In
engineering
Engineering is the use of scientific method, scientific principles to design and build machines, structures, and other items, including bridges, tunnels, roads, vehicles, and buildings. The discipline of engineering encompasses a broad rang ...
, damage tolerance is a property of a structure relating to its ability to sustain defects safely until repair can be effected. The approach to
engineering design
The engineering design process is a common series of steps that engineers use in creating functional products and processes. The process is highly iterative - parts of the process often need to be repeated many times before another can be entere ...
to account for damage tolerance is based on the assumption that flaws can exist in any structure and such flaws propagate with usage. This approach is commonly used in
aerospace engineering
Aerospace engineering is the primary field of engineering concerned with the development of aircraft and spacecraft. It has two major and overlapping branches: aeronautical engineering and astronautical engineering. Avionics engineering is si ...
,
mechanical engineering
Mechanical engineering is the study of physical machines that may involve force and movement. It is an engineering branch that combines engineering physics and mathematics principles with materials science, to design, analyze, manufacture, and ...
, and
civil engineering
Civil engineering is a professional engineering discipline that deals with the design, construction, and maintenance of the physical and naturally built environment, including public works such as roads, bridges, canals, dams, airports, sewage ...
to manage the extension of
cracks in structure through the application of the principles of
fracture mechanics
Fracture mechanics is the field of mechanics concerned with the study of the propagation of cracks in materials. It uses methods of analytical solid mechanics to calculate the driving force on a crack and those of experimental solid mechanics t ...
. A structure is considered to be damage tolerant if a maintenance program has been implemented that will result in the detection and repair of accidental damage,
corrosion
Corrosion is a natural process that converts a refined metal into a more chemically stable oxide. It is the gradual deterioration of materials (usually a metal) by chemical or electrochemical reaction with their environment. Corrosion engine ...
and
fatigue
Fatigue describes a state of tiredness that does not resolve with rest or sleep. In general usage, fatigue is synonymous with extreme tiredness or exhaustion that normally follows prolonged physical or mental activity. When it does not resolve ...
cracking before such damage reduces the
residual strength
Residual strength is the load or force (usually mechanical) that a damaged object or material can still carry without failing. Material toughness
In materials science and metallurgy, toughness is the ability of a material to absorb ene ...
of the structure below an acceptable limit.
History
Structures upon which human life depends have long been recognized as needing an element of
fail-safe
In engineering, a fail-safe is a design feature or practice that in the event of a specific type of failure, inherently responds in a way that will cause minimal or no harm to other equipment, to the environment or to people. Unlike inherent safe ...
ty. When describing his flying machine,
Leonardo da Vinci
Leonardo di ser Piero da Vinci (15 April 14522 May 1519) was an Italian polymath of the High Renaissance who was active as a painter, Drawing, draughtsman, engineer, scientist, theorist, sculptor, and architect. While his fame initially res ...
noted that "In constructing wings one should make one chord to bear the strain and a looser one in the same position so that if one breaks under the strain, the other is in the position to serve the same function."
Prior to the 1970s, the prevailing engineering philosophy of aircraft structures was to ensure that airworthiness was maintained with a single part broken, a redundancy requirement known as
fail-safe
In engineering, a fail-safe is a design feature or practice that in the event of a specific type of failure, inherently responds in a way that will cause minimal or no harm to other equipment, to the environment or to people. Unlike inherent safe ...
ty. However, advances in
fracture mechanics
Fracture mechanics is the field of mechanics concerned with the study of the propagation of cracks in materials. It uses methods of analytical solid mechanics to calculate the driving force on a crack and those of experimental solid mechanics t ...
, along with infamous catastrophic fatigue failures such as those in the
de Havilland Comet
The de Havilland DH.106 Comet was the world's first commercial jet airliner. Developed and manufactured by de Havilland in the United Kingdom, the Comet 1 prototype first flew in 1949. It featured an aerodynamically clean design with four d ...
prompted a change in requirements for aircraft. It was discovered that a phenomenon known as
multiple-site damage could cause many small cracks in the structure, which grow slowly by themselves, to join one another over time, creating a much larger crack, and significantly reducing the expected time until failure
Safe-life structure
Not all structure must demonstrate detectable crack propagation to ensure safety of operation. Some structures operate under the
safe-life design
In safe-life design, products are intended to be removed from service at a specific design life.
Safe-life is particularly relevant to simple metal aircraft, where airframe components are subjected to alternating loads over the lifetime of the ai ...
principle, where an extremely low level of
risk
In simple terms, risk is the possibility of something bad happening. Risk involves uncertainty about the effects/implications of an activity with respect to something that humans value (such as health, well-being, wealth, property or the environme ...
is accepted through a combination of testing and analysis that the part will never form a detectable crack due to fatigue during the service life of the part. This is achieved through a significant reduction of stresses below the typical fatigue capability of the part. Safe-life structures are employed when the cost or infeasibility of inspections outweighs the weight penalty and development costs associated with safe-life structures.
