CROSS-REFERENCE TO RELATED APPLICATION

This application claims priority benefit under 35 U.S.C. § 119(d) from French Patent Application No. 18 72185, filed Nov. 30, 2018, the disclosure of which is incorporated by reference herein in its entirety

FIELD

The field of the invention relates to the cracking of materials and more specifically to systems and methods for determining the characteristics of a crack detected in a material, in particular the direction as well as the position of the crack tip or tips.

BACKGROUND

Identification of a crack in a material is a major issue in industry today. Indeed, the propagation of a crack in an industrial structure can result in a malfunction. For example, the propagation of cracks in piping can result in undesirable leaks presenting a potential hazard.

When the characteristics of the crack are available, in particular the direction as well as the position of the tip or tips of the crack, the known formalism of linear elastic fracture mechanics (known by the acronym LEFM) is generally used to predict the propagation of the crack and the probability of this propagation. This predictive analysis makes it possible to evaluate the harmfulness of a crack and to implement the means necessary to prevent the detected crack from propagating in the threatened material. In addition to the possibility of reducing the mechanical loads applied, it is known to apply mechanical loads to the cracked material to hinder the propagation of the crack.

The principles of linear elastic fracture mechanics are particularly suitable for fragile or somewhat fragile materials such as glass, ceramics, or plexiglass. For this type of material, it is observed that the relative displacements of one point relative to another or the opening of the crack are linearly related to the mechanical loads applied. In addition to this property of linearity, the similarity in the crack-opening profile for various types of loads are observed, which makes it possible to describe them by a reduced number of parameters. However, the same principles can also be used to make a more approximate estimate for materials whose behavior exhibits greater non-linearity when a crack is present, such as concrete, graphite, or metals at low temperature.

In order to carry out a predictive analysis of the propagation of a crack based on the formalism of linear elastic fracture mechanics, it is essential to determine in advance the direction of the crack as well as the position of its tip or tips.

Among the non-destructive methods, there is the known method for example of digital image correlation (known by the acronym DIC) using an acquisition camera and which allows, under conditions of strong lighting and no movement of the acquisition camera, achieving a measurement accuracy that is more than satisfactory. Even so, the sensitivity of the digital image correlation to the movements of the acquisition camera as well as the need to have data processing facilities close to the threatened industrial structure make this method difficult to use for a very large number of industrial structures.

It is also known to scatter neutrons or X-rays in the material in which a crack has been identified. This method is also non-destructive, but is incompatible with a large number of industrial structures and is therefore not generally recommended in industry.

The present invention improves the situation.

SUMMARY

For this purpose, a method is provided for determining characteristics of a crack in a material. The presence of the crack is detected beforehand in an area of presence of the material, using a non-destructive process. The method comprises:determining at least two initial mechanical loads applied to the material, each mechanical load being defined by a vector field of known amplitude,applying a plurality of crack-opening mechanical loads to the material, each opening mechanical load being a linear combination of the at least two initial mechanical loads,measuring, using at least one measuring instrument, a relative displacement of a first point with respect to a second point induced by each opening mechanical load, the first and second points being located in the area of presence of the crack, each measuring instrument being associated with a pair of first and second points,applying a plurality of crack-closing mechanical loads to the material, each closing mechanical load being a linear combination of the at least two initial mechanical loads,measuring, using each measuring instrument, the relative displacement of the first point with respect to the second point induced by each closing mechanical load, andestimating the direction of the crack, by computer means, as a function of the amplitude of each opening and closing mechanical load applied to the material and of the relative displacements measured by the measuring instrument(s).

Such a method constitutes a non-destructive method for determining the characteristics of a crack, and in particular its direction, in a material. The determination, or at least the estimation, of the direction of the crack makes it possible to evaluate the harmfulness of the crack and hinder its propagation. Cracks represent a danger for an industrial structure when they affect a material of this structure.

According to one aspect of the invention, at least one measuring instrument is an extensometer.

According to another aspect of the invention, the opening and closing mechanical loads applied to the material are of small amplitude, thus mechanical loads ensuring the material to keep an elastic and linear mechanism behavior.

It is known to those skilled in the art that a mechanical load that a mechanical load is of small amplitude when its application does not alter the material and that the latter maintains an elastic behavior, even under the constraint of this mechanical load.

