Verein Deutscher Ingenieure

Failure analysis - Defects of thermoplastic products made of plastics caused by faulty design
 N VDI 3822 BLATT 2.1.1

Annotation

This guideline will deal with failures caused primarily by the faulty design of plastic components. As a rule, generally valid rules are to be followed in designing components and component groups; these apply to the process, dimensions, design and testing. In addition, there are extensive norms and literature dealing with the development and design of components. Specific publications that take the special characteristics of thermoplastic materials into consideration provide assistance in designing and planning plastic components (see Bibliography).

The design of plastic components is, however, subject, in large measure, to material-specific properties, which is why it is not possible to simply transfer designs originally developed for metallic materials. A common cause of failure in plastic components therefore results in the direct substitution of plastics in designs for metallic components without adapting them to the new materials or developing new components.

Generally applicable procedures and guidelines for design methodology, as found in the guidelines VDI 2221 and VDI 2222, also apply in the case of thermoplastic products. These aspects will not be addressed in this guideline.

In determining the dimensions and calculating the strength, the non-linear viscoelastic behaviour of thermoplastic materials must be taken into consideration. Other special considerations come to bear in conjunction with the design of screw and snap-lock joints, which are described in the literature. In individual cases, the failure analyst will need to re-evaluate the material data, re-calculation models and review the assumptions on which they were based, if this information is available. Tests for sufficient strength can, for example, be undertaken by using a computer aided FEM1 ) simulation and mechanical testing methods on sample components.