The research group in the UPC participates in the high temperature mechanical characterization of the materials. The researchers in the group have directed research efforts towards the characterization and understanding of the mechanical properties of new ceramic and metallic materials for more than 15 years. Thus, considerable expertise and infrastructure in the area of mechanical properties is available in this research group which is fundamental in its activity in the project. are several forms, use only many as you need
WORKPACKAGE TITLE. Mechanical response STARTING DATE (Month #) 0 DURATION (Months) 36 Effort (MM) ? OBJECTIVES i) Assess strength degradation due to the application of static and cyclic loads in a range of temperatures. ii) Evaluate the influence of surface interactions with a high-temperature environment on the fracture strength. This aspect is studied under cyclic, static and monotonic loads. iii) Develop a new model to compute fracture strength from fundamental crack growth results and microstructural features of the materials. This will provide a quantitative understanding of microstructural parameters which are controlled through processing with the mechanical properties used in the designing of the valves.
DESCRIPTION OF WORK / TASKS Strength characterization under gradually-increasing (monotonic) loads. Strength characterization under static and cyclic loads. Fracture toughness and fracture strength at temperatures in the range of 20 to 900 ºC. Microstructural and fractographical studies of the materials. In situ studies of environmental degradation. Strength degradation by environmental interactions.
DELIVERABLES A detailed knowledge of the influence of microstructural factors and temperature range in the mechanical response of the materials under different loading conditions.
MILESTONES AND CRITERIA Mechanical properties and crack growth studies
INTERRELATION WITH OTHER WORKPACKGES
This study aims to assess the mechanical response of candidate silicon nitride-based ceramics with intermetallic TiAl and Ni3AL matrix composites for use in the automotive industry. Attention will be given to the understanding of the combined role of temperature and environment in the degradation of the mechanical response of the materials. This investigation is central to the overall project as it will provide an evaluation of relevant mechanical properties and their link to processing-related microstructural features. The results of this work will also provide the necessary tools to substantiate the mechanical design of the valves based on endurance (life) considerations. The proposed investigations have a strong experimental component which is key to the validation of the models used in the simulation of the thermo-mechanical response of the valves during the design process.
The most relevant aspects of the present study with respect to the design of the engine valves are:
i) To assess strength degradation due to the application of static and cyclic loads in a range of temperatures. These results along with those of the fracture strength under gradually increasing (monotonic) loads will enable the application of strength statistics in the mechanical design of the valves. While most of the results available in the literature focus on crack growth, little work is available in the above aspects which are fundamental to mechanical design.
ii) To evaluate the influence of surface interactions with a high-temperature environment on the fracture strength. This aspect is studied under cyclic, static and monotonic loads. While this investigation is relevant to structural design with the material, not much attention has been given to it in the open literature.
iii) To develop a new model to compute fracture strength from fundamental crack growth results and microstructural features of the materials. This will provide a quantitative understanding of microstructural parameters which are controlled through processing with the mechanical properties used in the designing of the valves.
iv) The above points are studied in a range of temperatures relevant to the engine valves. This is important as much of the results available in the literature focus on temperatures in the range of 1200-1400 §C which are too high for the engine valves.
The study will be conducted in the following stages:
