scholarly journals Fracture asperity evolution during the transition from stick slip to stable sliding

2021 ◽  
Vol 379 (2196) ◽  
pp. 20200133
Author(s):  
Cheng Mei ◽  
Wei Wu
Keyword(s):  
Normal Stress ◽  
Contact Area ◽  
Deterministic Chaos ◽  
Acoustic Energy ◽  
Radius Of Curvature ◽  
Stick Slip ◽  
Transition Stage ◽  
Fracture Dynamics ◽  
Stress Dependent ◽  
Stable Sliding

Fracture asperities interlock or break during stick slip and ride over each other during stable sliding. The evolution of fracture asperities during the transition between stick slip and stable sliding has attracted less attention, but is important to predict fracture behaviour. Here, we conduct a series of direct shear experiments on simulated fractures in homogeneous polycarbonate to examine the evolution of fracture asperities in the transition stage. Our results show that the transition stage occurs between the stick slip and stable sliding stages during the progressive reduction in normal stress on the smooth and rough fractures. Both the fractures exhibit the alternative occurrence of small and large shear stress drops followed by the deterministic chaos in the transition stage. Our data indicate that the asperity radius of curvature correlates linearly with the dimensionless contact area under a given normal stress. For the rough fracture, a bifurcation of acoustic energy release appears when the dimensionless contact area decreases in the transition stage. The evolution of fracture asperities is stress-dependent and velocity-dependent. This article is part of the theme issue ‘Fracture dynamics of solid materials: from particles to the globe’.

Residual Strength: Does BS 5930 Help or Hinder?

1986 ◽  
Vol 2 (1) ◽  
pp. 279-282 ◽  
Author(s):  
A. B. Hawkins ◽  
K. D. Privett
Keyword(s):  
Stability Analysis ◽  
Normal Stress ◽  
Residual Strength ◽  
Clay Content ◽  
Failure Envelope ◽  
Strength Parameters ◽  
Effective Normal Stress ◽  
Ring Shear ◽  
Stress Dependent ◽  
Shear Box

AbstractBS 5930 offers little assistance to engineers wishing to use residual strength parameters in slope stability analysis. It wrongly suggests the ring shear gives lower parameters than the shear box.BS 5930 does not mention the fact that the residual strength is stress dependent, hence the failure envelope is curved and the parameters must be assessed using an appropriate effective normal stress. For this reason the correlation charts relating ϕ′R to plasticity index or clay content need replacing with a series of charts in which these properties are plotted against ϕ′R values obtained at a number of effective normal stress loadings. Even then such correlations should be treated with caution.

Paper 17: Microtopography of Surfaces

1967 ◽  
Vol 182 (11) ◽  
pp. 21-30 ◽  
Author(s):  
J. B. P. Williamson
Keyword(s):  
Gaussian Distribution ◽  
Contact Area ◽  
Surface Texture ◽  
Glass Surface ◽  
Surface Density ◽  
Radius Of Curvature ◽  
Height Distribution ◽  
Digital Analysis ◽  
Texture Parameters ◽  
Combining Data

This paper describes an approach to the study of surfaces based on the digital analysis of data obtained from profilometric examinations. This technique is used to determine several new surface texture parameters, including the surface density, height distribution, and mean radius of curvature of the asperities. Recent theories have shown that these are the parameters which control the nature of surface contact. The implications which these ideas have for the science of metrology are discussed. The study also shows that many surfaces have height distributions which are Gaussian, and in particular that the heights of the upper half of most surfaces closely follow a Gaussian distribution. By combining data obtained from many closely spaced parallel profiles it has been possible to reconstruct detailed maps of the surface texture. Two examples are discussed: bead-blasted aluminium, and a glass surface lightly blasted with alumina. One of the advantages of microcartography is that it permits the geometry of the contact between rough surfaces to be studied in detail. A map is given showing the manner in which the contact area between two bead-blasted aluminium surfaces splits into sub-areas, and how these sub-areas are distributed with respect to the surface features of the contacting solids.

Large-Scale Evaluation of Constrained Bearing Elements Made of Thermosetting Polyester Resin and Polyester Fabric Reinforcement

2006 ◽  
Vol 128 (4) ◽  
pp. 681-696 ◽  
Author(s):  
P. Samyn ◽  
W. Van Paepegem ◽  
J. S. Leendertz ◽  
A. Gerber ◽  
L. Van Schepdael ◽  
...  
Keyword(s):  
Plastic Deformation ◽  
Large Scale ◽  
Elastic Recovery ◽  
Small Scale ◽  
Practical Implementation ◽  
Fiber Failure ◽  
Stick Slip ◽  
Composite Surface ◽  
Pull Out ◽  
Stable Sliding

