Thermo-Mechanical Fatigue of Compacted Graphite Iron in Diesel Engine Components

Cast iron components in combustion engines, such as cylinder blocks and heads of trucks, are exposed for long periods of time to elevated temperatures. Moreover, the engines are started and stopped frequently during their operational life, constituting a large number of heating and cooling cycles. In geometrical complex components the sudden heating (starting the engine) and cooling (stopping the engine) lead to thermal gradients and thermal mismatch within the material, resulting in the local development of high stress levels. The many start-stop operations and their associated alternating stress levels can lead to a localized cracking phenomenon known as Thermo-Mechanical Fatigue (TMF). Compacted Graphite Iron (CGI) is a common material of choice for diesel engine cylinder heads of heavy trucks and is a type of graphitic cast iron, consisting of vermicular graphite particles embedded in a metal matrix of pearlite. This material provides a suitable combination of thermal and mechanical properties, satisfying the functional requirements of these engine components. The main aim of this research is to identify and understand the damage micro-mechanisms that control thermo-mechanical fatigue phenomena in cast iron (CGI). The acquired knowledge is of relevance for predicting the lifetime, improving the properties and increasing the reliability of diesel truck cylinder heads. The work of this study can roughly be categorized into three main subjects: (i) Microstructural evolutions of CGI at elevated temperatures, (ii) TMF crack growth characterization and (iii) precise microstructural analysis of the TMF-crack path. Microstructural Evolutions of CGI at Elevated Temperatures In a first series of experiments, time and temperature induced microstructural changes in CGI were characterized, in view of their possible role in the TMF behavior of CGI. During open air annealing of CGI at 420 °C microstructural changes take place in the material, which gave rise to volume expansion and weight increase. The weight increase can be explained by considering the formation of an oxide scale whereas the volume expansion can be attributed to the decomposition of pearlite into ferrite and graphite. It was observed that the atmosphere is of crucial importance in this process. Annealing in an open-air atmosphere produced ten times less volume expansion as compared to annealing in vacuum conditions. Internal oxidation was observed during annealing under atmospheric conditions and the presence of an internal oxidation layer largely inhibited the progress of pearlite decomposition. The observed oxide layers at the internal metal/vapour interface of cavities (left behind by denuded graphite) cause the obstruction of carbon diffusion and thus the suppression of the pearlite decomposition process. In addition it was found that the depth of the oxidized zone near the surface (the oxide penetration depth) was of the same order of magnitude as the eutectic cell size, i.e. the volume in which graphite particles are interconnected. This suggests that the interconnectivity of the graphite has a dominant influence on the kinetics of the oxidation process. The microstructural dependence of tensile and fatigue properties of CGI at room temperature were evaluated by an extended annealing treatment of 720 h at 420 °C. This extended annealing treatment leads to better tensile (increase in yield strength, ductility) and dynamic properties (fatigue lifetime) at room temperature. The variations of mechanical properties were observed both after annealing under atmospheric and vacuum conditions, but were more pronounced after vacuum annealing. This can be explained by the decomposition of the pearlite phase during annealing and the formation of new ferrite at the graphite/metal interface. It is assumed that the ferrite/graphite interface exhibits a stronger bond than the pearlite/graphite interface. As this stronger bond will be better resistant to delamination, it will strengthen the material both in static and dynamic loading. Such effects were far less pronounced in the open-air annealed material, which could be associated with the fact that it was shown that internal oxidation strongly reduced the kinetics of decomposition. TMF Crack Growth Characterization An important part of this study was to measure and analyze the TMF lifetime of CGI. For smooth and notched specimens, the TMF lifetime was measured in TMF tests under total-constraint conditions, with temperatures cycling between 50 °C and 420 °C. By considering the notch depth as the initial crack length, TMF lifetimes were reproduced numerically using the Paris equation for fatigue crack growth (da/dN = C (?K)m). The calculated lifetimes were found to be in good agreement with all experimental results, covering a wide range of TMF lifetimes from 30 to 1400 cycles. Also for smooth specimens the Paris model worked well by considering the typical graphite particle size as notch depth. It is one of the main conclusions of this work that graphite particles act as internal notches from which a TMF crack almost immediately starts to grow during the first TMF cycles. Hence, it was established that TMF lifetime in CGI is governed by crack growth and not by crack initiation. The relevance of the Paris growth law was further confirmed by meticulously measuring the actual crack growth rates for three typical values of the stress intensity factor. The resulting crack growth rates proved to be in reasonable agreement with the predicted values according to the Paris model. It was further shown that the cyclic plasticity of the bulk material, accumulated during TMF cycles, does not have a noticeable effect on TMF lifetime (i.e. crack growth rates are not affected). The notched dog-bone specimen geometry is proposed in this work as a valid alternative for monitoring the TMF behavior of CGI. By applying standard TMF tests with notched specimens, it was possible to significantly reduce both testing time and experimental data scatter, whilst preserving a realistic estimation of the lifetime of the smooth sample. The effect of prolonged holding times (HT) on TMF lifetime was studied by using notched specimens and a clear effect was observed. Extended holding times were accompanied by an increased relaxation of compressive stresses, causing higher tensile stresses to develop in the subsequent low temperature stages of the TMF cycles. So, extended HTs had an adverse effect on the sample lifetime with a saturating effect for HTs above 1800 s. The Paris fatigue-crack-growth model was used also to estimate the impact of extended HTs. According to the Paris growth law, using a fixed value of tensile stress at low temperature, it was estimated that an increase of holding time from 30 s to 18000 s (5 h) produced a drop of 45% in lifetime. In reality a 60% drop in lifetime was measured, though, which implies that a combined effect of (tensile) stress and microstructural evolution during TMF is responsible for the reduction of lifetime. Precise Microstructural Analysis of the TMF Crack Path To the purpose of precise characterization of the complex TMF-crack-path morphology in CGI in relation to local microstructural features and to find out how and by which mechanisms the cracks predominantly develop, 2D and 3D orientation contrast imaging was carried out on wide field sample volumes, covering several mm3 of imaged material. The data analysis revealed that the crystal planes that are parallel to the (local) crack plane are essentially of a random orientation. Conversely, it was found that graphite particles do not only play a crucial role in the crack initiation, but also are of primary significance for crack propagation. Quantitative analysis of the EBSD data in 2D and 3D showed that the distribution of graphite particles is very important for the crack propagation, as it was revealed that graphite particles enhance crack growth. It was statistically proven that the density of graphite particles in the crack plane is more than double of the density in an arbitrary plane. Our materials knowledge, based on the interpretation of test results in terms of quantifiable microstructural data functions, is of crucial importance to develop a microstructurally based TMF model.Materials Science & EngineeringMechanical, Maritime and Materials Engineerin

