Thursday, December 25, 2008

What are the Failure Of Engineering Material

Various theories of failure have been proposed, their purpose being to establish, from the behavior of a material subjected to simple tension or compression tests, the point at which failure will occur under any type of combined loading.

Structural Failure; many materials such as brick, glass, concreate and some metals are brittle, generally implying that when the limit of elasticity is reached they are close to disintegration or failure. Acceptable

Working stress for such material are obtained by loading them to destruction and then applying a suitable safty factor, this factor of safety should take into account that failure may be sudden or dramatic since plastic flow in them is not normally possible.

Often, a deficiency in engineering ethics is found to be one of the root causes of an engineering failure. An engineer, as a professional, has a responsibility to their client or employer, to their profession, and to the general public, to perform their duties in as conscientious a manner as possible. Usually this entails far more than just acting within the bounds of law. An ethical engineer is one who avoids conflicts of interest, does not attempt to misrepresent their knowledge so as to accept jobs outside their area of expertise, acts in the best interests of society and the environment, fulfills the terms of their contracts or agreements in a thorough and professional manner, and promotes the education of young engineers within their field.

Structural failure refers to loss of the load-carying capacity of a component or member within the structure or of the structure itself. Structural failure is initiated when the material is stressed to its strength limit, thus causing fracture or excessive deformations. The ultimate failure strength of the material, component or system is its maximum load-bearing capacity. When this limit is reached, damage to the material has been done, and its load-bearing capacity is reduced significantly and quickly. In a well-designed system, a localized failure should not cause immediate or even progressive collapse of the entire structure. Ultimate failure strength is one of the limit states that must be accounted for in civil engineering.

In conection with tests to failure of materials and of structural parts or members it is important to observe and record the type of failure and the characteristics of the fracture. Such observation shoiuld include not only the phenomena associated with final rupture but also all evidences of change of condition such as yield, slip, scaling, necking down, localized crack development, etc..

Two modes of fracture may occur in a metallic or crystalline material- a separation fracture as a shear or sliding fracture.

It is based on the assumption that failure occurs when the maximum principle. Sharp on an element reaches a limiting value.

Primary Causes of Engineering Disasters

The primary causes of engineering disasters are usually considered to be;

* human factors (including both 'ethical' failure and accidents)
* design flaws (many of which are also the result of unethical practices)
* materials failures
* extreme conditions or environments, and, most commonly and importantly
* combinations of these reasons

A recent study conducted at the Swiss federal Institute of technology in Zurich analyzed 800 cases of structural failure in which 504 people were killed, 592 people injured, and millions of dollars of damage incurred.

Almost any component that fails in service does so because it wore out, it corroded, or it broke. Some components surfer two or more modes of deterioration.

Many years of failure analysis work have shown that most service failure did not occur because of bad material. More often than not, some common-sense design factor was ignored of there was an error in fabrication.

Common problems and preventive measures

Underground piping- check soil conditions; use cathodic protection, plastic or bitumanstic coated pipe, or polyethylene wrapping if the soil shows conductivity. This type of soil produces abnormal potential for corrosion of ferrous pipes.

Under deposit attack- Design equipment to be cleanable. If chemical handling equipment is difficult to clean thoroughly, chances are it will not be cleaned. When chemicals are allowed to accumulate on surfaces, severe local corrosion can occur the deposit.

Water coolant system- wherever possible use a recirculating system and add proper corrosion inhibitors to the water.

Oil sumps- use vapor space inhibitors in the oil to prevent corrosion in the tank space above the oil level.

Corrosion in storage- use vapor space inhibitors inert gas shielding, desiccants, hermetically sealed containers, or moisture- displacing oils on metal surface that may be stored for long periods of time before use. Keep storage container full whenever possible.

Coating- when using metallic coating for corrosion protection, try to use a metal that is anodic to the substrate so that pitting will not occur in pinholes and scratches. Nonmetal coating should be checked for pinholes and scratches.

Stress corrosion cracking- the most common causes of Stress corrosion cracking is chlorides in contact with austenitic stainless steels. Stress corrosion cracking will not occur if welds are stress relieved or if operating stress levels are kept low by design.

Environmentally assisted cracking- plastic failure studies shows that 30% of plastic failure is due to environmental stress cracking. Some corrosion engineers believe that all plastic will eventually develop environmental stress cracks.

Intergranular corrosion: use low-carbon or stabilized grades of austenitic stainless steel when welding processes can cause sensitization; check welded tubing and pipe for sensitization (in manufacture) before use (ASTM A 708). Avoiding use of stainless steels sensitizing range from 800 to 1650 F.

High temperature Oxidation: avoid the use of metals at temperatures in excess of the maximum use temperature in air. Temperature above the maximum use temperature cause excessive scaling.

Welding- Avoid incomplete penetration welds. Unfused part of the joint can become a concentration cell. Invert gas backing or pickling after welding should be used to prevent weld-contamination corrosion of s.s.

Some concepts;

- premature wear failures can be minimized by designing parts to resist specific modes of wear.

- corrosion failure can be minimized by using materials with documented ability to withstand a particular environment.

- Mechanical failure can be minimized by calculating operating stresses cad keeping these stress rupture of the materials involved. It is also imperative to prevent the introduction of geometric stress concentrating should also be used to ensure that flaws from fabrication or service conditions do not exist.

- Nothing has to fail. Roads paved by the Romans 2000 years ago are still in use; many modern highways in the northeast united stated last less than 10 years. The difference is ,very simply, proper design, and Quality workmanship.


-Material in construction, G.D Taylor.
-Engineering Material properties and selection, 7th edition, Kenneth G. budinski, micheal K. budinski.
-Strength of materials, 2nd edition.
-The testing and inspection of Engineering material, Harmer E. Davis, Consulting Edition.

No comments:

Post a Comment