ES2361349B1 - MATERIAL TESTING PROCEDURE TO PREDICATE YOUR BEHAVIOR IN A CONFORMED BY FAST HEATING IN PROGRESSIVE PROCESSES. - Google Patents

Abstract

Material testing procedure to predict its behavior in a rapid heating process in progressive processes. # It comprises fixing a test piece (1) in a universal testing machine and starting a constant speed tensile test under a displacement control, if it has the tensile test and apply a heating on an area (3) of the specimen (1) for a short period of time, resume the test and obtain the test curve of force (P) displacement of the specimen (1) (D ), calculate the reduction of the elastic recovery suffered by the specimen (1) in the heated zone (3) from the test curve to determine the behavior of the specimen material when it is subjected to a rapid heating forming in processes

Description

Material testing procedure to predict its behavior in a rapid heating process in progressive processes.

Object of the invention

The present invention, as expressed in the statement of this specification, refers to a material testing procedure that is intended to predict the behavior of materials when subjected to a rapid heating forming process in progressive processes

The invention is applicable in any sector of the industry in which it is required to determine the behavior of a material in a progressive process formed by rapid heating, and more particularly in the automotive sector.

Background of the invention

The use of high-strength materials makes it possible to manufacture resistant components of low weight, which also have excellent performance in case of impact. However they accuse limited conformability.

The formability is affected by the high elastic limit of engineering alloys, by storing large amounts of energy in the form of elastic fields, which they later release in the form of distortions. Therefore, the final geometry of the shaped pieces always moves away from their desired geometry. This relaxation of stored energy is called "Spring Back" or spring effect due to the elastic recovery that occurs. The elastic recovery translates into a geometric inaccuracy, unacceptable in terms of product quality in many industries.

To alleviate this problem, a technique for performing assisted shaping by the simultaneous application of a laser beam, an induction coil, or any rapid heating technique at the same time as mechanical stress is applied is known.

The heating must occur in the areas of major main stresses of the sheet to be formed, and is based on the interaction between the stress field and the thermal, and on the dependence of the mechanical properties of the material with the temperature.

The method works best in a progressive process, that is, when the sheet is progressively and continuously deformed from its original form to the end by successive stages of point deformation. Compared to stamping or embossing methods, they are slower processes but require less energy. A quick point heating can make the material yield, eliminate its accumulated tensions and reduce the accumulated elastic energy, which allows extending the amount of deformation that the material will support without breaking, and above all, reducing the spring effect (spring- back) due to elastic recovery.

There is currently no precise way to anticipate the results of simultaneous processes of forming and rapid heating, such as the elastic recovery of the material, the ratio of tensions experienced by the piece with respect to cold forming, or the amount of energy needed that is required to carry out the process.

This circumstance is especially true in the case of thermally assisted forming in progressive processes, such as drilling, rotary lamination or progressive stamping, in which the material has been subjected to intense plastic deformation when intense heating is applied, and after He continues the deformation.

Consequently, the invention provides a material testing procedure that simulates exactly what happens to a point of the sheet undergoing a progressive process, when it undergoes rapid heating between two deformation stages, and assesses the effect of that heating allowing to predict its effect and adjust the parameters in the real process, avoiding damage to the material and its resistance and maximizing the benefit of the reduction of the elastic recovery and the greater geometric precision.

Description of the invention

In order to achieve the aforementioned objectives, the invention has developed a new material testing procedure that allows predicting its behavior in a rapid heating process in progressive processes, in which the first phase is conventional and consists of setting a tensile specimen, of the material to be tested in a universal testing machine, and to start a constant speed tensile test under a displacement or force control.

The novelty of the invention is that it also comprises a phase in which the tensile test is stopped maintaining the value of the total displacement, that is, stopping the displacement of the jaws of the universal machine in which the specimen is fixed. A heating is then applied on an area of the surface of the specimen for a short period of time between 0.05 and 0.5 seconds, so that a level of deformation of the heated part greater than the level of deformation caused in the specimen before heating, and a tension level in the unheated part less than the stress level caused in the specimen before heating, thereby reducing deformation of the unheated zone after heating.

Next, it is expected that the distribution balance of the tension levels along the specimen will occur after heating.

Then, the constant-speed tensile test is resumed until the piece is broken and the test curve that relates the levels of force or load to the displacement or elongation of the specimen is obtained, so that a continuation follows phase in which the calculation of the reduction of the elastic recovery that the specimen has undergone in the heated zone from the test curve obtained is carried out, to determine the behavior of the material when subjected to a conformation assisted by rapid heating in progressive processes

The process of the invention establishes two possibilities for calculating the reduction of elastic recovery.

