Fatigue Crack Tip Plasticity Associated With Overloads and Subsequent Cycling

1976 ◽  
Vol 98 (1) ◽  
pp. 17-23 ◽  
Author(s):  
J. Lankford ◽  
D. L. Davidson
Keyword(s):  
Growth Rate ◽  
Crack Tip ◽  
Threshold Value ◽  
Effective Stress Intensity Factor ◽  
Apparent Relationship ◽  
Key Factor ◽  
Crack Tip Plasticity ◽  
Crack Tip Opening ◽  
Crack Retardation ◽  
Zone Dimension

The plastic zones associated with single overloads of cyclically loaded specimens have been mapped using electron channeling patterns. The zones are asymmetric with respect to the crack tip, and are complex in shape. Crack retardation subsequent to an overload is closely related to the size and shape of the overload zone, but has no apparent relationship to the maximum zone dimension. Following an overload, cracks try to exit from the monotonic zone by moving toward the nearest elastic-plastic boundary. The size of the overload zone is predicted by a plane strain rather than plane stress relationship. The minimum retarded growth rate corresponds to an effective stress intensity factor no greater than the threshold value for Stage II growth. This is caused by crack closure, with minimal crack tip shear strains and an absence of crack tip opening and blunting. Since the crack growth rate quickly approaches the preoverload rate once the crack crosses the overload boundary, it appears that residual stress within the overload plastic zone is the key factor in governing crack retardation.

Fracture of WC-Co alloys: an example of spatially constrained crack tip opening displacement

1977 ◽  
Vol 356 (1687) ◽  
pp. 483-508 ◽  
Keyword(s):  
First Principles ◽  
Crack Tip ◽  
Mean Free Path ◽  
Crack Tip Opening Displacement ◽  
Opening Displacement ◽  
Plastic Yield ◽  
Crack Tip Plasticity ◽  
Crack Tip Opening ◽  
First Time ◽  
Co Alloys

This paper attempts, both experimentally and theoretically, to account quantitatively for the propagation of cracks in WC-Co alloys. Evidence is presented which shows that cracks propagate through the ductile Co phase. It is further shown that the square of the quasi static stress intensity factor ( K IC ) of a slowly propagating crack is linearly related to the Co mean free path. On the assumption that crack tip plasticity cannot extend beyond a single interlayer of Co it is found that the yield stress must be close to twice the theoretical shear stress of Co. Thus, for a particulate composition which consists of grains sufficiently resistant to cleavage and plastic yield, and is bonded by a matrix which is ductile and not susceptible to cleavage, K IC can be calculated. It therefore appears possible, for the first time, to predict accurately from first principles, the fracture toughness of a real elastic-plastic body.

Crack Tip Plasticity By Classic Dislocation Dynamics

2008 ◽  
Vol 33-37 ◽  
pp. 97-102
Author(s):  
Hiroomi Homma ◽  
Huu Nhan Tran
Keyword(s):  
Short Pulse ◽  
Crack Tip ◽  
Plastic Zone Size ◽  
Dislocation Dynamics ◽  
Crack Tip Opening Displacement ◽  
Opening Displacement ◽  
Discrete Dislocation Dynamics ◽  
Discrete Dislocation ◽  
Crack Tip Plasticity ◽  
Crack Tip Opening

Under very short pulse loads in range from 25 to 100 μs, crack tip plasticity a head of the crack tip in the mode I condition was investigated by discrete dislocation dynamics. The obtained dislocation array parameters such as the number of dislocations, dislocation distribution density, crack tip opening displacement and plastic zone size increase with the magnitude of stress intensity factor, KI and pulse durations. The numerical results were well compared with the experimental ones.

scholarly journals Biological scaling in green algae: the role of cell size and geometry

2021 ◽  
Vol 11 (1) ◽  
Author(s):  
Helena Bestová ◽  
Jules Segrestin ◽  
Klaus von Schwartzenberg ◽  
Pavel Škaloud ◽  
Thomas Lenormand ◽  
...  
Keyword(s):  
Growth Rate ◽  
Population Growth ◽  
Population Growth Rate ◽  
Internal Diffusion ◽  
Scaling Exponent ◽  
Power Scaling ◽  
Nutrient Distribution ◽  
Metabolic Scaling ◽  
Key Factor ◽  
Size And Morphology

