This instrument tests high velocity impact and analysis of materials and components. Source: Instron

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 also is indispensable in developing 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.

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 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.

This test system certifies the fracture toughness of the materials after production performing pre-cracking and K1C fracture toughness evaluation.

Types of tests

The biaxal TMF system has an integrated induction heater. Source: Instron

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 unacceptable 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, 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.

This testing system features a 100 kilonewton high-stiffness precision-aligned loading frame, and a single-ballscrew, 100 kilonewton electromechanical actuator. The combination of the two provides an aligned precision loading system, suitable for fatigue testing—up to 1 Hz—and slow strain rate testing. Source: Instron

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, such as fans, compressors and turbines.

Fracture mechanics testing shows design engineers the crack sensitivity of a material or a component to help specify inspection intervals. Fracture mechanics methods include dAdN Fatigue Crack Propagation to determine resistance to crack growth; K1C Fracture Toughness-Brittle Failure to ascertain critical load for catastrophic failure of a cracked specimen like brittle materials; and J1C Fracture Toughness-Ductile Failure shows the use of absorption of critical energy for catastrophic failure of cracked specimen like ductile materials. Fracture mechanics is relevant to the joints of an airplane, such as where the wing meets the body of the plane, but it also is applicable to the body of the plane.

LCF, HCF and fracture mechanics are routine or standardized analyses carried out in accordance with applicable international standards for aviation testing.

This test system is used for fracture mechanics testing and HCF. Source: Instron

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 per second 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.

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. Creep is the slow deformation of a material under a continuously affecting load. A creep test simulates this mode of loading by exposing a specimen to a constant load—mostly a dead weight—and to record the consequential deformation vs. the testing time. Among these tests, the rate of crack growth is of particular interest, as described by the C-Star test.

C-Star is an analysis procedure that considers major plastic deformations at the crack tip during a crack propagation. Those can be mostly observed in creep tests at higher temperatures.

In the jet engine industry many companies are interested in this type of test because of the extremely high temperatures associated with their operations. Consequently, this type of testing is often performed at temperatures ranging from 1,000 F to more than 2,000 F.

This machine has a 1,000 C furnace and mechanical reverse stress pullrods. Source: Instron

Tech tips

• The use and performance of new, advanced materials must be thoroughly evaluated.
• Fatigue tests are conducted to identify the fatigue behavior of a material under continuously oscillating loads.
• Fracture mechanics testing shows design engineers the crack sensitivity of a material or a component to help specify inspection intervals.

Aluminum and aerospace

The use of aluminum in structural engineering applications continues to grow. Aluminum is of interest to designers of automobiles and aircraft because of its lightweight properties. Replacing steel components with aluminum leads to weight loss of around 40% to 50%, even allowing for the lower inherent strength of the material. In addition, aluminum has excellent corrosion properties and can be recycled efficiently.

Supplying markets such as aerospace manufacturing places stringent demands on the test protocol of the primary material producer. In the air, material failure is not an option. A global producer of aluminum, supplying materials to the world’s leading aircraft manufacturers, uses Instron’s 8801 series test systems to certify the fracture toughness of the materials after production. Fracture toughness characterization is performed on each material batch, which means that the laboratory has to test more than 2,000 samples per month. The 8801 series test systems are run using a 24 hour-a-day, 7 day-a-week test program, performing precracking and K1C fracture toughness evaluation. In this environment, the Instron 8801 ensures aluminum is at its best for an aircraft.