Jeff Kliner, age 15, of Portland, Ore., for his question:

WHAT CAUSES METAL FATIGUE?

Engineers use the term fatigue to describe certain changes that occur in substances like metals. After extensive use, the substance gradually weakens.

Repeated stress, such as continued weight, tension or pressure, causes this weakening. Such stress often alters the molecular structure of the materials so that they bend or crack.

Metal fatigue usually starts at the surface of a metal piece where s small defects or even minute tool marks serve as a concentration point for stress. The crack spreads through the piece, making it too weak to carry its normal load.

Engineers allow for fatigue when planning airplanes, bridges and machinery.

The strength of a material depends on its mechanical properties which include elasticity, hardness and stiffness. Mechanical properties combine differently in every material. Each material differs in its ability to resist various types of force.