Лако-красочные материалы — производство

Tests to monitor the quality of the adhesive materials may relate to the fresh state (e. g. viscosity, sag, pot-life), and to the hardened state — requiring the measurement of mechanical properties.

Quality variations in the cured adhesive layer itself can occur as a result of a number of factors. Voids may be present due to air becoming trapped by the method of mixing, dispensing or application of adhesive to the substrate; small voids may be caused by volatiles. Surface voids may be present due to adhesive being applied to one adherend only before closing the joint. Local areas of uncured material may be present due to poor or incorrect mixing of two — component systems, or due simply to insufficient curing. Small cracks can appear in the adhesive layer due to shrinkage problems with cure, or of course due to large applied stresses when cured — particularly in a cold environment. In general, poor or insufficient cure is self-correcting with time inasmuch as chemical reaction will continue.

The presence of such defects may or may not be important, depending on their extent, nature, location and the operating stresses and environment to which a joint is subjected. For the majority of joints, the presence of relatively small defects is of little immediate significance unless they occur in regions of high stress transfer (e. g. at the ends of a bonded overlap). In the longer term such defects may allow faster accession of water or sites for fatigue crack nucleation, and as such it is important to detect them. The NDT methods available are claimed to give information on the following:

(1) density of the adhesive layer

(2) thickness of the adhesive layer

(3) interfacial debonds

(4) overall structural stiffness

Physical and mechanical tests. Three main sources of guidance which exist in the UK are:

(1) BS5350 — Methods of test for adhesives (7)

(2) BS6319 — Testing of resin compositions for use in construction

(8)

(3) FIP — Proposal for a standard for acceptance tests and verification of epoxy bonding agents for segmental construction

(9)

It may be useful to monitor the unreacted components of the adhesive system as well as the properties of the freshly mixed adhesive, partly as a check on the adhesive supplier and partly to establish the working characteristics of the adhesive. Techniques for measuring the viscosity of adhesives are defined in BS5350 : Part B8 : 1977, whilst aspects such as open time, pot-life and thixotropy (sag flow) may be established for the actual application. Suggestions relevant to the use of epoxy bonding agents for segmental construction are documented (9).

Tests for determining mechanical properties of the cured adhesive may take the form of the fabrication and testing of bulk adhesive specimens and adhesive joints. As discussed in chapter 4 the standard test procedures listed by ASTM, BSI, DIN and other official bodies are essentially for testing adhesives and surface treatments rather than joints. A number of methods therefore exist for the testing of bulk adhesive specimens — tension, shear, compression and thermal response properties being fairly easily determined. Interpretation of the results of joint tests must be done very carefully, requiring also a careful choice of test joint geometries. This topic was discussed fully in Chapter 4, and a list of standard test methods is given in Table 4.3. Some of the bulk and joint test procedures are elaborated in the Appendix. On the whole, joint tests are of limited value in assuring quality of the bonded assembly.

Non-destructive testing. NDT represents a large and diverse field in which a number of review papers have been published in recent years(6, 10-13). Brief mention only of the most important techniques is given below because few are currently readily applicable to adhesive joints in civil engineering. The techniques in general are void detectors.

Sonic methods The coin-tap technique is one of the oldest and still intuitively acceptable methods. The sound emitted by tapping the surface of well-bonded adherends differs from that over an unbonded area indicating voids, disbonds, and so on. Voids of two or three centimetres in diameter can be detected but it remains a very subjective test. Attempts have been made to quantify the result by exciting the bonded structure with an electromagnetic tapper, measuring its response and then analyzing the frequency spectrum(14, 15). Trials have been conducted on the experimental Wester Duntanlich Bridge which was fabricated using concrete-to-steel — bonded precast deck panels, and is discussed at the beginning of Chapter 8. Areas which were well-bonded gave a heavily damped response, while at small areas which were delaminated a less damped response was produced.

Safety

Ultrasonic methods There exist a variety of methods which are used widely in the aircraft industry. The technique measures changes in acoustic impedance caused by defects in a bonded assembly when an ultrasonic transducer is liquid-coupled to it. The high frequency sound waves are simply scattered by the presence of porosity or voids in the bondline. Interpretation of the data can be difficult, although substrate thickness is not generally a limitation.

Acoustic emission If a bond is mechanically or thermally stressed local perturbations of energy, or stress waves, may be released from discontinuities such as disbonds. The high frequency content of such stress waves may then be detected with a piezoelastic sensor. Unfortunately it is usually necessary to stress the joint to a considerable extent, which may often be impossible or inadvisable.

Thermography Heat-sensitive detectors may be used to ascertain the temperature distribution over a heated surface; disbonds will show up as hotter areas provided the adherends are relatively thin. Conversely, by heating one surface of a bonded sandwich structure and monitoring the opposite face, areas of disbond to which the heat has not been conducted will become apparent as cool spots.

Radiography Density variations in bonded components can be detected with X-rays, provided that metallic fillers or other special contrast indicators are present in the adhesive. X-ray inspection of bonded honeycomb sandwich panels may be used for establishing the location of the core, and for checking for damage of the honeycomb.

Holography Optical holographic interferometry is a technique used for measuring minute surface displacements. When load is applied to a bonded assembly, defects become apparent as local perturbations in the holographic interferogram. The technique is really only applicable to thin adherend structures. Acoustic holography uses pulsed echo ultrasound and focused transducers to enable a hologram of reflections from the bondline. With both techniques the equipment required is very expensive and the process time-consuming.