The determination of the elastic parameter values is based on the linear 

 part of the stress-strain curve. For ultimate stress design, the knowledge 

 of the nonlinear part of the stress-strain curve is very important. Investi- 

 gation is also being done on the stress interaction behavior of wood. 



One of the problems associated with the theoretical prediction of the 

 strength of wood is the lack of understanding of its mechanism of failure. 

 Fracture mechanics of wood as well as the concept of energy of distortion 

 limitation, are also being investigated. 



(2) Time -Dependent Characteristics . Because of the time-dependent 

 stress-strain behavior of wood a large amount of investigation has been 

 concerned with the rheological properties of this material. Nonlinear time- 

 dependent relationships, cyclic loads, and cyclic environmental factors all 

 complicate these relationships. Among the various rheological properties, 

 creep behavior appears to be the property most often needed in designing with 

 wood. The effect of duration of load on the strength properties also is 

 being investigated. Dynamic forces act for a very short duration and under 

 these conditions wood appears to be stronger and stiffer than under static 

 loading. 



(3) Wood Composites . When compared to other construction materials, 

 wood is one of the most efficient materials available on a pound per pound 

 basis in stiffness and strength along the grain. However, its efficiency is 

 much lower if across-the-grain direction is considered. Thus for specific 

 engineering purposes it is necessary to rearrange the wood in relation to its 

 natural form. This necessitates the manufacture of composites such as 

 laminated beams, plywood, particleboard, hardboard, and fiberboard. Further 

 modifications can be made by high density overlays and impregnations and 

 preservatives . 



Some investigation and testing of laminated wood utilizes a proof-loading 

 concept to establish the laminating combinations and their associated design 

 stress. The research is intended to determine what tensile proof load should 

 be used in order to justify strength levels and what percentage of the 

 tension zone laminations should be subject to the proof load level. In order 

 to utilize more flexibility in laminating combinations, a project is in 

 progress which will provide criteria for combining different species. 



g. Plastics . The number of plastic materials and resins available today 

 is so great and the variety of synthetics available in each family of 

 plastic resins is so large that it is virtually impossible to identify 

 significant investigation and research that is ongoing and of importance to 

 coastal engineering materials development. Investigations and development 

 occur in three general areas: processing and machinery, new resins, and resin 

 modifications by additives. 



(1) Processing and Machinery . The development of new resins will 

 not lead to an improved product until the machinery for processing such 

 resins or modified resins can he developed. The new machinery will control 

 the plastic manufacturing in a manner to properly produce a given product 

 with the required physical properties. 



390 



