Frictional Shear Stress Research Articles

Comparative study of a femoral amputated limb with and without implant

The contact pressure at the socket–residual limb interface is the most critical parameter for evaluating the comfort of a leg prosthesis. Experimental studies have analyzed this parameter for typical postures and during walking on a flat surface of an amputee; however, experimental tests require a real socket prototype equipped with transducers. To optimize socket design, this work presents a virtual approach based on a digital avatar of a patient wearing a lower limb prosthesis. Our study integrates two different types of simulations: the first concerns the femur with an implant, the second without an implant, and includes the evaluation of pressure at the socket–residual limb interface using finite element analyses (FEA). The objective of this study is to understand the distribution of loads and stresses at the interface between the residual limb and the prosthesis. The lower contact pressure is represented by CPRESS, while CSHEAR1 is the frictional shear stress component in the first local tangent direction, and CSHEAR2 is the frictional shear stress component in the second local tangent direction.

Simulation of Friction Stir Welding of AZ31 Mg Alloys.

Friction stir welding has been extensively applied for the high-quality bonding of Mg alloys. The welding temperature caused by friction and plastic deformation is essential for determining the joint characteristics, especially the residual stress and weld microstructure. In this work, a modified moving heat source model was proposed by considering the variations in heat generation caused by friction shear stress at both the side and bottom surfaces of the tool. The application of this model was further extended to the entire welding process, especially in the plunging stage. The relative errors between the experimental and simulated peak temperatures at characteristic points were small, with a maximum of 10%, thereby validating the model for accurate temperature prediction. Furthermore, the influence of welding and rotational speed on temperature fields was systematically investigated. At relatively low welding and rotational speeds, the welding temperature increased significantly with either an increase in rotational speed or a decrease in welding speed. However, this effect gradually diminished at higher welding and rotational speeds. These results provide some valuable guidelines for controlling heat generation to improve the quality of Mg alloy welds.

TiO2-hBN nanocomposite coating with excellent wear and corrosion resistance on Ti6Al4V alloy prepared by plasma electrolytic oxidation

Hexagonal boron nitride (hBN) is recognized for its promising application prospects in anti-friction and corrosion resistance due to its self-lubrication and excellent impermeability to gases and liquids. In this study, the TiO2-hBN nanocomposite coatings are prepared via the liquid plasma-assisted particle deposition sintering (LPDS) technology, enabling compact and uniform growth of hBN on the Ti6Al4V surface. Results indicate that the dense and stable plasma at the coating/electrolyte interface facilitates the deposition and sintering of hBN particles, effectively filling surface defects and achieving a coating density of 86.7 %. The friction coefficient of the TiO2-hBN nanocomposite coating significantly decreases from 0.54 (titanium alloy) to 0.28, remaining stable even after 2000 sliding cycles. Compared to the substrate, the wear rate (4.3 × 10−4 mm3N−1 m−1) of nanocomposite coating drops by 70.8 %, which is primarily attributed to the self-lubricating property of hBN, reducing the frictional shear stress. Moreover, the TiO2-hBN nanocomposite coating also has excellent corrosion resistance due to the hBN sheet filling internal defects and inhibiting the corrosion reaction. All these merits render the LPDS technology competitive in expanding the severe service conditions of titanium alloys in aerospace and marine engineering equipment.

Influence of corrugated/flat rolling on microstructure and tensile property of coarse-grained AZ31–0.25Ca cast alloy

Novel corrugated rolling technology (CR) and conventional flat rolling (FR) were employed for rolling the AZ31–0.25Ca cast alloy without prior predeformation. The impact of corrugated rolling and/or flat rolling on texture modification, microstructure evolution, and mechanical properties of the magnesium alloy during two-pass rolling was examined using optical microscopy (OM), X-ray diffraction (XRD), scanning electron microscopy (SEM), electron backscatter diffraction (EBSD), transmission electron microscopy (TEM), tensile tests at ambient temperature, and ABAQUS finite element simulation analysis. The results indicate that the AZ31–0.25Ca cast alloy, after homogenization with a coarse grain structure of ∼200 μm, can endure significant plastic deformation with a 60 % total rolling reduction in both corrugated-flat rolling (CFR) and flat-flat rolling (FFR) without experiencing edge cracking. Twinning is activated dramatically except for dislocation slip during CFR, which induced high-density dislocations piling up at the twin boundaries and grain boundaries of α-Mg grains. The CFR sheet exhibited a macroheterogeneous structure attributed to the distinctive corrugated roll profile. Elevated shear strain and intensified frictional shear stress were observed at the trough, resulting in the formation of an asymmetric cross shear band in the CFR sheet. Denser deformation twins and finer recrystallized grains are present at the trough, whereas fewer and coarser ones are observed at the peak, while there is a significant texture weakening effect for corrugated rolling in contrast to the conventional flat rolling. but the stress and strain heterogeneity and asymmetric cross shear band also induce strong stress concentration and result in a premature cleavage fracture and a lower tensile property in CFR sheet compared to that of the FFR sheet.

