FI126728B - A method for finding out the fatigue state in a damaged metal structure - Google Patents

Description

The present invention relates to a method for determining the fatigue state of a damaged metal structure around a failure site, such as a crack, wherein the hardness of the metal is measured at various points.

The invention relates to the fatigue phenomena of metals and their investigation. The background for this can be found in the dissertation Markku Käppi: "Development and Use of the Voltage Variation Collective in Vehicle Designing" from 2001 and in Finnish Patent 104761.

The metal structure of mobile devices is subject to a number of different dynamic stresses that can lead to fatigue of the metal over time. In practice, tertiary plate surfaces are a very difficult object. Typical examples of such. metal products include vehicle tanks, boats and ship structures, airplanes, etc. For example, when a ship is in motion, it is subjected not only to vibrations caused by engines and other equipment, but also to stress caused by waves and the use of weapons. The latter are very different in nature, but together they wear out the work resistance of the metal while fatigueing it. Together, these can quickly cause fatigue of the metal and lead, for example, to a rather rapid, even destructive, breakdown of the structure through the strengthening of the metal. For example, if a crack in the aluminum sheet becomes / grows due to fatigue, it can be welded over, but experience has shown that the structure tends to break quite quickly near the welded repair point, but not traditionally in the weld geometric discontinuity or heat-affected zone. Thus, damage to the metal structure is not limited only to the crack or weld but also to the indefinite area around it. This damage is typically due to a cyclic deformation of the metal, which usually results in either softening or hardening of the metal. Both result in fatigue loading, resulting in metal structure damage, typically cracks or breaks at the vague point. The problems described above are very typical for stress-strain curves, i.e., non-linear behavior metals such as aluminum and stainless steel. Aluminum has also been used extensively in vehicles, aircraft and shipbuilding, for example.

A similar situation occurs with the fatigue of some yield-limited, linearly-acting metals, particularly high-strength steels, near the crack tip. This is mainly due to the fact that in the region near the crack tip, the yield stress of the steel is locally exceeded and the so-called steel stress limit is exceeded. elastic recovery is not after that. area no longer possible. However, immediately outside the range, the steel behaves in an elastic / linear fashion. The situation with fatigue is very surprising and dangerous. From such a small / narrow area, for example, it is virtually impossible to take a biopsy.

Before deciding how to successfully repair a tired structure, it is necessary to determine the full extent, shape and quality of the fatigue damage of the structure and the cyclic strength change of the material. This gives you an idea of how much of the real structure, for example, needs to be replaced with a new one.

The extent of the damage can be tested by measuring hardness in the structure around the lesion or from a detached specimen. However, in practice, current hardness measurement methods are very cumbersome and expensive, especially in structures where working space is generally low and working position difficult. The precision presser probes on the equipment are extremely susceptible to damage, easily soiled, difficult to clean and expensive. In addition, cyclic deformations of the metal generally occur indefinitely in the zones, so they should rapidly obtain accurate measurement data over a fairly wide and even zone around the lesion area.

Hardness in technology refers to the ability of a material to resist foreign matter, scratching, wear and shearing. There is no unambiguous way to measure hardness, but it is defined as the result of the measurement method used. Traditionally, hardness measurement is divided into three parts: a) Scratch resistance, which is traditionally measured by the Mohs method. b) Compression hardness, which measures the area or depth left by the tip of the press, eg. Vickers, Rockwell, Brinell and Knoop. c) Tensile strength, eg Leeb's method.

The results of the different methods are very poorly comparable. It is an object of the present invention to provide a sufficiently simple, reliable and fast method for solving the fatigue state of a metal structure in a practical repair without removing the specimen. The method of the invention is characterized in that the hardness calibration result is taken from a non-fatigued or cyclically altered region of the metal structure under investigation, at a distance from the point of failure, and comparing the required results around the point of failure with a calibration result. For example, the calibration point can be searched for by moving away from the point of damage so far into the homogeneous region that the measurement result no longer changes.

