Figure 1

As kids most of you glued plastic models together such as jet planes, Old Ironsides, the Nautilus, and so on. Full size trucks are not much different, at least some parts of certain models. Major truck body parts like whole hoods with integral fenders may be molded in two or three section and adhesively bonded together.

I ran into a problem with the bonds which held heavy truck hoods together. The right and left halves of these heavy truck hoods with integral fenders were molded of sheet molding compound (SMC) which is a thermosetting plastic resin containing about 30 percent by volume of chopped glass fibers randomly oriented for reinforcement. The raw material comes in soft, pliable sheets that are cut to size, laid into molds, compressed to shape and thickness, and heated to cure into rigid complex shapes. These shapes, such as the right and left halves of a truck from the bumper to the windshield, are then bonded together with a thermosetting adhesive. The lap joint is typically at least 25 mm (1 in.) wide. The adhesive is supposed to spread throughout the joint area when the two parts are brought together and then is supposed to cure, holding the parts together. The parts in question were made by a first tier supplier and shipped to a truck assembly plant for final assembly into vehicles.

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The problem showed itself in the field where fleets of new trucks were falling apart. 

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Failures of the adhesive bond can occur from several causes including unclean surfaces, lack of adhesive, precure of the adhesive if the parts are not put together soon enough, and spring back of the parts if they are not clamped into position during the cure. The problem I ran into was compounded by all of these causes, not just one. Contamination could never be ruled out because of the shipping and handling routine. Adhesive was applied by hand with things like caulking guns so that areas could be missed in a hurry up routine. Workers could take a cigarette break between the application of the adhesive and the joining of parts. Because the parts were not clamped but simply set aside, gravity and mismatch could cause parting of the adhesive line in the adhesive curing at room temperature. And, compounding the problem still further, a relatively rapidly polymerizing adhesive was used so that the parts would not have much time to sag apart before curing. This attempt to circumvent the spring back problem (without the use of clamping jigs) exacerbated the precure problem if there were assemble delays.

The problem showed itself in the field where fleets of new trucks were falling apart. Failure rates up to 40 percent were experienced. Since these heavy trucks were supposed to be durable for industrial jobs, the truck company's reputation was on the line. To complicate the situation, the first tier supplier was secretly repairing adhesive bonds in the field without informing the warranty arm of the truck company. When we found out, we calculated the actual loss to the truck company at $250 000 a year plus a large multiple for damage to reputation.

The most obvious solution, namely to change processes or to change suppliers, was complicated by contractual obligations and the time to renegotiate and plan, probably two years. The situation was so bleak that that truck company management had issued an edict declaring the use of adhesively bonded SMC parts to be infeasible in manufactured products. The next step would have been an order to stop production, bringing heavy truck production to a screeching halt. The threat of this action was real as was its implementation.

At that time, it was obvious that an NDT method was necessary, there was not one available. The truck company wanted to be able to test bonded truck bodies as they arrived at the assembly plant and to retrofit such inspections into the first tier supplier's plant. The truck company wanted a field portable method for obvious reasons.

At that point, the only test method available to the truck company was a gross test for the absence of adhesive. A feeler gage shim was used as a probe between the two layers of SMC to detect whether adhesive was missing. The test proved ineffectual because many truck hoods were observed with the edges of adhesive joints buttered over with extra adhesive, which prevented the entry of the shim. Sawing up these hoods revealed that the adhesive was missing from within the joints. Besides, the shim method did not address the question of weak bonds containing adhesive.

The plastics design group of the truck company assembled a task force and looked up as many NDT methods and instruments as they could find, they but got no definitive off the shelf answers. They came to me as head of the NDT research, development, and applications group to evaluate these leads or to invent a new method.

I put Gilbert Chapman, II, on the job, and he singled out an ultrasonic instrument as having potential. This was the Sondicator Mk II, which was manufactured at that time by Automation Industries and has now been redesigned by Zetec. The instrument used lamb waves at approximately 25 kHz propagating between two closely spaced probe tips. Actually, the wave motion involved both propagating waves and evanescent waves analogous to resonance near the tips. The received signal was compared in both amplitude and phase with the input signal by means of built-in circuitry, and poor bonds were signaled by a red light and an audible tone burst. The instrument required calibration against acceptable reference standards of adhesively bonded material.

The device was immediately found to be capable of differentiating between well adhered adhesive in the lap joints and the lack of adhesive over moderate areas, including buttered over vacant regions. However further work was required to detect the present but nonadhered adhesive and also adhesive with weak bond(s).

