Estimation of Torsional Compliance from Free-Free FRF Measurements: ercf Theory

Estimation of Torsional Compliance from Free-Free FRF Measurements: ercf Theory Hasan G. Pasha, Randall J. Allemang, Allyn W. Phillips, Alexander Young University of Cincinnati Structural Dynamics Research Lab (UC-SDRL), Cincinnati, Ohio, USA Jeff Poland BMW Manufacturing Company, Spartanburg, South Carolina, USA Abstract. The enhanced rotational compliance function (ercf) is a useful concept for estimation of static torsional compliance/stiffness of a structure using measured frequency response functions (FRFs) from a structural system with free-free boundary conditions. The ercf is estimated using FRF measurements involving impact testing in which a four by four (4x4) FRF matrix is acquired at four separate, symmetric locations on a structure. This is in contrast to a traditional, static torsion test that involves constraints applied to two of these four locations and a static torque applied to the other two of these four locations. The traditional, static torsion test requires extensive instrumentation and a two day test procedure while the ercf method involves minimal instrumentation over several hours. Added masses can be utilized to acquire additional statistical data that estimates the same compliance (stiffness). The theoretical background is presented along with both modeling and experimental cases involving a rectangular plate structure. Keywords. Static Torsional Stiffness, Enhanced Rotational Compliance Function, Torsional Stiffness Sensitivity, Overhang, Statics-from-dynamics Notation Symbol Description σ Standard deviation Δ Static deflection (in) Θ Angular deflection (rad) f max f min l f l r Maximum frequency of the stiffness parameter estimation band (Hz) Minimum frequency of the stiffness parameter estimation band (Hz) Front spacing (in) Rear spacing (in) F Generalized force magnitude (lb f ) M Generalized moment magnitude (lb f in) [H(ω)] FRF matrix ( X F ) K T Static torsional stiffness (lb f in/deg) {V} Moment scaling vector DOF 1 Left front DOF, z direction DOF 2 Right front DOF, z direction DOF 3 Left rear DOF, z direction DOF 4 Right rear DOF, z direction DOF 5 Left mid-span DOF, z direction DOF 6 Right mid-span DOF, z direction ercf(ω) enhanced Rotational Compliance Function efrf(ω) enhanced Frequency Response Function

1. Introduction Static structural stiffness is an important criterion in automobile structure design as it impacts vehicle handling, ride comfort, safety and durability. Traditionally, automobile manufacturers used special test rigs to estimate stiffness, which require precise setup and expensive instrumentation. In the past decade, dynamic frequency response function (FRF) measurements with modeling techniques were used to estimate static stiffness. Though test setup time and expense associated with the instrumentation are considerably reduced, heroic technical measures in terms of user experience and data processing are required to get reasonable stiffness estimates. As static stiffness information is contained in the measured free-free FRFs, it can be extracted by utilizing spatial/geometric filtering and averaging techniques. A simple and efficient method to estimate static bending stiffness from free-free FRFs is developed in this paper. The enhanced Rotational Compliance Function (ercf) method presented here utilizes spatial/geometric filtering and averaging techniques to enhance response functions and estimate static stiffness. The stiffness estimates obtained from this method were comparable with the analytical results, while requiring significantly less resources. 2. Static Torsional Stiffness Torsional stiffness or rotational stiffness K T is defined as the ratio of applied torque to the angle of twist. (1) K T = T θ where, T is the torque (lb f in or N m) and θ is the angle of twist (deg). Torsional stiffness is expressed in lb f in/deg or N m/deg. Torsional stiffness can also be expressed in terms of the modulus of rigidity (a.k.a shear modulus, G: units psi or Pa), the torsion constant (J: units in 4 or m 4 ) and the characteristic length, L (distance between the fixed end to the point of application of the torque). The product of the torsion constant and modulus of rigidity, J G is more commonly known as torsional rigidity [8]. (2) K T = J G L It is evident from Eq. 2 that the torsional stiffness is inversely proportional to the characteristic length, in other words, the torsional stiffness value depends on the test geometry. As a result, multiple torsional stiffness estimates might be required for a structure that has multiple loading locations. The torsional stiffness of a structure, for example a rectangular plate shown in Fig. 1, can be calculated using the following expression by performing a static FE analysis or static testing. Consider that one end of the plate is constrained and a moment is applied at the other end. The torsional stiffness is defined as (3) K T = T θ = F L f/2 θ ( ) where, θ = tan 1 Δ L f /2, is the angular twist, which is expressed in terms of the vertical displacement and the moment arm L f. 3. Enhanced Rotational Compliance Function (ercf) 3.1. General procedure for estimating stiffness using enhanced compliance function method. The formulation of the enhanced rotational compliance function method for a particular load case involves the following steps: (1) Develop a free-free system in dynamic equilibrium, which is equivalent to the statically determinate constrained system used in static testing methods (2) Express the reactions and applied moments/forces in terms of a generalized moment to obtain the moment scaling vector {V} (3) Utilize the measured FRFs and the moment scaling vector to formulate the required enhanced compliance function

