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Phenolic composites are replacing metals in a number of automotive engine and transmission components. Phenolics’ high elastic stiffness and excellent creep resistance enable the design of bolted engine components without requiring costly and heavy metal mounting boss inserts. The creep performance of a phenolic composite has been characterized as a function of stress, time, temperature and degree of cure (Tg). Creep strain ε(t) is shown to be proportional to applied stress σ. The effect of time is characterized as “primary creep” (ε(t) = A + Btc), with C=0.25 providing a good fit across a broad range of conditions. The phenolic material's creep performance is shown to depend on temperature and Tg only through the reduced temperature variable (Tg - T). Design equations are presented which describe the measured elastic and creep strain versus stress, time, temperature and Tg.

Composite materials are replacing metals in a variety of engine and transmission components, including covers, manifolds, pumps and housings. In many applications, the cost and complexity of mounting boss inserts can be avoided by understanding composite creep performance and designing bolted joints for adequate bolt load retention. A lumped element model of a bolted joint is presented. Based on empirical equations describing creep performance, the present model yields equations to predict clamp load retention in bolted joints. Load retention is aided by the elastic compliance in the fastener, and is reduced by creep compliance in the boss and gasket. To ensure adequate load retention, the fastener must provide adequate elastic compliance to compensate for creep in the boss and gasket. The present model provides a means of choosing composite material, process conditions and fastener type based on application clamp load retention requirements.

Phenolic molding materials are increasingly being designed in to replace metal components in engines and transmissions. The effective use of phenolics requires the integration of part design, processing and material selection, not simply the substitution of a new material into an old design. In this paper we outline relevant design data and methods for Metal Alternative Design (MAD) with phenolic molding materials.