Carlos Engler-Pinto

Statistical Approaches Applied to Fatigue Test Data Analysis

The use of statistics is an important aspect in the analysis of fatigue test results. This paper describes the use of the Maximum Likelihood Estimate to analyze the fatigue results of staircase tests and Life-Regression Models (S-N curve). Two life-regression models are analyzed: a modified Basquin model and the Random Fatigue Limit (RFL) model.

Hold-Time Effect on Thermo-Mechanical Fatigue Life and its Implications in Durability Analysis of Components and Systems

Thermo-mechanical fatigue (TMF) resistance of engineering materials is extremely important for the durability and reliability of components and systems subjected to combined thermal and mechanical loadings. However, TMF testing, modeling, simulation, validation, and the subsequent implementation of the findings into product design are challenging tasks because of the difficulties not only in testing but also in results interpretation and in the identification of the deformation and failure mechanisms. Under combined high-temperature and severe mechanical loading conditions, creep and oxidation mechanisms are activated and time-dependent failure mechanisms are superimposed to cycle-dependent fatigue, making the life assessment very complex. In this paper, the testing procedures and results for high-temperature fatigue testing using flat specimens and thermal-fatigue testing using V-shape specimens are reported; emphasis is given to hold-time effects and the possible underlying mechanisms. The uncertainty nature and the probabilistic characteristics of the V-shape specimen test data are also presented. Finally, the impact of hold-time effect on current product design and validation procedure is discussed in terms of virtual life assessment.

Thermo-mechanical fatigue investigation of single crystal nickel base superalloy SRR99

This paper deals with the comparison between thermal fatigue (TF) and thermomechanical fatigue (TMF) testing for SRR99 superalloy. First, a measured TF mechanical strain‐temperature (epsilon(m)‐T) profile between 200 degrees C and 1100 degrees C has been reproduced on a TMF specimen to allow for an estimation of the stresses and inelastic strains which occur during TF. Thermal‐fatigue‐base (TFB) epsilon(m)‐T profiles were determined based on the measured TF profiles. These TFB cycles, together with the well‐known out‐of‐phase profile, were employed for all of TMF tests. The comparison of the results reveals that the TF and TMF out‐phase tests are more damaging in terms both of the applied stress and strain range, while the TMF out‐of‐phase test is more severe if the comparison is made in terms of the inelastic strain range. Finally, it is shown that the results can be rationalized quite well by plotting the fatigue lives as a function of the maximum stabilized stress (sigma(max)).

The effect of voids on the quasi-static tensile properties of carbon fiber/polymer-laminated composites

Static longitudinal/transverse tensile tests for unidirectional carbon fiber/polymer (T300/924) laminates and laminates with lay-ups [ 0 / ± 45 / 0 / 90 ] s at various void levels were conducted, and degradations in stiffness/strength were observed with the presence of voids. The void levels were controlled by compression pressure during the compression molding process. The characterization of voids was achieved by digital microscopy image analysis; the density distributions of equivalent diameters and aspect ratios were analyzed with respect to compression pressure. For the purpose of quantifying the effect of voids on the static mechanical properties of composites, a stiffness prediction method based on the Mori-Tanaka method and void geometric statistical data have been used with the implementation of a finite element model of the representative volume element for unidirectional composites; the prediction results show good correlation with experimental data. Finally, a modified continuum damage model for laminated composites with the presence of voids was proposed, the model is capable of capturing the effect of voids; and gradual damage analysis for carbon fiber/polymer composite laminates at different void levels was conducted to evaluate the effect of voids on their tensile properties.