WO2016087790A1

WO2016087790A1 - Method for characterising the thermal ageing of composite materials, especially composite materials with an organic matrix

Info

Publication number: WO2016087790A1
Application number: PCT/FR2015/053312
Authority: WO
Inventors: Arnaud DELEHOUZE; Thibaud CHOPARD; Nicolas PREUD'HOMME; Elsa RACAPE
Original assignee: Aircelle
Priority date: 2014-12-03
Filing date: 2015-12-03
Publication date: 2016-06-09
Also published as: FR3029630A1

Abstract

The invention relates to a method for characterising the thermal ageing of a part made of a composite material, especially a composite material with an organic matrix, the method comprising the steps of: taking (50) a sample of composite material from the part; subjecting (51) the sample to a thermogravimetric analysis in an oxidising atmosphere; determining (52), from the deconvolution of the thermogram obtained, the intensity of a given range peak, said intensity being characteristic of the thermal ageing; and determining (53) the thermal ageing of the composite material by comparing the intensity of the peak with a reference nomogram.

Description

Process for the characterization of thermal aging of composite materials, in particular of organic matrix composite materials

The invention relates to a method for characterizing the thermal aging of composite materials, in particular organic matrix composite materials. The invention finds an advantageous application in the field of aeronautics, a field in which the parts made of composite materials are generally subjected to severe thermal stresses.

Among the materials used in aeronautics, composite materials represent an increasingly important part. The composite material parts of an aircraft are subject to constraints, including thermal stresses, which can lead to premature aging of the parts concerned.

These constraints relate in particular to propulsion systems, namely turbojet engines and nacelles surrounding turbojet engines. Indeed, the elements of turbojets and nacelles made of composite materials are subject to significant thermal variations. For example, in the case of a nacelle, these elements are at room temperature on the ground, and undergo in flight either very low temperatures (for the elements located in the so-called cold zone of the nacelle), or very high temperatures ( for the elements located in the so-called hot zone of the nacelle).

The composite material parts of the nacelles are qualified according to minimum mechanical characteristics for exposure at a first given temperature over the lifetime of the aircraft, and at a second given temperature (a peak temperature, greater than the first) on a short duration of cumulative exposure during the life of the aircraft.

In practice, it happens that the composite materials are subjected to temperatures greater than or equal to the second threshold for a period resulting in an unacceptable decrease in mechanical properties.

It is therefore essential to be able to accurately and rapidly characterize the thermal aging of a composite material element, in particular to evaluate the level of loss of the mechanical characteristics of this element.

Methods are known for characterizing thermal aging by evaluating the degree of discoloration of a paint so-called primary. These are, however, of relative precision because the quality of the evaluation is highly dependent on the application of the primary paint (variations in thickness, surface condition, etc.). In addition, these methods are naturally applicable only to painted parts, which represents a significant disadvantage with regard to the presence of many pieces of unpainted composite material.

In addition, semi-destructive control methods are known. These methods require the taking of a sample of material whose mechanical characteristics are then analyzed, for example dynamic mechanical analysis methods (also known as DMA, for "Dynamic Mechanical Analysis"). However, this type of process requires taking a sample of a large size, the subsequent repair does not fall within the scope of so-called "cosmetic" repairs.

The present invention aims to meet existing needs by providing a method for accurately characterizing the thermal aging of composite material parts, including unpainted parts.

For this purpose, the invention relates to a method for characterizing the thermal aging of a composite material part, in particular an organic matrix composite material, the method comprising the steps of:

- take a sample of composite material on the part;

- subject the sample to thermogravimetric analysis under oxidizing atmosphere;

determining, from the deconvolution of the thermogram obtained, the intensity of a peak of given rank, this intensity being characteristic of thermal aging;

- Determine the thermal aging of the composite material by comparing the intensity of the peak with a reference chart.

Thus, the invention makes it possible to precisely determine the actual level of thermal aging of a composite material part, in particular an aircraft part (for example a part of the propulsion unit). The small sample size necessary for the implementation of the method according to the invention allows the repair of the nacelle element subsequent to the sample to enter the field of so-called "cosmetic" repairs. (category defined by the regulations in force). This is an important advantage because so-called cosmetic repairs are much less time-consuming and costly to implement than so-called "structural" repairs.

In one embodiment, the reference chart makes it possible to determine the residual mechanical strength of the composite material as a function of the minimum mechanical properties required.

