PSI - Issue 66

Davide D’Andrea et al. / Procedia Structural Integrity 66 (2024) 449–458 D’Andrea et al./ Structural Integrity Procedia 00 (2025) 000–000

450

2

with powders has become faster, more precise, and economically accessible if compared to Selective Laser Sintering (SLS) [1]. MJF consist of selectively applying ink jetting liquid agents (melting and detailing agents) to regulate locally powder absorptivity and fusion energy required, which is given homogenously through IR lamp as a heat source to each slicing layer [2]. Geometries are built layer by layer repeating the process until the model is finished. The unfused powder serves as a substitute for traditional support structures, simplifying the model slicing phase. Polyammide-12 (PA12) is the most used material in MJF process. It finds application in many industrial fields, such as biomedical [3,4] and automotive [5] due to its biocompatibility, chemical resistance, impact resistance and thermal stability. However, its mechanical properties present large variations depending on printing parameters, such as build orientation [6] and cooling rate [7]. This wide dispersion of results associated with the large processing window necessitates those parts produced through additive manufacturing to be tested to verify their static and fatigue mechanical properties behaviour. Since fatigue characterization of material process needs significant time and resources, it is essential to use alternative methods which significantly reduce the effort required. Energy methods, by considering material’s phenomena conventionally ignored when standards are followed, can provide valuable insights into the fatigue life of the material. Infrared thermography is a full field measurement technique that enables the quantification of the energy release of a material subjected to various types of loads. Risitano’s Thermographic Method (RTM) allows to determine fatigue life of material by monitoring specimen’s surface temperature during constant amplitude [8] and stepwise [9] fatigue tests. Static Thermographic Method (STM) [10], which consists in monitoring and analysing specimen’s surface temperature during a static tensile test, returns a critical stress level corresponding to the onset of the first damage caused by irreversible micro-plasticity. It was demonstrated how fatigue limit and Static stress limit are correlated and how this value could be used as a design parameter [11]. Thermographic Methods have already been used on material treated through different AM processes such as Selective Laser Sintering (SLS), Fused Deposition Modelling (FDM), Laser Beam Melting (LBM) and Powder Bed Fusion (PBF) , both on metals [12,13] and plastics [14]: results demonstrated that these well-established methodologies, typically applied to conventionally manufactured specimens, can also be effectively extended to materials produced through additive manufacturing even if specimens’ surface thermal trend is influenced by microstructural characteristics [15]. Aim of this work, is to define how the PA12 MJF tested specimens’ mechanical behaviour differs from the one reported in productor’s datasheet and to determine a reliable parameter for fatigue design purposes with alternative methodologies in order to optimize time and resource efficiency.

Nomenclature c

specific heat capacity of the material [J/kg.K]

E

Young’s Modulus [MPa] thermoelastic coefficient [MPa -1 ]

K m

N, N i

number of cycles

R

stress ratio

R 2

coefficient of determination

t

test time [s]

T, T i

instantaneous value of temperature [K]

T 0

initial value of temperature estimated at time zero [K]

v α

displacement velocity [mm/min] thermal diffusivity of the material [m 2 /s]

ΔT s ΔT st

absolute surface temperature variation during a static tensile test [K]

stabilization temperature during a stepwise fatigue test [K] ΔT I-II Transition temperature between first and second phases during a static tensile test [K] ΔT 1 estimated value of temperature for the first set of temperature data [K] ΔT 2 estimated value of temperature for the second set of temperature data [K] ε, ε f strain, strain at failure Φ Energy parameter