WO2022070004A1

WO2022070004A1 - Computer implemented method for measuring local elastic properties of a shell material and measuring system thereof

Info

Publication number: WO2022070004A1
Application number: PCT/IB2021/058641
Authority: WO
Inventors: Davide Salvatore PAOLINO; Carlo BOURSIER NIUTTA; Andrea TRIDELLO
Original assignee: Politecnico Di Torino
Priority date: 2020-09-29
Filing date: 2021-09-22
Publication date: 2022-04-07

Abstract

A computerized method for measuring elastic properties of a component ready to be assembled or already assembled comprises the steps of: - Applying and maintaining a perimeter constraint to one face of a slab area of the product, wherein the constraint delimits a closed perimeter path (7) within which the slab area is identified; - Applying excitation by means of an exciter device to the slab area in order to excite at least a first mode of vibration of the slab area; - Detecting at least a first natural frequency of the slab area by means of a vibration sensor in the slab area; - Calculating at least one elastic constant of the material of the slab area based on at least a first natural frequency and a mathematical model of the vibrational dynamics of the peripherally constrained slab area.

Description

"Computer implemented method for measuring local elastic properties of a shell material and measuring system thereof"

DESCRIPTION

TECHNICAL FIELD

The present invention relates to a non-destructive computer assisted method for measuring local elastic properties, e.g. elastic modules and Poisson's ratio, and/or defects impacting the vibrational behaviour of a slab material or manufactured product comprising slab portions, possibly shaped, such as body panels of a land vehicle or substantially two-dimensional frame portions. It is also possible to apply the invention to aeronautical components e.g. fuselage elements or wings, nautical e.g. hull and sports industry e.g. downhill skis, surfboards, water skis and snowboards. Advantageously, the present invention is also applied when the slab material is anisotropic and non-metallic, e.g. a polymer matrix composite. The method is also applicable to isotropic slab materials.

BACKGROUND OF THE INVENTION

It is well known to measure one or more local elastic properties of an isotropic metallic material in a slab comprising the steps of applying a perturbation impulse and acquiring through a vibration sensor, e.g. a microphone, the response of the slab. In particular, the technique called Impulse Excitation Technique is an example and is the subject to ASTM standards. Any defects in the slab area identified by comparing the vibrational data collected during the measurement with known reference data in the literature or performed on a specimen having known elastic characteristics or by computer simulations . For example , a defect such as a crack or delamination can be identi fied, since the presence of such a defect impacts the vibration frequencies . It is therefore possible , starting from the vibrational data collected, to calculate one or more elastic properties of a material and to determine the presence and other characteristics of slab defects that impact the vibrational behaviour of the slab .

This technique i s applied to slab, possibly also of anisotropic material , which l imits its application to the supplier quality control sector and not , for example , to the diagnostics of existing products after having suf fered damage , such as in the case of components of a land vehicle after suf fering an accident .

SUMMARY OF THE INVENTION

The obj ect of the present invention is to present a non-destructive diagnostic method for ready to assembly or already assembled components .

The obj ect of the present invention is achieved by a method for measuring elastic properties of a component ready to be assembled or already assembled, comprising the steps of :

- Applying and maintaining a perimeter constraint to one face of a slab area of the product , wherein the constraint delimits a closed perimeter path within which the slab area is identi fied;

- Applying an impulse through an exciter device to the slab area

- Using a vibration sensor to detect the slab area

Wherein the perimeter constraint comprises a contact surface surrounding the perimeter path and extending radially or spaced from the perimeter path in such a way as to constrain the slab area in an interlocking manner .