A three-dimensional CFD simulation of corium jet breakup in intensive vapor generation condition

The complexity of ex-vessel phenomena during a severe accident limits the most previous CFD applications only to hydrodynamic aspects. The present study performs numerical analysis of jet breakup and debris bed formation under intensive steam generation using the STAR-CCM + code. The CFD prediction of the MATE06 experiment demonstrates jet breakup progression patterns consistent to the experiment results. The predicted jet breakup lengths are in good agreement with the MATE 06 data in earlier stage. However, some disparities of the leading-edge position between the MATE 06 and the simulation are predicted in late stage. This is attributed to non-periodic repetitions of the detachment and reattachment of some fragmented segments to the jet column. The difference of frictional force or shear stress between the experiment and CFD simulation also causes uncertainty in the amount of steam generation. In overall, the present study becomes significant to simulate successfully the series of jet breakup process and debris bed formation under intensive steam generation condition. In future studies, it is required to upgrade the current model through more evaluation of experiments and to develop much sophisticated models which provide an enhanced realistic simulation of ex-vessel phenomena.

Bioinspired Lubricity from Surface Gel Layers.

Surface gel layers on commercially available contact lenses have been shown to reduce frictional shear stresses and mitigate damage during sliding contact with fragile epithelial cell layers in vitro. Spencer and co-workers recently demonstrated that surface gel layers could arise from oxygen-inhibited free-radical polymerization. In this study, polyacrylamide hydrogel shell probes (7.5 wt % acrylamide, 0.3 wt % N,N'-methylenebisacrylamide) were polymerized in three hemispherical molds listed in order of decreasing surface energy and increasing oxygen permeability: borosilicate glass, polyether ether ketone (PEEK), and polytetrafluoroethylene (PTFE). Hydrogel probes polymerized in PEEK and PTFE molds exhibited 100× lower elastic moduli at the surface ( = 80 ± 31 and = 106 ± 26 Pa, respectively) than those polymerized in glass molds ( = 31,560 ± 1,570 Pa), in agreement with previous investigations by Spencer and co-workers. Biotribological experiments revealed that hydrogel probes with surface gel layers reduced frictional shear stresses against cells (τPEEK = 35 ± 15 and τPTFE = 22 ± 16 Pa) more than those without (τglass = 68 ± 15 Pa) and offered greater protection against cell damage when sliding against human telomerase-immortalized corneal epithelial (hTCEpi) cell monolayers. Our work demonstrates that the "mold effect" resulting in oxygen-inhibition polymerization creates hydrogels with surface gel layers that reduce shear stresses in sliding contact with cell monolayers, similar to the protection offered by gradient mucin gel networks across epithelial cell layers.

Mixing-phase model for shear-induced contractive/dilative effects in unsteady water-sediment mixture flows

Among the geophysical surface processes, mud and debris flows show one of the most complex and challenging behaviour for scientists and modellers. These flows consist of highly-unsteady gravity-driven movements of water-sediment mixtures with non-Newtonian rheology where the solid concentration could be about 40%–80% of the flow volume and which occur along steep and irregular terrains. Furthermore, the appearance of dynamic pressures in the fluid filling the intergranular pores increases the complexity and dominates the behaviour of the fluidized water-sediment material, leading to the appearance of significant density gradients during the movement. The dynamic pressure in the pore-fluid changes the effective normal stress within the mobilized material, affecting the frictional shear stress between grains and leading to the solid phase dilation/contraction. This must be properly accounted for when developing realistic models for water-sediment surface flows. In this work, a novel physically-based approach for modelling multi-grain dense-packed water-sediment flows is presented. A novel closure formulation for the pressure distribution within the pore-fluid during the movement of dense-packed water-sediment materials has been derived. This closure allows to relate the appearance of shear-induced dynamic pore pressures to the contractive/dilative behaviour of the solid aggregate. The resultant system of depth-averaged conservation laws includes continuity of the density-variable water-sediment material and the different solid classes transported in the flow, as well as the linear momentum equation for the fluidized bulk material, and it is solved using a well-balanced fully-coupled Finite Volume (FV) method. The resultant simulation tool is faced to synthetic, laboratory and real-scale benchmark cases to test its robustness and accuracy. The presence of dynamic pore pressures within the pore-fluid leads to the appearance of a deviatoric contribution to the solid flux, which causes the shear-induced separation of the solid and liquid phases and sustains the flow mobility for long distances, as it has been observed in real mud and debris events.

