We use integrated digital image correlation (IDIC) with gradient-free optimization to identify the in-plane, linear-elastic material parameters of carbon fiber-reinforced polymer (CFRP) plates. With IDIC, a monolithic least-squares problem is formulated, based on the video capture of a tension/compression test, with the material parameters as unknowns. Transverse deformations are induced by using holeplate specimens, so that all parameters are identified with one tensile test. We also demonstrate how an invariant-based CFRP model, parameterized by the Tsai modulus, further enhances the robustness of IDIC characterization.

In warm pre-stressing (WPS), the fracture resistance of cracked steel components is raised when subjected to certain temperature-load histories. WPS’s beneficial effects enhance safety margins and potentially prolong fatigue life. However, understanding and predicting the WPS effects is crucial for employing such benefits. This study utilised pre-cracked compact tension specimens made from steam turbine steel for WPS and baseline fracture toughness testing. Two typical WPS cycles were investigated (L-C-F and L-U-C-F), and an increase in fracture resistance was observed for both cycles. The WPS tests were simulated using finite element analysis to understand its effects and predict the increase in fracture resistance. A local approach was followed based on accumulative plastic strain magnitude ahead of the crack tip. Since cleavage fracture is triggered by active plasticity, the WPS fracture is assumed when accumulated plasticity exceeds the residual plastic zone formed at the crack tip due to the initial pre-load.

In laser powder bed fusion (L-PBF), most powders are not melted in the chamber and collected after the printing process. Powder reuse is appreciable without sacrificing the mechanical properties of target components. To understand the influences of powder reuse on mechanical performance, a nickel-based superalloy, IN738LC, was investigated. Powder morphology, microstructure and chemical compositions of virgin and reused powders were characterized. An increase in oxygen content, generally metallic oxides, was located on the surface of powders. Monotonic tensile and cyclic fatigue were tested. Negligible deterioration in strength and tensile ductility were found, while scattered fatigue performance with regard to fatigue life was shown. Deformation and fatigue crack propagation mechanisms were discussed for describing the powder degradation effects.

The effect of using thin plies to increase the bearing strength of composite laminates was investigated. Five different composite laminates were manufactured using a single material system with varying proportions of thin plies (100% thick-ply, 50% thin-ply and 100% thin-ply). Bearing tests were performed and the results from the tests are investigated. The results show that performance in terms of bearing strength at onset of damage, and ultimate bearing stress increase proportionally with the increasing amount of thin plies within the stack. Microscopic examination of the failure modes for all laminates was performed at the center of the hole to determine the dominant failure mode. Transition zone was investigated where both thin and thick plies were designed so that the thin plies are used only when more strength is required. © 2022 Loukil et al.

The deformation mechanisms of a single crystal nickel-base superalloy with and without Ru-doped have been investigated under out-of-phase thermomechanical fatigue. The Ru-doped alloy exhibits a thermomechanical fatigue life more than twice as high compared to the Ru-free alloy and a difference in thermomechanical fatigue behavior is also displayed. Microstructure studies by scanning electron microscopy and transmission electron microscopy revealed that the deformation mechanism of the Ru-free alloy in the initial stage is the movement of dislocations in the γ matrix. In the later stage of the thermomechanical fatigue test, large amounts of twins are formed in the material, and a large number of stacking faults and dislocations are sheared into the γ' precipitates. By comparing with the Ru-free alloy, the Ru-doped alloy has a higher matrix strength due to the solid solution strengthening effect of Ru, and is also prone to different deformation mechanisms. For example, the stacking faults are formed in the initial thermomechanical fatigue cycles and remain in the matrix throughout the entire thermomechanical fatigue process. The formation of twins, on the other hand, is suppressed by Ru addition. Such effects are believed to extend the thermomechanical fatigue life effectively.

