OS-C501

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Offshore Standard DNV-OS-C501, November 2013 Sec.5 Materials – sandwich structures – Page 82 Table 5-5 Shear strength correction factors f tc is a correction factor for the effect of core thickness t of a sandwich beam f i is a correction factor for the in-plane width w of a sandwich beam f ip is a correction factor for the in-plane size b of square panels f ip is a correction factor for the in-plane size ab of rectangular panels f b is a correction factor accounting for the effect of bending τ ref is the mean value of the shear strengths measured from the reference sandwich specimen. f b can be derived as follows: The ratio between shear strain and bending strain for a beam subject to four point bending is given by the following formula (derived from sandwich beam theory) ε l bG ct c = γ t E d where ε γ is the ratio between extensional in-plane strain and shear strain occurring in the core. A simple failure criterion in terms of shear strain and in-plane normal strain can be chosen ε γ + = 1 C C where C ε and C γ are empirical constants. These empirical constant C ε and C γ are determined by fitting the previous equation to measured data. Solving the equations simultaneously for and multiplying by G c , one obtains the shear stress as a function of the ε ration where a 1 and a 2 are constants. γ τ G c f b = = τ ε ref a1 + a 2 γ Guidance note: The coefficients are based on Weibull theory. The theory states that where σ i is the uniform stress at failure acting over a volume V i . ε γ σ σ 1 2 ε The equation describes the dependence of the failure stress on the loaded volume, and was originally developed for the failure of brittle materials such as ceramics. In a balsa-cored sandwich beam, one can expect the core failure of a shear-loaded beam to be controlled by randomly distributed defects within the loaded volume. For a 4-point bending specimen, the shear-loaded volume is V=2L b wt c . ---e-n-d---of---G-u-i-d-a-n-c-e---n-o-t-e--- corrected 2.5.10 Other methods to correct the shear strength may be used if they are backed by experimental evidence. f 2.5.11 For specifically predicting the shear strength at failure of balsa-cored sandwich beams or panels made out of end-grain balsa type of density 150 kg/m 3 , and provided that the ratio between extensional in-plane strain ε and shear strain occurring in the core, , remains between 0.37 and 1.1 , the following correction factors may γ be used: γ f ⎛ V2 ⎞ = ⎜ V ⎟ ⎝ 1 ⎠ 1 m f f f f f tc i τ = τ τ = τ ip,square ip,rec tan gular b = ref corrected τ τ ref τ = τ ref ⎛ t = ⎜ ⎝ t ⎛ l = ⎜ ⎝ corrected ref τ = τ = a corrected 1 ref ref c slref l sl G c ε + a γ ⎞ ⎟ ⎠ w w 1 m tc ref ⎛ b = ⎜ ⎝ b ref ⎛ 2b = ⎜ ⎝ ab 2 ⎞ ⎟ ⎠ ⎟ ⎠ ⎞ 2 ref 1 mi 2 m ip ⎞ ⎟ ⎠ 1 m ip DET NORSKE VERITAS AS
Offshore Standard DNV-OS-C501, November 2013 Sec.5 Materials – sandwich structures – Page 83 τ ref = 1.52 f tc ⎛ 50 ⎞ = ⎜ ⎟ ⎝ tc ⎠ 1 3.7 f i ⎛17600 ⎞ = ⎜ ⎟ ⎝ lsl w ⎠ 1 7.9 f and ⎛17600 ⎞ = ⎜ ⎟ ⎝ lsl w ⎠ where, ε : the ratio between extensional in-plane strain and shear strain occurring in the core γ t c : the core thickness l sl : the shear-loaded length w : width G c : the core shear modulus t f : the face thickness E f : the face in-plane elastic modulus In the above equations shear stress values are in MPa and lengths in mm. Guidance note: Each correction factor is independent. When no correction is needed, for example when a size effect does not occur because of identical dimensions between reference specimen and structure to design, the corresponding correction factor shall be set to 1 in the above equations. ---e-n-d---of---G-u-i-d-a-n-c-e---n-o-t-e--- Guidance note: When using data from the literature, it shall be checked that the geometrical, physical and loading characteristics are proper for the structure under consideration. ---e-n-d---of---G-u-i-d-a-n-c-e---n-o-t-e--- 2.5.12 Characteristic values of the shear strength, τ char , shall be based on the corrected shear strength values, τ corrected . Calculations shall be done as described in Sec.4 [2.4]. 2.6 Core skin interface properties 2.6.1 Good bonding between skin and cores shall be insured. 2.6.2 The shear strength, transverse tensile strength and peel strength are usually the critical parameters that should be checked for sandwich structures. 2.6.3 If it can be documented that the interface is stronger than the core, core properties can be used to describe the interface. For many sandwich structures made of foam core the interface is stronger than the core and interface failure is actually a failure inside the core close to the interface. 2.6.4 Test methods to obtain interface properties are described in the App.D. 2.6.5 The general requirements for interfaces described in Sec.7 should also be considered. 3 Properties under long term static and cyclic loads 3.1 General 1 7.9 f b = ε ip 22 + 89 ε lslG ct c = γ t E d f f d = t c + t f 104 γ 3.1.1 The structure shall be analysed for the elastic constants before time-dependent damage and for the elastic constants with time-dependent damage. 3.1.2 If the structure is exposed to through thickness time-dependent loads or deformations, a degradation of the through thickness properties shall be considered. Experimental evidence shall be provided since no other guidance can be given today (year 2003). 3.1.3 The single logarithmic representation of fatigue data shall be preferred; the scale of the x-axis should be logarithmic function of the cycle number. DET NORSKE VERITAS AS
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OS-C501

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