Study of the plastic behavior around the crack tip by means of thermal methods

AbstractIn this work, the behaviour of two cracked stainless steel AISI 410 and C3FM was studied by means of a new procedure based on thermographic methods. A temperature model in time domain was considered in order to obtain information about the first and the second order harmonic of the temperature signal. Interesting results were obtained in term of possibility to describe the plastic phenomena at the crack tip

Influence of Second-Order Effects on Thermoelastic Behaviour in the Proximity of Crack Tips on Titanium

The Stress Intensity Factor (SIF) is used to describe the stress state and the mechanical behaviour of a material in the presence of cracks. SIF can be experimentally assessed using contactless techniques such as Thermoelastic Stress Analysis (TSA). The classic TSA theory concerns the relationship between temperature and stress variations and was successfully applied to fracture mechanics for SIF evaluation and crack tip location. This theory is no longer valid for some materials, such as titanium and aluminium, where the temperature variations also depend on the mean stress. The objective of this work was to present a new thermoelastic equation that includes the mean stress dependence to investigate the thermoelastic effect in the proximity of crack tips on titanium. Westergaard’s equations and Williams’s series expansion were employed in order to express the thermoelastic signal, including the second-order effect. Tests have been carried out to investigate the differences in SIF evaluation between the proposed approach and the classical one. A first qualitative evaluation of the importance of considering second-order effects in the thermoelastic signal in proximity of the crack tip in two loading conditions at two different loading ratios, R = 0.1 and R = 0.5, consisted of comparing the experimental signal and synthetic TSA maps. Moreover, the SIF, evaluated with the proposed and classical approaches, was compared with values from the ASTM standard formulas. The new formulation demonstrates its improved capability for describing the stress distribution in the proximity of the crack tip. The effect of the correction cannot be neglected in either Williams’s or Westergaard’s model

New data analysis to evaluate defects in composite materials using microwaves thermography

Abstract In this work microwave sources were used as heating sources to evaluate the damages areas in composite materials using thermographic techniques. In particular, lock-in thermography and microwave lock-in thermography tests were carried out on CFRP damaged specimens. A new data analysis was developed to processing the thermographic data obtained by microwave tests and various algorithm were used to processing lock-in thermography tests. Data obtained were compared with non destructive tests performed with other techniques such as x-ray. A numerical model was developed for microwave heating simulation

Effects of Anacetrapib in Patients with Atherosclerotic Vascular Disease

BACKGROUND: Patients with atherosclerotic vascular disease remain at high risk for cardiovascular events despite effective statin-based treatment of low-density lipoprotein (LDL) cholesterol levels. The inhibition of cholesteryl ester transfer protein (CETP) by anacetrapib reduces LDL cholesterol levels and increases high-density lipoprotein (HDL) cholesterol levels. However, trials of other CETP inhibitors have shown neutral or adverse effects on cardiovascular outcomes. METHODS: We conducted a randomized, double-blind, placebo-controlled trial involving 30,449 adults with atherosclerotic vascular disease who were receiving intensive atorvastatin therapy and who had a mean LDL cholesterol level of 61 mg per deciliter (1.58 mmol per liter), a mean non-HDL cholesterol level of 92 mg per deciliter (2.38 mmol per liter), and a mean HDL cholesterol level of 40 mg per deciliter (1.03 mmol per liter). The patients were assigned to receive either 100 mg of anacetrapib once daily (15,225 patients) or matching placebo (15,224 patients). The primary outcome was the first major coronary event, a composite of coronary death, myocardial infarction, or coronary revascularization. RESULTS: During the median follow-up period of 4.1 years, the primary outcome occurred in significantly fewer patients in the anacetrapib group than in the placebo group (1640 of 15,225 patients [10.8%] vs. 1803 of 15,224 patients [11.8%]; rate ratio, 0.91; 95% confidence interval, 0.85 to 0.97; P=0.004). The relative difference in risk was similar across multiple prespecified subgroups. At the trial midpoint, the mean level of HDL cholesterol was higher by 43 mg per deciliter (1.12 mmol per liter) in the anacetrapib group than in the placebo group (a relative difference of 104%), and the mean level of non-HDL cholesterol was lower by 17 mg per deciliter (0.44 mmol per liter), a relative difference of -18%. There were no significant between-group differences in the risk of death, cancer, or other serious adverse events. CONCLUSIONS: Among patients with atherosclerotic vascular disease who were receiving intensive statin therapy, the use of anacetrapib resulted in a lower incidence of major coronary events than the use of placebo. (Funded by Merck and others; Current Controlled Trials number, ISRCTN48678192 ; ClinicalTrials.gov number, NCT01252953 ; and EudraCT number, 2010-023467-18 .)

