Tightening control by ultrasound

Transcription

1 19 th World Conference on Non-Destructive Testing 16 Tightening control by ultrasound Farid BELAHCENE 1, Pierre SAMSON 1 1 ULTRA RS, 145 Breviandes, France Contact f.belahcene@ultrars.com Abstract. Control of screws and bolts tightening takes a growing importance in different sectors, where assembly defects caused by insufficient, excessive or heterogeneous tightening could induce hazardous consequences. To avoid or mitigate the failures related to tightening, the measurement and the control of tightening pretension should be performed in order to ensuree perfect sealing as well as assembly stiffness which allow withstanding external static and dynamic solicitations. This paper describes a new ultrasonic method of the tighteningg control, principle of the ultrasonic measurement and some experimental results. This technology is based on the measurement of the travel time of the ultrasonic wave along the elongated screw during the tightening. An ultrasound sensor is thus positioned inside a socket in order to measure the screw travel time in live. The information of the travel time combined with the calibration coefficient of the screw allows measuring the actual tightening force applied. 1. Introduction One of the major problems with the use of bolted joints is the precision, with regard to achieving an accurate preload. It is essential to apply an adapted tightening force to prevent the detachment of the assembled parts under the influence of external forces, and nullify any additional stresses due to vibration, shock... Insufficient, excessive or heterogeneous tightening represent about 3% of static failures and bad tightening conditions represent about 45% failures in fatigue. They are a recurrent problem in all industries: transport, oil and gas, nuclear, shipbuilding etc..., causing a strong financial and human impact. Therefore, it is necessary to control the tightening process with precise and reliable measurement tools. There was a strong need to evaluate the tightening force in bolted joints non-destructively with the aim to better control the assembly processes and increase the lifetime of the assemblies. Portable or transportable instruments have been developed to provide solutions to these needs, and are now commercially available. They provide information (torque, stress, elongation, and angle) on the quality of the assembly. To control the tightening of the assembled parts, there are different methods. These include the use of torque wrench and methods that determine the elongation of fasteners (bolt, screw, rods) using either mechanical strain gauges or more sophisticated means such as ultrasonic method. The torque wrench method adapted for this purpose has been reported to show enormous intrinsic errors in bolt tension measurement by up to 3% %. It s dissipated in License: 1 More info about this article:

2 overcoming friction under the bolt head or the nut face (whichever face that is rotated). Identical bolts, when tightened to identical torque values, vary substantially in their actual tensions [1]. The ultrasonic method is independent on the friction. This technique is accurate, non-destructive, easy to implement, and can be used to control the tightening in real time. However, the limits of the ultrasonic method (single wavelength) are imposed by the need of the original length of the bolts before tightening and material constants such as Young's modulus to determine the actual tensile load from 'ultrasonic elongation' of the bolts.. Principal of the ultrasonic stress measurement The linear theory of elasticity is generally adequate to describe the elastic behaviour of materials, by using Hooke s law. In this approach, the elastic strain energy is developed to the second-order terms for isotropic media. However, the theoretical description of the acoustoelastic effect, which refers to the change in velocity of ultrasonic waves propagating in strained solids, is only possible by considering the non-linear theory of elasticity. Using Murnaghan theory [], which takes into account third-order terms in the strain energy, Hughes and Kelly derived expressions for the velocities of elastic waves in a stressed solid [3]; in the following form: ρ V 11 ρ V 1 ρ V 13 = λ + µ + (l + λ) θ + (4m + 4λ + 1µ ) ε 1 = µ + ( λ + m) θ + 4µε1 + µε nε3 1 = µ + ( λ + m) θ + 4µε1 + µε 3 nε 1 (1) Where: V 1j is the velocity of waves in the direction 1 with particle displacement in the j direction, ρ is the initial density, θ =ε1 +ε+ ε3 are the components of the homogeneous triaxial principal strains, λetµ: second order elastic constants, l, m, n : Third order elastic constants. In the case of uniaxial stress, ε1 = ε and ε = ε3 = νε, where ν is the Poisson ratio. By linearizing the equation (1) in the first order, we can write: V 1 j = V1 j ( 1+ A1 j σ11) () Where: V 1j : velocity of the wave in the direction 1 with zero stress, σ 11 : stress in the direction 1, V 1j : velocity of the wave in the direction 1, in the presence of stresses (σ 11 ), A 1j : acoustoelastic constant.

