טכניון מכון טכנולוגי לישראל
הטכניון מכון טכנולוגי לישראל - בית הספר ללימודי מוסמכים  
M.Sc Thesis
M.Sc StudentStivi Nofar
SubjectConstitutive Model For Describing the Mechanical Behavior
Of a Glassy Polymer
DepartmentDepartment of Mechanical Engineering
Supervisors Professor Daniel Rittel
Dr. Arieh Sidess
Full Thesis text - in Hebrew Full thesis text - Hebrew Version


Abstract


This research investigates the mechanical behavior of a glassy polymer at different loading methods in room temperature and develop a constitutive model. The investigated polymer is Hysol 4183, a thermosetting viscoelastic-viscoplastic polymer.

The mechanical characterization including two types of loading: short-time (constant strain rate) and long-time (creep). The first part consists of uniaxial tension and compression tests at different strain rates, as well as uniaxial compression loadings at constant strain rate combined with confining pressure applied through confining sleeve. The second part consists of uniaxial creep tests experiments (loading and unloading) with various stress levels and limited number of creep compression with confining pressures.

The main results and findings from this research are:

(a)   The mechanical properties in tension and compression of the polymer are strongly depended, as expected, on the loading rate. As the strain rate increases the failure-stress increases as well, while the failure-strain decreases. This dependency is typical for viscoelastic or viscoplastic materials. Major differences were observed for the mechanical response in tension compared to compression.

(b)   Constant strain rate loading with confining pressure has a significant effect on the polymer’s behavior relative to uniaxial constant strain rate loading.  A linear relation was derived between confining pressure to the failure stress and strain.

(c)   For the constant strain rates tests a constitutive model was developed and its parameters were calibrated for each loading type. A good fitting was obtained between the measured and predicted results according to the calibrated model.

(d)   In creep tests elastic, viscoelastic and viscoplastic strains are developed during the loading time. In the unloading duration the elastic and viscoelastic strains are recoverable.

(e)   The elastic and viscoplastic strain developed in the creep tests are linear to the stress level as opposed to the non-linearity of the viscoelastic strain.

(f)    In the creep tests, it is obtained that at high stress level the dominant strain component is the viscoplastic one. However, in the case of low stress level the dominant strain components are the elastic and viscoelastic.

(g)   A constitutive model was developed based on the creep tests. The parameters of the model strain components were calibrated with the measured creep results. A good fitting was obtained between the measured and the predicted results, according to the calibrated model, for all stress levels both in the loading duration and the unloading duration.

(h)   The creep phenomenon is significantly affected by the confining pressure. The maximal strain developed in creep with confining pressure is almost 7 times smaller than uniaxial creep at the same stress level.

(i)   The most dominant component in the creep with confining pressure tests is the viscoplastic strain while the viscoelastic strain is negligible. This phenomenon is expected, considering the fact that confining a viscoelastic material stiffens its behavior and reduces its ductility and flexibility.