In fatigue life prediction of single crystal gas turbine blades, the risk of rapid crystallographic crack growth along the close-packed planes poses a large uncertainty. A criterion is proposed to predict the transition from mode I to crystallographic crack growth, which is necessary for reliable prediction of the number of cycles from crack initiation to the onset of crystallographic crack growth. The proposed criterion is calibrated against tests performed under a wide range of conditions representative for a gas turbine blade, including isothermal fatigue crack growth tests and thermomechanical fatigue crack growth tests, some including hold times and pre-test aging.

In this study, the difference between in-phase (IP) and out-of-phase (OP) thermomechanical fatigue (TMF) cycling at 100–850 °C of a single-crystal superalloy is investigated both from a mechanical response and resulting microstructure perspective. Results indicate that there is no significant difference in fatigue lives between IP and OP TMF when similar strain ranges and crystal orientations are considered. The deformation mechanisms occurring during IP and OP TMF are similar where the main deformation mechanism for this alloy is localized deformation bands and crack initiation is preferred to these bands. Other TMF mechanisms, such as recrystallization and oxidation, are also discussed.

Light weight metal parts produced with additive manufacturing have gained increasing interest from the aerospace industry in recent years. However, light weight parts often require thin walls which can have different material properties compared to thick bulk material. In this work, the fatigue properties of Ti-6Al-4 V produced by electron beam powder bed fusion have been investigated for samples with three different wall thicknesses ranging from 1.3 to 2.7 mm and in three different directions; 0°, 45°, and 90° relative to the build plate. Generally, the 90° specimens show worse fatigue life compared to both 0° and 45°. It was found that the fatigue strength is lower for thin samples compared to thicker samples when the stress is calculated from nominal thickness or calliper measurements. However, since materials produced by electron beam powder bed fusion often have a rough as-built surface, the load bearing area is not easy to determine. In this paper, four different methods for determining the load bearing area are presented. It is shown that if the surface roughness is considered when calculating the stress levels, the influence from specimen thickness decreases or even disappears. 

Oxidized MC carbides which act as main crack initiation sites in a polycrystalline superalloy under thermal-mechanical fatigue (TMF) conditions at 850 degrees C were studied. Microstructural observations in the TMF tested specimens were compared to findings from bulk samples exposed isothermally in air at 850 degrees C for 30 hours in the absence of any external applied load. Carbides were found to oxidize rapidly after exposure at 850 degrees C for 30 hours resulting in surface eruptions corresponding to oxidation products, from where micro-cracks initiated. Plastic deformation due to volume expansion of the often porous oxidized carbides led to high dislocation densities in the adjacent matrix as revealed by controlled electron channeling contrast imaging. The high dislocation density facilitated the dissolution kinetics of gamma precipitates by segregation and diffusion of chromium and cobalt along the dislocations via pipe diffusion, resulting in the formation of soft recrystallized grains. Atom probe tomography revealed substantial compositional differences between the recrystallized grains and the adjacent undeformed gamma matrix. Similar observations were made for the TMF tested alloy. Our observations provide new insights into the true detrimental role of oxidized MC carbides on the crack initiation performance of polycrystalline superalloys under TMF.

In this study, the influuence from crystal orientation on the thermomehanical fatigue (TMF) behaviour of the recently developed single-rystal superalloy STAL-15 is considered, both from an experimental and a nite element (FE) perspective. Experimental results show that there is a strong inuence from the elastic stiffness, with respect to the loading direction, on the TMF life. However, the results also indicate that the number of active slip planes duringdeformation inuence the TMF life, where specimens with a higher number of active slip planes are favoured compared to specimens with fewer active slip planes. The higher number of active slip planes results in a more widespread deformation compared to a more conentrated deformation when only one slip plane is active. Deformation bands with smeared and elongated  γ-precipitates together with deformation twinning were found to be major deformation mechanisms, where the twins primarily were observed in specimens with several active slip planes. From an FE-perspective, therystal orientation with respect to the loading direction is quantied and adopted into a framework which makes it possible to describe the internal crystallographic arrangement and its entities in a material model. Further, a material model which incorporates the crystalorientation is able to predict the number of slip planes observed from microstructural observations, as well as the elasticstiness of the material with respect to the loading direction.