Experimental and model study of swirling fluid flow in a converging channel as a simulation of blood flow in the heart and aorta

The study of swirling flows in channels corresponding to the static approximation of flow channels of the heart and major vessels with a longitudinal-radial profile zR2 = const and a concave streamlined surface at the beginning of the longitudinal coordinate has been carried out. A comparative analysis of the flow structure in channel configurations zRN = const, where N = –1; 1; 2; 3, in the absence and presence of a concave surface was carried out. The numerical modelling was compared with the results of hydrodynamic experiments on the flow characteristics and the shape of the flow lines. The numerical model was used to determine the velocity structure, viscous friction losses and shear stresses. Numerical modelling of steady-state flows for channels without a concave surface showed that in the channel zR2 = const there is a stable vortex flow structure with the lowest viscous friction losses. The presence of a concave surface of sufficient size significantly reduces viscous friction losses and shear stresses in both steady state and pulsed modes.

Characterization of Interlaminar Friction during the Forming Processes of High-Performance Thermoplastic Composites

Friction is a pivotal factor influencing wrinkle formation in composite material shaping processes, particularly in novel thermoplastic composites like polyetheretherketone (PEEK) and low-melting polyaryletherketone (LM-PAEK) matrices reinforced with unidirectional carbon fibers. The aerospace sector lacks comprehensive data on the behavior of these materials under forming conditions, motivating this study’s objective to characterize the interlaminar friction of such high-performance thermoplastic composites across diverse temperatures and forming parameters. Differential scanning calorimetry (DSC) and dynamic mechanical analysis (DMA) were employed to analyze the thermomechanical behaviors of PEEK and LM-PAEK. These data guided friction tests covering room-to-forming temperatures. Horizontal pull-out fixed-plies tests were conducted to determine the friction coefficient and shear stress dependency concerning temperature, pressure, and pulling rate. Below the melting point, both materials adhered to Coulomb’s law for friction behavior. However, above the melting temperature, PEEK’s friction decreased while LM-PAEK’s friction increased with rising temperatures. These findings highlight the distinct responses of these materials to temperature variations, pulling rates, and pressures, emphasizing the need for further research on friction characterization around glass transition and melting temperatures to enhance our understanding of this phenomenon.

Frictional shear stress-induced incomplete martensitic transformation in mono-crystalline B2-CuZr spherulites

An austenitic B2-CuZr phase, when deformed, can undergo a martensitic transformation (MT) that produces a martensitic B19′-CuZr phase, resulting in remarkable transformation-induced plasticity (TRIP) and shape memory effects in B2-structured alloys. However, the shear mechanism as well as the crystallographic characteristics, which underlie the MT and, in turn, the TRIP and shape memory effects, remain unclear and somewhat controversial. Here we report that a single shear on {110}B2<001>B2, along with {1 1 5.5}B2 being the habit plane, are responsible for the MT in B2-CuZr, where {110}B2 and <001>B2 are the shear plane and the shear direction, respectively. These were revealed by determining unambiguously the orientation relationship between B2 and B19′, co-existing in individual spherulites contained in a B2-CuZr-containing bulk metal glass-matrix composite (BMGC). This exotic microstructure, featuring a core-shell like distribution in each single spherulite in which all the 12 possible martensite (B19′) variants constitute the core and the parent B2 forms the shell, was made from an incomplete MT in originally mono-crystalline B2 spherulites of the as-cast BMGC when subjected to moderately applied frictional shear stresses upon mechanical grinding. Finite element analysis suggests that a core-shell like shear stress distribution inside a B2 spherulite, caused by the confinement of the stiffer glass matrix, may account for it. These results provide direct experimental evidence to elucidate the B2→B19′ MT and its variant selections under various stress states, and pave the way for developing ductile B2-structured alloys, including but not limited to, shape memory intermetallics, high entropy alloys and B2-containing composite materials.

