Adherend shaping

THE EFFECT OF ADHEREND SHAPE—SCARFED, BEVELLED AND STEPPED ADHERENDS... 

Tapers (internal or external), or more complex adherend shaping, are excellent methods to reduce the peel stresses at the ends of the overlap and, therefore, to increase the joint strength. Internal tapers with a fillet seem to be the more efficient way to have a joint increase, especially with brittle adhesives and when composites are used. The EE method is a convenient technique for the determination of the optimum adherend geometry however, the complexity of the geometry achieved is not always possible to realize in practice. 

Peel tests are accompHshed using many different geometries. In the simplest peel test, the T-peel test, the adherends are identical in size, shape, and thickness. Adherends are attached at thek ends to a tensile testing machine and then separated in a "T" fashion. The temperature of the test, as well as the rate of adherend separation, is specified. The force requked to open the adhesive bond is measured and the results are reported in terms of newtons per meter (pounds per inch, ppi). There are many other peel test configurations, each dependent upon the adhesive appHcation. Such tests are well described in the ASTM hterature. 

A similar specimen design uses a sandwich construction with a dissimilar material bonded between the two cylindrical halves of the button specimen. This design is commonly used to measure the tensile strength of adhesives between dissimilar materials or if the adherend does not have the strength or characteristics to be machined into the shape of the button specimen. With some modifications in the dimensions, the button tensile test has also been adapted for testing adherence of honeycomb-cover sheets to the core (ASTM C 297). 

This procedure is repeated for each incremental crack length until the contour for the entire length of the CDCB specimen is obtained. When an interface of dissimilar materials is considered, the contour has to be developed for each adherend, since the stiffness of the adherends would be different and the shape of the contour would be different on the two sides of the interface bond in order to achieve the same rate of compliance for each half of the specimen. 

Brushing is often used when the adherends have complex shapes, or when it is desired to apply the adhesive to selected areas of a surface without the use of masks. With brushing the control of film thickness is limited and the resultant adhesive films are often uneven and blobbed. Brushing is generally not suited to rapid assembly work. Stiff brushes provide the best results. Good brushes may be used repeatedly. 

Blister—An elevation of the surface of an adherend, somewhat resembling in shape a blister on the human skin its boundaries may be indefinitely outlined and it may have burst and become flattened. A blister may be caused by insufficient adhesive, inadequate curing time, temperature or pressure, or trapped air, water, or solvent vapor. 

In the case of laminate thicknesses below 5 mm the use of scarf and step-lap configurations shall be considered with care, as producing the required shapes on the adherends may become complicated. Also the bonding may become impossible if adequate jigging cannot be provided, see 5.3,5,11. 

In adhesively bonded joints copper wires with a diameter of 0.34 mm were used in between the laminates to control the adhesive layer thickness. The copper wires were placed in areas to be discarded. Adhesive fillets with a 45° angle were formed at the joint ends. The adherend ends were not shaped. Test specimens with 25 mm width were cut from the bonded laminates. Typically five specimens from each batch were tested. 

With plasma treatment, surface wettability can be readily induced on a variety of normally non-wettable materials as shown in Table P. 5. Certain polymeric surfaces, such as the polyolefins, become crosslinked during plasma treatment. The surface skin of polyethylene, for example, will become crosslinked so that if the polymer were placed on a hot plate of sufficient heat, the interior would turn to a molten liquid while the crosslinked outer skin held a solid shape. Other polymers have their critical surface energy affected in different ways. Plasma-treated polymers usually form adhesive bonds that are two to four times the strength of nontreated polymers. Table P.5 presents bond strength of various plastic adherends pretreated with activated gas and bonded with an epoxy or urethane adhesives. 

Contrary to the long duration of fatigue and creep tests, impact tests occur in seconds. Samples can be made of wood or metal adherends and are block-shaped, as described in ASTM D950-82 (see Figure 18). Loading is administered by a pendulum arm which travels at a velocity of 11 ft/s. The arm shears off the top half of the block and the impact energy in foot-pounds per square inch of bond area is recorded. It should be noted that impact values are very sensitive to the adhesive layer thickness thicker layers will allow more deformation and an increase in energy absorption. 

Nondestructive Testing of Adhesive Joints. Many commercially available adhesives are based on polymers. NDT of adhesive joints (180) poses challenges because of (2) the small size of possibly deleterious defects (eg, porosity), (2) the thinness of the adhesive layers, and (3) sometimes, the size, shape, and material of the adherends. Acousto-ultrasonics (181) and various types of ultrasonics (182,183) are applicable to adhesive joints. Ultrasonics can be combined with noncontact optical methods (184) and pulsed thermography (185). However, the assessment of the quality or of the quantitative strength of the adhesive bond based on the NDT data often proves difficult. 

Adhesive bonding consists of interposing a layer of an adhesive composition chemically different from the plastics to be bonded between the adherends. It is one of the most popular and versatile methods to join plastics. Some of its advantages are uniform distribution of stresses over the assembled areas, a high strength/weight ratio, a simplified means of assembling smaU or complex-shaped plastic parts, and in some instances thermal or electrical conductivity. 

Pig. 4. Deformed shape of the elastic layer in an adhesively bonded butt joint with rigid adherends when subjected to a positive-shear loading. 

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