Advances in crash dynamics for aircraft safety
Introduction
Crashworthiness is the ability of an aircraft structure and its internal systems to protect occupants from injury in an event of crash. Specifically, it means that the integrity of the passenger cabin should be maintained, that the accelerations the passengers are subjected to should be survivable in case of a crash, and that fires should be prevented.
Human tolerance to impacts has been studied for many years and a number criteria have been developed to predict injury [1,2]. A well-known criterion is the head injury criterion (HIC):where a is the linear acceleration of the center of mass of the head expressed as a multiple of the gravitational acceleration , t is the time in seconds, and is the time interval. The HIC is the maximum value of this moving average during the duration of the impact and is limited to 1000 when or to 700 when . This criterion predicts brain injury while accounting only for the linear acceleration when it is known that angular accelerations have a significant effect. One of many criteria for mild traumatic brain injury (MTBI) is based on P, the power of the loads applied to the head. where m is the mass, is the linear acceleration vector, is the linear velocity vector, is the inertia tensor, is the angular acceleration and is the angular velocity. The Head Impact Power (HIP) injury criterion introduced by Newman can be written as:where m is the mass of the head and and are its principal moments of inertia. In Ref. [3], m = 4.2 kg, kg.m2, kg.m2, and kg.m2. Slightly different values are used in other publications [4].
Eiband [5] summarized previously available test results dealing with the effect of acceleration on the whole body. A typical chart for the effect of acceleration in the vertical direction on a seated occupant (Fig. 1) shows that for durations under 0.03 s, no injuries were reported for uniform accelerations below 18 G.
To prevent or limit injury to the spine, CS Part 25.562c [7] stipulates that the lumbar load should not exceed 1500 pounds.
The Dynamic Response Index (DRI) developed Air Force Wright Laboratory to estimate the probability of compression fractures in the lower spine due to acceleration in a pelvis-to-head direction [8,9]. Fig. 2 shows a single degree of freedom model of a seated human with a mass m connected to a seat by spring k and dashpot c.
The relative motion , the difference between x, the displacement of the body and y, the displacement of the seat, is governed by:
Given the input acceleration , one can determine the maximum deflection and calculate the Dynamic Response Index:where g is the acceleration of gravity. The DRI is used to evaluate the potential for spinal injury during aircraft crashes [[10], [11], [12], [13]] and also in the operation of high speed marine craft [14]. In this model, the natural frequency and the damping ratio are taken to be 52.9 rad/s and 0.224 respectively. As the maximum DRI value increases from 20 to 23, the spinal injury rate increases from 16 to 50%.
A typical section of the fuselage for a transport aircraft include frames, stringers and skin, the passenger floor, the cargo floor and struts or stanchions. During a crash landing, the kinetic energy should be dissipated in a controlled way by the crushing of the structure below the passenger floor. Energy is dissipated by stanchions connecting the frames and beams on the cargo floor (Fig. 3), by longitudinal sine wave beams below the subfloor beams (Fig. 4) or sine wave beams below cargo floor beams (Fig. 5).
In another design (Fig. 6), cargo floor beams are connected to the frames while the bottom frame of the fuselage is replaced by a splice plate [18]. A special honeycomb shaped material is inserted between the cargo floor and the clip plate for dissipating energy during a crash (see Fig. 7).
The height of the sub-cargo structure in helicopters and light fixed-wing aircraft is typically in the range of 200 mm [19]. For wide-body transport aircrafts category provides significantly more distance between the cargo floor and the ground is of the order of 600 mm [19]. Assuming a constant deceleration a, the crushing distance is given by . If the initial velocity v is 30 ft/s and the acceleration is 20 times the acceleration of gravity, the crushing distance is 0.7 ft or 213 mm. The necessary space is not available in helicopters and small aircraft. In that case the landing gear and the seats must dissipate a significant part of the kinetic energy.
In the following, Section 2 describes previous research dealing with structural elements such as frames, struts and sine wave beams. It also presents an overview of tests of full size aircraft and fuselage sections, the facilities where these tests are conducted and the software used for the simulation of these tests. Section 3 reports how the aircraft safety agencies advice the compliance with the structural requirement. Section 4 discusses the use of composite materials in aircraft structures and the mathematical models of the behavior of these materials. Sections 5 Evolution of stanchion as an absorber mechanism in the cargo subfloor, 6 Sub-cargo demonstrator test, 7 Finite element evaluation discusses the design and analysis of a particular aircraft to resist crash landing.
Section snippets
Frames
Fuselage frames provide a significant contribution to the resistance to crash landings and dissipate a large portion of the kinetic energy of the aircraft. A number of experimental and analytical studies consider composite frames with various cross-sectional shapes. Collins and Johnson [20] determined the natural frequencies and mode shapes of semi-circular frames with I shape or channel sections, Fig. 8. A special beam finite element was developed and predictions are in very good agreement
Compliance to the crashworthiness requirements
Airplane crashworthiness is dominated by the impact response characteristics of the fuselage. No specific rule addresses means of compliance for the impact response characteristics for what could be considered survivable crash conditions. Therefore, special conditions are necessary to ensure a level of safety equivalent to that provided by Part 25 rules, [7].
Given the similarity of the fuselage impact response characteristics in the longitudinal direction compared with the other aircraft, it is
Advanced composite materials
Currently, composite materials are becoming more important in the construction of the aircraft parts [180]; they were developed during the 1960s for their weight saving opportunity over aluminum parts. New generation large aircraft are designed with all composite fuselage and wing structures, and the repair of these advanced composite materials requires an in-depth knowledge of composite structures, materials, and tooling [181], and [182]. The primary advantages of composite materials are their
Evolution of stanchion as an absorber mechanism in the cargo subfloor
In order to define a structural subcomponent that is specifically designed to absorb a crash condition, a comprehensive experimental study was performed to investigate the energy absorbing capabilities of the stanchions in the subfloor structure of aircraft section. In the sketch of the subfloor (Fig. 22), it is possible to distinguish the beam for the cargo floor and the skin of the fuselage. The stanchion is the element where the dynamic impact load is mostly concentrated. It is made up of
Sub-cargo demonstrator test
The full lower lobe section including the previous optimized version of the stanchions is shown in Fig. 25, a representative section of an A321 aircraft fully made of CFRP. This configuration presents a different installation of the subfloor compared to that installed on the actual lower lobe of the A321, as reported in ref [198].
To validate the development process of a crash sized structure, the lower lobe has been subjected to a drop test to evaluate the energy absorbing devices, to evaluate
Finite element evaluation
The finite element model of the lower lobe fuselage section (Fig. 27), includes the outer skin, three fuselage frames, longitudinal stringers and stanchions (see Fig. 28).
The assignment of a consistent contact is important for the analysis of the crash event, which is essential to allow the propagation of the load between the structure and the rigid truss. It also allows to predict the load transmission among the different parts of the structure that are in contact during the deformation, and
Conclusion
The necessity of showing that a fuselage has acceptable crashworthiness capabilities under foreseeable survivable impact events may be demonstrated via a combination of test and analysis is a valid alternative to follow. Absolutely, test the fuselage so to provide a level of crash survivability comparable to that achieved on previously certificated wide-body transports of similar size is easier if in conditions of conventional materials and configurations, more difficult in case of innovations
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