Adhesive joints uniform shearing

The kinetics of strength variation for adhesive joints, under water or oil, had not previously been investigated. When formed in liquids, PN-1, PU, and AK compositions do not display adhesion properties. The adhesion strength for underwater fixing with Sprut-5M is 14 and 12 MPa under uniform fracture and shear for VAK adhesive the values are 13 and 12 MPa. 

Typically, the yardstick for qualitatively measuring the internal resistance of an adhesive bond to an external load has been the determination of the strain distribution in the adhesive and adherends. This is a difficult task. Even in simple lap joints, the actual stress-strain distributions under load are extremely complex combinations of shear and tensile stresses, and are very prone to disturbance by non-uniform material characteristics, stress concentrations or locaUzed partial failures, creep and plastic yielding, etc. It is extremely difficult to accurately measure the strains in adhesive joints with such small glue Une thicknesses and such relatively inaccessible adhesive. Extensometers, strain gauges, and photoelasticity are being used with limited success." ... 

The most common method for reducing stress concentration in lap joints is by tapering the adherends, in a tapered lap joint (Figure 7.22a). Stress at the joint ends is reduced, allowing for a more uniform stress distribution. Modeling studies indicate that both adhesive peel and shear stresses... 

At first sight, it might seem a simple test to analyse with uniform tensile stress throughout the adhesive layer. In practice, the stress distribution is not uniform the disparity of modulus and Poisson s ratio between the cylinders and the adhesive means that shear stresses are introduced on loading (see Stress distribution Poisson s ratio). Thus, the failure stress is not independent of the dimensions of the joint. 

Chen and Cheng [31] show a development of their earlier work [32] in which they extend their theory of lap-joint behaviour to include non-identical adherends. In order to allow the resulting system of equations to be solved using closed form methods, they assume a uniform shear stress distribution across the adhesive layer thickness. A complementary energy method was then employed to solve the final equations. This theory is also presented by Wu et al. [33], who also show that it decomposes to Goland and Reissner s solution if further simplifying assumptions are made. 

A recently developed method is the so-called thick adherend test , (ASTM D 3983-81) in which it is attempted to minimize the effects of differential straining using stiff, thick, metallic or wooden adherends. In this form of joint, there is a considerable increase in the flexural stiffness of the adherends. The combination of these two properties has led to the popular belief that the adhesive is now in a state of uniform shear and there are no significant transverse peeling loads (but see Renton, 1976). 

Johnson WS (ed) Adhesively bonded joints testing, analysis and design, ASTM STP 981. American Society for Testing and Materials, Philadelphia, pp 28-38 Wycherley GW, Mestan SA, Grabovac I (1990) A method for uniform shear stress-strain analysis of adhesives. J Test Eval 18 203... 

Adherend stresses in weldbonded joints are lower and more uniform than those for comparable spot welded joints. This provides increased in-plane tensile shear and/or compressive buckling load-carrying ability for a given joint design. The presence of the spot weld provides enhanced out-of-plane load-carrying capability compared to adhesive bonding only. 

A standard test report usually documents the resulting measurements, such as tensile shear strength and peel strength. It should also indicate all the pertinent conditions that are required to ensure reproducibility in subsequent testing. It is often very useful to describe the failure mode of the tested specimens. An analysis of the type (or mode) of failure is an extremely valuable tool to determine the cause of adhesive failure. The failed joint should be visually examined to determine where and to what extent failure occurred. The percent of the failure that is in the adhesion mode and that in the cohesion mode should be provided. A description of the failure mode itself (location, percent coverage, uniformity, etc.) is often quite useful. The purpose of this exercise is to establish the weak link in the joint to better understand the mechanism of failure. 

The design analysis of a scarf may be considered similar to a single lap bonded joint, detailed analysis of which can be found in MIL-HDBK 17-3E 3. The analysis of a bonded joint is made complex, however, by the modulus difference of the adhesive compared to the adherends and the relative thicknesses of both which causes a non-linear distribution of the shear forces in a lap joint with peak stresses at the ends [1]. Scarf repairs provide a more uniform stress distribution however, to achieve this an adequate scarf angle is required [24] shown in Eigure 14.6. 

Bond strengths of PU adhesives were obtained from single lap-shear determinations. Substrate/adhesive/substrate joints were made with ABS and PVC coupons, 8 x 2.5 cm, as substrates the overlap area was 4.5 cm. The reference PU here was compounded with glass beads to provide adhesive layers with uniform thicknesses of 1.27 mm. Lap... 

Shear tests are very common because samples are simple to construct and closely dupUcate the geometry and service conditions for many structural adhesives. As with tensile tests, the stress distribution is not uniform and, while it is often conventional to give the failure shear stress as the load divided by the bonding area (Table 11.1), the maximum stress at the bond line may be considerably higher than the average stress. The stress in the adhesive may also differ from pure shear. Depending on such factors as adhesive thickness and adherend stiffness, the failure of the adhesive shear joint can be dominated by either shear or tensioa ... 

In instrumented creep tests taken to failure, one learns not only how long specimens last but also how deformation increases throughout the creep process. For lap joints, delay times have been seen in creep tests, probably due to the increasing uniformity of the shear stress state, as predicted by the shear lag model as the creep compliance of the adhesive increases with time. In other situations, no such delay time is seen. A schematic illustration of a creep curve for an adhesive bond consisting of a butt joint bonded with a pressure sensitive foam tape is shown in Fig. 2, exhibiting classical primary, secondary and tertiary regions of creep behaviour. 

In conventional Shear tests, there is considerable non-uniformity of stress throughout the joint. The napkin ring test was introduced to provide a test where the variation of shear stress was minimal. It consists of two thin-walled tubes joined end to end by a thin layer of adhesive. The torque required to break the joint is recorded. 

Many researchers have performed subsequent analysis of lap shear joints.Plantema combined the results of Volkersen and Goland and Reissner and also included shear effects.Cornell analyzed the lap shear joint and characterized the adherends as simple beams while considering the adhesive to behave as a system of shear and tension springs.The stress analysis of an unusual lap joint consisting of circular tubes was studied by Lubkin and Reissner. Lubkin was able to define conditions for uniform stress distributions in both flat and tubular lap joints, specifically when the adhesive is perfectly elastic. ... 

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