Fatigue limit evaluation of martensitic steels with thermal methods

Fatigue behavior is a discriminating feature in choice of steels for extreme conditions such as high or low temperatures and corrosive environments. This is the case of martensitic steels considered in this work, ASTM A182 grade F6NM, and VIRGO 39. These kind of steels are characterized by temperature changes of material related to fatigue damage very low. The fatigue behavior of these steels was investigated with two thermal methods based on temperature surface monitoring and phase variation of thermoelastic signal (TPA method). Two procedures were developed to process thermographic data and to assess fatigue limit of materials

Fatigue behaviour of stainless steels: A multi-parametric approach

In recent years different experimental methods have been experienced to enhance the fatigue characterisation of materials with the aim to overcome the Standard long-lasting tests, i.e. Wohler curve determination. Standard fatigue treatment requires at least 15 specimens being tested to get an estimation of material fatigue limit and it is worth noting that this kind of tests do not provide any information on damage phenomena occurring in the material. Thus, topic to be addressed in this research have to do with development of lock-in infrared measurement based thermal method for rapid evaluation of fatigue limit. By performing a single test, the adopted method leads to match different parameter information. The Assessed parameters are in number more than the ones provided by TSA, as well. Moreover, the adopted technique points to study damage by analysing the different phenomena involved in fatigue and in this regard, the aim of this paper is to show how a thermal technique can attain an early assessment of the failure processes during a cyclically loading test. The author is, also, focused on to illustrate the strong points of a method based on infrared measurements for assessing endurance limit for both austenitic and martensitic stainless steels while considering, as reference, the Standard Test methods

A thermoelastic stress analysis general model: Study of the influence of biaxial residual stress on aluminium and titanium

All the studies on the thermoelastic behaviour of materials, including the revised higher order theory on the thermoelastic effect, are based on several assumptions that limit the application of such theory to the cases of isotropic materials in the presence of uniaxial residual stresses and undergoing uniaxial applied loads. These assumptions lead to some discrepancies in the description of the real thermoelastic behaviour of materials in the presence of residual stresses. In this work, by rewriting the thermoelastic equation in a different way, it was possible to study the behaviour of homogeneous and non-isotropic materials undergoing any loading conditions and residual stresses. Firstly, the error made by the calibration procedures of thermoelastic stress analysis (TSA) data in the presence of residual stresses has been investigated. Then, a statistical analysis was carried out to determine the minimum value of residual stress which would lead to significant and measurable variations in the thermoelastic signal. The simulations involved two non-ferrous metals: AA6082 and Ti6Al4V, which exhibit a specific thermoelastic behaviour

Study of the thermo-elastic stress analysis (TSA) sensitivity in the evaluation of residual stress in non-ferrous metal

The Thermoelastic Stress Analysis (TSA) is a contactless technique able to determine the superficial stress of a component subjected to a dynamic load in a linear elastic field. In these conditions the thermoelastic effect shows the generation of small reversible temperature variations. In this work, a general equation was obtained for the evaluation of the thermoelastic signal. The proposed equation is valid under adiabatic and isentropic conditions, for generalized homogeneous and anisotropic materials in any load condition. By using the proposed generalized equation, TSA sensitivity to the variation of the physical and mechanical material characteristics and TSA sensitivity in the determination of residual stresses were studied. The case studies for performing numerical simulations were represented by AA6082 and Ti6Al4V non-ferrous metals. The results were then compared with the data obtained from experimental tests performed on AA 6082 samples