Modelling and mechanical design of a flexible tube-guided SMA actuator

Shape memory alloy (SMA) wires are excellent candidates for wearable actuators since they are thin, low weight and have a high actuation force. The main drawbacks are that the wire should be kept straight and needs to be relatively long to enable a large enough actuation stroke. Embedding the SMA wire in a flexible tube largely enhances its applicability since then the counter forces are transferred by the tube material and the tube can be rolled up or attached to flexible surfaces or clothing layers. The performance of such tube-guided SMA actuators is, however, more complicated since it not only depends on the SMA behaviour but also on the tube materials and the actuator construction. In this research, a simple end-state model for a tube-guided SMA actuator system is proposed. We measure and model both the SMA and tube material properties, including tube creep effects, and derive an approximate prediction for the actuator stroke. Validation experiments showed that the predicted stroke during the second heating and cooling experiments agreed well with the measurements and that the average deviation is 9.6%, even though the deviation is much larger (27.3%) for the maximum applied force.Emerging Material

Development of a Knitted Strain Sensor for Health Monitoring Applications †

As an emerging technology, smart textiles have attracted attention for rehabilitation purposes to monitor heart rate, blood pressure, breathing rate, body posture and limb movements. Compared with traditional sensors, knitted sensors constructed from conductive yarns are breathable, stretchable and washable, and therefore, provide more comfort to the body and can be used in everyday life. In this study, knitted strain sensors were produced that are linear with up to 40% strain, sensitivity of 1.19 and hysteresis of 1.2% in absolute values, and hysteresis of 0.03 when scaled to the working range of 40%. The developed sensor was integrated into a wearable wrist-glove system for finger and wrist monitoring. The results show that the wearable was able to detect different finger angles and positions of the wrist.Emerging Material