A preferred embodiment of the method according to the invention is characterized in that the hardness is measured by impacting the pits of the metal under investigation in a controlled manner with constant impact force, followed by measuring their surface and / or depth and comparing it with the calibration result.

Another advantageous embodiment of the method according to the invention is characterized in that the wells are struck with a self-opening so-called. with an automatic point stick with constant striking force, which uses the same tip throughout the measurement.

Another preferred embodiment of the method according to the invention is characterized in that the hardness is determined by scratching the surface of the metal structure being examined with a carbide tip at various points and comparing the slip caused by the hardening or softening of the surface with the calibration point. Such slippage can be detected quickly and with sufficient accuracy, even at such narrow places that no other experiments / measurements can be seen or performed.

Advantages of the invention may be mentioned that by means of the method it is simple and advantageous to solve the state of fatigue of the metal as a whole with sufficient reliability and over a wide range. This provides quick and reliable information on how and to what extent damage to a metal structure would be most cost effective. This can be seamlessly combined with immediate repair welding, etc. Similarly, the method can be used to deduce otherwise invisible directions of cyclic loads, i.e., what is wrongly designed / done in the structure, that the damage has occurred at all. Thus, the structure itself can be reliably improved in the future, and such problems will no longer arise. Just visible cracking growth with directions is usually not enough for this.

In addition to being illustrative, the advantage of experimenting with the actual structure method is that, for example, the plate's Vals dry direction and all manufacturing factors and cyclic deformations are automatically taken into account in the actual structure.

In the following, the invention will be explained in more detail by means of preferred embodiments with reference to the accompanying drawing, in which the metal structure and the resulting crack are shown.

Thus, reference numeral 1 denotes a metal structure, for example an aluminum plate, which has developed a visible crack or crack 2 due to fatigue of the metal. The aluminum plate may be, for example, a plate or structure used in a tank shell, ship side structure or aircraft. Here, it is shown in rectangular form, but the shape is not relevant to the invention. It can also be a support structure, stiffener, beam, etc.

The traditional method of operation would be to weld the crack 2 or replace a vague or, for safety reasons, a sufficiently large piece of aluminum sheet with a new one. However, it is difficult to know what is a sufficiently large and properly shaped piece because the extent and exact quality of the damage is unknown. Thus, in practice, an unnecessarily large but completely incorrectly shaped piece is usually replaced for safety reasons, and the repair fails.

The method of the invention seeks to find out what has happened to the metal in the vicinity of a crack or similar damage site. Namely, welding is of no use if there is an indeterminate softening or curing of metal around the crack due to fatigue of the metal, but the damage is usually quickly or unexpectedly renewed. This is particularly true for metals with a stress-free stress-strain curve, that is, non-linear behavior.

In the method according to the invention, small pits are punched on the surface of a 3 metal at different distances from the damage / crack 2. The punch is always made with the same constant striking force and the same tip in each measurement. The surface and / or depth of the wells thus formed are proportional to each other. The punch is performed with a constant force with an automatic point stick or similar simple device.

When it comes to finding out what has happened to a metal, it is best to take a reference, that is, a calibration result from an area where the metal is in perfect condition, that is, far enough away from the damaged area. If the material used is known, the additional reference point may also be taken from a corresponding non-tiring material, for example an unused plate from a storage shelf. The lowest point in the picture could be such a reference point. From here, we start approaching the actual damage site. When a well 3 is punched into the metal surface of the well 3, the surface and / or depth of the resulting well depends on the hardness of the material in question. section. The softer the metal, the bigger the pit will naturally be. Conversely, the harder the metal, the smaller the pit will be. The size of the pits is comparable when always struck with the same force and the same tip. Thus, this is accomplished simply and quickly with an auto-point stick. The measured wells are compared with the calibration result to determine whether the metal has softened, strengthened or remained intact. mittauskohdas-SA.

