Fracture mechanics and fatigue strength.

Fatigue strength analyses, widely used in many industrial sectors today, have a long history reaching back to the 19th century and pioneering studies on the fracture mechanics of railway components. The 20th century brought a dynamic development of this science and engineering discipline. Today, one of the most significant drivers of progress in this area is the aerospace industry (covering both aviation and space). However, fatigue strength research is far more widespread than space flights alone. In this article, we briefly introduce its fascinating history and key milestones in the development of this field. We also present examples of the applications of such analyses, their resulting benefits, and highlight the areas of specialization at Endego in this domain.

History of work on fracture mechanics and fatigue

The beginnings of fracture mechanics research can be traced to the work of pioneers in the 19th century who analyzed phenomena occurring in the railway industry. Over subsequent years, engineers working in the automotive, marine, aerospace, and space industries also contributed to the development of this field.

19th century – the railway industry as a driver of fracture mechanics research

The key events leading to the establishment of fracture mechanics as a science and its further development were the spread of railways and the pioneering work necessary for progress in this sector. Among the milestones, the following events are worth mentioning:

20th century – fracture mechanics in aviation, marine, automotive, and space industries

The century, marked by the Wright brothers’ flight experiments and the widespread of automobiles, turned out to be a period of dynamic growth in knowledge and research on fracture mechanics and fatigue strength. From this perspective, the key events are at least:

• the use of optical microscopes to study slip lines and slip planes in metal structures at the turn of the 19th and 20th centuries,
• the work of Gough, Griffith, and Taylor on fracture mechanics in aviation applications,
• Haigh’s publication on the fatigue strength of metals in marine applications (Haigh diagram),
• Almen’s research in General Motors’ laboratories on automotive fatigue strength applications,
• Neuber’s works at German universities and in the aviation industry, resulted in the creation of a calculation manual that introduced the plastic correction for linear-elastic calculations (Neuber correction),
• Palmgren’s proposal to use the linear fatigue damage rule for ball bearings,
• Miner’s proposal to apply the linear fatigue damage rule and the damage accumulation in aviation,
• US Air Force issues the guideline document for the design of new fighter aircrafts,
• NASA develops computer programs for stress and strain state simulations, followed by programs for fracture mechanics phenomena simulations and for fatigue life calculations.

Even today, the aviation and space industries play the most significant role in fracture mechanics research.

Application examples of fracture mechanics knowledge

The fracture mechanics knowledge, that was developed over centuries, along with the methodological frameworks for performing calculations and with the software that facilitates their execution, are today being widely used in modern industries. Fatigue strength analyses are being performed for applications such as:

1. Design of safety switches or safety devices that have mechanical samples with various notches; these samples fail before failure occurs in the device under protection or in a specific component, such as a turbine rotor.
2. Allowable crack depths are determined in thick-walled turbine casings to plan the timing of casing replacement or to decide when and how repairs should be performed.
3. Extremely lightweight landing gear components for fighter aircrafts, for a limited number of takeoff-landing cycles, are being designed to be replaced with new ones shortly before they collapse.
4. Inspecting railway wheels with a hammer to detect fatigue cracks through changes in sound before the cracks grow and cause failure.
5. Examining fracture surfaces of broken components to determine the cause of failure and to improve the design for the future.
6. Shaping technological processes where parts are separated or divided by impact-induced cracking.
7. Design of ship structures taking vibrations into account to avoid resonances that, in unexpected situations, could lead to the vessel sinking.

Benefits when including fracture mechanics into strength calculations

Simulations and calculations of fracture mechanics and fatigue strength provide numerous benefits. Among the most significant advantages are:

• The ability to guarantee safe equipment usage through service life predictions,
• Increased safety for drivers and passengers through crash resistance testing,
• Improved durability of structures exposed to cyclic vibration loads.

Below, we describe how fracture mechanics research contributes to achieving benefits in these specific areas.

Equipment and device usage safety through service life predictions

Service life predictions, based on test results of material samples under cyclic loading, have now become a standard component of strength calculations. Alongside static calculations, they are being performed for relevant parts of new vehicles and many machines and equipment, to guarantee their safe operation.

Improving road safety through crash resistance investigations

The behavior of vehicles and safety devices during collisions is now being routinely tested at the design stage of car structures (e.g. car crumple zones, airbags). Collision calculations and simulations, that include fracture mechanics, are now being widely performed and this way, they reduce the risk of death or severe injury caused by accidents.

Increasing structure resistance to shocks and vibrations

Cyclic loads in the form of vibrations, induced periodically or by impact, occur during both, earthquakes and extraordinary events such as explosions. These may occur in civilian life situations, such as on oil platforms, or in military scenarios involving explosive weapons, such as on warships. Calculations of these phenomena help improve the resistance of structures under investigation.

CAE Endego team – specialists in fracture mechanics calculations

CAE Endego team specializes in simulations and calculations based on fracture mechanics and fatigue strength and performs them for industries such as:

• thermal turbines – consulting on the structural strength of turbine parts and assemblies,
• rail vehicles – design (structure and electro), strength prove, consulting,
• Ship and offshore structures – equipment vibrations, hull design, numerical prove, optimization, consulting,
• Automotive and heavy equipment – design (structure, electro, lighting), strength prove, virtual tests, consulting.

The extensive experience of Endego’s engineering team allows us to adapt advanced computational techniques to any business requiring their application, as well as to its specific needs.

We invite you to contact us.

Grzegorz Wiśniewski

CAE Technical Expert

Senior specialist in areas such as mechanics and strength of materials, non-linear issues, plasticity (PhD ETH Zürich), strength of thermal turbines, rail vehicles, ship structures. He has professional experience in conducting and coordinating development projects in the above-mentioned fields, as well as teaching and implementing the Finite Element Method (including ETH Zürich and Gdansk University of Technology).