Showing papers on "Paris' law published in 2018"

Mechanics Of Fatigue

Abstract: Mechanics of Fatigue addresses the range of topics concerning damage, fatigue, and fracture of engineering materials and structures. The core of this resource builds upon the synthesis of micro- and macro-mechanics of fracture. In micromechanics, both the modeling of mechanical phenomena on the level of material structure and the continuous approach are based on the use of certain internal field parameters characterizing the dispersed micro-damage. This is referred to as continuum damage mechanics.The author develops his own theory for macromechanics, called analytical fracture mechanics. This term means the system cracked body - loading or loading device - is considered as a mechanical system and the tools of analytical (rational) mechanics are applied thoroughly to describe crack propagation until the final failure.Chapter discuss:preliminary information on fatigue and engineering methods for design of machines and structures against failures caused by fatiguefatigue crack nucleation, including microstructural and continuous modelstheory of fatigue crack propagationfatigue crack growth in linear elastic materials subject to dispersed damagefatigue cracks in elasto-plastic material, including crack growth retardation due to overloading as well as quasistationary approximationfatigue and related phenomena in hereditary solidsapplication of the theory fatigue crack growth considering environmental factorsunidirectional fiber composites with ductile matrix and brittle, initially continuous fiberslaminate compositesMechanics of Fatigue serves students dealing with mechanical aspects of fatigue, conducting research in fracture mechanics, structural safety, mechanics of composites, as well as modern branches of mechanics of solids and structures.

Fatigue crack growth anisotropy, texture and residual stress in austenitic steel made by wire and arc additive manufacturing

Abstract: Wire based additive manufacturing of metals is a novel and cost-effective method for the production of large-scale metallic parts in a wide range of engineering applications. While these methods display excellent tensile properties, relatively little is known of the microstructure-to-property relationships of wire-based additively manufactured stainless steel builds as they relate to fatigue and fracture behavior. Stainless steel alloy 304L walls were fabricated using wire and arc additive manufacturing and subjected to mechanical tests to characterize location and orientation dependant properties and microstructural features affecting crack growth. Fatigue crack growth rate analysis in the high cycle fatigue regime was undertaken on horizontally- and vertically-oriented single-edge notch bend specimens extracted at several positions from the wall. The R ratio was 0.1 and the test frequency was 10 Hz. Paris Law behavior similar to that observed wrought steel alloys has been achieved with vertical orientations showing the greatest crack growth resistance. In conjunction with mechanical testing, scanning electron microscopy and electron backscatter detection were used to assess microstructural effects on crack growth within the build.

Predicting the 3D fatigue crack growth rate of small cracks using multimodal data via Bayesian networks: In-situ experiments and crystal plasticity simulations

Abstract: Small crack propagation accounts for most of the fatigue life of engineering structures subject to high cycle fatigue loading conditions. Determining the fatigue crack growth rate of small cracks propagating into polycrystalline engineering alloys is critical to improving fatigue life predictions, thus lowering cost and increasing safety. In this work, cycle-by-cycle data of a small crack propagating in a beta metastable titanium alloy is available via phase and diffraction contrast tomography. Crystal plasticity simulations are used to supplement experimental data regarding the micromechanical fields ahead of the crack tip. Experimental and numerical results are combined into a multimodal dataset and sampled utilizing a non-local data mining procedure. Furthermore, to capture the propensity of body-centered cubic metals to deform according to the pencil-glide model, a non-local driving force is postulated. The proposed driving force serves as the basis to construct a data-driven probabilistic crack propagation framework using Bayesian networks as building blocks. The spatial correlation between the postulated driving force and experimental observations is obtained by analyzing the results of the proposed framework. Results show that the above correlation increases proportionally to the distance from the crack front until the edge of the plastic zone. Moreover, the predictions of the propagation framework show good agreement with experimental observations. Finally, we studied the interaction of a small crack with grain boundaries (GBs) utilizing various slip transmission criteria, revealing the tendency of a crack to cross a GB by propagating along the slip directions minimizing the residual Burgers vector within the GB.

