Inputs, outputs and limits

Browser and access

Nothing to install: Skalnav runs in the browser. Current desktop versions of Chrome, Edge and Firefox are tested; Safari is supported on a best-effort basis; phones and tablets are limited, because the application is built for desktop work. JavaScript and cookies must be enabled.

Units

SI and imperial presets: mm and MPa with kN or N, or inches with ksi and kips, or psi and lbf. The file formats below keep fixed units.

FE load import (CSV)

Export the section resultants at the cut you care about from your global FE model; native solver files are not read. FE load import needs Professional or higher (Professional and 30-Day Project Access: up to 5 load cases per import; Team and above: no such cap). If a file holds more load cases than your plan allows, the dialog says how many stay behind before you import.

What your post-processor must write

The banner # skalnav-fe-loads/1 first; a newer major version is refused, not guessed at.
A header row, then one row per load case and section, in kN, kN·m and MPa. Comma, semicolon or tab; a decimal point, no thousands separator, no quoting.
Sign: compression positive. A tension-positive export is flipped once you declare it in the dialog; an all-negative axial column is an error.
Frame: resultants about the cylinder axis.
The # units:, # sign: and # frame: lines only pre-select the dialog, where you attest the convention: a file cannot attest to its own convention; the person whose name goes on the report does that.
Optional # geometry: and # material: lines are cross-checked against the shell on screen; a mismatch is a red finding, and a factor of exactly two is named as a radius/diameter mix-up.

# skalnav-fe-loads/1
# units: force=kN, moment=kNm, pressure=MPa
# sign: compression_positive
# frame: cylinder_axis
# geometry: radius_mm=1800, thickness_mm=4, length_mm=3200
# material: E_MPa=73100, sigma_y_MPa=393, nu=0.33
load_case,section_id,N_kN,M_kNm,p_int_MPa,p_ext_MPa,T_kNm,notes
lc1,S1,1800,2400,0.25,,,
lc2,S1,2450,3900,0.25,,,
lc3,S1,950,,,,180,

Example file with illustrative values.

Columns of skalnav-fe-loads/1

Column	Required	Unit	Meaning
`load_case`	yes	—	Becomes the load-case name
`section_id`	yes	—	The cut the row belongs to; you pick one section at import
`N_kN`	yes	kN	Axial force, compression positive
`M_kNm`	no	kN·m	Bending moment, as a magnitude
`p_int_MPa`	no	MPa	Internal pressure, unsigned
`p_ext_MPa`	no	MPa	External pressure or vacuum, unsigned
`T_kNm`	no	kN·m	Torsion
`notes`	no	—	Free text, carried into the load case; it must not contain the delimiter

Empty optional cells count as zero. Unrecognised columns are ignored and named in a warning. Each pair of load case and section may appear only once.

Each row becomes one load case: the narrowest combination that covers every non-zero resultant. Combinations Skalnav has no load case for are not imported — internal pressure alone, internal and external pressure together, axial force with bending and external pressure, and torsion with anything other than axial force or bending alone — and the dialog names row and reason. Torsion with axial force is assessed through a linear interaction, flagged as an approximation; rows with bending and external pressure, or with bending and torsion, are imported but get no reserve factor yet.

Element fluxes: `skalnav-fe-fluxes/1`

Skalnav can also condense element fluxes into section resultants, in the cylinder frame only: element-local frames are rejected.

Format, example file and columns of skalnav-fe-fluxes/1

One row per element, with its station x_mm, hoop angle theta_deg and fluxes in N/mm (Nx, Ny, Nxy) and N·mm/mm (Mx, My, Mxy). The # frame: cylinder_axis line and the radius are mandatory, and each station must cover the full ring. Hoop compression is reported, never applied as a pressure.

