Method for characterising a part made of a composite material

Abstract

The invention consists in a method of characterizing a part made of composite material (30), the method comprising a step of determining a characteristic of a longitudinal ultrasound wave (41) traveling along a path within the part (30), and being characterized in that the travel time of a longitudinal ultrasound wave (42) transmitted by the part (30) is measured (E4).

Claims

1. A method of characterizing a part made of composite material, the method comprising a step of determining a characteristic of a longitudinal ultrasound wave traveling along a path within the part, and being characterized in that a travel time of a longitudinal ultrasound wave transmitted by the part is measured, and the travel time of the transmitted wave is measured by observing the beginning of the transmitted wave, wherein a propagation time of an ultrasound wave transmitted through a liquid in the absence of the part is measured, and the propagation times of ultrasound waves reflected respectively by a first face of the part and by a second face of the part are measured with a first transducer facing the first face and a second transducer facing the second face of the part in order to determine a dimension of the part traveled by the longitudinal ultrasound wave traveling along the path within the part, wherein an amplitude of the transmitted wave is also measured in order to determine an overall or unit length attenuation to which the longitudinal ultrasound wave is subjected on traveling within the part, wherein an overall attenuation (.sub.2) of the longitudinal ultrasound wave in the part is determined according to the following equation: ${}_2=\frac{1}{x_2}\left(\mathrm{Ln}\left(\frac{Y_1}{Y_2}.Math.t_{12}{t}_{21}\right)+{}_1{x}_2\right)$ where Y.sub.1/Y.sub.2 is an amplitude ratio, t.sub.12 is an amplitude transmission coefficient from the liquid to the composite material, t.sub.21 is an amplitude transmission coefficient from the composite material to the liquid, 1 is a value of an attenuation of the transmitted wave in the liquid, and x.sub.2 is the dimension of the part traveled by the transmitted wave traveling along the path within the part.

2. A method according to claim 1, wherein a propagation speed of the longitudinal ultrasound wave in the part following the path within the part is determined.

3. A characterization method according to claim 1, performed for a part made of 3D woven composite material.

4. A method according to claim 1, wherein x.sub.2 is determined according to the equation:
x.sub.2=(t.sub.X1+X2+X3t.sub.X3t.sub.X1)V.sub.liquid where V.sub.liquid is a propagation speed of the wave in the liquid, t.sub.X1+X2+X3 is the propagation time of the ultrasound wave transmitted through the liquid in the absence of the part, t.sub.X1 is the propagation time of the ultrasound wave reflected by the first face of the part, and t.sub.X3 is the propagation time of the ultrasound wave reflected by the second face of the part.

5. A method according to claim 1, wherein t.sub.12t.sub.21 is determined according to the equation: $t_{12}{t}_{21}=\frac{4Z_1{Z}_2}{{\left(Z_1+Z_2\right)}^2}$ where Z.sub.1 is an acoustic impedance of the liquid, and Z.sub.2 is an acoustic impedance of the composite material.

6. A method according to claim 5, wherein the acoustic impedance of the liquid, Z.sub.1, and the acoustic impedance of the composite material, Z.sub.2, are calculated according to equations:
Z.sub.1=.sub.1V.sub.1
Z.sub.2=.sub.2V.sub.2 where .sub.1 is a density of the liquid, .sub.2 is a density of the composite material, and V.sub.1 is a propagation speed of the transmitted wave within the liquid, and V.sub.2 is a propagation speed of the transmitted wave within the composite material.

Description

BRIEF DESCRIPTION OF THE FIGURES

(1) FIG. 1 shows a preliminary operation in the context of performing a method of the invention.

(2) FIG. 2 shows the three steps of a thickness measuring stage performed in the invention.

(3) FIGS. 3 to 5 show the signals recorded during the three steps of FIG. 2.

(4) FIG. 6 shows the step of observing a transmitted wave, during a method of the invention.

(5) FIG. 7 shows the signal measured during the step of FIG. 6.

(6) FIGS. 8 to 10 show signals obtained during the steps of FIGS. 2 and 6 for a spacer made of composite material.

DETAILED DESCRIPTION OF AN IMPLEMENTATION

(7) With reference to FIG. 1, two plane ultrasound sensors operating in transmission mode are put into alignment. This putting into alignment constitutes a preliminary step E0. The sensors are separated by a liquid, such as water. The transducer 10 operates in emission mode, and the sensor 20 in reception mode. The signal received by the sensor 20 passes through a maximum after successive adjustments of the axes Oy and Oz, and also the angles and .