Figure 1: Configuration for estimating torsional stiffness A system in dynamic equilibrium, which is equivalent to a statically determinate system should be developed for a particular load case. This is achieved by applying equal and opposite force (reaction forces) at the constraint locations. The boundary constraints (fixed to free) at front and rear load/deflection DOFs have no effect on the stiffness estimate as long as the motion at all DOFs is accounted for [3]. The static rotational stiffness is sensitive only to relative motion, end-to-end. Therefore, the boundary constraints can be replaced by reaction moments/forces. The estimation of static stiffness using an enhanced compliance function involves the following steps: (1) Measure the accelerance FRFs of the free-free system (2) Derive the compliance or dynamic stiffness FRFs from the measured accelerance FRFs (3) Form a reduced (sieved) dataset based on the desired test configuration. A 4x4 system for torsional stiffness estimation is formed. (4) Compute the enhanced compliance function formulated for the load case, using the measured FRFs and moment scaling vector (5) Fit an appropriate model to the low frequency region of the enhanced compliance function (6) Use the model parameters from Step 5 to estimate the static compliance/stiffness 3.2. Formulation of the ercf for Torsion Load Case. In order to simulate a pure torsion load while performing static testing, a commonly used configuration consists of constraints at the rear DOFs. A moment is applied at the front DOFs and the deformations are measured, as illustrated in Fig. 2(a). The stiffness constraints are applied at the rear DOFs (DOFs 3 and 4), while a moment is applied at front DOFs (DOFs 1 and 2). An equivalent configuration without stiffness constraints for the torsion test is shown in Fig. 2(b). The constraints at DOFs 3 and 4 are now replaced by a reaction moment (M r = F r L r ) at the rear. When the moments are applied on the structure, the sum of the applied moments should be equal to zero for the structure to be in equilibrium: + ΣM x = 0 (4) (5) M f = M r F f L f = F r L r where, M f, F f (= F 1, F 2 ), and M r, F r (= F 3, F 4 ), are the moments and forces applied at the front and rear DOFs respectively. Beginning with the fundamental input/output FRF equation, (6) {X(ω i )} 4x1 = [H(ω i )] 4x4 {F(ω i )} 4x1.

(a) Fixed-free configuration (b) Equivalent free-free configuration Figure 2: Equivalent configurations for estimating torsional stiffness Figure 3: Load/displacement orientations

