Tensile Butt Joint Test

The tensile butt joint test is less commonly used as an adhesive test method this may be due to the perception that it is more difficult to prepare the test specimens ... 

Fig. 81. Tensile butt joint tests (a) poker chip test (thin adhesive film) (b) and (c) schematic for thick adhesive test (d) gauge length and deflections in...

Fig. 4.11. Tensile (cylindrical) butt-joint test, (a) Unloaded, (b) Loaded, (c) Stress distribution.

Degradation rate in tensile butt joints is more sensitive to the bond area when compared with the lap shear test, therefore, smaller diameter joints allow a more rapid indication of durability. The absence of distortion and the general uniformity of fracture surface provide a good facility for analysis of the locus of failure. This makes this type of test particularly suitable for detailed analytical studies on specific surface treatments. 

On the other hand, bulk specimens are not susceptible to the problems inherent in butt joint tests and are more suitable for providing reliable data on the response of adhesives to various known states of stress although care should be exercised in controlling the curing schedule. If a particular adhesive is not suitable for making into large bulk specimens, it should be possible to obtain uniaxial tensile stress-strain data by testing an unsupported thin film of the adhesive. 

The ASTM D 897 tensile button test is widely used to measure the tensile strength of a butt joint made with cylindrical specimens (Fig. 20.3). The tensile strength of this bond is defined as the maximum tensile load per unit area required to break the bond (measured in pounds per squre inch). The cross-sectional bond area is usually specified to be equal to 1 in.2. The specimen is loaded by means of two grips that are designed to keep the loads axially in line. The tensile test specimen requires considerable machining to ensure parallel surfaces. 

Finite-element methods have also been used to evaluate new test equipment to measure shear strength under impact loads. In the equipment, two rectangular plates, bonded opposite faces of a vertical hexagonal prismatic rod, bear on a firm surface. The top of the central rod is subjected to an impact load. To prove the validity of the method, the maximum shear stress was compared with the impact shear strength, which was measured using a cylindrical butt joint subjected to impact torsional loads (see Tensile tests). 

There has, however, been concern raised over the stability of the CAA oxide on Ti6A14V when exposed to dry conditions for prolonged periods (up to 1200 h) at 330 °C or shorter periods (up to 24 h) at 400 °C. A brittle zone has been shown to form at the oxide-metal interface, which results in cohesive failure of joints at low loads, below 0.7 MPa, when measured in a tensile butt test. Cohesive failure in this instance occurred within this brittle zone. 

The two main types of filled joints are butt joints and lap joints (Fig. 2) (see Shear tests and Tensile tests) each has advantages and disadvantages. The lap joint gives the sealant some protection against the weather, and a further advantage of this type of joint... 

Another tensile test, ASTM D2095-72, involves the testing of bar and rod butt joint specimens. ASTM 2094-69 describes the preparation of these specimens (see Figure 3). This test and the samples can be used with substrates comprised of metals, plastics, or reinforced plastics. Loads are applied through fixtures connected to the samples by dowel pins. The standard test rate is 2400 to 2800 lb per square inch of bond area. The maximum load at failure is used to calculate the tensile strength, the same as for the pi-tensile specimens. 

Fig. 9. Tensile strengths at various test temperatures for butt joints made with polished or sandblasted aluminum adherends and an epoxy adhesive. (Taken from Ref. 76, published by Gordon and Breach Science Publishers.)...

Butt joints for testing adhesives in tension are also usually designed with a circular cross-section to facilitate manufacture and to maintain symmetry. In this case, it would appear to make little difference whether they are annular (Fig. 63(a)) or solid (Fig. 63(b)). In a butt joint subjected to a tensile load, the adhesive is restrained in the radial and circumferential directions by the adherends. In the absence of this restraint, the adhesive would tend to contract radially with respect to the adherends because of its much lower modulus. The presence of the adherends has the effect of inducing radial and circumferential stresses in the adhesive, so increasing the stiffness of the joint. The simplest analysis makes the assumption that the radial and circumferential strains in the adherend and the adhesive are zero, in which case the radial and circumferential stresses are given by ... 

Sometimes called the poker chip test.) This is effectively a butt joint to which tensile loads are applied, as shown in Fig. 81(a). At first sight, this appears to be a simple test in which the adhesive is loaded uniformly in tension. Unfortunately, this is far from the case. If the adhesive layer were thick (i.e. many times its lateral dimensions) as... 

ASTM list two specifications for the tensile testing of butt joints. The first, ASTM D 897-78 uses short, stubby, circular specimens of metal (or wood) with a cross-sectional area of 1 in, while the second (ASTM D 2095-72) allows for longer specimens of metal or reinforced plastics. For the latter test, specimens (round or square) should be prepared according to ASTM D 2094-69 which designates alignment tolerances and fixtures to minimize misalignment problems. 

In tension, the recommended test is the bulk tensile test. The axially loaded butt joint has poor repeatability and suffers from constraining effects. 

In a modification of the napkin ring shear test, the adherends are solid bars and the adhesive forms a penny-shaped slab similar to the butt joint tensile test. Such a test will give the relationship between torque and twist, but whereas in the napkin ring test it may be assumed that all the adhesive is at the same stress and strain, with a solid butt joint there is a radial variation of strain, but a non-linear variation of stress after yield. It is then necessary to use the Nadai correction (see Adams and Wake") to determine the true stress-strain curve of the adhesive. 

Despite the fact that adhesive joints are rarely designed to be loaded directly in tensile mode, tensile tests are eommon for evaluating adhesives. The axially-loaded butt (or poker chip ) joint geometry, as recommended by ASTM D897(52), is depicted in Fig. 4.11. 

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