FR3133923A1 - process for characterizing a weld bead - Google Patents

Abstract

A method of characterizing a structure comprising a weld bead, the method comprising steps of: acquiring (100) data representing a profile of a surface of the weld bead in a sectional plane of the structure; construction (106) of a mesh in a two-dimensional area delimited by the profile, the two-dimensional area representing the interior of the structure; and determining (108) a stress concentration coefficient (Kt) of the structure in the cutting plane, by applying a finite element method to the mesh. Figure for abstract: Fig. 1

Description

process for characterizing a weld bead FIELD OF THE INVENTION

The present invention relates to a method for characterizing a weld bead.

STATE OF THE ART

The fatigue life of a weld bead depends on its geometry. To evaluate the fatigue resistance of a weld bead, it was proposed to determine certain geometric parameters of this bead.

A first geometric parameter that can be used for this purpose is a radius at the base of the weld bead. This radius is that of a circle which tangents the surface of the cord at a point corresponding to the foot of the cord. A second parameter that can be used for this purpose is a weld angle.

However, these geometric parameters are extremely sensitive to noise. In particular, a slight inaccuracy in estimating the radius at the base of the weld bead can lead to very disparate diagnoses. Thus, two successive implementations of a process based on such geometric parameters are likely to provide very different results.

An aim of the invention is to characterize the fatigue resistance of a weld bead in a reliable manner.

For this purpose, according to a first aspect, a method is proposed for characterizing a structure comprising a weld bead, the method comprising steps of:
- acquisition of data representing a profile of a surface of the weld bead in a sectional plane of the structure,
- construction of a mesh in a two-dimensional zone delimited by the profile, the two-dimensional zone representing the interior of the structure,
- determination of a stress concentration coefficient of the structure in the cutting plane, by application of a finite element method to the mesh.

The method according to the first aspect may include the following optional characteristics, taken alone or combined with each other whenever technically possible.

Preferably, the acquisition is carried out using a profilometer, or even an optical scanning profilometer.

Preferably, the profile is an open profile which is completed, before the construction of the mesh, by additional data representing a line forming with the open profile a closed contour of the two-dimensional zone.

Preferably, the mesh is a mesh of order 2.

Preferably, the acquisition, construction and determination steps are repeated for different sectional planes of the structure, so as to obtain stress concentration coefficients of the structure which are respectively associated with the different sectional planes of the structure. .

Preferably, the method according to the first aspect comprises filtering the stress concentration coefficients of the structure, so as to smooth a curve representing each stress concentration coefficient of the structure as a function of a position of the associated cutting plane.

Preferably, the method according to the first aspect further comprises a step of determining a fatigue class of the weld bead from the stress concentration coefficients of the structure.

It is also proposed, according to a second aspect, a system for characterizing a structure comprising a weld bead, the system comprising:
- an acquisition device configured to acquire data representing a profile of a surface of the weld bead in a sectional plane of the structure,
- a processing unit configured to construct a mesh in a two-dimensional zone delimited by the profile, the two-dimensional zone representing the interior of the structure, and to determine a stress concentration coefficient of the structure in the cutting plane, by application of a finite element method to the mesh.

Preferably, the acquisition device is or includes a profilometer, or even an optical scanning profilometer.

DESCRIPTION OF FIGURES

Other characteristics, aims and advantages of the invention will emerge from the description which follows, which is purely illustrative and not limiting, and which must be read with reference to the appended drawings in which:

There schematically illustrates a characterization system according to one embodiment of the invention.

There is a perspective view of a structure comprising a weld bead, and of an acquisition device according to one embodiment.

There is a flowchart of steps of a characterization process, according to one embodiment.

Figures 4 to 8 are images which represent data produced at different states of the process of characterizing the .

Figures 9 and 10 represent curves showing the evolution of a stress concentration coefficient Kt as a function of a longitudinal position.

In all the figures, similar elements bear identical references.

DETAILED DESCRIPTION OF THE INVENTION

In reference to the , a characterization system 1 comprises an acquisition device 10 and a processing unit 12.