Measuring plasticity with orientation contrast microscopy in aluminium 6061-T4

Orientation contrast microscopy (i.e., electron backscattered diffraction, EBSD) was employed to monitor the plastic strain in loaded tensile samples of aluminium alloy Al6061 in T4 condition. The kernel average misorientation (KAM) is known to be an appropriate parameter in orientation contrast microscopy which has the potential to characterise the plastic strain by monitoring the local misorientations. This technique was applied here to gauge the extent of the plastic zone around a fatigue crack. To establish the magnitude of strain (which can be identified by the KAM parameter), a series of tensile samples were strained in the range of 1% to 25%. KAM maps were compared, and the average misorientations were related to the tensile strain values. The KAM distribution functions for all the strained samples were derived from a large scanned area. In addition, Vickers microhardness tests were conducted for these series of samples. This allowed the comparison of the mesoscopic plastic strain measured by Vickers microhardness with the micro plastic strain locally obtained by KAM. Noise was observed in the average KAM values up to a plastic strain of 1.5%. For the plastic strain exceeding 1.5%, noise no longer dominates the KAM map, and a positive—though not linear—correspondence between plastic strain and KAM was observed. The observed plastic zone at the tip of the fatigue crack by micro-Vickers hardness measurements was about an order of magnitude higher than the plastic zone observed on the KAM maps. In view of the calibration of KAM values on the tensile samples, it could be concluded that in the larger area of the plastic zone, the strain did not exceed the critical value of 1.5%(OLD) MSE-1(OLD) MSE-

Quantitative correlation between slip patterning and microstructure during tensile elongation in 6xxx series aluminum alloy

To the purpose of evaluating the effect of deformation on the microstructure, aluminum structures were analyzed on tensile strained samples extended to 25% elongation. In the substructure of these deformed samples linear slip patterns were observed, generally confined to the bulk of the grain. In order to study the crystallographic aspect of these slip patterns, two methods were applied based on orientation contrast microscopy (EBSD). The first method is the statistical analysis of stereological nature, which allows us to determine the incidence of certain crystallographic planes with the slip patterns. In other to corroborate the statistical method, also a 3D analysis was carried out on two perpendicular planes of observation (TD and ND sections). The results of both methods were in a very good agreement. It was found that the linear features are predominantly parallel to the {111} crystal planes, although the frequency of {111} planes was not exclusive; also other crystal planes such as {112} and {110} are involved. These observations give a stronger statistical basis for similar observations earlier made by TEM on much smaller fields of observation.Materials Science and EngineeringMechanical, Maritime and Materials Engineerin

Review of magnetic shape memory polymers and magnetic soft materials

Magnetic soft materials (MSMs) and magnetic shape memory polymers (MSMPs) have been some of the most intensely investigated newly developed material types in the last decade, thanks to the great and versatile potential of their innovative characteristic behaviors such as remote and nearly heatless shape transformation in the case of MSMs. With regard to a number of properties such as shape recovery ratio, manufacturability, cost or programming potential, MSMs and MSMPs may exceed conventional shape memory materials such as shape memory alloys or shape memory polymers. Nevertheless, MSMs and MSMPs have not yet fully touched their scientific-industrial potential, basically due to the lack of detailed knowledge on various aspects of their constitutive response. Therefore, MSMs and MSMPs have been developed slowly but their importance will undoubtedly increase in the near future. This review emphasizes the development of MSMs and MSMPs with a specific focus on the role of the magnetic particles which affect the shape memory recovery and programming behavior of these materials. In addition, the synthesis and application of these materials are addressed.Industrial Design EngineeringEmerging Material

Understanding the effects of root structure on the mechanical behaviour of engineered plant root materials

