verification: implicit contact patch test and Hertz line contact

Two oracle cases: the contact patch test on non-matching meshes in small and finite sliding,
and Hertz's cylinder on a flat under force control (peak pressure, pressure distribution,
subsurface stress). docs/fem.md describes the formulation, the literature survey its papers;
ROADMAP M7 and parity are updated.

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01UhhZMBTviDQxZvwnyP89fo
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jyvikoandClaude Opus 5.5 authored and Irakli Kourtis committed 2026-10-14 14:58:32 -04:00
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{
"title": "Contact patch test: two blocks with non-matching meshes in uniaxial compression (implicit mortar contact)",
"kind": "theory",
"source": "The contact patch test: a contact discretisation must transmit a uniform pressure across a non-matching interface exactly; Taylor & Papadopoulos, On a patch test for contact problems in two dimensions, Nonlinear Computational Mechanics, Springer 1991, 690–702; Crisfield, Re-visiting the contact patch test, IJNME 48:435–449, 2000; passed by mortar methods: Puso & Laursen, CMAME 193:601–629, 2004. Uniaxial stress between frictionless symmetry planes is uniform in both blocks",
"quantities": [
{
"id": "small_lower_corner",
"quantity": "Small: σ_zz of the lower block's corner element under the interface (element 10) over that of the upper block's corner element over it (element 19)",
"measure": {
"command": "solve.probe",
"params": {
"step": "Small",
"elements": [
10
]
},
"pointer": "/elements/0/stress/0/2",
"over": {
"command": "solve.probe",
"params": {
"step": "Small",
"elements": [
19
]
},
"pointer": "/elements/0/stress/0/2"
}
},
"value": 1.0,
"tolerance": {
"relative": 1e-06
},
"reason": "small sliding is linear and its mortar integrals exact on these rectangular faces: a uniform stress is the discrete solution up to the convergence tolerance (10⁻¹⁰)"
},
{
"id": "small_lower_centre",
"quantity": "Small: σ_zz of the lower block's centre element under the interface (element 14) over that of element 19",
"measure": {
"command": "solve.probe",
"params": {
"step": "Small",
"elements": [
14
]
},
"pointer": "/elements/0/stress/0/2",
"over": {
"command": "solve.probe",
"params": {
"step": "Small",
"elements": [
19
]
},
"pointer": "/elements/0/stress/0/2"
}
},
"value": 1.0,
"tolerance": {
"relative": 1e-06
},
"reason": "as for the corner element"
},
{
"id": "small_lower_base",
"quantity": "Small: σ_zz of the lower block's bottom corner element (element 1) over that of element 19",
"measure": {
"command": "solve.probe",
"params": {
"step": "Small",
"elements": [
1
]
},
"pointer": "/elements/0/stress/0/2",
"over": {
"command": "solve.probe",
"params": {
"step": "Small",
"elements": [
19
]
},
"pointer": "/elements/0/stress/0/2"
}
},
"value": 1.0,
"tolerance": {
"relative": 1e-06
},
"reason": "as for the corner element"
},
{
"id": "small_upper_top",
"quantity": "Small: σ_zz of the upper block's top far corner element (element 26) over that of element 19",
"measure": {
"command": "solve.probe",
"params": {
"step": "Small",
"elements": [
26
]
},
"pointer": "/elements/0/stress/0/2",
"over": {
"command": "solve.probe",
"params": {
"step": "Small",
"elements": [
19
]
},
"pointer": "/elements/0/stress/0/2"
}
},
"value": 1.0,
"tolerance": {
"relative": 1e-06
},
"reason": "as for the corner element"
},
{
"id": "small_lateral",
"quantity": "Small: σ_xx of element 14 over its σ_zz (uniaxial stress: 0)",
"measure": {
"command": "solve.probe",
"params": {
"step": "Small",
"elements": [
14
]
},
"pointer": "/elements/0/stress/0/0",
"over": {
"command": "solve.probe",
"params": {
"step": "Small",
"elements": [
14
]
},
"pointer": "/elements/0/stress/0/2"
}
},
"value": 0.0,
"tolerance": {
"absolute": 1e-06
},
"reason": "frictionless symmetry planes and a frictionless interface leave the blocks free to expand sideways; the non-matching interface must not hold them"
},
{
"id": "small_pressure",
"quantity": "Small: the contact pressure at the secondary node (0.5, 0.5, 1) over −σ_zz of element 19",
"measure": {
"command": "solve.probe",
"params": {
"step": "Small",
"at": [
[
0.5,
0.5,
1.0
]
]
},
"pointer": "/nodes/0/contact/0/pressure",
"over": {
"command": "solve.probe",
"params": {
"step": "Small",
"elements": [
19
]
},
"pointer": "/elements/0/stress/0/2",
"scale": -1.0
}
},
"value": 1.0,
"tolerance": {
"relative": 1e-06
},
"reason": "the mortar multipliers are the nodal values of the contact pressure, uniform and equal to the stress across the interface"
},
{
"id": "small_force",
