Joints shear

St. Clair et. al. investigated a series of maleimide and nadimide terminated polyimides and developed LARC-13 [8,9]. Changing the terminal group from maleimide to nadimide, the value of the lap shear strength of a titanium lap shear joint increased from 7 to 19 MPa [9]. They also added an elastomeric component to the adhesive formulation. The introduction of 15 wt% of a rubbery component, ATBN (amine terminated butadiene nitrile polymer) and ADMS (aniline terminated polydimethyl siloxane) enhanced the adhesive properties as follows 19 MPa to 25 MPa (ATBN) titanium T-peel strength 0.2 kN/m to 1.4... 

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. 

The lap shear test involves measuring the adhesive shear strength between two surface fluorinated polyolefin sheet tokens that are adhesively secured with a reinforcement resin. The tokens are individually reinforced with steel backing plates to eliminate flexural distortion in the shear joint. Lap shear tests carried out with various reinforcing polyester-type resins, contrasting fluorination and oxyfluorination as surface treatment, are shown in Table 16.8. 

Regarding the development of a structural model of the refinery subsurface, it was observed that the discontinuities present in the area can be grouped into three distinct systems. With the exception of stratification (primary discontinuity), all three of these systems are represented by joints (fractures where the two opposing faces do not shift with respect to one another), shear joints (where the two faces do shift with respect to one another) and faults (fault planes or zones, marked by cataclasis, rubble and mylonite with clear evidence of shifting of the two faces). 

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

FIGURE 14.7 Illustration of dielectric heating for the bonding of an electrogalvanized steel (EGS)/ SMC lap shear joint.30... 

A comparison of experimentally determined failure times for different stress levels and those predicted by the above equation for epoxy-aluminum lap shear joints aged at 60°C and 95 percent RH is presented in Table 15.3. These results indicate that the reaction rate method is satisfactory for predicting the effects of temperature and stress on the lifetime of adhesive bonds, provided that failure is cohesive within the adhesive. This, of course, should be validated by prototype testing. 

Bodnar, M. J., and Wegman, R. F., Effect of Outdoor Aging on Unstressed, Adhesive Bonded Aluminum to Aluminum Lap Shear Joints, Technical Report No. 3689, Picatinny Arsenal, Dover, NJ, May 1968. 

TABLE 16.14 Joint Strengths for Polystyrene-Aluminum Tensile Shear Joints Exposed to Various Environments74... 

Fresh pig skin was harvested from the back of a pig within 5 hours postsacrifice and the fat attached to the inside skin surface trimmed away. The skin was cut into 2x1 coupons and covered by saline moist paper towels before use. The external surface of the skin was used as the bonding surface to prepare the lap shear joint samples. 

The coupons were dried prior to forming a lap shear joint. About 100 pi of the experimental adhesive was deposited to one coupon and smoothed to cover a 1/2 X 1 area. Another coupon was placed over the area of the initial coupon and a 1 lb weight placed on top. It was cured for 20 to 30 min and the strength of the joint... 

Another aspect to be considered is the difficulty in producing curved structures with the same fibre content as flat laboratory panels. This effect is shown in Figure 16, at the comer the laminate thickness is larger than at the flat section and fibre content is rather lower. This will affect the bending stiffness of the arm and the predicted failure load. This figure also shows the fillet, which is critical to initiation in the specimens without implanted defects. It is well known that fillets can significantly alter the load path in lap shear joints and increase the failure loads (see [1] and Figure 3 for example). If a fracture mechanics approach is to be applied this effect must be considered. Some recent studies on stress intensity factors for such cases may allow this to be addressed [22]. 

Through observation of induced microseismic events and examination of well logging data, it has been understood that the permeable zone seems to be created by migration of shearing joints during hydraulic fracturing test, and that microseismic multiplets is considered to be seismic events associated with shearing of fractures which are favourably oriented to stress field. 

The shear adhesion strength of the PU formulations was tested by single lap shear joints, according to ASTM D 1002-94 (at a rate of 1.3mmmin ). Peel strength was evaluated by T-Peel (ASTM 1876-95) (at a rate of 254 mm min ). 

Figure 4 Standard lap shear geometries (a) simple lap joint test, ASTM D-1002 (b) laminated lap shear joint test, ASTM D-3165 (c) double lap joint test, ASTM D-3528.

To apply an external or internal patch (single lap joint) the surface is cleaned and lightly abraded prior to laying up the repair. Single lap shear joints are not as... 

Fracture A general term to include any kind of discontinuity in a body of rock if produced by mechanical failure, whether by shear stress or tensile stress. Fractures include faults, shears, joints, and planes of fracture cleavage. 

---