Chapman made a breakthrough on this challenge by making one important discovery. The instrument would reject almost all industrially made bonds if it was calibrated against perfectly made bonds in the laboratory. In reality, many of the industrially made bonds were strong enough to survive in the field. The test in this stage of development would have rejected all of production. Chapman's conclusion was that the perfect laboratory calibration standard was worthless. It followed that he had to create a calibration standard containing the requisite degree of imperfection to just barely accept bonds and reject the bonds that were actually made but unacceptably weak.

Chapman solved the problem of the creation of sufficiently imperfect reference standards by applying statistics to a large family of bond samples made in the supplier's factory by hourly personnel under production conditions. These samples Chapman tested and rank ordered with the instrument modified to give quantitative read out, not just the red light and tone burst no go alarm of its regular operation. Physical tensile pull tests then determined the instrument level corresponding to the rejectable strength level. The reference standard was born as the type of sample just good enough to exceed the minimum specifications of the pull test. With the reference standard, the no go test could be used.

Chapman then taught the method at the plant where the trucks were assembled. The truck company also instructed the first tier supplier on the use of the method and taught its own quality assurance surveillance agents to use the method so that high quality could be assured at the supplier and so that the nonconforming product would not be shipped to the assembly plant.

The quality management office of the truck manufacturer accepted the method after Chapman wrote it up in the standard format. The method then served to define a specification for an adequate adhesive lap join on a per unit length basis. No such specification had existed in the industry previously. The Chapman specification is now accepted as an exact parallel to the spot weld specification for steel.

The edict declaring adhesively bonded SMC to be infeasible in a manufacturing context was rescinded just weeks before the order to stop truck production was to have been issued. One can imagine the magnitude of disruption that would have occurred if the company had been forced to revert to steel truck bodies. It would have impacted the plastics industry, the company's stamping plants, steel sheet orders, fuel economy, corrosion lifetimes of bodies, and all the future designs for a variety of SMC parts for further trucks and cars. As feasibility of adhesive bonding of SMC was reestablished, the use of SMC was extended to other parts and other car lines, thus improving corporate average fuel economy mileage and durability. The rescuing of SMC and the elimination of all the above problems is directly attributable to NDT applied with imagination and the requisite degree of smarts.

The cost of NDT for keeping the SMC bonding process under surveillance for a year was about $25 000 including wages and the cost of the instrument. The first tier SMC supplier reduced its failure rate from 40 percent to five percent simply because it became cognizant that it could be monitored by the NDT police function. Other parts went into production in later years because their bonding quality could be assured. NDT paid for itself many times over.

The references contain articles by Gilbert Chapman, II, on the method he developed as well as an economic analysis by Emmanuel Papadakis. For further study of proper management of NDT in industry and for further study of the cost of quality when detrimental conditions occur, one would be advised to take short courses on quality, finances, and NDT.

 

References
Chapman, G.B., II, "Nondestructive Inspection for Quality Assurance of Fiber Reinforced Plastic Assemblies," Paper No. 820226, SAE Transactions, Vol. 91, 1982, pp. 887-896.

Chapman, G.B., II, "A Nondestructive Method of Evaluating Adhesive Bond Strength in Fiberglass Reinforced Plastic Assemblies," STP 749, Philadelphia, PA, American Society for Testing and Materials, 1983, pp. 32-60.

Chapman, G.B., II, "Practical NDI for Fiber-Reinforced Plastics," Materials Engineering, October 1982, pp. 72-73.

Chapman, G.B., II, E.P. Papadakis, and F.J. Meyer, "A Nondestructive Testing Procedure for Adhesive Bonds in FRP Assemblies," Body Engineering Journal, Fall 1964, pp. 11-22.

Ford Manufacturing Staff, Ford Laboratory Test Method FLTM BU 17-1, "Nondestructive Inspection (NDI) of Adhesive Bonds," Dearborn, MI, Ford Motor Co., July 27, 1980.

Papadakis, E.P., "The Deming Inspection Criterion for Zero of 100 Percent Inspection," J. Quality Technology, Vol. 17, No. 3, July 1985, pp. 121-127.

 

* Quality Systems Concepts, Inc., 379 Diem Woods Dr., New Holland, PA 17557; (717) 355-9809; fax (717) 355-9812; e-mail papadakis@desupernet.net.

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