Visual inspection is one of the most important nondestructive inspection techniques used today. However, it must be done carefully.
We have all heard the old adage, “Believe none of what you hear and half of what you see and you will get along just fine.”
One of the most important nondestructive inspection techniques used today is visual inspection. This is primarily because operators use visual methods during all processes of inspection.
For 20 years, I served in the United States Navy, and during that time, I progressed from a seaman apprentice to Navy chief petty officer and Level III examiner in all nondestructive testing (NDT) disciplines for the fleet. We were in a fast-paced repair, overhaul and fabrication unit for nuclear-powered submarine and deep submergence vessels.
Mistakes were not an option because our submariner shipmates’ lives depended on the decisions we made. It was critical that we adhere to correct inspection procedures, methods and acceptance criteria in all of our inspections. It was during my service in the U.S. Navy that I was first introduced to the various applications of visual NDT: radiography, ultrasonic, dye penetrant and magnetic particle inspection. Since then, I have continued to see a single common error in NDT: the misuse of visual inspection methods and practices.
Visual Inspection Trouble
One of the earliest examples of this phenomenon occurred for me during a routine submarine hull surveillance inspection at the sub base in Guam. We were in the torpedo room of a nuclear submarine where we noticed stalactites on a main ballast tank blow pipe fitting. We issued a corrective action requesting a gamma ray (Ir 192) inspection on the pipe joints and other areas where stalactites had occurred. As a result, more than 100 joints were rejected with massive holes in what was then 316 stainless steel piping, considered the best noncorrosive material available at that time.
After analysis, all submarines using 316 stainless steel piping for the main ballast blow system were required to replace this piping with Monel (NiCu). The finding was that 316 stainless steel, used for corrosion resistance and chemical tanks, was not suitable for the blow down system. There was a check valve designed to keep the salt water out of the piping system, but it had one design flaw-it leaked.
The combination of salty water and the 4,500 pounds of pressure to blow the ballast tanks in order for the submarine to surface caused severe corrosion in the 316 stainless steel. The inspection and repair cycle took about two months of 12 hours on, 12 hours off shifts. Fatigue soon set in, causing two injury accidents. We also found that X-ray film interpretation was about 50% incorrect between the hours of midnight and 5 a.m. As a result, final acceptance of film shot at night would not be made until the fresh new crew came in.
Do We See What We See?
Because all inspections require a visual inspection, let us look at methods used to perform inspections and evaluate their accuracy and effectiveness in a logical method.
First, here is a quick refresher on visual data collection. Visual inspection has two steps:
1) The entire field of vision is processed, an automatic function of the brain; and then
2) Focus on the specific object.
Based on this concept, it is theorized that various light patterns reaching the eyes are simplified and encoded as lines, spots, edges, shadows, colors, orientations and reference locations within the entire field of view. These are then combined with a comparison between visual data and the long-term memory of previously collected data.
This pattern, when scanned from left to right, makes it difficult to separate the reverse Ls from the upright Ts, however, the tilted Ts are easily recognized. Source: Lean Quality Systems Inc.
One of my clients was experiencing a challenge during the inspection of a specific part. This particular part was an injection mold and the target defect was flash, or excess material that is formed with and attached to the component along a seam or mold parting line.
The part to be inspected was the size of a silver dollar with an extrusion that snapped into another part when assembled. Flash on the part had stopped the customer’s automated assembly line, resulting in the return of 400,000 parts. The presence of the flash also had caused the re-inspection of more than a million parts from the same lot that was in the company’s warehouse stock.
As can be expected, the decision was to re-inspect and/or sort out the existing stock in order to find the good parts, or start a new production run that would require raw material, and at least three days of setup and validation to produce the 1.5 million new parts. This was a tough decision to make, and the proper training and use of visual inspection could have helped the company avoid this scenario.
One company wanted to inspect a part that was the size of a silver dollar with an extrusion that snapped into another part when assembled. Source: Lean Quality Systems Inc.
Key to Nondestructive Inspection
Visual inspections are the key to all nondestructive inspections because they all depend on visual physiological and psychological processing. Both play a large part when looking for one target defect. When looking for a number of defects, the error factor is compounded.
There are specific education requirements for NDT Level III examiners on the fundamentals of visual testing, and the methods and techniques of visual inspection are based on environmental and physiological factors, as well as performance standards. In the majority of companies I visit, there are no training programs that address these factors, which are quite possibly the most important aspects of NDT at the Level I (operator) or Level II (inspector) levels.
Another client that rebuilds cylinder heads inspects springs before they are reused and requires 100% inspection to determine if the spring still maintains its spring, and possesses discontinuities or defects that could shorten the life span. A visual inspection of a spring must determine the diameter, free length, squareness, rust, overheat discoloration, scores, bits, broken coils and any face damage.
This procedure establishes the acceptance requirements: hundreds of the same type of spring are cleaned and aligned as a group and inspected simultaneously. This is the ideal method for this small company because scanning the stationary group of springs can show anomalies or identify a target spring that is not fit for use.
There also are production runs of new products that require sampling and oftentimes accelerated inspections, otherwise known as 100% inspections. In the medical device industry, 100% inspections may be defined as a requirement for key characteristics by the customer, statutory or regulatory agencies.
The most difficult type of visual inspection occurs when hundreds of products are being produced in large quantities, or hundreds of parts require 100% inspection on a moving assembly line. This can task the inspector and make the process more error-prone.
For a client that rebuilds cylinder heads and inspects springs before they are reused, 100% inspection is required. Source: Lean Quality Systems Inc.
The Forgotten Para-Nondestructive Prerequisites
The greatest challenge in the NDT field is that many of the inspections on the production lines are automated and monitored by operators who may not be able to identify defective parts, but are able to respond to a bell ringing or red light alarm. Further, the machine may be programmed to kick out a suspected defective part during a production run.
With this in mind, let us look at variables that define a defective part in an automated machine. If you have ever had to set up or monitor an automated production line, you know that the most important step is the setup and validation of the machine. The tooling dies/molds and machine used all have to be in sync. With the proper dies and machine in place, the next step is line clearance, which is part of the validation. Line clearance establishes the set-up parameters, temperatures, pressures and cycle times necessary to deliver the drawing and specification required configuration.
The person on the front line is usually responsible for identifying potential problems; however, this operator is typically the least-skilled employee. He is simply given instructions and pictures to assist him in doing his job, but that is where the education often stops. As such, we are looking at the new breed of inspectors because most have less than a high school education.
What today’s manufacturing organizations must do is add the environmental factors and physiological factors, as well as performance standard methods to all NDT training programs. These are the most obvious and most overlooked training topics for Level I and Level II. Because there is no requirement for a Level III ASNT TC-1A examiner to be onsite for ultrasonic inspections, many companies are ignorant about the devastating effects and company liability associated with not being properly trained in visual inspections. NDT
Tech TipsVisual inspection has two steps: the entire field of vision is processed, followed by focus on the specific object.
It is theorized that various light patterns reaching the eyes are simplified and encoded, as lines, spots, edges, shadows, colors, orientations and reference locations within the entire field of view.
Light patterns are combined with a comparison between visual data and the long-term memory of previously collected data.