FORD BO 127-03 Stress Crack Test for Plastics

The standard “FORD BO 127-03 Stress crack testing for plastics,” developed by Ford Motor Company (FORD), describes a procedure for determining the resistance of thermoplastic materials to stress cracking when used in direct contact with other materials such as oil, grease, cleaners, flexible PVC and elastomers.

All amorphous materials should be evaluated for potential incompatibility with other materials they may come into contact with. Common automotive resins are listed in the table provided in the standard. If in doubt or if the material does not appear in the table, materials engineering should be contacted.

For example, a greased metal fastener is used on a PC component or part, so the PC must be tested for stress cracking with production grease. ABS is tested because of an armrest substrate molded from ABS wrapped in PVC (vinyl). PVC can cause plasticizer migration into ABS.

The necessary devices used in these tests are:

• Mechanical convection oven: A Type II A oven must be used as described in ASTM E145-19. An oven with forced ventilation and temperature uniformity within plus or minus 1 percent of the difference between oven and ambient temperatures. The oven temperature must be controllable to within plus or minus 2 degrees Celsius unless otherwise specified in the Ford specification.
• Test fixtures: A mandrel is constructed from chemically inert materials such as wood or aluminum with the required radius to achieve the required percent strain. Specimens should be clamped onto the mandrel in a manner that will not affect the test results.
• AATCC whisk meter cloth (style 3): 80 x 84 thread count, combed cotton, unsized, bleached (no optical brighteners or finishing materials).
• Chemical Substances: Test substances must be determined and approved by both contracting parties, Ford Product Development Engineering, and the test laboratory, respectively. The test substance must consist of chemicals specific to the environment for which the product is designed. Possible chemicals are listed in the table provided in the standard.

The procedures described in the FORD BO 127-03 standard are as follows:

• Method A: Molded tensile test specimens (material testing): Prepare Type 1A and 1B bars according to ISO 527-2. A minimum of four tensile bar specimens should be prepared. In the case of materials suspected of exhibiting anisotropy, specimens should be obtained parallel and perpendicular to the material flow and identified as such.

Test fixtures should be carefully cleaned before each test to prevent possible contamination from previous tests. Tensile bars are mounted in the test fixtures and a 1 percent strain level is achieved. The back of the specimens must fully contact the test fixture to achieve the desired strain level. A bend radius of 198 mm (based on a 4 mm thickness) should be used for Type 1A and 1B specimens.

The samples are examined for cracks along the gauge length using a 20X stereo microscope (after the stress is applied but before the chemical is applied). This is to determine the initial stress state.

The chemical agent is applied to at least three sample tensile bars by the applicable application method in the table within 2 hours of mounting the test bar in the test fixture.

Samples are exposed to 80°C plus/minus 2°C for 72 hours. The control sample is exposed in a separate oven to prevent contamination.

At the end of the test period, remove the chemical by wiping or scraping with a soft cloth. No solvents are used.

The filtered sample is conditioned for 1 hour at a temperature of 23 degrees plus/minus 2 degrees and a relative humidity of 50 percent plus/minus 5 percent.

Using a stereo microscope, the strained sample is examined for cracks and fissures at 20X magnification while the sample is still in the test fixture.

The chemical substance, its quantity, frequency of application and the presence of cracks or crevices are reported.

• Method B: Part pedigree test specimens (performance testing): A 150 mm x 20 mm sample is prepared from a flat section of the existing part, either by water jetting or smooth cutting. The part must be free of burrs or sharp edges that could cause fracture. At least four specimens should be prepared. For materials suspected of exhibiting anisotropy, specimens should be obtained from multiple parts and from the part's location and orientation as agreed upon with Ford Materials Engineering.

Test fixtures must be carefully cleaned before each test to prevent possible contamination from previous tests. Specimens are placed in the test fixtures and strained to a 1 percent strain level. The back of the specimens must be in full contact with the test fixture to achieve the desired strain level.

Due to the inherent variability of the part, the average thickness of the part (at least three points along the sample) should be measured to calculate the bend radii required to achieve a 1 percent strain. Specimen thickness measurements should not exceed plus/minus 0,5 mm, as this will affect the strain applied to the sample. If the thickness variation exceeds plus/minus 0,5 mm, materials engineering should be contacted.

Samples are examined for cracks using a 20X stereo microscope through the center of the part sample (after the stress is applied but before the chemical is applied). This is to determine the initial stress state.

From this point on, Method A is followed. Method A may be tested in place of Method B in a performance specification at the discretion of the materials engineer.

Our organization, which has been supporting businesses across all sectors for years through a wide range of testing, measurement, analysis, and evaluation activities, has a strong team of employees who closely follow global developments in science and technology and are constantly improving themselves. In this context, we also provide testing services in accordance with the "FORD BO 127-03 Stress Crack Test for Plastics" standard.