1. Introduction
The term “bird strike” most commonly denotes a collision between birds and aircraft. The problem of bird strikes is one of the major threats faced by airplanes during takeoff and landing. Research estimates suggest that approximately 90% of instances involving foreign-object damage (FOD) to aircraft structures are attributable to bird encounters [
1]. Data from the Federal Aviation Administration (FAA) indicate that bird strikes result in considerable downtime and economic losses to the aviation industry [
2]. Scholars are currently working to address this issue in two main ways:
Scientific and comprehensive assessments of the risk of bird strikes to prevent bird strikes;
Predicting the response of and damage to aeronautical structures after bird strikes to improve bird strike resistance.
In the domain of bird strike risk assessment, numerous scholars have been continuously engaged in conducting studies from innovative perspectives. Budgey et al. [
3] introduced a methodology to evaluate the risk of bird strikes within flocks, employing a stereo camera for flock photography to ascertain flock density and bird distribution. Lopez-Lago et al. [
4] devised a real-time bird strike risk assessment model grounded in pertinent factors, including bird detection radar and flight data. Coccon et al. [
5] formulated generalized linear models (GLMs) with a binomial distribution to further quantify the level of risk associated with bird strikes at airports. Metz et al. [
6] calculated bird strike risk by predicting bird flights and evaluating the severity of collisions. Additionally, they assessed the impact on airport safety and capacity when implementing a bird strike advisory system.
According to statistics, accidents resulting from engine strikes by birds constitute 44% of all bird strikes [
7]. Fan blades, which are a critical component of an engine, are situated at the forefront of the engine and are particularly vulnerable to bird strikes during aircraft take-off, landing, and flight. Bird strikes can result in various forms of damage to the blades, including craters, tears, and curling, which may lead to more serious aviation accidents [
8]. Many scholars have extensively investigated the responses and damage issues related to bird strikes on fan blades. Prakash et al. [
9] conducted numerical simulations of the bird strike response of rotating blades with and without convex shoulders. Their findings suggest that blades with convex shoulders exhibit reduced deformation damage at the strike position compared to those without. Zhang et al. [
10] utilized the SPH method to create a detailed bird model and studied the impact response of and damage to a rotating engine fan struck by a bird, considering bird geometry and impact direction. Liu et al. [
11] examined the effects of the bird strike problem on rotating fan blades through a combination of experiments and numerical simulations, with a focus on impact parameters affecting engine blade damage. Their study revealed that bird mass and engine blade rotational speed significantly influenced the deformation of and damage to the fan blades. Puneeth et al. [
12] investigated the effect of bird mass and impact height on blade response, identifying the critical impact height of the fan blades under study. Yella et al. [
13] explored the bird strike resistance of hybrid fiber composite blades, particularly emphasizing the impact of the bird’s position and the length of the combined region of the two materials on the blade’s response.
Prior studies on bird strike response damage in aerostructures have predominantly centered on single bird strikes. However, in actuality, aircraft are frequently confronted with flocks of birds. The Federal Aviation Administration (FAA) reported 141,067 bird strikes from 1990 to 2020, revealing that multiple bird strikes constituted 13.8% of total incidents [
14]. A prominent example is the 1975 incident involving an American Overseas Airlines DC10 airliner, which tragically crashed in New York following the ingestion of a flock of seagulls during takeoff [
15]. This accident further reinforced the concern about the airworthiness of high-bypass-ratio turbofan engines for bird-absorbing. Simultaneously, the U.S. authorities issued FAR 33.77 AMDT6, further refining the criteria governing the weight and quantity of birds used in airworthiness verification tests for bird ingestion, thereby enhancing the standards [
16]. In 2000, the FAA issued AMDT20, mandating the execution of engine bird ingestion validation tests with a mass ranging from 1.5 to 2.5 lb and a maximum of six birds, determined by parameters such as the engine’s windward area. Parameters such as the engine’s windward area were used to establish the test requirements [
17]. Subsequently, in 2007, AMDT23/24 elevated the performance criteria post bird ingestion and introduced validation for ingesting large flocks of birds [
18]. A number of scholars have also carried out studies related to multiple bird strikes. Wu et al. [
19] investigated the effects of bird flock strikes on an engine rotor system under a variety of scenarios. Rezaei et al. [
20] redesigned an aircraft windshield to improve its mechanical resistance against simultaneous bird strikes. It is evident that in recent years, researchers have increasingly focused on the issue of multiple bird strikes, and there has been a gradual enhancement in the standards of related airworthiness verification experiments.
In a multiple-bird-strike incident, a single fan blade is prone to experiencing multiple bird strikes. Existing studies on bird strike risk assessment predominantly address the aircraft as a unified structure, lacking a recognized systematic approach to assess the risk of multiple fan blade impacts. Current blade strength designs often overlook the possibility of multiple bird strikes, which can lead to damage in real multiple-bird-strike accidents. Random bird placement is primarily employed in airworthiness verification experiments, making it challenging to simulate the most hazardous impact scenarios. Additionally, the issue of blade impact by multiple birds is more intricate, involving the accumulation of blade damage and dynamic response coupling after each impact. There is also very limited research on the dynamic response of fan blades struck by multiple birds.
This paper investigates the aforementioned issues. The paper begins by introducing the bird slicing process and subsequently proposes a probability model for assessing the risk of multiple impacts on fan blades. Subsequently, a simulation model for bird strikes is established, with validation accomplished through the solution of rotational prestressing of the blade and its impact on the rigid flat plate. Ultimately, this paper considers factors such as bird strike position and impact time interval, which may influence the dynamic response of the blade. The effects of these factors on a blade’s dynamic response are investigated through numerical simulation.