OS-C501

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Offshore Standard DNV-OS-C501, November 2013 App.D Test methods for sandwich materials – Page 220 D.1 General APPENDIX D TEST METHODS FOR SANDWICH MATERIALS D.1.1 Introduction D.1.1.1 This appendix describes recommended and preferred test methods for core materials alone and sandwich components (including the face/core adhesive joints and the faces). Other test methods than the ones described here may be used if they measure the same physical properties under the same conditions. D.1.1.2 If no standard tests exist and no test methods are suggested, tests shall be chosen that measure the desired properties with no or small side effects from specimen size and geometry. It shall be ensured that the test results are correct or conservative with respect to the way they are used in the design. D.1.1.3 The complete list of mechanical static properties (for core and adhesive materials) needed for structural analysis in this standard is shown in Sec.5 [2] 'static properties'. D.1.1.4 Guidelines for the testing of face materials alone are presented in App.C. D.1.1.5 Health and safety shall be considered when performing tests. This standard does not address these aspects and reference to applicable health and safety regulations shall be made. D.1.2 General testing information D.1.2.1 For anisotropic material, mechanical properties should be determined relative to the relevant direction of anisotropy. D.1.2.2 Tests should be carried at a loading rates corresponding to about 1% per minute, unless specified differently in the standard. D.1.2.3 For the preparation of test samples, curing conditions, surface treatment and application procedure shall be according to the specifications as described in Sec.5 [1]. D.1.2.4 The evaluation of stress vs. strain curves is described in App.C, [C.3] for brittle, plastic and ductile materials. D.1.2.5 The use of strain gauges for the measurement of deformation in the core is difficult. Suitable adhesive should be used to bond strain gauges to the core in order to avoid stress concentrations. Guidance note: If strain gauges are bonded with epoxy resin, for example, stress concentration will arise due to the difference of between the adhesive stiffness and the (typically) low core stiffness. Alternatively, an extensometer could also be used. D.2 Core materials - static tests D.2.1 Tensile tests ---e-n-d---of---G-u-i-d-a-n-c-e---n-o-t-e--- D.2.1.1 The specimen dimensions should be sufficiently large to avoid end-effects. D.2.1.2 For anisotropic material, test should be carried on specimen having their long axis parallel and normal to the direction of anisotropy. D.2.1.3 The entire stress-strain curve should be recorded. D.2.1.4 The recommended test methods are ASTM D 1623-78(1995) for in-plane properties and ISO 1926:1979 for through-thickness properties. Other test methods are also available: ASTM D 638-93, ISO/DIS 1798, and ISO 527-2:1993. D.2.2 Compressive tests D.2.2.1 For anisotropic material, test should be carried on specimen having their long axis parallel and normal to the direction of anisotropy. D.2.2.2 The entire stress strain curve should be recorded. D.2.2.3 A typical schematic of a stress-strain curve for a material exhibiting crushing behaviour, like foam cores, is shown in Figure D-1. These materials have a compressive strength and a crushing strength (σ crush ). The elastic parameters are the same as for a brittle material see App.C [C.3]. DET NORSKE VERITAS AS
Offshore Standard DNV-OS-C501, November 2013 App.D Test methods for sandwich materials – Page 221 σ u lt σ cru sh C o m p r essiv e S tress E lin C o m p ressiv e S tra in ε u lt : S tress-str a in rela tio n Figure D-1 Compressive test and crushing strength D.2.2.4 The recommended test methods are ASTM D 1621-00. Other test methods may be used such as ASTM C 365-00, ISO 844:1978, ISO/DIS 844, ISO 1856:1980, ISO 3386-1:1986, ISO 3386-2:1986. D.2.3 Shear tests D.2.3.1 Tests cover the determination of the shear properties of the core when loaded in shear parallel to the plane of the faces. D.2.3.2 The entire stress-deformation and stress-strain curves should be recorded. D.2.3.3 The recommended test methods is the block shear test described in the ISO 1922:1981. The test produce pure shear loading. Small deviation from perfect parallel plates can produce undesirable loading and can cause errors in the calculation of the shear properties. Plate thickness should be sufficient to prevent bending of the plates near the attachment points. For balsa cores see [D.2.3.5] and [D.2.4]. D.2.3.4 An other possible test method is ASTM C 273-00. This test does not produce pure shear, but secondary stresses can be minimised. The test can also be unfavourable because of stress concentration developing at corners causing premature final failure. D.2.3.5 The block shear test from [D.2.3.3] and [D.2.3.4] should not be used to obtain design shear strength of balsa cored sandwich beams, panels and high density core materials. The flexural test method described in ASTM C 393-00 should be used instead, see [D.2.4]. Guidance note: Block shear test, such as the one used in ASTM C 273-00 or ISO 1922, should not be used to obtain design shear strength of balsa cored sandwich beams and panels. Using the block shear tests data to obtain the shear strength of balsa beams and panels will in many relevant cases overestimates the shear strength by a factor of 2 to 4. ---e-n-d---of---G-u-i-d-a-n-c-e---n-o-t-e--- D.2.3.6 The possible corrections of measurements described in Sec.5 [2.5] shall be considered. D.2.4 Shear test for balsa and high density cores D.2.4.1 Shear strength and modulus should be tested by two different methods. Strength measurements are described in [D.2.4.2]-[D.2.4.6]. Stiffness measurements are described in [D.2.4.7]. D.2.4.2 The recommended test method to determine shear strength is ASTM C 393-00. D.2.4.3 Typically, the 3-point-bending and the 4-point-bending test methods are used for the determination of properties of flat sandwich constructions subjected to transverse loading. D.2.4.4 One can ensure shear fracture of the core, or (compressive or tensile) failure of the faces laminates by suitable specimen design. D.2.4.5 The entire load-deformation curve should be recorded. D.2.4.6 The possible corrections of measurements described in Sec.5 [2.5] shall be considered. DET NORSKE VERITAS AS
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OS-C501

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