Low Cycle Fatigue (LCF) at elevated temperatures (up to 1000°C) has been an important area of testing in the aerospace industry for decades. The objective is to simulate the aggressive loading conditions seen by materials in critical turbine components, generating data for use in material or process selection, batch release, or more recently for simulation.

A drive for greater fidelity in this simulation has led to a renewed focus on LCF test data. One of the underlying motivations is demand for lower carbon emissions. A small increase in usable temperature in the right place can provide measurable improvement in efficiency. While the use of structural composites has delivered impressive aircraft weight reductions, improving the efficiency of engines is an easy win for emissions.

What does better data mean?

All material fatigue behavior shows scatter or distribution, so design and simulation use statistical confidence limit values to account for this. However, not all of the scatter is true material variation alone, but some additional randomness is introduced by the test procedure itself. Variation in the operation of the test system (either from mechanics or from the operator) means that even if it were possible to re-test the same specimen, it will probably give a slightly different result. This means that sometimes design allowables can be improved simply by collecting better data!

As a community of test engineers, we often focus obsessively on simple accuracy, but the realization has hit home that this alone is not enough; repeatability is vital. The routes to improvement distill down to better mechanical and thermal controls, but their implications may not be what you expect. Working in collaboration with our customers, Instron has engineered solutions in three key areas that have challenged manufacturers and test practitioners over past decades.

Firstly, gripping and alignment of specimens is a major source of variability. Many theoretically sound grip designs have been used in the past, but they do not necessarily help repeatable loading. The essentials are using appropriate locating features on the specimen, applying tight tolerances to both grip and specimen, and a clamping action without any off-axis motion. Simultaneously, high lateral stiffness of the whole system is needed to retain that alignment during a test — it is important to realize that most of the lateral compliance at the specimen is not from the load frame, but rather from flexure of pull rods and drive train.

Specimen misalignment

Secondly, the fidelity of mechanical test control is important. This may seem obvious, but it is not trivial to achieve, unless the test frame's control system is tuned effectively and consistently. It has been common practice to allow the first 10 fatigue cycles considerable latitude to “envelope” toward the target loading, yet these early cycles are often the most critical, when the material is changing most rapidly (hardening or softening). Tuning was historically a skilled operation, where “good” was highly subjective. Instron’s latest patented stiffness-based tuning algorithms remove any risk of overloading or pre-cycling a specimen during test setup, while achieving near identical test control between different operators, machines, and even laboratories.

Thirdly, Instron has completely automated the control of specimen temperature. Furnace control hardware in most laboratories has remained surprisingly primitive, manually controlling the furnace's three zones independently — requiring extensive, skilled, and time-consuming pre-test work. Instron’s latest furnace control systems are fully pre-tuned and use multiple specimen temperature measurements to adaptively control the furnace without any operator intervention. Each test is automatically conducted at the intended temperature, with faster heating times, while minimizing temperature overshoot and specimen gradient. The fully computerized interface means that features such as thermocouple calibrations can be added and a wide variety of metrics can be logged, greatly improving traceability.

What is generally considered “best practice” within the aerospace sector is still significantly ahead of public international standardization such as ISO, ASTM, JSA (Japanese Standards Association), and BSI (British Standards Institute). Obviously, particular companies or groups use internal test specifications or commercial standards, but these are not published openly to the whole community. Nadcap requirements are moderately stringent, but they are limited by the fact that their role is only to ensure good implementation of any particular test specification. The essence of standards such as ASTM E606 and ISO 12106 is well-established, although there is ongoing discussion on how to maintain and improve them. Some of the considerations are around specimen geometry and finish requirements — especially how to implement them in the context of additive manufactured specimens — but also about improving the robustness of quality requirements for test control. For established workers, it is worth noting that a slightly more stringent method of alignment verification for fatigue testing equipment (ISO 23788) is available, and in 2022, ISO published a technical specification for verification of specimen temperature measurement (ISO 21913).

Primary test laboratories for the aerospace industry are well aware of this and provide clear and stringent requirements for new equipment, but many small or new research laboratories do not have any knowledge of these more demanding requirements. Consequently, it has become more important than ever for equipment suppliers to maintain a good understanding of market needs, helping new laboratories to join the testing community at the “state of the art.”

Today’s environmental and social demands on industry are combining with years of improvement in design and simulation to promote development of higher efficiency in gas turbines and power generation in general. In the materials testing industry, we must respond in supporting the engineering sector with equipment that provides the highest possible confidence in data and facilitates good scientific practice in ensuring reproducibility of experiments. The recent advances in equipment design discussed in this article exemplify that principle and have addressed industrial need to improve the consistency of test data by removing the hidden effects of alignment, test control, and temperature control.

Peter Bailey, Ph.D. | Dr. Bailey is the principal applications specialist for fatigue and fracture mechanics testing at Instron. He is based in High Wycombe, UK, at the company’s factory and centre of excellence for dynamic testing. With more than 20 years of experience in experimental materials science, he also participates in standards development, as a working group convener and UK delegate to ISO technical committee TC164, and contributor to ASTM committee E08.