Elsevier

Composite Structures

Volume 276, 15 November 2021, 114545
Composite Structures

An innovative digital image correlation technique for in-situ process monitoring of composite structures in large scale additive manufacturing

https://doi.org/10.1016/j.compstruct.2021.114545Get rights and content

Highlights

Abstract

As additive manufacturing (AM) continues to develop and become a standardized manufacturing method, there will be a continued need to provide in-situ monitoring during the manufacturing of polymer composite printed components. Thermal residual stress is a primary cause of failures such as interlayer disbonds or delamination, micro cracking, and dimensional instability, which can occur during or after the build. This study reports a novel digital image correlation (DIC) adaptation to monitor thermal residual stresses during the entire print process for large-scale AM. In this work, DIC has been investigated (a) by the natural speckle produced by the polymer surface for correlation, (b) to monitor AM build, and (c) to evaluate the effect of thermal residual stress on warpage of the printed component. The natural speckle pattern of the AM material resulted in a respectable 3.57% error compared to the traditional painted speckle pattern of 3.05% error. DIC measured a 190% increase in vertical displacement at the edge of the wall compared to the center, indicating warpage during AM. This work is a step towards a non-intrusive residual stress measuring technique using DIC for large-scale AM.

Introduction

AM is an emerging manufacturing process in several industrial sectors. The digital nature of the AM process provides flexibility for integration into Industry 4.0 platforms, offering integration of intelligent production systems with advanced information technologies [1]. In recent years, compared to traditional manufacturing methods, AM has significantly improved the manufacture of complex geometries ranging from engine fuel nozzle [2], [3], thermoplastic autoclave molds [4], and printed large-scale demonstrations such as the Shelby Cobra [5] and the US Navy’s first printed submarine hull [6].

The large-scale polymer AM method used in this research has a history for rapid prototyping [7] but is now being used for semi-structural components [8]. Recent development of this large-scale AM process has led to larger machines which have build envelopes up to 30.5 m long by 7.6 m wide by 6.1 m tall (100 ft × 25 ft × 20 ft) with high deposition rates of 453 kg/hr (1000 lb/hr) or higher [9]. With the current rapid demand and innovation of the technology, the aerospace industry is using this process for large molds, trim tools and fixtures [10].

Early in-situ detection of defects and anomalies during the printing process can lead to significant savings in time, material waste, and cost. Defects that are associated with the printing process include porosity/voids, inclusions, weak interlayer adhesion/lack of fusion, microcracking/delamination, dimensional tolerance inaccuracies, and residual stress(es) [11], [12]. Residual stresses are the “locked-in” stresses within the printed component leading to warped, distorted geometries and debonding/delamination failures between layers of the AM process. Residual stresses are a primary cause of flaws such as microcracking, disbonds, delaminations, dimensional tolerance deviation, warping, shrinkage, and overall failed builds [13]. Residual stress failures occur in both metal and polymer AM techniques [14], [15]. These failures are caused by the high thermal expansion coefficients of common print materials, which produce a thermal expansion/contraction mismatch between the extruded polymer and the previously deposited layers [16], [17]. The thermal mismatch of printed layers produces a shear strain within the polymer bead. This leads to the formation of thermal residual stress(es) [16], [18]. Fig. 1 shows the formation of residual stress during the print process.

Several methods have been investigated to evaluate thermal residual stresses in polymer AM. Destructive characterization methods, such as the hole drilling method, are paired with the electronic speckle pattern interferometry (ESPI) performed by Caterina et al. [17]. Other methods are nondestructive, [19] such as interferometric techniques [20], [21], Xray/Neutron Diffraction [22], [23], [5], ultrasonics [24], [25] and infrared thermography [16]. Seppala et al. [26] used infrared (IR) thermography with reflection corrections to monitor the thermodynamics and kinetic states of the extruded polymers for weld formation. During printing, it was noticed that the temperature of the extruded layer rapidly decreased primarily through convection with little heat transfer to the sub-layer. Insufficient weld formation between polymer layers seems to occur due to the rapidly decreasing weld temperature, thereby resulting in residual stresses.

