Femap neutral file (.neu)

The neutral file is Femap's documented ASCII interchange format, and the only open route into or out of a Femap model: a .modfem model is exported to it by Femap (Femap .modfem route, v16.13.0). Solvers write their results to it too: NX Nastran through Femap, MYSTRAN and EMSolution. It is a sequence of data blocks. Each opens with a line holding -1, then the block id, and closes at the next -1 line. Records are comma-separated.

Format name	femap
Extensions	.neu (also recognised by content: a lone -1 line, then 100)
Read / Write	✓ (mesh, groups and results) / ✓ (mesh, groups and results; a time series as one output set per step since v16.17.0)
Extra dependencies	—

Reading & writing

Native comma fields view the mapped/buffered source and real numbers use bounded parsing. Block traversal shares a private record cursor while retaining grammar, line diagnostics and owned titles/regions/results. Writer bytes are unchanged; see the token-view comparison and cursor consolidation evidence.

import meshioplusplus

mesh = meshioplusplus.read("model.neu")                   # the first output set
last = meshioplusplus.read("model.neu", time_step=-1)     # the last one
times = meshioplusplus.femap.time_values("model.neu")     # one value per output set
meshioplusplus.write("out.neu", mesh)                     # Femap 8.2 layout, mesh and results

read also takes points_only and arrays, which narrow which output vectors are read. Both engines (the C++ core and the pure-Python reference) read the same meshes and write the same bytes.

Blocks

Record layouts change with the Femap version in block 100. The reader reads every record by position and length rather than by version, so one reader covers Femap 4.41 to 2401 (24.1):

Block	Read as
100 header	the version
403 nodes	points: x, y, z are fields 11–13 in every version
404 elements	cells. A record is seven lines, then, from version 4.5, one node list per non-zero list flag (rigid and weld elements), each ending at a -1 line. The element's property and type become the femap:property and femap:type cell data
402 properties	their titles name the property_<id> cell regions (tag = property id). From Femap 2401 a record repeats its value count and lists as many integers (function references) before the outline counts; the reader recognises that list by its shape
408 groups	a group's node list becomes a point region and its element list a cell region of the group's title (tag = group id). Its rules are not evaluated: the lists are what the group holds
450 output sets	steps: time_step selects one, and its value is field_data["meshio:time"] (with femap:set holding its id). The lines that follow a set's notes change with the version (none up to 9, one in 11.0, three from 11.2, seven in 2020.1), so the reader finds the next record by its shape
451 / 1051 output vectors	the selected set's vectors become point_data (nodal, entity type 7) or cell_data (elemental, 8), named by their titles, NaN where a vector has no value. 451 holds id,value records; 1051 holds start,end,value… ranges, and in Femap 11 also id,value records
anything else (materials, loads, constraints, views…)	skipped

Element topologies

Femap stores every element's nodes in 20 slots laid out as a degenerate brick: corners 0–3 on the bottom and 4–7 on the top, mid-edge nodes 8–11 on the bottom edges, 12–15 on the vertical edges and 16–19 on the top edges. A tetrahedron's apex is therefore slot 4, and its mid-edge nodes are the brick's.

Topology	meshio++ type	Slots, in meshio++ node order
0 / 1	line / line3	0 1 / 0 1 2
2 / 3	triangle / triangle6	0 1 2 / 0 1 2 4 5 6
4 / 5	quad / quad8	0–3 / 0–7
6 / 10	tetra / tetra10	0 1 2 4 / 0 1 2 4 8 9 10 12 13 14
7 / 11	wedge / wedge15	0 1 2 4 5 6 / 0 1 2 4 5 6 8 9 10 16 17 18 12 13 14
8 / 12	hexahedron / hexahedron20	0–7 / 0–11 16–19 12–15
9	vertex	0
14	pyramid	0–4

Rigid (13, 18), multi-list (15), contact (16) and weld (17) elements are skipped with one warning per topology, as is the 13-node pyramid (19), whose slots are not documented anywhere open.

