Failure, adherend

Failure, adherend— Joint failure by cohesive failure of the adherend. 

The principal type of shear test specimen used in the industry, the lap shear specimen, is 2.54 cm wide and has a 3.23-cm overlap bonded by the adhesive. Adherends are chosen according to the industry aluminum for aerospace, steel for automotive, and wood for constmction appHcations. Adhesive joints made in this fashion are tested to failure in a tensile testing machine. The temperature of test, as weU as the rate of extension, are specified. Results are presented in units of pressure, where the area of the adhesive bond is considered to be the area over which the force is appHed. Although the 3.23-cm ... 

Rider and Amott were able to produce notable improvements in bond durability in comparison with simple abrasion pre-treatments. In some cases, the pretreatment improved joint durability to the level observed with the phosphoric acid anodizing process. The development of aluminum platelet structure in the outer film region combined with the hydrolytic stability of adhesive bonds made to the epoxy silane appear to be critical in developing the bond durability observed. XPS was particularly useful in determining the composition of fracture surfaces after failure as a function of boiling-water treatment time. A key feature of the treatment is that the adherend surface prepared in the boiling water be treated by the silane solution directly afterwards. Given the adherend is still wet before immersion in silane solution, the potential for atmospheric contamination is avoided. Rider and Amott have previously shown that such exposure is detrimental to bond durability. 

Eqs. 1-5 hold whether failure is interfacial or cohesive within the adhesive. Furthermore, Eq. 5 shows that the reversible work of adhesion directly controls the fracture energy of an adhesive joint, even if failure occurs far from the interface. This is demonstrated in Table 5, which shows the static toughness of a series of wedge test specimens with a range of adherend surface treatments. All of these samples failed cohesively within the resin, yet show a range of static toughness values of over 600%. 

Adhesives need to produce high peel forces when the adherends are pulled apart — preferably high enough to induce substrate failure. To accomplish high peel, the... 

At elevated temperatures where titanium alloys could be the adherend of choice, a different failure mechanism becomes important. The solubility of oxygen is very high in titanium at high temperatures (up to 25 at.%), so the oxygen in a CAA or other surface oxide can and does dissolve into the metal (Fig. 12). This diffusion leaves voids or microcracks at the metal-oxide interface and embrittles the surface region of the metal (Fig. 13). Consequently, bondline stresses are concentrated at small areas at the interface and the joint fails at low stress levels [51,52]. Such phenomena have been observed for adherends exposed to 600°C for as little as 1 h or 300°C for 710 h prior to bonding [52] and for bonds using... 

In-service issues. As mentioned previously, many early service failures of bonded structure were due to adherend surface treatments that were unstable in long-term exposure to water. A majority of these problems were resolved by the adoption of surface treatments such as chromic and phosphoric acid anodize for aluminum details. The remaining few were alleviated by the adoption of phosphoric acid anodized honeycomb core and foaming adhesives resistant to water passage. Other service durability issues such as the cracking of brittle potting compound used to seal honeycomb sandwich assemblies, and subsequent delamination, have been minor in scope. 

Samples constructed from adherends which had been alkaline cleaned, lubricated or left untreated exhibited similar joint strength values and durability trends (Figure 10). Adhesive joints placed in the room temperature control environment or the 23 C water bath retained lOOZ and 92% of initial joint strength, respectively. Failure remained cohesive within the adhesive for all of the control samples and for the first 20 days of exposure in the 23 C water bath. After 20 days, some failure began to initiate at both the primer/steel and primer/topcoat interfaces. The adhesive/topcoat interface proved to be more durable than those found between the substrate/primer/topcoat layers. Samples exposed to the more severe salt fog, 60 C water bath and cycle tests were able to retain 70% to 50% of their initial strength over a 60-day exposure period. 

While a non-phosphated topcoat/adhesive interface provided an excellent, moisture resistant, occlusive seal even under the most severe cycle testing, phosphated ZM adherends did not prove to be as durable in comparison (Figure 11). The reason for this lies in the fact that phosphate coverage on Zincrometal is incomplete. Partially crystalline phosphates are non-uniformly interspersed on randomly exposed zinc dust spheres at the surface. Consequently, the moisture resistance normally provided at the adhesive/topcoat interface was reduced due to the incomplete sealing between the topcoat/ adhesive surfaces. This became apparent as most of the failures examined after aging in these environments were concentrated at the adhesive/phosphate/paint interface. Results obtained on these samples were similar to those obtained for phosphated CRS joints, indicating that the locus of failure occurred at phosphate crystal sites. Note, however, that the durability of these joints was still considered to be very good in comparison to other metallic oxide/ adhesive interfaces. 