The friction-induced microstructures changes of 18Cr-8Ni austenitic stainless steels with different grain sizes

Abstract The friction and wear performance, wear morphology and friction-induced subsurface microstructural characteristics of 18Cr-8Ni austenitic stainless steels with coarse-grained (CG), heterogeneous ultrafine-grained (HUFG), and nano/ultrafine-grained (NG/UFG) microstructures after dry sliding wear under room temperature were studied. The results reveal that HUFG steel with a good match between hardness and plasticity exhibits the best wear performance, followed by CG steel, while NG/UFG steel with the highest hardness exhibits the poorest wear performance. From the element distribution map, the contents of O and Si in the delamination and wear debris are relatively high. O is relatively evenly distributed on the whole wear surface of HUFG steel, and there is a continuous oxide layer on its wear surface. After the wear test, the hardness increment near the wear surface of the CG sample is the largest, and the depth affected by sliding is the largest, followed by the HUFG sample, and those of the NG/UFG steel are the smallest. The repeated frictional shear stress causes the formation of cracks between the mechanical mixed layer and the plastic deformation layer, and the continuous expansion of cracks can help oxygen elements diffuse deeply, causing the deformation layer materials to fall off.

Experimental and Model Study of a Swirling Fluid Flow in a Converging Channel As a Simulation of Blood Flow in the Heart and Aorta.

Study of swirling flows in channels corresponding to the static approximation of flow channels of the heart and major vessels with a longitudinal-radial profile zR2 = const and a concave streamlined surface at the beginning of the longitudinal coordinate has been carried out. A comparative analysis of the flow structure in channel configurations zRN = const, where N = -1, 1, 2, 3, in the absence and presence of a concave surface was carried out. The numerical modeling was compared with the results of hydrodynamic experiments on the flow characteristics and the shape of the flow lines. The numerical model was used to determine the velocity structure, viscous friction losses, and shear stresses. Numerical modeling of steady-state flows for channels without a concave surface showed that in the channel zR2 = const there is a stable vortex flow structure with the lowest viscous friction losses. The presence of a concave surface of sufficient size significantly reduces viscous friction losses and shear stresses in both the steady state and pulsed modes.

Phenomenon-Based Model for Virtual Hot Strip Rolling

Every year digitalization is taking a bigger role in the steel industry. Models for predicting metallurgical phenomena, roll forces and microstructure have been commonly used in development of novel steel grades. These individual models may predict certain phenomena thoroughly, but input values are usually based on an assumption or on a “good guess”. To produce reliable boundary conditions for these models of individual phenomena, a virtual rolling model is developed. This model computes the whole process of the hot strip mill from roughing to accelerated water cooling on a run-out table. Strip location and temperature evolution is calculated continuously. Thermal and thermo-mechanical (rolling stands) boundary conditions are according to process layout. Input data for the model is automatically read from raw process data. Rolling parameters are calculated using a coupled ARCPRESS model, which is developed by authors, and calculates normal and frictional shear stress distributions in the roll gap to predict roll forces and displacements of the work roll surface. Recrystallization is considered when calculating the flow stress of the rolled strip. Phase fractions during water cooling are calculated as well. The virtual rolling model minimizes the need for parameter speculation as all parameters are calculated throughout the process. All the input values are read from actual process data and the metallurgical and mechanical state of the strip are computed throughout the whole process. As required by the state-of-art virtual rolling model, this model is based on generally accepted theories and experimentally studied metallurgical and physical phenomena along with the thermo-mechanical response of the actual rolling process.