Kousiatza et al. [13] integrated fiber Bragg grating (FBG) sensors for in-situ and real-time monitoring of residual strain during polymer-extruded printing. However, the FBG technique is invasive due to the integration of a fiber-optic sensor within the printed structure. Compton et al. [16] utilized IR thermography to monitor the thermal evolution during a large-scale polymer AM process. A 1D thermal finite difference model was used to simulate the build process and was verified with experimental IR measurements. They indicated the importance of maintaining the top layer above the glass-transition temperature (Tg) before depositing a new layer, to reduce the effects of warping and microcracking. While maintaining the Tg of the top layer as a pass/fail criterion, they showed the most important parameter for a successful print is to increase the ambient temperature within the build chamber. A numerical simulation of a full-size car printed on a large-scale AM system was developed by Talagani et al. [18] to reduce warpage distortion and residual stress effects. Kim et al. [27] investigated a de-homogenization modeling technique for monitoring warpage. A model was validated by IR thermography to measure the temperature field while a linear variable differential transformer (LVDT) to measure the warpage of the printed part.

DIC is a non-intrusive optical metrology technique that, unlike traditional strain measurements using strain gages, provides full-field displacement/strain measurements by implementing a tracking technique [28], [29], [30]. DIC has been used extensively in multiple applications to measure deformation gradients of an object’s surface [29], [31], [32]. Since residual stress is subsequently calculated from the warping deformation, DIC provides the ability for full-field thermal residual stress monitoring during the AM build process. The hole drilling technique per ASTM E837 [33] evaluates the displacement from the release of the residual stress when a hole is drilled through the medium, typically measured using a specialized radial rosette strain gauge. DIC has been applied to this hole drilling process to measure full-field displacement during the release of the residual stress [34], [35]. Even though this hole drilling technique is a cheap method for manufacturers to measure residual stress, it only provides a localized area of measurement and is evidently a destructive technique.

DIC was used for post-process characterization of the residual stress in a polymer printed structure by Zhang et al. [36]. It was found that higher print speeds caused higher warpage, while a raster angle of ± 45⁰ and introduction of fiber reinforcement reduced warpage. Warpage was monitored post-process with DIC during the sectioning of printed specimens and subsequent release of residual stress. Biegler et al. [37] monitored laser metal deposition (LMD) AM technique in-situ, using DIC. They were successfully able to monitor a portion of the LMD process but were not able to monitor the entire build area/volume. In their work, a 30 × 100 mm2 wall was printed, cleaned, and painted with a speckle pattern for DIC. Then, newly-printed layers were deposited on top while DIC captured distortion of the wall below.

This work presents the ability to use DIC to monitor thermal residual stress distortion in-situ, during a large-scale AM process. The ability to monitor thermal residual stress distortion during the printing process is inhibited by the inability to apply a painted speckle pattern to the print surface during the printing process itself. In this research we used the natural texture of the AM part as a speckle pattern for correlation. The natural roughness of the AM print material produces a high contrast (black/white) speckle pattern mimicking traditional painted patterns. To the best of the authors knowledge, no such work has been performed. Two studies are presented within this paper: (1) a natural speckle correlation study, and (2) an in-situ DIC monitoring for large-scale AM study. These studies demonstrate the feasibility of DIC monitoring by (a) confirming the natural surface speckle pattern is viable for correlation, (b) demonstrating that DIC can monitor during the AM build, and (c) capturing thermal residual stress trends by measuring part warpage during the printing process itself.