The surface / depth measurement of the well 3 can be made by any known technique, for example, digital imaging and transferring the image to a computer for enlarging it. The image can be used to measure the diameter of wells of circular cross-section or other major dimensions of a well formed by a probe of another shape, such as a cone, a ball, etc. Another option is to use different 'loupes' for measuring, that is, magnifying glasses with a scale.

For example, when examining or searching for a disc or structure that has become cyclically inhomogeneous, or especially for densified / formed flow areas, the elliptical cross-section of the probe is very useful in showing the interdependence of the direction of the well axis and the resulting size. The new knowledge thus acquired is of paramount importance for repair and, in particular, for improving design or raising the level.

The invention provides a fairly accurate picture of the state of the damaged metal structure. If the structure is undamaged except for the crack, then the crack should be welded. If, on the other hand, the cyclic deformation caused by metal fatigue is within a wider range, the metal structure may be removed from this area and welded to a new site or, in some cases, left unrepaired.

It should be noted that the pits 3 should not be struck too close to each other as they easily begin to deform the metal and distort the measurement result. The minimum safety distance between the wells is about 3 mm.

When the target to be examined is a thin plate, the sharp point of the auto-point stick may pass through the plate with a slight force of impact. This results in too much deformation, resulting in damage to the structure and not always reliable research results. In this case, the slippage of the surface with a carbide tip or similar is examined. This is quite easily discerned by a skilled / experienced person. First, take the 'calibration slip' far enough from outside the fatigue range. Paint or similar coating must first be removed from the surface. The hardened surface is harder and feels much slippery. On a softened surface, the tip even bumps and feels rough and not as slippery.

The test tip may also be made of the metal to be tested. This will cause the tip to become damaged / dull in the stiffened area but leave a clear mark on the softened area. With less experience than this, you can do this.

It will be clear 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. Under certain conditions, the method can also be used in the vicinity of fatigue cracking with yield-stressed, i.e. linear-acting steels, in particular high-strength steels. In this case, the yield point is exceeded locally at the tip of the crack and the reversal of the load variation does not occur linearly in the vicinity of the crack, although the surrounding area behaves more linearly.

It is clear that several different tips can be used to examine the same area of damage. In this case, the comparison is, of course, always made in the respective case. to the tip calibration result.

If necessary, the symbols which may be presented in the specification together with other characteristics may also be used separately.

Claims (2)

1. Menetelmä vaurioituneen metallirakenteen (1) väsymis-tilan selvittämiseksi vauriokohdan, kuten halkeaman (2), ympäriltä, jossa menetelmässä metallin kovuutta mitataan eri kohdista, tunnettu siitä, etä kovuituen kalibuuden väsynyt tai syklisesti muuttunut, etäisyyden päässä vauriokohdasta, että vertailutu-loksia otetaan tarvittava määrä vauriokohdan ympäriltä yes verrataan niitä kalibrointitulokseen, joka kovuus mitataan iskemällä tutkittavaan metalliin kuoppia (3) hallitusti vakio lyöntivoimalla, jonka jälkeen mitataan niiden pinta-ala ja / tai syvyys yes verrataan calibrointituloxene, jolloin kuopat (3) isketään itselaukeavalla ns. automataat-tipistepuikolla vakio lyöntivoimalla, jossa käytetään koko mittauksen ajan ellipsin muotoista kärkeä. CLAIMS A method for finding out the fatigue state of a damaged metal structure (1) in the vicinity of a point of damage, such as a crack (2), in which the method of measuring the hardness of the metal in different places, characterized by taking a calibration result on hardness in an area of the investigated metal structure (1) where the metal is not fatigued or cyclically altered, at a distance from the damage point, and that a necessary amount of comparison result is taken in the environment around the damage point and compared with the calibration result, which hardness is measured by that pits (3) are controlled in a controlled manner with a constant impact force in the investigated metal, after which they measure their area and / or depth and compare with the calibration result, whereby the pits (3) are struck by means of a self-triggering automatic runner with a constant impact force that uses a tip in elliptical form throughout the measurement.