An aircraft endures various load forces and harsh environments during flight. In order to ensure the optimal performance of materials used in aerospace applications, it is necessary to fully understand the materials'' behavior under various loading conditions. The list of loading forces and harsh environments is seemingly endless. Stress, strain, impact, fatigue, and susceptibility to fracture only begin to scratch the surface.
Superior materials and improved manufacturing methods have played an integral role in the increased reliability of aircraft components, but playing an equally vital role is the extensive testing that these vehicles undergo -- from both a materials and structural perspective. In fact, testing not only ensures the safety of aircraft currently in use, it is also indispensable in the development of new aircraft designs. It is necessary for air-transportation providers -- including government, military, and commercial -- to operate the aircraft economically, as each kilogram saved in weight ensures less fuel consumption as well as the ability to carry more freight. As a result, the use and performance of new, advanced materials must be thoroughly evaluated. (Shown here, thermo-mechanical fatigue testing using an integrated Instron system.)
As the heaviest individual component in an aircraft, jet engines represent a critical area of investigation. Besides producing lightweight design concepts, the aim is to gain increased efficiencies, i.e., to endure higher temperatures during flight. In order for planes to be able to fly higher, faster, and further at reduced cost, intensive testing is required, in basic research and quality control.
The number and type of tests performed on aircraft components are as varied as the components themselves.
Fatigue tests are conducted to identify the fatigue behavior of a material under continuously oscillating loads and are usually illustrated by an endurance curve, which separates acceptable from non-acceptable loading levels. There are two distinct types of fatigue tests: low cycle fatigue (LCF) and high cycle fatigue (HCF).
LCF testing involves three stages. The first stage is designed to detect crack initiation on a polished specimen. The second stage is propagation life, which occurs after initiation. The third stage is failure, which is usually determined by some percentage of load drop from a stable condition. Most airplane components should be subjected to LCF tests, with turbine blades and helicopter rotor blades being the most critical.
HCF results from vibratory stress cycles at frequencies which can reach thousands of cycles per second and can be generated from various mechanical sources. Vibratory stress is typical in aircraft gas turbine engines and has led to the premature failure of major engine components (fans, compressors, turbines). (Illustrated, Instron Dynatup 9250HV testing instrument for high velocity impact testing and analysis of materials and components.)
Fracture mechanics testing shows the design engineer the crack sensitivity of a material or a component to help specify inspection intervals. Fracture mechanics methods include dAdN Fatigue Crack Propagation, used to determine resistance to crack growth; K1C Fracture Toughness -- Brittle Failure, to ascertain critical load for catastrophic failure of a cracked specimen (brittle materials); and J1C Fracture Toughness -- Ductile Failure, which shows the use of absorption of critical energy for catastrophic failure of cracked specimen (ductile materials). Fracture mechanics is relevant to the "joints" of an airplane (i.e., where the wing meets the body of the plane) but it is also applicable to the "bulk" of the plane.
LCF, HCF, and fracture mechanics are examples of routine or standardized analyses that are carried out in accordance with applicable international standards for aviation testing.
Thermo-Mechanical Fatigue (TMF) simulates the combined effects of mechanical fatigue and thermal cycling experienced by an airplane''s turbine blades. In a TMF test the temperature history changes rapidly in parallel with the mechanical load; heating and cooling rates of 50°C and higher are common. As a result, the design engineer receives information about the influence of simultaneous changes in temperatures and mechanical loading, ensuring that the turbines can withstand arduous operating conditions such as emergency shutdowns and afterburner activation. (Shown, Instron Biaxial TMF system with integrated induction heater.)
It is not just variable loads that are of interest in testing. Constant loads should be considered as well, and they are effectively simulated in creep tests. Among these tests the rate of crack growth is of particular interest, as described by the C-Star (C*) test. In the jet engine industry many companies are interested in this type of test due to the extremely high temperatures associated with their operations. Consequently, this type of testing is often performed at temperatures ranging from 1000°F to over 2000°F.
Instron, a leading provider of testing equipment designed to evaluate the mechanical properties of materials and components, provides complete mechanical testing solutions to the global aerospace industry with a product range encompassing tension, compression, impact, and fatigue test systems, through to full-scale structural testing.