rustytorch/crates/specialized/rtx-materials/src/elastic.rs

//! Linear elastic material models.
//!
//! Provides linear elastic material properties commonly used in finite element analysis.

use serde::{Deserialize, Serialize};

/// Linear elastic material properties.
///
/// This is a simple isotropic elastic material defined by Young's modulus
/// and Poisson's ratio.
#[derive(Debug, Clone, Copy, PartialEq, Serialize, Deserialize)]
pub struct LinearElastic {
    /// Young's modulus (E) in Pa
    pub youngs_modulus: f64,
    /// Poisson's ratio (ν), typically 0.0 to 0.5
    pub poissons_ratio: f64,
    /// Density in kg/m³
    pub density: f64,
}

impl LinearElastic {
    /// Create a new linear elastic material.
    ///
    /// # Arguments
    /// * `youngs_modulus` - Young's modulus (E) in Pa
    /// * `poissons_ratio` - Poisson's ratio (ν)
    /// * `density` - Density in kg/m³
    ///
    /// # Panics
    /// Panics if Poisson's ratio is outside [0, 0.5] or if E or density are negative.
    pub fn new(youngs_modulus: f64, poissons_ratio: f64, density: f64) -> Self {
        assert!(
            youngs_modulus >= 0.0,
            "Young's modulus must be non-negative"
        );
        assert!(
            (0.0..=0.5).contains(&poissons_ratio),
            "Poisson's ratio must be between 0 and 0.5"
        );
        assert!(density >= 0.0, "Density must be non-negative");

        Self {
            youngs_modulus,
            poissons_ratio,
            density,
        }
    }

    /// Create a material from shear modulus (G) and bulk modulus (K).
    pub fn from_shear_bulk(shear_modulus: f64, bulk_modulus: f64, density: f64) -> Self {
        let youngs_modulus =
            9.0 * bulk_modulus * shear_modulus / (3.0 * bulk_modulus + shear_modulus);
        let poissons_ratio =
            (3.0 * bulk_modulus - 2.0 * shear_modulus) / (6.0 * bulk_modulus + 2.0 * shear_modulus);
        Self::new(youngs_modulus, poissons_ratio, density)
    }

    /// Create a material from Lamé parameters (λ, μ).
    pub fn from_lame(lambda: f64, mu: f64, density: f64) -> Self {
        let youngs_modulus = mu * (3.0 * lambda + 2.0 * mu) / (lambda + mu);
        let poissons_ratio = lambda / (2.0 * (lambda + mu));
        Self::new(youngs_modulus, poissons_ratio, density)
    }

    /// Calculate shear modulus (G or μ).
    pub fn shear_modulus(&self) -> f64 {
        self.youngs_modulus / (2.0 * (1.0 + self.poissons_ratio))
    }

    /// Calculate bulk modulus (K).
    pub fn bulk_modulus(&self) -> f64 {
        self.youngs_modulus / (3.0 * (1.0 - 2.0 * self.poissons_ratio))
    }

    /// Calculate first Lamé parameter (λ).
    pub fn lame_lambda(&self) -> f64 {
        let e = self.youngs_modulus;
        let nu = self.poissons_ratio;
        e * nu / ((1.0 + nu) * (1.0 - 2.0 * nu))
    }

    /// Calculate second Lamé parameter (μ), same as shear modulus.
    pub fn lame_mu(&self) -> f64 {
        self.shear_modulus()
    }

    /// Calculate P-wave modulus (M = K + 4G/3).
    pub fn p_wave_modulus(&self) -> f64 {
        self.bulk_modulus() + 4.0 * self.shear_modulus() / 3.0
    }

    /// Calculate P-wave velocity.
    pub fn p_wave_velocity(&self) -> f64 {
        (self.p_wave_modulus() / self.density).sqrt()
    }

    /// Calculate S-wave velocity.
    pub fn s_wave_velocity(&self) -> f64 {
        (self.shear_modulus() / self.density).sqrt()
    }
}

impl Default for LinearElastic {
    fn default() -> Self {
        // Typical soft tissue properties
        Self {
            youngs_modulus: 3000.0, // 3 kPa
            poissons_ratio: 0.49,   // Nearly incompressible
            density: 1000.0,        // ~water density
        }
    }
}

/// Common material presets
impl LinearElastic {
    /// Brain tissue (approximate)
    pub fn brain_tissue() -> Self {
        Self::new(3000.0, 0.49, 1040.0)
    }

    /// Cortical bone
    pub fn cortical_bone() -> Self {
        Self::new(17e9, 0.3, 1900.0)
    }

    /// Cancellous bone
    pub fn cancellous_bone() -> Self {
        Self::new(1e9, 0.3, 600.0)
    }

    /// Soft tissue (general)
    pub fn soft_tissue() -> Self {
        Self::new(5000.0, 0.49, 1000.0)
    }

    /// Steel
    pub fn steel() -> Self {
        Self::new(200e9, 0.3, 7850.0)
    }

    /// Aluminum
    pub fn aluminum() -> Self {
        Self::new(70e9, 0.33, 2700.0)
    }
}

#[cfg(test)]
mod tests {
    use super::*;

    #[test]
    fn test_elastic_creation() {
        let mat = LinearElastic::new(200e9, 0.3, 7850.0);
        assert_eq!(mat.youngs_modulus, 200e9);
        assert_eq!(mat.poissons_ratio, 0.3);
        assert_eq!(mat.density, 7850.0);
    }

    #[test]
    fn test_shear_modulus() {
        let mat = LinearElastic::new(200e9, 0.3, 7850.0);
        let g = mat.shear_modulus();
        // G = E / (2 * (1 + ν)) = 200e9 / (2 * 1.3) ≈ 76.92e9
        assert!((g - 76.92307692307693e9).abs() < 1e6);
    }

    #[test]
    fn test_bulk_modulus() {
        let mat = LinearElastic::new(200e9, 0.3, 7850.0);
        let k = mat.bulk_modulus();
        // K = E / (3 * (1 - 2ν)) = 200e9 / (3 * 0.4) ≈ 166.67e9
        assert!((k - 166.66666666666666e9).abs() < 1e6);
    }

    #[test]
    fn test_from_shear_bulk() {
        let g = 76.92307692307693e9;
        let k = 166.66666666666666e9;
        let mat = LinearElastic::from_shear_bulk(g, k, 7850.0);
        assert!((mat.youngs_modulus - 200e9).abs() < 1e6);
        assert!((mat.poissons_ratio - 0.3).abs() < 0.001);
    }

    #[test]
    fn test_lame_parameters() {
        let mat = LinearElastic::new(200e9, 0.3, 7850.0);
        let lambda = mat.lame_lambda();
        let mu = mat.lame_mu();

        // Verify round-trip
        let mat2 = LinearElastic::from_lame(lambda, mu, 7850.0);
        assert!((mat2.youngs_modulus - mat.youngs_modulus).abs() < 1e3);
        assert!((mat2.poissons_ratio - mat.poissons_ratio).abs() < 0.0001);
    }
}