Todd Letcher

Failure Analysis and Mechanical Characterization of 3D Printed ABS With Respect to Layer Thickness and Orientation

In contrast to conventional subtractive manufacturing methods which involve removing material to reach the desired shape, additive manufacturing is the technology of making objects directly from a computer-aided design model by adding a layer of material at a time. In this study, a comprehensive effort was undertaken to represent the strength of a 3D printed object as a function of layer thickness by investigating the correlation between the mechanical properties of parts manufactured out of acrylonitrile butadiene styrene (ABS) using fused deposition modeling and layer thickness and orientation. Furthermore, a case study on a typical support frame is done to generalize the findings of the extensive experimental work done on tensile samples. Finally, fractography was performed on tensile samples via a scanning digital microscope to determine the effects of layer thickness on failure modes. Statistical analyses proved that layer thickness and raster orientation have significant effect on the mechanical properties. Tensile test results showed that samples printed with 0.2 mm layer thickness exhibit higher elastic modulus and ultimate strength compared with 0.4 mm layer thickness. These results have direct influence on decision making and future use of 3D printing and functional load bearing parts.

Material Property Testing of 3D-Printed Specimen in PLA on an Entry-Level 3D Printer

An entry level consumer priced 3d-printer, the MakerBot Replicator 2x, was used to print specimen to conduct tensile, flexural and fatigue testing. Average priced, generic brand PLA material was used (similar to the filament a home user may purchase). Specimen were printed at raster orientation angles of 0°, 45° and 90° to test orientation effects on part strength. PLA filament was also tensile tested.Tensile testing of the 3d-printed specimens showed that the 45° raster orientation angle made the strongest specimen at an ultimate tensile strength of 64 MPa. The 0° and 90° raster orientation were not much less at 58 MPa and 54 MPa. A 3-point bending fixture was used to conduct flexural testing on printed specimen. For this type of testing, the 0° raster orientation produced the strongest parts with an ultimate bending stress of 102 MPa. Both the 45° and 90° raster orientations had similar results at 90 MPa and 86 MPa. For the fatigue testing, there was no clear best option, but there was a clearly worst option, the 90° raster orientation. This orientation clearly had lower fatigue lives than either of the other two raster orientations. The other two raster orientations, 0° and 45°, were very similar. PLA filament testing using bollard style grips, showed that the PLA filament exhibited mechanical properties similar to that of printed specimen — when tested at high enough strain rates that creep damage didn’t play a significant role. This may lead to implications for recycling failed 3d-print jobs and turning it back into reusable filament.Copyright

Experimental Study of Mechanical Properties of Additively Manufactured ABS Plastic as a Function of Layer Parameters

In this study, a preliminary effort was undertaken to represent the mechanical properties of a 3D printed specimen as a function of layer number, thickness and raster orientation by investigating the correlation between the mechanical properties of parts manufactured out of ABS using Fused Filament Fabrication (FFF) with a commercially available 3D printer, Makerbot Replicator 2x, and the printing parameters, such as layer thickness and raster orientation, were considered. Specimen were printed at raster orientation angles of 0°, 45° and 90°. Layer thickness of 0.2 mm was chosen to print specimens from a single layer to 35 layers. Samples were tested using an MTS Universal Testing Machine with extensometer to determine mechanical strength characteristics such as modulus of elasticity, ultimate tensile strength, maximum force and maximum elongation as the number of layers increased. Results showed that 0° raster orientation yields the highest mechanical properties compared to 45° and 90° at each individual layer. A linear relationship was found between the number of layers and the maximum force for all three orientations, in other words, maximum force required to break specimens linearly increased as the number of layers increased. The results also found the elastic modulus and maximum stress to increase as the number of layers increased up to almost 12 layers. For samples with more than 12 layers, the elastic modulus and maximum stress still increased, but at a much slower rate. These results can help software developers, mechanical designers and engineers reduce manufacturing time, material usage and cost by eliminating unnecessary layers that do not increase the ultimate stress of the material by improving material properties due to the addition of layers.Copyright

