PSI - Issue 68

Amy Milne et al. / Procedia Structural Integrity 68 (2025) 666–673 Milne et. al. / Structural Integrity Procedia 00 (2025) 000–000

668

3

= ; *+,

[3] where is the sample thickness, !"# the net sample thickness between the side grooves, the applied load and the sample width. Under widespread creep conditions, the ∗ parameter is used to characterize the CCG rate of an alloy. A power law relationship is expected between the CCG rate, ̇ , and ∗ in exponential form, as shown in Eqn [4], ȧ = D 0 C * ∅ [4] where % is a temperature dependent constant and the power-law exponent, ∅ , is expected to be close to unity. The ∗ parameter evaluated experimentally for C(T) samples using the relationship shown in Eqn [5] Nikbin et al. (1984). ∗ = ∆ /̇ *+, ( − ) + 1 [5] where represents the remote applied tensile load, ∆ &̇ is creep contribution to the LLD rate, is the specimen width and is the instantaneous crack length and is a geometric factor obtained from Davies et al. (2007). For the C(T) specimens = 2.2 , and is the uniaxial creep power-law stress exponent, Davies et al. (2007). 2.1. CCG Mechanism Creep cracks can initiate from a pre-existing defect, which acts as a stress concentrating feature in an alloy at elevated temperatures. The intensified stress at the tip of the defect causes relatively high creep strain rates in the alloy grains. These grains will deform, leading to the nucleation, growth and coalescence of voids ahead at the grain boundaries to form micro-cracks. These voids and micro-cracks can generally be referred to as ‘creep damage’. These microcracks can link up with the main defect generating a new creep crack tip where the stress is now at its highest. Creep strain, and hence damage, accumulates at the new creep crack tip and hence the process continues such that the crack progressively grows through the geometry. Creep damage and hence creep cracks are intergranular in nature, whereas fatigue cracks tend to be trans-granular as reported by G. A. Webster (1994). 3. MATERIALS AND TEST METHODS 3.1. Sample Geometries In this work, 316L SS C(T) samples were manufactured by LPBF using a Renishaw AM250 machine, using the standard parameters detailed in Williams et al. (2020a). A ‘stripes’ scanning strategy over successive layers with a rotation angle of 67° was used. The samples were manufactured in three orientations, as defined in the inset in Figure 1. For the ‘X’ samples, the crack plane normal is in the z direction and the crack grows in the x direction, along the build layers. The ‘Y’ samples crack plane normal is in the y direction and the crack grows in the x direction, and the ‘Z’ samples crack plane normal is in the x direction and the crack grows in the y direction (through the build layers). Note that the principal stress direction in the Y and Z samples are often referred to as the ‘horizontal’ direction, where the loading direction is perpendicular to the build direction. For the X sample, the principal stress direction is often denoted the ‘vertical’ where the build direction is parallel to the loading direction. Two nominally identical build plates were manufactured denoted Plate 1 and Plate 2, hence all samples are samples ID’s start with either a ‘1’ or a ‘2’.