Non-isothermal Histories Research Articles

Thermodynamics and kinetics of laser induced transformation in zirconium based bulk metallic glass

Laser-material interaction for a Zirconium based bulk metallic glass was investigated. The change in glass transition, crystallization temperature and enthalpies along with microstructure of the amorphous alloy was explained by the non-isothermal history experienced during processing. Varying volume fractions of amorphous and crystalline phases were found through the cross-section. A finite element based multi-physics modeling approach was employed to predict the temperature history and associated cooling rates during laser processing. Maximum devitrification occurred in sub-surface regions that experienced suitable combination of thermodynamic and kinetic conditions during laser treatment. It was demonstrated that the location and length scale of crystallized zone can be controlled by tailoring the laser processing parameters.

Long-term creep of polyphenylene sulfide (PPS) subjected to complex thermal histories: The effects of nonisothermal physical aging

The long-term viscoelastic behavior of polymeric materials used below the glass transition temperature ( T g ) is greatly affected by physical aging. In contrast to isothermal physical aging, long-term response under nonisothermal history has received far less attention. This paper reports experimental results and analytical methods of long-term creep behavior of polyphenylene sulfide (PPS) subjected to complex thermal histories in a temperature range below T g . To characterize the effects of aging, creep tests were performed using a dynamic mechanical analyzer (DMA). Besides the long-term data, short-term creep tests in identical thermal conditions were also analyzed; these were utilized with effective time theory to predict long-term response under both isothermal and nonisothermal temperature histories. The long-term compliance after a series of temperature changes was predicted by the effective time theory using the KAHR- a te model to obtain nonisothermal physical aging shift factors. Comparison of theoretical predictions with experimental data shows good agreement for various thermal histories.

Recovering Nonisothermal Physical Aging Shift Factors Via Continuous Test Data: Theory and Experimental Results

For isothermal physical aging, a few simple tests to characterize the aging shift factors allow reasonable prediction of the mechanical response. In this paper, a new technique is developed to extract aging shift factors from creep data during a nonisothermal history. Previous methods have generated discrete experimental shift factors by a series of short-term creep tests, in which the load portion alone is used for evaluation; this is particularly time consuming for nonisothermal histories, since many data points (requiring several tests) may be needed for an adequate characterization of the response. This paper presents a new continuous shift factor (CSF) method, based on the validity of effective time theory, which generates a continuous experimental shift factor curve from a single test. Results are presented for this method when applied to a polyimide/carbon fiber composite material tested in shear under temperature jump conditions; this nonisothermal aging data for a polymer matrix composite is shown to exhibit similar response to that of homogeneous polymers. The new CSF technique will be useful in the development of models to predict the shift factor due to coupled aging and thermal history.