Multiple Reference Impact Test Research Articles

Frequency response characteristics and failure model of single-layered thin plate rock mass under dynamic loading

In underground engineering, disturbance of dynamic load can change layered rock mass stress state and induce accidents. Traditional elastic mechanics can’t effectively solve the complex deformation problem. However, Hamiltonian mechanics system can overcome this problem. Dual variables are introduced in symplectic space to solve the deflection equations of single-layered thin plate rock mass. Comparing vibration parameters, it’s found the 1st, 5th and 6th order are effective vibration modes. The resonance characteristics of thin plate are obtained with three dynamic loads. It’s found the thin plate is most likely to resonate and damage due to the smallest resonance frequency interval and the largest vibration amplitude by impact wave and rectangular wave respectively. Then, the vibration mode of multi-layered rock mass is analyzed through Multiple Reference Impact Testing. The failure of fine sandstone is caused by the resonance of effective vibration modes by hammer excitation. Finally, the failure mechanism of thin plate is obtained by the failure theory and LS-DYNA. It’s found the four sides and corners suffer tensile shear failure and shear failure respectively. When tensile failure occurs in central, the main crack and secondary crack propagate along long axis and short axis to form “O-十” failure mode.

Experimental investigation of a method for diagnosing wall thinning in an artificially thinned carbon steel elbow based on changes in modal characteristics

Curved cylindrical structures such as elbows have a non-uniform thickness distribution due to their fabrication process, and as a result have a number of complex mode shapes, including circumferential and axial nodal patterns. In nuclear power plants, material degradation is induced in pipes by flow accelerated erosion and corrosion, causing the wall thickness of carbon steel elbows to gradually thin. The corresponding frequencies of each mode shape vary according to the wall thinning state. Therefore, the thinning state can be estimated by monitoring the varying modal characteristics of the elbow. This study investigated the varying modal characteristics of artificially thinned carbon steel elbows for each thinning state using numerical simulation and experimental methods (MRIT, Multiple Reference Impact Test). The natural frequencies of specified mode shapes were extracted, and results confirmed they linearly decreased with increasing thinning. In addition, by comparing single FRF (Frequency Response Function) data with the results of MRIT, a concise and cost effective thinning estimation method was suggested.

Natural characteristics analysis and experimental test of spiral bevel gear pair in the tractor transmission system

Spiral bevel gear is widely used in various mechanical transmission systems, such as tractor transmission system. Because it is mainly used in the heavy-load conditions, it would most likely resonate within the rated speed, resulting in tooth fatigue damage. In this paper, based on the principle of meshing and gear tooth machining, the spherically involute tooth profile equation of spiral bevel gear is deduced and the precise modeling method based on the CATIA is studied. The natural frequency and modal shape under free vibration are obtained by the finite element method (FEM), the influence of web thickness and web hole on the natural frequency of driven gear plate is analyzed as well. In addition, the experimental modal of bevel gear pair is carried out based on a multiple-reference impact test, Modal Assurance Criterion (MAC) is calculated, the three-dimensional modeling accuracy and the finite element analysis reliability are verified. The results show that the error between the measured frequency of bevel gear pair and the calculated frequency of the finite element simulation are both within 5%, and the MAC is above 0.8. The fourth-order natural frequency is the most sensitive to the web thickness, the second-order natural frequency is the most sensitive to the web hole.

Simultaneously identifying displacement and strain flexibility using long-gauge fiber optic sensors

Abstract The method of simultaneously identifying displacement and strain flexibility matrices of the structure is proposed by performing the multiple reference impact test and using long-gauge (LG) fiber optic sensors. This method has the following promising features: (1) The impact test is performed in which not only structure responses but also the input force are measured, which ensures the consistency between the identified Frequency Response Function (FRF) and the analytical one, thus it has the advantage of identifying structural flexibility. In contrast, the ambient vibration test cannot output FRF magnitudes. (2) The long-gauge fiber optic sensors are used to measure structure strain responses, which has the merit of reflecting both the macro and micro characteristics of a structure. Both strain and displacement flexibility are identified simultaneously by establishing the relation between LG strain and displacement through an improved conjugate beam method. (3) The subspace identification method in the time domain is developed for structural flexibility identification. It has the merit to deal with the test data with multi-reference multi-peak impacting forces which can better excite the dynamic characteristics of the structure, while traditional frequency domain methods generally are difficult to do that due to the leakage problem. Numerical and experimental examples have been studied to verify the validity and demonstrate the promising features of the proposed method.