(8) In FIG. 2, a measurement is made of the thickness of the material of the part under study, referenced 30. The measurement needs to be accurate to within one micrometer.

(9) A first step E1 consists in measuring the travel time of the wave transmitted through the water between the two transducers 10 and 20, in the absence of the part. A second step consists in measuring the travel time of the wave reflected by the first surface, referenced 31, of the part 30, with the transducer 10 operating as a transceiver and facing the surface 31. A third step consists in measuring the travel time of the wave reflected by the second surface, referenced 32, of the part 30, with the transducer 20 operating in turn as a transceiver and facing the surface 32.

(10) The travel time is measured on each occasion by observing the beginning of the signal, and not an arch of the signal. This makes it possible for the operator to ignore any phenomenon associated with possible phase shifting of the signal. Specifically, in the presence of multiple reflections, phase shifts appear. This also happens when, after a reflection, the signal is inverted. The shape of the arches of the signal is modified, and it is difficult to obtain an accurate value for the travel time. That is why it is proposed to measure the signal by observing solely the beginning of the signal.

(11) Since the propagation speed of the wave in water V.sub.water is known, it is possible by subtraction to obtain the thickness of the part from the steps E1, E2, and E3, by using the formula X.sub.2=(t.sub.X1+X2+X3t.sub.X3t.sub.X1)V.sub.water, where X1 is the distance between the transducer 10 and the surface 31, X2 is the thickness of the part at the point of impact of the beam, and X3 is the distance between the transducer 20 and the surface 32, and where t.sub.X1+X2+X3, t.sub.X1 and t.sub.X3 are the travel times measured during the steps E1, E2, and E3 respectively.

(12) FIGS. 3 to 5 show the plots displayed during steps E1, E2, and E3 respectively, with water at 22 C., a wave at a frequency of 5 megahertz (MHz) (giving a propagation speed of 1486.45 meters per second (m/s) in water), for a spacer having a thickness of 76.20 millimeters (mm) and made of TA6V titanium. The travel time of the wave is measured on the basis of the beginning of the wave, given respective references 100, 110, and 120.

(13) The following results are obtained:

(14) t.sub.X1+X2+X3=92.72 microseconds (s)

(15) t.sub.X3=52.98/2=26.49 s

(16) t.sub.X1=29.94/2=14.97 s

(17) X2=(t.sub.X1+X2+X3t.sub.X3t.sub.X1)V.sub.water

(18) X2=(92.7210.sup.626.4910.sup.614.9710.sup.6)1486.54

(19) X2=76.20 mm

(20) The thickness measured with calipers is indeed 76.20 mm, i.e. 3.

(21) FIG. 6 shows the step E4 during which the wave transmitted by the part 30 is observed. Thus, the transducer 10 is operating in emitter mode, while the transducer 20 is operating in receiver mode. The incident wave is referenced 40 in the figure, the wave propagating in the part 30 is referenced 41, and the transmitted wave is referenced 42.

(22) The travel time of the wave in the part 30 is expressed in the form t.sub.X2=t(t.sub.X1+t.sub.X3). Knowing X2 as determined beforehand, the propagation speed of the wave in the material is expressed in the form V.sub.material=X2/t.sub.X2.

(23) FIG. 7 shows the signal observed during the step E4 for the 76.20 mm thick space spacer made of titanium (TA6V), still with a wave at 5 MHz. The travel time of the wave is measured on the basis of the beginning of the wave, referenced 130.

(24) The values obtained are as follows:

(25) t=53.80 s

(26) t.sub.X2=(53.8010.sup.626.4910.sup.614.9710.sup.6)

(27) t.sub.X2=12.34 s

(28) V=76.2010.sup.3/12.3410.sup.6

(29) And finally, the numerical value of the speed is V=6175.04 m/s. This value is verified with a conventional propagation speed measurement in order to validate the method.

(30) FIGS. 8 to 10 show the scans obtained for the steps E2, E3, and E4 for a composite stepped spacer having a thickness of 47.09 mm, with a transducer emitting at 1 MHz. The travel time of the wave is measured on the basis of the beginnings of the waves, given respective references 140, 150, and 160.

