Demonstration And Standardization Of The Method Of Measuring Fiber Dynamic Elastic Modulus By Sound Velocity Method

The modulus of elasticity is one of the fundamental physical quantities of textile fibres.The modulus of elasticity is usually calculated using the tensile stress strain method,but the tensile process is often accompanied by relaxation and the constant stress applied does not reflect the state of the material in actual use,and the specimen is eventually destroyed.However,the acoustic pulse method（acoustic velocity method）,which is based on the slight perturbation of the fibres by acoustic waves to cause small deformations in the fibre material,is a good alternative to the tensile method because it is easy to operate,the material is undamaged and the test results are sensitive,stable and accurate.This paper explores the empirical evidence and standardisation of the sound velocity method using a variety of fibres as raw materials.For polypropylene fiber feedstock,28 sets of average acoustic wave propagation times were measured and recorded for polypropylene monofilaments at eight separation distances and various pre-tensions using a fiber orientation meter.Delay time,sound velocity,acoustic modulus and orientation parameters（including orientation degree and orientation angle）were calculated by the two-point and multi-point methods.The good agreement between the scatter plots and regression lines of the calculated data indicates that the acoustic wave method provides a reliable and accurate determination of the target parameters.In practice,the zero time of acoustic wave propagation depends heavily on the separation distance.The optimal range for pre-tensioning is 0.1 gf/den-0.2 gf/den and the optimal values for separation distances are 200 mm and 400 mm.This allows simple and fast measurements by the two-point method,suitable for industrial applications.The more complex multi-point method is the preferred method for scientific research.PET-POY filaments were made by quasi-static stretching into 12 sets of samples with different elongations.The properties and structures of PET-POY filaments at different elongation rates were systematically investigated,and their internal molecular orientation,sound velocity,mechanical properties,and crystallization properties were characterized.The specific conclusions are as follows: The sound velocity orientation test shows that the fiber orientation increases with the increase of elongation,but the slope of the increase slows down,and its sound velocity is also increasing,which indicates that its dynamic elastic modulus is also increasing.By comparing with the traditional stress-strain curve to calculate the initial modulus,the dynamic elastic modulus tested by acoustic pulse method is more sensitive and accurate and stable.The results of the mechanical property tests show that the initial modulus of the fiber increases with the increase of elongation,but the elongation at break decreases,but the change of strength at break is not obvious.the DSC results show that the crystallinity is increasing with the increase of elongation,which indicates that the growth of spherical crystals of the specimen is greatly promoted by the external stretching.For water-insoluble vinyls（IPVA）and water-soluble vinyls（SPVA）,the acoustic velocity and delay time,as well as the dynamic elastic modulus,were calculated at eight and 36 sets of transducer spacings,respectively,using the acoustic pulse method test principle and the multi-point extrapolation method.Combined with the experimental data,the results of the calculations based on the known fiber density of 1.3 g/cm3 show that the sound velocity of IPVA is 2.28-3.34 km/s,the sound modulus is 6.67-14.50 GPa,the preferred pretension interval is 0.175-0.275 gf/den,and the preferred transducer separation distances are 150 mm and 300 For the same specimen and instrument environmental conditions,both specimen pretension and transducer separation（specimen measurement length/acoustic wave propagation distance）have an effect on the sound velocity.For the same specimen and instrument environmental conditions,both specimen pre-tension and transducer separation（specimen measurement length/acoustic wave propagation distance）have an effect on the sound velocity measurements,but pre-tension has less effect,while measurement distance significantly changes the delay time.