Section snippets

Materials and methods

This research was performed on the Big Area Additive Manufacturing (BAAM) system at the Oak Ridge National Laboratory – Manufacturing Demonstration Facility (ORNL-MDF), Oak Ridge, Tennessee, USA. The BAAM system has a build envelope of 6.1 m long, 2.4 m wide, and 1.8 m tall (20 ft × 8 ft × 6 ft). It utilizes a polymer extrusion deposition process, using a single-screw extruder to deposit polymer material. An acrylonitrile butadiene styrene (ABS) polymer reinforced with 20% fiber weight fraction

Natural speckle correlation

A translation experiment was performed on a speckled and unspeckled printed surface during a 5 mm displacement. The subset size and subset spacing were optimized during the DIC evaluation. For the natural surface speckle, a subset size of 67 pixels and a step size of 5 pixels yielded an average CI of 0.00476 pixels and a standard deviation of 0.000646 pixels. The painted speckle had a subset size of 125 pixels and a step size of 5 pixels, yielding an average CI of 0.00259 pixels and standard

Conclusions

The monitoring of a large-scale polymer additive manufacturing process utilizing digital image correlation was explored as a way to monitor thermal residual stress of printed parts. Each of the three goals were achieved in this investigation:

  • (1)

    The ability of the natural speckle contrast of an AM material surface to be used for DIC correlation was confirmed through the natural speckle correlation study. The translation experiment resulted in sufficient correlation and a CI value below 0.01

CRediT authorship contribution statement

Ryan Spencer: Methodology, Formal analysis, Investigation, Data curation, Writing - original draft, Visualization. Ahmed Arabi Hassen: Resources, Conceptualization, Writing - review & editing, Visualization, Supervision. Justin Baba: Conceptualization, Methodology, Validation, Investigation, Resources, Writing - review & editing. John Lindahl: Investigation, Resources. Lonnie Love: Conceptualization, Project administration, Funding acquisition. Vlastimil Kunc: Resources, Project administration,

Declaration of Competing Interest

The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.

Acknowledgement

Research sponsored by the U.S. Department of Energy, Office of Energy Efficiency and Renewable Energy, Advanced Manufacturing Office, under contract DE-AC05-00OR22725 with UT-Battelle, LLC. Part of this work was funded in part by the Office of Energy Efficiency and Renewable Energy (EERE), U.S. Department of Energy, under Award Number DE-EE0006926.

We gratefully acknowledge the Institute of Advanced Composites Manufacturing Innovation (IACMI) and the Manufacturing Demonstration Facility (MDF),

Data availability

The raw/processed data required to reproduce these findings cannot be shared at this time due to technical or time limitations.

References (41)

  • M. Biegler et al.

    In-situ distortions in LMD additive manufacturing walls can be measured with digital image correlation and predicted using numerical simulations

    Addit Manuf

    (2018)
  • Kellner T. ge.com,“ General Electric (GE), 13 Nov 2017. [Online]. Available:...
  • Milberg E. “ORNL, Boeing Achieve First in Autoclave 3-D Printing,” CompositesManufacturingMagazine.com, 16 May 2016....
  • S. Babu et al.

    Additive Manufacturing of Materials: Opportunities and Challenges

    Mater Eng: Propell Innov

    (Dec 2015)
  • Milberg E. US Navy and ORNL Develop Military’s First 3D-Printed Submarine Hull,“ 24 July 2017. [Online]. Available:...
  • Caffrey T, Wolher T. “Wohlers Report,”...
  • J. Lindahl et al.

    Large-Scale Additive Manufacturing with Reactive Polymers

    CAMX - The Composites and Advanced Materials Expo, Dallas, TX

    (2018)
  • B. Post et al.

    “Big Area Additive Manufacturing Applications In Wind Turbine Molds

    Solid Freeform Fabrication An Additive Manufacturing Conference

    (2017)
  • L. Love et al.

    There's Plenty of Room at the Top

    Addit. Manuf.

    (2021)
  • J. Waller

    Nondestructive Testing of Additive Manufacturing Metal Parts Used in Aerospace Applications - ASTM International Webinar

    (2018)
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    Notice of Copyright This manuscript has been authored by UT-Battelle, LLC under Contract No. DE-AC05-00OR22725 with the U.S. Department of Energy. The United States Government retains and the publisher, by accepting the article for publication, acknowledges that the United States Government retains a non-exclusive, paid-up, irrevocable, world-wide license to publish or reproduce the published form of this manuscript, or allow others to do so, for United States Government purposes. The Department of Energy will provide public access to these results of federally sponsored research in accordance with the DOE Public Access Plan (http://energy.gov/downloads/doe-public-access-plan).

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