Standards Development

Scope

This proposal specifies terms, definitions, symbols, principles, apparatus, specimen, test procedure, data processing, result evaluation, report and other content for the very high cycle fatigue test under resonant axial loading at an ultrasonic frequency that an accelerated method permits affordable time consuming to obtain the fatigue result.

This proposal is applicable to the determination of fatigue properties or to the comparing of fatigue resistance of metallic materials at large number of cyclic loading within the range from 10^7 to 10^11 cycle, which are applied for the study of durability problems, such as analysis of wheel set safety based on numerical simulation in railway industry, design of key elements in power plant for long term service in energy industry, numerical predictive assessment of structure integrity in aviation industry, etc.

Purpose

While pursuing higher requirements for safety and integrity, many industrial components and structures are required to possess fatigue resistance in the very high cycle regime where loading repetitions may exceed 10 cycles.

Typical examples include aviation engine components designed for over 10,000 hours of service life and railway axles designed for operational mileage exceeding 500,000 kilometers. Recent catastrophic failures in long-term service equipment, such as in-flight aircraft engine blade fractures and high-speed train derailments caused by wheel fractures, have been conclusively linked to very high cycle fatigue (VHCF) failures through accident investigations. These incidents have prompted heightened attention from researchers and engineers toward VHCF performance. Notably, aviation engine design specifications in many countries now mandate material fatigue testing up to at least 10 cycles. Indeed, the VHCF has remained a prominent focus in fatigue research in last two decades, yielding significant progress and consensus. The distinct classification of VHCF as an independent research domain primarily arises from two novel characteristics that challenge conventional understanding of fatigue failure mechanisms: First, the Sub-fatigue-limit Failure: Fractures can occur under stress amplitudes below the traditional fatigue limit after ultra-high cycle loading, contradicting the classical fatigue theory that assumes infinite life below this threshold. These stress amplitudes may even be less than half of the material's elastic limit, raising critical design considerations regarding fatigue evaluation under low-amplitude variable loads. Second, the internal crack initiation. Fractographic analysis of highstrength steels and titanium alloys reveals characteristic fish-eye patterns with internal crack origins, contrasting with traditional fatigue striations typically originating from surface defects. This subsurface initiation mechanism cannot be explained by conventional surface-based crack formation theories involving defects or slip extrusion.

The investigation of VHCF properties necessitates addressing critical testing methodology challenges. Conventional standard testing methods DIN 50100 / ASTM E466 / ISO 1099 ISO prove impractical due to excessive time requirements - even at 20Hz loading frequency, achieving single test up to 10 cycles would require over 1.5 years. Furthermore, current ISO1099 standards explicitly limit applicability to frequencies below 300Hz.

Since the 1990s, ultrasonic resonant testing has emerged as the predominant solution for VHCF evaluation. This methodology employs hourglass-shaped specimens subjected to ultrasonic vibration (typically 19-20 kHz) through single-end excitation, generating maximum cyclic stress at the specimen's minimum cross-section through resonance design. Widely adopted in both research and failure analysis, this technique has been documented by leading material testing institutions and forms the basis for an international triennial conference series now in its ninth iteration.

Despite established standards of Chinese national standard GB/T43896-2024, Japanese wielding industry standards WES1112, and ongoing ASTM work item WK75240 in the United States, no unified international standard currently exists for ultrasonic resonant fatigue testing. This standardization proposal aims to harmonize testing methodologies for determining fatigue properties and comparing fatigue resistance of metallic materials within the 10-1010 cycle range, enabling efficient evaluation within practical time frames.