Failure modes tensile test

One of the simplest criteria specific to the internal port cracking failure mode is based on the uniaxial strain capability in simple tension. Since the material properties are known to be strain rate- and temperature-dependent, tests are conducted under various conditions, and a failure strain boundary is generated. Strain at rupture is plotted against a variable such as reduced time, and any strain requirement which falls outside of the boundary will lead to rupture, and any condition inside will be considered safe. Ad hoc criteria have been proposed, such as that of Landel (55) in which the failure strain eL is defined as the ratio of the maximum true stress to the initial modulus, where the true stress is defined as the product of the extension ratio and the engineering stress —i.e., breaks down at low strain rates and higher temperatures. Milloway and Wiegand (68) suggested that motor strain should be less than half of the uniaxial tensile strain at failure at 0.74 min.-1. This criterion was based on 41 small motor tests. 

The strength of adhesion between the fiber and matrix could also be expected to play a role in this change in failure mode. The interfacial testing system (ITS) provides comparative data on the interfacial shear strengths of the bare and sized E-glass fibers in real composites. A handbook value of 76 GPa [19] was used for the tensile modulus of E-glass fibers and the matrix shear modulus was previously determined as 1.10 GPa. Table 4 lists the mean interfacial shear strength, standard deviation (SD), and number of fiber ends tested for the two fiber types. 

The increase in ILSS for the epoxy-sized fibers over the bare fibers is 12.4%, approximately 50% of the increase observed in the interfacial shear strength as measured by ITS testing. Changes in the failure mode at the fiber-matrix interface may account for the differences. The sized fibers produced large matrix cracks that grew quickly to catastrophic size under load. This would tend to limit the increase in composite shear properties if at every fiber break in the tensile surface of the coupon a matrix crack was created. The presence of these matrix cracks... 

As can be seen, some larger fragments are seen in the sample held at elevated temperature, and sodium has segregated to the surface. Such segregation is very common in high temperature cured specimens, where sodium is often found at the failure surface in an adhesive failure mode. ISS/SIMS data from the adhesive side of a titanium-epoxy failure interface from a tensile test specimen are shown, in Figure 8. 

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. 

Figure 4 shows typical failure surfaces obtained from tensile tests of the co-cured single and double lap Joint specimens. In the case of the co-cured single lap Joint, as the surface preparation on the steel adherend is better, a greater amount of carbon fibers and epoxy resin is attached to the steel adherend. Failure mechanism is a partial cohesive failure mode at the C ply of the composite adherend. In contrast with the co-cured single lap joint, failure mechanism of the co-cured double lap joint is the partial cohesive failure or interlaminar delamination failure at the 1 ply of the composite adherend because interfocial out-of-plane peel stress... 

A number of ASTM tests and practices involve durability, but one of the most important is the Wedge Test, ASTM D3762-03. In this method, a wedge is forced into the bond line of a flat-bonded aluminum specimen, thereby creating a tensile stress in the region of the resultant crack tip. The stressed specimen is exposed to an aqueous environment at an elevated temperature, or to any other desired environment. The resultant crack growth with time and failure modes is then evaluated. The test is primarily qualitative, but it is discriminatory in determining variations in adherend surface preparation parameters and adhesive environmental durability. 

As for the tensile test, the flexural test results are in general agreement with the values predicted theoretically, except for S2 but the large difference here can be attributed essentially to its particular failure mode, which is induced by its geometry (i.e. thick upper skin and non-symmetric sandwich construction). 

Some tests specify the use of two circular plates that may attach directly to the straining frame. Rubber is bonded between the plates, which are then separated in a mode perpendicular to the plane of the plates (see Tensile tests). Very high loads should be expected in this type of test. On failure of a satisfactory bond, there should be no sign of bonding agent - rubber or bonding agent - substrate separation. Details are in ASTM D 429, Method A. 

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