The peripheral interlocking constraint allows to delimit and isolate the vibrating area and, in particular, to make the dynamic response of the vibrating area independent from the overall geometry of the product . In this way, it is possible , in particular when the perimeter constraint defines a simple geometry, to compare the measured data with values calculated through analytical models of the slab area, since these models are solved by considering a perimeter interlock . It should be noted that experimental evidence shows that even when the constraint is not a perfect fit , reasonably accurate results are obtained i . e . independent from the geometry of the product surrounding the slab area . The important characteristic of the constraint is to allow a frequency response of the slab area independent of the complete geometry of the component , with particular reference to the first natural frequency and, according to one embodiment , at least also to the second natural frequency . By means of this speci fic removable constraint method, multiple slab areas of a single component can be measured and any defect can be identi fied within the perimeter constraint or the calculation of the elastic properties can be performed in multiple areas of the same component . It is important that the constraint is applied only on one face as this greatly simpli fies the measurement process and still provides good quality results . Furthermore , with the proposed method, it is suf ficient to measure or calculate from the sensor data only the first natural frequency of the slab area and this has the advantage of f inding a good compromise between precision and computing power : in fact , the results are suf ficiently precise and do not make it necessary to calculate the natural frequencies following the first one .

According to a preferred embodiment , the method comprises the further steps of performing a first vibrational measurement of the slab area with a first configuration of the perimeter constraint , rotating the perimeter constraint to delimit a further slab area, blocking this second configuration of the perimeter constraint in the same way interlocking and applying a further impulse to the further slab area .

By modi fying the boundary conditions and, at the same time , keeping the relative position with the component of the excitation device unchanged, it is possible to obtain additional information with particular reference to anisotropic slab materials . In fact , when the slab area is defined by an anisotropic material , the modi fication of the boundary conditions allows to obtain data thanks to which the elastic characteristics along orthogonal directions or the known directions of the anisotropic reinforcements , e . g . oriented carbon fibres , Kevlar etc . , can be to calculated, in particular when the anisotropy originates from a multilayer material . When the method is applied to slab areas of isotropic material , the rotation of the interlocking perimeter can be useful to obtain redundant measurements . It should be noted that , according to a preferred embodiment , the vibrating area is excited by a pulse to excite , at least theoretically, all the natural frequencies and the free response is measured . Therefore , the position of the exciter can change but , preferably, remains fixed with respect to the product between a first and a second measurement .

Other advantages of the present invention are discussed in the description and mentioned in the dependent claims .

BRIEF DESCRIPTION OF THE FIGURES

The invention is described below based on non-limiting examples illustrated in the following figures , which relate respectively to :

Fig . 1 a sectional perspective view of a measuring device for carrying out the method of the present invention;

- Fig . 2 a perspective view of a component ready to be assembled or already assembled on which to apply the device of figure 1 ; - Fig . 3 a flow chart of the main steps of the method according to the present invention; and

Fig . 4 is a set of diagrams illustrating respective acquisitions of vibrational data in di f ferent angular positions of the measuring device applied to an anisotropic unidirectional material .

DETAILED DESCRIPTION OF THE INVENTION

Figure 1 shows a device for measuring the vibrations of a slab area in a component , such as a portion of a frame of a land vehicle ( Figure 2 ) , bodywork elements etc .

The device 1 comprises a preferably releasable constraint unit 2 for attaching the device 1 to a face of a component and delimiting e . g . a rectangular perimeter of a slab area of the component when the constraint unit 2 locks the device 1 on the face , an exciter device 3 for applying a pulse to the slab area and a vibration sensor 4 for detecting the vibrational response of the s lab area to the impulse . The vibration sensor may comprise a microphone , a piezoelectric sensor, a laser vibrometer, an accelerometer etc . Preferably, the releasable constraint unit 2 defines an axial symmetrical perimeter and the exciter device 3 is arranged on axis with the symmetry axis of the perimeter . Alternatively, or in combination, the unit 2 is rotatable and the exciter device 3 is coaxial with the rotation axis of the unit .

In particular, the constraint 2 comprises a contact surface 5 extending around the slab area both in the perimeter direction of the area itsel f and in a radial direction starting from the vibration sensor 4 so that the slab area is inscribed within the contact surface 5 . In use , a pressure is applied to the contact surface 5 to anchor the device 1 to the component and make the portion of the component external to the slab area delimited inside the device 1 negligible for the purposes of the vibrational calculations . The constraint 2 therefore performs a barrier function so that the response to the stresses applied to the slab area by the exciter device 3 can be considered at least as a first approximation to be limited to the slab area itsel f and independent of the shape , thickness , etc . of the component outside the slab area . In particular, the action of constraint 2 on the component during the measurement is such as to create an ideal continuous interlocking condition along the periphery of the slab area . This is achieved, for example , by a suitable extension of the contact area in the radial direction with respect to the sensor 4 or in the centre of the slab area .

