Fracture, Fatigue, and Structural Integrity of Metallic Materials and Components Undergoing Random or Variable Amplitude Loadings

Most metallic components and structures are subjected, in service, to random or variable amplitude loadings. There are many examples: vehicles subjected to loadings and vibrations caused by road irregularity and engine, structures exposed to wind, off-shore platforms undergoing wave-loadings, and so on. Just like constant amplitude loadings, random and variable amplitude loadings can make fatigue cracks initiate and propagate, even up to catastrophic failures. Engineers faced with the problem of estimating the structural integrity and the fatigue strength of metallic structures, or their propensity to fracture, usually make use of theoretical, numerical, or experimental approaches. This reprint collects a series of recent scientific contributions aimed at providing an up-to-date overview of approaches and case studies—theoretical, numerical or experimental—on several topics in the field of fracture, fatigue strength, and the structural integrity of metallic components subjected to random or variable amplitude loadings

Probabilistic multiscale models and measurements of self-heating under multiaxial high cycle fatigue

WOSInternational audienceDifferent approaches have been proposed to link high cycle fatigue properties to thermal measurements under cyclic loadings, usually referred to as “self-heating tests.” This paper focuses on two models whose parameters are tuned by resorting to self-heating tests and then used to predict high cycle fatigue properties. The first model is based upon a yield surface approach to account for stress multiaxiality at a microscopic scale, whereas the second one relies on a probabilistic modelling of microplasticity at the scale of slip-planes. Both model identifications are cost effective, relying mainly on quickly-obtained temperature data in self-heating tests. They both describe the influence of the stress heterogeneity, the volume effect and the hydrostatic stress on fatigue limits. The thermal effects and mean fatigue limit predictions are in good agreement with experimental results for in and out-of phase tension-torsion loadings. In the case of fatigue under non-proportional loading paths, the mean fatigue limit prediction error of the critical shear stress approach is three times less than with the yield surface approach

Dialogue essais-modèle pour l'interprétation des liens entre mesure d'auto-échauffement sous chargement cyclique et fatigue polycyclique

WOSInternational audienceUne démarche de développement d'un modèle probabiliste à deux échelles pour la fatigue HCF des aciers est proposée. Elle est basée sur l'utilisation de mesure d'autoéchauffement sous chargements cycliques et est validée sur la base de la prévision des courbes de Wöhler d'un acier dual-phase pour différents rapport de charge

Rapid multiaxial high cycle fatigue limit predictions using self-heating-based probabilistic multiscale models

Thermal measurements under multiaxial cyclic loadings are used herein to predict multiaxial fatigue properties. Two models describing random microplasticity activation via a Poisson Point Process. The thermal response is interpreted as the “mean” behaviour of the microplastic activity, whereas the fatigue limit relies on the weakest link assumption. The first model is based upon a yield surface approach to account for stress multiaxiality at a microscopic scale. The second one relies on a probabilistic modelling of microplasticity at the scale of slip-planes. Both models are identified on thermal results and a uniaxial mean fatigue limit, and then validated using fatigue limits as well as thermal responses in the case of tension-torsion loadings on tubular specimens made of medium carbon steel. They predict well hydrostatic stress, volume and proportional multiaxial effects. The model with microplasticity described at the scale of slip-planes also offers a good prediction of nonproportional mean fatigue limits (~ 5% error) whereas the other model is less predictive (~ 17% error)

Cruciform specimens’ experimental analysis in ultrasonic fatigue testing

In this work, two special aluminium cruciform specimens are designed and tested in an ultrasonic fatigue machine. They were designed based on Single-Input-Multiple-Output (SIMO) modal analysis to induce in-plane biaxial stress combinations (in-phase T-T and out-of-phase C-T) when at resonance at 20 kHz. The geometries were subjected to both numerical analysis and experimental testing to understand if they can indeed create the intended biaxial state of stresses. Both numerical and experimental results showed an impact of nearby resonant modes of non-interest on the correct functioning of the specimens, especially regarding the T-T specimen where a large deviation from the mode of interest was measured. This means that future work includes re-designing T-T specimens taking into account these mode shapes. Only out-of-phase specimens demonstrated to work properly and tests until failure were conducted. The first failure results showed to be consistent with literature when out-of-phase biaxial stress is applied cyclically

