the linear region the load or stress variation is in principle inversely proportional to the applied material thicknesses.

Thanks to the invention, stress measurements are simpler and more economically advantageous than before. In addition, the measurement results are more reliable. It is also an advantage of the invention that extrapolation can be performed, for example, when using multiple stretch strips, at any position, and especially within the shell structures, within certain limits, taking into account the constriction, in any predetermined direction. In practice, this means that extrapolation from a strip very far away (or even from a reliably calculated value) can be performed with due care. In principle, only one strip is needed. In practice, the so-called. the ribbon is very useful. Because the tension clevis cracks are known as the direction of the main stress, an additional advantage is obtained at both the 15 strips and at the hot spot: unless the stresses are, for some reason, the necessary correction calculation. This is, for example, a very common situation in the corners of shell structures, especially in the case of break welds. The method also has the advantage that the threshold stress of the elongation coating used for calibration is not significant, so that inexpensive elongation coatings and other advantageous brittle coatings can be used, e.g.

. chalk wool or cheaper hard (cracking) lacquers instead of tension varnish.

In the following, the invention will be explained in more detail with reference to some embodiments of your embodiment.

25 - - - ________ ______? The basic method could be characterized as a method of measuring the number of cracks in the elongation pin of a structure of durable length (e.g.

* 2 mm thick). The same measurement is also made for a thinner structure.

In this case, more cracks will appear for the corresponding length of the study. Strain-30 variations are proportional to the number of cracks mentioned above, so the 6104761 method can simply be used to perform rapid, rapid tests to determine whether further investigation is warranted. For example, a particular burst density (e.g., 6 pieces / cm) may be selected for which densities are greater than that. fatigue analysis.

5 The fatigue analysis with the elongation coating is carried out by measuring the elongation strip at the selected crack density (eg 4 pcs / cm). The behavior measured from the strip (e.g., tension) is extrapolated directly to the desired position by means of an elongation coating. For example, if at a desired point 10, the burst density is 8 pieces / cm, the stress variation is also doubled.

If the limit of fatigue analysis is 6 pcs / cm, the analysis should be done.

In the preliminary screening step, the elongation coating can be used as follows: if any point in the structure is suspected, it may be sprayed with suitably cracked or up to 15 detachable elongation coatings. The need for further investigation can be deduced from the crack density of the stretch coating. In practice, the cracking density of the elongation coating gives a sufficiently reliable indication of the elongation variation of the sheet surface. Unless an elongation coating, such as chalkboard or other material, is used. the substance suitable for the purpose is cracked at all, is below its threshold, and the said substance is the site is not susceptible to fatigue or an inappropriate coating is used. Easily detachable coating effectively reveals multi-axis load. areas. For example, a hardened chalk wool works out when it collapses. After that, it can be done. the area is subjected to more accurate stretch coating measurements.

If necessary, the examination of the notch shape number and the notch effect number can be performed as follows: The notch shape number is the ratio of the increased stress / rated stress caused by the discontinuity locally occurring. This can often be calculated directly in the given case. section with densified cracks. The notch impact index * · is the ratio of the fatigue strengths of the notched and notched body. In determining the groove effect number, a similar comparison is made by bending the crack density increase obtained by the discontinuity ai-30 to a "smooth" piece.

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The elongation coating method according to the invention is also well suited for determining the aforementioned hot spot stress. The burst density of the stretch coating can be used to extrapolate the tension of the strip to the hot spot 5. In addition to the hot spot stress, the same method can be used to examine the peak of tension at the edge of the weld, e.g. This is accompanied by the appended drawing, in which:

Figure 1 illustrates generally the principle of measuring and extrapolating hot spot elongations at 10 welds.

Fig. 2 is a top view of the weld joint of Fig. 1, after the elongation coating test, with three unidirectional stretch tabs.

Fig. 3 shows essentially the same as Fig. 2, but under a different stress condition.