In a first embodiment, the method comprises calculating the deformation and in the tension level of the unheated area after the heating performed, as well as calculating the deformation of the heated part after finishing said heating, all this by means of algorithms whose parameters are obtained at from the test curve obtained.

In another embodiment, the calculation of the reduction in elastic recovery comprises calculating the load of the test piece at the moment when the tensile test is stopped and the load of the test piece once the balance of the tension levels has occurred after the heating, all from the test curve obtained in such a way that the reduction of elastic recovery is calculated from an algorithm based on said calculated loads.

The invention makes it possible to calculate the instantaneous deformation rates under the action of heating from the test curve obtained and calculate the probability of dynamic recrystallization by high-speed deformation and simultaneous heating from said obtained test curve.

The universal test machine is selected between a hydraulic machine and a conventional electromechanical machine of the type from which they obtain the test curve and algorithms are applied to perform the aforementioned calculations.

In the preferred embodiment of the invention the heating is carried out by means of a laser pulse with a constant power that heats the surface of the specimen to the required temperature, so that said power allows to establish the amount of energy required that would be required in a conformed by rapid heating in progressive processes.

The laser comprises an optics that is provided with a beam homogenizer to generate a rectangular focal spot in the specimen having a width equal to that of said specimen and a height between 10 and 15 mm.

Next, in order to facilitate a better understanding of this descriptive report and forming an integral part thereof, a series of fi gures are attached in which the object of the invention has been represented by way of illustration and not limitation.

Brief description of the fi gures

Figure 1.- Shows a perspective view of jaws of a universal testing machine in which a specimen is retained on which the method of the invention is applied, which includes heating the specimen by means of a laser.

Figure 2.- Shows the curve of the test carried out that relates the tensile force to which the test piece is subjected to the displacement or elongation that it suffers. In this same curve, the comparison with the cold behavior of the specimen and its behavior at the maximum heating temperature is represented.

Figure 3.- It shows the previous curves in greater detail to allow explaining a way in which the calculation of the reduction of the elastic regulation of the specimen is performed when heat is applied.

Figure 4.- It shows a graph equivalent to that of the previous figure, but to explain another way of calculating the elastic recovery reduction suffered by the specimen when heating.

Description of the preferred embodiment

A description of the invention based on the aforementioned figures is made below.

The process of the invention comprises fi xing a tensile specimen 1, with standard measurements of the material to be tested, on jaws 2 of a universal testing machine, and initiating a constant speed tensile test under control of displacement of jaws 2 causing the traction of the test piece 1, as is conventionally carried out in the tests of materials carried out in the state of the art.

For this, the universal machine, apart from the jaws 2, must include means of control in closed loop of force and displacement control, means of measurement of displacement of the jaws, means of measurement of the force applied by means of load cell, means to obtain the force-displacement graph of the test, and programming means of different ramps and loading stages.

The universal machine can be hydraulic or electromechanical interchangeably.

Once the test has started, it is stopped by maintaining the value of the deformation or elongation of the specimen 1, for which the stoppage of the movement of the jaws 2 is carried out, and then a heating is applied on a zone 3 of the surface of the specimen 1 for a short period of time between 0.05 and 0.5 seconds.

As a heating source, a direct diode laser source 4 is used, with sufficient power to reach the required temperature in the specimen 1 for a short period of time. As an example, it can be mentioned that in the case of steel it is required that it reaches a temperature of 400 ° C in a time of the order of 0.2 seconds. The power required depends on the optical properties of the material, but in the case of uncoated steels, approximately 1.5 kW per millimeter thickness of specimen 1 may be required.

The laser radiation is focused on the specimen 1 with an optics 5, which is provided with a beam homogenizer, to generate a rectangular focal spot 3 of the width of the specimen and a height between 10 and 15 mm.

The heating is carried out by means of a laser pulse at constant power so that the parameters to be controlled in the laser 4 are the duration of the pulse and the maximum temperature that is applied on the surface of the specimen 1.

Once the heating has been carried out, a time is expected until the equilibrium of the redistribution of the forces or stresses in the test tube 1 occurs. A time greater than 1.5 seconds is sufficient for this equilibrium. The redistributed force is P1 in the graphs.

Once the time has elapsed until the commented equilibrium is obtained, the tensile test is resumed until the rupture of the test piece 1 occurs.

This test simulates the application of a fl ash heating in an intermediate phase of a forming process, and the effect of the consequent deformation on the material after being heated by laser.