According to the embodiment illustrated in Figure 1 , the contact area 5 is multi-component and has a first peripherally closed area 6 and a second peripherally closed area 7 inscribed in the area 6. The areas 6 and 7 are spaced apart in a radial direction to define an annular chamber 8 wherein , according to a preferred embodiment , vacuum is applied to generate a pressure on the contact area 5 such as to delimit the slab area within the zone 7 and make it negligible for the purposes of the response the remaining part of the component is vibrational . Furthermore , the vacuum in the chamber 8 keeps the device 1 bound to the component and, for this purpose , the contact area 5 has an adequate number of gaskets (not shown) whose seats are generically indicated in the figure with the number 9 . Alternatively, to the vacuum action, the pressure on the contact area 5 can also be applied by means of a magnetic or electromagnetic field, in particular i f the component is ferromagnetic, or by adhesives .

In order to provide adequate insulation from the remaining portion of the component so that the vibrations propagate in the slab area in a manner similar to a plate continuously stuck on its periphery, the contact area 5 has , at least during the vibration measurement , a f ixed and rigid geometry to provide a relative rigid constraint .

For example , when the device 1 is applied to a component made o f an anisotropic multilayer material of known geometry with reference to the oriented reinforcement fibres , it is possible to perform two or more measurements by modi fying the angular position of the constraint unit 2 . In this way it is possible collect data of slab areas having di f ferent orientations from which elastic parameters can be calculated along orthogonal axes or otherwise incident on the basis of the disposition of the oriented reinforcement fibres . Preferably, the sensor 4 rigidly rotates with the constraint unit 2 and in this way presents , for each measurement , the same relative position with respect to the periphery of the vibrating area . At the same time , further embodiments wherein the relative position between the sensor 4 and the constraint unit varied due to rotation, would produce equally reliable results .

According to the preferred embodiment illustrated in Figure 1 , the area 6 and the area 7 of the contact area 5 have an adj ustment of the relative angular position . For example , in use , the zone 6 remains stationary in contact with the component and the zone 7 , for example by means of a handle 10 or other grip, is rotated so as to modi fy the angular position of the slab area with respect to the elongated fibres reinforcing the layers compris ing the anisotropic material .

In particular, figure 1 shows an embodiment of the device 1 wherein the area 6 is defined by a peripheral block 11 defining a circular cavity, the area 7 is defined by a central body 12 carrying the handle 10 and guided in rotation by the circular cavity of the block 11 . Furthermore , the central body 12 comprises a head portion 13 on which the handle 10 is fixed and disposed in support of a closing wall 14 of the block 11 which closes the cavity at the top . On the opposite side of the head portion 13 with respect to the closing wall 14 , the central body 12 comprises inside the block 11 a measuring unit 15 configured to carry the sensor 4 and define the contact area 7 to peripherally delimit the slab area . In particular, the measuring body 15 is hollow to allow the slab area to vibrate and a spring 16 is arranged between the head wall 13 and the measuring unit 15 so as to press the area 7 against the face of the component when the device 1 is blocked, for example by applying vacuum to chamber 1 . Suitably, a structure , e . g . screws 17 , rigidly connects the measuring portion 15 to the head portion 13 and openings , e . g . slots , are defined by the closing wall to allow the screws to protrude from the block 11 and connect to the head wall 13 .

According to an aspect of the present invention, the vibrational data acquired by the sensor 4 are subsequently processed to calculate either at least an elastic characteristic or to provide an integrity estimate in the case of a component that has suffered a shock, for example as a result a road traf fic accident .

