Adhesive type failure

Two types of composite physical property tests were conducted to measure properties which are sensitive to the degree of adhesion and failure mode of the fiber-matrix interphase. Short beam shear tests (ASTM D2344-84) were conducted on 18 ply unidirectional laminates. The support span-to-thickness ratio... 

In a similar manner, differences in adhesive thickness also complicate the comparison between adhesive joints. Say, for example, a person was comparing the lap joint strength of two adhesives in which the amount of overlap and the adherends were identical, but the thickness of the adhesives differed. If the first adhesive thickness was 1.3 mm and the second was 6.4 mm. the adhesive thickness effect would likely swamp any differences due to the adhesive type. Similar effects of thickness are noted for tensile tests. Not only does the relative thickness of the adhesive affect the load at failure, but it may also influence the point from which cracks are likely to grow. 

Adhesive type Dry test (MPa) Wood failure (%) Cold water soaking test (MPa) Wood failure (%) Boiling test (MPa) Wood failure (%) Formaldehyde emission (mg/100 g)... 

The adhesive failures can be categorized as adhesive type and cohesive type. In adhesive type, the failure occurs along the interface of the adhesive and the adherent in, while in cohesive type, failure is contained within the adhesive layer. Cohesive failure occurs more commonly in the heterogeneous materials than neat polymer adhesives. 

The use of Auger and XPS analytical techniques enabled the locus of failure to be determined. This was found to be interfacial between the metallic oxide and the epoxy adhesive at the edges of the joint, becoming cohesive within the adhesive layer towards the center of the joint. The relative amounts of interfacial and cohesive type failure depended on the length of time that the joint was immersed in water. 

Four different types of belt are considered synchronous belts, V-belts, V-ribbed belts and conveyor belts. Each type of belt has a distinct set of failure modes and so each is considered in turn below. Only failures which can be considered to be belt failures have been considered, rather than belt/pulley system failures, so that failures as a result of pulley misalignment, for instance, have not been included. For adhesion related failure modes, methods of predicting belt failure where they have been developed, are outlined. 

Assembly time (min.) Press time (min.) Wood failure by adhesive type ( Vb) ... 

Unstable damage development of thick bond-lines (lower plastic strain development) resulted in adhesive cracking in multiple locations with a cohesive-type failure and lower failure strengths. 

XPS images recorded from the interfacial failure surfaces of an adhesively bonded aluminum joint, prepared using an organosilane primer, with an instrument of the type shown inO Fig. 9.14. The optical mirror images are complementary views of the fracture surfaces and are mirrored along the dotted line. Visually the failure appears to be interfacial with the fracture path moving from one interface to the other at the boundary between metal and adhesive interfacial failure surfaces... 

Sancaktar and Narayan (1999) stated that an attention to the type of stress (i.e., normal, transverse or shear) provided information relevant to the type of failure (i.e., brittle- or ductile-type failures). The authors also introduced newly defined design parameters stress times substrate volume, stress divided by substrate volume, and stress (strain) gradients along and across the adhesive joint, as a general methodology to compare adhesive joints for design optimization. 

Before pursuing the implications of this point further, consider first the related case of fracture between two dissimilar materials. In contrast to the former case in which the fracture separated the same material, thus leading to a cohesive type failure, this debonding of different materials may be described as adhesive fracture. From the point of view of continuum mechanics, the problem and energy balance are essentially the... 

Film adhesives are applied by removing a paper or plastic separator/ protective film and laying the adhesive on the facing surface smoothly, taking care not to allow wrinkles to develop, or air to become entrapped between the adhesive and the substrate surface. A common workmanship error is failure to remove the separator film before assembly of the detail parts. Some bonders utilize special check-off points to insure its removal. The batch number, lot number, time and date of application, and adhesive type should be logged into the inspection record for traceability should a failure occur. The shop-life expiration date and time should also be logged to aid in controlling assembly and cure of the adhesive [5]. 

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 ... 

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. 

Another important application area for PSAs in the electronic industry focuses on the manufacturing, transport and assembly of electronic components into larger devices, such as computer disk drives. Due to the sensitivity of these components, contamination with adhesive residue, its outgassing products, or residue transferred from any liners used, needs to be avoided. Cleanliness of the whole tape construction becomes very critical, because residuals like metal ions, surfactants, halogens, silicones, and the like can cause product failures of the electronic component or product. Due to their inherent tackiness, acrylic PSAs are very attractive for this type of application. Other PSAs can be used as well, but particular attention has to be given to the choice of tackifier or other additives needed in the PSA formulation. The choice of release liner also becomes very critical because of the concern about silicone transfer to the adhesive, which may eventually contaminate the electronic part. 

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