Section 1 Calculation procedure
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Clasifications Register Guidance Information - Guidance Notes for the Classification of Special Service Craft – Calculation Procedures for Composite Construction, July 2013 - Chapter 4 Design of Sandwich Panel Laminates - Section 1 Calculation procedure

Section 1 Calculation procedure

1.1 Consider the model of a side shell sandwich panel having dimensions 700 mm x 2000 mm shown in Figure 4.1.1 Sandwich panel laminate example model. Assume a design pressure of 50 kN/m2 and a panel of negligible curvature.

Figure 4.1.1 Sandwich panel laminate example model

1.2 The panel is to be designed in accordance with the requirements of Pt 8, Ch 3 Scantling Determination for Mono-Hull Craft of the Rules and Regulations for the Classification of Special Service Craft (hereinafter referred to as the Rules for Special Service Craft). Firstly it is necessary to estimate the core thickness using Pt 8, Ch 3, Pt 8, Ch 3, 1 General of the SSC Rules assuming a nominal value of tensile modulus of 11000 N/mm2 for sandwich panel facings:

Hence, select 20 mm core as a typically available size.

1.3 From Pt 8, Ch 3, Pt 8, Ch 3, 1 General of the Rules for Special Service Craft the maximum core shear at the mid-point along the edge of the sandwich panel is given by:

1.4 The panel aspect ratio correction factor is given in Pt 8, Ch 3, Pt 8, Ch 3, 1 General of the Rules for Special Service Craft. A R = 2000/700 = 2,86, hence k S = 1.

No correction factor applied since A R > 2.

1.5 To achieve an allowable shear stress fraction of 0,30, see Pt 8, Ch 7, Pt 8, Ch 7, 3 Stress control of the Rules for Special Service Craft, this design would require the selection of 200 kg/m3 foam core. Clearly being unacceptable the conclusion is to adopt a 50 mm thick core to reduce the core shear stress. The stress varies inversely with core thickness:

1.6 To achieve the required allowable shear stress fraction the foam core must have a core shear strength of 0,35/0,30 = 1,17 N/mm2. This may be achieved using a 100 kg/m3 foam core using 90 per cent of the manufacturer’s quoted value.

1.7 However, there are numerous solutions to achieve an acceptable arrangement. The options include:

  1. Repositioning the stiffeners to reduce panel sizes.

  2. Inclusion of shear ties in accordance with Pt 8, Ch 3, Pt 8, Ch 3, 1 General of the Rules for Special Service Craft.

  3. Accept the higher density core solution.

1.8 From Pt 8, Ch 3, Pt 8, Ch 3, 1 General of the Rules for Special Service Craft the skin thicknesses may be re-calculated allowing for the change in core thickness:

Clearly, the increase in core thickness gives a substantial increase in panel stiffness. Consequently the skin thicknesses default to the minimum requirement quoted in Pt 8, Ch 3, Pt 8, Ch 3, 3 Shell envelope laminate of the Rules for Special Service Craft. The minimum side shell sandwich skin thicknesses are:

Outer 4mm
Inner 3mm.

1.9 The minimum skin thicknesses relate to an assumed fibre content, f c, of 0,5. Where the fibre content by weight is less than 0,5 the required minimum thickness are to be determined from Pt 8, Ch 3, Pt 8, Ch 3, 2 Minimum thickness requirements of the Rules for Special Service Craft.

1.10 In order to comply with the Rules for Special Service Craft, chopped strand mat reinforcements are required against the core. The initial proposal to meet the requirement, features such CSM reinforcements together with a CSM against the gel coat surface.

1.11 The proposed sandwich skin laminate schedule for the outer skin is:

1 x 450 g/m2 CSM @ Gc = 0,286 = 1,112 mm
3 x 600 g/m2 WR @ Gc = 0,5 = 2,202 mm
1 x 300 g/m2 CSM @ Gc = 0,33 = 0,625 mm
Total outer skin thickness = 3,94 mm
Total reinforcement weight = 2550 g/m2.

1.12 By transforming the relationship given in Pt 8, Ch 3, Pt 8, Ch 3, 1 General of the Rules for Special Service Craft the equivalent fibre content by weight, f c, is 0,42.

The adjusted minimum skin thickness for the outer skin is:

t outer = t 0,5(1,65 – 1,3f c)
= 4 x (1,65 – 1,3 x 0,42)
t outer = 4,42 mm.

1.13 The proposed outer skin is deficient by 4,42 – 3,94 = 0,48 mm. Hence one additional woven roving reinforcement may be added to achieve the minimum requirement. The actual thickness of outer skin is 4,67 mm.

1.14 The proposed sandwich skin laminate schedule for the inner skin is:

1 x 300 g/m2 CSM @ G c = 0,33 = 0,625 mm
4 x 600 g/m2 WR @ G c = 0,5 = 2,936 mm
Total inner skin thickness = 3,56 mm
Total reinforcement weight = 2700 g/m2

The equivalent overall fibre content of the inner skin is 0,473.

The adjusted minimum skin thickness for the inner skin is:

t inner = t 0,5(1,65 – 1,3f c)
= 3 x (1,65 – 1,3 x 0,473)
t inner = 3,1 mm.

1.15 The proposed arrangement which meets both core shear and minimum sandwich skin requirements is given in Table 4.1.1 Tabulation of sandwich panel calculations.

Table 4.1.1 Tabulation of sandwich panel calculations

  Ref No. Description G c Weight t lever @ E E.t E.t.x I @ EI @
(g/m2) (mm) base, x (mm) (N/mm2)     Base base
Inner skin 1 WR 0,5 600 0,734 57,87 14000 10276 594641 24579 344105690
  2 WR 0,5 600 0,734 57,13 14000 10276 587099 23959 335431718
  3 WR 0,5 600 0,734 56,40 14000 10276 579556 23348 326868472
  4 WR 0,5 600 0,734 55,67 14000 10276 572014 22744 318415951
  5 CSM 0,33 300 0,625 54,99 7200 4500 247435 18896 136054699
Core 6 100 kg/m3 - - 50,000 29,67 83,7 4185 124182 544410 45567128
  7 CSM 0,33 300 0,625 4,36 6950 4344 18941 119 827333
  8 WR 0,5 600 0,734 3,68 14500 10643 39177 100 1446879
  9 WR 0,5 600 0,734 2,95 14500 10643 31365 64 929103
  10 WR 0,5 600 0,734 2,21 14500 10643 23553 36 526005
Outer Skin 11 WR 0,5 600 0,734 1,48 14500 10643 15741 16 237588
  12 CSM 0,286 450 1,112 0,56 6290 6994 3889 5 28830
Totals         58,23     103699 2837592   1510439396
Inner skin in compression/outer skin in tension
Parent topic: Chapter 4 Design of Sandwich Panel Laminates
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