The mechanical behaviour of a new single crystal superalloy suitable for power generation applications is considered. Effects of alloying with either Si or Re are elucidated. Out-of-phase thermal-mechanical fatigue is emphasised, although to clarify the effects arising some static creep deformation tests are also carried out. A significant Si-effect is found: a modest addition of 0.25 wt. % Si increases the TMF life by a factor of 2. Thinner deformation bands which traverse the γ'-phase are promoted by Si alloying, with a concomitant greater resistance to recrystallization and cracking along them. Alloying with Re, whilst improving the creep behaviour more markedly than Si, does not have such a strong effect on TMF life. The results provide insights into the composition/performance relationships relevant to the TMF performance of single crystal superalloys.

In this study, the TMF stress relaxation and creep behavior at 1023 K and 1223 K (750 °C and 950 °C) have been investigated for a Ni-based single-crystal superalloy. Specimens with three different crystal orientations along their axes were tested; 〈001〉, 〈011〉, and 〈111〉, respectively. A highly anisotropic behavior during TMF stress relaxation was found where the 〈111〉 direction significantly shows the worst properties of all directions. The TMF stress relaxation tests were performed in both tension and compression and the results indicate a clear tension/compression asymmetry for all directions where the greatest asymmetry was observed for the 〈001〉 direction at 1023 K (750 °C); here the creep rate was ten times higher in compression than tension. This study also shows that TMF cycling seems to influence the creep rate during stress relaxation temporarily, but after some time it decreases again and adapts to the pre-unloading creep rate. Creep rates from the TMF stress relaxation tests are also compared to conventional constant load creep rates and a good agreement is found.

In this study, the difference between in-phase (IP) and out-of-phase (OP) thermomechanical fatigue (TMF) cycling from 100 to 750 °C has been investigated for the Ni-based single-crystal superalloy MD2. In addition, two different crystal orientations were studied, the ⟨001⟩ and ⟨011⟩ orientations respectively. When comparing IP and OP TMF lives, a strain range dependency is found for the ⟨001⟩ direction. For high strain ranges, IP cycling leads to a higher number of cycles to failure compared to OP. However at lower strain ranges, OP cycling leads to a higher number of cycles to failure compared to IP. Microstructure investigation shows that for the ⟨001⟩ direction, deformation twinning within the γ/γ′-microstructure is much more pronounced during OP conditions compared to IP. However for the ⟨011⟩ direction, the opposite is observed; twinning is more pronounced during IP TMF. From the microstructure investigation it is also visible that intersections between twins seems to trigger formation of TCP phases and recrystallization. These intersections also work as initiation points for TMF damage.

In this study, low-cycle fatigue (LCF) tests at 500 degrees C in the < 001 >, < 011 > and < 111 > directions have been performed for the Ni-based single-crystal superalloy MD2. All tests were carried out in strain control with R-is an element of = -1. The < 001 > direction has the lowest stiffness of the three directions and also shows the best fatigue properties in this study followed by the < 011 > and < 111 > directions, respectively. It is well recognised that Ni-based single-crystal superalloys show a tension/compression asymmetry in yield strength and this study shows that a tension/compression asymmetry is also present during LCF conditions. At mid-life, the < 001 > direction generally has a higher stress in tension than in compression, while the opposite is true for the < 011 > direction. For the < 111 > direction the asymmetry is found to be strain range dependent. The < 011 > and < 111 > directions show a cyclic hardening behaviour when comparing cyclic stress-strain curves with monotonic stress-strain curves. In addition, the < 011 > and < 111 > directions show a serrated yielding behaviour for a number of cycles while the yielding of the < 001 > direction is more stable.

The thermomechanical fatigue (TMF) stress relaxation of the single-crystal nickel-base superalloy MD2 has been analysed and modelled in this paper. In-phase and out-of-phase TMF experiments in the nominal [001],[011] and [111] crystal orientations have been performed. The TMF cycle consists of two loadings each with a 100 h long hold-time. A simple crystallographic creep model, based on Norton’s creep law, has been developed and used in conjunction with a crystal plasticity model. The model takes anisotropy and tension/compression asymmetry into account, where the anisotropic behaviour is based on the crystallographic stress state. The values of the creep parameters in the anisotropic expression were determined by inverse modelling of the conducted TMF experiments, a parameter optimisation were performed. The developed model predicts the stress relaxation seen in the TMF experiments with good correlation.

