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without destroying parts. Vigorous global directed govern- ment and industry research is seeking ways to validate the adhesive bond-line of structures. When a viable method for bond-line inspection and validation is found, fasteners will be signifi cantly reduced. The elimination or signifi cant reduction of fasteners would be a boon to future airplane manufactur- ing by reducing cost and operations. Fasteners and the pro- cess to create a conforming hole for their insertion drives as much as 65% of the cost of an airframe; 80% of the defects; and 80% of lost-time injuries at the assembly level.


Laser bond inspection (LBI) is an inspection technology that provides a method to evaluate the relative strength of adhesive bonds in bonded structures. LBI is a local- ized testing method that applies a well-controlled dynamic tensile stress to a composite structure, and then senses the response of the adhesive bonds to the tensile stress. LBI can detect the pres- ence of weak regions in adhesive bonds. Many classes of substan- dard adhesive bonds (adhesive bonds with insuffi cient strength) are undetectable using conventional NDT techniques. Examples of hard- to-sense adhesive bond manufac- turing defects include a “kissing” bond which has good mechanical contact but no strength, a poor adhesion due to inadequate surface preparation, poor adhesion due to contamination (e.g. mold release agent), and weak adhesive due to improper mixing or curing. LBI can detect these defi ciencies. LBI can verify that the manufacturing process is under control, and confi rm that bonds are still adequate in subsequent depot level mainte- nance inspections.


There are a number of advantages that bonding has, in particular composite bonded structures produce greater stiffness. It allows uniform load distributions, forms smoother aerodynamic lines and surfaces, and has the possibility of eliminating rivet holes or signifi cantly reducing them. Bonded structures do require a reliable inspection method for char- acterizing the strength of composite bonds. One solution is Laser Bond Inspection (LBI), a pulsed laser system. LBI creates a stress wave that can test the strength of an adhesive interface in a compos- ite bond. The strength of the stress wave can be selected by varying the energy and pulse width of the laser beam. The strength of the refl ected tensile stress wave produced can be selected to fail weak bonds while strong bonds are unaf- fected.


LBI can be set to detect weak bonds, it is non-destruc- tive to strong bonds, it can also detect kissing bonds—bonds in which the bond line is in intimate contact with the laminate that is around it where there is no gap between the bond line and the composite material.


Figure 2: A hole delamination undetected by conventional gages but revealed by a noncontact ring laser derived point cloud.


LBI will also detect variations in bond quality, different types of surface preparation, adhesive mixing, and contaminations in the bond line material. It is a


localized testing method of bond strength. It measures the strength of the bond that is in the region of the diameter of the laser beam being used to interrogate the bonded struc- ture.


Advanced composite structures and bonded materials are now incorporated into a growing number of aircraft used for both civilian and military applications by all of the major aero- space manufacturers. They are also used in primary structures in UAVs. For the industry to certify these structures it is neces- sary to determine the strength of the adhesive bonds. Con- ventional non-destructive testing (NDT) methods are unable to assure that the bond strength is acceptable for service.


Laser bond inspection creates an internal tensile stress wave that tests the strength of an adhesive interface of a bonded structure, detecting weak bonds at the adhesive/ composite interface. The strength of the stress wave is se- lected by varying energy and pulse width of the laser accord- ing to the requirements of the particular composite material being investigated. The strength of the refl ected tensile wave is selected to fail a weak bond while strong bonds are unaf- fected. LBI can inspect bonded structures that are less than one inch thick.


141 — Aerospace & Defense Manufacturing 2015


Image courtesy United Sciences.


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