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Designs of composite laminates are investigated for hydrogen tanks in cryogenic environments. Large residual strains, which can develop due to thermal mismatch between matrix and bers, result in matrix cracking at cryogenic temperatures and increase hydrogen leakage through the tank wall. To reduce thermal mismatch, ply angles need to be close to each other, but this leads to a substantial weight increase under biaxial loading. First deterministic optimization is used to investigate possible weight reduction measures. Reducing axial loads on walls by auxiliary stiffening mechanisms led to signi cant weight reduction. Reliability-based optimizations were performed to identify the uncertainties in composite material properties with the largest in uences on the optimum design. Then measures for reducing uncertainty in important parameters are examined. The results indicate that the most effective measure for reducing thickness is quality control. Nomenclature E; ¾ , CV = mean value, standard deviation, and coef cient of variation (¾=E) of the random variables E 1 , E 2 , G 12 = elastic modulus along and transverse to ber direction and shear modulus of a composite ply h = total laminate thickness T zero = stress-free temperature t 1 , t 2 = thickness of plies with angles µ 1 and µ 2 , respectively ® 1 ; ® 2 = coef cient of thermal expansion along and transverse to ber direction " 1 ; " 2 ;°1 2 = strains along and transverse to ber direction and shear strain of a composite ply µ ; µ 1 ; µ 2 = ply orientation angles ¹ 12 = major Poisson ratio of a composite ply Subscripts l = lower limit u = upper limit

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