AbstractThe Metabolic Scaling Theory (MST), hypothesizes limitations of resource-transport networks in organisms and predicts their optimization into fractal-like structures. As a result, the relationship between population growth rate and body size should follow a cross-species universal quarter-power scaling. However, the universality of metabolic scaling has been challenged, particularly across transitions from bacteria to protists to multicellulars. The population growth rate of unicellulars should be constrained by external diffusion, ruling nutrient uptake, and internal diffusion, operating nutrient distribution. Both constraints intensify with increasing size possibly leading to shifting in the scaling exponent. We focused on unicellular algae Micrasterias. Large size and fractal-like morphology make this species a transitional group between unicellular and multicellular organisms in the evolution of allometry. We tested MST predictions using measurements of growth rate, size, and morphology-related traits. We showed that growth scaling of Micrasterias follows MST predictions, reflecting constraints by internal diffusion transport. Cell fractality and density decrease led to a proportional increase in surface area with body mass relaxing external constraints. Complex allometric optimization enables to maintain quarter-power scaling of population growth rate even with a large unicellular plan. Overall, our findings support fractality as a key factor in the evolution of biological scaling.

scholarly journals Study on the Influence of the Gurson–Tvergaard–Needleman Damage Model on the Fatigue Crack Growth Rate

Metals ◽  
2021 ◽  
Vol 11 (8) ◽  
pp. 1183
Author(s):  
Edmundo R. Sérgio ◽  
Fernando V. Antunes ◽  
Diogo M. Neto ◽  
Micael F. Borges
Keyword(s):  
Growth Rate ◽  
Fatigue Crack ◽  
Crack Growth Rate ◽  
Fatigue Crack Growth ◽  
Plastic Strain ◽  
Crack Growth ◽  
Damage Model ◽  
Crack Tip ◽  
Strength Parameter ◽  
Stress Intensity Factor Range

The fatigue crack growth (FCG) process is usually accessed through the stress intensity factor range, ΔK, which has some limitations. The cumulative plastic strain at the crack tip has provided results in good agreement with the experimental observations. Also, it allows understanding the crack tip phenomena leading to FCG. Plastic deformation inevitably leads to micro-porosity occurrence and damage accumulation, which can be evaluated with a damage model, such as Gurson–Tvergaard–Needleman (GTN). This study aims to access the influence of the GTN parameters, related to growth and nucleation of micro-voids, on the predicted crack growth rate. The results show the connection between the porosity values and the crack closure level. Although the effect of the porosity on the plastic strain, the predicted effect of the initial porosity on the predicted crack growth rate is small. The sensitivity analysis identified the nucleation amplitude and Tvergaard’s loss of strength parameter as the main factors, whose variation leads to larger changes in the crack growth rate.

scholarly journals The Abundance of Microcystin-Producing Genotypes Correlates Positively with Colony Size in Microcystis sp. and Determines Its Microcystin Net Production in Lake Wannsee

2003 ◽  
Vol 69 (2) ◽  
pp. 787-795 ◽  
Author(s):  
Rainer Kurmayer ◽  
Guntram Christiansen ◽  
Ingrid Chorus
Keyword(s):  
Growth Rate ◽  
Colony Size ◽  
Population Growth Rate ◽  
Size Class ◽  
Net Production ◽  
Cell Numbers ◽  
Size Classes ◽  
Key Factor ◽  
Genotype Composition ◽  
Working Hypotheses

ABSTRACT The working hypotheses tested on a natural population of Microcystis sp. in Lake Wannsee (Berlin, Germany) were that (i) the varying abundance of microcystin-producing genotypes versus non-microcystin-producing genotypes is a key factor for microcystin net production and (ii) the occurrence of a gene for microcystin net production is related to colony morphology, particularly colony size. To test these hypotheses, samples were fractionated by colony size with a sieving procedure during the summer of 2000. Each colony size class was analyzed for cell numbers, the proportion of microcystin-producing genotypes, and microcystin concentrations. The smallest size class of Microcystis colonies (<50 μm) showed the lowest proportion of microcystin-producing genotypes, the highest proportion of non-microcystin-producing cells, and the lowest microcystin cell quotas (sum of microcystins RR, YR, LR, and WR). In contrast, the larger size classes of Microcystis colonies (>100 μm) showed the highest proportion of microcystin-producing genotypes, the lowest proportion of non-microcystin-producing cells, and the highest microcystin cell quotas. The microcystin net production rate was nearly one to one positively related to the population growth rate for the larger colony size classes (>100 μm); however, no relationship could be found for the smaller size classes. It was concluded that the variations found in microcystin net production between colony size classes are chiefly due to differences in genotype composition and that the microcystin net production in the lake is mainly influenced by the abundance of the larger (>100-μm) microcystin-producing colonies.

Sign in / Sign up

Export Citation Format

Share Document