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Product failure analysis is an important discipline in the production industry and it is used in development of new products and improvement of existing products on the market. 

Complex problems on the production line are not easily solved. In such cases, often the companies do not have enough resources, knowledge and analytical methods to ensure a  comprehensive approach to the problem. LOTRIČ Metrology laboratories are your partner in problemsolving, using chemical and mechanical methods in the field of chemistry, mechanics, polymer technology and metallurgical sciences.

DETERMINING THE CAUSE OF FASTENERS CRACKING

Problem

The customer asked LOTRIČ Metrology to determine why screws are cracking at relatively very low torque moment.

Initial investigations

First, we performed a penetrant analysis and checked that  no cracks in the screws have occurred before the screw was installed. We have found that the material has the required hardness and that the screws were surface-protected according to the instructions. By SEM analysis, we looked at the fracture surfaces and determined that this was an intercrystalline fragile fracture, suggesting that the most probable cause is either temper brittleness due to inadequate heat treatment or hydrogen embrittlement due to excessive irrigation in surface protection agents. 

Confirming hypotheses

In order to determine whether this is a case of hydrogen embrittlement, we have carried out a tensile test of screws tempered at different temperatures. Some screws were exposed for 5 hours at 200 ° C, resulting in diffusion of hydrogen from the metal and were then compared with the control group. It has been shown that the mechanical properties are somewhat improved, but remain substantially worse than expected. 
We found that the temperature of austenitization and hardening and the method of tempering correspond to those in the literature. According to the client's data, however,  the screws were tempered at a temperature of 350 ° C, which was almost 200 ° C higher than the actual temperature. In order to confirm the hypothesis that temper brittelness is responsible for the failure of the screws, the untreated screws were irrigated at the recommended temperatures and subsequently tempered at two different temperatures - we carried out low temperature deterioration at temperatures below the area of ​​the appearance of temper brittleness and high temperature deterioration in this region. Then the same tensile test we performed on the screws which were cracking, was carried out on these treated screws. The test showed a strong increase in mechanical properties, especially strength and elongation after break. The SEM analysis was used to examine the breaks produced by the tensile tests. It turned out that the fracture morphology at other tempering temperatures is completely different.

Conclusion

Analyses have shown that the cause of the cracking of the screws is the incorrect heat treatment of the screws resulting in temper brittleness , while a small part of the failure can also be ascribed to hydrogen embrittlement. Thus, we proposed tempering at higher temperatures to prevent temper brittleness. Had material hardness been the problem, we could also use lower tempering temperatures, but this is a sensitive issue. In any case, we could choose a more appropriate material which would achieve the prescribed hardness in an area where it would not be sensitive to temper brittleness.

CHANGE OF CHARACTERISTICS OF ABS MATERIALS UNDER THE IMPACT OF ARTIFICIAL AGING

Problem

The manufacturer of refrigerating technology incorporates 4 different ABS plastic materials into its products. We needed to establish  why some refrigerators come to break in parts with ABS material after a few years.

Methodology

Since most of the affected plastic parts were mounted on visible parts of the refrigerators, we hypothesized that the ABS material was fragile due to the regular cleaning of the cooling cabinets.

From all four samples of ABS material we produced standard specimens according to ISO 527-2 / 1A and subjected them to the following conditions before carrying out the tensile test:

1. No additional exposure before the test

2. 24 hours exposure of test pieces to  six most common clenaing agents 

3. 24 h of test pieces exposed to six most common cleaning agents + pre-tensioned condition.
We decided to use a detergent and a pre-tensioned state because some plastic elements in the product are mounted in pre-tensioned or pre-stressed state.

Results

All four tested plastic materials had a similar response to the test conditions. Table 2 presents the results for one material. The results in the table show how varied the effect of cleaning agents on ABS material is. When the test specimens were exposed to CIF, FROSCH and AJAX for 24 hours, the mechanical properties of the material did not change or suffered only minute changes. During the 24 hours exposition at HOLLU LM, W5, HOLLU F9,however, there is a change in the elongation at break.

We have assumed that the results at the 24 hour exposure to the medium in a pre-tensioned state would be similar only that the differences would be more pronounced, but this was not the case. Mechanical properties have significantly deteriorated in test subjects which were exposed to HOLLU LM and HOLLU F9. When exposed to CIF and AJAX, there is a significant reduction in elongation at break. This means that the material has retained its tensile strength but has become much more fragile. Exposure to  W5 in pre-tensioned condition leads to an improvement in elastic properties. The material retained its tensile strength while the elongation at break had improved. The same effect can be seen in exposure to  FROSCH, but the intensity is lower.

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