OS-C501

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Offshore Standard DNV-OS-C501, November 2013 App.C Test methods for laminates – Page 216 Guidance note: The Young’s modulus can be measured well with all test arrangements. Measuring strength and strain to failure can be more complicated, because stress concentrations at the grips or shoulder may cause premature failure. However, choosing a non-optimised test method gives conservative results for static tests. C.2.2 In-plane compression tests ---e-n-d---of---G-u-i-d-a-n-c-e---n-o-t-e--- C.2.2.1 Compressive tests can be performed on straight specimens with or without tabs or on specimens with reduced cross-sections in the middle (dogbone or dumbbell tests). An anti-buckling devise should be used in most cases. C.2.2.2 The recommended test procedure is ASTM D3410 using the Celanese test fixture. C.2.2.3 It should be ensured that test specimens do not buckle in the test fixture. Guidance note: One way to ensure that a specimen does not buckle is to place at least one strain gauge or extensometer at each side to ensure that the specimen does not buckle. Strain readings should not deviate by more than 10%. C.2.3 In-plane shear tests ---e-n-d---of---G-u-i-d-a-n-c-e---n-o-t-e--- C.2.3.1 In-plane shear tests tend to show non-linear stress vs. strain curves and the interpretation of the results is often difficult. C.2.3.2 Recommended test procedures are the 2-rail shear test ASTM D4255 or a tensile or compressive test on a laminate with (±45) s reinforcement. C.2.3.3 Test results from axial tests on (±45) s laminates must be converted into ply properties using laminate theory. The following formulas shall be used to convert measurements of laminate stress in load direction σ x , laminate strain in load direction ε x and laminate strain perpendicular to the load direction ε y into ply shear stress τ 12 and strain γ 12 : τ 12 = σ x /2 and γ 12 = -( ε x - ε y ) C.2.3.4 The non-linear stress vs. strain curve looks usually like a ductile curve, see [C.6.3]. The results shall be analysed accordingly. C.2.4 Through thickness tensile tests C.2.4.1 Test can be performed on straight, cylindrical or dogbone specimens. C.2.4.2 To increase the length of the specimens, sections of laminates may be glued together. However, care shall be taken that in such case the adhesive properties do not influence the results. If the adhesive fails, the test values may be used as a conservative estimate of the laminate properties. C.2.4.3 A widely used test is ASTM C297. C.2.5 Through thickness compressive tests C.2.5.1 Tests are usually performed on straight or cylindrical specimens. ASTM D1621 could be used for these tests. C.2.5.2 An anti-buckling devise is usually not required, provided the specimen is not too slender. C.2.5.3 Specimens should not be too short. Short specimens may show wrong stress distributions if the crosssectional surfaces are not totally parallel. In addition, the surfaces touching the test fixture cannot widen due to friction like the rest of the specimen. Due to this effect a too high modulus can be measured if the specimen is too short. C.2.5.4 To increase the length of the specimens, sections of laminates may be glued together. C.2.6 Interlaminar shear tests (through thickness) C.2.6.1 The recommended test method is the interlaminar shear test (ILSS) according to ISO 4585 or ASTM D2344. C.2.6.2 The ILSS test is not a pure shear test and the results are, therefore, not ideal for use in design. Other tests may be used if better results can be obtained with them. DET NORSKE VERITAS AS
Offshore Standard DNV-OS-C501, November 2013 App.C Test methods for laminates – Page 217 C.2.7 In-plane fracture toughness tests C.2.7.1 In-plane fracture toughness of laminates can be described by the point stress criterion, see Sec.6 [3.6]. C.2.7.2 The point stress criterion needs a critical distance that is a material parameter. The critical distance can be measured on specimens with different crack length or hole sizes. At least three different crack or hole sizes should be measured. C.2.7.3 The critical distance for cracked specimens may differ from the critical distance for specimens with holes. The two tests should not be mixed. C.2.7.4 It is recommended to prepare specimens with cracks or holes that resemble closely the real application. C.2.7.5 The specimen width should be wider than the defect size, to avoid measuring edge effects. C.2.7.6 No standard tests are known for measuring fracture toughness. C.2.8 Interlaminar fracture toughness tests C.2.8.1 The Double Cantilever Beam (DCB) test is recommended to obtain the Mode I fracture toughness G IC . C.2.8.2 The End Notch Flexure (ENF) test is recommended to obtain the Mode I fracture toughness G IIC . C.2.8.3 No standards are developed for these tests. C.2.8.4 Both specimens use a starter crack to initiate crack growth. C.2.8.5 The starter crack should be introduced by placing a thin film between the two plies that shall delaminate. The film should not adhere to the matrix. C.2.8.6 It is best to grow a natural starter crack from the film by applying a few low amplitude fatigue cycles. Static testing can then be performed based on the crack length of the artificial crack and the naturally grown crack. C.2.8.7 If tests are performed based on the artificial crack only, the film should be very thin, less than 13 µm. C.3 Tests to obtain properties under long term static and cyclic loads C.3.1 C.3.1.1 The same tests as for measuring static properties can be used. C.3.1.2 Loads and test environments shall be carefully controlled over the entire test period. C.3.1.3 The strain rate should be kept constant for cyclic fatigue tests of visco elastic materials. This means the test frequency can be increased for lower strain (test) amplitudes. C.3.1.4 The temperature of the specimens should be monitored to avoid heating of the specimen due to testing at too high frequencies. C.4 Tests to obtain the fibre fraction C.4.1 C.4.1.1 The fibre volume fraction should be obtained using one of the following standards: — ASTM D3171 and ASTM D3553 Fibre volume fraction, digestion method. Guidance note: This method is used to find the fibre volume fraction by digestion of the matrix. The choice of chemical to digest the matrix depends on the matrix and fibres. Suggestions for chemicals are given in the standards. The composite weight is determined before and after the digestion of the matrix. The fibre volume fraction can be calculated from those measurements. Calculations are based on the density of the fibres. However, this density can vary, especially for carbon fibres. In this case only approximate values can be obtained. ---e-n-d---of---G-u-i-d-a-n-c-e---n-o-t-e--- — ASTM D3171 Fibre volume fraction, burnoff method — ISO 1172 DET NORSKE VERITAS AS
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OS-C501

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