The extent of coating adhesion failure was found to be dependent upon the resistance of the polymer in the coating to hydrolysis by corrosion generated hydroxide. In this study, similar trends have been observed for adhesives. Table I shows the results of salt spray corrosion on a series of bonds between cold rolled steel adherends and adhesives of varying chemistry. The results show that there is a direct correlation between the chemistry of the adhesive polymer and the durability of the series of adhesive bonds studied. The locus of adhesion failure also appears to be related to the type of adhesive chemistry. In this study, adhesives based on polymers having a wide range of hydrolysis resistance were examined. 

In a specific example of adhesive bonds between cold rolled steel and SMC adherends (Table II) an adhesive based on hydrolysis resistant epoxy chemistry (i.e., adhesive E) was compared with an adhesive based on hydrolysis prone urethane chemistry (i.e., adhesive C) in composite to cold rolled steel bonds. After corrosion testing, a significant difference in both retention of initial bond strength and locus of failure was observed. For bonds prepared with adhesive E, little if any reduction of the initial bond strength was observed after corrosion testing. The locus of failure for both the tested and untested bonds was largely in the... 

Although distinct "metal" and "adhesive" sides were apparent upon visual examination of the debonded surfaces treated with 100 ppm NTMP, SEM analysis showed the presence of an adhesive layer on the "metal" side. XPS analysis indicated low A1 and 0 and identical high C levels on both debonded sides, confirming a failure within the adhesive layer (cohesive failure), i.e., the best possible performance in a given adherend-adheslve system. This result is similar to that obtained using a 2024 A1 alloy prepared by the phosphoric acid-anodization (PAA) process (16) and indicates the importance of monolayer NTMP coverage for good bond durability (Fig. 4). 

The two predominant mechanisms of failure in adhesively bonded joints are adhesive failure or cohesive failure. Adhesive failure is the interfacial failure between the adhesive and one of the adherends. It indicates a weak boundary layer, often caused by improper surface preparation or adhesive choice. Cohesive failure is the internal failure of either the adhesive or, rarely, one of the adherends. 

Data have been obtained at PicArsn on the adhesive bonding of HE s to themselves and to other adherends. The expls investigated include 75/25 cyclotol, RDX-polystyrene HMX-cellulose nitrate compns. Whether the bonded expl assembly was tested statically or dynamically, failure always occurred in the expl, never in the adhesive. If proper adhesives bonding procedures are used, failure should occur in the expl... 

Abstract—The structure of films formed by a multicomponent silane primer applied to an aluminum adherend and the interactions of this primer with an amine-cured epoxy adhesive were studied using X-ray photoelectron spectroscopy, reflection-absorption infrared spectroscopy, and attenuated total reflectance infrared spectroscopy. The failure in joints prepared from primed adherends occurred extremely close to the adherend surface in a region that contained much interpenetrated primer and epoxy. IR spectra showed evidence of oxidation in the primer. Fracture occurred in a region of interpenetrated primer and adhesive with higher than normal crosslink density. The primer films have a stratified structure that is retained even after curing of the adhesive. 

Abstract—The effects of metal alkoxide type and relative humidity on the durability of alkoxide-primed, adhesively bonded steel wedge crack specimens have been determined. Aluminum tri-sec-butoxide, aluminum tri-tert-butoxide, tetrabutyl orthosilicate, and titanium(IV) butoxide were used as alkoxide primers. Grit-blasted, acetone-rinsed mild steel adherends were the substrates bonded with epoxy and polyethersulfone. The two aluminum alkoxides significantly enhanced the durability of the adhesively bonded steel, while the titanium alkoxide showed no improvement in durability over a nonprimed control. The silicon alkoxide-primed samples gave an intermediate response. The failure plane in the adhesively bonded samples varied with the relative humidity during the priming process. 

There is an apparent optimum relative humidity level required to achieve good adhesion and durability. Priming the steel adherends at 18% RH caused failure in the wedge samples within the steel (oxide) layer. Adherends primed at 34% RH failed within the alkoxide primer layer, whereas at 51% RH failure occurred primarily within the adhesive layer. This change in locus of failure with humidity was not evident using the wedge crack test when the adherends were primed with aluminum alkoxides. A peel-type test would probably be more sensitive in detecting these shifts in failure mode. 

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