By applying small-amplitude mechanical loads to the material, the material will adopt an elastic and linear behavior. A material to which the formalism and principles of linear elastic fracture mechanics apply has certain properties useful for determining the direction of the crack. One will in fact observe that the relative displacements of one point of the material relative to another point of the material, or the opening of the crack, are linearly related to the mechanical loads applied. In addition, similarity in the crack opening profile is observed for various types of load, which makes it possible to describe them by a small number of parameters.

According to another aspect of the invention, weighting coefficients for the linear combination of the initial mechanical loads are real numbers between −1 and 1.

According to another aspect of the invention, the measurement of the relative displacement of a first point with respect to a second point comprises measuring the relative displacement of the first point with respect to the second point in a first direction and measuring the relative displacement of the first point with respect to the second point in a second direction orthogonal to the first direction.

It is advantageous to determine the direction of the crack in a plane. We therefore consider a projection of the material in a plane rather than considering the material in three dimensions. The characteristics of the projection of the crack in the chosen plane are then considered, and the first and second directions considered are directions according to orthogonal vectors defining the plane. This plane is advantageously a previously selected plane of interest. In other words, determining the direction of the crack corresponds in fact to determining the direction of the projection of the crack in the plane. Similarly, as detailed hereinafter, determining the position of the tip or tips of the crack corresponds in fact to determining the position of the projection of the tip or tips of the crack in the plane.

According to another aspect of the invention, the method further comprises the determination, for each pair of first and second points, of an opening surface representing the amplitude of the opening mechanical load applied to the material as a function of the relative displacement of the first point with respect to the second point in the first direction and of the relative displacement of the first point with respect to the second point in the second direction in a coordinate system (O, x, y, z), where x is the displacement in the first direction, y is the displacement in the second direction, and z is the amplitude of the load applied to the material.

According to another aspect of the invention, the opening surface is similar to a cone of revolution truncated along a plane orthogonal to the base of the cone and comprising the vertex of the cone.

Indeed, for a material that fully satisfies the hypotheses from applying the formalism and principles of linear elastic fracture mechanics, the opening surface obtained, for pairs of measurement points located one on each side of the crack in close proximity to one another, will be a truncated cone. However, in practice, these theoretical hypotheses in which the material adopts elastic and linear behavior so that the opening surface approaches or is similar to a truncated cone are not fully satisfied. In addition, with only the area of presence of the crack being known, the pairs of measurement points will not necessarily all be located one on each side of the crack.

According to another aspect of the invention, the method further comprises the determination, for each pair of first and second points, of a closing surface representing the amplitude of the closing mechanical load applied to the material as a function of the relative displacement of the first point with respect to the second point in the first direction and of the relative displacement of the first point with respect to the second point in the second direction in a coordinate system (O, x, y, z), where x is the displacement in the first direction, y is the displacement in the second direction, and z is the amplitude of the load applied to the material.

According to another aspect of the invention, the closing surface is similar to a plane.

Indeed, for a material that perfectly satisfies the hypotheses from applying the formalism and principles of linear elastic fracture mechanics, the closing surface obtained, for pairs of measurement points located one on each side of the crack in close proximity to one another, will be a plane. However, in practice, for the same reasons as those mentioned above for the opening surface, these theoretical hypotheses in which the closing surface approaches or is similar to a plane are not fully satisfied.

According to another aspect of the invention, the determination of the direction of the crack comprises:determining the line of intersection of the truncation plane of the cone of revolution corresponding to the opening surface and of the plane corresponding to the closing surface,determining the orthogonal projection of the line of intersection on plane (Oxy) and an angle characterizing the direction of the projected line obtained.

The angle in question corresponds to the angle formed by the projected line obtained, thus the orthogonal projection of the line of intersection of the truncation plane of the cone of revolution and the plane corresponding to the closing surface, and by the axis corresponding to the relative displacement along the axis x or the axis corresponding to the relative displacement along axis y.

In a first case, the crack is a surface crack. The crack thus has an opening at the boundary between the interior and the exterior of the material. In this situation, a single measuring instrument is installed to measure the relative displacement of a first point with respect to a second point, the first and second points being located one on each side of the crack at the level of the opening at the boundary between the interior and the exterior of the material. The angle characterizing the direction of the projected line then corresponds to the direction of the crack.

