ReviewStudy of impact on helicopter blade
Highlights
► Steel ball impact on a helicopter blade is discussed experimentally and numerically. ► Blade damage kinematics is defined from tests performed at different energy levels. ► An original finite elements model is defined with a change of scale. ► The numerical model is validated by comparison with experimental results.
Introduction
To reduce certification and development costs, computational methods are required by the aeronautical industry that are able to predict the structural integrity of composite structures under impact from various bodies, such as birds, hailstones, small metal parts and other soft or hard objects.
The complexity of impact problems is increased when composite materials are involved, due to the distinct failure mode that may occur. The comprehensive review by Abrate [1], [2], [3] discusses impact failure mechanisms and summarises impact modelling approaches, based mainly on analytical models.
Recent advances in understanding the damage mechanisms of laminated composite [4], [5], [6], [7], [8], [9] coupled with development of the performance and capability of computers, offer the possibility of avoiding many experimental tests by using impact simulation. However, numerical results should be examined cautiously. They must, in addition, be validated by experimental tests. A key point in this research is the development of models that are usable in an industrial structure modelling.
The objective of this study is to propose a methodology of calculation that can be used by industry for the analysis of a rigid body impact on complex composite structures such as helicopter blades. Very few impact studies have been conducted on complex composite structures. They deal generally with soft body impact, such as gelatine, hail or ice [10], [11], [12].
In this study, impact tests on structures similar to the blade sections are performed. They are carried out using a steel ball and for a wide range of speed. These tests allow the identification of the kinematic mechanisms of degradation. Following this, a model is proposed which represents the observed experimental phenomena. It relies on a multi scale approach. Some models of realistic damage that can be used on industrial structures are proposed. The numerical model is validated by comparison with experimental results.
Section snippets
Experimental test
As shown on the schema, the studied specimens are composed of a main spar with unidirectional glass–epoxy, a skin, a rib with hybrid composite glass–epoxy and carbon–epoxy, a polyurethane foam core, and a protective stainless steel that covers the front edge (see Fig. 1). The length of the specimen was 378.2 mm, and with a maximum thickness of 22 mm.
The tests were conducted using a gas gun. The projectile used was a steel ball with a diameter of 30 mm and a mass of 110 g. This steel ball was
Numerical modelling
An F.E. model of the impact on the blade’s structure was developed. It must represent the physical phenomena observed in experimental tests. One difficulty results in the compromise between mesh sizing and convergence/calculation time. This can be very limiting for the modelling of the complete blade. The complexity of the phenomena that must be modelled and the scales distinct to which the phenomena of damage appear, force the use of very fine meshing to obtain an acceptable modelling, which
Results and discussion
The analysis of results obtained from the model allows us to understand the kinematics of the impact. Fig. 9 shows the behaviour of the blade section during the impact for an impact energy of 550 J. The penetration of the front edge at the impact point is on the curve as a function of time.
The image at t = 0.06 ms shows the generation of the wave on the front edge caused by the contact with the ball. The amplitude of this wave increased and the wave propagated in the direction of the impact and
Conclusion
Many experimental tests were carried out to further understand the phenomena concerned during and after a frontal impact to a blade. From the observations, the mechanism of damage could be defined. Firstly, the front edge is the first element that undergoes damage, and then the rise of the impact energy level causes a plastic deformation of the stainless steel protection, which leads to the delamination skin–foam of the structure. Moreover, the penetration of the projectile damages the roving
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