Open access peer-reviewed chapter

Hot Compression Tests Using Total Lagrangian SPH Formulation in Energy-Based Framework

By Kadiata Ba

Submitted: November 7th 2018Reviewed: March 18th 2019Published: April 10th 2019

DOI: 10.5772/intechopen.85930

Downloaded: 304


Limitations of the finite element method (FEM) in some cases involving large deformations as in forging or high compression tests are overcome nowadays by meshless methods such as the smoothed particle hydrodynamic (SPH) method. This paper presents a corrected total Lagrangian SPH formulation for problems encountering large deformations in solid mechanics. The continuum is modeled as a Hamiltonian system of particles (energy-based framework). The total Lagrangian formulation proposed overcomes some problems faced by standard SPH in simulating solid mechanic problems such as tensile instability. Numerical applications compared with experimental results are presented to show the capabilities of this novel formulation.


  • SPH
  • Hamiltonian system
  • total Lagrangian
  • thermomechanical
  • solid mechanics

1. Introduction

The use of the smoothed particle hydrodynamic (SPH) method [1, 2, 3, 4, 5] (Figure 1) in solid mechanics is quite recent (in the 1990s) compared to the SPH fluid formulation. Libersky and Petschek [6] and Libersky et al. [4] are cited as the first to use SPH in solid mechanics, for impacts modeling at high speeds and phenomena of rupture, perforation, and fragmentation. As SPH is a meshless method, there is no mesh to distort; it can efficiently handle large deformations. SPH is an efficient numerical method for applications in forging processes [7], machining [8, 9, 10], and welding [11]. Classical approach is widely used to describe SPH equations but faces drawbacks such as lack of completeness and tensile instability (numerical fragmentation). Total Lagrangian corrected SPH formulation is then used to fix the cited problems. In this paper, a Hamiltonian formulation is proposed for dynamic and steady-state problem simulation focusing on numerical efficiency such as accuracy and simulation time. The governing equations are derived following a Lagrangian variational principle leading to a Hamiltonian system of particles [12, 13, 14]. With the Hamiltonian SPH formulation, local conservation of momentum between particles is respected, and they remain locally ordered during the process as wanted in solid mechanic problems.

Figure 1.

(a) Schematic representation of the discretization of the domain Ω by a set of points i [15] and (b) seen in the space of a B-spline [16].

Total Lagrangian formulation reduces also the computational time by avoiding the search for neighboring particles for the construction of the kernel function at each time step. Through axial and lateral compression tests, the accuracy of the new formulation is shown. Results are compared to a classical formulation based on differential equations for solid mechanic applications.


2. Discrete equations of motion from energy-based formulation

The governing equations are derived following a Lagrangian variational principle leading to a Hamiltonian system of particles (energy-based) [12, 17, 18, 19] where the motion of each particle is given by the classical Lagrange equations. Therefore, as explained by Bonet et al. [18], constants of the motion such as linear and angular momentum are conserved.

For each particle, the physical quantities are calculated through interpolation over neighbor particles. Every particle is considered as a moving thermodynamic subsystem [12]. The volume of each particle is given by


where miis the mass of the particle and ρiits density.

To proceed with a variational formulation of the equations of motion of the continuum, the kinetic K, internal int, and external energy extof the system need to be defined.

With dissipative effects such as plasticity, the equations of motion of the system of particles representing the continuum can be evaluated following the classical Lagrangian formalism. For more details, readers can refer to Ba and Gakwaya [12]:


where xiand viare the spatial position and velocity of the particle i, Πdisis the dissipative energy, and L is the Lagrangian given by


By substituting Eq. (3) into Eq. (2), it leads to


The total kinetic energy of the system can be approximated as the sum of the kinetic energy of each particle:


For a common case where the external forces result from a gravitational field g, the total external energy is


The total internal energy can be expressed as the sum of the products of particle masses by the amount of energy accumulated per unit mass πthat depends on the deformation, density, or other constitutive parameters:


The dissipative energy can also be expressed as


where πdispis the dissipative energy per unit mass and dis the rate of deformation tensor.


with the velocity gradient given by


where Wxixjhis the SPH kernel function and his the smoothing length.

3. Corrected total Lagrangian SPH formulation for solid mechanics

Total Lagrangian formulation [20, 21] is well suited for solid mechanic problems as the SPH particles change less often their neighbors than in fluid mechanics [12]. The SPH kernels and their gradients are then expressed in the initial configuration (material coordinates Xare used). The proposed corrected kernel is to address the lack of completeness and interpolation consistency; the smoothing length his considered as a functional variable in the calculation of the gradient of the kernel function W[19].

Lagrangian and spatial coordinates are connected through the gradient of deformation tensor F:


where uis the displacement of a material point.

The expression of the corrected gradient of deformation tensor F, in total Lagrangian formulation, is given by


where Xjis the gradient with respect to a material point X, Vj0is the initial volume of particle j, h0is the initial smoothing length, and Iis the identity matrix.

Bis the expression of the correction of the gradient expressed as [20]


The corrected mass conservation equation for particle i is


where Jand ρ0are the Jacobian and the initial density.

The corrected momentum equation for a particle i is


where a, W˜, and fiare the acceleration, the normalized smoothing function, and the body force.

Pis the first Piola-Kirchhoff stress.


where σis the Cauchy stress tensor and FTis the inverse of the transpose of the gradient of deformation tensor.

The corrected energy conservation equation for particle i is


where e˙is the energy rate, rplis the mechanical contribution (heat generated by the plastic dissipation), k is the conductivity, and T is the temperature of the particle.

