| Flexible composite have widely been used in different fields such as transportation, protection and civil construction, military and so on, due to their very good flexibility and mechanical performance. Traditional flexible composites are based on the woven structures. However, the flexible composites based on the biaxial warp knitted structures have rapidly been developed in the recent years. Besides higher production compared to woven structures, the biaxial warp knitted structures also provide better mechanical properties in tension and tearing propagation resistance. In a biaxial warp knitted structure, straight and un-crimped yarns are respectively inserted in the warp (90°) and weft (0°) directions in order to enhance their in-plane mechanical properties. The most important structural feature of a biaxial warp knitted structure is that the warp and weft yarns are only bound by knitted loops and are not interlaced among them. Because of this particular structural feature, the mechanical behaviors of flexibility composite based on the coated biaxial warp knitted fabrics could be different from those of woven fabrics.In the practical applications, the flexible composites normally withstand simultaneous loads coming from biaxial loads or multi-axial loads. In addition, little cracks could occasionally be produced by contacting with pointed objects or under undesired mechanical actions during transportation, installation or utilization. The existence of cracks can result in the reduction. of mechanical performance of the coated fabrics.The present study deals with the experimental investigation of tensile properties of PVC coated flexible biaxial warp knitted composites which are subjected to mono-axial, bi-axial and multi-axial loads. The mechanical properties of the composites were analyzed and characterized in details. The stress-strain curves and failure mechanisms of the samples under mono-axial, bi-axial and multi-axial loads were compared. Understanding of the mechanical behavior of the flexible composites was extended from mono-axial to bi-axial and multi-axial fields. The mono-axial testing has offered necessary data foundation and constitutive model for the following FEM analysis.The tearing behaviors of the composites were studied by using three different tearing methods, i.e., Trapezoidal, Tongue and Precracked tearing. The correlations among the results obtained from these different tearing methods were established. The typical tearing load-extension curves and the influences of different tearing testing conditions were analyzed. The precracked tearing testing results of the composites under mono-axial, bi-axial and multi-axial loads were compared. The mechanical properties of flexible composite with and without initial cracks under mono-axial, bi-axial and multi-axial loads were studied. The precracked tearing failure mechanisms of the composites under mono-axial, bi-axial and multi-axial loads were analyzed and developed.Finally, the finite element analysis method was used to simulate the tensile behavior of the composites under bi-axial and multi-axial loads based on the testing data of mono-axial testing results. The FEM models were developed. The stress-strain relationship based on the FEM models were calculated. The calculating results from FEM models were compared with the experimental results.The finite element analysis method was also used to simulate the precracked tearing behavior of the flexible composites under bi-axial and multi-axial loads. The tearing behavior, particularly the influence of crack length and direction under biaxial tensile loads was simulated. Using finite element analysis method can simulate not only the stress-strain curves of the flexible composite under bi-axial and multi-axial loads for a given load ratio which can tested by instrument, but also the stress-strain behavior of the flexible composite under bi-axial and multi-axial loads for a given load ratio which can not tested by instrument.
|
PDF Full Text Request