When the pulse of the laser radiation reaches the test tube 1, the local heating causes a decrease of the elastic limit, whereby a greater plastic deformation is located in the heated zone. As the control loop of the machine maintains the total deformation of the specimen 1 during heating, the tension is redistributed in the specimen to maintain the homogeneous stress, so the time must be waited until the distribution equilibrium occurs. of that effort.

Next, the curve that relates the force or tension to the distance or elongation of the test is obtained, a curve shown in Figure 2 with reference 6.

As can be seen in said Figure 2, less force is used to maintain the same global deformation, since a part is absorbed as plastic deformation in the heated zone 3. This behavior is reflected in the graph of the test as a decrease in the P0-P1 force, with which a reduction in forming energy can be observed. When the test continues, the test tube 1 has two different regions3 and 9 due to metallurgical transformations in the zone 3 affected by laser, the decrease in the breaking load or the rupture located in the heated zone shows that the heating-cooling cycle is not Suitable for the material. Otherwise, if a stress drop occurs and then a recovery of the resistant properties, a good behavior of the material is indicated.

In addition, in graph 2 with reference 7, the test curve applied to a high-strength steel in its cold behavior is shown, while with reference 8 the behavior curve for the maximum heating temperature by the laser is shown , and reference 6, as already mentioned, represents the test curve for this same steel when the method of the invention is used, so that a comparison is established.

From the curve obtained, the behavior of the specimen 1 can be determined in a procedure formed by rapid heating in progressive processes, in the manner described below.

At the time of applying the laser, the closed control loop maintains the total deformation of the specimen 1.

By having different temperatures, the heated and unheated zones 9 of the test tube 1 also have different deformation values in equilibrium before the same load.

In the case of a continuous medium uniaxially loaded at the ends, the load is the same throughout the entire specimen 1, whereby the initial deformation ε0 is redistributed between the two mentioned parts3 and 9.

The graph in Figure 3 shows the evolution separately of each of the parts: starting from the specimen 1 in its initial situation, in which all of it is subjected to a load P0, and all of it undergoes a level of deformation ε0 = D0 / L, then the laser is applied. Then the laseada part evolves extending along the section of line 6b to a level of unit deformation εp, verifying that εp> ε0. On the other hand, the area without lasear 9 relaxes elastically to a tension level σ1 lower than the original σ0 corresponding to the moment before applying the heating, reducing the deformation so that ε1 <ε0.

There are three conditions that are verified at the moment when the laser is influencing on the specimen 1, and which are used in the calculation to determine the distribution of the deformation.

The load is the same in both sections: σp = σ1 <σ0

-

The original offset D0 remains:

-

In addition, the relaxation of the load in the non-lashed area occurs along a straight line of known slope E (elastic discharge):

What happens is represented in Figure 3.

The reference 6a represents the behavior before the laser pulse, the 6b the behavior of the area heated by the laser (fl uence), the 6c the behavior of the unheated zone during the laser pulse (elastic discharge).

If we define the material treatment temperature curve as the Hollomon equation σ = K · εn that models how a metal flows under high temperature stresses, it must be

where T is the temperature, σ the tension, K is a property of the material that represents the elastic component of its behavior and n is also a property of the material that represents the plastic fl uence. Therefore, Kynson constants in the Hollomon fl uence model.

Then you have three equations to get the three unknowns σ1, ε1 and εp. The value of σ0, ε0 is known and comes from the graph, and the values of K, n and E can be known by characterization of the material.

The calculations made are as follows; from Eq. 2 you get:

Applying Eq. 3. to Eq. 1 .: Applying the elastic discharge line (Eq.4) to the previous one:

Therefore (relocating terms):

with A = KE · (L-A), a constant that depends on the properties of the

material. It is therefore possible to calculate the deformation of the heated power ε1 from the known values σ0, ε0, K, nyE.

Since the heating has a kinetics lower than that of deformation, since the incremental displacements are small and the diffusion of heat requires a time greater than the incremental deformation, the

strain rate at

t being the time of the laser pulse.

With this, the instantaneous deformation rates can be estimated under the action of the laser in the actual process, and the possibility of dynamic recrystallization by simultaneous high-speed deformation and heating, as well as any other effect dependent on the instantaneous deformation velocity.

This test can be used in turn to calculate the reduction in elastic recovery experienced by the material when it is formed by heating fl ash simultaneously with the application of progressive deformation.

What happens in the test (storage of elastic energy in the form of plastic deformation in the injured material, and perhaps in the form of recrystallization energy, as has been proposed) can be understood as a reduction of the energy available to produce the elastic recovery , which is mechanical work.