Figure 1 shows a cross-section, for example, of a container wall 1 and a sheet 2 welded thereto or the like. Reference numeral 3 represents a weld. The arrows 20 show stress situations at various points on the container wall under stress. In order to detect stresses, three strain gauges A, B and C are attached to the surface of the container.

Figures 2 and 3 are schematic diagrams of the determination of hot spot stress by means of stretching cracks 4. Thus, 25 stretching agents or the like have been applied to the test site, after which the test piece is preferably loaded in a manner appropriate to the application. This will cause cracks to occur in the elongation coating as soon as its threshold elongation is exceeded. The elongation can be extrapolated from any of the values of strip A, B, or C in the desired direction and position using the cracks. In Fig. 2, the crevices 4 are parallel to the weld boundary line and in Fig. 3, the crevices are at a plate about 30 ° at an angle of 104761 δ with respect to the weld boundary line. In the area of the weld, the cracks turn about 30 ° more. This is not possible, for example, with the measurement of a ribbon strip (without the stretch coating method) because the strip cannot be attached to the weld. This is a typical and dangerous situation, for example in the case of a disc welding of a plate structure. The arrows 5 illustrate these directions of the main stretches.

The previously mentioned problem of additional heavy measurements in the vicinity of strain gauges can also be solved by the method of the invention; since the stress state of the previously measured strip site is known, it can subsequently be extrapolated very precisely to the desired location, e.g., at the weld edge, by means of cracks in the stretch coating applied to the same area. With the main tension obliquely (e.g. at 45 °) to the weld, it is often difficult to know at which angle it exceeds the weld. If the tension turns parallel to the weld, the situation is not as dangerous as in the opposite situation. In the worst case scenario, the main stress may be at or near perpendicular to the weld. It is not always easy to deduce this in the shell structure. The tension space can be easily and reliably secured even afterwards by means of a stretch coating.

The method described above can be utilized even in stress comparisons between products and in the investigation of the load history of previous products / structures. Performing the measurement is a small job as all you need is spray or other application of the varnish. and load. In practice, they can be carried out in the field, eg in a garage. In order to verify and calibrate the method under engineering conditions, tests must have been carried out on areas previously measured on strips. Also, simple bending tests on pieces, plate strips, welding tests made of them, etc., can easily be performed with the aid of a stretch coating. Although the experiments are relatively simple, their information value is very high.

Determination of local elongation variation at the bottom of the notch (e.g., weld root defect) 30 is usually achieved directly by the elongation coating method. Notch compression stretch 9 104761 does not need to be calculated using the relatively cumbersome and inaccurate methods traditionally used by Neuber and Glinka, where local stretch is usually defined by Ram-ber-Osgood-shaped stress and elongation dependence.

* 5 In the SAE fatigue design, the local stress peak should be taken into account as closely as possible. This invention defines a local elongation at, for example, the bottom of a notch at the weld boundary line. cracks with a suitable elongation coating.

It will be apparent to one skilled in the art that the invention is not limited to the exemplary embodiments set forth above, but may be varied within the scope of the appended claims.

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Claims (3)

104761 A method for determining the elongations and changes thereof in the surface of articles (1), 5 such as metal containers, etc., in which a brittle material, so-called, is applied to the surface of the article. a stretching coating, for example a tension varnish, while the body is at rest, and then subjected to a stress, preferably at least equivalent to the use situation, whereby the stretching threshold elongation causes cracks (4) to determine 10 of the stretching strip or various strips (A, B, C). , e.g. a ribbon strip for positioning for more accurate measurement or elongation determination, characterized in that the elongation coating cracks (4) extrapolate the value indicated by at least one elongation strip into desired directions, for example fatigue tabs. 15 Method according to Claim 1, characterized in that the number of cracks in the elongated coating over a length under test is measured, the same measurement is made for example of a thinner structure, whereby cracks (4) occur more over the length under study, corresponding elongations / elongations are (4). 3. A method according to claim 1, characterized in that the strain gauge is measured at the selected crack density and the behavior measured at least one of the strain gauges (A, B, C), such as stress variation, is extrapolated by stretch strain cracks directly proportional to the desired locations and directions. 11 104761