In one embodiment, the reference chart is obtained by comparison between:

first data, resulting from a study to determine the reduction of the mechanical properties of the composite material as a function of thermal aging; second data, resulting from a study making it possible to determine the thermal aging as a function of the intensity of the peak.

In one embodiment, the first data is obtained by analyzing a first population of samples that have been subjected to different temperatures for different durations, preferably according to a DMA type method.

In one embodiment, the second data are obtained by analysis, according to a thermogravimetric analysis method, of a second population of samples, these samples having been subjected to different temperatures for different durations, the different time / temperature pairs of the first one. population of samples being identical to the time / temperature pairs of the second population of samples.

In one embodiment, thermogravimetric analysis is performed at temperatures below 1100 ° C, preferably below 500 ° C.

In one embodiment, thermogravimetric analysis is performed at temperatures below 350 ° C.

In one embodiment, the mass of the sample taken is between 1 and 5 grams.

In one embodiment, the sample is taken from a part of an aircraft, in particular a part of an aircraft propulsion unit.

In one embodiment, a cosmetic type of repair is performed on the part after sampling. The present invention will be better understood on reading the detailed description which follows, made with reference to the appended drawings, among which:

FIG. 1 is a schematic representation of a thermogravimetric analysis device;

FIG. 2 is a thermogram obtained by thermogravimetric analysis of a sample of an organic matrix composite material;

FIG. 3 represents curves obtained by deconvolution of the thermogram of FIG. 2;

FIG. 4 represents, for different samples, the value of the ratio between the intensity of a peak of the thermogram of a given sample and the intensity of the same peak for a reference sample;

- Figure 5 is a diagram showing the steps for determining the thermal aging of a composite material part according to the invention;

- Figure 6 is a diagram showing the steps for constructing a reference chart according to the invention. Figure 1 is a diagram of a thermogravimetric analysis device 1. The device 1 comprises a sealed enclosure 2 designed to contain a sample 3 to be analyzed. The sealed enclosure 2 is placed in a furnace 4 making it possible to subject the enclosure 2 and thus the sample 3 which it contains to the temperature necessary for the analysis. The atmosphere of the chamber 2 is controlled by injection of a gas, and one can thus subject the sample to different types of atmosphere: oxidizing, reducing, inert ...

The device 1 further comprises a weighing module of the sample 3. The weighing module is disposed inside a sealed enclosure 5. In this example, it is a so-called "weighing module". flail ", which thus comprises a beam 6, a counterweight 7 and a hanger 8, at one end of which the sample 3 is hooked. The atmosphere of the enclosure 5 is controlled by injection of an inert gas (such as dinitrogen or argon).

The device 1 makes it possible to implement the thermogravimetric analysis of the sample 3. Such an analysis consists in following the evolution of the mass of the sample 3 when it is subjected to a profile. determined thermal (isothermal stage, thermal cycle, rise or fall in temperature), in the controlled atmosphere.

FIG. 2 shows an example of a thermogram obtained by thermogravimetric analysis of a sample made of organic matrix composite material. Curve 20 in FIG. 2 represents the time derivative of the mass loss of the sample (referred to as "intensity" on the graph of FIG. 2 and plotted on the ordinate) as a function of time (the time being measured in abscissa on the graph of Figure 2).

In the context of the invention, the thermogram of FIG. 2 is applied to a deconvolution treatment. The result of this operation is shown in FIG. 3. The initial signal (curve 30), corresponding to the curve 20 of FIG. 2, is thus decomposed into several lines, or elementary curves (curves 31, 32, 33, 34, 35), these lines being associated either with the organic matrix of the composite material (typically a resin) or with the reinforcement of the composite material (typically fibers, especially carbon fibers). In the example of Figure 3, the curves 31, 32 and 33 are associated with the matrix while the curves 34 and 35 are associated with the reinforcement. By "associated" is meant that the curves considered respectively account for the oxidation of the matrix or the reinforcement. This classification of the different lines results from oxidation tests at different temperatures. It is observed in Figure 3 that each elementary curve has a peak, which can be determined in particular the expectation, the width at half-height and the relative intensity.

