Failure surfaces

The primary drawback to the application of XPS in adhesion science is associated with the limited spatial resolution of the technique. This can make it difficult to study processes that are highly localized, such as corrosion, or to accurately characterize certain types of failure surfaces where, for example, the locus of failure may pass back and forth between two phases. 

Fig.. 24. Schematic drawing of the visual appearance of the failure surfaces of lap joints prepared from hot-dipped galvanized steel substrates. Reproduced by permission of John Wiley and Sons from Ref. [41].

Tsai and Wu postulated that a failure surface in six-dimensional stress space exists in the form... 

Figure 63 Scanning electron micrographs of the failure surface of the white backing layer of the bad packaging sample. Four layers were clearly visible in the SEM images. They are labeled as follows (1) outer clear polyester layer, (2) middle opaque polyethylene layer,... 

To visualize the stress states at which failure occurs, a failure surface may be constructed. The six stress components may be resolved into three orthogonal principal stresses. Plotting failure in principal stress space, any stress state which exists in the bounded space containing the... 

Mehldahl (65) depicts several failure surfaces by photographs of various three-dimensional models. Figure 23 illustrates three such surfaces taken from Ref. 110, which shows geometries which are symmetrical about the space diagonal, oi = triaxial compression octant should be open ( because hydrostatic compression cannot lead to failure in the ordinary sense ). 

Figure 23. Various failure surfaces in principal stress space (a) cylinder, (b) cone, and (c) paraboloid (110)...

Figure 24. Parabolic failure surfaces coaxial to the hydrostatic axis...

Other coordinate systems may be used for failure surface representations in addition to stress space. Blatz and Ko (11) indicate that either stress (Stress space is most commonly used because the failure surface concept was originally applied to metals, for which stress and strain are more simply related. Viscoelastic materials, on the other hand, may show a multitude of strain values at a given stress level, depending on test conditions. 

Some materials might produce a unique failure surface providing measurements could be conducted under first stretch conditions in a state of equilibrium. Tschoegl (110), at this writing, is attempting to produce experimental surfaces by subjecting swollen rubbers to various multiaxial stress states. The swollen condition permits failure measurements at much reduced stress levels, and the time dependence of the material is essentially eliminated. Studies of this type will be extremely useful in establishing the foundations for extended efforts into failure of composite materials. 

Figure 25. Failure surface in principal stress space for a composite solid...

Bond failure may occur at any of the locations indicated in Fig. 1. Visual determination of the locus of failure is possible only if failure has occurred in the relatively thick polymer layer, leaving continuous layers of material on both sides of the fracture. The appearance of a metallic-appearing fracture surface is not definite proof of interfacial failure since the coupling agent, polymer films, or oxide layers may be so thin that they are not detectable visually. Surface-sensitive techniques such as X-ray photoelectron spectroscopy (XPS) and contact angle measurements are appropriate to determine the nature of the failure surfaces scanning electron microscopy (SEM) may also be helpful if the failed surface can be identified. 

The locus of failure after peel testing was determined using XPS analysis of both the silicon wafer and the polyimide failure surfaces. This analysis was also done as a function of the humidity. 

Figure 4. XPS locus of failure analysis of PMDA-ODA on SiO,. (A) PI failure surface C(ls) and N(l ) data, (B) SiO, failure surface C(ls) and N(l ) data.

The N(ls) peak on the sapphire failure surfaces indicates presence of imide (peak at 400.7-401.0 eV), which suggests presence of polyimide on the sapphire failure surface. This is supported by the C(ls) data which show the presence of the carbonyl C(ls) species as well as jr-to-jr transition peak due to aromatic species. Since the polyimide failure surface shows only C(ls), N(ls), and O(ls) peaks in relative abundance, characteristic to the polyimide, it can be said that... 

The locus of failure in all PMDA-ODA/MgO cases is consistently a mixed mode type leaving ceramic on the polyimide failure surface, and leaving polyimide on the ceramic failure surface. APS application prior to polyimide coating does not change the failure locus or the peel strength behavior of these interfaces. 

The failure surfaces of the wedge samples were analyzed using X-ray photoelectron spectroscopy (XPS). Spectra were obtained using a PHI 5300 ESC A system with a magnesium X-ray anode at a power of 400 W. 

The failure surfaces were analyzed using XPS. The results from the XPS analysis of the samples primed with each of the four alkoxides at 34% RH and the control sample, each bonded with PES and immersed in DI water at 100°C are listed in... 

XPS analysis of failure surfaces of wedge samples primed at 34% RH and bonded with PES and placed in a 100°C water bath... 

Table 2. The two failure surfaces are denoted as A and B In the case of the A1TSB and A1TTB pruned samples, the atomic concentration of carbon was about 60% on both sides and is possibly due to carbon originating from partially unreacted alkoxide or residual adhesive. The concentration of oxygen was about 25%. Very little iron was detected on either side, which indicates that Mure did not occur close to the steel substrate. The atomic concentration of sulfur for the A1TSB case is not high enough to conclude that failure occurred primarily within the adhesive. The atomic concentration of aluminum was about 14% on each of the surfaces of both samples. The XPS results support the assignment that failure occurred mainly within the alkoxide layer. The Ti(IV)B primed and TBOSi primed samples also failed mainly within the alkoxide layer.

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