The equation of motion and the equation of the thermal energy of each particle can be put after discretization and evaluation of all the interactions in the following forms:


where fintiand fextiare the internal and external forces, hintikand hextikare the internal and the external heat flux, and Cis the capacitance matrix.

The expression for the internal force for a given particle can be expressed by differentiating the internal energy per unit mass with respect to the nodal positions as


where Gis the gradient function and contains the corrected kernel gradients Wat the initial reference configuration.


Internal heat flux can be expressed as


where kis the heat conductivity matrix and Tthe vector of nodal temperatures.

Explicit finite difference method is used to solve numerically the differential equation presented in this section through explicit dynamic algorithm to update the velocity, position, and temperature of each SPH particle.

4. Temporal integration scheme

A typical integration scheme used for integrating SPH equations is the leapfrog algorithm (Figure 2), an extension of the Verlet algorithm with low storage memory during computation.

Figure 2.

SPH code structure [23].

The heat transfer equations are integrated using the explicit forward-difference time integration rule [22].


T˙tis computed at the beginning of the increment by


The stability time is given by


where Δrminis the smallest interparticle distance and αis the diffusivity of the material.


where kis the conductivity and sis the specific heat.

For the mechanical part, an explicit central-difference integration rule is used to integrate the equation of motion. The nodal accelerations u¨at time t is given by


where M, Pt, and Itrepresent the mass matrix and the external and internal forces.

The integration leads to the nodal velocity u˙(Eq. 28) and the nodal displacement u(Eq. 29).


The stable time is calculated as follows:


where Leand cdare, respectively, the characteristic length of the element and the dilatational wave speed of the material.

The whole thermomechanical problem is solved by explicit coupling; both the forward-difference (for the thermal problem) and central-difference (for the mechanical problem) integrations are explicit.

The structure of the SPH code is described below (Figure 2). The time integration routine is the main subroutine. It calculates the new variables (density, acceleration, external force, internal force).

5. Material behavior

Johnson-Cook model [24, 25, 26] is used in this work, and the flow stress is expressed as follows:


where εis the plastic strain, ε̇s1is the plastic strain rate, ε̇0s1is the reference plastic strain rate, Tmis the melting temperature, Tris the reference temperature, Tis the current temperature, Ais the yield stress, Bis the coefficient of strain hardening, Cis the coefficient of strain rate hardening, n is the strain hardening exponent, and m is the thermal softening exponent.

The material used for the simulations (see Section 6) is an Al-Zn-Mg-Cu aluminum alloy. The Johnson-Cook material parameters are shown in Table 1.

A (MPa)B (MPa)cnmε̇0(s−1)Tm (°C)Tr (°C)

Table 1.

Johnson-Cook material parameters [12].


6. Applications

6.1 Axial compression test

A cylindrical sample (diameter, 25.4 mm; length, 25.4 mm) was subjected to the uniaxial compression test at constant velocity (2.54 mm s−1) and high temperature (400°C). Both experimental and numerical tests were performed (Figure 3). The aim of this test is to demonstrate the efficiency of the proposed total Lagrangian SPH formulation. We compared the numerical stress-strain curve with the experimental ones to verify the accuracy and the stability of the code (see Figure 4). To confirm the validity of the experimental result, the tests were repeated three times.

Figure 3.

Axial compression test setup in SPH.

Figure 4.

Stress-strain curves: experimental vs. SPH.

6.2 Lateral compression test

Lateral compression test is performed in this section (Figure 5). Eulerian and Lagrangian simulation time are compared, and tensile instability is verified. The test is carried out at 30 mm s−1, and cylindrical sample (diameter 25.4 mm, length 25.4 mm) with 5313 particles was used. This is a case of a large deformation test; the initial diameter of the sample was reduced over 50% during the test. See comparison of results at Table 2.

Figure 5.

Lateral compression test: Eulerian vs. Lagrangian.

Simulation timeNumber of particles
Eulerian SPH4 h 04 min5313
Lagrangian SPH1 h 36 min5313

Table 2.

Simulation time comparison.

6.3 Discussion

Figures 4 and 5 and Table 2 gather the tests results. From Figure 4 (axial compression test), we can see that the SPH result is very accurate compared to the experimental ones. Less than 5% of error is noted between the curves. The simulated sample shows no clustered particles, meaning there is no tensile instability.

Figure 5 and Table 2 show the results of the lateral compression test and confirm the previous result. Even in very large deformation test, particles keep their initial neighbors and do not suffer from tensile instability. In addition, the simulation time is very interesting compared to classical SPH formulation; simulation time is reduced drastically (from 4 h 04 min to 1 h 36 min); a good numerical efficiency is reached.

7. Conclusion

A corrected SPH particle approximation in energy-based framework is presented. Stability (no tensile instability), accuracy, and fast result production are shown leading to the conclusion that the total Lagrangian SPH formulation is very well suited to simulate solid mechanic problems. This is particularly interesting in simulating large deformation problems with physical fragmentation where the numerical fragmentation (tensile instability) will not corrupt the results.


The author wishes to acknowledge Augustin Gakwaya for his appreciated help.

© 2019 The Author(s). Licensee IntechOpen. This chapter is distributed under the terms of the Creative Commons Attribution 3.0 License, which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited.

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Kadiata Ba (April 10th 2019). Hot Compression Tests Using Total Lagrangian SPH Formulation in Energy-Based Framework, Progress in Relativity, Calin Gheorghe Buzea, Maricel Agop and Leo Butler, IntechOpen, DOI: 10.5772/intechopen.85930. Available from:

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