| In recent years,short glass fiber reinforced PA66 composites have been selected to replace metal material in the application of automotive parts,which can not only ensure the safety performance and low cost but also provide excellent thermal intensity for cars,thus it means that the research on glass fiber reinforced PA66 composites is of great significance for promoting the development of"automobiles lightening".Meanwhile,since the short glass fiber tend to be oriented along the flow direction during the injection molding process,its products have significant anisotropy,if its anisotropy advantages can be utilized to meet the requirements of mechanical properties in a specific direction,the application advantages of the material can be greatly exerted.The object of this research is GF35/PA66(short glass fiber reinforced PA66 composite with glass fiber mass fraction of 35%)and GF50/PA66(short glass fiber reinforced PA66 composite with glass fiber mass fraction of 50%)manufactured by injection molding process,and five angles of dumbbell-shaped tensile specimens were prepared from the injection molded rectangle plates:parallel to the flow direction(0°specimens),normal to the flow direction(90°specimens),and other off-axis angles relative to the flow direction(30°,45°,60°specimens).Tensile,bending,notched impact and compression tests were carried out on specimen of each off-axis angle respectively at the room temperature(23℃)and high temperature(85℃),and the internal fiber arrangement and the fiber-resin interface were observed by scanning electron microscopy(SEM),the dynamic thermal mechanical properties of each sample(DMA)was also tested.In addition,the volume fraction of the glass fiber,the residual length of the glass fiber after injection molding,and the fiber orientation in the material were measured and analyzed.Combined with the experiment value,the critical fiber length(L_c)of the material was calculated by the modified Kelly-Tyson formula to obtain the interfacial shear strength(τ_c).The shear modulus of the material was also calculated with the 45°off-axis stretching method.Finally,combined with the experimental data,the off-axis elastic modulus and the off-axis tensile strength of short fiber reinforced composites are predicted by related formulas.The conclusions were summarized as follows:1)Compared with GF35/PA66 composite,the mechanical properties of GF50/PA66 composite have been significantly improved;2)The mechanical properties of the sample in the normal temperature environment are obviously superior to the high temperature environment,and the non-linearity of the tensile and bending curves of the materials was more obvious in high-temperature environments,in which the maximum stress of the samples decreased significantly,while the fracture strain increased significantly.3)The mechanical properties of 0°specimen are obviously better than those of other angles.When the off-axis angle is increased from 0°to 30°,the mechanical properties of the material are greatly reduced,and when the off-axis angle get larger than 30°,the downward trend of performance becomes weaker with the increase of the off-axis angle.After reaching 60°,it gradually becomes stable,then the effect of fiber orientation on the mechanical properties is almost negligible.4)The DMA test results show that when the glass fiber content increases and the off-axis angle approaches 0°(the fiber arrangement is closer to the flow direction),the storage modulus(E′)of the material increases,and tanδdecreases,which means the elasticity of the composite increases and the rigidity is enhanced.5)The residual fiber length of GF35/PA66 composite and GF50/PA66 composite after injection molding decreased by 74%and 76%,respectively,indicating that the glass fiber length in the material was greatly reduced after the injection molding process,and the average fiber length in the material decreases as the fiber mass fraction increases;6)Compared with GF35/PA66 composites,the calculated GF50/PA66 composites have bigger L_c and smallerτ_c.7)The off-axis elastic modulus calculated by the engineering elastic constant is basically consistent with the change trend of the test value,and the relative deviation is also small;8)The off-axis tensile strength value predicted by the Hoffman failure criterion is significantly closer to the experimental value than the modified Kelly-Tyson criterion,the predicted tensile strength of 30°specimen is slightly lower than the experimental value,but still within a reasonable range,which indicates that the Hoffman criterion has a good applicability to GF/PA66 composites from an engineering point of view. |
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