An example of a safe-life component is the
helicopter rotor
A helicopter main rotor or rotor system is the combination of several rotary wings (rotor blades) with a control system, that generates the aerodynamic lift force that supports the weight of the helicopter, and the thrust that counteracts aerody ...
blade. Due to the extremely large numbers of cycles endured by the rotating component, an undetectable crack may grow to a critical length in a single flight and before the aircraft lands, result in a catastrophic failure that regular maintenance could not have prevented.
Damage tolerance analysis
In ensuring the continued safe operation of the damage tolerant structure, inspection schedules are devised. This schedule is based on many criteria, including:
*assumed initial damaged condition of the structure
*stresses in the structure (both
fatigue
Fatigue describes a state of tiredness that does not resolve with rest or sleep. In general usage, fatigue is synonymous with extreme tiredness or exhaustion that normally follows prolonged physical or mental activity. When it does not resolve ...
and operational maximum stresses) that cause
crack growth from the damaged condition
*geometry of the material which intensifies or reduces the stresses on the
crack tip
*ability of the material to withstand cracking due to stresses in the expected environment
*largest crack size that the structure can endure before catastrophic failure
*likelihood that a particular inspection method will reveal a crack
*acceptable level of
risk
In simple terms, risk is the possibility of something bad happening. Risk involves uncertainty about the effects/implications of an activity with respect to something that humans value (such as health, well-being, wealth, property or the environme ...
that a certain structure will be completely failed
*expected duration after manufacture until a detectable crack will form
*assumption of failure in adjacent components which may have the effect of changing stresses in the structure of interest
These factors affect how long the structure may operate normally in the damaged condition before one or more inspection intervals has the opportunity to discover the damaged state and effect a repair. The interval between inspections must be selected with a certain minimum safety, and also must balance the expense of the inspections, the weight penalty of lowering fatigue stresses, and the opportunity costs associated with a structure being out of service for maintenance.
Non-destructive inspections
Manufacturers and operators of aircraft, trains, and civil engineering structures like bridges have a financial interest in ensuring that the inspection schedule is as cost-efficient as possible. In the example of aircraft, because these structures are often revenue producing, there is an
opportunity cost
In microeconomic theory, the opportunity cost of a particular activity is the value or benefit given up by engaging in that activity, relative to engaging in an alternative activity. More effective it means if you chose one activity (for example ...
associated with the maintenance of the aircraft (lost ticket revenue), in addition to the cost of maintenance itself. Thus, this maintenance is desired to be performed infrequently, even when such increased intervals cause increased complexity and cost to the overhaul. Crack growth, as shown by
fracture mechanics
Fracture mechanics is the field of mechanics concerned with the study of the propagation of cracks in materials. It uses methods of analytical solid mechanics to calculate the driving force on a crack and those of experimental solid mechanics t ...
, is exponential in nature; meaning that the crack growth rate is a function of an exponent of the current crack size (see
Paris' law
Paris' law (also known as the Paris–Erdogan equation) is a crack growth equation that gives the rate of growth of a fatigue crack. The stress intensity factor K characterises the load around a crack tip and the rate of crack growth is experime ...
). This means that only the largest cracks influence the overall strength of a structure; small internal damages do not necessarily decrease the strength. A desire for infrequent inspection intervals, combined with the exponential growth of cracks in structure has led to the development of
non-destructive testing
Nondestructive testing (NDT) is any of a wide group of analysis techniques used in science and technology industry to evaluate the properties of a material, component or system without causing damage.
The terms nondestructive examination (NDE), n ...
methods which allow inspectors to look for very tiny cracks which are often invisible to the naked eye. Examples of this technology include
eddy current
Eddy currents (also called Foucault's currents) are loops of electrical current induced within conductors by a changing magnetic field in the conductor according to Faraday's law of induction or by the relative motion of a conductor in a mag ...
,
ultrasonic
Ultrasound is sound waves with frequencies higher than the upper audible limit of human hearing. Ultrasound is not different from "normal" (audible) sound in its physical properties, except that humans cannot hear it. This limit varies fr ...
,
dye penetrant, and
X-ray
An X-ray, or, much less commonly, X-radiation, is a penetrating form of high-energy electromagnetic radiation. Most X-rays have a wavelength ranging from 10 picometers to 10 nanometers, corresponding to frequencies in the range 30&nb ...
inspections. By catching structural cracks when they are very small, and growing slowly, these non-destructive inspections can reduce the amount of maintenance checks, and allow damage to be caught when it is small, and still inexpensive to repair. As an example, such repair can be achieved by drilling a small hole at the crack tip, thus effectively turning the crack into a
keyhole-notch.
References
Further reading
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{{DEFAULTSORT:Damage Tolerance
Aerospace engineering
Fracture mechanics
Mechanical failure
Mechanical failure modes