Flexibility identification and deflection prediction of a three-span concrete box girder bridge using impacting test data

Abstract Deflection is an important parameter for condition assessment of in-service bridges. However, accurately measuring bridge deflections especially that of long span bridges is still a challenging problem. In this article, field tests including multiple reference impact test (MRIT) and static test were performed on a three-span post-tensioned concrete box girder bridge to study structural deflections. Firstly, a new impacting device, which can generate high-magnitude and wide-frequency-band forces, has been developed and employed to execute the MRIT on the bridge. The output data from MRIT was utilized to estimate structural Frequency Response Function (FRF) and to identify deep-level structural parameters including modal scaling factor and structural flexibility, from which structural deflections under arbitrary loads were predicted. Field test results have shown that the developed impacting device can produce FRF with much better reciprocity than the FRF estimated from traditional sledge hammer, which demonstrates the superiority of the developed impacting device for bridge excitation and deflection prediction. Secondly, static test was also conducted on the bridge. The developed long gauge Fiber Bragg Grating (LG-FBG) strain sensors were mounted on the bottom of the box girder bridge. The measured long-gauge strains have been adopted to calculate structural deflections through a modified conjugate beam method which has high precision and there is no error accumulation from other span. Finally, data processing strategy was carried out according to the procedure of structural flexibility identification from MRIT data, and the identified flexibility matrix has been used to predict structural deflections by multiplying a static force vector corresponding to the tested truck load. The well agreement between the predicted deflections and the calculated deflections indicates that the proposed methods can be utilized to obtain structural deflections.

Structural Identification by Processing Dynamic Macro Strain Measurements

In contrast to traditional modal identification methods processing structural accelerations for modal parameter identification, a multiple reference impact testing method using long-gauge fiber optical sensors and the related data processing approach are proposed. First, macro strains of key structural elements during the impact testing are measured through long-gauge fiber optic sensors, which subsequently are used to estimate the strain frequency response functions (FRFs) of the structure. Second, an approach is proposed to estimate strain mode shapes from the strain FRFs, and corresponding displacement mode shapes are calculated by applying an improved conjugate beam method. Finally, the strain flexibility character of the structure is identified by using the identified strain and displacement mode shapes with normalization. The advantage of long-gauge fiber optic sensors measuring averaged strains in a long length (e.g., 1 meter) provides the opportunity for strain modal analysis. The output of the proposed method, strain flexibility, is meaningful for static strain prediction and structural capacity evaluation. Examples investigated successfully verified the effectiveness of the proposed method for structural strain flexibility identification.

Multiple Reference Impact Testing for Bridge Assessment with Drop Hammer

A new drop hammer was designed and used for multiple reference impact testing of two bridges. The drop hammer utilizes a multiple-rebound control system for successfully applying a single impact with repeatable high-level force for bridge testing. Signal analysis results indicate that the drop hammer provides a higher signal-noise ratio and a better coherence even when the response signals are subject to pollution due to traffic on the bridge. Modal flexibility was computed by using the impact data from the drop hammer, and meaningful deflected shapes could be generated demonstrating the potential of the envisioned structural assessment system.

Structural Identification of an Old Bridge by Multiple Reference Impact Test Method

Experimental data was collected for structural identification (St-Id) of an old bridge: (1) static displacement and strain measurements taken under proof-load level, and; (2) multi-reference impact test (MRIT) data from one of the spans of a three span, cast-in-place reinforced concrete (RC) T-beam Bridge. MRIT was used to generate the modal data for computation of modal flexibility and displacement profiles. The St-Id procedure used during this application was designed to mitigate epistemic uncertainty in the data interpretation process. Successful results from MRIT demonstrated the reliability of applications for bridge condition assessment based on impact testing.

Condition assessment of steel stringer highway bridges using experimental modal analysis

An experimental modal testing procedure has been developed for assessing the condition of steel stringer highway bridges using an optimized rapid impacting testing procedure. A 30 m to 90 m long two lane bridge can be instrumented and tested in a 5- to 6-h period using a multiple reference impact testing procedure. A special spatial domain multiple reference parameter estimation algorithm is used to obtain a modal model from the measured multiple reference impact data. The modal model is used to predict the static response of the bridge to various loading conditions. This predicted flexibility of the bridge is a good indication of the state of the bridge and can be use to assess the bridge. In the development phase of this procedure, the reliability of the resulting flexibility was verified by comparing measured truckload displacements against simulated truckload displacements, which were computed by virtually loading the modal flexibility matrix with a truckload. Furthermore, induced damage scenarios revealed that modal flexibility was an excellent objective, kernel index for both global and local condition assessment. The modal model is also used in the calibration of a nominal 3-D–FE model which is used in evaluating the rated load capacity of the bridge.