Page 38 - Mechanical Behavior of Materials

Section 1.6 Summary 37

over a longer time period through research and development—that is, by obtaining new knowledge
and developing ways to put this knowledge to work. And the final roughly one-third would
be difficult to eliminate without major research breakthroughs. Hence, noting that two-thirds
of these costs could be eliminated by improved use of currently available technology, or by
technology that could be developed in a reasonable time, there is a definite economic incentive
for learning about deformation and fracture. Engineers with knowledge in this area can help
the companies they work for avoid costs due to structural failures and help make the design
process more efficient—hence more economical and faster—by early attention to such potential
problems. Benefits to society result, such as lower costs to the consumer and improved safety and
durability.

1.6 SUMMARY

Mechanical behavior of materials is the study of the deformation and fracture of materials. Materials
tests are used to evaluate the behavior of a material, such as its resistance to failure in terms of the
yield strength or fracture toughness. The material’s strength is compared with the stresses expected
for a component in service to assure that the design is adequate.
Different methods of testing materials and of analyzing trial engineering designs are needed for
different types of material failure. These failure types include elastic, plastic, and creep deformation.
Elastic deformation is recovered immediately upon unloading, whereas plastic deformation is
permanent. Creep is deformation that accumulates with time. Other types of material failure
involve cracking, such as brittle or ductile fracture, environmental cracking, creep rupture, and
fatigue. Brittle fracture can occur due to static loads and involves little deformation, whereas
ductile fracture involves considerable deformation. Environmental cracking is caused by a hostile
chemical environment, and creep rupture is a time-dependent and usually ductile fracture. Fatigue
is failure due to repeated loading and involves the gradual development and growth of cracks.
A special method called fracture mechanics is used to specifically analyze cracks in engineering
components.
Engineering design is the process of choosing all details necessary to describe a machine,
vehicle, or structure. Design is fundamentally an iterative (trial and error) process, and it is necessary
at each step to perform a synthesis in which all concerns and requirements are considered together,
with compromises and adjustments made as necessary. Prototype and component testing and
monitoring of service experience are often important in the later stages of design. Deformation
and fracture may need to be analyzed in one or more stages of the synthesis, testing, and actual
service of an engineered item.
Advancing and changing technology continually introduces new challenges to the engineering
designer, demanding more efficient use of materials and improved materials. Thus, the historical
and continuing trend is that improved methods of testing and analysis have developed along with
materials that are more resistant to failure.
Deformation and fracture are issues of major economic importance, especially in the motor
vehicle and aircraft sectors. The costs involved in avoiding fracture and in paying for its
consequences in all sectors of the economy are on the order of 4% of the GNP.