SPRING — 2-node longitudinal spring#

A linear longitudinal spring of stiffness \(k\) connecting two nodes. Two end nodes × 3 translational DOFs = 6 DOFs per element. No mass contribution.

Spec: ELEMENTS.SPRING

Verified vs analytical series-spring + spring-mass natural frequency

Cross-vendor mapping#

Solver	Element name	Notes
femorph-solver	`ELEMENTS.SPRING`	linear axial spring; stiffness-only
ANSYS Mechanical APDL	`COMBIN14`	longitudinal spring-damper (no damper here)
NX / MSC Nastran	`CELAS1` / `CELAS2`	PELAS property carries stiffness; CELAS2 inline
Abaqus	`SPRING1` / `SPRING2`	SPRING2 = two-node axial (matches our case)
LS-DYNA	`ELEMENT_DISCRETE`	DRO=0 (translational discrete element)

Restrictions#

Use a different element when:

Rotational / torsional stiffness is needed — SPRING here carries only translational stiffness; the rotational analogue isn’t shipped yet (use a stiff BEAM2 segment as a workaround).
Damping is required for transient / harmonic analysis — SPRING is stiffness-only; Rayleigh damping at the model level is the current path.

Stiffness#

The local 2 × 2 axial stiffness:

\[\begin{split}\mathbf{K}^{\mathrm{loc}} = k\, \begin{bmatrix} 1 & -1 \\ -1 & 1 \end{bmatrix},\end{split}\]

rotated into the global 6 × 6 by a direction-cosine block.

The spring carries only the axial component along the node-to-node direction; transverse motion is unconstrained (zero stiffness, zero coupling). When a single SPRING element is the only member at a free node, the solver’s zero-pivot guard pins the free transverse DOFs automatically.

Mass#

None. SPRING is a stiffness-only kernel — every entry in \(\mathbf{M}^{\mathrm{loc}}\) is zero.

Real constants#

REAL[0] — \(k\), axial stiffness in N / m.

Verification cross-references#

SPRING — two springs in series — series of two springs vs analytical equivalent stiffness \(k_{\mathrm{eq}} = k_1 k_2 / (k_1 + k_2)\).
POINT_MASS — single-DOF spring-mass oscillator — single-DOF spring-mass system, \(f = (1/2\pi)\sqrt{k/m}\).

Implementation: femorph_solver.elements.spring.

References#

Cook, R. D., Malkus, D. S., Plesha, M. E., Witt, R. J. (2002) Concepts and Applications of Finite Element Analysis, 4th ed., Wiley, §2.2 (spring element).
Bathe, K.-J. (2014) Finite Element Procedures, 2nd ed., §3.4.1 (extension to springs).