The acquisition device 10 is, generally speaking, configured to acquire data representing a profile of a structure in a cutting plane. Even if other means can be considered, the acquisition device can be a profilometer.

Preferably, the acquisition device 10 comprises an optical scanning profilometer. Such a profilometer makes it possible to determine a profile of a structure without contact with the structure examined.

The characterization system 1 may also include means for moving the acquisition device 10 relative to a structure to be examined. These means of movement can for example include a guide of linear, circular, or even more complex shape. This guide, which may for example include a rail or a slide, can be motorized in order to automate the movement of the acquisition device 10 and to guarantee a homogeneous distance between two acquisition positions. In the following, this distance is called acquisition step.

The processing unit 12 is configured to apply certain processing and calculations to data acquired by the acquisition device 10, which will be described below. For this purpose, the processing unit 12 comprises at least one processor configured to execute the code instructions of a program.

The characterization system 1 also includes a memory 14 adapted to store data produced by the processing unit 12. The memory is of any type (RAM, EEPROM, Flash, HDD, SSD, etc.).

We illustrated on the an example of structure 2 comprising a weld bead 20 extending in a longitudinal direction. The structure comprises a first sheet 21 and a second sheet 22 connected together by the weld bead 20. The bead 20 is a fillet weld bead.

Structure 2 has a free surface capable of being observed by the acquisition device 10. This free surface includes in particular:

• a surface 24 of the weld seam 20,
• a surface 25 of the first sheet, hereinafter referred to as the first surface 25,
• a surface 26 of the second sheet, hereinafter referred to as second surface 26.

The surface 24 of the weld bead connects the first surface 25 to the second surface 26.

In the example shown in , the two surfaces 25 and 26 are planar and mutually orthogonal. The two sheets 21, 22 together form a rectilinear corner covered by the weld bead 20.

In reference to the , a method for characterizing this structure using the characterization system 1 described above comprises the following steps.

In an acquisition step 100, the acquisition device 10 acquires data representing a profile of the surface 24 of the weld bead in a section plane of the structure. This acquisition step is carried out while the free surface of structure 2 comprising surfaces 24, 25 and 26 is facing the acquisition device 10.

For example, the acquisition device 10 is oriented relative to the structure so that the cutting plane is perpendicular to the weld bead 20 (therefore perpendicular to the longitudinal direction in which the weld bead 20 extends).

More generally, the data acquired by the acquisition device 10 representing a profile 30 of the surface of the structure including the surfaces 24, 25 and 26.

When a profilometer is used as an acquisition device, the acquired data is in the form of a set of points each having 2D coordinates in a coordinate system (X, Y) whose axes define the cutting plane. This set of 2D points forms a line, as in the example in .

Profile 30 is an open profile, in the sense that the line formed by the 2D points is an open line, in other words a line not closed on itself. The open line corresponds to the projection on the structure 2 of a laser beam F emitted by the profilometer 10. The beam F has the shape of a sheet, as shown in the .

More precisely, the data representing the profile 30 of the free surface of structure 2 include:

• data representing the profile 34 of the surface 24 of the weld bead 20; these data are 2D points forming a central line in the cutting plane having two opposite ends;
• data representing the profile 35 of the first surface 25; these data are 2D points forming a first peripheral line which extends the central line at one of its ends, and which extends in a first direction in the cutting plane;
• data representing the profile 36 of the second surface 26; these data are 2D points forming a second peripheral line which also extends the central line at the other of its ends, and which extends in a second direction in the cutting plane.

In an alignment step 102, the processing unit modifies the data acquired in step 100 so as to move the profile 30 into a reference position. During this step, the acquired data undergoes a rotation in the frame (X, Y) and, if necessary, a translation. In practice, this step modifies the coordinates of the 2D points which constitute profile 30.

In the reference position, the profile of one of the two free surface portions is arranged parallel to a reference direction. That is, the acquired data is rotated so that the first direction or the second direction corresponds to the reference direction. In addition, the profile of the other of the two surfaces can be translated so as to occupy a certain position, in this reference direction. There represents a possible outcome of this alignment step. In this example, the reference direction is parallel to the abscissa axis X of the coordinate system (X, Y).