"quantity": "Small: the integrated contact pressure over the reaction at the pushed top",
"measure": {
"from": "small",
"pointer": "/contacts/0/normal_force",
"over": {
"command": "results.reactions",
"params": {
"step": "Small",
"sets": [
"top"
]
},
"pointer": "/sets/0/force/2",
"scale": -1.0
}
},
"value": 1.0,
"tolerance": {
"relative": 1e-06
},
"reason": "the pressure carries the whole load across the interface"
},
{
"id": "finite_lower_corner",
"quantity": "Finite: σ_zz of the lower block's corner element under the interface (element 10) over that of the upper block's corner element over it (element 19)",
"measure": {
"command": "solve.probe",
"params": {
"step": "Finite",
"elements": [
10
]
},
"pointer": "/elements/0/stress/0/2",
"over": {
"command": "solve.probe",
"params": {
"step": "Finite",
"elements": [
19
]
},
"pointer": "/elements/0/stress/0/2"
}
},
"value": 1.0,
"tolerance": {
"relative": 0.0002
},
"reason": "finite sliding is exact up to the geometric terms of the gaps the augmentation leaves (10⁻⁵ of the element thickness) and the default convergence tolerances"
},
{
"id": "finite_lower_centre",
"quantity": "Finite: σ_zz of the lower block's centre element under the interface (element 14) over that of element 19",
"measure": {
"command": "solve.probe",
"params": {
"step": "Finite",
"elements": [
14
]
},
"pointer": "/elements/0/stress/0/2",
"over": {
"command": "solve.probe",
"params": {
"step": "Finite",
"elements": [
19
]
},
"pointer": "/elements/0/stress/0/2"
}
},
"value": 1.0,
"tolerance": {
"relative": 0.0002
},
"reason": "as for the corner element"
},
{
"id": "finite_lower_base",
"quantity": "Finite: σ_zz of the lower block's bottom corner element (element 1) over that of element 19",
"measure": {
"command": "solve.probe",
"params": {
"step": "Finite",
"elements": [
1
]
},
"pointer": "/elements/0/stress/0/2",
"over": {
"command": "solve.probe",
"params": {
"step": "Finite",
"elements": [
19
]
},
"pointer": "/elements/0/stress/0/2"
}
},
"value": 1.0,
"tolerance": {
"relative": 0.0002
},
"reason": "as for the corner element"
},
{
"id": "finite_upper_top",
"quantity": "Finite: σ_zz of the upper block's top far corner element (element 26) over that of element 19",
"measure": {
"command": "solve.probe",
"params": {
"step": "Finite",
"elements": [
26
]
},
"pointer": "/elements/0/stress/0/2",
"over": {
"command": "solve.probe",
"params": {
"step": "Finite",
"elements": [
19
]
},
"pointer": "/elements/0/stress/0/2"
}
},
"value": 1.0,
"tolerance": {
"relative": 0.0002
},
"reason": "as for the corner element"
},
{
"id": "finite_lateral",
"quantity": "Finite: σ_xx of element 14 over its σ_zz (uniaxial stress: 0)",
"measure": {
"command": "solve.probe",
"params": {
"step": "Finite",
"elements": [
14
]
},
"pointer": "/elements/0/stress/0/0",
"over": {
"command": "solve.probe",
"params": {
"step": "Finite",
"elements": [
14
]
},
"pointer": "/elements/0/stress/0/2"
}
},
"value": 0.0,
"tolerance": {
"absolute": 0.0002
},
"reason": "frictionless symmetry planes and a frictionless interface leave the blocks free to expand sideways; the non-matching interface must not hold them"
},
{
"id": "finite_pressure",
"quantity": "Finite: the contact pressure at the secondary node (0.5, 0.5, 1) over −σ_zz of element 19",
"measure": {
"command": "solve.probe",
"params": {
"step": "Finite",
"at": [
[
0.5,
0.5,
1.0
]
]
},
"pointer": "/nodes/0/contact/0/pressure",
"over": {
"command": "solve.probe",
"params": {
"step": "Finite",
"elements": [
19
]
},
"pointer": "/elements/0/stress/0/2",
"scale": -1.0
}
},
"value": 1.0,
"tolerance": {
"relative": 0.0002
},
"reason": "the mortar multipliers are the nodal values of the contact pressure, uniform and equal to the stress across the interface"
},
{
"id": "finite_force",
"quantity": "Finite: the integrated contact pressure over the reaction at the pushed top",
"measure": {
"from": "finite",
"pointer": "/contacts/0/normal_force",
"over": {
"command": "results.reactions",
"params": {
"step": "Finite",
"sets": [
"top"
]
},
"pointer": "/sets/0/force/2",
"scale": -1.0
}
},
"value": 1.0,
"tolerance": {
"relative": 0.0002
},
"reason": "the pressure carries the whole load across the interface"
},
{
"id": "small_stress",
"quantity": "Small: σ_zz of element 19 (uniaxial stress E ε = −10 for the 1% squeeze)",
"measure": {
"command": "solve.probe",
"params": {
"step": "Small",
"elements": [
19
]
},
"pointer": "/elements/0/stress/0/2"
},
"value": -10.0,
"tolerance": {
"relative": 0.0001
},
"reason": "the squeeze less the penetration the augmented Lagrangian leaves (up to 10⁻⁵ of the element thickness, a few 10⁻⁵ of the squeeze)"
}
]
}