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Abstract Frictional sliding along grain boundaries in brittle shear zones can result in the fragmentation of individual grains, which ultimately can impact slip dynamics. During deformation at small scales, stick–slip motion can occur between grains when existing force chains break due to grain rearrangement or failure, resulting in frictional sliding of granular material. The rearrangement of the grains leads to dilation of the granular package, reducing the shear stress and subsequently leading to slip. Here, we conduct physical experiments employing HydroOrbs, an elasto-plastic material, to investigate grain comminution in granular media under simple shear conditions. Our findings demonstrate that the degree of grain comminution is dependent on both the normal force and the size of the grains. Using the experimental setup, we benchmark Discrete Element Method (DEM) numerical models, which are capable of simulating the movement, rotation, and fracturing of elasto-plastic grains subjected to simple shear. The DEM models successfully replicate both grain comminution patterns and horizontal force fluctuations observed in our physical experiments. They show that increasing normal forces correlate with higher horizontal forces and more fractured grains. The ability of our DEM models to accurately reproduce experimental results opens up new avenues for investigating various parameter spaces that may not be accessible through traditional laboratory experiments, for example, in assessing how internal friction or cohesion affect deformation in granular systems.
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Characterizing friction for fiber reinforced composites manufacturing: Method development and effect of process parameters
In automated layup manufacturing processes of fiber-reinforced polymer composites, the quality of the manufactured part is strongly dependent on frictional behavior. Improper control of frictional forces can lead to defect formation. Frictional sliding rheometry tests provide an innovative methodology to accurately characterize the tool-ply friction of unidirectional (UD) prepreg employing unique annular plate geometries. The effect of processing parameters (temperature, velocity, and normal force) on the frictional response of a carbon fiber prepreg was studied. Moreover, utilizing custom designed plate geometries coupled with optically transparent fixtures allowed for in-situ quantification of the prepreg-rigid surface contact area along with simultaneous characterization of the process parameter-dependent frictional mechanisms. Our findings highlight the reduction in frictional forces with increasing temperature, attributed to the increased resin flowability, while increases in sliding rates resulted in a pronounced increase in the frictional forces. The effect of applied load on the frictional characteristics was more complicated due to contributions from both the adhesive and normal forces. Finally, the results were interpreted in light of the contact area measurements performed at different temperatures, normal force, and sliding rate.
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- Award ID(s):
- 1854185
- PAR ID:
- 10573509
- Publisher / Repository:
- ElSevier
- Date Published:
- Journal Name:
- Composites Part B: Engineering
- Volume:
- 236
- Issue:
- C
- ISSN:
- 1359-8368
- Page Range / eLocation ID:
- 109777
- Subject(s) / Keyword(s):
- Carbon fiber Prepreg Rheological properties Optical microscopy Friction
- Format(s):
- Medium: X
- Sponsoring Org:
- National Science Foundation
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Abstract The frictional properties of faults control the initiation and propagation of earthquakes and the associated hazards. Although the ambient temperature and instantaneous slip velocity controls on friction in isobaric conditions are increasingly well understood, the role of normal stress on steady‐state and transient frictional behaviors remains elusive. The friction coefficient of rocks exhibits a strong dependence on normal stress at typical crustal depths. Furthermore, rapid changes in normal stress cause a direct effect on friction followed by an evolutionary response. Here, we derive a constitutive friction law that consistently explains the yield strength of rocks from atmospheric pressure to gigapascals while capturing the transient behavior following perturbations in normal stress. The model explains the frictional strength of a variety of sedimentary, metamorphic, and igneous rocks and the slip‐dependent response upon normal stress steps of Westerly granite bare contact and synthetic gouges made of quartz and a mixture of quartz and smectite. The nonlinear normal stress dependence of the frictional resistance may originate from the distribution of asperities that control the real area of contact. The direct and transient effects may be important for induced seismicity by hydraulic fracturing or for naturally occurring normal stress perturbations within fault zones in the brittle crust.more » « less
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Nearly all frictional interfaces strengthen as the logarithm of time when sliding at ultra-low speeds. Observations of also logarithmic-in-time growth of interfacial contact area under such conditions have led to constitutive models that assume that this frictional strengthening results from purely time-dependent, and slip-insensitive, contact-area growth. The main laboratory support for such strengthening has traditionally been derived from increases in friction during “load-point hold” experiments, wherein a sliding interface is allowed to gradually self-relax down to subnanometric slip rates. In contrast, following step decreases in the shear loading rate, friction is widely reported to increase over a characteristic slip scale, independent of the magnitude of the slip-rate decrease—a signature of slip-dependent strengthening. To investigate this apparent contradiction, we subjected granite samples to a series of step decreases in shear rate of up to 3.5 orders of magnitude and load-point holds of up to 10,000 s, such that both protocols accessed the phenomenological regime traditionally inferred to demonstrate time-dependent frictional strengthening. When modeling the resultant data, which probe interfacial slip rates ranging from 3 . μ m · s − 1 . to less than 10 − 5 μ m · s − 1 , we found that constitutive models where low slip-rate friction evolution mimics log-time contact-area growth require parameters that differ by orders of magnitude across the different experiments. In contrast, an alternative constitutive model, in which friction evolves only with interfacial slip, fits most of the data well with nearly identical parameters. This leads to the surprising conclusion that frictional strengthening is dominantly slip-dependent, even at subnanometric slip rates.more » « less
- Free Publicly Accessible Full Text
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Accepted Manuscript1.0
- Journal Article:
- https://doi.org/10.1016/j.compositesb.2022.109777