1. Strength distributions under monotonic loading.
In this section, the objective is to develop a fundamental understanding of the interplay between microstructural factors and the fracture strength of silicon nitride-based ceramics. This knowledge will provide tools for microstructural tailoring of the materials for improving their mechanical response. The experiments undertaken here will also provide the essential mechanical properties of the candidate materials needed for sound mechanical design of the valves. Experiments will be carried out to measure the fracture strength (Weibull) statistics of the candidate materials. A precise knowledge of these statistics is necessary when using probabilistic design schemes which allow the computation of the failure (fracture) probability at a given stress level (i.e., percentage of valves that will be capable of withstanding a given stress level). Experiments will also be conducted to assess the R-curve behavior of the materials. A rising R-curve is indicative of a gradual increase in fracture toughness (resistance) as a preexisting critical crack growths in the material. The occurrence of such phenomenon has been reported in silicon nitride-based ceramics, providing the possibility of tailoring the microstructural features of the material against the attainment of unstable crack growth (catastrophic failure) in a ceramic component. Research efforts will be directed to the identification of crack-nucleating sites, such as porosity, in smooth specimens. The distribution of pore sizes, remnant from the manufacturing process of the material, will be measured. It is expected that the overall results will allow the development of a new methodology for computing the fracture strength statistics of silicon nitride-based ceramics from the distribution of strength- controlling features (such as pores), and the R-curve behavior. By contrast to strength distributions, which are measured using a large number of specimens, these two material-related aspects can be evaluated independently using a small number of specimens. A similar study will be carried out at high temperatures. This investigation will provide the fracture strength statistics of the materials as the temperature increases and thus, the necessary data to conduct mechanical design of the valves at temperatures other than room temperature. The evaluation of the R-curve at the temperature range of choice will allow us to check the soundness of the above-mentioned methodology, as well as to propose possible modifications for its use at high temperatures. ,
2. Environmental effects in fracture strength
This study builds upon the basic knowledge developed in the first section of the project. The results will provide the design properties necessary to ensure the structural reliability of the valves when subjected to service conditions involving the combined action of mechanical loads and temperature. In situ observations of surface degradation processes taking place in the materials under the action of a high-temperature environment will be conducted. This will allow us to establish relevant degradation mechanisms and propose alternatives for improving the processing of the materials. Then, the specimens will be fractured under externally applied (monotonic) loads to measure the fracture strength statistics. The results will be compared to those found in the first stage of the work (i.e., without a high-temperature environment). Special attention will be given to link the expected decrease in fracture strength to the inducement of crack-nucleating sites at the surface of the material due to environmental interaction. This section of the project requires a quantitative evaluation of surface degradation taking place in the material, so that the severity of the environmental interaction is assessed. Upon exposure of the materials for different periods of time to the environment of choice, fracture surface observations will enable to locate crack- nucleating sites. These sites will be compared to those detected in the materials whose surfaces have not interacted with the high-temperature environment. This will allow the extension of the methodology for the prediction of the fracture strength statistics, developed in the first stage of the work, to cases where the materials are subjected to environmental interactions.
3. Fatigue and stress-corrosion cracking
It is well known that many ceramic materials fail to withstand a mechanical load when applied for long periods of time. It has also been established that advanced ceramics, such as silicon nitride-based ceramics, are susceptible to the application of cyclic loads (i.e., loads which fluctuate over time). Catastrophic failure under these conditions is triggered after shorter periods of time than in cases where the externally applied load is held constant. As a result of these findings, it is now accepted that mechanical design using ceramic materials should consider the above delayed-fracture effects both under constant or cyclically applied loads. Testing under such loading conditions provides the necessary data to perform mechanical design of a load-bearing structure for finite or infinite life. While the term "stress-corrosion cracking" indicates the underlying mechanism by which a ceramic fails under constant loads, the term fatigue (also mechanical fatigue or cyclic fatigue) is used to designate the occurrence of failure under cyclic loads where various strength-degradating mechanisms develop. This part of the project will provide the stress (stress amplitude)-time (number of cycles) curves for the prospective silicon nitride-based materials which are necessary to substantiate the reliability of the engine valves under real service conditions (i.e., constant or cyclic loads at different temperatures). Within the temperature range of choice, the role of processing-sensitive microstructural factors in the occurrence of both fatigue and stress corrosion cracking will be assessed by detailed observations of fracture features and crack-initiation sites. Fracture statistics under cyclic and constant loads will be determined from room to high temperatures. For a given stress (or stress amplitude) level, these statistics will indicate the number of specimens failing over a time period of load application. These experiments will allow the use of probabilistic design methodologies when the valves are subjected to real thermo-mechanical service conditions. In a similar manner as indicated under monotonic loads (see section 1 of the project), present results will allow the design of the valves with a pre-established failure probability or endurance criterion.
II. DURABLE EQUIPMENT
The present work requires the use of different mechanical testing systems and microscopes. It is noted that all of the following durable equipment is currently available to conduct the present project. In the case of the present study, none of the durable equipment will be charged to the project.