The force vector {F} is rewritten in terms of the generalized moment magnitude (M), applied to the front and the rear of the structure. The frequency notation is dropped for simplicity (7) {F} = F 1 F 2 F 3 F 4 where, {V} is the moment scaling vector. Therefore, = + M L f M L f M L r + M L r = M (8) {X} 4x1 = M [H] 4x4 {V} 4x1. Pre-multiplying both sides by {V} T, (9) {V} T 1x4 {X} 4x1 = M {V} T 1x4 [H] 4x4 {V} 4x1. Expanding and reorganizing the terms on the LHS, (10) (11) + 1 L f 1 L f 1 L r + 1 L r = M{V} LHS = {V} T 1x4 {X} 4x1 = X 1 L f X 2 L f X 3 L r + X 4 L r = X 1 X 2 L f X 3 X 4 L r. Expressing the angular deflections in terms of the relative displacements and by applying the small angle assumption, the rotation angles are: ( ) θ 1 = tan 1 X1 X 2 X 1 X 2 (12) L f L f ) (13) Therefore, the LHS of Eq. 9 becomes θ 2 = tan 1 ( X3 X 4 L r X 3 X 4 L r. (14) {V} T 1x4 {X} 4x1 = θ 1 θ 2 = θ. Organizing the equation into the final form by using the expression in Eq. 14 in Eq. 9, (15) (16) θ = M {V} T 1x4 [H] 4x4 {V} 4x1 ercf(ω i ) = θ M = {V}T 1x4 [H(ω i )] 4x4 {V} 4x1. It has to be noted that in the above equations, all forces, moments, displacements and rotations obey a right hand coordinate rule. Therefore, these terms must be entered into the equations using a common right hand coordinate system and should have positive/negative signs accordingly. 4. Experimental Validation A rectangular steel plate structure was fabricated and FE models of the plate were validated. The validated model was then used to predict the static torsional stiffness using ANSYS R. The measured FRFs from structural tests on the rectangular plate were used to estimate the static torsional stiffness by applying the enhanced rotational compliance function formulated in the previous section. The analytical predictions were compared with the estimates from the enhanced compliance function method (using both synthesized and measured FRFs). The steps involved in validating the enhanced rotational compliance function method and the results obtained are discussed in detail in the subsequent sections.

Figure 4: DOF map of the rectangular plate for torsional stiffness estimation Table 1. Rectangular plate DOF description for torsion configurations; L = 10 in for both the configurations DOF Label Distance Sl Description Left Right (in) 1 135 23 14 Front DOFs for Configurations 1 and 2 2 140 28 14 Rear DOFs for Configuration 1 (equal spacing) 3 156 12 18 Rear DOFs for Configuration 2 (unequal spacing) 4 145 1 20 Mass attachment DOFs for 1x and 2x mass cases 4.1. Analytical Method. The static torsional stiffness of the rectangular plate can be estimated analytically using the expression in Eq. 3. To analytically predict the torsional stiffness, four points on the validated plate model are chosen such that they are symmetric about the centerline. Two configurations were chosen for estimating the torsional stiffness as shown in Fig. 4. The geometric details of the selected configurations are given in Table 1. The rotation angle meets the small angle criteria, θ = 2Δ L f rad, where is the vertical deflection at the front DOF. The vertical deflection at the front DOFs is retrieved and the static torsional stiffness is estimated using Eq. 3, The static torsional stiffness was estimated to be 3549.33 lb f in/deg for the equal spacing configuration (Configuration 1) and 3619.11 lb f in/deg for the unequal spacing configuration (Configuration 2) (for the no added mass case). 4.2. Enhanced Rotational Compliance Function (ercf) Method. As mentioned in the previous section, two different plate configurations, one with equal front and rear spacing and another with larger rear spacing were considered. FRF data for three perturbed mass cases (no mass, 1x mass and 2x mass cases) for both the configurations was collected. The perturbed mass cases were utilized to acquire additional statistical data that refined the torsional stiffness estimates. It is evident from the ercf plots for both the configurations that adding perturbed masses shifted the peaks towards the low frequency region. However, the global characteristics (specifically static torsional stiffness) do not vary considerably. In order to utilize the ercf method to estimate the torsional stiffness, a dataset containing the driving-point and the cross-point FRFs should be measured at the locations where the torsional stiffness is to be estimated. Typically, the measured FRFs are accelerance FRFs ( ) ( A F obtained from an impact test from which compliance functions X ) F are derived. A stiffness estimation toolkit, in which the theoretical ercf equation (Eq. 16) was implemented, was developed using Matlab R. The FRF data was acquired in British units for the rectangular plate. Therefore, a calibration value of 386.088 for converting g s to in/s 2 was applied on the accelerance FRFs ( A F ). The distances between front and