In a profile closing step 104, the processing unit determines a two-dimensional zone 50 delimited by the profile 30. This zone 50 is a theoretical representation of a section of the structure 2 in the cutting plane in the vicinity of the bead of weld 20. This zone 50 does not necessarily respect the rear profile of the structure 2. Nevertheless, this representation remains faithful to the structure in the vicinity of the weld bead 20.

To obtain the two-dimensional zone 50, the processing unit 12 completes the data representing the profile 30 with additional data, typically three additional 2D points.

We represented in an example of a two-dimensional zone 50 obtained at the end of step 104. This zone 50 is delimited by the profile 30, and by a complementary line 40, the profile 30 and the complementary line 40 together constituting a closed contour of the zone two-dimensional 50.

Preferably, the complementary line 40 is made up of segments, for example only four segments. In the example shown on the , the drawn line is made up of the following four segments:

• a first segment 41 parallel to the profile 35 of the first surface,
• a second segment 42 parallel to the profile 36 of the second surface, and connected to the first segment 41,
• a third segment 43 connecting profile 35 to segment 41, parallel to profile 36
• a fourth segment 44 connecting profile 36 to segment 42, parallel to profile 35.

We understand that the two-dimensional zone 50 represented on the can be described entirely on the basis of the data representative of the profile 30 acquired in step 100, supplemented with only three additional points determined in step 104: the junction point between the segments 40 and 41, the junction point between the segments 41 and 42, and the junction point between segments 42 and 44.

In a meshing step 106, the processing unit 12 constructs a mesh 60 of zone 50. The mesh obtained includes elements defined by points and edges.

The constructed mesh can be of order 1, in which case an element of the mesh is triangular (it is characterized by three vertices and three rectilinear edges). But the mesh constructed by the processing unit is preferably an order 2 mesh. In an order 2 mesh, a finite element has three vertices and three curved edges. Constructing an order 2 mesh has the advantage of being less sensitive to local geometry than an order 1 mesh, which reduces the variability of the results.

The mesh 60 of the zone 50 is constructed so as to present a greater density in the vicinity of the profile 30 than in the vicinity of the complementary line 40. In other words, the elements of the mesh 50 are smaller in the vicinity of the profile 30 than in the vicinity of the complementary line 40. in the vicinity of the complementary line 40. Elements of the mesh 50 in the vicinity of the profile 34 of the surface of the weld bead are small, and other elements of the mesh 50 near the segments 41 and 42 are comparatively larger. This makes it possible to have sufficient calculation fineness in the areas of interest of the mesh while reducing the number of elements in areas of less interest, which makes it possible to reduce the duration of the calculations which are subsequently carried out on the base of mesh 50, and which will be discussed below. There illustrates an example of mesh produced in step 106.

The maximum size of the mesh elements (corresponding to the size of the elements which are closest to segments 41 and 42) is for example chosen equal to the distance between two measuring points of the profilometer.

In a step 108, the processing unit determines a stress concentration coefficient Kt of structure 2 in the acquisition cutting plane, by application to the mesh 50 of the finite element method. This method is known to those skilled in the art.

During this step 108, the zone is subjected to a tensile force in the reference direction. The finite element method is based on prior knowledge of the materials of structure 2, in particular the Young's modulus and the Poisson's ratio of the sheets 21 and 22, which are for example made of steel. During step 108, symmetry boundary conditions are applied along the complementary line 40 and are symbolized by triangles with two circles on the . The arrows indicate the normalized force applied to the end of the meshed zone 50 making it possible to obtain a normalized principal stress, for example at 1 MPa. The whole is calculated in order to obtain the first principal stress at each node of the mesh, which directly gives the coefficient Kt, as indicated in . The calculation requires few resources and time due to the fact that it is carried out on a zone 50 in two dimensions located in the cutting plane of the acquisition 100.

The coefficient Kt thus obtained is stored in memory 14.