Modeling and analysis of the cutting temperature of titanium alloys for quasi-intermittent vibration assisted swing cutting

EVC has proven to be an effective means of machining difficult-to-process materials. QVASC inherits EVC's intermittent cutting and friction reversal advantages, eliminating EVC cutting residues and improving machining quality. In order to deeply understand the cutting mechanism of QVASC, it is necessary to understand the cutting heat generation and tool surface temperature distribution in the process of QVASC. In this paper, the kinematic and frictional characteristics of the QVASC machining process are analyzed, the heat generation and heat transfer model of QVASC machining process is established, and the cutting temperature prediction model considering the characteristics of QVASC machining is proposed. According to whether there is heat convection and frictional shear stress distribution between the tool and the environment, the heat flow distribution of the front surface of the tool is divided into four regions, and the heat flux of the tool is calculated respectively. On this basis, the tool temperature of QVASC is solved by using the heat balance equation in the form of the partial differential equation. Finally, a systematic comparison experiment was carried out, and the effects of cutting depth, cutting speed, tool swing frequency, and amplitude on cutting temperature were studied by single factor method. The results show that the average cutting temperature of QVASC is 10 % lower than that of OC, which verifies the advantage of QVASC in reducing cutting temperature. Under different cutting depth, cutting speed, tool swing frequency and amplitude, the change trend of theoretical and experimental cutting temperature is the same, and the average error between them is 9 %, 8 %, 10 % and 9 %, respectively, which is within the acceptable range, proving the validity of the theoretical model. This research contributes to a deep understanding of the cutting process of QVASC and effectively determines the optimal cutting parameters.

Effect of graphene nano-additives on the fatigue limit of fiber-reinforced polymer hybrid composites—A micromechanical modeling procedure

In this article, the effect of graphene nanoadditives on the fatigue limit of unidirectional glass fiber-reinforced polymer (GFRP) composites has been investigated. A micromechanical model based on the crack growth has been used to analyze the fatigue limit response of unidirectional GFRP hybrid composite containing nanoadditives. The influence of different percentages of graphene nanosheets (GNS) and different types of nanofiller dispersion has been investigated by the micromechanical model. The fatigue limit of unidirectional GFRP hybrid composites has been plotted in terms of glass fiber volume fraction, interfacial friction shear stress, interfacial fracture energy and reference length. The fatigue limit of unidirectional composites can be improved by addition of GNS into the polymer matrix of such composite systems. The obtained results on the GNS dispersion type demonstrate that the uniform dispersion of GNS into the polymer matrix increases the fatigue limit of unidirectional GFRP hybrid composites. As the agglomeration density of GNS is low, the fatigue limit of GFRP hybrid composites will be improved more than when the density of agglomeration is high.

Mechanism of pin thread and flat features affecting material thermal flow behaviors and mixing in Al-Cu dissimilar friction stir welding

Three different features of pin models in Al-Cu dissimilar friction stir welding processes are established to analyse the effects of threads and flats on the two-phase material thermal flow behaviors. Based on computational fluid dynamics (CFD), a simulation methodology that calculates pins with complex morphologies through an adaptive force boundary is proposed. The pin characteristics are gradually optimized from “conical contour” to “conical-thread” and then to “thread-flats”. For dissimilar Al-Cu materials, the physical properties are weighted by the volume fractions of two-phase materials in the mixing zone. To build a dynamic adaptive contact model that varies with the material flow state, the interfacial friction shear stress is calculated by using the relative slip between the tool and material. The linkage effects between the “thread + flats” on the dissimilar Al/Cu welding mechanisms are systematically analysed. The enhancing mechanisms of the thread and flats on the material flow are different. The thread mainly reinforces the longitudinal migration of the Al/Cu materials, which is perpendicular to the horizontal plane. The flats effectively expand the plastic flow range and material mixing zone on the horizontal plane. These flow expanding effects are observed on the Z = 1 mm plane since the radial range of material flow enlarges from 2 mm to 3 mm under the action of the flats, while the horizontal flow range undergoes little change with the thread. The coupled mechanism of the “thread + flats” shows 0.06 s periodic strengthening, and the fluctuation threshold of the material flow velocity is wider, significantly improving the upwards stirring of the bottom material. The enhanced horizontal flow and vertical migration are beneficial to the uniform dispersion of the second-phase material (Cu) and local material softening for the formation of Al-Cu heterogeneous joints. Moreover, the accuracy of the model is verified by comparing the simulated and experimental TMAZ boundaries.

