Size Effect And Yield Behavior Of Aluminum Alloy Foams

Aluminum alloy foams,as a type of lightweight materials,are wildly used in engineering fields,such as aerospace,transportation,safety protection,and so on,because of their high specific stiffness,high specific strength,abilities of energy absorption and noise reduction.Due to the various and complex service conditions for the material,it is important to investigate the mechanical properties of aluminum alloy foams systematically for optimal design and being applied in practical engineering.In this thesis,experimental investigation,numerical calculation,and theoretical analysis were combined to carry out this research of the sample size effect on the properties of aluminum alloy foams and yield behavior of materials.A relative density-dependent macroscopic phenomenological constitutive model was established for describing the mechanical properties of aluminum alloy foam with different relative densities.In order to investigate the effect of sample size on the mechanical properties of aluminum alloy foam sample and determining the least sample size which can represent the properties of materials,a 3D Voronoi Finite element model(FE model)was established.The validity and reasonability of the FE model were validated by being compared with experimental results and theoretical analysis.By using the FE method,the sample size effect was studied under uniaxial compression loading,shear loading,and bending loading.The results show that the sample size affects the mechanical properties of samples until the sample size is large enough.For uniaxial compression loading condition and bending loading condition,the dominant factor of the sample size effect is ‘weak cell layers'.The weak cell layer is a layer of cells which are less constrained and located at edges of samples.For shear loading condition,the dominant factor is the combination of ‘strong boundary layers' and ‘weak cell layers'.The strong boundary layer is a layer of cells which is tied to the rigid platen.According to the results of experiments and numerical calculations,the least sample size was determined as L/ d ?7.Moreover,based on this conclusion,the design of experimental setup and establishment of macroscopic phenomenological constitutive model for aluminum alloy foam can be ensured.Because aluminum alloy foams are at complex stress state when they are applied in practical engineering,a series of tests,including simple stress state tests(uniaxial compression tests,uniaxial tension tests,and shear tests)and complex stress state tests(compression-shear combined tests and triaxial compression tests)were carried out to determine the initial experimental yield loci of aluminum alloy foam with relative densities of 10%,15%,20%.Moreover,the loading rate effect was investigated in compression-shear combined tests.The experimental results demonstrate that the loading rate effect is negligible when the loading rate is in range of 3~300 mm/min and the relative density of the material is in range of 10%~20%.Based on experimental results,an initial yield criterion which is associated with the first invariant of stress tensor and second and third invariants of deviator stress tensor was proposed.The experimental yield loci and prediction of the proposed model were compared for validation.By introducing characteristic stress and characteristic strain which are work conjugate,a macroscopic phenomenological constitutive model was established for volume compressible solid based on classical rate-independent elastoplastic constitutive model.The relationships between relative density of materials and parameters in the model were analyzed.By introducing the effect of relative density of materials,a relative density-dependent macroscopic phenomenological constitutive was established,which can predict the mechanical behavior of aluminum alloy foam at ‘elastic stage' and ‘platform stage'.The validity of the proposed constitutive model was validated by a comparison between experimental data and prediction of model.