Elsevier

Engineering Structures

Volume 153, 15 December 2017, Pages 180-190
Engineering Structures

An experimental investigation on dynamic response of composite panels subjected to hydroelastic impact loading at constant velocities

https://doi.org/10.1016/j.engstruct.2017.10.029Get rights and content

Highlights

  • Dynamic structural response of composite structures under slamming impact.

  • Experimental investigation on the slamming impact water of composite wedge.

  • Damage progressive in composites under slamming impact is presented.

  • Assessment to meet the specific requirements of marine vessels.

Abstract

Generally, when marine vessels encounter the water surface on entry and subsequently re-enter the water at high speed, this can subject the bottom section of the vessels to high hydrodynamic loads, especially over very short durations. This phenomenon generates high hydrodynamic loads, which can cause a catastrophic failure in the structure. In contrast, the interaction between deformable structures and free water surface can be modified the fluid flow and changed the estimated hydrodynamic loads comparing with rigid body, due to appearance of hydroelastic effects. These effects are considered active challenge areas in structural ship design. This work presents an experimental study of the water impact for composite laminate wedge at different constant entry velocities. The aim of this study is to investigate the dynamic structural response of panels and predicts the hydrodynamic loads to meet the specific requirements of marine vessels. In order to better describe hydroelastic influence, two composite panels with different thicknesses namely 8 mm and 13 mm are subjected under constant impact velocities of 4, 6 and 8 m/s with the deadrise angle of 10°. The obtained experimental results were indicated that more flexible panels had a higher peak force and significant dynamic noise compared with higher stiffness panels. In addition, the maximum deformation occurred in the centre and close to the chine edge of the panel due to changes in local velocity and local deadrise angle. For this reason, special attention requires in both design phase and operation phase.

Graphical abstract

Figure: Composite panels under slamming impact: panels and Servo-test machine.

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Introduction

Water entry problem is the impact loads between the free water surface and structure which is considered one of critical design issues in ship structures. Therefore, it is recommended to determine the global and local loads. The main early efforts in this area are intensified analytical works in the case of the rigid structure to predicate hydrodynamic forces [1], [2], [3].

There are several experimental attempts which have studied the slamming phenomenon to further the understanding of influence of the hydrodynamic loads against ship structures. However, these attempts have concentrated on rigid bodies and most of these have been achieved using drop test impact.

Chuang [4] performed a series of the experiment tests for a wedge-shaped steel body with deadrise angles less than 15° to determine the effect of trapped air. He showed that the trapped air has important influence on the slamming phenomenon with deadrise angles β (0° < β ≤ 15°). Wu et al. [5] analysed the hydrodynamic problem numerically and experimentally for symmetric wedges with different weights, entering velocities and deadrise angles by means of the drop test. They noted that the comparisons between the numerical and experimental results were in good agreement, and that the divergence became increasingly apparent for the deadrise angle less than 45°. They explicated this discrepancy by the air cushion, which leads to a change in the acceleration before and after the wedge enters the water. Tveitnes et al. [6] studied the water entry problem for wedge-shaped section with constant velocity to further understanding of planning and slamming of marine vessels. El Malki et al. [7], [8] performed experimental studies for both axisymmetric and pyramidal rigid bodies under constant impact velocity up to 20 m/s using a high-speed shock machine.

The main difference in the hydrodynamic loads between the rigid and deformable structures is the presence of the hydroelastic influence along the fluid-structure interface. This explains why the hull flexibility has significant effects on the design of these structures, which can change the behaviour of the fluid-structure interaction. Moreover, a deadrise angle between the water and the structure is considered to be an important factor to presence this phenomenon, especially small deadrise angles [9], [10]. The hydroelastic effects exert of both dynamic and kinematic influences. The dynamic effects occur due to the interaction between the water and the structure, while the kinematic effects are produced due to the inertia effect and the change in the local deadrise angle along fluid-structure interface [11].

The fast development of composite materials in the last decade has encouraged the use of these materials in naval structures, due to their lightweight, high strength and stiffness to density ratios. For this reason, many researchers have studied their behaviour to ascertain the performance and reliability over the life time. Consequently, this assists ship designers to estimate and specify hydroelastic effects and damage mechanism before suggesting a final design load.