Let us call W the work done in zone 9, not laseada of the test piece 1 when relaxing, and that we will consider that it is the work that cannot be used later in the elastic recovery.

Since it is unloaded along a line, it is very easy to calculate the work done:

The reduction of elastic recovery will be related to the relationship between total energy. This can be obtained from the previous equation, or from operating on the areas under S1 and S2, the curve directly as shown in Figure 4, this calculation is the accumulated deformation energy in the specimen. S1 is the elastic energy stored before the laser pulse, and S2 is released when heating is applied. The energy is calculated as an area under the test curve, represented in the axis of ordinates the tension and in the axis of abscissa the deformation.

The value of the elastic recovery reduction (SBR) is dimensionless, and can be obtained according to any of the following expressions:

In short, the% SBR can be predicted directly from the load drop in the P-D diagram of the test. The previous calculations show that: - The test is significant and provides useful information to understand and quantify the forming process, even to simulate it and predict its operation before putting it into practice.

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There is a possibility of dynamic recrystallization and other effects due to the high deformation rate in the heated zone, and the deformation rate of the laserated zone can be estimated when it is subjected to constant displacement conditions, typical of lamination and drilling.

-

The test is simple to perform and uses a conventional testing machine, only the calculations mentioned must be obtained to interpret the results.

Claims (10)

1. Procedure for testing materials to predict their behavior in a rapid heating process in progressive processes, which includes setting a tensile specimen (1) of the material to be tested in a universal testing machine, and initiating a tensile test of constant speed under a control selected between a force control and a displacement control; It is characterized because it also includes:

-

Stop the tensile test keeping the value of the fixed displacement parameter and apply a heating on an area (3) of the surface of the specimen (1) for a short period of time between 0.05 and 0.5 seconds, to cause a level of deformation (εp) of the heated part (3) greater than the level of deformation (ε0) caused in the specimen before its heating and a tension level (σ1) in the unheated part (9) less than the tension level (σ0) caused in the test piece (1) before heating reducing the deformation (ε1) of the unheated area after heating;

-

wait for the distribution balance of the tension levels along the test piece (1) to occur after heating;

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Resume the constant speed tensile test until the test piece breaks (1).

-

obtain the force (P) -shift (D) curve of the test

-

Calculate the reduction of the elastic recovery suffered by the specimen (1) in the heated zone (3), from the test curve to determine the behavior of the specimen material (1) if it is subjected to a heating forming Fast in progressive processes.

2.

Method of testing materials to predict their behavior in a rapid heating process in progressive processes, according to claim 1, characterized in that it comprises calculating the deformation (ε1) and the tension level (σ1) of the unheated zone (9) after heating performed and calculate the deformation (εp) of the heated part after finishing said heating, from the test curve obtained to calculate the elastic recovery reduction.

3.

Procedure for testing materials to predict their behavior in a rapid heating process in progressive processes, according to claim 1, characterized in that it comprises calculating the load of the test piece (1) at the moment when the tensile test and the load of the test piece (1) once the equilibrium of the tension levels after heating has taken place, from the test curve obtained, to calculate the reduction of the elastic recovery from the calculated loads.

Four.

Method of testing materials to predict their behavior in a rapid heating process in progressive processes, according to claim 1, characterized in that it comprises calculating the instantaneous deformation rates under the heating action from the obtained test curve.

5.

Method of testing materials to predict their behavior in a rapid heating process in progressive processes, according to claim 1, characterized in that it comprises calculating the probability of dynamic recrystallization by high speed deformation and simultaneous heating from the test curve obtained.

6.

Procedure for testing materials to predict their behavior in a rapid heating process in progressive processes, according to claim 1, characterized in that the heating phase is carried out by means of a device selected from a laser device (4), an induction device and a source of power

7.

Procedure for testing materials to predict their behavior in a rapid heating process in progressive processes, according to claim 6, characterized in that the heating phase is carried out by means of a laser pulse with a constant power.

8.

Method of testing materials to predict their behavior in a rapid heating process in progressive processes, according to claim 6, characterized in that the laser heating phase comprises generating a rectangular focal spot in the specimen of the width of the specimen and of a height comprised between 10 and 15 mm.

