Vibrophores test aerospace parts

Materials World magazine
,
2 Mar 2016

Alan Thomas looks at the history of metal fatigue and argues for greater use of vibrophores to ensure safety and reliability in high-performance fastener applications in the aerospace industry.

Metal fatigue is the name given to a phenomenon whereby a metal component, subjected to repeated loading, fails, even though it has never experienced a force greater than that which it could safely withstand as a single sustained force. It is a term that is known to the general public, having hit the headlines in the early 1950s as a result of a series of air disasters involving the first jet airliners.

The earliest scientific paper on the study of fatigue was published in 1837. Between 1852–1870, a significant amount of experimental work was conducted on railway axles and on small samples by August Wöhler, who is sometimes referred to as ‘the father of fatigue.’ His research established a relationship between the magnitude of the applied stress and the number of cycles required to cause fracture by fatigue.

In 1849, the Institution of Mechanical Engineers concluded that fatigue was due to ‘crystallisation’ of the metal. This misconception was founded on the frequent observation of a crystalline appearance in the final fracture face. In fact, all metals are crystalline and the development of fatigue does not generally involve any gross changes in the bulk properties of the material. The crystallisation theory was eventually disposed of in the early 20th Century.

Fatigue is now known to involve the development of cracks as a result of cyclic plastic deformation, even though it may be confined to very small volumes of metal in a component, which appears to be behaving within the elastic regime. Fatigue cracks propagate with each loading cycle until a critical crack size is reached, at which point the component fails.

In spite of extensive research over a period of more than 150 years, coupled with the introduction of a series of ever more sophisticated fatigue design approaches, fatigue still accounts for a large proportion of engineering failures.

In the air

The aerospace industry uses a wide variety of standard fastener types, including screws, rivets, bolts, pins and collars. The key difference in this industry, compared with fasteners used in many other applications, is quality. Aerospace products need to be more durable to withstand a range of high pressure and temperature environments such as leaving the earth’s atmosphere or exposure to burning engine fuel. Of course, aerospace products must also be lightweight to assist with lift and reduce fuel costs. Aircraft fasteners are safety critical components, with large aircraft employing up to 3 million fasteners in the assembly process where high quality bolts comprise up to 25% of the total fasteners used, with rivets making up the remainder.

Dynamic forces from engine operation and aircraft manoeuvre loads are superimposed onto fastener installation loads, resulting in a constantly changing stress environment. The fatigue strength of fasteners in this cyclic loading environment is a critical factor in the design process and quality control of the manufactured part. While static tensile tests provide a measure of fastener performance under peak load conditions, and microscopic examination can be used to characterise basic material properties, laboratory testing that simulates actual operational conditions is crucial to obtaining a thorough understanding of the fastener’s performance over its anticipated service life. For aircraft manufacturers and fastener suppliers, conformance to industry standards, such as NASM 1312-7 and ASTM E 466, as well as to individual manufacturer specifications, can be guaranteed only by testing with systems that replicate realistic dynamic loading conditions.

Advanced fatigue testing systems 

Fatigue failures are the result of cumulative stress cycles, which can be applied by thermal, mechanical, or vibratory effects. Particularly important for aerospace fasteners is how they perform under vibratory loading over the course of numerous cycles at stresses well below the ultimate strength of the material. These cycles may alternate tension and compression loading, or lie completely within the tensile or compressive loading regimes.

High-cycle fatigue (HCF) refers to the effect of low-amplitude, high-frequency vibration within the elastic strain region for a number of load cycles N, where typically N > 106. While the applied stress is within the material’s elastic region, plastic deformation can still take place on a microscopic level as the part ages, eventually leading to failure of the component. A component or material’s fatigue characteristics can be quantified by generating the graph of stress versus cycles at a given load, known as the Wöhler curve, where fatigue strength is determined from the maximum stress the fastener can withstand for a specified number of cycles. The endurance limit of the fastener is then defined as the stress level below which failure does not occur, meaning the component has theoretically infinite life. Because fatigue failures can occur quickly if the endurance limit is exceeded, fastener performance must be guaranteed by demonstrating adequate fatigue strengths through cyclical testing that simulates installed conditions.  

Historically, the use of servo-hydraulic testing machines has satisfied the need for component fatigue testing. These machines generally comprise a load frame incorporating a servo-hydraulic actuator with servo-valve, force and position transducers, a hydraulic power unit with some form of oil cooling facility and a control system with an associated software package to control, monitor and log the test parameters. Such machines offer a versatile fatigue testing capability but can be challenging to house in a laboratory and expensive to operate, especially if high forces and high test frequencies are required to complete a given test procedure in a relatively short time-frame. A viable, more economical alternative solution is offered by vibrophores, which are magnetic resonance fatigue testing systems.

Systems for the future

The operating principle of the vibrophore is based on the concept of a mechanical resonator with electro-magnetic drive. The mean force is applied by displacement of the upper test machine crosshead via the lead-screw drive, while the dynamic load is generated through an oscillating system working in full resonance mode. In this way, test frequencies of up to 300Hz are possible, provided specimens are sufficiently stiff. Such systems incorporate two drives, to accommodate dynamic and for monotonic testing, which are controlled separately, so that any stress ratios (R-ratios) are possible. Tests can be force, displacement or strain-controlled.

Because testing is in the resonance range, the vibrophore can also detect developing and growing cracks in the specimen at an early stage through minimal changes in the test frequency. The signal form of the dynamic load applied always corresponds to a sine wave. 

Such machines are ideal for HCF testing as they are capable of completing fatigue tests far faster than an equivalent servo-hydraulic machine and have the added benefit of operating at significantly lower running costs. This increased efficiency is a major advantage where batch testing of components is necessary to facilitate fastener production release.

It's been more than 100 years since the first aircraft was built. In that time, we have developed new engines, new structures and new resistant materials, but making things join together uses almost the same techniques or systems as a century ago. Few enhancements have been made.

Estimates suggest that 85% of all fastener failures in service are due to fatigue, often as a result of insufficient tightening and lack of proper clamping force that causes movement between the parts. In time, cracks will progress and the load will no longer be supported by the fastener. This failure of fasteners costs lives and money. For fastener manufacturers, and for end users working in production environments, accelerated fatigue testing is essential to maintaining quality assurance. Vibrophores could provide an economical and easy-to-use high-cycle fatigue testing option that meets the needs of designers, component producers and airframe manufacturers. 

Alan Thomas is the Marketing Manager at Zwick Testing Machines Limited, UK, with specialist knowledge relating to the testing requirements of the global aerospace and automotive industries.