inspection and engineering unit 11.3.1

8/13/2019 Inspection and Engineering Unit 11.3.1

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INSPECTION AND ENGINEERINGUNIT 11.3.1

ASTM D 4541

Objective

The objective of this training unit is to give a thorough overview of ASTM D4541 which is thetest method for determining the pull-off strength of coatings using a portable adhesion tester.

The test methods cover procedures for assessing the adhesion of coating films to metallicsubstrates by applying a force to a dolly attached to the coating surface by an adhesive.

Coating Adhesion

Adhesion is fundamental to the performance of an applied coating system.

Because adhesion is an important factor in a coating life, a number of methods are available toasses the adhesion of applied and cured coatings on metallic substrates.

This method is semi-quantitative and provides a value for the pull-off adhesion strength.

It can be used to determine whether the coating meets the requirement of a value written in aspecification or as a pass/fail.

It is important that the person conducting the tests understands the requirements, has a copy of therelevant standard and has the experience to conduct the test, interpret the results and produce thetechnical report.

Copyright 2010 Fitz-Coatings Ltd




Summary of Test Method

A dolly is attached perpendicular to the coated surface with an adhesive.

After curing, the test apparatus is attached to the dolly and aligned to apply tension normal to thetest surface.

A gradually increasing force is applied to the dolly until the coating breaks and detaches.

The force (pull-off strength) at the time of detachment can be calculated (normally read from a scaleor dial on the instrument).

The mode of failure is assessed as the percent of adhesive and cohesive failures, with a descriptionof the interfaces involved.

Results with different instruments may vary.

Adhesion Test Equipment - Pull-Off Test

To conduct an adhesion test in accordance with ASTM D 4541 the following items are required:

• A copy of the ASTM test method. This document can be purchased online.

• Adhesion test apparatus.

• "Dolly" with a flat surface for attaching to the coated surface with adhesive and a means

of attaching to the tester at the other.

• Base on the tester or an annular ring to ensure that the force is applied normal to the

surface.

• Timer or means of controlling the rate of stress to less than 1MPa/s (150psi/s).





Adhesion Test Equipment - Pull-Off Test Continued.

• Force indicator (normally part of the test apparatus).

• Solvent or other means to clean the dolly surface.

• Fine sandpaper.

• Adhesive.

• Magnetic or mechanical clamps.• Cotton swabs.

• Circular hole cutter.

Pull-off strength measurements depend on material and instrumental requirements.

There are five types of instrument (2 fixed alignment and 3 self alignment) described in the standard

with there own variations on the general test procedure and they may produce different results.

They apply the force to the dolly either manually, hydraulically or by use of pressurized gas.





All of the testers are available in a number of ranges of tensile force to suit different coating systemsand different substrates.

Curved dollies are available for surfaces such as pipelines.

Test Preparation

Prior to undertaking the test the following requirements apply:-

The test area should be flat and each test site should be separated by at least the area of the size ofthe apparatus.

A minimum of three sites are required for each test area.

The thickness of the substrate can affect the results e.g. <3.2mm the results are reduced.

Clean the test area only if necessary and with fine sandpaper if loose contaminants are present.

Test Preparation continued





Clean the dolly. See ASTM D 2651 and 3933 for guidance to avoid glue line failures.

Prepare the adhesive, apply in accordance with the manufacturers recommendations and positionthe dolly on the test area. Remove excess adhesive and apply constant pressure on the dolly with aclamp until cured.

If scoring around the dolly is carried out extreme care should be taken, as scoring violates the testprocedure it should be documented.

Note the temperature and relative humidity.

General Test Procedure

There are five test methods related to the five different types of adhesion tester.

The general procedure is set out below and the user should refer to the standard for the variationsapplicable to each tester.

• Place the bearing ring and shims, if required, around the dolly.• Carefully connect the tester to the dolly.

• Align and set to zero in accordance with the manufacturers instructions.

• Increase the load in a smooth continuous manner at a rate of < 1 Mpa/s (150psi/s) so that

failure occurs or maximum stress is reached in about 100 s or less.





Results

• Record the force at failure or maximum force applied.

• If paint is detached, label and store the dolly.

• Note any departure from the procedure.

If instructed by the manufacturer, calculate, or use the calibration chart for the instrument to

calculate the relative stress applied. Refer to the standard for details.

Estimate the percent of adhesive and cohesive failure and their position within the coating system.

Disregard any test where glue failure is > 50% of the area. However in the case of a pass/failcriterion if a glue failure occurs at a pull-off strength greater than the criterion the result should bereported as a pass.

Reporting





The following information is required:-

• Generic type of coating for each coat.

• Temperature, relative humidity and other relevant environmental conditions.

• Apparatus type, dolly type and dimensions and bearing ring type and dimensions.

• Substrate type and thickness.

• Adhesive used.

• Pull-off strengths.

• Date and location.• For pass/fail, the stress applied and nature of failure.

• Nature and location of failure e.g. adhesive/cohesive.

• Any omitted results and reasons.

• If scoring was carried out.


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