rear DOFs should be measured and converted in the base displacement units (inch in this case). Synthesized FRFs (obtained from the validated FE model), as well as measured FRFs were used to generate the ercf for the plate torsion configurations. The ercfs obtained using synthesized FRFs are shown in Fig. 5. Individual data fit was performed for each of the perturbed mass case and the plots were overlaid for both plate configurations, as shown in Fig. 6. From the ercf plots obtained using measured FRFs, it is evident that the effects due to the rigid body modes modal response and the noise in the low frequency region are reduced when compared to the impedance modeling method. 5. Results and Discussions 5.1. Static Torsional Stiffness Estimation Sensitivity. The stiffness is estimated by fitting an appropriate model to the ercf. Typically, reasonable estimates should be obtained by fitting a SDOF model to the first peak of the enhanced compliance function and then extrapolating the value at 0 Hz. The accuracy of the stiffness estimates is dependent on carefully choosing the frequency range where the data is fit. It is required to identify the peak, minimum and maximum frequencies in the desired frequency range for the compliance/stiffness parameter estimation processes. If a poor choice of frequency bandwidth is made, it results in an increased variation in the stiffness estimates. It is recommended to choose the minimum frequency such that the corresponding phase is nominally 0 to avoid including rigid body response and measurement noise in the model. The maximum frequency should be chosen in the vicinity of the peak being fit. It was anticipated that by fitting more than just one peak (that predominantly contributes to the static stiffness), a better estimate of torsional stiffness can be achieved. The effect of fitting multiple peaks on the stiffness estimates using various MDOF models along with SDOF models (fitted with different frequency bandwidths) was studied for the ercf method. A residual flexibility constant was used in all the models. The peaks selected in the different models used to fit the ercf are listed in Table 2. Table 2. Details of the models selected to fit the ercf method for Configurations 1 and 2 Model SDOF I SDOF II MDOF I Peaks chosen First peak First peak Peaks one and two The peaks selected in the different models used to fit the ercf method for the rectangular plate are listed in Table 2. A residual flexibility constant to account for the effect of the high frequency peaks was used in all the models while fitting the ercfs for the rectangular plate Configurations 1 and 2 (including the perturbed mass cases). A value below the frequency of the first peak was chosen as the maximum frequency (f max ) for SDOF I model, the exact value varied depending on which mass perturbed case was being fit. Allowing the maximum frequency to include the region beyond the resonance should yield acceptable results as the data is usually not contaminated with measurement noise near the peaks. The effect of choosing a maximum frequency value beyond the first peak was studied with SDOF II model. The first two peaks were chosen in the MDOF I model. Example plots to illustrate the minimum and maximum frequencies and the peak (peaks) that was (were) fitted for SDOF I, SDOF II and MDOF I models are shown in Figs. 7, 8 and 9 and the torsional stiffness estimation sensitivity for the rectangular plate is presented in Tables 3, 4 and 5 respectively. Comparing the results obtained for these models, MDOF I model had the least variation in stiffness estimates in terms of the nominal (average) torsional stiffness value (±1.43%). It was also observed that selecting a maximum frequency value above the first peak (SDOF II model) did not significantly yield better results for the torsional stiffness values as compared to SDOF I model.

(a) Configuration 1 Equal spacing L f = L r = 14 in (b) Configuration 2 Unequal spacing L f = 14 in and L r = 18 in Figure 5: ercf for rectangular plate generated with synthesized FRFs (no added mass cases)

(a) Configuration 1 Equal spacing L f = L r = 14 in (b) Configuration 2 Unequal spacing L f = 14 in and L r = 18 in Figure 6: ercf for rectangular plate generated with measured FRFs

Figure 7: ercf method parameter estimation with SDOF I model; Configuration 1, no mass case Table 3. Parameter estimation sensitivity of ercf method with SDOF I model (for different f min values); Configuration 1: K Tref = 3549.00 lb f in/deg and Configuration 2: K Tref = 3619.11 lb f in/deg Static torsional stiffness K T (lb f in/deg) Config.# f max (Hz) 10 Hz-f max 15 Hz-f max 20 Hz-f max 25 Hz-f max 30 Hz-f max Average KT Std. Dev. σ σ/k T % 1 No mass 37.95 3609.40 3609.80 3663.20 3665.50 3666.90 3642.96 30.48 0.84 1x mass 35.74 3592.20 3682.30 3664.20 3639.00 3693.90 3654.32 40.44 1.11 2x mass 33.53 3595.70 3651.30 3640.90 3598.70 3419.70 3581.26 93.64 2.61 2 No mass 38.88 3743.30 3732.00 3697.10 3700.60 3674.90 3709.58 27.74 0.75 1x mass 34.85 3715.70 3701.40 3702.40 3685.00 3781.80 3717.26 37.69 1.01 2x mass 32.90 3731.70 3660.80 3640.10 3626.90 2955.70 3523.04 319.72 9.08