# skalnav-fe-fluxes/1
# units: force_flux=N/mm, moment_flux=Nmm/mm; length=mm; angle=deg
# sign: tension_positive
# frame: cylinder_axis
# geometry: radius_mm=1800, thickness_mm=4, length_mm=3200
load_case,element_id,x_mm,theta_deg,Nx,Ny,Nxy,Mx,My,Mxy,notes
lc1,e0,800,0,-252.800,-3.600,0,0,0,0,
lc1,e1,800,15,-250.756,-3.600,0,0,0,0,
lc1,e2,800,30,-244.762,-3.600,0,0,0,0,
# … one row per element, the full ring at every station

Column	Required	Unit	Meaning
`load_case`	yes	—	Groups the element rows; becomes the load-case name
`element_id`	no	—	Element label
`x_mm`	yes	mm	Axial station; values within about 1 % of the span form one station
`theta_deg`	yes	deg	Hoop position; the full ring must be present
`Nx`	yes	N/mm	Axial membrane flux, in the cylinder frame
`Ny`	no	N/mm	Hoop membrane flux; mean compression is reported, not applied
`Nxy`	no	N/mm	Membrane shear flux; gives the torsion resultant
`Mx`, `My`, `Mxy`	no	N·mm/mm	Plate-bending fluxes
`notes`	no	—	Free text

The condensed rows then pass the same sign attestation, cross-check and mapping as skalnav-fe-loads/1.

Measurement and batch files

Measured imperfections (from Professional). A point cloud, contour image or Fourier field becomes the imperfection amplitude.

Batch studies (from Team). A skalnav-batch/1 CSV holds one complete cylinder case per row.

Formats and limits

Point clouds from CMM, laser or photogrammetry as plain-text XYZ (.txt, .csv, .xyz, .asc, .pts) up to 40 MB, for cylinders and spheres; a published contour plot of an unrolled cylinder as PNG or JPEG up to 20 MB, with its colour-bar range (required).

Batch rows carry geometry, material, imperfection amplitude and loads in mm, MPa, kN and kN·m, compression positive, up to 500 rows; an empty geometry or material cell takes the shell on screen.

Outputs

Reports: PDF and Word, from Professional, with inputs, method, design basis, reserve factors, the engineering disclaimer and the engine version.
Data: CSV and JSON exports of the results.
Imperfection fields: from the measured classes of the Imperfection Hub, exported to seed your FE model.
Validation Hub: a campaign export (JSON or text) with its sources.
Report archive: issued reports are kept without an edit path, for re-download and a re-check against today’s engine.

API

A limited v1 REST API for Enterprise Design Partners: token-authenticated ping and MDC cylinder axial-compression compute.

GET /api/v1/ping — checks the token and returns the caller’s identity
POST /api/v1/compute/cylinder — the MDC knockdown and design load of a cylinder under axial compression, on the design basis you choose

Tokens are created in the app and sent as a bearer token or an X-API-Key header; responses carry the engine version. There is no availability commitment for the API. See Enterprise.

Known limits

Skalnav supports engineering decisions; responsibility stays with the engineer, and results are not a sole basis for approval.
Design-code comparisons are implementations of the cited documents, not official certificates.
Materials: isotropic metals, with the MDC plasticity correction; composite laminates as stated below.
Imperfection amplitude: cylinders and cones are served up to w/t = 8; larger amplitudes are refused. Spheres and torispherical heads are fitted up to w/t = 2; above it the knockdown is extrapolated, and the app warns that the result may be non-conservative. A Batdorf Z outside the fitted range is clamped to its edge, and the app shows a banner.
Very thick walls in the plastic range (roughly R/t below 50) lie outside the main fitting range; read results there conservatively.
External pressure with an out-of-roundness above about 2 % needs explicit treatment: the MDC knockdown does not cover it, and the app’s optional ovality check is advisory.
Cutouts: cylinders under axial compression, calibrated around R/t ≈ 330, Z 100–5,000, opening ratio up to 0.7 and up to 8 openings.
Composite laminates: the anisotropy correction is Beta and calibrated for cylinders under axial compression only; elsewhere the app marks values as not for design.
The app warns when a design lies outside the range of the tests MDC is checked against (the Heritage Check, with Professional).

Which shells and load cases carry a released MDC knockdown: the coverage matrix.