A mechanical design of an ultra-light log trailer was performed. The design process necessitated the experimental measurement of dynamic loads exerted on a log trailer and a comprehensive stress analysis of the structure.

Two light-weight, prototype, log trailers were selected to be studied, after an exhaustive survey of the existing designs. Finite element models (FEM) were developed for each of the trailers using three-dimensional tapered unsymmetrical beam elements. Static stress analysis was performed to identify critical spots in the structures and to estimate stresses encountered at these locations. These spots were selected as strain gage placements for the experimental dynamic stress analysis.

A special data acquisition system based on the STD bus computer was designed, assembled, programmed, and tested for the experimental force and strain measurements.

Experimental stress analysis of each of the selected trailers was performed. Dynamic loads and the resultant strains at critical locations were measured, both while simulating extreme situations, and during typical work cycles. The recorded service stress-time histories were then used to identify peak values of the maximum dynamic loading and the structure response. Stress distributions throughout the structures were obtained using the FEM models. The recorded service load spectra were then utilized to assess a fatigue life of each of the tested designs.

A survey of log trailers for fatigue cracks was conducted when the dynamic stress analyses indicated that a log trailer is most likely to fail due to repeated loading. The causes for fatigue cracking of log trailers were then investigated through elastic shell element FEM modeling.

Finally, an efficient and economically feasible ultra-light design was developed based on the formulated recommendations.

The most important features of the proposed design are:

1. Reduced tare weight of 7,770 pounds.

2. Sound structural integrity.

3. Material of high strength and toughness used throughout.

4. Improved fatigue resistance (e.g., welds were replaced by elastic friction joints).

5. Movable bunks.

6. Replaceable bolsters and standards.

7. Constant tensioning device for load binders.