Patent Application
20150330949
The invention lies in the field of methods of characterizing parts made of composite material, in the engineering industry, and in particular in the aviation industry.
While a given part is being developed, it is necessary to know the fiber content and the resin content in a given zone of the part. To do this, it is known to measure the propagation speed and the attenuation of a longitudinal ultrasound wave passing through the part.
One method of measuring these magnitudes is to use an ultrasound transducer in transceiver mode. Attention is then given to a zone of the part that is defined by mutually parallel front and back surfaces. The longitudinal wave is directed so as to propagate orthogonally to the two surfaces, being partially reflected and also being attenuated in the material of the part. There are thus observed a first echo, coming from the front surface, and a second echo coming from the back surface and referred to as the back echo. The transducer receives the reflected wave, and it is then possible by observing the two reflected components to deduce both the propagation speed and the attenuation of the wave in the material.
Nevertheless, that solution is unsuitable for materials that absorb ultrasound waves strongly. This applies for example to three-dimensional 3D woven composites of a structure that is inhomogeneous and anisotropic. For parts of industrial thicknesses, no back echo is visible in recordings made on those materials, because of the strong absorption.
It is thus necessary to develop a method suitable for application to parts made of composite materials and enabling a large number of parts to be characterized, independently of their thickness or of their absorbent nature.
The invention relates to 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 the travel time of a wave transmitted by the part is measured.
By means of this technique, the problem associated with the absence of a back echo in transceiver mode measurements is overcome.
According to an advantageous characteristic, the travel time of the transmitted wave is measured by observing the beginning of the wave.
By means of this characteristic, it is possible to ignore greatly amplified problems of phase shifting and of deformation of the sinusoidal signal of the ultrasound wave used, as caused by thick materials, or as caused by the complex, inhomogeneous, and anisotropic structure of certain composite materials.
In an implementation, the propagation speed of the longitudinal ultrasound wave following a path in the part is determined.
This provides information that is useful for determining the fiber content and the resin content of a composite material, which information can be used in the development of the part under study.
In another implementation, which may be combined with the preceding implementation, the amplitude of the transmitted wave is also measured in order to determine the overall or unit length attenuation to which the longitudinal ultrasound wave is subjected on traveling in the part. This provides information that is useful for determining the pore content, which can be used in the development of the part under study.
Preferably, the propagation time of an ultrasound wave transmitted in the absence of the part is measured, as are the propagation times of ultrasound waves reflected respectively by a first face of the part and by a second face of the part, in order to determine the dimension of the part passing the longitudinal ultrasound wave traveling along a path in the part.
By means of this characteristic, which is optional but advantageous, an accurate measurement is obtained of the dimension of the part that passes the transmitted wave, whereas such a dimension is quite variable in parts made of composite material, so it can therefore be useful to know an exact value for a given part, for the particular path followed by the ultrasound wave used.
In particular, the method is performed for a part made of 3D woven composite material.
Such materials are particularly challenging to characterize because of their inhomogeneity and because of their anisotropy. By means of the invention, it is possible to study them quickly and reliably, particularly while parts are under development.
With reference to
In
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.
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.
Since the propagation speed of the wave in water Vwater 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 X2=(tX1+X2+X3−tX3−tX1)×Vwater, 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 tX1+X2+X3, tX1 and tX3 are the travel times measured during the steps E1, E2, and E3 respectively.
The following results are obtained:
tX1+X2+X3=92.72 microseconds (μs)
tX3=52.98/2=26.49 μs
tX1=29.94/2=14.97 μs
X2=t(X1+X2+X3−tX3−tX1)×Vwater
X2=(92.72×10−6−26.49×10−6−14.97×10−6)×1486.54
X2=76.20 mm
The thickness measured with calipers is indeed 76.20 mm, i.e. 3″.
The travel time of the wave in the part 30 is expressed in the form t′X2=t−(tX1+tX3). Knowing X2 as determined beforehand, the propagation speed of the wave in the material is expressed in the form Vmaterial=X2/t′X2.
The values obtained are as follows:
t=53.80 μs
t′X2=(53.80×10−6−26.49×10−6−14.97×10−6)
t′X2=12.34 μs
V=76.20×10−3/12.34×10−6
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.
The values obtained are as follows:
tX1+X2+X3=90.22 μs
t=74.90 μs
tX3=52.42/2=26.21 μs
tX2=64.68/2=32.34 μs
X2=tX1+X2+X3−tX3−tX1)×Vwater
X2=(90.22×10−6−26.21×10−6−32.34×10−6)×1486.54
X2=31.67×10−6×1488.76
X2=47.078 mm
t′X2=t−(tX1+tX3)
t′X2=(74.90×10−6−26.21×10−6−32.34×10−6)
t+X2=16.35 μs
Vcomposite=X2/t′X2
Vcomposite=47.078×10−3/16.35×10−6
And finally, the numerical value of the speed is Vcomposite=2879.4 m/s.
Attention is then given to the attenuation of the longitudinal wave in the material.
The expression for the amplitude of the wave transmitted from the emitter to the receiver is written as follows: Y1=Amaxe−α1.(X1+X2+X3), where Amax represents the maximum amplitude at the surface of the transducer and α1 is the attenuation of the wave in water.
The expression for the amplitude of the wave transmitted from the emitter to the receiver after passing through the material is written as follows: Y2=Amaxe−α1(X1+X3)e−α2X2t12t21, where α2 is the attenuation of the wave in the material, t12 is the amplitude transmission coefficient from water to the material, and t21 is the amplitude transmission coefficient from the material to water.
The expression for the product t12×t21 is a function of the acoustic impedance of the material Z2=ρ2×V2 and of the acoustic impedance of water Z1=ρ1×V1. In the acoustic impedance expression, ρ represents density and V represents the propagation speed of the longitudinal wave at the frequency under consideration.
The amplitude ratio Y1/Y2 is written as follows:
From which it is possible to deduce the expression for attenuation in the material:
A first implementation relates to the spacer of composite material having thickness of 47.09 mm, using a wave at 2.25 MHz.
The numerical values for this implementation are as follows:
ρ2=1525.71 kilograms per cubic meter (kg/m3)
V2=2946.75 m/s
Z2=4.39316 megOhms alternating current (MΩac)
ρwater=997.77 kg/m3
Vwater=1486.54 m/s
Zwater=1.48322 MΩac
t12×t21=0.75478
x2=47.078 mm (accurate ultrasound measurement)
Y1=643.2 millivolts (mV)
Y2=15.885 mV
αwater2.25MHz=0.972 nepers per meter (Np/m)
α2=73.61 Np/m.
A second implementation relates to the spacer of composite material having thickness of 47.09 mm, using a wave at 1 MHz.
ρ2=1525.71 kg/m3
V2=2879.39 m/s
Z2=4.39311 MΩac
ρwater=997.77 kg/m3
Vwater=1486.54 m/s
Zwater=1.48322 MΩac
t12×t21 =0.75479
x2=47.078 mm (accurate ultrasound measurement)
Y1=370.25 mV
Y2=16.395 mV
αwater1MHz=0.682 Np/m
α2=60.92 Np/m.
The invention is not limited to the implementations described but extends to any variant within the scope of the claims.
| Number | Date | Country | Kind |
|---|---|---|---|
| 1262155 | Dec 2012 | FR | national |
| Filing Document | Filing Date | Country | Kind |
|---|---|---|---|
| PCT/FR2013/052985 | 12/6/2013 | WO | 00 |
