Factors influencing impact force profile and measurement accuracy in drop weight impact tests

The impact behavior and dynamic response of RC beams are critical in determining their overall performance. For a long time, drop weight impact tests have been widely used to investigate the dynamic responses of different specimens. These tests involve impacting the specimens with a drop weight falling from a predetermined distance and at the desired velocity to measure the reaction and dynamic forces and midspan displacement that are of great significance in the dynamic response analysis. However, different researchers use different test setups and configurations that potentially result in inconsistent test observations, thus triggering the need to understand various factors influencing such tests.

Different factors reportedly affect the drop weight impact tests. The geometry of the drop head, its inclination angle, and the impact interlayer combinedly affect the beam failure modes, impact duration, and reaction and impact forces. Similarly, other factors like impact force measurement methods exhibit considerable influence on the measurement accuracy and overall test results. This necessitates the need for an effective approach for performing drop weight tests and analyzing resulting data to improve the feasibility and accuracy of the obtained results. Additionally, accurate measurement is essential in determining the capacity of the beams and ensuring accurate numerical modeling calibration.

While extensive research has been conducted to understand the influence of various test configurations on the accuracy of force measurements and results, factors influencing the impact force profiles of RC beams are sparsely reported. Consequently, most research on RC beams concentrates on the effects of the drop weight mass on the impact peak force rather than on the whole force profile. To this end, correlating the impact force profiles with respective impacting conditions for impulse quantification and determining the dynamic responses is necessary.

Recently, Dr. Huawei Li, Professor Wensu Chen and Professor Hong Hao from Curtin University in Australia developed a new numerical method for determining the influence of different test setups and configurations on the accuracy of the measured impact forces and the impact force profile of RC beams under drop-weight impact. In their approach, analytical derivations were carried out to determine the actual impact force on the structures. The mass distributions of drop weight on the impact forces were examined to assess the results accuracy and structural performance of the beam. Also, simulation experiments were conducted to establish the relationship between the impact profile forces and the mass ratio of drop weight to beam. Their work is currently published in the journal, International Journal of Impact Engineering.

The authors found out that the drop weight mass distribution significantly influenced the measured impact forces when the load cell was embedded into the drop weight. The impact forces measured by the load cells agreed well with the actual contact forces for the mass ratio of weight to drop head (α_d) higher than 20, and deviated by more than 10% when the value of α_d was less than 20. Moreover, different impact force profiles were observed when the load cell was mounted between the drop weight and beam, leading to an increase in the stiffness and higher impact force peaks. Type I impact force profiles were associated with a mass ratio of 0.25, type II with mass ratios 0.5, 0.75 and 1.0, while type III was 2.0, 3.0 and 4.0.

In summary, the study reported an investigation of different impact force measurement methods and their corresponding effects on the measurement accuracy in drop weight impact tests of RC beams. The drop weight mass ratio was identified as one of the key factors influencing the relative velocity between the beam and drop weight, which further influenced the impact force profiles. In a statement to Advances in Engineering, the authors explained the study will lead to future improvement of the accuracy and reliability of drop weight impact tests used in studying the behaviors of RC beams.