A Review of the Fatigue Properties of Additively Manufactured Ti-6Al-4V

Abstract: Various additive manufacturing (AM) technologies have been used to fabricate Ti-6Al-4V. The fatigue performance of Ti-6Al-4V varies from process to process. In this review, fatigue properties of Ti-6Al-4V alloys made by different AM technologies and post-fabrication treatments were compiled and discussed to correlate with the materials’ characteristic features, primarily surface roughness and porosity. Microstructure anisotropy and porosity effects on fatigue crack growth and fatigue life are also presented and discussed. A modified Kitagawa–Takahashi diagram developed from current available fatigue data was used to quantify the influence of defects on fatigue strength. This review aims to assist in selecting/optimizing AM processes to achieve high fatigue resistance in Ti-6Al-4V, as well as provide a better understanding of the advantages and limitations of current AM techniques in producing titanium alloys.

Anisotropy of fatigue crack growth in wire arc additive manufactured Ti-6Al-4V

Abstract: The effects of crack growth direction on the fatigue crack growth of wire arc additive manufactured Ti-6Al-4V were studied. The fatigue crack growth rate (FCGR) of horizontal and vertical samples has different stress intensity factor transition point, ΔKT, which is 11.3 MPa m1/2 and 10.3 MPa m1/2, respectively. The FCGR of vertical sample is 5% higher than that of horizontal sample when the stress intensity factor, ΔK is smaller than ΔKT. The difference in FCGR is resulted from the microstructure characterization and the fatigue crack growth direction.

Toward identifying crack-length-related resonances in acoustic emission waveforms for structural health monitoring applications

Abstract: In this study, we focus on analyzing the acoustic emission waveforms of the fatigue crack growth despite the conventional statistics-based analysis of acoustic emission. The acoustic emission monit...

Relating porosity to fatigue failure in additively manufactured alloy 718

Abstract: Additive manufacturing (AM) has the potential to revolutionize the way parts are designed and manufactured; however, AM also produces defects that influence the performance of the components. In order to ensure the quality of the manufactured parts, the processing-structure-property-performance (PSPP) relationship must be understood. In this study, the porosity created during the AM process is investigated, and its influence on performance is quantified with respect to the PSPP framework. Test specimens were fabricated with different processing pedigrees, and the porosity populations within each specimen was characterized. The fatigue life of the specimen was predicted based on the size and location of porosity using a fatigue crack growth approach. Results show that the fatigue life can be successfully predicted, when the appropriate crack growth behavior is used. The insight gained in this study will inform future AM fatigue studies and will lay the groundwork for design and qualification of fracture-critical AM components.

Evaluation of size effect on strain-controlled fatigue behavior of a quench and tempered rotor steel: Experimental and numerical study

Abstract: Combining the weakest-link theory with fatigue crack growth modeling, this study presents a mechanical-probabilistic modeling of specimen size effect for 30NiCrMoV12 steel in a low cycle fatigue (LCF) regime. Particularly, the influence of specimen size on fatigue life is quantified by experiments in strain-controlled fatigue and crack propagation. Experimental results from replica tests with three geometrical specimens indicate that nearly all of its fatigue life consists of multiple surface cracking with mutual interactions and coalescences. A probabilistic procedure for multiple surface fracture simulation is then established by incorporating random processes of crack formation, propagation and coalescence between dispersed surface cracks. Moreover, an evaluation of surface damage evolution is elaborated based on statistical physics for different structural sizes/volumes, which showed good agreement between analytical life distributions and test results.

On the theoretical modeling of fatigue crack growth

Abstract: Although fatigue is by far the most common mode of failure of structural materials, mechanistic understanding is still lacking. For example, the fundamental Paris law which relates the crack growth rate to stress-intensity factor range is still phenomenological and no reliable mechanistic model has been established for a given material or class of materials despite numerous investigations over a half a century. This work is an attempt to theoretically model fatigue crack propagation induced by alternating crack-tip plastic blunting and re-sharpening in the mid-range of growth rates on the basis of inputs from experiments that measure macroscopic material behavior, e.g., response to uniaxial cycling loading. In particular, we attempt to predict Paris law behavior by accounting for the material constitutive behavior in response to cyclic loading by modeling crack advance solely in terms of the underlying plastic dissipation. We obtain the steady-state condition for crack growth based on plastic dissipation, numerically using finite element analysis, which involves a methodology to address plastic closure upon unloading. For a given stress-intensity range, we calculate the crack propagation rate from the steady-state condition through each cycle of loading and unloading of a cracked compact-tension specimen, without resorting to any specific criterion for crack advance.

