The amount prediction of concrete fragments after impact using smoothed particle hydrodynamics

Highlights

• A model was developed for prediction of amount of fragmentation of concrete median barrier after impact loading using SPH (Smoothed Particle Hydrodynamics).

• The amount of fragmentations can change depending on different velocity-to-mass at a fixed local impact energy.

• Using the results of the analysis, MRA (Multiple Linear Regression Analysis) was conducted.

• The MRA showed a rather low correlation coefficient compared with the SPH analysis results.

Abstract

Concrete median barriers on highways are typical road safety facility that requires predicting correctly the amount of fragments generated during a vehicle collision. The fragmented pieces from the median barrier can cause secondary accidents to a vehicle coming from the opposite lane. Therefore, predicting the amount of fragments depending on the impact severity is important to prevent any secondary accident.

Many researchers have studied to predict the damaged area and strain of concrete. Such predictions of concrete structural behavior following impact loads mostly used FEM. However, FEM has a limitation in predicting the fragmentation amount since it simulates fragmentation through element deletion. As an alternative, Smooth Particle Hydrodynamics (SPH) can be used for predicting the amount of fragments or the motion of fragments since these are not affected by the mesh.

In the present study, impact analysis was performed to predict the amount of concrete fragments due to vehicle collision. The obtained results of SPH analysis showed that the amount of fragments can change depending on different velocity-to-mass ratios at a fixed local impact energy. Using the results of the SPH analysis, multiple regression analysis (MRA) was conducted further. The MRA showed a rather low coefficient of determination (R²) compared with the SPH analysis results. Therefore, as a future study, with the expectation of improvement, a method such as ANN (Artificial Neural Network) that can predict the amount of fragments including uncertainty is necessary.

Introduction

Concrete median barriers (CMB) are considered as one of the important highway facilities as they prevent trespassing and run-over by a vehicle coming from the opposite direction. Therefore, strength of a CMB is an important parameter to be evaluated against vehicle collisions. According to Lee et al. [1], 310 accidents occurred between 2001 and 2005 which were related to broken pieces from shade net due to collision, and this number accounted for more than 58% of total shade net accidents. Besides these, a number of run-over accidents and secondary accidents also occurred from damaged fenders installed on top surface of 810 mm CMBs.

Therefore, to increase the height of CMBs up to 1,270 mm, new designs were proposed and developed after field tests. In S. Korea, the CMBs present are similar to New Jersey CMBs. At present, most of the CMBs in S. Korea have the height of 1,270 mm. From field tests on 1,270 mm CMBs, three-step impact stages were repeatedly observed due to increased height, and most of the broken fragments formed were from second impact [1], [2]. Fig. 1 shows an example of damaged CMB after 2nd impact.

To note, the upper part of a CMB is the weakest. The second impact in Fig. 1 was due to collision of the lower corner zone of a steel cargo compartment, the stiffest part of the vehicle, with the upper zone of the CMB. If a CMB is lower than 1,200 mm, the corner zone of the steel cargo of truck cannot contact the upper part of the CMB since the lower corner zone of the steel cargo is normally at a height about 1,200 mm. From this context of fragmentation, a short height CMB would therefore be safer. However, considering anti-glare function, 1,270 mm height is significant.

If there is no opposite direction traffic across the barrier, the broken fragments are not any safety issue. However, with opposite direction traffic, highway safety is greatly threatened by CMB fragments. In S. Korea, in the past, a severe accident occurred in which broken CMB fragments flew and hit the windshield of a running sedan on the road in the opposite direction, causing two causalities. This accident shows the importance of prediction of CMB fragmentation under impact load.

Several collision models to predict fragments were then developed and reported as found in Lee et al. [1], Kim et al. [2], Kim et al. [3], and Lee et al. [4]. Fragments were predicted under different impact severity such as SB5-B, SB6 and SB5-B(20A) using finite element (FE) method along with field tests [2]. The PHD (Post-impact head deceleration) and THIV (Theoretical head impact velocity) values obtained from the FE model matched well with the field test data and the behavior of colliding vehicle can be well predicted. The amount of fragments could also be satisfactorily predetermined from Kim et al. [2]. However, the 2nd impact could not be simulated using the FE model. The model was developed and calibrated based on the previous study [4], [2]. It was found that the obtained results were strongly dependent on some selected values of the key parameter, such as “repow” and “erode” values, as mentioned in Kim [5]. Especially, ‘‘erode’’ controls removal of damaged elements after impact, and fragments were estimated from the amount of removed elements due to severe damages. The elements were eroded when the concrete damage exceeded 0.99 and the principal strain of the concrete element exceeded 0.2 when, for example, the erode value was set to 1.2. Typically, the erode value is a key factor to control the results of simulation. However, it remains as an unknown factor until field test is carried and calibration of the developed model is done. Therefore, many researchers use various erodes to simulate damaged concrete in FE analysis [6], [7], [8].

In the present study, alternative method Smoothed Particle Hydrodynamics (SPH) model was developed. The SPH model is one of methods for continuum media. It was originally developed for astrophysics and started to be used in the field of fluid dynamics extensively [9]. In solid mechanics, Libersky and Petschek [10] pioneered the basic concept based on elasticity model, while Rabczuk and Eibl [11] applied the concept of SPH to the prediction of concrete fragments caused by impact load. In the present study, SPH model using LS-DYNA was developed to predict concrete fragments. The developed SPH model was first verified with pre-existing test data and then used to predict the number of fragments from CMBs. Total 486 cases were obtained using the developed SPH model and multi-regression analysis were conducted on the SPH analysis results to develop predictive equation for the number of concrete fragments from CMBs under impact load. Finally, field test was performed under SB4, SB5-B and SB6 impact severity and the results were compared with the SPH analysis ones for validity check [5], [12], [13].