Phase-field study of crack nucleation and propagation in elastic - perfectly plastic bodies

Crack initiation and propagation in elastic - perfectly plastic bodies is studied in a phase-field or variational gradient damage formulation. A rate-independent formulation that naturally couples elasticity, perfect plasticity and fracture is presented, and used to study crack initiation in notched specimens and crack propagation using a surfing boundary condition. Both plane strain and plane stress are addressed. It is shown that in plane strain, a plastic zone blunts the notch or crack tip which in turn inhibits crack nucleation and propagation. Sufficient load causes the crack to nucleate or unpin, but the crack does so with a finite jump. Therefore the propagation is intermittent or jerky leaving behind a rough surface. In plane stress, failure proceeds with an intense shear zone ahead of the notch or crack tip and the fracture process is not complete

Phase-field study of crack nucleation and propagation in elastic-perfectly plastic bodies

Crack initiation and propagation in elastic–perfectly plastic bodies is studied in a phase-field or variational gradient damage formulation. A rate-independent formulation that naturally couples elasticity, perfect plasticity and fracture is presented, and used to study crack initiation in notched specimens and crack propagation using a surfing boundary condition. Both plane strain and plane stress are addressed. It is shown that in plane strain, a plastic zone blunts the notch or crack tip which in turn inhibits crack nucleation and propagation. Sufficient load causes the crack to nucleate or unpin, but the crack does so with a finite jump. Therefore the propagation is intermittent or jerky leaving behind a rough surface. In plane stress, failure proceeds with an intense shear zone ahead of the notch or crack tip and the fracture process is not complete

Analysis of rolling contact spall life in 440 C steel bearing rims

The results of a two year study of the mechanisms of spall failure in the HPOTP bearings are described. The objective was to build a foundation for detailed analyses of the contact life in terms of: cyclic plasticity, contact mechanics, spall nucleation, and spall growth. Since the laboratory rolling contact testing is carried out in the 3 ball/rod contact fatigue testing machine, the analysis of the contacts and contact lives produced in this machine received attention. The results from the experimentally observed growth lives are compared with calculated predictions derived from the fracture mechanics calculations

Cracklike Dynamics at the Onset of Frictional Sliding

We propose an elasto-plastic inspired friction model which incorporates interfacial stiffness. Steady state sliding friction is characterized by a generic nonmonotonic behavior, including both velocity weakening and strengthening branches. In 1D and upon the application of sideway loading, we demonstrate the existence of transient cracklike fronts whose velocity is independent of sound speed, which we propose to be analogous to the recently discovered slow interfacial rupture fronts. Most importantly, the properties of these transient inhomogeneously loaded fronts are determined by steady state front solutions at the {\em minimum} of the sliding friction law, implying the existence of a new velocity scale and a "forbidden gap" of rupture velocities. We highlight the role played by interfacial stiffness and supplement our analysis with 2D scaling arguments.Comment: 4 pages, 2 figure

How collective asperity detachments nucleate slip at frictional interfaces

Sliding at a quasi-statically loaded frictional interface can occur via macroscopic slip events, which nucleate locally before propagating as rupture fronts very similar to fracture. We introduce a novel microscopic model of a frictional interface that includes asperity-level disorder, elastic interaction between local slip events, and inertia. For a perfectly flat and homogeneously loaded interface, we find that slip is nucleated by avalanches of asperity detachments of extension larger than a critical radius $A_c$ governed by a Griffith criterion. We find that after slip, the density of asperities at a local distance to yielding $x_\sigma$ presents a pseudo-gap $P(x_\sigma) \sim (x_\sigma)^\theta$, where $\theta$ is a non-universal exponent that depends on the statistics of the disorder. This result makes a link between friction and the plasticity of amorphous materials where a pseudo-gap is also present. For friction, we find that a consequence is that stick-slip is an extremely slowly decaying finite size effect, while the slip nucleation radius $A_c$ diverges as a $\theta$-dependent power law of the system size. We discuss how these predictions can be tested experimentally

Life prediction of materials exposed to monotonic and cyclic loading: A new technology survey

