Adherend lap shear

Chromic acid-etched A1 adherends. Lap-shear, 90° peel and honeycomb joint geometries. Two years exposure in temperate, hot-dry, and hot-wet regions. 

For thick adherend lap shear test specimens a surface treatment was performed. The aluminium adherends were 10 mm thick and 25 mm wide. After the specimens were cured and postcured, the adherends were cut to form a test area with an 8 mm lap length. 

Thick adherend lap shear tests (TALST) were not based on any standard. However, standard practice used in the tests was followed. The displacement rate used was 0.3 mm/min. The adhesive layer displacement was measured using two in-house built KGR-1 type extensometers. 

Adhesive Adherends Lap shear strength, MPa (psi) Peel strength, N/cm (piw)... 

Several testing techniques are possible which use different specimen configurations, different loading modes, and various loading frequencies. Marceau et al. have described results for thick adherend lap shear and double cantilever beam (DCB) specimens exposed to 140 F/100%... 

Figure 2 is a schematic drawing of the KGR-1 with the thick adherend lap shear specimen currently in use. Two instruments are used they are mirror images of each other and are mounted on opposite sides of the specimen. The signal of either individual instrument can be read, or their average signal can be obtained by combination. 

FIGURE 3. KGR-1 extensometer mounted on a thick adherend lap shear specimen. 

Fig. 2. Illustrations of forces to which adhesive bonds are subjected, (a) A standard lap shear specimen where the black area shows the adhesive. The adherends are usually 25 mm wide and the lap area is 312.5 mm. The arrows show the direction of the normal apphcation of load, (b) A peel test where the loading configuration, shown by the arrows, is for a 180° peel test, (c) A double cantilever beam test specimen used in the evaluation of the resistance to crack propagation of an adhesive. The normal application of load is shown by the arrows. This load is appHed by a tensile testing machine or other...

The principal type of shear test specimen used in the industry, the lap shear specimen, is 2.54 cm wide and has a 3.23-cm overlap bonded by the adhesive. Adherends are chosen according to the industry aluminum for aerospace, steel for automotive, and wood for constmction appHcations. Adhesive joints made in this fashion are tested to failure in a tensile testing machine. The temperature of test, as weU as the rate of extension, are specified. Results are presented in units of pressure, where the area of the adhesive bond is considered to be the area over which the force is appHed. Although the 3.23-cm ... 

Load bearing capabiUties are dependent upon the adherend, joint design, rate of loading, and temperature. Values given represent the type of adherends normally used at room temperature. Lap shear values approximate those obtainable from an overlap of 3.2 cm. ... 

Surface cleaning/etches. As with aluminum and titanium, the most critical test for bonded steel joints is durability in hostile (i.e., humid) environments. The fact that the problem is a serious one for steel was illustrated in a study [117] that compared solvent cleaned (smooth) 1010 cold-rolled steel surfaces with FPL aluminum (microrough) substrates. Although the dry lap-shear strengths were not markedly different, stressed lap-shear joints of steel adherends that were exposed to a humid environment failed in less than 30 days, whereas the aluminum joints lasted for more than 3000 days. 

We have also looked at the lap shear strength of selected EB-ciirable epoxy adhesives. Because the adhesives being developed were being used for both aluminum-to-aluminum and composite-to-composite applications the lap shear strengths for both adherends was measured. Aluminum adherends were T2024 phosphoric acid anodized according to ASTM 3933. The composite adherends... 

Effects on lap shear strength of EB-cured epoxy adhesives from different surface preparations on aluminum and composite adherends... 

Since the m,m -DABP polyimide is known to be an outstanding adhesive, lap shear strength tests employing titanium-titanium adherends and metal ion filled polyimides were conducted. Tests were performed at room temperature, 250°C and 275°C employing either DMAC or DMAC/Diglyme as the solvent. At room temperature regardless of the metal ion employed adhesive strength is de-... 

Table IV Lap Shear Tests Titanium-Titanium Adherend BTDA-m,m -DABP...

Figure 3. Graph of shear strength vs. exposure time for lap shear joints constructed from untreated, alkaline cleaned or lubricated CRS adherends.

Metal ion modified polyimide films have been prepared to obtain materials having mechanical, electrical, optical, adhesive, and surface chemical properties different from nonmodified polyimide films. For example, the tensile modulus of metal ion modified polyimide films was increased (both at room temperature and 200 0 whereas elongation was reduced compared with the nonmodif ied polyimide (i). Although certain polyimides are )cnown to be excellent adhesives 2) lap shear strength (between titanium adherends) at elevated temperature (275 0 was increased by incorporation of tris(acetylacetonato)aluminum(III) (2). Highly conductive, reflective polyimide films containing a palladium metal surface were prepared and characterized ( ). The thermal stability of these films was reduced about 200 C, but they were useful as novel metal-filled electrodes ( ). 

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