Figure 8: ercf method parameter estimation with SDOF II model; Configuration 1, no mass case Table 4. Parameter estimation sensitivity of ercf method with SDOF II model (for different f min values); Configuration 1: K Tref = 3549.00 lb f in/deg and Configuration 2: K Tref = 3619.11 lb f in/deg Static torsional stiffness K T (lb f in/deg) Config.# f max (Hz) 10 Hz-f max 15 Hz-f max 20 Hz-f max 25 Hz-f max 30 Hz-f max Average KT Std. Dev. σ σ/k T % 1 No mass 50.085 3589.50 3580.00 3461.80 3319.30 3123.10 3414.74 196.36 5.75 1x mass 50.085 3656.60 3596.00 3622.80 3699.00 3841.40 3683.16 96.48 2.62 2x mass 50.085 3627.80 3591.40 3562.20 3532.60 3448.20 3552.44 68.10 1.92 2 No mass 50.085 3643.50 3602.70 3582.90 3501.30 3399.40 3545.96 96.92 2.73 1x mass 50.085 3592.00 3539.30 3487.50 3428.10 3345.20 3478.42 96.14 2.76 2x mass 50.085 3509.80 3490.60 3412.70 3313.20 3282.00 3401.66 102.31 3.01

Figure 9: ercf method parameter estimation with MDOF I model; Configuration 1, no mass case, 2 peaks fitted Table 5. Parameter estimation sensitivity of ercf method with MDOF I model (for different f min values); Configuration 1: K Tref = 3549.00 lb f in/deg and Configuration 2: K Tref = 3619.11 lb f in/deg Static torsional stiffness K T (lb f in/deg) Config.# f max (Hz) 10 Hz-f max 15 Hz-f max 20 Hz-f max 25 Hz-f max 30 Hz-f max Average KT Std. Dev. σ σ/k T % 1 No mass 98.89 3611.00 3603.30 3565.90 3538.10 3519.40 3567.54 39.86 1.12 1x mass 98.89 3599.50 3553.50 3528.50 3500.20 3467.50 3529.84 50.45 1.43 2x mass 98.89 3675.50 3656.80 3639.30 3618.30 3587.70 3635.52 34.09 0.94 2 No mass 98.89 3640.80 3621.50 3610.40 3587.10 3569.70 3605.9 28.04 0.78 1x mass 98.89 3611.40 3589.40 3569.70 3551.20 3530.40 3570.42 31.67 0.89 2x mass 98.89 3558.20 3546.80 3519.00 3495.10 3500.10 3523.84 27.93 0.79 5.2. Comparison with results from the theoretical model. It is evident from the results for the static torsional stiffness that the theoretical error/uncertainty of the stiffness estimates from the ercf method is less than ±3 % of the nominal torsional stiffness value. 5.3. Issues with the ercf approach. The quality of the measured data is critical. As double impacts and overloads in high frequency region could severely affect this method, the force/response spectrum and overload indicators should be checked and monitored while acquiring the data. In addition, calibration of the impact hammer and sensors should be performed carefully as they directly affect the stiffness value estimates. 5.4. Structure with overhung segments. In the load case analyzed for the ercf formulation, the supports and moment application locations were at the vertices of the structure. Consider the structure shown in Fig. 10. Let this structure have the same characteristic length and front/rear spacing as the structure shown in Fig. 2(a) but with overhung structural segments (shaded in blue). In other words, the supports and load application locations at the interior of the structure.

