2.1 Introduction The stiffness of a part is defined as the relationship between the load and the deflection of a part. This Chapter will discuss what modifications can be made to a part in order to influence and optimize stiffness. It will also give some guidelines of how the stiffness of a part can be calculated. 2.2 How to determine the stiffness In general the stiffness of a part is determined by its material and its geometry. 2.2.1 Material The most important material property for stiffness is the stress/strain curve. In general, the Young’s modulus, which is determined from the stress/strain curve, is the best parameter to be used when comparing the stiffness of materials. However, when the Young’s modulus is used, the stress/strain curve is assumed to be linear. Especially for thermoplastics, the range in which the stress/strain curve can be estimated with a straight line is limited. For this reason, when a stiffness calculation of a part is made, it is necessary to check if the occurring stresses and strains still allow a linear approach. If this is not the case, it is advised to use a secant modulus for the stiffness calculation. Consider n FIGURE 2. Suppose that the stiffness of a part is calculated using the Young’s modulus Y. Suppose that a verification of the occurring stress results in the value s. This value is clearly out of the range in which a linear approximation of the stress/strain curve is justified. In this case it is better to recalculate the stiffness of the part with the secant modulus Y*. Furthermore, it is important to consider the temperature at which the load is applied. For thermoplastics the stress/ strain curves are heavily dependent on temperature. It is advised to consider the stress/strain curve at the temperature at which the load is applied for the calculation of part stiffness. Time also plays a role in the determination of the stiffness of a part. It can influence stiffness in one of two ways: (a) the material is loaded for a long time, (b)the material is loaded in cycles, (c) the material is loaded during a very short time. The phenomena (a) and (b)are typically known as creep and fatigue. How these effects must be taken into account can be found in Chapter 4 ‘Design for behaviour over time’. Phenomenon (c) is known as impact. ...... www.m448 中国最大的资料库下载