In the case of calculation of the elasticity parameters , proceed as in figure 3 and, preferably, 4 rotations of zone 7 of 30 ° each are performed with respective data acquisitions i f the multilayer composite compri ses layered elongated fibres arranged at 0 , 30 ° , 60 ° and 90 ° from layer to layer, as often happens in the automotive and aerospace industries .

In particular, for each rotation of zone 7 the first resonance frequency is calculated by means this frequency, e . g . with the application of the Rayleigh- Ritz method and an optimi zation algorithm, it is possible to calculate through iterations the values El l of the elastic modulus of the slab area in the direction of the fibres , E22 of the elastic modulus in the direction perpendicular to the fibres , G12 of the tangential elastic modulus in the plane of the slab area and one of the Poisson ' s ratio in the plane of the slab area ( the other Poisson ' s ratio in the plane is calculated on the basis of said elastic characteristics) . Since there are 4 unknown elastic parameters, it is necessary to proceed with 4 acquisitions e.g. of the first resonant frequency in the four angular orientations indicated above. In more detail, the 4 measured resonance frequencies are compared to those obtained through a numerical simulation e.g. by means of finite elements or, in the simplest cases, analytics. In particular, the frequencies obtained through the calculation have the unknown elasticity constants as independent variables and through the solution e.g. numerical of an optimization problem that minimizes the difference between the measured and the calculated frequencies, allows to calculate the unknown elastic constants of the vibrating area. According to an embodiment, the optimization problem is identified by:

Where x = [E₁₁,E₂2> G₁₂, v₁₂], f_eXp are the measured frequencies and f_num are the frequencies calculated using mathematical models. In particular, the frequencies are measured and calculated for the different orientations of the constraint unit 2.

Furthermore, numerical finite element simulations for the calculation of the first resonance frequency must reproduce characteristic parameters of the vibrating area, such as the orientation of the constraint unit 2 with respect to the one or more layers of oriented reinforcement fibers, the reciprocal orientation of these fibers layer by layer, the thickness of the vibrating zone and the constraint condition applied i.e. the continuous interlocking along the periphery. In the case of isotropic material, the unknowns are reduced, i.e. bending elastic modulus, torsional elastic modulus, and Poisson's ratio or even one, i.e. the elasticity modulus, if the Poisson's ratio and the tangential elastic modulus are assumed. For example, if there are two unknowns, the constraint unit 2 is used to measure two natural frequencies, e.g. the first and second natural frequencies. In this case, the minimum number of measurements decreases as does the complexity of the numerical solution of the optimization problem.

In the case of measuring slab of anisotropic material, it is possible to correlate the shape of the constraint unit 2 to the disposition of the oriented reinforcement fibres to increase the sensitivity and thus the accuracy of the frequency measurements. For example, in the case of anisotropic material with unidirectionally oriented fibres, a perimeter of the rectangular area 7 of the constraint unit 2 wherein the difference between the longer side and the shorter side increases, increases the sensitivity of the measurement. In fact, when the rectangle has its long side parallel to the orientation of the fibres, the resonant frequency is more influenced by the E22 module and, correspondingly, when the fibers and the elongated rectangle are oriented at 90°, the resonance frequency is more influenced by Ell. In strongly anisotropic materials, the resonance frequencies of the first way of vibrating i.e. flexural way, they vary more with the orientation of the perimeter and if the shape factor of the perimeter helps to increase this diversity, the accuracy of the measurement improves. It should be noted that the method of the present invention is applicable to various configurations of oriented reinforcement elements, e.g. even in the case of reinforcement elements arranged orthogonally to each other. In this case, if the fibres are the same and four measurements are performed every 30° starting from the orientation of one of the two groups of fibres, and the first and fourth measurements of the first natural frequency are the same or very similar.

Therefore, for the purposes of the present invention, a form factor of the slab area i.e. the ratio between a maximum transversal dimension and a minimum transversal dimension of the slab area, must be lengthened, i.e. different from 1. For example, the form factor is 1 in the case of a circumference, 1.41 in the case of a square, the ratio between major and minor semiaxis in an ellipse etc.