Huera-Huarte et al. [12] experimentally investigated the water entry problem on flat sandwich panels at impact velocities over than 5 m/s. They reported that the loads predicted by the asymptotic theory were in good estimation with the experiment data for deadrise angles greater than 5°. For rigid body with small angles, the cushioning effect produced by trapped air appearance in the initial impact duration. Panciroli et al. [13], [14] investigated the water impact problem for the deformable wedge, using experimental and numerical approaches. Their results showed that under different boundary conditions, the hydroelastic influence depended highly on the ratio (R) between the first natural frequency of the structure and the wetting time. Therefore, they used a variant of the panel’s stiffness and cantilever boundary condition to obtain a large deformation which made the hydroelastic effects easy to measure and investigate. From these results, they supposed that the hydroelastic influence was important for values of R less than 1, and that the same hydrodynamic pressure was produced when comparing with rigid bodies.

The centre for Advanced Composite Materials, of the University of Auckland [15], [16], [17], [18] conducted experiments in order to investigate the hydroelastic effects due to fluid-structure interaction of the flexible composite panels in high speed marine craft. They characterized the variations in the pressure and the response of these panels using the servo-hydraulic slam testing system (SSTS) for over 6 m/s impact velocity and deadrise angle of 10°. They observed that panel flexibility has an influence on the total force, and that the lower stiffness panels have a high peak force. They attributed that the variation of the acting force due to the change in local velocity and local deadrise angle along the fluid-structure interface.

In the present work, experimental investigation of the flexible laminate (panels with different stiffness) namely semi-flexible (SF) and flexible (F) and different velocities of impact were implemented to characterise the response of structures subjected to slamming impacts. This was done by using the high velocity shock machine which is capable of keeping approximately constant velocity through impact duration. The panel deformations and the hydrodynamic forces are investigated as indicators to describe the hydroelastic effect during water-structure interaction.

Section snippets

Servo-test machine and instruments

A high speed shock machine with velocity control system was used to calibrate and maintain the velocity approximately constant throughout the water impact, as illustrated in Fig. 1. The performance specifications of the machine were distinct from other traditional machines, since could apply more than 100 kN (200 kN) and achieve velocities of up to 20 m/s (10 m/s). The fixture system was made of steel 355S with weight 58.5 kg, which was attached to the hydraulic piston, while total weight of

Calibration parameters of shock machine

To prove the validity of the experimental results, a procedure was followed to calibrate the machine performance to ensure that robustness of the test was satisfactory in terms of stability and approximately constant velocity. Moreover, take into consideration the inertial force caused by the panel and system fixture mass. All data were manipulated (post-processing) to convert them to respective units from the voltage unit using the Matlab software. To show how the data obtained was interpreted

Structural response and hydroelastic effects

The structural response was analysed with regard to hydrodynamic force acting on the panel. A hydrodynamic force in the end of the panel edges has regarded as significant source of local damage in composite panels due to normal and transverse shear stress. Hence, it was generally considered to be a critical region in the marine structure design.

In order to investigate how the hydroelastic influence depends on the impact velocity, bending stiffness, a nondimensional parameter was used to frame

Hydrodynamic force

Fig. 13a shows the evolutions of the hydrostatic force for different impact velocities in the case of flexible (t = 8 mm) and semi-flexible (t = 13 mm) panels. It can be seen that the flexibility of the tested panels has a great influence on the profile and the amplitude of the maximum hydrodynamic load. At the same impact velocities, the more flexible panels have higher peak force and a clear imprint on the structural oscillations compared with more stiffness panels along the impact direction,

Conclusions

This paper discusses the experimental investigation of water entry problem for composite laminate. Different velocities and panels rigidities were tested to identify the hydroelastic influence and to better understanding the dynamic response behaviour of composite material wedges. Therefore, flexible and semi-flexible composite panels have tested (effect of structural stiffness) at different impact velocities to identify the hydroelastic influence. The structural response including the

Acknowledgement

This work has been financially supported by ministry of higher education and scientific research of Iraq, university of technology, Baghdad, and ENSTA Bretagne, France. The authors want to thanks their financial support.

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