Writing

• The Femap 8.2 layout, the one the fixtures verify: blocks 100 (its title carries the provenance tag), 402, 403, 404, 408, and 450/451 for the results (v16.11.0).
• Each element's property is femap:property (else 1) and its type femap:type (else 1 rod, 17/18 linear/parabolic plate, 25/26 linear/parabolic solid, or 27 mass, by topology). A property's title comes from the cell region the reader makes of it: same tag, same cells. Other point and cell regions become groups, and a point region and a cell region with the same name share one group.
• Coordinates are written with 17 significant digits, which round-trips a double.
• Results (v16.11.0): one output set (450) whose id is femap:set (else 1) and whose value is meshio:time (else 0), holding one 451 vector per numeric point or cell data array, and one per component of a multi-component array (<name>_0, <name>_1, …): point arrays as nodal vectors (entity 7), cell arrays as elemental ones (entity 8). femap: arrays are not results; NaN values are left out of a vector, as Femap leaves out entities with no value. Reading the file back gives the same arrays, component by component; femap_neutral_parser reads every written value.
• Dropped, with a warning and a provenance note: non-numeric or ragged data arrays and other field data, side regions, and cells with no Femap topology (quad9, hexahedron27, polygons…).
• Deliberate limit: a multi-component array is written as one vector per component and reads back that way (U_0, U_1, U_2), not as one (n, 3) array. Femap's own files link a vector's components to a total vector (the component lines of a 451 record), which the reader leaves apart so that a Femap file reads as its vectors; writing the links would change how every Femap file reads.

Time series

meshioplusplus.write_sequence("run.neu", [(0.0, m0), (0.5, m1), (1.0, m2)])
with meshioplusplus.femap.SeriesWriter("run.neu") as w:   # the same, pushed step by step
    for time, mesh in steps:
        w.write(time, mesh)

Since v16.17.0 a series goes into one neutral file: the mesh blocks once, from the first step, then an output set (450) with its vectors (451) per step, the interleaved layout of the MYSTRAN and EMSolution files the reader already takes. read(..., time_step=k), femap.time_values and read_sequence give the steps back. A step's set id is its femap:set when positive and unused, else the next free one; its value is the step's time; a vector keeps its id across steps (by title and entity), and a step without vectors is still an output set, so there are as many sets as steps. Every step must have the first step's cells (a WriteError names the step that does not; write one file per step with {step} instead); a step whose points moved is written with the first step's, with a warning. Only one mesh is held at a time. Both engines write the same bytes: the C++ FemapSeriesWriter, bound as _core.FemapSeriesWriter, and the Python twin the core falls back to when it declines to open the file. The native sequence_to_timeseries and pipeline fan-in take .neu too.

Errors

A file with no nodes (a geometry-only or results-only file has no mesh to read), a truncated or malformed record, a node or element id defined twice, an element that names an undefined node or leaves a slot its topology needs empty, and an output set that does not exist raise ReadError, naming the line. A group record that does not fit the layout ends that block's reading with a warning; the groups read before it are kept.

Verification

No Femap installation was available. The layouts follow FrontISTR's neu2fstr converter and were checked against real files, all MIT-licensed:

• 36 Femap 8.2 models from FrontISTR, covering every solid topology, with node groups. The tests check that every mid-edge node lands on its meshio++ edge midpoint.
• EMSolution's 4.41 files, including pyramids, with 13 output sets written both as 451 and as 1051, which the tests read to the same arrays.
• Result blocks written by Femap 2020.1, Femap 8.2 and MYSTRAN, from femap_neutral_parser. All 2,770 values of the 8.2 and MYSTRAN files equal femap_neutral_parser's own reading.

Real Femap 9.1–11.1 exports and Femap 2401 files (public, but without a licence, so not in the tests) were also read locally (v16.11.0): the 11.0 and 11.1 laminate models with their 1051 results, and the 2401 property layout, which led to the change above. Still unverified: element and group records from Femap 10 and from 12 on (the 2401 files hold geometry and properties but no elements), and the 13-node pyramid; no public file has one. A written file has not been imported into Femap.