- Electromechanical testing system for conducting mechanical tests under monotonic and static loads from room to high temperatures.
- Resonant testing system for conducting fatigue-life studies from room to high temperatures.
- Environmental scanning electron microscope for in situ observations of surface degradation from room to elevated temperatures under the action of different environments.
- Scanning electron microscope for conducting fracture surface and microstructural observations.
- Laboratory for metallographic preparation.
III. COSTS OF THE PROJECT
1. Personnel Costs
These costs are calculated considering that a Technician and two Professors will work on the project. The salary of the Technician will be 2.500 euro/month. This personnel will (i) perform sample and specimen preparation, (ii) conduct the mechanical tests for the project, (iii) conduct fracture surface and microstructural observations through electron microscopy, and (iv) use in situ electron microscopy to assess high temperature environmental action.
An Assistant Professor and Full-Professor will devote 6 and 2 hrs/week to the project, respectively. Along with the Technician, they will (i) coordinate the project, (ii) perform data analysis, (iii) provide technical input, and (iv) take the necessary steps to ensure that the overall objectives are met. Labor costs are calculated as 16 and 34 euro/hr for the Assistant and Full-Professor, respectively.
The consumables charged to the project are calculated in the basis of the consumed polishing wheels and diamond paste used in the surface preparation of the ceramic specimens, as well as on expenses in photographic paper. The costs of heating elements for the furnaces are also taken into account. The total costs for consumables are estimated as 1.200 euro/year.
IV. PUBLICATIONS RELEVANT TO THE PROJECT
1) J. Alcal , L. Llanes and M. Anglada, "Fracture Characteristics of Silicon Nitride at Elevated Temperatures," Fourth Euro-Ceramics, Vol 3, 211-218, Grupo Edit. Faenza Editrice, Italia, 1995.
2) J. Alcal and M. Anglada, "Fatigue and Static Crack Propagation in Yttria-Stabilized Tetragonal Zirconia Polycrystals: Crack Growth Micromechanisms and Precracking Effects," Journal of the American Ceramic Society, Vol 80, No. 11, 2759-2772, 1997.
3) J. Alcal and M. Anglada, "Indentation Precracking in Y-TZP: Implications to R- Curves and Strength," Materials Science and Engineering A, 245, 267-276, 1998.
4) J. Alcal and M. Anglada, "High-Temperature Crack Growth in Y-TZP," Materials Science and Engineering A, 232, 103-109, 1997.
5) J. Alcal , L. Iturgoyen and M. Anglada, "Crack Growth by Cyclic Fatigue in Yttria- Tetragonal Zirconia and Silicon Nitride Ceramics," Reliability and Structural Integrity of Advanced Materials, European Conference on Fracture ECF-9, 144-149, EMAS, 1992.
6) J. Alcal , L. Iturgoyen and M.Anglada, "Cyclic Fatigue Crack Growth of Indentation Cracks in Silicon Nitride and Y-TZP," Fatigue 93, 1269-1274, EMAS, Canada, 1993.
7) L. Llanes, J. Alcal , J. Gonzalez and M. Anglada, "Cyclic Fatigue and Fracture Mechanisms of Silicon Nitride," Elaboration, Thermomechanical and Physicochemical Properties of Ceramics: Fourth Interregional Colloquium on Ceramics, 53-58, Spain, 1993.
8) D. Casellas, C.Baudin, M. Osendi, L. Llanes and M. Anglada, "Fracture Resistance of Mullite Under Static and Cyclic Loads," Scripta Materialia, 38, 39-44, 1998.
9) F.L. Cumbrera, F. Sanchez Bajo, R. Fernandez and L. Llanes, "Microstructural Effects in the X-Ray Powder Diffraction Profile of 9 mol% Mg-PSZ," Journal of the European Ceramic Society, 18, 2247-2252, 1998.
10) J.M. Sabadell and M. Anglada, "Influence of the Load Ratio on the Subcritical Crack Propagation in Alumina Under Cyclic Compressive Loads," Journal of Materials Science Letters, 9, 964-966 (1990).