Reliability And Its Requirement, Reliability Vs. Quality Control, Bathtub Curve
1.0 Reliability and its equirement:
Reliability is the ability of an item to perform a required function under stated conditions for a stated period of time. The following factors make it almost essential that a product go through a reliability enhancement programme.
Rapid evolution of new materials, components, and processes.
Complexity of the product being manufactured.
Need to meet customer demands such as availability, safety, warranty.
To meet desired MTBF, product costs.
Compliance with the Governmental regulations.
2.0 Reliability Vs. Quality Control:
The inspectors view of reliability is that the product is assessed against a specification or set of attributes and having passed is delivered to the customer. However, this approach provides no measure of quality over a period of time. Therefore, the need for a time based concept of quality is developed. The inspectors concept is not time-dependent. The product either passes a given test or it fails. On the other hand, reliability is usually concerned with failures in the time domain. This distinction marks the difference between traditional quality control, and the approach to reliability.
Quality control (QC) of manufacturing processes obviously makes an essential contribution to the reliability of a product. QC can be considered as an integral part of an overall reliability programme. Reliability is generally concerned with failures during the life of a product.
3.0 The bathtub Curve:
Figure 1 shows how they are related to the reliability bathtub curve. The bathtub curve is a plot of failure rate vs. time. It is characterized by three regions in time: an initial region of decreasing failure rate, and intermediate region of relatively constant failure rate, and a final region of increasing failure rate. The bathtub curve is actually a composite curve, made up of the sums of three smaller curves: infant mortality, random failures, and wear out. Infant mortality failures are caused by defects in the product which cause it to fail early in its lifetime. They are also called intrinsic failures, since they are due to causes internal to the product. This type of failures decrease sharply with time. Random failures occur at a somewhat constant rate over the entire life of the product. Ideally, in a mature product, where the design and processes are good, the failures are usually due to forces external to the product, such as mishandling, external interface failures, or accidents. They are therefore called extrinsic failures. In the time period after infant mortality, but before the beginning of wear out, random failures dominate. This region is also called the useful life region of the product.
The final region of the bathtub curve represents the time when the product begins to wear out because it has reached the end of its useful life. There is usually only a few wear out failure mechanisms, which results from the stresses accumulated over the life of the product. These failures are intrinsic, and the failure rate increases in this region.