我国是世界上重要产棉国家之一有悠久的植棉历 史和广阔植棉地区东起长江三角洲沿海棉区及辽河 流域,起迄新疆塔里木盆地西缘,南自海南,北至新 疆玛纳斯垦区均可植棉。各种棉花病害也随之遍及各 棉区。 棉花病害是制约棉花生产的重要因素。据联合国粮 农组织估计,由于病害造成的损失达24% 美国棉花病害协会19521990年的统计,因病害造 成棉花的损失年平均为13.28%

Recall the definition of stress: o stress =\intensity of internal force at a point\ Figure 3.1 Representation of cross-section of a general body

Sources of Stresses and Strains Depends on type of structure Aircraft

As we've previously noted, we may often want to describe a structure in various axis systems. This involves... Transformations (Axis, Deflection, Stress, Strain, Elasticity Tensors) e.g., loading axes material principal axis Figure 7.1 Unidirectional Composite with Fibers at an Angle

Summarizing what we've looked at in elasticity, we have: 15 equations in 15 unknowns 3 equilibrium -6 strains -6 strain-displacement -3 displacements

The logical extension of discrete mass systems is one of an infinite number of masses. In the limit, this is a continuous system. Take the generalized beam-column as a generic representation:

Return to the simplest system the single spring-mass This is a one degree-of-freedom system with the governing equation

Before we look specifically at thin-walled sections, let us consider the general case (i.e, thick-Walled) Hollow thick-walled sections Figure 12.1 Representation of a general thick-walled cross-section 中=c2 on one boundary φ=c1 on one boundary This has more than one boundary(multiply-connected do=0 on each boundary

Earlier looked at Simple Beam Theory in which one considers a beam in the x-z plane with the beam along the x-direction and the load in the z-direction Figure 14.1 Representation of Simple Beam Now look at a more general case Loading can be in any direction

For a number of cross-sections we cannot find stress functions. However, we can resort to an analogy introduced by Prandtl(1903) Consider a membrane under pressure p, Membrane\. structure whose thickness is small compared to surface