29-08-2014, 03:52 PM
This report provides a review of the visual inspection literature from the 1950s to the present. Although various reviews have appeared in book chapters and journal articles over the years, the findings from the literature may not be reported such that Nuclear Security Enterprise (NSE) employees can immediately understand and apply the concepts to nuclear weapons inspection work. Further, some reviews have focused on only one or two inspection topics (e.g., individual differences in inspection performance) and do not provide comprehensive summaries of the literature as a whole. Drury (1992) published the last comprehensive review of the visual inspection literature in the second edition of the Handbook of Industrial Engineering. The corresponding chapter in the third edition of the book published in 2007 is limited to the topics of human factors and automation in test and inspection. The current paper aims to extend the review to the present and provide a document that is readily available and useable for NSE personnel.
2.1.1. APPROACH
Two hundred twelve inspection-related documents written between 1958 and 2012 werereviewed and summarized during the course of the literature search. Of these documents, 140 were available in existing archives in the Human Factors and Statistics Department at Sandia National Laboratories in New Mexico. These documents were published between 1958 and 2004. The remaining 72 documents published 2004 or later were located by conducting four different keyword searches via Google Scholar:
· Visual inspection and human factors
· Visual inspection and defect detection
· Visual inspection and precision manufacturing
· Multiple inspections
2.2. CHARACTERIZATION OF VISUAL INSPECTION
The dictionary definition of inspection is a careful and critical examination, especially for flaws. The term itself derives from the Latin inspectionem, meaning to examine carefully or look closely into. In fact, inspection is typically a deliberate, in-depth, exacting process that requires more than mere looking or scanning. As Drury and Prabhu (1992) point out; precision, depth, and validity are critical elements of the definition. Inspection processes require a large amount of mental processing, concentration, and information transmission, along with extensive use of both short-term and long-term memory (Gallwey, 1982). For example, short-term memory is required to remember which areas of an item have already been inspected and which have not. Long-term memory may be required to recall the standard dictating what a good item should like. In addition, inspection tasks are usually inherently stressful for inspectors. Inspection must typically be completed quickly, and critical defects that may require rework must be identified early. In addition, multiple defects at various severity levels and locations may be present, adding to the complexity of the task
2.2.1. INSPECTION IN INDUSTRY AND BEYOND
Inspection is a major quality control component for many industrial tasks. The intent of conducting inspection is to verify that a product is free of defects before being distributed for use. Inspection plays a role in many different fields, most of which are characterized as high Consequence:
Food industry
Aircraft maintenance
Printed circuit assemblies
Airport baggage screening
Medicine (radiology, pharmaceuticals, histology)
Nuclear weapons
For most of these fields, the inspection process is exacerbated by the fact that defects tend to be rare occurrences, while the costs of missing them is high. As one example, if tainted meat ismissed during final inspection, people who consume the meat may fall seriously ill or die. Further, defects may be somewhat nebulous and difficult to define precisely. Airport baggage screening provides a good example of the challenges involved in defining “defects.” The catalog of potential threats changes constantly for airport baggage screeners. While the general nature of the threat is known (e.g., guns, knives, incendiaries); the actual size, shape, and characteristics change over time. Threats may also be deliberately concealed. The field of radiology provides another example of the difficulties inherent in inspection. For example, the incidence of breast cancer is very low, rare signs of potential cancer must be detected at their earliest presentation (which means detecting extremely small image changes), and cancer in the breast image may be masked by image background density (which varies with a woman’s age and physiology)
2.2.2. IMPORTANCE OF INSPECTION
The criticality of inspection in manufacturing and production becomes evident when the potential consequences of missed defects are examined. In some cases, missed flaws can haveserious consequences ranging from injury to fatality. The field of aviation maintenance and inspection provides three examples:
In 1988, an Aloha Airlines Boeing 737 lost part of its upper fuselage after cracks that were missed during inspection failed while the plane was in the air—one fatality and 65 injuries occurred.
In 1989, a crash landing in Sioux City, Iowa, took place after a cracked engine fan disc that went undetected during inspection failed completely—111 fatalities occurred.
In 1996, an engine failed during takeoff at the Pensacola, Florida, airport after a crack in the engine’s front compressor fan hub was not detected during inspection. There were two fatalities and one serious injury.
In other instances, inspection errors may not cause injuries or fatalities, but they can translate directly into costs for the company. On the one hand, a defective product may be shipped, which may negatively impact customer satisfaction and reduce the likelihood of repeat business. On the other hand, a good item may be classified as defective and have to be reworked or scrapped, resulting in unnecessary expenses for materials and labor. As just one example, a Malaysian factory that produced printed circuit assemblies launched an investigation in 2004 to resolve inspection errors because too many defective products were being distributed and subsequently returned. The distribution of defective products degraded customer satisfaction and led to an annual loss of nearly $300k.
Finally, as in the case of nuclear weapons work, an inspection error could conceivably lead to both types of consequences. A flaw in the system that goes undetected might lead to a situation in which the weapon system detonates unintentionally, causing serious injuries or fatalities. In some instances, if radioactive material is released, the injuries or fatalities may not be immediate. Such an incident would also incur steep costs for the Department of Energy in terms of lost materials, medical expenses, investigations, and recoveryto be defective. By contrast, modern production processes tend to de-emphasize or even criticize.