Figure 10: Structure with overhung segments; overhung segments are shaded in blue When the overhung segments are significant, they would have a significant stiffening effect due to moments on the supports, which would render the stiffness estimates from the ercf method inaccurate. However, when there are negligible overhung segments, the torsional stiffness of the structure with overhung segments would be just slightly higher than the structure without overhung segments. The results of a study performed to analyze the effect of removing overhung segments on the torsional stiffness are presented in detail in Appendix A.1. 6. Conclusions The enhanced rotational compliance function (ercf) is a simple and efficient method to accurately estimate static torsional compliance/stiffness using measured frequency response functions (FRFs) from a structural system with free-free boundary conditions. The FRF measurements are obtained from impact testing at selected DOFs quickly with minimal instrumentation. These tests involve obtaining a 4x4 FRF matrix at four separate, symmetric locations on a structure for a particular torsional stiffness estimate. Traditional static torsional tests involve constraints applied to two of these four locations and static loads (moment) applied to the other two of these four locations. These traditional, static torsion tests require extensive instrumentation and a time-consuming procedure. These traditional tests are likely sensitive to the overhung structure problem discussed in this paper. The theoretical background for ercf was presented along with validations using both modeling and experimental cases involving a rectangular plate structure. The ercf method involves minimal instrumentation. Fitting a MDOF model had the least variation in stiffness estimates in terms of the nominal (average) torsional stiffness value (±1.43%). It was also observed that selecting a maximum frequency value above the first peak did not significantly yield better results for the torsional stiffness values as compared to an SDOF model in which the maximum frequency was selected below the first peak.

Appendix A.1. Effect of Removing Overhung Structural Segments on Torsional Stiffness The effect of removing overhung structural segments on torsional stiffness is analyzed in this section. With other parameters being held constant, when material is removed from a structure, it is expected that the stiffness of the structure would reduce. In the experimental validation of the ercf method in Sec. 4.2, two plate configurations with equal (L f = L r ) and unequal (L r > L f ) front and rear spacing were considered. In both these cases the force application locations are interior, resulting in outboard material (overhanging segments) in the front and the rear. The effect of removing the outboard material on the torsional stiffness value is an important factor to be considered while performing a torsion test in order to obtain an accurate estimate. This effect is studied for the following two configurations: (1) Symmetric loading cases (same amount of outboard material on both front and rear A = B = 12 in as shown in Fig. 11(a)) (2) Asymmetric loading cases (amount of outboard material on the front is more than in the rear A = 14 in and B = 10 in as shown in Fig. 11(b)) (a) Symmetric configuration (A = B) (b) Asymmetric configuration (A > B) Figure 11: Torsion load cases with overhung segments For both the configurations, the initial case (maximum outboard material case, a = 0 in) was considered as the reference. The effect of cutting outboard material on torsional stiffness for both the configurations is compared in Fig. 12. It is evident that in the presence of outboard material, the torsional stiffness estimates are consistently higher compared to the case in which there is no outboard material. When 50% of the front outboard material was removed, there was a 3.78% reduction in the torsional stiffness value for the asymmetric configuration and a 5.21% reduction in the torsional stiffness values for the symmetric configuration. It was emphasized that the torsional stiffness value depends on the characteristic length, as a result it is an important factor while comparing the torsional stiffness values of two similar structures. It can be concluded from this study that it is also important to consider the effects due to outboard material when comparing the torsional stiffness results of two structures with the same characteristic length and front/rear spacing but different support locations.

Figure 12: Effect of removing outboard material on torsional stiffness References [1] R.J. Allemang and A.W. Phillips, Static Stiffness from Dynamic Measurements, 2012 [2] R.J. Allemang and A.W. Phillips, Alternative Methods for Determining Painted Body Stiffness, 2012 [3] H.G. Pasha, Estimation of Static Stiffnesses from Free Boundary Dynamic (FRF) Measurements, Ph.D. Dissertation, University of Cincinnati, 2014 [4] D. Griffiths, A. Aubert, E.R. Green and J. Ding, A Technique for Relating Vehicle Structural Modes to Stiffness as Determined in Static Determinate Tests, SAE Technical Paper Series (2003-01-1716) [5] J. Deleener, P. Mas, L. Cremers and J. Poland, Extraction of Static Car Body Stiffness from Dynamic Measurements, SAE Technical Paper Series, (2010-01-0228) [6] B. Rediers, B. Yang and V. Juneja, Static and Dynamic Stiffness One test, both results, Proceedings of the International Modal Analysis Conference (IMAC) XVI A Conference and Exposition on Structural Dynamics, 1998 [7] F.P. Beer and E.R. Johnston Jr., Vector Mechanics for Engineers: Statics and Dynamics, McGraw Hill Publishing Company, USA, pp 314, 2004 [8] F.P. Beer, E.R. Johnston Jr., J. DeWolf and D. Mazurek, Mechanics of Materials, McGraw Hill Publishing Company, USA, 2011