3 Application of the acoustoelastic theory for the tightening measurement: Figure 1 represents the configuration of the tightening tension measurement in the screw. In order to, an ultrasonic signal is applied on the head of the screw. The signal propagates through the screw until reaching the air / steel interface at the end of its rod, and then returns to its head. The time between the 1 st and the nd echo is used to measure the length of the screw. Fig. 1. Configuration of the tightening tension measurement. When a screw is tightened, two effects will increase the travel time of the ultrasonic wave. Firstly, the acoustoelastic effect, which is caused directly by the tensile stress s in the screw. It tends to decrease the propagation velocity of the wave and, therefore, increase its travel time. Secondly, the effect of lengthening of the screw increases the wave path and thus increases the travel time. We consider a simple one-dimensional model to calculate the plane-wave response to the axial load (Figure ). The total length of the bolt without stress is L and the longitudinal wave is V in the stress-freee conditions; l N and l H are the lengths of thee nut and the bolt head. When the tensile stress σ is applied along the length, a small segment dx in the bolt shank is elongated to (1+σ/E).dx, where E is the Young modulus of o the material. Assuming that the wave velocity V varies linearly with axial stress σ, thatt is: V = V (1 + K. σ ) (3) V is the wave velocity without stress, and K is the acoustoelastic constant. The travel time in the small segment dx becomes: [ ( ) ] dx 1 1 dt = 1+ E K σ V. The higher-order terms are neglected in the derivation. The round trip travel time along a the bolt is obtained by integration, and is given by the following formula: (4) 3

4 t = t The formula (5) can also be written as follows: (1 +. σ ) (5) C B ave Here: t t t = C B.σ ave (6) t = L V C B = E 1 K σ ave = β.σ max C B is the calibration coefficient which depends on the acoustoelastic effect and the elongation effect of the screw. The effective length ratio β iss defined as: ln + lh β = 1 (7). L Fig.. One-dimentional axial-stress model [4]. 3. Influences of external parameters on the ultrasonic measurement 3.1 The effect of the variation of the tightened length The calibration coefficient depends on the tight length (Ls). The change of Ls (Figure ) has a non-negligible influence onn the results of calibration coefficient C B and therefore on the accuracy of the measurementt of the tightening tension. Figure 3 represent an example of the calibration coefficient C B obtained on a steel screw (M4) with two different values of Ls. This result shows that the relationship of proportionality between the relative change in travel time of the longitudinal wave and the applied stress (C B ) ranges from to MPa -1 (about % difference) respectively for Ls of 54 mmm and 4 mm. 4

5 The formula (6) requires the knowledge of the right value of Ls to obtain an accurate coefficient C B and an accurate tightening stress. Fig. 3. Calibration coefficient C B obtained for two tight lengths Ls [1]. 3. The effect of the variation of the temperature The temperature is an important factor on the application of the ultrasonic method for measuring the tightening stress. The influence of the temperature on the propagation velocities of the ultrasonic waves is well known [5]. The velocity of the ultrasonic wave, (independently of the tightening stress) is defined as: V = E(1 ν ) ρ(1 + ν )(1 ν ) (8) Where V is the velocity, E is Young's modulus, ν is the Poisson's ratio and ρ is the density. It is clear that E and ρ are dependent of the temperature. Therefore, a change in temperature influences the velocity of the wave and changes the accuracy of the tightening stress measured. A linear increase in temperature causes a linear increase of the measured stress (error expressed in MPa). For example, the stress measured by the longitudinal wave increases by 8 MPa per degree in the rod M48*3-34CrNiMo6. To correct the temperature effect on the measurement, the stress equation can be rewritten: σ reel = σ mes + 8( T T ) (9) Where: σ mes = σ ave (formula 6) T : initial temperature during the first measurement T: temperature at time t Therefore, the measurement of the tightening stress on the screw is realized by reading the travel time of ultrasonic waves propagating on the screw and correcting the travel time depending on the temperaturee of the component under evaluation. 5

6 3.3 The effect of the parasitic bending The bending stress can be caused by a variation of the thickness of a washer or the presence of a weld point under the head of the screw. The ultrasonic measurement of the tightening stress will be erroneous if the screw bends: the ultrasonic signal will be influenced by the presence of the bend in the screw. Table 1 represent an example of measurement obtained on a steel screw (M16x6) with a washer of inconstant thickness. A variation in the thickness of a washer of 1 mm may induce a bend in the screw and result in overestimation of stress measured by ultrasound. For this example, the relative difference between strain gauge and ultrasound is equal to 11 %. Torque applied with a torque wrench (N.m) Stress measured by strain gauge (MPa) Stress measured by ultrasound (MPa) error(%) = F SG F F SG US With bending stress 13 4,8 45,46 11,4 Without bending stress 13 N.m 36,4 35,75-1,8 13 N.m ,59,6 13 N.m 3,9 33,3 1, Table 1. Stress measured in a steel screw M16 x 6 - effect of the variation of the thickness of a washer. 4. Calibration Procedure The calibration is necessary to know the coefficient C B connecting the measured travel time of the ultrasonic waves to the imposed tightening stress. This coefficient depends on the screw diameter, the tight length Ls and mechanical characteristics of the assembly. For a given tightening configuration (diameter, tensile length Ls, materials), we must perform a calibration to determine the coefficient C B. The screw is tightened in the steel block (figure 4) and increasing levels of different loads are applied (step by step) with an hydraulic cylinder and measured with a force transducer. The ultrasonic sensor is placed on the screw head to measure the relative change in travel time of the longitudinal wave. An example of calibration is represented above in figure 3. 6