Polymer composites are increasingly used as sliding materials for high-loaded bearings, however, their tribological characteristics are most commonly determined from small-scale laboratory tests. The static strength and dynamic coefficients of friction for polyester/polyester composite elements are presently studied on large-scale test equipment for determination of its bearing capacity and failure mechanisms under overload conditions. Original test samples have a diameter of 250 mm and thickness of 40 mm, corresponding to the practical implementation in the sliding surfaces of a ball-joint, and are tested at various scales for simulation of edge effects and repeatability of test results. Static tests reveal complete elastic recovery after loading to 120 MPa, plastic deformation after loading at 150 MPa and overload at 200 MPa. This makes present composite favorable for use under high loads, compared to, e.g., glass-fibre reinforced materials. Sliding tests indicate stick-slip for pure bulk composites and more stable sliding when PTFE lubricants are added. Dynamic overload occurs above 120 MPa due to an expansion of the nonconstrained top surface. A molybdenum-disulphide coating on the steel counterface is an effective lubricant for lower dynamic friction, as it favorably impregnates the composite sliding surface, while it is not effective at high loads as the coating is removed after sliding and high initial static friction is observed. Also a zinc phosphate thermoplastic coating cannot be applied to the counterface as it adheres strongly to the composite surface with consequently high initial friction and coating wear. Most stable sliding is observed against steel counterfaces, with progressive formation of a lubricating transfer film at higher loads due to exposure of PTFE lubricant. Composite wear mechanisms are mainly governed by thermal degradation of the thermosetting matrix (max. 162°C) with shear and particle detachment by the brittle nature of polyester rather than plastic deformation. The formation of a sliding film protects against fiber failure up to 150 MPa, while overload results in interlaminar shear, debonding, and ductile fiber pull-out.

Adhesive and normal stress-dependent dynamic friction of a gelatin hydrogel

2021 ◽  
pp. 135065012110446
Author(s):  
Nitish Sinha ◽  
Arun Kumar Singh ◽  
Vinit Gupta ◽  
Jitendra Kumar Katiyar
Keyword(s):  
Experimental Data ◽  
Normal Stress ◽  
Scaling Law ◽  
Shear Velocity ◽  
Friction Model ◽  
Dynamic Friction ◽  
Practical Applications ◽  
Soft Solids ◽  
Gelatin Hydrogel ◽  
Stress Dependent

Adhesion and friction of soft solids on hard surfaces are the important properties for a variety of practical applications. In the present study, Coulomb's law of friction is used for characterizing adhesive friction as well as normal stress-dependent dynamic friction of a gelatin hydrogel on a fixed glass surface. The experimental data, concerning normal stress-dependent dynamic friction of different shear velocity, are obtained from literature. It is observed that both components of friction increase with shear velocity. More importantly, the scaling law shows that adhesive stress varies almost linearly with corresponding coefficient of friction of the hydrogel. A dynamic friction model is also used to analyze the same experimental data to predict a negative normal stress at which dynamic friction reduces to zero, and this result matches closely with the experimental value.

What controls b-value variations: insights from a physics based numerical model

2020 ◽  
Author(s):  
Pierre Dublanchet
Keyword(s):  
Creep Rate ◽  
Normal Stress ◽  
Acoustic Emissions ◽  
Slip Rate ◽  
B Value ◽  
Power Exponent ◽  
Power Law Distribution ◽  
Differential Stress ◽  
B Values ◽  
Stress Dependent

<p>The magnitudes of earthquakes are known to follow a power-law distribution, where the frequency of earthquake occurrence decreases with the magnitude. This decay is usually characterized by the power exponent, the so-called b-value. Typical observations report b-values in the range 0.5-2. The origin of b-value variations is however still debated. Seismological observations of natural seismicity indicate a dependence of the b-value with depth, and with faulting style, which could be interpreted as a signature of a stress dependence. Within creeping regions of major tectonic faults, the b-value of microseismicity increases with creep rate. Stress dependent b-value of acoustic emissions is also commonly reported during rock failure experiments in the laboratory. Natural and laboratory observations all support a decrease of b-value with increasing differential stress. I report here on the origin of b-value variations obtained in a fault model consisting in a planar 2D rate-and-state frictional fault embedded between 3D elastic slabs. This model assumes heterogeneous frictional properties in the form of overlapping asperities with size-dependent critical slip distance distributed on a creeping segment. This allows to get complex sequences of earthquakes characterized by realistic b-values. The role of frictional heterogeneity, normal stress, shear stress, and creep rate on the b-value variations is systematically explored. It is shown that the size distribution of asperities is not the only feature controlling the b-value, which indicates an important contribution from partial ruptures, and cascading events. In this model cascades of events (and thus b-value) is strongly influenced by frictional heterogeneity and normal stress through fracture energy distribution. If the decrease of b-value with differential stress is reproduced in these simulations, it is also shown that part of the b-value fluctuations could be attributed to changes of nucleation length and stress drop with normal stress. A slight increase of b-value with slip rate exists but remains an order of magnitude smaller than the observations.</p>

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