Testing for Residual Strains in 3D Printed Components using High Definition Distributed Fiber Optic Sensing (HD-FOS)

  In the May 2015 issue of the Harvard Business Review, Richard D’Aveni states that additive manufacturing is on the brink of completely transforming the design and production ecosystems for manufacturers of tangible goods.   https://hbr.org/2015/05/the-3-d-printing-revolution  Additive manufacturing frees both designers and manufacturers from many of the constraints of the normal product design, prototyping and industrialization process.  The cost and lead time for tooling can be eliminated and designers can create greater numbers of increasingly more complex parts, seemingly limited only by their imaginations. According to the author, the result will be a complete re-thinking of how business operations are conducted. While the rapid advancements of additive manufacturing technology have begun to change the paradigm for product design and manufacturing, the requirements for test and design validation remain the same. If anything, the growing penetration of additive manufactured parts into structural components along with an ever expanding range of available materials are combining to make traditional test methods completely inadequate for validating designs of additively manufactured components.  In particular is the problem of internal residual stresses that can accumulate during the buildup of material during the printing process.  Residual stress can cause layer delamination, part distortion and cracking and are a significant barrier to the wider adoption of additively manufactured parts in structural applications. Fortunately, commensurate with the coming 3-D printing revolution is an equally revolutionary technology from Luna Innovations that can be used to measure strain on complex surfaces and, and by embedding the sensor during printing, actually measure strain inside the component.  The fiber optic sensor, with a diameter of only 155 micro meters, can be embedded un-obtrusively during printing and not impact the components inherent characteristics.

To demonstrate this capability Luna engineers constructed a block using a 3D printer. The block was 3 inches in height with a base of 4 x 1.5 inches and constructed using ABS material. During printing, the operation was paused and the head lifted to allow laying a section of the fiber lengthwise across the block. A segment of the fiber was embedded every 0.3 inches resulting in a total of 9 layers vertically spaced 0.3 inches apart. Figure 1 shows the dimensions and pattern of embedded fiber.
Figure 2 shows the ABS block, nearly completed with a single fiber sensor embedded during its construction.
Figure 2 shows the ABS block, nearly completed with a single fiber sensor embedded during its construction.

Strain measurements were recorded using Luna’s ODiSI high definition fiber optic sensing system both during the printing process and also after its completion once the fabricated part cooled to ambient temperature.  The scan of strain taken at ambient temperature showed a significant amount of residual strain concentrated near the centerline of the ABS block and diminishing in proportion to the distance from the blocks center line.  Figure 3 shows the strain measurements plotted vs the height of the sensor segment from the base of the block.  The strain data shows clearly a very high level of residual stress existing within the bloc

Residual stresses and strains accumulate in the 3D printing fabrication process during the build-up of material and can have a significant and detrimental effect on the mechanical strength of a part. These residual stresses can couple additively to stresses from external loading, resulting in unexpected or premature failure.  Residual strains and stresses can be mitigated through a combination of material selection and through a careful optimizing of the various fabrication parameters.  The ODiSI high definition fiber optic sensing system when used in conjunction with this iterative process optimization can ensure parts built with additive manufacturing not only meet the design requirements, but can be built in volume production with a known and consistent process capability for critical parameters. The data and test methods from Luna’s experiment have been shared with researchers at Oak Ridge National Laboratory (ORNL) and Luna engineers are now working in partnership with ORNL to experiment with embedding sensors in 3D printed components. If 3D printing is truly going to revolutionize manufacturing and change the way businesses operate then paving the way will be equally revolutionary methods to test and validate these rapid advancements in 3D printing technology.  The accumulation of residual strains and stresses need to be controlled in order to maintain design integrity, reliability and quality.  These stresses and strains can only be controlled if they can be measured and Luna’s ODiSI system, with its ability to embed sensors and provide high definition strain measurements is the perfect solution.

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