Still in this first case, the length of the crack corresponding to the distance between the opening and the tip of the crack is determined using a non-destructive process. According to another aspect of the invention, the method further comprises the determination of the tip of the crack as a function of the direction and length of the crack.

Indeed, when the crack is a surface crack, it has an opening at the boundary between the interior and the exterior of the material so that the exact position of at least a portion of the crack is known, namely the points located at the opening. This information makes it possible to directly select at least one pair of first and second relevant points, meaning points located at the opening, one on each side of the crack. The surface character of the crack also makes it possible to implement a non-destructive process for deriving its length. The combination of information about the direction and length of the crack then makes it possible to determine, or at least estimate, the position of the tip of the crack.

Alternatively, in a second case, the crack is an embedded crack and a plurality of measuring instruments is installed. According to one aspect of the invention, the method further comprises the generation of a histogram of angle values obtained for each pair of first and second points, the direction of the crack being determined on the basis of the histogram.

Still in this second case, according to another aspect of the invention, the direction of the crack corresponds to the angle of a local maximum of the histogram. More specifically, the local maximum for determining the direction of the crack is the local maximum furthest from 0°.

When the crack is embedded, less information is available than when the crack is open to the surface. In particular, only one area of presence of the crack is known and it is not possible to guarantee that the pairs of measurement points will be selected so that, for each of these pairs, the first point and second point will be located one on each side of the crack. It is therefore necessary to cover the area of presence of the crack as much as possible and to make relative displacement measurements for each pair of points in the plurality of pairs of measurement points. A histogram then makes it possible to transform a two-dimensional problem, where the points are in a plane, to a one-dimensional problem where the points are then in a line corresponding to the direction of the crack. Indeed, the angle obtained for the pairs of measurement points located one on each side of the crack is significantly close to the angle value characterizing the direction of the crack, so a histogram providing a statistical assessment of the obtained angle values makes it possible to determine the direction of the crack.

According to another aspect of the invention, the method further comprises the determination of the respective positions of the tips of the crack, the determination comprising:plotting a curve representing, along the direction of the crack, the angle obtained for each pair of first and second points located in the direction of the crack,determining, based on the angle corresponding to the direction of the crack, a plateau in the plotted curve, such that the two pairs of first and second points marking the two ends of the plateau respectively correspond to the tips of the crack.

The pairs of points located in the direction of the crack typically correspond to the pairs of measurement points for which the angle αiobtained is in a neighborhood of the angle α. For example, the range of angle values αifor which the pairs of measurement points are considered relevant, i.e. in the direction of the crack, corresponds to the range of values from the largest local minimum less than α to the smallest local minimum greater than α.

As previously explained, the histogram makes it possible to determine the direction of the crack as well as the pairs of relevant measurement points. We then know that the tips of the crack are located on a straight line. By identifying a plateau in the curve representing the angle obtained for each pair of measurement points located one on each side of the crack, it is then possible to deduce the position of the tips of the crack.

According to another aspect of the invention, the method further comprises the prediction of the propagation of the crack, using the direction, the position of the tip or tips of the crack, and the model of linear elastic fracture mechanics, in order to evaluate its harmfulness.

The invention also relates to a computer program comprising instructions for implementing the method described above, when the instructions are executed by at least one processor.

Finally, the invention also provides a system for determining characteristics of a crack in a material. The presence of the crack is detected beforehand in an area of presence of the material by means of a non-destructive process. The system comprises:a diagnostic module for the material, arranged to determine at least two initial mechanical loads applied to the material, each mechanical load being defined by a vector field of known amplitude,a device for applying mechanical loads, arranged for:applying a plurality of crack-opening mechanical loads to the material, each opening mechanical load being a linear combination of the at least two initial mechanical loads,applying a plurality of crack-closing mechanical loads to the material, each closing mechanical load being a linear combination of the at least two initial mechanical loads,at least one measuring instrument arranged to measure a relative displacement of a first point with respect to a second point induced by each opening load and by each closing load, the first and second points being located in the area of presence of the crack, each measuring instrument being associated with a pair of first and second points, anda processing unit arranged to estimate the direction of the crack as a function of the amplitude of each opening and closing mechanical load applied to the material and of the relative displacements measured by the measuring instrument(s).

