Validation of a Critical Nitrogen Curve for Summer Maize in the North China Plain

The concept of critical N concentration(N_c)has been widely used in agronomy as the basis for diagnosis of crop N status,and allows discrimination between field situations of sub-optimal and supra-optimal N supply.A critical N dilution curve of N_c=34.0W^(-0.37),where W is the aboveground biomass(Mg DM ha^(-1))and N_c the critical N concentration in aboveground dry matter(g kg^(-1)DM),was developed for spring maize in Europe.Our objectives were to validate whether this European critical N dilution curve was appropriate for summer maize production in the North China Plain(NCP)and to develop a critical N dilution curve especially for summer maize production in this region.In total 231 data points from 16 experiments were used to test the European critical N dilution curve.These observations showed that the European critical N dilution curve was unsuitable for summer maize in the NCP,especially at the early growth stage.From the data obtained,a critical N dilution curve for summer maize in the NCP was described by the equation of N_c=27.2W^(-0,27),when aboveground biomass was between 0.64 and 11.17 Mg DM ha^(-1).Based on this curve,more than 90%of the data for the N deficiency supply treatments had an N nutrition index(NNI)<1 and 92%of the data for the N excess supply treatments had an NNI>1.

Finite Element Analysis of the Base

This paper describes the structure of the base,on which two working arms are installed simultaneously.To ensure structura safety,the fatigue failure analysis and statics analysis are finished using the finite element method.The calculation can make sure that the structure of the base meets the design standard,and the material can be reduced one grade.

Performance of Steel Structures under Fatigue Cyclic Loading

A component or structure, which is designed to carry a single monotonically increasing application of static load, may fracture and fail if the same load or even smaller load is applied cyclically a large number of times. For example a thin rod bent back and forth beyond yielding fails after a few cycles of such repeated bending. The fatigue failure is due to progressive propagation of flaws in steel under cyclic loading. This is partially enhanced by the stress concentration at the tip of such flaw or crack. The presence of a hole in a plate or simply the presence of a notch in the plate has created stress concentrations at the center points. These stress concentrations may occur in the material due to some discontinuities in the material itself. At the time of static failure, the average stress across the entire cross section would be the yield stress. However when the load is repeatedly applied or the load fluctuates between tension and compression, the center points experience a higher range of stress reversal than the applied average stress. These fluctuations involving higher stress ranges, cause minute cracks at these points, which open up progressively and spread with each application of the cyclic load and ultimately lead to rupture. Fatigue failure can be defined as the number of cycles and hence time taken to reach a pre-defined or a threshold failure criterion. Low cycle fatigue could be classified as the failures occurring in few cycles to a few tens of thousands of cycles, normally under high stress/strain ranges. High cycle fatigue requires about several millions of cycles to initiate a failure. The type of cyclic stresses applied on structural systems and the terminologies used in fatigue resistant design are illustrated in this paper. The common form of presentation of fatigue data is by using the S-N curve, where the total cyclic stress （S） is plotted against the number of cycles to failure （N） in logarithmic scale. The point at which the S-N curve flattens off is called the ＂endurance limit＂. To carry out fatigue life predictions, a linear fatigue damage model is used in conjunction with the relevant S-N curve.