In this paper the TMF crack initiation behaviour of the single-crystal nickel-base superalloyMD2 is investigated and modelled. TMF tests were performed in both IP and OP for varying mechanicalstrain ranges in the [001] crystallographic direction until TMF crack initiation was obtained. Acrystal plasticity-creep model was used in conjunction with a critical-plane approach, to evaluate thenumber of cycles to TMF crack initiation. The critical-plane model was evaluated and calibrated ata stable TMF cycle, where the effect of the stress relaxation had attenuated. This calibrated criticalplanemodel is able to describe the TMF crack initiation, taking tension/compression asymmetry aswell as stress relaxation anisotropy into account, with good correlation to the real fatigue behaviour.

Thanks to their excellent mechanical and chemical properties at temperatures up to 1000 °C, nickel-based superalloys are used in critical components in high-temperature applications such as gas turbines and aero engines. One of the most critical components in a gas turbine is the turbine blade, and to improve the creep and fatigue properties of this component, it is sometimes cast in single-crystal form rather than in the more conventional poly-crystalline form. Gas turbines are most commonly used for power generation and the turbine efficiency is highly dependent on the performance of the superalloys.

Today, many gas turbines are used as a complement for renewable energy sources, for example when the wind is not blowing or when the sun is not shining. This means that the turbine runs differently compared to earlier, when it ran for longer time periods with a lower number of start-ups and shut-downs. This new way of running the turbine, with an increased number of start-ups and shut-downs, results in new conditions for critical components, and one way to simulate these conditions is to perform thermomechanical fatigue (TMF) testing in the laboratory. During TMF, both mechanical strain and temperature are cycled at the same time, and one fatigue cycle corresponds to the conditions experienced by the turbine blade during one start-up and shutdown of the turbine engine.

In the work leading to this PhD thesis, TMF testing of single-crystal superalloys was first performed in the laboratory and this was then followed microstructure investigations to study the occurring deformation and damage mechanisms. Specimens with different crystallographic directions have been tested in order to investigate the anisotropic behaviour shown by these materials. Results show a significant orientation dependence during TMF, in which specimens with a low elastic stiffness perform better. However, it is also shown that specimens with a higher number of active slip planes perform better during TMF compared to specimens with less active slip systems. This is because a higher number of active slip systems results in a more widespread deformation and seems to be beneficial for the TMF life. Further, microscopy shows that the deformation during TMF is localised to several deformation bands and that different deformation and damage mechanisms prevail according to in which crystal orientation the material is loaded. Deformation twinning is shown to be a major deformation mechanism during TMF, and the interception of twins seems to trigger recrystallization. This work also studies the effects of alloying a single-crystal superalloy with Si or Re, and results show a significant Si-effect where the TMF life increases by a factor of 2 when Si is added to the alloy.

Finally, this research results in an increased knowledge of the mechanical response as well as a deeper understanding of the deformation and damage mechanisms that occur in single-crystal superalloys during TMF. It is believed that in the long-term, this can contribute to a more efficient and reliable power generation by gas turbines.

Since Ni-based single-crystal superalloys are anisotropic materials, their behavior in different crystal orientations is of great interest. In this study, the yielding behavior in both tension and compression for 〈001〉, 〈011〉 and 〈111〉 oriented materials at 500 °C has been investigated. The 〈011〉 direction showed a serrated yielding behavior, a great tension/compression asymmetry in yield strength and visible deformation bands. However, the 〈001〉 and 〈111〉 directions showed a more homogeneous yielding, less tension/compression asymmetry in yield strength and no deformation bands. Microstructure investigations showed that the serrated yielding behavior of the 〈011〉 direction can be attributed to the appearance of dynamic strain aging (DSA) and that only one slip system is active in this direction during plastic deformation.

