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        Case Studies

        Failure Analysis

        Surgical Cutter/Stapler

        A client’s stapler had a variable pitch power screw drive mechanism that would “lock” in the steep pitch, fast advance portion of the thread.  The Stress Engineering Services (SES) team developed a system of governing equations that characterized the lifting force generated by the power screw and frictional forces generated in the device drive train.  These equations were used to examine the effects of manufacturing & assembly tolerances and lubrication on device performance.  Finally, results from ‘what-if’ calculations performed in Excel were used to identify changes to the thread pitch and requirements for tolerances needed to make the device function without locking.

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        Computer Housing

        SES was tasked with investigating the internal boss and external shell cracking of a computer housing to determine the cause of the failures and provide recommendations for mitigating the cracking issues.  Following our failure investigation process, the SES team conducted a detailed inspection of both new and failed parts and identified critical aspects of the manufacturing process, materials and part design.  Inspection of the cracks revealed that many were in close proximity to corners, often near dents or dings on the outer surface.  Testing was completed on the base material to understand its response to sustained load, temperature and moisture.  Additionally, SES made calculations to understand the strain in the material resulting from the assembly process and usage loads.  Ultimately, the cracks were found to be the result of reduction in the material’s ductility caused by sustained strain, temperature and humidity.

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        failure analysis

        Medical Device Retention Feature

        An SES client was experiencing failures of the retention tabs on one of their medical components. The tabs were cracking after a period of time and, in some cases, breaking off completely.  Our investigation team conducted a physical examination of both new and failed parts along with dimensional measurements of the various components in the assembly.  Using this information, we made calculations to determine interference between the parts and completed finite element analysis (FEA) to determine the stress after assembly.  Combining the results of these engineering calculations with environmental stress cracking and fatigue tests on the material, demonstrated that strain-induced aging caused the failures.  SES provided recommendations for design modifications to eliminate the failures.

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        Seismic Sensor

        This sensor product is intended for use in very harsh environments, where temperatures and moisture levels can be quite extreme.  SES’s client began hearing of cracking issues with the product after only a fraction of its intended design life.  The housing contains sensitive electronics, which are destroyed if they are exposed to water.  SES evaluated failed samples and found cracks in many locations on the sensor housing.  The housing is insert molded to embed the metal tip into the plastic in order to provide a secure seal and intimate contact between the tip and the electronics.  Our investigation included assessing aging effects on the materials, reviewing the part design, evaluating the manufacturing process and calculating the stress on the parts after manufacturing and during use.  The material was found to be sensitive to moisture and is significantly weaker at weld lines. The investigation suggested that the failure of the product was the result of residual stresses induced by the molding process and compounded by elevated temperature and exposure to water.

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