Page 35 - Mechanical Behavior of Materials

36 Chapter 1 Introduction

while designing hardware in a manner that accommodates their still relatively low fracture
toughness.
In general, the challenges of advancing technology require not only improved materials, but
also more careful analysis in design and more detailed information on materials behavior than
before. Furthermore, there has recently been a desirable increased awareness of safety and warranty
issues. Manufacturers of machines, vehicles, and structures now find it appropriate not just to
maintain current levels of safety and durability, but to improve these at the same time that the other
technological challenges are being met.

1.5 ECONOMIC IMPORTANCE OF FRACTURE

A division of the U.S. Department of Commerce, the National Institute of Standards and Technology
(formerly the National Bureau of Standards), completed a study in 1983 of the economic effects
of fracture of materials in the United States. The total costs per year were large—specifically,
$119 billion in 1982 dollars. This was 4% of the gross national product (GNP), therefore
representing a significant use of resources and manpower. The definition of fracture used for the
study was quite broad, including not only fracture in the sense of cracking, but also deformation
and related problems such as delamination. Wear and corrosion were not included. Separate studies
indicated that adding these to obtain the total cost for materials durability would increase the total
to roughly 10% of the GNP. A study of fracture costs in Europe reported in 1991 also yielded an
overall cost of 4% of the GNP, and a similar value is likely to continue to apply to all industrial
nations. (See the paper by Milne, 1994, in the References.)
In the U.S. fracture study, the costs were considered to include the extra costs of designing
machines, vehicles, and structures, beyond the requirements of resisting simple yielding failure
of the material. Note that resistance to fracture necessitates the use of more raw materials, or
of more expensive materials or processing, to give components the necessary strength. Also,
additional analysis and testing are needed in the design process. The extra use of materials and other
activities all involve additional costs for manpower and facilities. There are also significant expenses
associated with fracture for repair, maintenance, and replacement parts. Inspection of newly
manufactured parts for flaws and of parts in service for developing cracks involves considerable cost.
There are also costs such as recalls, litigation, insurance, etc., collectively called product liability
costs, that add to the total.
The costs of fracture are spread rather unevenly over various sectors of the economy. In the U.S.
study, the sectors involving the largest fracture costs were motor vehicles and parts, with around 10%
of the total, aircraft and parts with 6%, residential construction with 5%, and building construction
with 3%. Other sectors with costs in the range of 2 to 3% of the total were food and related products,
fabricated structural products, nonferrous metal products, petroleum refining, structural metal, and
tires and inner tubes. Note that fatigue cracking is the major cause of fracture for motor vehicles
and for aircraft, the two sectors with the highest fracture costs. However, brittle and ductile fracture,
environmental cracking, and creep are also important for these and other sectors.
The study further found that roughly one-third of this $119 billion annual cost could be
eliminated through better use of then-current technology. Another third could perhaps be eliminated