In accordance with the invention, the thermogravimetric analysis in an oxidizing atmosphere of samples of an organic matrix composite material, including a reference sample and samples having previously been subjected to different temperature / time combinations, makes it possible to characterize the level of thermal aging. FIG. 4 is a bar graph giving, for different samples (curves 41, 42, 43, 44, 45), the value of the ratio between the intensity of the peak of a given rank line (in the example the line of rank 1, ie the elementary curve 31 of FIG. 3) for the sample in question and the intensity of the peak of the line of the same rank for the reference sample. The samples (with the exception of the reference sample) were previously subjected to a given temperature T1, respectively for a duration D1, D2, D3, D4 and D5. The comparison of the different curves represented in FIG. note the evolution of the peak characterizing the thermal aging, in particular with respect to the height of the reference peak of the reference sample (curve 40). It is found that for a given temperature T1, as the duration of exposure increases, the ratio between the peak intensity of the sample considered and the peak intensity of the reference sample decreases. Similar evolutions are also observed for exposure at different temperatures for a given time.

As shown in FIG. 6, the thermogravimetric analysis of samples is carried out for a certain number of temperature / time pairs (step 60) and the results obtained are compared (step 62) with the data of a reference study determining the degradation of mechanical properties as a function of thermal aging. This comparison makes it possible to extract an abacus 63 giving the residual mechanical strength of the composite material concerned as a function of the peak intensity as shown in particular in FIG. 4. The data relating to the thermal aging of the reference study will have been obtained. by any suitable method (step 61), for example by means of DMA type tests on a population of samples having been subjected to the same temperature / time pairs as the population of samples tested by thermogravimetric analysis.

The chart 63 obtained in accordance with the invention is directly usable by the personnel in charge of maintenance. Thus, it becomes possible, thanks to this chart, to precisely characterize the level of aging of the composite material constituting a part embedded on an aircraft (and more particularly a part located in the propulsion unit). As shown in FIG. 5, it is sufficient for the personnel to take a sample on the part concerned (step 50) and to subject this sample to thermogravimetric analysis (step 51), so as to determine the intensity of the peak as described above. high (step 52). This value is then compared with the reference chart 63, which makes it possible to deduce the level of thermal aging of the sample taken (step 53).

The invention makes it possible to characterize thermal aging from a sample of very small size (approximately 1 to 5 grams), so that the repair subsequent to the taking of a sample enters the field of so-called cosmetic repairs. In addition, the method according to the invention is applicable to both painted and unpainted parts. Although the invention has been described in connection with a particular embodiment, it is obvious that it is not limited thereto and that it comprises all the technical equivalents of the means described and their combinations if they are within the scope of the invention.

Claims

1. A method for characterizing the thermal aging of a composite material part, in particular an organic matrix composite material, the method comprising the steps of:

- take (50) on the piece a sample of composite material;

subjecting (51) the sample to thermogravimetric analysis under an oxidizing atmosphere;

determining (52), from the deconvolution of the thermogram obtained, the intensity of a peak of given rank, this intensity being characteristic of thermal aging;

- determining (53) the thermal aging of the composite material by comparing the intensity of the peak with a reference chart (63).

2. Method according to claim 1, wherein the reference chart (63) makes it possible to determine the residual mechanical strength of the composite material according to the minimum mechanical properties required.

3. Method according to claim 1 or 2, characterized in that the reference chart is obtained by comparison between:

first data, resulting from a study to determine the reduction of the mechanical properties of the composite material as a function of thermal aging;

second data, resulting from a study making it possible to determine the thermal aging as a function of the intensity of the peak.

4. Method according to claim 3, characterized in that the first data are obtained by analysis of a first population of samples having been subjected to different temperatures for different durations, preferably according to a DMA type method.

5. Method according to claim 4, characterized in that the second data are obtained by analysis, according to a method of analysis thermogravimetric, a second population of samples, these samples having been subjected to different temperatures for different durations, the different time / temperature pairs of the first population of samples being identical to the time / temperature pairs of the second population of samples .

6. Method according to one of the preceding claims, wherein the thermogravimetric analysis is carried out at temperatures below 1100 ° C, preferably below 500 ° C.

7. Method according to one of the preceding claims, wherein the thermogravimetric analysis is carried out at temperatures below 350 ° C.

8. Method according to one of the preceding claims, wherein the mass of the sample taken is between 1 and 5 grams.

9. Method according to one of the preceding claims, wherein the sample is taken from a part of an aircraft, in particular a part of an aircraft propulsion unit.

10. Method according to the preceding claim, wherein a cosmetic type of repair is performed on the piece after sampling of the sample.