A developable surface is a special ruled surface which has the same tangent plane at all points along a generator [13,222,120,32,326,252,329]. Since surface normals are orthogonal to the tangent plane and the tangent plane along a generator is constant, all normal vectors along a generator are parallel. This is shown in Fig. 9.11.
A developable surface has following differential geometry properties [412]:
A developable surface can be formed by bending or rolling a planar surface without stretching or tearing; in other words, it can be developed or unrolled isometrically onto a plane. Developable surfaces are also known as singly curved surfaces, since one of their principal curvatures is zero. Developable surfaces are widely used with materials that are not amenable to stretching. Applications include the formation of ship hulls, ducts, shoes, clothing and automobile parts such as upholstery, body panels and windshields [120].
As indicated by Munchmeyer and Haw [281], a developable surface can be shaped purely by rolling and should be fed to the roller so that the direction of the zero principal curvature is parallel to the rolls. However, when the sheet reaches a line of inflection, it can no longer be fed into the roller in the same direction because the direction of bending changes. Therefore, it is beneficial for planning the fabrication process to determine the lines of inflection prior to such a process.
Surface inflection of a developable surface was studied by Hoitsma [172] who showed that a surface has an inflection at a point if and only if its mean curvature changes sign in the neighborhood of . Maekawa and Chalfant [251] further extended this result and derived two theorems (Theorem 9.7.1 and 9.7.2).
Since the Gaussian curvature of a developable surface is zero
everywhere [412,76], the maximum and minimum
principal curvatures (3.49) and (3.50)
of a developable surface can be written as
(9.77) |
In the following we establish some elementary differential geometry
properties of developable surfaces. We
assume that the developable surface is regular and the
iso-parametric
line corresponds to the generator of the developable surface or, in
other words, the straight line ruling is in the
direction. With
this assumption,
, and hence the second
fundamental form coefficient (see (3.28))
vanishes. From (3.46), since
Gaussian curvature of a developable surface is zero,
Next, we will show that the parametric straight lines become the lines of zero curvature. This can be seen from the fact that i) the iso-parametric straight lines have zero normal curvature, and ii) no other direction has zero normal curvature. The second fact comes from Euler's theorem introduced in Sect. 3.6. When , (3.87) reduces to , which becomes zero only when or , corresponding to the direction of . Similarly, when , only when or , corresponding to the direction of .
Theorem 9.7.1. A developable surface does not possess generic isolated flat points 9.5 but rather may contain a line of non-generic flat points along a generator [251].
Proof:
From (9.79) and (9.82),
vanishes
at a flat point
where both principal curvatures are zero.
Therefore from the first equation of (3.27) we have
(9.86) |
For a developable surface, the inflection line is a generator
which consists of a line of flat points and the nonzero principal
curvature changes sign. The inflection line can be detected by
finding
such that
where
is an arbitrary
constant between 0 and 1.
can be
written as
The local approximation (8.73) will now be applied to developable surfaces.
Lemma 9.7.1. A developable surface is, in general, locally a parabolic cylinder except at an inflection line, where it becomes a cubic cylinder, provided that [251].
Proof: Let us consider an orthogonal Cartesian reference frame
-
attached to the surface
at an
arbitrary point
with
being
. We
choose unit vectors
,
and
at
as the directions
of
,
and
axes such that the
axis coincides with the
generator
, the
axis coincides with the surface normal vector and
the
axis is orthogonal to both axes. Therefore the local
coordinates
,
and
are given by
The second fundamental form coefficient
in terms of the height
function
is given in (3.65) as
. Since
on a
developable surface (see (9.81)), we have
. Furthermore, all the second and higher order partial derivatives
with respect to
vanish, since the
axis corresponds to the
generator, which is linear in
. Thus
(8.73) reduces to
A developable surface is said to have contact of order with the tangent plane along the generator if the Taylor expansion for starts with terms of degree . The ordinary inflection line (see (9.93)) thus has a contact of order . If the tangent plane has contact of order with the surface along a generator, a developable surface may not look like a cubic cylinder at an inflection line.
If the developable surface has a contact of order
with the tangent
plane,
is zero or equivalently
is zero for
along the entire generator. Accordingly its variation
in
also vanishes; hence
for
along the
entire generator. Since all the second and higher order partial
derivatives with respect to
vanish, the Taylor expansion of the
height function along the higher order contact line reduces to