Reviewed and evaluated technical abstracts for about 100 significant documents are reported relating primarily to life prediction for structural materials exposed to monotonic and cyclic loading, particularly in elevated temperature environments. The abstracts in the report are mostly for publications in the period April 1962 through April 1974. The purpose of this report is to provide, in quick reference form, a dependable source for current informatio

Peridynamics modelling of weibull distributions for nuclear fuel fracture

Peridynamics is a non-local continuum mechanics modelling method, with fundamental equations built upon integrals as opposed to partial differentials, which gives benefits when modelling brittle fracture relative to other continuum mechanics modelling techniques. Notably absent from peridynamics literature is an investigation of the effect of fracture strength distributions (an important element of brittle fracture) in peridynamics. This thesis outlines a method for appropriately including fracture strength distributions in peridynamics, and presents a model of a UO2 fuel pellet fracturing in service using this method. It was shown that using a Weibull distribution in peridynamics without adjusting the distribution of strengths to account for the difference in size between bonds and the part to be modelled produces inaccurate results. Using Weibull scaling to account for this did not alone solve this problem, as there was still a disconnect between the stress at which the first bond fails (stage 1 failure) and the stress at which the overall part modelled fails (stage 2 failure). Bond strengths were localised by linking bond strength to the material points they are connected to. Combining this localisation with using the most extreme strengths, the shape of the Weibull curve was accurately recreated in 1D peridynamics. The method was applied in two dimensions, and it was shown that the method which had worked in one dimension is no longer adequate. It was found that edge length is the most appropriate size-scaling criteria, as opposed to total area of the two-dimensional model. The model was able to recreate Weibull distributions of fracture strain in a two dimensional tensile test using a Weibull modulus of 10, but was less accurate with lower Weibull moduli. The effect of Weibull distributions on radial crack numbers in in-service UO2 nuclear fuel pellets was investigated. It was found that using a Weibull distribution of fracture strains in a peridynamics model of fuel pellets allows the model to more accurately predict the number of cracks expected at a given power. The model was compared to low-burnup post irradiation examination data.Open Acces

Energy and Thermodynamically Based Approaches for Analysis of Damage in Contact Problems

A novel methodology for analysis of damage in contact problems in presented. Two important categories of contact damage are studied, namely crack nucleation and adhesive wear. The proposed methodology is primarily based on the laws of energy and thermodynamics, and as such offers great advantages by unifying the analysis of damage in a variety of contact configurations. Crack nucleation is the first damage type analyzed. For this purpose, the line-contact fretting configuration is chosen. A thermodynamically-based continuum damage mechanics (CDM) approach is employed. Intense stress gradients in the contact region are found to be highly influential in the crack nucleation process. A methodology is proposed that identified the averaging zone as a function of the contact and loading parameters. The predicted crack nucleation lives are verified by comparing against the published experimental data for two different alloys. The utility of the proposed methodology is also investigated for the case of rough surface contact. The deterministic approach is employed to investigate the effect of roughness on the surface tractions and contact stresses. A special averaging technique, proposed for the case of smooth surface, is adopted. The predictions of the crack nucleation life for different roughness values are compared with relevant experimental data in literature, and confirm the validity of the analysis. Adhesive wear is another type of contact damage investigated. The study is carried out for three different contact conditions. These include disk-on-disk unidirectional dry sliding, pin-on-flat reciprocating dry sliding and pin-bushing grease-lubricated oscillatory sliding. It is shown that the wear rate is linearly related to the power dissipation as well as the entropy generation rate. The linear correlations are verified experimentally. Degradation coefficient is obtained and a simple approach is proposed for prediction of wear in dry sliding configuration. The proposed technique can be employed for prediction of wear in circumstances where the direct measurement of power dissipation is encumbered by practical limitations. Also investigated are the relationship between the system’s wear rate, power dissipation and thermal response. The wear-energy dissipation coefficient (WED) is identified as an important property of tribo-systems. The methodology relies on measurement of temperature rise in the sliding system. It is shown that the correlation between the frictional power dissipation and temperature rise can be obtained through thermal analysis of the system. The proposed methodology is shown to be capable of predicting the wear rate under a wide range of loading conditions