The role of intergranular fracture on hydrogen-assisted fatigue crack propagation in pure iron at a low stress intensity range

Abstract: Hydrogen-assisted fatigue crack growth (HAFCG) in pure iron at a relatively low stress intensity range exhibits brittle-like intergranular (IG) fracture, while the macroscopic crack acceleration is not significant. The present study focuses on the mechanism of IG fracture in terms of the microscopic deformation structures near the crack propagation paths. We found that the IG fracture is attributed to hydrogen-enhanced dislocation structure evolution and subsequent microvoid formation along the grain boundaries. The impact of such IG cracking on the macroscopic fatigue crack growth (FCG) acceleration is evaluated according to the dependency of IG fracture tendency on the hydrogen gas pressure during testing. It is demonstrated for the first time that increased hydrogen pressure results in a larger fraction of IG fracture and correspondingly faster FCG. On the other hand, the gaseous hydrogen environment also has a positive role in decelerating the FCG rate relative to air due to the absence of oxygen and water vapor. The macroscopic crack propagation rate in hydrogen gas is eventually determined by the competition between the said positive and negative influences.

Tensile, Creep, and Fatigue Behaviors of 3D-Printed Acrylonitrile Butadiene Styrene

Abstract: Acrylonitrile butadiene styrene (ABS) is a widely used thermoplastics in 3D printing. However, there is a lack of thorough investigation of the mechanical properties of 3D-printed ABS components, including orientation-dependent tensile strength and creep fatigue properties. In this work, a systematic characterization is conducted on the mechanical properties of 3D-printed ABS components. Specifically, the effect of printing orientation on the tensile and creep properties is investigated. The results show that, in tensile tests, the 0° printing orientation has the highest Young’s modulus of 1.81 GPa, and ultimate strength of 224 MPa. In the creep test, the 90° printing orientation has the lowest k value of 0.2 in the plastics creep model, suggesting 90° is the most creep resistant direction. In the fatigue test, the average cycle number under load of 30 N is 3796 cycles. The average cycle number decreases to 128 cycles when the load is 60 N. Using the Paris law, with an estimated crack size of 0.75 mm, and stress intensity factor is varied from 352 to 700 $$N\sqrt m$$ , the derived fatigue crack growth rate is 0.0341 mm/cycle. This study provides important mechanical property data that is useful for applying 3D-printed ABS in engineering applications.

The signatures of acoustic emission waveforms from fatigue crack advancing in thin metallic plates

Abstract: The acoustic emission (AE) waveforms from a fatigue crack advancing in a thin metallic plate possess diverse and complex spectral signatures. In this article, we analyze these waveform signatures in coordination with the load level during cyclic fatigue. The advancing fatigue crack may generate numerous AE hits while it grows under fatigue loading. We found that these AE hits can be sorted into various groups based on their AE waveform signatures. Each waveform group has a particular time-domain signal pattern and a specific frequency spectrum. This indicates that each group represents a certain AE event related to the fatigue crack growth behavior. In situ AE-fatigue experiments were conducted to monitor the fatigue crack growth with simultaneous measurement of AE signals, fatigue loading, and optical crack growth measurement. An in situ microscope was installed in the load-frame of the mechanical testing system (MTS) to optically monitor the fatigue crack growth and relate the AE signals with the crack growth measurement. We found the AE signal groups at higher load levels (75%?85% of maximum load) were different from the AE signal groups that happened at lower load levels (below 60% of load level). These AE waveform groups are highly related to the fatigue crack-related AE events. These AE signals mostly contain the higher frequency peaks (100 kHz, 230 kHz, 450 kHz, 550 kHz). Some AE signal groups happened as a clustered form that relates a sequence of small AE events within the fatigue crack. They happened at relatively lower load level (50%?60% of the maximum load). These AE signal groups may be related to crack friction and micro-fracture during the friction process. These AE signals mostly contain the lower frequency peaks (60 kHz, 100 kHz, 200 kHz). The AE waveform based analysis may give us comprehensive information of the metal fatigue.