Plant root growth can be altered by introducing obstacles in the path of growth. This principle is used in design to produce planar grid structures composed of interweaving roots. The Engineered Plant Root Materials (EPRMs) grown with this method have the potential to serve as environmentally sensitive alternatives for conventional materials, but their applications are delimited by their material properties. To bridge the gap in the wider application of these materials, the role of plant root structure and an agar-agar matrix are explored in relation to the mechanical properties of the EPRMs. Tensile tests were performed on five root configurations, ranging from single roots to grids of varying sizes. Heterogeneities in each configuration suggest poor load distribution throughout the structure. Agar-agar was introduced as a biopolymer matrix to improve load distribution and tensile properties. Digital microscopy at the intersection of grid cells suggests a correlation between cell size, root tip density, and material strength. The largest cell size (2 cm) had the highest root tip density and yield strength (0.568 ± 0.181 roots/mm2 and 0.234 ± 0.018 MPa, respectively), whereas the structure with the least root tips (1 cm) was 31 % weaker.Design for SustainabilityEmerging Material

Thermo-mechanical fatigue lifetime assessment of spheroidal cast I\iron at different thermal constraint levels

In previous work on the thermo-mechanical fatigue (TMF) of compacted graphite iron (CGI), lifetimes measured under total constraint were confirmed analytically by numerical integration of Paris’ crack-growth law. In current work, the results for CGI are further validated for spheroidal cast iron (SGI), while TMF tests at different constraint levels were additionally performed. The Paris crack-growth law is found to require a different CParis parameter value per distinct constraint level, indicating that Paris’ law does not capture all physical backgrounds of TMF crack growth, such as the effect of constraint level. An adapted version of Paris’ law is developed, designated as the local strain model. The new model considers cyclic plastic strains at the crack tip to control crack growth and is found to predict TMF lifetimes of SGI very well for all constraint levels with a single set of parameters. This includes not only full constraint but also over and partial constraint conditions, as encountered in diesel engine service conditions. The local strain model considers the crack tip to experience a distinct sharpening and blunting stage during each TMF cycle, with separate contributions to crack-tip plasticity, originating from cyclic bulk stresses in the sharpening stage and cyclic plastic bulk strains in the blunting stage.Emerging Materials(OLD) MSE-3(OLD) MSE-

Shape memory alloy actuators for haptic wearables: A review

Devices delivering sophisticated and natural haptic feedback often encompass numerous mechanical elements, leading to increased sizes and wearability challenges. Shape memory alloys (SMAs) are lightweight, compact, and have high power-to-weight ratios, and thus can easily be embedded without affecting the overall device shapes. Here, a review of SMA-based haptic wearables is provided. The article starts with an introduction of SMAs, while incorporating analyses of relevant devices documented in the literature. Haptic and SMA materials fields are correlated, with haptic perception insights aiding SMA actuator design, and distinct SMA mechanisms offering diverse haptic feedback types. A design process for SMA haptic wearables is proposed based on material-centered approach. We show SMAs hold potential for haptic devices aiding visually impaired people and promise in immersive technology and remote interpersonal haptic communication.Emerging MaterialsHuman Information Communication Desig

Development of Low Hysteresis, Linear Weft-Knitted Strain Sensors for Smart Textile Applications

In recent years, knitted strain sensors have been developed that aim to achieve reliable sensing and high wearability, but they are associated with difficulties due to high hysteresis and low gauge factor (GF) values. This study investigated the electromechanical performance of the weft-knitted strain sensors with a systematic approach to achieve reliable knitted sensors. For two elastic yarn types, six conductive yarns with different resistivities, the knitting density as well as the number of conductive courses were considered as variables in the study. We focused on the 1 × 1 rib structure and in the sensing areas co-knit the conductive and elastic yarns and observed that positioning the conductive yarns at the inside was crucial for obtaining sensors with low hysteresis values. We show that using this technique and varying the knitting density, linear sensors with a working range up to 40% with low hysteresis can be obtained. In addition, using this technique and varying the knitting density, linear sensors with a working range up to 40% strain, hysteresis values as low as 0.03, and GFs varying between 0 and 1.19 can be achieved.Emerging MaterialsTechnical Suppor