Since no material is perfectly rigid, the imposition of a road always produces a deformation between the particle and wall in a collision. A typical particle–wall contact process can be divided into the following three steps:
Elastic-plastic compression: Once the wall strain is greater than its yield limit, as shown in
Figure 1b, a plastic deformation zone is first produced in the contact area. As the particles continue to compress, as shown in
Figure 1c, the plastic deformation zone continues to expand toward the periphery of the contact area until the normal velocity of the particles drops to 0 m/s. The indentation strain of the wall reaches the maximum value.
Elastic recovery: When the particle velocity reaches the minimum, the elastic potential energy stored in the wall deformation zone is released, thereby pushing the particles in the opposite direction. In the release process, if the particle is not affected by external forces, theoretically it will always be in contact with the wall. However, if the particle is subjected to other contact forces (liquid forces, other particle collision forces, etc.), or non-contact forces (van der Waals forces, electrostatic forces, etc.), the particle and wall may come out of contact before the wall elastic deformation completely recovers.
2.1. Calculation of Normal Indentation
Based on Hertzian contact theory [
20], The normal stress and deformation when the rigid particles are in contact with the wall surface can be expressed as,
where 1/
E* = (1 −
ν2)/
E + (1 −
νi2)/
Ei. According to the relationship between the maximum depth of elastic indentation
h1 and normal deformation, one has
Substituting Equation (2) into Equation (3), we have,
Comparing the items in the polynomial, we get the expressions of
R and
h1,
According to the geometric formula
R2 =
rp ×
h1, the maximum stress on the contact surface is
And the total normal pressing force is expressed as,
The relationship between h
1 and normal squeeze force is,
By means of kinetic energy theorem, the motion equation of a particle during wall elastic deformation can be presented as,
where
vy0 is the initial velocity since the particles are in contact with the surface;
vy1 is the particle velocity at which the elastic indentation reaches maximum. Equation (10) can be recast as,
If we set the velocity
vy1 to 0 m·s
−1, it means that all the kinetic energy of the particles is converted into the elastic potential energy of the wall. Using the Equation (11) to calculate the initial particle velocity, which refers to the critical initial velocity (
vy0) of a particle corresponding to the elastic-plastic deformation of the wall. If
vy >
vy0, the plastic deformation of wall will take place, otherwise, it will not occur. Therefore, we set
vy1 to 0 m·s
−1, the critical velocity
vy0 can be presented as,
According to the critical velocity of a particle before plastic deformation of the wall, the particle momentum equation is built to calculate the elastic contact time, which can be expressed by,
Substituting Equation (8) into Equation (13), it is recast as,
Integrating Equation (14), the elastic contact time is,
If 15
mp/16
E*·
rp1/2·
h15/2 =
λ, Equation (15) is recast as,
In this case, the elastic contact time is dominated by the initial velocity of a particle and the maximum depth. When the wall is plastically strained, the contact stress should be equal to the yield stress of the material (i.e.,
σ0 =
σy). The elastic contact time is expressed by
σy as shown below,
Similar to the calculation of elastic contact time of indention, using momentum theorem to establish a formula for calculating elastic-plastic contact time (
t2) and rebound time (
t3) between a particle and the wall. The specific process can refer to
Appendix A. The elastic-plastic contact time is obtained after simplification, which is shown below,
The calculation of rebound time can be expressed by,
where the final velocity of the particle rebound is
v’
y1 = (16
E*·
rp1/2·
h15/2/15
mp)
1/2. Therefore, the total contact time between a particle and the wall is available from solving the simultaneous Equations (17)–(19). During this contact process, the particle not only compress normally but also tangentially extrude the wall, which causes the material to protrude in one direction.
Through the overall analysis of the three processes, we get the expression of momentum change in the process of particle impact. Since the deformation can be recovered during the elastic indentation and recovery, the normal indentation depth
h is approximately equal to the elastic-plastic indentation depth
h2. Replace
h2 with
h in Equation (A8), the total indentation depth is,
In this equation, the second term is the amount of particle velocity change in the elastic indentation process, which is approximately equal to 10
−6~10
−8 m·s
−1. This velocity change is negligible compared to the initial particle impact velocity. Therefore, Equation (20) is recast and simplified as a relation between impact energy
Ep and indentation depth
h, which is expressed by,
where
,
.
Solve the monadic quadratic equation and take the positive solution to get the total indentation depth as shown below,
2.2. Tangential Indentation under No Sliding Contact
Normally, a particle impacts the wall at an angle
α, so depending on the angle of impact, two types of crater shapes under no sliding contact and sliding contact are shown in
Figure 3.
Figure 3a shows that a particle impacts the wall at a large angle
α. The large normal pressure causes no slip contact between the particle and the wall. Due to continuous particle extrusion, the wall material is pushed in one direction to form a material lip. At this time, the shape of the crater is asymmetrical, and the tangential displacement of the particle contact point is
L. Unlike the no sliding contact crater, the energy of the particles is dissipated in the sliding friction, resulting in a reduction in the indentation depth. As shown in
Figure 3b, impact crater with sliding contact approximates symmetrical shape. Therefore, by comparing the maximum static friction force with the tangential contact force, it can be judged whether there is sliding or no sliding contact between the particle and the wall.
Based on Equation (A6), the maximum static friction of the wall can be expressed as,
By means of kinetic energy theorem, the relationship between the tangential velocity of a particle and the tangential force is,
where
t is the contact time of the indentation. When the final velocity in tangential direction is treated as 0 m·s
−1 (i.e.,
vx1 = 0 m·s
−1), Equation (25) is recast as,
Therefore, substituting the no sliding contact condition, i.e.,
Fx ≦
Fx,max, to Equations (24) and (25), one has,
Similar to the calculation of elastic-plastic contact time of normal indention (Equation (A8)), the relationship between the scratch length
L and tangential particle velocity
vx is expressed by,
where
,
,
,
According to geometry of an impact crater as shown in
Figure 2, the volume of the eroded crater is expressed by
L and
R, and it is shown below,
2.3. Tangential Indentation under Sliding Contact
When the particles are in sliding contact with the wall, the contact areas can be divided into adhesive areas and sliding areas (
Figure 4). The adhesive areas are mainly affected by normal extrusion force, and the range of sliding areas are controlled by tangential force transformed by particle kinetic energy. By means of Hertzian contact theory, the total tangential force of the adhesive and sliding areas is [
21],
where
and
where
l =
L/2.
The relationship between stress and strain is shown below in the adhesive and sliding regimes,
By means of Coulomb’s friction law, the shear stresses in the adhesive and sliding regimes can be treated as
τ1 =
μσ0 and
τ2 =
μσ0R/
l, respectively. The total tangential force can be expressed by,
Simplifying Equation (A6) and ignoring the small amount in the first item, we obtained the relationship between normal and tangential contact forces in the adhesive regime according to
Fy =
σyπR2 and
σ0 =
σy,
Therefore, the tangential maximum length of a crater is,
where
Fx is obtained from Equation (26), and
Fy is obtained from Equation (A6). The depth of a cater caused by sliding contact is less than no sliding contact, but it is contrary for crater length. Therefore, the crater geometry under sliding contact is closer to the symmetrical vertebral body (
Figure 3b). The volume of the eroded crater is expressed by,