Failure Analysis on suitcase
The object
The object whose causes of breakage are being investigated is a travel suitcase. It is found to be non-compliant on the front side, characterized by the presence of anomalies (cracks, scratches…), while on the back side there is no damage. From the information provided by the client, the suitcase was never used, but it was subjected to a leak test. The test in question consisted of having the suitcase take an airplane trip, performing manipulations (opening/closing) and dragging it to the ground.
The anomalies, observed only on the front side of the case, led our client to want to investigate the possible differences of this case from those produced in the past, which were characterized by greater resistance to stress in operation.
The analyses
The analyses performed to determine the causes of component failure are:
- Fractographic study. Observations using the electron microscope (SEM) and the stereo-microscope
- Fluid contact by manifestation of internal stresses
- DSC thermal analysis (differential scanning calorimetry)
- Thermogravimetric analysis (TGA)
DSC and TGA analysis indicate that the case is made of a thermoplastic material mix: amorphous recyclable polyetheretheraphthalate, ABS (acrylonitrile-butadiene-styrene), and ABS and polycarbonate (PC) blends.
All the materials involved are polymeric materials, characterized by macromolecules interacting with each other through physical bonds (Van der Waal forces and hydrogen bridges). This characteristic allows for materials that can be recycled. Such recycling will have to be done properly, respecting the thermal strength parameters of the material, so as not to cause damage (loss of molecular weight) in the material.
Fractographic study
The suitcase as delivered to us by the client is characterized by the presence of defects (cracks) in the center of the front side. No similar defects are present either on the back of the case or on its side parts. Each part of the artifact under examination consists of a rough blue plastic layer on the outside and a smooth gray layer on the inside. From initial observations under the stereo-microscope, it is observed that the defect basically develops on the blue rough outer layer.
As for electron microscope (SEM) observations, they were made both frontally to the component and after opening the fracture surface of one of the cracks.
From the frontal observations, it should be noted that the end of each crack is characterized by plastic deformation, which can be correlated with the action caused by a tool and/or other with an etching effect on the case surface.
As for the fracture surface, it is characterized by a ductile, plastically deformed morphology with marks that can be correlated with an abrasion phenomenon of the material.
It should be noted that all thedefects found are attributable to the handling and handling that the case underwent in the leak test.
Contact with certain fluids for verification of internal stresses
The materials constituting the two parts of the case are amorphous thermoplastic materials. This type of plastic material is characterized by the possibility of accumulation of internal stresses at the macromolecular level. These stresses originate during the transformation process, that is, during the obtaining of the case itself. Unfortunately, the internal stresses exhibited by these materials are sometimes the cause of failures during operation. For this reason, it is recommended to conduct detensioning of the material to eliminate the stresses.
In the particular case of the suitcase, portions of the front and back were subjected to contact with some fluids, suitable for tensioned areas to emerge on the surface. From the analyses conducted, the material of the two parts of the sample under investigation shows no internal tensioning. Therefore, it is believed that the material is not tensioned and this is not the cause of the cracks that occurred.
DSC thermal and thermogravimetric analysis (TGA)
These analyses have the following functions:
- DSC analysis is a verification of the thermal behavior of the material. From the graph we will obtain typical thermal parameters of an amorphous thermoplastic material, i.e., its glass transition temperature (Tg).
- Thermogravimetric analysis (TGA) provides at a quantitative level all the fractions that make up the polymer compound: volatiles and semi-volatiles fraction, polymer fraction, and inorganic fraction (filler and/or reinforcement).
From the analyses conducted, no qualitative differences were found between the two parts that make up the case. Both consist of the mix indicated above (RPET, ABS and ABS/PC blend).
It should be pointed out, however, that with the above analyses, a percentage difference was found between the polymeric materials in the mix. In particular, there is a higher content of the ABS/PC blend on the front part of the case and a lower content of ABS than what was found on the back part, which, let us recall, is the one that had no damage.
Conclusions
From the results obtained, the two constituent parts of the case show a quantitative difference in the constituent polymer fractions. This difference is responsible for the different behavior that the two parts have regarding two important stresses during use: wear resistance and coefficient of friction.
For good physical-mechanical behavior, both parts of the case should exhibit high wear resistance and low coefficient of friction. If we consider the characteristics of each material in the mix with respect to the above physicomechanical parameters, the plastic material with the highest wear resistance property and the lowest coefficient of friction is R-PET. In contrast, with regard to the ABS material and the ABS/PC blend, the latter is characterized by worse behavior regarding the above parameters. Since the front part is the one with the highest content of ABS/PC blend, it is explained that it performs worse during the tightness tests undergone by the case itself.
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