Suitability of Stiffened Sheet Metal as Against a Plate Metal in Fabrication Works: A scientific approach towards metal craft work

Suitability of Stiffened Sheet Metal as Against a Plate Metal in Fabrication Works: A scientific approach towards metal craft work

Subramanya D. Sanbhat

Fabrication Technology & Erection Engineering Department Fr. Agnel Polytechnic, Vashi

Navi Mumbai, India

AbstractThis paper is an attempt to deduce on the economical selection of a plate versus sheet metal for use in fabrication works. The method adopted is theory and practical applications of the principles of Strength of Materials (SOM) to a plate, a sheet metal and a stiffened sheet metal. The results confirmed that in spite of less rigidity due to less thickness; sheet metal strength could be enhanced considerably as against the strength of a thick plate by appropriately applying the principles of stiffening. This ability of increasing the strength/weight ratio by stiffening sheet metals helps in cost control on the shop floor.

KeywordsDeduce, Plate, Sheet metal, SOM, Stiffening

1. INTRODUCTION

   Material	Elastic Moduls, E MN/m2	Density, r Kg/m3	Tensile Stiffness Index, Ew= E/r MNm/kg	Bending Stiffness Index, Sw= Ew/12.r2 Nm7/kg3
   Steel	210000	7800	25	0,03
   CF composites	200000	2000	100	2,00
   Paper	15600	700	22	3,70

   Material	Elastic Moduls, E MN/m2	Density, r Kg/m3	Tensile Stiffness Index, Ew= E/r MNm/kg	Bending Stiffness Index, Sw= Ew/12.r2 Nm7/kg3
   Steel	210000	7800	25	0,03
   CF composites	200000	2000	100	2,00
   Paper	15600	700	22	3,70

   The basic principle of stiffening can be illustrated by supporting a tumbler of water on a piece of note paper bridging two other tumblers as shown in the Fig. 1 and Fig. 2 below. From the figures it is clear that a sheet metal panel will not support a very great load due to the thinness of the material. A metal plate of the same surface area will support a fairly substantial load because of its extra thickness. But, although the metal plate is much more rigid than the sheet metal panel, this rigidity is obtained at the expense of considerable additional weight. Therefore, more the weight, more the rigidity and as per the prevailing market cost rate per kg, more would be the cost.

   Original piece of note paper corrugated by folding

   Figure 2. Note paper stiffened by corrugation

   Strength/weight ratio is an important factor in the fabrication industry and by stiffening the sheet metal in an appropriate manner, it is possible to produce a multiplicity of light fabrications which are very rigid and strong. The comparative parameters as from [1], to justify the above illustration for stiffening using a corrugated note paper in lieu of a sheet metal are as shown in Table 1. below.

   TABLE 1. COMPARISON OF STEEL AND NOTE PAPER

   Note paper

   Empty tumblers

   Figure 1. Note paper

   Tumbler of water

   1. Literature Review

      The following conclusions reviewed from [2] [14] on the sheet metals types, alternative materials, fabrication processes, methods for stiffening sheet metal components, etc. are compiled in the Table 2 to be utilized for defining the objectives of this paper.

      TABLE 2. METHODS OF STIFFENING

      	Pros	Cons	Remarks
      A. Plate component	More rigidity More stiffness	More weight More cost	Not economical
      B.	Less weight	Less rigidity	Not
      Sheet	Less cost	Less stiffness	economical
      component			unless used
      used as it is			in proper
      (without			orientation
      stiffening)			with respect
      			to applied
      			load
      C.	Less weight	Additional	Economical
      Sheet	Less cost	cost of	but subject to
      component	More rigidity	fabrication or	conditions in
      fabricated (less	More stiffness	changeover to composite	fabrication process used
      thickness)		materials	(Refer Figure
      			5.)
      D.	More weight	Additional	Economical
      Sheet	More cost	cost of	but subject to
      component fabricated (medium	More rigidity More stiffness	fabrication or changeover to composite	conditions in fabrication process used.
      thickness		materials	(Refer Figure
      but less			6.)
      than for a
      plate)
      E.	Less weight	Additional	Economical
      Web	Less cost	cost of	but subject to
      stiffeners	More rigidity	fabrication or	conditions in
      (structural sections)	More stiffness	changeover to composite	fabrication process used.
      		materials	(Refer Figure
      			7.)

   2. Objective

   Study of the stiffening methods by applying the principles of Strength of Materials and hence conclude their suitability for fabrication in metal craft works with relevant illustrations that depict;

   • Improved strength and rigidity for the intended function

   • Safe edge for handling

   • Aesthetic look for marketing

2. RESULTS AND DISCUSSION

   First method: Utilize the existing geometry available to improve strength but by employing a different orientation and same loading as shown in Fig. 3 and the comparison of the deflection parameter for both the cases is as shown in Table 3.

   Figure 3. Stiffening using change in orientation

   TABLE 3. COMPARISON OF TWO ORIENTATIONS FOR LOADING

   /tr>

   Sr. No s.	Variables	Assumed values	Deflection y*10-6mm	Moment of Inertia I
   1	Loading perpendicul ar to L*W	L= 50mm, B= 15mm, W= 20mm, E=2*105 N/mm2, P= 10N	9.26	14062.5mm4
   2	Loading perpendicul ar to B*W		0.02	156250mm4

   Thus, the second position with a higher Moment of Inertia results in a lesser amount of downward deflection. The results so obtained may be deduced that the deflection of a component under a load reduces when the extreme fibers of the component lies far away from the neutral axis. The more farther the distance more is the ability to sustain any bending load. This obviously implies that any plate material having considerable thickness with respect to other dimensions is able to withstand bending loads very easily unlike a sheet metal. But, the fabricability of a sheet metal to be folded, joined by bolts, rivets, adhesives, etc. enables the above condition to be achieved as shown in Fig. 4.