If it is intended to provide an estimate of the residual elastic properties after a shock that could have damaged the component, the elastic characteristics calculated as above or the relative natural frequencies measured are compared with the respective values from the technical literature or from finite element simulations. If the elastic characteristics calculated on the basis of the measurements of the device 1 are lower than the reference values, this difference is an indication of the degradation of the elastic properties of the slab area immediately following the shock of the component . Furthermore , it is possible that the comparison is carried out by means of an electronic computer programmed to generate an alert message when the di f ference exceeds a predefined threshold . In this case , in fact , the measured material may present damage that compromises its structural performance such as a delaminated area .

The calculations mentioned in the present description are performed by means of a programmed electronic processor and a memory connected in data exchange to the processor .

Claims

1 . A computer implemented method for measuring elastic properties of a component ready to be assembled or already assembled comprising the steps of :

- Applying and maintaining a perimeter constraint to one face of a slab area of the product , wherein the constraint delimits a closed perimeter path ( 7 ) within which the slab area is identi fied;

- Applying an excitation through an exciter device to the slab area in order to excite at least a first mode of vibration of the slab area ;

- Detecting at least a first natural frequency of the slab area by means of a vibration sensor in the slab area ;

- Calculating at least one elastic constant of the material of the s lab area on the basis of the at least a first natural frequency and a mathematical model of the vibrational dynamics of the peripherally constrained slab area ;

Wherein the perimeter constraint comprises a contact surface ( 5 ) surrounding the perimeter path ( 7 ) and extended radially or spaced from the perimeter path in such a way as to define an interlocking perimeter constraint such that the measurement of the at least one first natural frequency is substantially independent of the overall shape of the component .

2 . Method according to claim 1 , wherein said steps are repeated for a plurality of angular positions of the perimeter path ( 7 ) around a fixed axis with respect to said component so as to detect respective di f ferent natural frequencies due to the anisotropy of the slab area .

3. Method according to claim 2, wherein said steps are repeated a number of times equal to the unknown elastic constants.

4. Method according to claim 3, wherein the unknown elastic constants are at least two between a longitudinal elastic modulus Ell, a transversal elastic modulus E22, a tangential elastic modulus G12 and a Poisson's ratio.

5. Method according to claim 4, wherein the step of calculating comprises the step of solving the following problem numerically through a programmed processor and a memory:

Where x= [E₁₁,E₂₂,G₁₂,v₁₂], f_exp are measured frequencies and f_num are frequencies expressed through the said mathematical model.

6. Method according to any one of the preceding claims, comprising the step of identifying one or more directions of oriented fibres reinforcing the material and orienting the closed perimeter path (7) based on the direction of the fibres to obtain different natural frequencies due to the anisotropy of the slab area.

7. Method according to any one of the preceding claims, wherein a form factor of the slab area is different from 1. 17

8 . Method according to any one o f the preceding claims , comprising a step of comparing said at least one elastic constant and/or natural frequency measured with a reference value and generating an alert message when a di f ference between the elastic constant and/or natural frequency measured and the reference value reaches or exceeds a predefined threshold .

9 . System for measuring elastic properties of a component ready to be assembled or already assembled comprising a measuring group ( 1 ) having a perimeter constraint configured to maintain itsel f in contact with a face of a slab area of the product , wherein the constraint delimits a closed perimeter path ( 7 ) within which the slab area is identi fied; an exciter device ( 3 ) configured to apply an excitation to the slab area so as to excite at least a first way of vibrating the slab area ; a vibration sensor ( 4 ) for detecting at least a first natural frequency of the slab zone ; and an electronic microprocessor controller programmed to calculate at least one elastic constant of the material of the slab area on the basis of the at least a first natural frequency and a mathematical model of the vibrational dynamics of the slab area peripherally interlocked;

Wherein the perimeter constraint comprises a contact surface ( 5 ) surrounding the perimeter path ( 7 ) and extended radially or spaced from the perimeter path in such a way as to define a perimeter constraint similar to an interlocking so as to make the measurement of at least one first natural frequency 18 substantially independent of the overall shape of the component .