(31) The values obtained are as follows:

(32) t.sub.X1+X2+X3=90.22 s

(33) t=74.90 s

(34) t.sub.X3=52.42/2=26.21 s

(35) t.sub.X2=64.68/2=32.34 s

(36) X2=t.sub.X1+X2+X3t.sub.X3t.sub.X1)V.sub.water

(37) X2=(90.2210.sup.626.2110.sup.632.3410.sup.6)1486.54

(38) X2=31.6710.sup.61488.76

(39) X2=47.078 mm

(40) t.sub.X2=t(t.sub.X1+t.sub.X3)

(41) t.sub.X2=(74.9010.sup.626.2110.sup.632.3410.sup.6)

(42) t+.sub.X2=16.35 s

(43) V.sub.composite=X2/t.sub.X2

(44) V.sub.composite=47.07810.sup.3/16.3510.sup.6

(45) And finally, the numerical value of the speed is V.sub.composite=2879.4 m/s.

(46) Attention is then given to the attenuation of the longitudinal wave in the material.

(47) The expression for the amplitude of the wave transmitted from the emitter to the receiver is written as follows: Y.sub.1=A.sub.maxe.sup.1.(X1+X2+X3), where A.sub.max represents the maximum amplitude at the surface of the transducer and .sub.1 is the attenuation of the wave in water.

(48) The expression for the amplitude of the wave transmitted from the emitter to the receiver after passing through the material is written as follows: Y.sub.2=A.sub.maxe.sup.1(X1+X3)e.sup.2X2t.sub.12t.sub.21, where .sub.2 is the attenuation of the wave in the material, t.sub.12 is the amplitude transmission coefficient from water to the material, and t.sub.21 is the amplitude transmission coefficient from the material to water.

(49) The expression for the product t.sub.12t.sub.21 is a function of the acoustic impedance of the material Z.sub.2=.sub.2V.sub.2 and of the acoustic impedance of water Z.sub.1=.sub.1V.sub.1. In the acoustic impedance expression, represents density and V represents the propagation speed of the longitudinal wave at the frequency under consideration.

(50) $t_{12}{t}_{21}=\frac{4.Math.Z_1{Z}_2}{{\left(Z_1+Z_2\right)}^2}$

(51) The amplitude ratio Y.sub.1/Y.sub.2 is written as follows:

(52) $\frac{Y_1}{Y_2}=\frac{e^{-1X2}}{e^{-2X2}.Math.t_{12}{t}_{21}}.$

(53) From which it is possible to deduce the expression for attenuation in the material:

(54) ${}_2=\frac{1}{x_2}\left(\mathrm{Ln}\left(\frac{Y_1}{Y_2}.Math.t_{12}{t}_{21}\right)+{}_1{x}_2\right).$

(55) A first implementation relates to the spacer of composite material having thickness of 47.09 mm, using a wave at 2.25 MHz.

(56) The numerical values for this implementation are as follows:

(57) .sub.2=1525.71 kilograms per cubic meter (kg/m.sup.3)

(58) V.sub.2=2946.75 m/s

(59) Z.sub.2=4.39316 megOhms alternating current (M.sub.ac)

(60) .sub.water=997.77 kg/m.sup.3

(61) V.sub.water=1486.54 m/s

(62) Z.sub.water=1.48322 M.sub.ac

(63) t.sub.12t.sub.21=0.75478

(64) x.sub.2=47.078 mm (accurate ultrasound measurement)

(65) Y.sub.1=643.2 millivolts (mV)

(66) Y.sub.2=15.885 mV

(67) .sub.water2.25MHz=0.972 nepers per meter (Np/m)

(68) .sub.2=73.61 Np/m.

(69) A second implementation relates to the spacer of composite material having thickness of 47.09 mm, using a wave at 1 MHz.

(70) .sub.2=1525.71 kg/m.sup.3

(71) V.sub.2=2879.39 m/s

(72) Z.sub.2=4.39311 M.sub.ac

(73) .sub.water=997.77 kg/m.sup.3

(74) V.sub.water=1486.54 m/s

(75) Z.sub.water=1.48322 M.sub.ac

(76) t.sub.12t.sub.21=0.75479

(77) x.sub.2=47.078 mm (accurate ultrasound measurement)

(78) Y.sub.1=370.25 mV

(79) Y.sub.2=16.395 mV

(80) .sub.water1MHz=0.682 Np/m

(81) .sub.2=60.92 Np/m.

(82) The invention is not limited to the implementations described but extends to any variant within the scope of the claims.