Mechanical Properties of Additively Manufactured PEEK Components Using Fused Filament Fabrication

Polyether ether ketone (PEEK) is introduced as a material for the additive manufacturing process called fused filament fabrication (FFF), as opposed to selective laser sintering (SLS) manufacturing. FFF manufacturing has several advantages over SLS manufacturing, including lower initial machine purchases costs, ease of use (spool of filament material vs powder material), reduced risk of material contamination and/or degradation, and safety for the users of the equipment. PEEK is an excellent candidate for FFF due to its low moisture absorption as opposed to other common FFF materials, such as Acrylonitrile Butadiene Styrene (ABS).PEEK has been processed into a filament and samples have been manufactured using several build orientations and extrusion paths. The samples were used to conduct tensile, compression, flexural, and impact testing to determine mechanical strength characteristics such as yield strength, modulus of elasticity, ultimate tensile strength and maximum elongation, etc. All tests were conducted at room temperature. A microscope analysis was also conducted to show features on the failures surfaces. The mechanical property results from this study are compared to other published results using traditional thermo-plastic manufacturing techniques, such injection molding.Tensile testing was conducted at three raster orientations, 0°, 90° and alternating between 0° and 90°. Average ultimate tensile stresses were determined to be 73 MPa for 0° orientation, and 54 MPa for 90° orientation, with alternating 0°/90° orientations of 66.5 MPa. Compression testing was conducted at two raster orientations, 0° and alternating between 0° and 90°. Average ultimate strength for the single orientation direction was 80.9 MPa with the alternating orientations at 72.8 MPa. Flexural testing was conducted at three raster orientations, 0°, 90° and alternating between 0° and 90°. Ultimate flexural stress was determined to be 111.7 MPa for 0°, 79.7 MPa for 90°, and 95.3 MPa for orientations alternating between 0° and 90°. Finally, impact testing was conducted at three raster orientations, 0°, 90° and alternating between 0° and 90°. Average impact energy absorbed was determined to be 17.5 Nm in the 0° orientation, 1.4 Nm in the 90° orientation, and 0.7 Nm for the alternating 0° and 90° orientations.Copyright

Effects of Defects on the Performance of Hierarchical Honeycomb Metamaterials Realized Through Additive Manufacturing

Cellular metamaterials are of immense interest for many current engineering applications. Tailoring the structural organization of cellular structures leads to new metamaterials with superior properties leading to low weight and very strong/stiff materials. Incorporation of hierarchy to regular cellular structures enhances the properties and introduces novel tailorable metamaterials.For many complex cellular metamaterials, the only realistic manufacturing process is additive manufacturing (AM). The use of AM to manufacture large structures may lead to several types of defects during the manufacturing process, such as missing/broken cell walls, irregular thickness, flawed joints, missing (partial) layers, and irregular elastic plastic behavior due to toolpath. For large structures, it would be beneficial to understand the effect of defects on the overall performance of the structure to determine if the manufacturing defect(s) are significant enough to abort and restart or whether the material can still be used.Honeycomb structures are used for the high strength to weight ratio applications. These metamaterials have been studied and several models have been developed based on idealized cell structures to explain their elastic plastic behavior. However, these models do not capture real-world manufacturing defects resulting from AM. The variation of elastic plastic behavior of regular honeycomb structures with defects has been studied, but the performance of hierarchical honeycomb structures with defects is still unknown. In this study, the effects of missing cell walls are investigated to understand the elastic behavior of hierarchical honeycomb structures through simulations using finite element analysis. Regular (zero order), first order and second order hierarchical honeycombs have been investigated in this study. The first level of hierarchy has been implemented by changing each three edge vertex of a regular hexagonal honeycomb lattice by adding another smaller hexagon. The second level of hierarchy is created by adding another smaller hexagon at each three edge vertex of the hexagons added for the first order hierarchy. For the hierarchical cases, the overall density of the honeycomb is held constant to the parent structure (zero order or regular) by reducing the thickness of the cell wall in the first and second order structures. ANSYS® was used to develop finite element models to analyze the performance of both perfect and defected regular, first order and second order hierarchical honeycombs. Defects were added to the model by randomly removing cell walls. Hierarchical honeycombs demonstrated more sensitivity to missing cell walls than regular honeycombs. On average, the elastic modulus decreased by 45% with 5.5% missing cell walls for regular honeycombs, 60% with 4% missing cell walls for first order hierarchical honeycomb and 95% with 4% missing cell walls for second order hierarchical honeycombs.Copyright