7 Ultrasonic socket Screw Washer Hydraulic cylinder Force transducer Steel block Fig. 4. Experimental mock up for the calibration. 5. Tightening Stress Procedure The ultrasonic device used to control the tightening stress is represented in figure 5. It consists of: - the hydraulic torque wrench piloted by ultrasonic device, - the socket instrumented by ultrasonic sensor, - and the ultrasonic device. In order to optimize the tightening operation, the coupling of the ultrasonic technique with the technique of hydraulic tightening would be interesting in order to ensure an accurate and rapid homogeneous tightening. To address this problem, a project was born, which involved the development of a fully automated system: hydraulic torque wrench piloted by ultrasonic device: using ultrasound as a means of stopping the hydraulic tightening. The operator will be able to fix a value of the tightening force in the software; the hydraulic torque wrench will then proceed to tightening the screw until the desired value is obtained. In addition, the tightening operations can be performed simultaneously on a several screws. This provides a uniform tightening force on all screws concerned (homogenous assembly). Figure 5 represent the mock-up used to qualify this technique. The calibration coefficients of the screws were previously determined. The screw is tightened to a specified load and the actual force is measured by both the strain gauge (which is considered as the reference) and the US equipment (FUS). 7

8 Fig. 5. Experimen ental setup for measurement of tightening force. As shown in the fig.6 and table tab this method enables to accurately tighte hten a screw up to a specified force. error (%) = FSG FUS FSG Target value FSG (kn) FUS (kn) 5 5,3 49,,6% 1 1,7 99,5 1,% 15 15,5 149,1,9%,3,5,1% 5 47,5 49,9 1,% 3 95, 99,6 1,5% 5 51,6 49,3 4,5% 1 1, 99,8,4% ,6,9% 199,9,% 5 48, 5,4,9% 3 98, 3,,6% 5 5,7 49, 3,4% ,1,8% ,5 151, 1,5% 3,7 1,3 1,% 5 53,7 5,4,5% 3 31,3 31,5,1% Table. Stress measured steel screw M4 x 3. The tightening procedure is: the operator fix a target value of the tighte htening force in the software, the hydraulic torqu rque wrench will then proceed to tightening th the screw and stop the tightening when the ultras rasonic force value (FUS) is reach the target valu lue. 8

9 Fig. 6. Tightening at ambient temperature - Error versus tightening force. As show in the table, the values of the forces measured by ultrasound are very close to target values. 6. Conclusion The accurate control of the tightening force is often seen as a secondary operation in the design of mechanical equipment, and yet the insufficient, excessive or heterogeneous tightening is the cause of about 3% of static failures and the bad tightening conditions are the cause about 45% of failures by fatigue. Therefore, the tightening operation must be controlled through measuring instruments, precise, reliable, easy to implement, enabling a homogeneous tightening of all the bolts with a minimum of operations. Ultrasound is one of the techniques to address this issue: the ultrasonic determination of the tightening force is providedd by experimental devices that were designed to refine the methodology for measuring the travel time (speed) of the ultrasonic wave. The choices that were made at the experimental elements are: - The use of specific ultrasonic transducer to obtain better reproducibility of measurements, - Calibrations of acoustoelastic effect for selecting the longitudinal wave for measurement, -The use of the UltraRS software for the effective treatment of measurement signals in order to minimize the error and improve the acquisition speed. References [1] BELAHCENE (F). - Contrôle de tension de serrage par ultrasons - Techniques de l'ingénieur, Référence R44 Date de publication :1 nov. 15. [] MURNAGHAN (D.). - Finite deformation of an elastic solid, John Willey, New York (1951). [3] HUGUES (D.S.) et KELLY (J.L.). - Second-order elastic deformation of solid, Physical Review, vol. 9, no 5,p à 1149 (1953). [4] Masahiko (H) and Hirotsugu (O). -EMATS for science and industry: noncontacting ultrasonic measurements. Kluwer Academic Publishers, Boston/Dordrecht/London 3. [5] ZHOU (X). Etude paramétriques pour la détermination des contraintes résiduelles par la méthode ultrasonore,thèse de doctorat, UTT, France (6). 9