Inverse Stress Relaxation and Viscoelastic Recovery of Multifilament Textile Yarns in Different Test Cycles
The article presents the results of the experimental research of inverse stress relaxation (IR) and viscoelastic recovery (VR) that take place in acetate and polyester multifilament yarns in dependance on the mechanical pre-history. The above-mentioned time-effects are investigated in two different – stress relaxation (R-) and creep (C-) testing cycles. The fact that inverse stress relaxation process takes place in Ctest cycle, i.e. after previous sustaining the specimen at constant load is experimentally confirmed. It is shown that viscoelastic recovery is the slower process than the inverse stress relaxation. At identical elongations of the yarns at the end of loading period the inverse relaxation and viscoelastic recovery processes go on similarly regardless of the character of testing cycle.
Effects of Dynamic Loading of Sewing Process and Viscoelastic Property of Threads on Seam Puckering
Dynamic loading of sewing process causes the reduction in strength and weakening of visicoelastic property of threads during sewing. The reduction in thread strength during sewing is technically unwanted effect since this affects garment productivity and seam performance, particularly at high speed of sewing. In present study it is considered that sewing thread strength reduction during sewing is an important on the other hand for controlling seam puckering which, caused by sewing thread relaxation and recovery after sewing. To ascertain this idea with experimental work, three types of sewing threads (100 % spun polyester, core spun of polyester- cotton and core spun of polyester-polyester) were selected as the study material. For making stitches polyester –cotton (40/60) blend woven suiting fabric was used. The stitched threads were carefully taken out of the seam for testing strength, extension, relaxations, recovery and contraction of threads for the purpose of comparing the changes for them before and after sewing. The strength of threads was reduced in the range of 26.6-31.1 % for the three types of threads after sewing. For extension test made at equal load, high extension was resulted after sewing. Stress-relaxation index and recovery of threads were reduced after sewing. The reduction in recovery was ranged from 25 % to 58% after sewing for the three types of threads against the recovery results of threads before sewing. The contractions were greater in the range of 55.2- 344.4 % after sewing for all threads considered under the study. The increase in extension and contraction, as well as the decrease in relaxation index and recovery after sewing are important factors in controlling seam puckering, which caused by sewing threads. All these were caused by dynamic loading of sewing process and viscoelastic property of threads, through strength reduction and weakening of viscoelastic properties of threads after sewing. 
Key words: Dynamic load, sewing process, viscoelastic property, stress-relaxation, inverse stress-relaxation.
Modeling Mechanical Behavior of Fabric and Warp Yarn During Loom Stops
This paper proposes a model for calculating relaxation behavior after dynamic load ing of fabric and of warp yarns. The model is based on fitting the parameters of an extended nonlinear Maxwell model to experimental data using nonlinear regression. Combining the models for fabric and warp yarns can yield the viscoelastic behavior of the fabric-yarn combination, as well as the cloth fell displacement during the re laxation period.