Investigating the Structural Properties of Corn Stover at Macro and Fiber Levels

The purpose of this study is to analyze structural properties of biomass materials, namely corn stover. The structural properties of the biomass corn stover are examined at macro and fiber levels by performing a series of tests including three-point bending and tensile strength. Results of the stated tests are statistically analyzed. The goal of this analysis is to test the strength under loading from various directions to gather a full understanding of the structural properties of corn stalk fibers. Tests are performed using universal testing machines (UTMs). The results of these studies will be used to compile a database of the structural properties of biomass. These properties have the potential to be used in finite element computer simulations for structural analysis and bulk solid flows. The bulk fluid motion of the pulverized/chopped biomass can be simulated in storage and transportation equipment, including auguring screws and pneumatic conveyance systems, as well as devices for feeding biomass feedstocks in biorefineries. Traditional biochemical and thermochemical reactors operate as batch systems because of the difficulty of feeding the biomass feedstock in a continuous manner. Having a clearer background about the structural and rheological properties of biomass feedstock will help simulate and design the bulk-solid flows within storage bins and conveyance systems.Copyright

Applying an Energy-Based Fatigue Life Prediction Method to Unnotched and Notched Al6061-T6 Specimen

There is a strong relationship between fracture mechanics and fatigue. Recently, an energy-based fatigue life prediction method has been studied as a method to quickly, but still accurately determine an SN curve for new materials. In the development of this energy-based fatigue life prediction theory, efforts have concentrated on monitoring stress/strain hysteresis loops only to make life predictions. Thus far, no attempts have been made to link knowledge of fracture mechanics to advances in the energy-based fatigue lifing theory. In this study, notched and unnotched AL6061-T6 flat specimens were fatigued with fatigue monitored by an extensometer. In order to prevent from buckling during hysteresis strain loops, R = −0.5 stress ratio was used. In addition, efforts will concentrate in the low cycle fatigue (LCF) region to support future works on monitoring crack length in fracture mechanics investigation. The goal of this study is to understand how specimens behave in the context of the energy-based fatigue life theory when notches/cracks are present.Copyright

Hysteresis Strain Energy Behavior of Al6061-T6 With Multi-Fatigue Load Levels as Applied to an Energy-Based Fatigue Life Prediction Method

Fatigue testing is a time and resource-consuming task. Historically, SN testing was conducted at many stress levels on simple representative specimen in order to determine an SN curve, which could then be used to design a component from the same type of material. Recently, an energy-based fatigue life prediction method has been in development. The goal of this method is to quickly determine a material’s fatigue characteristics using simple test procedures. The main theory behind the energy-based fatigue life prediction method is that the strain energy in a monotonic tensile test is equal to the cumulative hysteresis energy of a cyclic test. This theory has always been tested using a single stress level on each specimen. The hysteresis loop information was then used to make fatigue life predictions at other stress levels. Further testing has been done to learn more about the hysteresis energy behavior throughout the lifetime of a specimen, but only for a single stress value. In this study, several stress levels were tested on a single specimen. This new information will help make fatigue life predictions by completely removing the difficult and inconsistent process of determining experimental curve fit coefficients traditionally used in the energy-based fatigue life prediction method.Copyright