Marcos Lugo

Effects of microstructural inclusions on fatigue life of polyether ether ketone (PEEK).

In this study, the effects of microstructural inclusions on fatigue life of polyether ether ketone (PEEK) was investigated. Due to the versatility of its material properties, the semi-crystralline PEEK polymer has been increasingly adopted in a wide range of applications particularly as a biomaterial for orthopedic, trauma, and spinal implants. To obtain the cyclic behavior of PEEK, uniaxial fully-reversed strain-controlled fatigue tests were conducted at ambient temperature and at 0.02 mm/mm to 0.04 mm/mm strain amplitudes. The microstructure of PEEK was obtained using the optical and the scanning electron microscope (SEM) to determine the microstructural inclusion properties in PEEK specimen such as inclusion size, type, and nearest neighbor distance. SEM analysis was also conducted on the fracture surface of fatigue specimens to observe microstructural inclusions that served as the crack incubation sites. Based on the experimental strain-life results and the observed microstructure of fatigue specimens, a microstructure-sensitive fatigue model was used to predict the fatigue life of PEEK that includes both crack incubation and small crack growth regimes. Results show that the employed model is applicable to capture microstructural effects on fatigue behavior of PEEK.

Fatigue Behavior of Ti-6Al-4V Alloy Additively Manufactured by Laser Engineered Net Shaping

Fatigue behavior of Ti-6Al-4V specimens manufactured via Laser Engineered Net Shaping (LENS) is investigated in this study. Since the ultimate mechanical behavior of metallic parts is related to the microstructure as affected by the time-temperature history, additive manufactured components will have different properties from their wrought counterparts. The appropriate additive manufacturing process parameters are achieved by examining the built coupons with X-ray CT for minimum defects (i.e voids). Fully reversed strain-controlled fatigue tests are conducted on LENS produced Ti-6Al-4V speciments. Fractography of fractured surface is performed using Scanning Electron Microscopy (SEM) to capture the crack initiation site as well as failure mechanism. In order to predict fatigue life of additive-manufactured titanium alloys, a microstructural-sensitive multi-stage fatigue (MSMF) model can be calibrated.

Fatigue Behavior and Modeling for Thermoplastics

In this study, the cyclic behavior and the microstructure under cyclic loading of a semi-crystalline thermoplastic polyether ether ketone (PEEK) are investigated. Macroscopic strain life experiments under uniaxial loading condition were conducted at strain amplitudes ranging from 0.02 mm/mm to 0.04 mm/mm and at ambient temperature. Scanning electron microscopy (SEM) analysis was performed on fatigue specimens to examine crack incubation sites and microstructural inclusions responsible for incubating fatigue cracks. Other microstructural inclusion properties including inclusion type (pore or particle), inclusion size, and nearest neighbor distance of inclusions were also obtained. Based on the experimental results, the strain-based fatigue model was employed to obtain strain-life fatigue properties and e-N fatigue curves of PEEK.

Investigation of Cyclic Behavior and Structure-property Relations of a 304 Stainless Steel

In this research, we investigated the microstructure properties and its effects on the cyclic behavior of a rolled 304 stainless steel alloy. Fully reversed, strain control fatigue tests were conducted on specimens over a wide strain amplitude range to evaluate the fatigue properties of this alloy. Scanning electron and optical microscopy was employed to establish structureproperty relations between microstructure and cyclic damage. The scanning electron microscopy analysis revealed that fatigue cracks predominantly initiated at cracked intermetallic particles. It is important to note that heterogeneities found on the material microstructure correlate well with variations observed in total fatigue life. Finally, fatigue lives were predicted based on microstructure properties.

Microstructural Relationship in the Damage Evolution Process of an Az61 Magnesium Alloy

The damage evolution process of magnesium AZ61 alloy under monotonic tensile loading conditions is investigated. Specimens that have been subjected to interrupted tensile loading were examined under optical microscopy to quantify the number density of cracked intermetallic particles as a function of applied strain. Digital image analysis of the optical images was employed to automatically quantify damage by separating cracked from non-cracked particles. Lastly, an internal state variable damage model was shown to adequately capture the experimentally-observed damage of intermetallic particles in the magnesium AZ61 alloy.