Shear-induced crystallization behavior of isotactic polypropylene (iPP) was investigated using a rotational rheometry covering a shear rate range of 1.2×10-4～1 s-1 and temperatures between 135℃ and 145℃.As the specimen was sheared at a constant rate

the viscosity and normal force were constant at first

then started to increase sharply after a certain time.At higher shear rates the transition of the normal force occurred considerably earlier than that of the viscosity

and at medium shear rates the two transition times were coincident.However at extremely low shear rates the data of normal force became noisy and only the data of viscosity was reliable.It was suggested that the earlier time in transitions for the normal force and viscosity was used to define an onset time （ton）which characterized the crystallization process.Furthermore

it is proposed that the plot of ton versus shear rate can be converted into a temperature-invariant curve by dividing the y axis by quiescent onset time ton

q and multiplying the x axis by ton

q.