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This study investigates the effects of pores on the mechanical properties of metals produced by additive manufacturing, which can limit strength and ductility. This research aims to both measure and model the rate of crack growth emanating from these pores in additively manufactured Ti-6Al-4 V fabricated with laser powder bed fusion. Uniaxial tensile samples containing intentionally embedded penny-shaped pores were mechanically tested to failure, and loading was interrupted by a series of unload steps to measure the stiffness degradation with load. The factors contributing to reduction in stiffness, namely (1) elastic and plastic changes to geometry, (2) the effect of plastic deformation on modulus, and (3) crack growth, were deconvoluted through finite element modeling, and the crack size was estimated at each unloading step. The stiffness-based method was able to detect stable crack growth in samples with large pores (1.6% to 11% of the cross-sectional area). Crack growth as a function of strain was fit to a model where the crack driving force was based on equivalent strain and a model where the crack driving force was based on energy release rate. Significant crack growth occurred only after the onset of necking in samples containing small pores, while samples containing large pores experienced continuous crack growth with strain.
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Degradable Plastics Are Vulnerable to Cracks
A plastic may degrade in response to a trigger. The kinetics of degradation have long been characterized by the loss of weight and strength over time. These methods of gross characterization, however, are misleading when plastic degrades heterogeneously. Here, we study heterogeneous degradation in an extreme form: the growth of a crack under the combined action of chemistry and mechanics. An applied load opens the crack, exposes the crack front to chemical attack, and causes the crack to outrun gross degradation. We studied the crack growth in polylactic acid (PLA), a polyester in which ester bonds break by hydrolysis. We cut a crack in a PLA film using scissors, tore it using an apparatus, and recorded the crack growth using a camera through a microscope. In our testing range, the crack velocity was insensitive to load but was sensitive to humidity and pH. These findings will aid the development of degradable plastics for healthcare and sustainability.
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- Award ID(s):
- 2011754
- PAR ID:
- 10499987
- Publisher / Repository:
- Elsevier
- Date Published:
- Journal Name:
- Engineering
- Volume:
- 7
- Issue:
- 5
- ISSN:
- 2095-8099
- Page Range / eLocation ID:
- 624 to 629
- Format(s):
- Medium: X
- Sponsoring Org:
- National Science Foundation
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- Free Publicly Accessible Full Text
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Accepted Manuscript1.0
- Journal Article:
- https://doi.org/10.1016/j.eng.2021.02.009