SPANISH OFFICE OF THE PATENTS AND BRAND Application no .: 200803692 SPAIN Date of submission of the application: 24.12.2008 Priority Date: REPORT ON THE STATE OF THE TECHNIQUE 51 Int. Cl.: G01N3 / 18 (2006.01) G01N3 / 60 (2006.01) RELEVANT DOCUMENTS

Category

Documents cited Claims Affected

TO

JP 7260654 A (TANAKA PRECIOUS METAL IND et al.) 13.10.1995, Summary and figures of the EPODOC database. Recovered from EPOQUE. 1-8

TO

US 7409869 B1 (KOTECKI DAMIAN J et al.) 12.08.2008, summary; column 1, line 53 - column 6, line 27; claims 1-4.7; figures 5.7. 1-8

TO

JP 2007278779 A (SHIMADZU CORP et al.) 25.10.2007, Summary and Figures 1 and 3 of the EPODOC database. Recovered from EPOQUE. 1-8

TO

JP 3152441 A (SAGINOMIYA SEISAKUSHO INC) 28.06.1991, Summary and figure 1 of the EPODOC database. Recovered from EPOQUE. 1-8

Category of the documents cited X: of particular relevance Y: of particular relevance combined with other / s of the same category A: reflects the state of the art O: refers to unwritten disclosure P: published between the priority date and the date of priority submission of the application E: previous document, but published after the date of submission of the application

This report has been prepared • for all claims • for claims no:

Date of realization of the report 01.06.2011

Examiner B. Weaver Miralles Page 1/4

REPORT OF THE STATE OF THE TECHNIQUE Application number: 200803692 Minimum documentation sought (classification system followed by classification symbols) G01N Electronic databases consulted during the search (name of the database and, if possible, terms of search used) INVENES, EPODOC, NPL, INTERNET State of the Art Report Page 2/4  WRITTEN OPINION Application number: 200803692 Date of Written Opinion: 01.06.2011 Statement

Novelty (Art. 6.1 LP 11/1986)

Claims Claims 1-8 IF NOT

Inventive activity (Art. 8.1 LP11 / 1986)

Claims Claims 1-8 IF NOT

The application is considered to comply with the industrial application requirement. This requirement was evaluated during the formal and technical examination phase of the application (Article 31.2 Law 11/1986).  Opinion Base.- This opinion has been made on the basis of the patent application as published. State of the Art Report Page 3/4  WRITTEN OPINION Application number: 200803692 1. Documents considered.- The documents belonging to the state of the art taken into consideration for the realization of this opinion are listed below.

Document

Publication or Identification Number publication date

D01

JP 7260654 A (TANAKA PRECIOUS METAL IND et al.) 13.10.1995

D02

US 7409869 B1 (KOTECKI DAMIAN J et al.) 12.08.2008

D03

JP 2007278779 A (SHIMADZU CORP et al.) 10.25.2007

D04

JP 3152441 A (SAGINOMIYA SEISAKUSHO INC) 06.28.1991

2. Statement motivated according to articles 29.6 and 29.7 of the Regulations for the execution of Law 11/1986, of March 20, on Patents on novelty and inventive activity; quotes and explanations in support of this statement Document D01 discloses a method for performing a high temperature tensile test in which, prior to performing the test, only the central part of the specimen is heated in a small oven. In this way, the fracture takes place in the center of the specimen and thus measures its resistance at high temperature. It differs from the first claim in that mainly, a charge is not applied first and the test is stopped to heat the sample and then resume it, but the central part of the sample is heated before starting the test. The technical effect achieved is a balance of distribution of the tension levels along the specimen after heating. The technical problem to solve is how to reproduce a progressive process of forming a sheet when it undergoes rapid warming between two stages of deformation. No document has been found in the state of the art that solves the technical problem posed, so said claim presents novelty and inventive activity according to articles 6 and 8.1 of patent law 11/1986. Document D02 discloses a method for evaluating resistance to thermal fatigue, which consists in placing a sample in an understanding-tensile testing machine, heating the sample to a certain temperature while applying a compression load, cooling the sample while a tensile load is applied. It differs from the first claim in that mainly, a charge is not applied first and the test is stopped to heat the sample and subsequently resume it, in addition to not heating a single part of the sample. Document D03 describes a thermal fatigue test without disturbing the temperature distribution between the reference points of the sample, avoiding induction heating of the jaws of the specimen, using a hydraulic servo testing machine testing machine with a coil of induction heating. Document D04 shows a thermal fatigue test in which heating and cooling cycles alternate and where a control circuit measures the deformation suffered by the sample. In none of the aforementioned documents, which reflect the prior art state closest to the object of the application, have all the technical characteristics defined in claim 1 of the application been found. Likewise, it is considered that the main differential characteristic does not seem to derive in an evident way from any of the cited documents, either individually or through an evident combination between them. Therefore, claim 1, and consequently, all its dependents, would satisfy the requirements of novelty and inventive activity according to article 6 and 8.1 of patent law 11/1986. State of the Art Report Page 4/4