Superalloys are a group of materials that are used in high temperature applications, for example gas turbines and aero engines. Gas turbines are most commonly used for power generation, and it is only the very critical components which are exposed to the most severe conditions within the turbine, which are made from superalloy material.

Today, energy consumption in many parts of the world is very high and is tending to increase. This implies that all power generating sources, including gas turbines, must aim for higher efficiency. For the gas turbine industry, it is a continuous challenge to develop more energy-efficient turbines. One way to do this is to increase the temperature within the hot stage of the turbine. However, increased temperature in the hot stage also challenges the materials that are used there. Today’s materials are already pushed to the limit, i.e. they cannot be exposed to the temperatures which are required to further increase the turbine efficiency. To solve this problem, research which later can lead to better superalloys that can withstand even higher temperatures, has to be conducted within the area of superalloys.

The aim of this licentiate thesis is to increase our knowledge about  deformation and damage mechanisms that occur in the microstructure in superalloys when they are subjected to high temperatures and loads. This knowledge can later be used when developing new superalloys. In addition, increased knowledge of what is happening within the material when it is exposed to those severe conditions, will facilitate the development of material models. Material models are used for FEM simulations, when trying to predict life times in gas turbine components during the design process.

This licentiate thesis is based on results from thermomechanical fatigue (TMF) testing of Ni-based single-crystal superalloys. Results show that the deformation within the microstructure during TMF is localized to several deformation bands. In addition, the deformation mechanisms are mainly twinning and shearing of the microstructure. Results also indicate that TMF cycling seems to influence the creep rate of single-crystal superalloys.

The purpose of this paper is to investigate the differences in mechanical response and microstructural behavior when the single-crystal Ni-based superalloy CMSX-4 is subjected to thermomechanical fatigue (TMF) in two different crystallographic directions, <001> and <011>. An out-of-phase (OP) straincontrolled TMF cycle with R=-∞ in the temperature range 100 to 850 °C was used. As expected, the material exhibited, when loaded in the <001> direction, a higher number of cycles to failure compared to the <011> direction, when equivalent strain ranges were compared. High strain ranges led to crystallographic fractures along one of the {111} planes while low strain ranges led to non-crystallographic fractures. This result was valid for both <001> and <011> directions. Specimens with random fractures also showed recrystallization close to the fracture surface. Twinning was found to be a major deformation mechanism for most specimens. A change in deformation mechanism from twinning to shearing was found in specimens subjected to loading in the <011> direction when going from low to high strain ranges. This investigation also indicated that crack propagation is a consequence of recrystallization and not the other way around.

The thermomechanical fatigue (TMF) behaviour of a new high Cr-containing single crystal superalloy, known as STAL-15, is investigated. This is a candidate alloy for future industrial gas turbine (IGT) applications. TMF involves complex interactions between high and low temperature deformation mechanism, and this study highlights important factors controlling their interrelationship. Emphasis is placed particularly on the microslructural aspects which control deformation. It is demonstrated that the TMF performance of the new alloy is comparable to that of a second generation alloy such as CMSX-4, despite the absence of Re alloying. An addition of 0.25wt% Si significantly improves the resistance to TMF further; this is attributed to a greater resistance to recrystallization and cracking along the deformation bands that develops across the cross section of the specimen during testing. The TMF resistance increases with increasing primary γ′ size, but the degree of solutioning of the γ′ phase caused by the solution heat treatment seems to have no significant effect.

The fatigue crack initiation in a notched single-crystal nickel-base superalloy component at 500 °C was investigated and analysed. A critical plane approach in combination with a critical distance method has been adopted, in which the total shear strain ranges on the discrete slip planes are evaluated. Furthermore, a Coffin-Manson type of expression is used to predict the number of cycles to fatigue crack initiation. The experimental test specimens were studied by microscopy to determine on which crystallographic plane the fatigue initiation occurred. A good correlation between the experimental results and the simulated results were found.