Fracture of Additive Manufactured Polymers
Written by: Dave Bourell, University of Texas at Austin and David Leigh, Harvest Technologies
The service applications of AM parts is increasing. This is the case for a number of polymeric materials with some being considered for or used in structural applications. Laser sintering (LS) is quite popular, and, while most aspects of this article will apply to other AM techniques, the focus here is polymeric parts built on LS fabricators.
Using best practice, it is possible to produce parts with consistent, build-direction-independent mechanical properties that are equivalent to those for compression molded parts. However, under certain circumstances, parts are built that fail unexpectedly at low loads. This is shown in the photos of fracture surfaces of two LS polyamide 11 ASTM D638 tension test specimens. The part on the left failed with very little lateral contraction consistent with the lack of any material ductility. The part on the right shows full ductility with a shear fracture morphology characterized by significant ductile dimpling of the fracture surface.
An explanation is offered here to provide understanding of the issues giving rise to this highly variable material response and to guide production of AM LS polymeric parts that avoid the brittle failure mode. The explanation is based on a fracture mechanics approach. Fracture mechanics was developed over the last 50 years or so to explain brittle behavior in materials that were designed to manifest ductile behavior. Implicit to fracture mechanics is both the assumption of the presence of defects or flaws in the material as well as the existence of a calculated “fast fracture stress”, the magnitude of which depends on the size and distribution of the defects. Highly defected materials have a low fast-fracture stress. If the calculated fast-fracture stress exceeds the actual yield and tensile strength of the material, then fracture mechanics does not apply, and the part will fail in a fully ductilemanner, similar to the dotted schematic stress-strain curve below. If the fast fracture stress is less than either the yield or tensile stress, then the part will fail in a fast-fracture mode once the fast-fracture stress is reached. The solid line on the figure represents this response for an extreme case.
The micrograph shows an extreme case of a highly defected LS part. It is a cross section of a polyamide-11 (PA-11, nylon) part which is highly defected. There is insufficient bonding between layers which has generated a series of defects in the material. The authors have engineered defects of increasing intensity in AM PA-11 tension specimens. The plot below shows the results of these tests. It is noteworthy that the mechanical behavior follows a master stress-strain curve, with fracture occurring at a variety of stresses along the curve depending on the degree of defecting in the specimen.
It is possible to quantify the effect. The relationship for the fast-fracture stress as a function of the defect structure is given by the basic equation of fracture mechanics:
where sf is the fast-fracture stress, KIC is the fracturetoughness (a material property), Y is a geometry factor detailing the defect distribution effect (Y increases with increasing levels of defecting) and a is the defect size. It is apparent from this relationship that to avoid fast fracture, the material should possess high intrinsic toughness and small, widely dispersed defects.
If one considers the denominator of the equation,, to be a measure of the intensity of the defect effect, then a schematic plot may be created to illustrate the observed ductile and brittle behavior in LS polymeric parts. The fast-fracture stress is shown to decrease with increasing defect intensity, while the intrinsic ductile yield and tensile stress of the polymer are plotted as defect independent quantities. At high defect intensity, the fast-fracture stress is less than the yield stress, and fast fracture occurs with minimal plastic deformation (0-5% elongation to failure). This is what occurred for the part shown at the beginning of this article in the left figure. At intermediate levels of defecting, the fast-fracture stress exceeds the yield strength, but the part still fails in a fast-fracture mode. In this case, there is some amount of elongation (5-20% elongation to failure) depending on when fracture occurs, but it is always less than what one might normally expect for a fully ductile material. Finally, for minimal defect presence, the fast-fracture stress exceeds the ultimate tensile strength (UTS) of the polymer, and full ductile behavior results (>50% elongation to failure).
Based on these results, it is imperative that sufficient energy be made available and maintained during AM (LS) processing to insure full melting and reflow of the polymer with minimal formation of voids. Factors that contribute to this for LS of polymers are increases in laser power, decreases in scan speed and scan spacing, double scanning, use of fresh powder and increasing of the processing temperature.