Evaluation of nonmetallic thermal protection materials for the manned space shuttle. Volume 1, task 1: Assessment of technical risks associated with utilization of nonmetallic thermal protection system

Technical problems of design and flight qualification of the proposed classes of surface insulation materials and leading edge materials were reviewed. A screening test plan, a preliminary design data test plan and a design data test plan were outlined. This program defined the apparent critical differences between the surface insulators and the leading edge materials, structuring specialized screening test plans for each of these two classes of materials. Unique testing techniques were shown to be important in evaluating the structural interaction aspects of the surface insulators and a separate task was defined to validate the test plan. In addition, a compilation was made of available information on proposed material (including metallic TPS), previous shuttle programs, pertinent test procedures, and other national programs of merit. This material was collected and summarized in an informally structured workbook

Microstructure-sensitive fatigue modeling of heat treated and shot peened martensitic gear steels

High strength secondary hardening lath martensitic steel is a strong candidate for high performance and reliable transmission systems in aircraft and automotives. The fatigue resistance of this material depends both on intrinsic microstructure attributes, such as fine scale (M2C) precipitates, and extrinsic attributes such as nonmetallic primary inclusions. Additionally, the aforementioned attributes are affected by processing history. The objective of this research is to develop a computational framework to quantify the influence of both extrinsic (primary inclusions and residual stresses) and intrinsic (martensite laths and carbides) microstructure attributes on fatigue crack formation and the early stage of microstructurally small crack (MSC) growth that dominate high cycle fatigue (HCF) lifetime. To model the fatigue response at various microstructure scales, a hierarchical approach is adopted. A simplified scheme is developed to simulate processing effects such as shot peening that is suitable to introduce representative residual stresses prior to conducting fatigue calculations. Novel strategies are developed to couple process route (residual stresses) and microstructure scale response for comprehensive analysis of fatigue potency at critical life-limiting primary inclusions in gear steels. Relevant microstructure-scale response descriptors that permit relative assessment of fatigue resistance are identified. Fatigue crack formation and early growth is highly heterogeneous at the grain scale. Hence, a scheme for physically-based constitutive models that is suitable to investigate crack formation and early growth in martensitic steel is introduced and implemented. An extreme value statistical/probabilistic framework to assess the influence of variability of various microstructure attributes such as size and spatial distribution of primary inclusions on minimum fatigue crack formation life is devised. Understanding is sought regarding the relative role of microstructure attributes in the HCF process, thereby providing a basis to modify process route and/or composition to enhance fatigue resistance. Parametric studies are conducted to assess the effect of hot isostatic pressing and introduction of compliant coatings at debonded inclusion-matrix interface on enhancement of fatigue resistance. A comprehensive set of 3D computational tools and algorithms for hierarchical microstructure-sensitive fatigue analysis of martensitic gear steels is developed as an outcome of this research; such tools and methodologies will lend quantitative and qualitative support to designing improved, fatigue-resistant materials and accelerating insertion of new or improved materials into service.Ph.D.Committee Chair: David L. McDowell; Committee Member: G. B. Olson; Committee Member: K. A. Gall; Committee Member: Min Zhou; Committee Member: R. W. Ne

Fatigue and Fracture of Traditional and Advanced Structural Alloys

The fatigue behavior of traditional and advanced materials is a very relevant topic in different strategic applications impacting and affecting our daily lives. The present Special Issue invites papers to update readers on the state of the art on this important topic. Both review and original manuscripts are welcome. Special attention will be dedicated to innovative materials and innovative manufacturing processes or post-treatments able to improve the fatigue life and reliability of a structural component. Scale effect will be also fully treated focusing on different applications and multiscale approaches aimed at understanding structural integrity under cyclic loadings. This state of the art perspective will help engineers, designers and people from the academy gain an updated view on this very challenging topic which is nowadays very important due to the advances in manufacturing technologies that allow complex new materials to be fabricated