Section 4 Stresses in facings
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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 4 Stresses in facings

Section 4 Stresses in facings

4.1 Consider the revised model, see Figure 4.4.2 Sandwich skin laminate details, with 50 mm thick and 100 kg/m3 density core having the proposed side shell sandwich skins which comply with the Rules for Special Service Craft minimum requirements. As indicated in Figure 4.4.1 Regions of tension (T) and compression (C) in example model there will be positions where tension and compression considerations will apply. The relevant elastic modulus has been applied to the element dependent upon its relative position in the sandwich. The proposed schedule together with the tabular calculations are given in Table 4.1.1 Tabulation of sandwich panel calculations. Such calculations are ideally suited to computer based investigation.

4.2 The stiffness, EI, per 1 cm width is determined using the parallel axis theorem:

In general,

where

y = distance of neutral axis above base (mm).

4.3 It should be noted that the factor 10 (width in mm) is introduced to correct the value of area used in the parallel axis theorem, since a 1 cm wide strip of material is considered in the calculations.

4.4 From the tabulated calculations the overall stiffness of the section is calculated:

EI sect = 1510439396 – (103699 x 10 x 27,362)
= 1510439396 – 776259189 Nmm4/mm2
= 734200000 Nmm4/mm2
= 73,4 x 103 Ncm4/mm2.

4.5 From Pt 8, Ch 3, 1 General of the Rules for Special Service Craft the individual layer stresses are determined from:

The calculation of the tensile stress in the individual layers becomes:

where

E i = modulus of elasticity of layer (N/mm2)
y i = distance of layer from the neutral axis (mm).

Figure 4.4.1 Regions of tension (T) and compression (C) in example model

4.6 Consider the model as shown in Figure 4.4.1 Regions of tension (T) and compression (C) in example model.

  1. Consider wet surface in tension at the panel boundary:

    1. Consider the CSM reinforcement in the outer ply (450 g/m2 and G c = 0,286)

      σCSMult = 82,2 N/mm2
      E t = 6290 N/mm2
      y i = 27,36 mm
      σCSM = 23,2 x 10–6 E t y i N/mm2
      = 23,2 x 10–6 x 6290 x 27,36 N/mm2
      = 3,99 N/mm2
      Stress factor = 3,99/82,2 = 0,05 < 0,33 hence accept.

    2. Consider the WR reinforcement in the in outer plies (600 g/m2 and G c = 0,5)

      σWRult = 190 N/mm2
      E t = 14500 N/mm2
      y i = 27,36 – 1,112 = 26,25 mm
      σWR = 23,2 x 10–6 E t y i N/mm2
      = 23,2 x 10–6 x 14500 x 26,25 N/mm2
      = 8,83 N/mm2
      Stress factor = 8,83/190 = 0,046 < 0,33 hence accept.

  2. Consider inner surface in compression at the panel boundary:

    1. Consider top WR reinforcement in compression (600 g/m2 and G c = 0,5)

      σWRult = 147 N/mm2
      E c = 14000 N/mm2
      y i = 58,23 – 27,36 = 30,87 mm
      σWR = 23,2 x 10–6 E c y i N/mm2
      = 23,2 x 10–6 x 14000 x 30,87 N/mm2
      = 10,03 N/mm2
      Stress factor = 10,03/147 = 0,068 < 0,33 hence accept.

    2. Consider CSM reinforcement in compression (300 g/m2 and G c = 0,33)

      σCSMult = 122 N/mm2
      E c = 7200 N/mm2
      y i = 30,87 – (4 x 0,734) = 27,93 mm
      σCSM = 23,2 x 10–6 E c y i N/mm2
      = 23,2 x 10–6 x 7200 x 27,93 N/mm2
      = 4,67 N/mm2
      Stress fraction = 4,67/122 = 0,038 < 0,33 hence accept.

Figure 4.4.2 Sandwich skin laminate details

4.7 Stress fractions in this example are considerably lower than those required by the Rules for Special Service Craft and it is evident that the design is controlled by and core shear considerations and minimum skin thickness requirements.

4.8 As indicated in Chapter 2, Section Ch 3, 2 Concluding remarks 2.4 of these Guidance Notes for Single Skin Laminates, consideration must be given to the strain compatibility of the reinforcements incorporated in the sandwich skins.

Parent topic: Chapter 4 Design of Sandwich Panel Laminates
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