Steps 100, 102, 104, 108 are repeated for different cutting planes of structure 2. Thus, data relating to different profiles of the free surface of the structure. For example, the acquisition device 10 is moved in a longitudinal direction parallel to the weld bead between two successive acquisitions, so that the different data acquired relate to cutting planes of the structure which are all parallel, having positions respective different in the direction of movement of the acquisition device 10.

The acquisition step, that is to say the distance in the longitudinal direction between two adjacent cutting planes, is for example of the order of 0.2 millimeters.

Steps 102 to 109 can be carried out in sequence so as to produce a coefficient Kt, before moving the acquisition device relative to structure 2 and repeating these steps once again. Alternatively, the acquisition step 100 can be repeated in different positions of the acquisition device 10, before implementing steps 102, 104, 106, 108.

At the end of the repetition of steps 100 to 108, the processing unit 12 produces a plurality of stress concentration coefficients Kt respectively associated with the different cutting planes of the structure 2, and which are stored in the memory 14.

The plurality of Kt coefficients can be represented by a curve taking on the abscissa the longitudinal positions of the cutting planes, and on the ordinate the values of the associated Kt coefficients. The presence of calculation or measurement noise can make this curve very irregular, as shown in . To counter this phenomenon, the processing unit applies filtering adapted to the plurality of coefficients Kt to smooth this curve. There shows the result of this smoothing step applied to the coefficients of the .

The local maxima of the smoothed curve constitute information which provides information on the fatigue resistance of the weld bead. In particular, this information can be used to determine a fatigue class of the weld bead.

The process described above is not limited to fillet welds; it can be generalized to any form of welding.

Furthermore, the use of a profilometer is not obligatory, even if it is particularly advantageous. The acquisition device 10 could in fact include a 3D camera adapted to acquire a 3D image of the surface of the weld bead; a plurality of 2D profiles of the surface of the weld bead can then be extracted from such a 3D image. As we can see, this embodiment makes it possible to obtain 2D profiles of the weld bead, but in a less direct manner and more computationally expensive than with a profilometer.

Claims (10)

Method for characterizing a structure comprising a weld bead, the method comprising steps of:
- acquisition (100) of data representing a profile of a surface of the weld bead in a sectional plane of the structure,
- construction (106) of a mesh in a two-dimensional zone delimited by the profile, the two-dimensional zone representing the interior of the structure,
- determination (108) of a stress concentration coefficient (Kt) of the structure in the cutting plane, by application of a finite element method to the mesh. Method according to the preceding claim, in which the acquisition (100) is carried out using a profilometer, preferably an optical scanning profilometer. Method according to one of the preceding claims, in which the profile is an open profile which is supplemented, before the construction of the mesh, by additional data representing a line forming with the open profile a closed contour of the two-dimensional zone. Method according to one of the preceding claims, in which the mesh is an order 2 mesh. Method according to one of the preceding claims, in which the steps of acquisition (100), construction (106) and determination (108) are repeated for different sectional planes of the structure, so as to obtain concentration coefficients stresses (Kt) of the structure which are respectively associated with the different cutting planes of the structure. Method according to claim 5, comprising filtering (110) the stress concentration coefficients of the structure, so as to smooth a curve representing each stress concentration coefficient of the structure as a function of a position of the associated cutting plane. Method according to one of claims 5 and 6, further comprising a step of determining (112) a fatigue class of the weld bead from the coefficients of stress concentration coefficients (Kt) of the structure. System (1) for characterizing a structure comprising a weld bead, the system comprising:
- an acquisition device (10) configured to acquire data representing a profile of a surface of the weld bead in a sectional plane of the structure,
- a processing unit (12) configured to construct a mesh in a two-dimensional zone delimited by the profile, the two-dimensional zone representing the interior of the structure, and to determine a stress concentration coefficient of the structure in the cutting plane , by application of a finite element method to the mesh. System according to the preceding claim, in which the acquisition device (14) is or comprises a profilometer. System according to the preceding claim, in which the profilometer is an optical scanning profilometer.