   Figure 4. Principle of stiffening

   The above method of corrugation of the note paper (analogous to a sheet metal) helps to separate the extreme fibers to lie much more beyond the neutral axis as compared to before the corrugation. This is similar to a plate metal which also has its extreme fibers far away from the neutral axis. So, stiffening obtained improves strength with less weight as compared to a similar plate with similar strength but more weight as seen from Table 4. below.

   TABLE 4. COMPARATIVE STIFFNESS FOR CORRUGATED SHEET

   METAL

   Sandwich panel	Load applied (kN)	Total deformation (mm)	Stiffness (kN/mm)	Total weight (N)
   3 curve	13.5	0.215	62.79	161.7
   4 curve	13.5	0.042	321.43	168.7

   (Source: IJERT, Vol. 1 Issue 8, October 2012)

   Hence, for a given length and height of the structure, increasing the number of curved waves (3 waves to 4 waves) the strength increases effectively. For increase of 4% weight, the strength is increased to 66% and increase in stiffness to 80.47%. Hence, from the above discussion when the number of curved waves increases to infinity (i.e. manifests as a solid plate) the tremendous increase in strength, stiffness and weight are evident.

   Second method: In this method, the same reasoning as in the First method is achieved by adopting fabrication methods on the sheet metal by folding/hem (single and/or double), punching a lightening hole, etc. Consider a single fold given to the sheet metal on all the four sides to attain strength and rigidity in both planes of bending as in Fig. 5. Now applying the principles of strength of materials we have the comparative figures in favor of the fold as in Table 5.

   Figure 5. Stiffening by fold

   Sr. Nos.	Variables	Assumed values	Deflection y mm	Moment of Inertia I
   1	Loading perpendicular	t= 2mm, W=	0.0099	53.33 mm4
   	to W*t	80mm
   	(W= 80mm)	(50mm
   2	Loading perpendicular	after	0.0025*10-2	5102.39 mm4
   		folding),

   Sr. Nos.	Variables	Assumed values	Deflection y mm	Moment of Inertia I
   1	Loading perpendicular	t= 2mm, W=	0.0099	53.33 mm4
   	to W*t	80mm
   	(W= 80mm)	(50mm
   2	Loading perpendicular	after	0.0025*10-2	5102.39 mm4
   		folding),

   TABLE 5. COMPARISON OF TWO ORIENTATIONS FOR LOADING

   	to W*t (W= 50mm)	E=2*105 N/mm2, P= 10N
   2	Loading perpendicular to L*t (Assume, L= W= 50mm)		0.0025*10-2	5102.39 mm4

   Third method: In this method, applicable for thicknesses that disable the sheet metals to undergo folding and/or bending operations, the above reasoning is again achieved by applying stiffeners/reinforcements at the required locations to attain strength and rigidity in both planes of bending as in Fig. 6 and comparative figures in favor for the same as in Table 6. The necessary stiffeners may be fabricated separately and then assembled by means of rivets or bolted joints.

   Figure 6. Stiffening by hem or applied stiffener

   TABLE 6. COMPARISON OF TWO ORIENTATIONS FOR LOADING

   Sr. No s.	Variables	Assumed values	Deflection y mm	Moment of Inertia I
   1	Loading perpendicular	t= 2mm, W= 80mm	0.0099	53.33 mm4
   	to W*t	(50mm
   	(W= 80mm)	after
   2	Loading perpendicular to W*t	folding), E=2*105 N/mm2,	0.064*10-2	203.33 mm4
   	(W= 50mm)	P= 10N
   2	Loading perpendicular		0.064*10-2	203.33 mm4
   	to L*t
   	(Assume, L=
   	W= 50mm)

   Another category in this method involves stiffening of structural sections, especially I sections/columns/beams with added stiffeners which may be conceptualized as a combination of I – section and a rectangular solid beam as in Fig. 7 and then compared as shown in Table 7.

   Little

   depth Web

   Sideways movement (Thrust)

3. CONCLUSION

Fabrication works make use of plate and sheet metals. The various fabrication processes involved are marking/measuring, cutting, folding/bending, rolling, presswork, joining, cleaning/painting. Depending upon the end use/application of the fabricated component an evaluation of the strength of stiffened sheet metal in terms of reduced deflection subjected to a point load as against a plate material helps offset the associated disadvantages of sheet metal. This implies that the cost of employing the appropriate stiffening operation (material & labor cost) ought to be less than cost of the plate material. This again implies that the stiffening operation ought to be as simple as possible. Some illustrations from [5] for the same are as given below to complete the objectives of the paper.

Greater depth

Web

Stiffening methods for small thickness are labor intensive and involves manual interventions as shown in Fig. 8 below.

Sideways movement (Thrust)

Figure 7. Web stiffening

TABLE 7. COMPARISON OF STRUCTURAL SECTION

Not economical

ALTERNATIVES

1. Folds/Hems

2. Diamond break Figure 8. Sheet component fabricated (less thickness)

Stiffening methods for more thickness are also labor intensive and involves manual/ machine interventions as seen in the Fig. 9 and Fig. 10 (for structural sections) below.

1. Circular component stiffening (before or after rolling)

   1. Large panel stiffening

2. Stiffening thick sheet metals

   Figure 9. Sheet component fabricated (medium to less than plate thickness)

   1. Welded joints

   2. I- beam of plates bolted/riveted Figure 10. Methods of web stiffening

Stiffening a flange is done when a single fillet weld has to be used, the other side being inaccessible for welding and if otherwise this single weld would be subjected to bending as shown in the Fig. 11 below.

1. Stiffening for welded flanged connection

2. Stiffening of welded connection between two I-section members Figure 11. Stiffening with gusset plates

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