Gradient alternating deformation mechanism of two metals and interface bonding mechanism of cu/Al cold rolling composite process

Cu/Al clad plates, with a reduction of 51%, were prepared by cold roll bonding (CRB). The bonding process, microstructural evolution, and bonding mechanism, in the rolling deformation zone of the clad plate, were studied. The interface of the deformation zone was analyzed qualitatively by experimental and numerical simulation methods. The results show that the two metal layers underwent gradient alternating deformation after the metal components enter the roll gap, grain refinement, grain boundary strengthening, and dislocation strengthening occur successively in the metal layers. Strong frictional shear stress at the interface has been produced by the gradient alternating deformation of the metal layers, which accelerated the fracture of the brittle layers of the interface. From the microscopic morphology of the shear sections, many fresh metal areas have been exposed on the surface of the metal layers before the interface began to bond. It can be seen that the cracking of the brittle layers of the metal surface and the bonding process of the interface were not simultaneous. During the cold roll bonding process, the interface undergoes the following stages: (1) brittle layer fracture; (2) fresh metals are squeezed into cracks; (3) fresh metals begin to contact with each other; (4) cracks propagation, sufficient and effective contact between fresh metals; (5) a large number of atomic vacancies are generated and the interface is firmly bonded.

Highly variable friction and slip observed at Antarctic ice stream bed

The slip of glaciers over the underlying bed is the dominant mechanism governing the migration of ice from land into the oceans, with accelerating slip contributing to sea-level rise. Yet glacier slip remains poorly understood, and observational constraints are sparse. Here we use passive seismic observations to measure both frictional shear stress and slip at the bed of the Rutford Ice Stream in Antarctica using 100,000 repetitive stick-slip icequakes. We find that basal shear stresses and slip rates vary from 104 to 107 Pa and 0.2 to 1.5 m per day, respectively. Friction and slip vary temporally over the order of hours, and spatially over 10s of metres, due to corresponding variations in effective normal stress and ice–bed interface material. Our findings suggest that the bed is substantially more complex than currently assumed in ice stream models and that basal effective normal stresses may be significantly higher than previously thought. Our observations can provide constraints on the basal boundary conditions for ice-dynamics models. This is critical for constraining the primary contribution of ice mass loss in Antarctica and hence for reducing uncertainty in sea-level rise projections.

Revealing the mechanism of tool tilting on suppressing the formation of void defects in friction stir welding

Experiments have proven tool tilting in friction stir welding (FSW) can effectively suppress the formation of void defects. However, the mechanism of tool tilting on suppressing the formation of void defects has not been revealed. In this study, a CFD model considering the influence of tool tilting is established. A non-uniform distribution of normal pressure at the tool-workpiece contact interfaces is proposed to relate to the tool tilt angle. The discrete particle tracing method is used to characterize the formation of void defects in FSW. The heat generation, temperature distribution and plastic flow behaviors between the case without (i.e. 0° tool tilt angle) and with (i.e. 2.5° tool tilt angle) tool tilting are quantitatively compared and analyzed. The results show the tool tilting in FSW leads to higher heat flux and temperature near the pin side in the middle and low parts of the workpiece. Moreover, the frictional shear stress at the pin side/workpiece contact interface significantly increases when the tool is tilting, leading to a higher driving force for the plastic material to flow. As a result, the material flows farther to the advancing side of the workpiece after bypassing the tool when the tool is tilting at 2.5°, which helps to heal the voids on the advancing side of the FSW joints. The higher temperature with higher frictional shear stress enhances the plastic material flow in the middle and low part of the joints when the tool is tilting, which is attributed to suppressing the void defects. The model is validated by experimental results.

New unsaturated erosion model for landslide: Effects of flow particle size and debunking the importance of frictional stress

Flow-type landslides, such as debris flows and rock avalanches, erode soil bed material as they flow downslope. The eroded material increases the volume of a landslide, and thus, its destructive potential. Field reports of debris flow show that collisional stresses generally occur at the flow front while frictional stresses generally occur at the base of the flow body. However, it remains unclear which of these competing effects is more dominant in terms of soil bed erosion. Moreover, existing theories do not consider the flow basal slip condition on the erosion dynamics at the flow-bed interface, nor do they consider the effects of volumetric response on the shear strength of unsaturated soil beds. In this study, an erosion model that considers the effects of basal slip condition and the volumetric response of unsaturated soil beds is proposed and evaluated with a unique set of experiments. Boundary collisional stresses drive soil bed erosion, while boundary frictional shear stresses are counteracted by the soil strength mobilized by overburden induced by volume contraction. The flow particle size and Froude number are shown to be adequate indicators of the boundary collisional stresses that govern the soil bed erosion rate. Findings from this study hint at the conjecture that existing erosion models may not be equipped with the necessary physics to provide realistic hazard assessments of flow-type landslides eroding soil beds.