Bond quality, testing

EN 13354 2008 Solid Wood Panels — Bonding quality — Test method. 

In addition to the direct measurements of fiber-matrix interface properties discussed in Section 3.2, a number of testing techniques have been devised to assess the fiber-matrix interface bond quality by inference from the gross mechanical properties such as interlaminar shear strength (ILSS), translaminar or in-plane shear strength, and transverse tensile strength. These testing techniques invariably employ... 

Apart from the short beam shear test, which measures the interlaminar shear properties, many different specimen geometry and loading configurations are available in the literature for the translaminar or in-plane strength measurements. These include the losipescu shear test, the 45°]5 tensile test, the [10°] off-axis tensile test, the rail-shear tests, the cross-beam sandwich test and the thin-walled tube torsion test. Since the state of shear stress in the test areas of the specimens is seldom pure or uniform in most of these techniques, the results obtained are likely to be inconsistent. In addition to the above shear tests, the transverse tension test is another simple popular method to assess the bond quality of bulk composites. Some of these methods are more widely used than others due to their simplicity in specimen preparation and data reduction methodology. 

Other non-destructive tests have been suggested to estimate bond quality, but such techniques as holography and radiography, and also ultrasonics, have mostly been used in the rubber industry for detection of flaws in tyres. It is not considered appropriate to cover non-destructive flaw detection in general here but an account of applications to polymers has been given by Gros in Handbook of Polymer Testing37. 

With thermal surface impedance testing, heat is injected into the test objects surface from a hot gas pulse. The resulting surface temperature transient is analyzed to determine the bond quality in nearly real time. The surface temperature transients are sensed using an innovative noncontacting, emissivity-independent infrared sensor.27 This method is not adversely affected by surface blemishes or roughness. 

It is shown that the composite adhesion or bond quality is dependent on the PA-6 modification as examined by tensile test. The processing parameters are not optimised to obtain the highest values for the tensile strength. The aim of this investigation was the determination of the influence of the increase of carboxylic acid... 

Specific Column Quality Tests for Chemically Bonded... 

Figure 4.33. Typical routine column quality test chromatogram for a 30 cm x 4.6 mm column packed with an octadecylsiloxane-bonded silica packing of 10 p,m particle diameter. Test mixture resorcinol (0.55 mg / ml), acetophenone (0.025 mg / ml), naphthalene (0.20 mg/ml) and anthracene (0.01 mg / ml) in acetonitrile, 10 p.1 injected. Isocratic separation at 23°C with acetonitrile-water (55 45) as the mobile phase with a flow rate of 1.5 ml/min.

The interfaces in bonded structures influence the mechanical behavior of components significantly. Therefore, an important task in nondestructive testing (NOT) is the investigation of the interaction forces in adhesive joints and the development of techniques to evaluate the bond quality. The load capacity of such joints is often limited by regions of weak bonding. As in all materials, the... 

In summary, bonded repairs are the preferred approach for manufacturing repairs to both honeycomb sandwich and monolithic secondary stmcture. However, for the FAA and European Airworthiness and Safety Administration, the main reason for withholding certification of bonded repairs for primary structure is the lack of certainty over bond quality as it is not possible to assess strength and durability of bonded joints without destructive testing. 

All the hoards were tested under dry (normal) condition and wet (immersion) condition according to European Norm EN 314 1993 Plywood-Bonding Quality (Part 1 Test Methods). As for immersion test, the specimens were soaked in water for 24 h at 20 °C. 

Furthermore, the application of existing European or national test and performance standards for epoxy bonded products are much too penalising, since they merely impose severe conditions that are not verified in service, or are inadequate because they were developed originally for other adhesives, namely for phenolic and aminoplastic adhesives used in very thin bondlines [7, 52-58]. Moreover, current standard proposals developed for gap-filling adhesives focus only on the initial bond quality control [59-63]. The lack of standards in this field impedes the objective evaluation of the reliability of a bonded-in rod connection, causing engineers to avoid this type of approach altogether. 

The current European standards for wood-based panels deal with evaluation of properties of end-products such as internal bond strength or bending strength. In case of plywood, the bonding quality can be evaluated by tensile shear testing according to EN 314-1 (2005) [1]. These properties are fully linked to adhesives used and process parameters. Nevertheless, no standard exists in Europe for the assessment of adhesives for wood-based panels. 

Plywood panels produced in Europe for building purposes use urea-formaldehyde (UF), melamine-urea-formaldehyde (MUF) and phenol-formaldehyde (PF) adhesives. Standards requirements are not focused on direct properties of these adhesives but properties of the end-products (closely linked to adhesive properties and process parameters). Nevertheless, EN 314-1 (2005) [1] defines a test method for the evaluation of the bonding quality of plywood panels, but also blockboards, laminated boards and laminated veneer lumbers (LVL), by shear testing before and after ageing treatments. Relevant requirements are specified in EN 314-2 (1993) [5] which also defines the ageing treatments required for the intended service class of the tested panel as shown in Table 1. 

Test samples are designed to determine the bonding quality of each ply tested in tensile shear, according to Fig. 1. 

For wood-based panels, European standards are focused on general properties of end-products. The only standard really linked to adhesion is EN 314-1 (2005) [1] which defines a test method for evaluation of the bonding quality of plywood panels. 

EN 314-1 2005, Plywood, Bonding quality — Part 1 Test methods. 

It is strongly recommended that lap-joint samples are prepared and tested to control the bond quality (for test methods see Chapter 7). The samples shall have identical surface treatments to the actual bond surfaces and shall be cured identically to the actual bond. 

The stresses in this type of test are not uniform and are not concentrated at the bond interface, which makes it difficult to judge bond quality, unless it is very poor. The mould is more expensive than that for a peel test. 

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