Taguchi optimization and ultrasonic measurement of residual stresses in the friction stir welding

The main goal of this study is optimization of residual stresses produced by friction stir welding (FSW) of 5086 aluminum plates. Taguchi method is employed as statistical design of experiment (DOE) to optimize welding parameters including feed rate, rotational speed, pin diameter and shoulder diameter. The optimization process depends on effect of the welding parameters on longitudinal residual stress, which is measured by employing ultrasonic technique. The ultrasonic measurement method is based on acoustoelasticity law, which describes the relation between acoustic waves and internal stresses of the material. In this study, the ultrasonic stress measurement is fulfilled by using longitudinal critically refracted (LCR) waves which are longitudinal ultrasonic waves propagated parallel to the surface within an effective depth. The ultrasonic stress measurement results are also verified by employing the hole-drilling standard technique. By using statistical analysis of variance (ANOVA), it has been concluded that the most significant effect on the longitudinal residual stress peak is related to the feed rate while the pin and shoulder diameter have no dominant effect. The rotational speed variation leads to changing the welding heat input which affects on the residual stress considerably

Analysis of the damage mechanisms in mixed-mode delamination of laminated composites using acoustic emission data clustering

In this study, acoustic emission (AE) technique is used to investigate different time-to-failure mechanisms of delamination in glass/epoxy composite laminates. Woven and unidirectional layups were subjected to the double cantilever beam, end notch flexure, and mixed-mode bending tests and the generated AE signals were captured. Discrimination of the AE events, caused by different types of the damage mechanisms, was performed using wavelet packet transform (WPT) and fuzzy clustering method (FCM) associated with a principal component analysis (PCA). The FCM and WPT analyses identified three dominant damage mechanisms. Furthermore, different interface layups and different GII/GT modal ratio values (ratio of mode II strain energy release rate per total strain energy release rate) indicated different time-to-failure mechanisms incidence. Additionally, the damaged mechanisms were observed using scanning electron microscopic (SEM) analysis. The results showed that the dominant damage mechanisms in all the specimens are matrix cracking and fiber–matrix debonding. Besides, some fiber breakage appeared during the tests, and the percentage of this damage mechanism in the unidirectional specimens and mode I condition was higher than those in the woven specimens and mode II. SEM observations were also in good agreement with the obtained results. It was found that the presented methods can be utilized to improve the characterization and discrimination of damage mechanisms in the actual occurring modes of delamination in composite structures

Using ultrasonic waves and finite element method to evaluate through-thickness residual stresses distribution in the friction stir welding of aluminum plates

The main goal of this study is ultrasonic measuring of through-thickness residual stresses in friction stir welding of aluminum plates. A 3D thermo-mechanical finite element analysis is used to evaluate residual stresses caused by friction stir welding of 5086 aluminum plates. The finite element (FE) model has been validated by the hole drilling method. Residual stresses obtained from the FE analysis is then compared with those obtained from ultrasonic stress measurement. The ultrasonic measurement technique is based on acoustoelasticity law which describes the relation between the acoustic waves and the stress of material. The ultrasonic stress measurement is accomplished by using longitudinal critically refracted (LCR) waves which are propagated parallel to the surface within an effective depth. Through-thickness distribution of longitudinal residual stresses is evaluated by employing the LCR waves produced by four different frequencies (1. MHz, 2. MHz, 4. MHz and 5. MHz) of ultrasonic transducers. Utilizing the FE analysis along with the LCR method (known as FELCR method), the through-thickness distribution of longitudinal residual stress could be achieved. The comparison between ultrasonic and FE results show an acceptable agreement, hence the FELCR method could be successfully applied on the FSW plates. It has been concluded that the longitudinal residual stresses of aluminum plates joined by friction stir welding can be evaluated by using the FELCR method. The good potential of FELCR as a nondestructive method is also confirmed in through-thickness stress measurement of aluminum plates

Comparison between contact and immersion ultrasonic techniques to evaluate longitudinal residual stress in friction stir welding of aluminum plates

Ultrasonic stress measurement is a nondestructive technique that has increasingly been employed to evaluate the welding residual stress of different materials. This study compares contact and immersion ultrasonic waves employed to measure residual stresses that are produced by friction stir welding (FSW) of 5086 aluminum plates. The ultrasonic stress measurement technique employs critically refracted longitudinal (LCR) waves produced by 2 MHz contact and immersion ultrasonic probes. The LCR waves are longitudinal ultrasonic waves propagated parallel to the surface within an effective depth inside the material. A finite element simulation of the welding process, verified by a hole-drilling technique, was also used to validate the results of ultrasonic stress measurement. The combination of using finite element analysis along with the LCR technique is known as the FELCR technique. The ultrasonic technique is able to measure the average of stresses within the depth of material; hence, subsurface stress analysis by the finite element technique was used for comparison with the ultrasonic measurement results. The distribution of longitudinal residual stress was determined by employing the FELCR technique and utilizing the contact and immersion probes separately. The results were then compared, showing no considerable difference between using the contact and immersion probes in ultrasonic stress measurement of FSW on the aluminum plates.Scopu

Change in skeletal muscle stiffness after running competition is dependent on both running distance and recovery time: a pilot study

Long-distance running competitions impose a large amount of mechanical loading and strain leading to muscle edema and delayed onset muscle soreness (DOMS). Damage to various muscle fibers, metabolic impairments and fatigue have been linked to explain how DOMS impairs muscle function. Disruptions of muscle fiber during DOMS exacerbated by exercise have been shown to change muscle mechanical properties. The objective of this study is to quantify changes in mechanical properties of different muscles in the thigh and lower leg as function of running distance and time after competition. A custom implementation of Focused Comb-Push Ultrasound Shear Elastography (F-CUSE) method was used to evaluate shear modulus in runners before and after a race. Twenty-two healthy individuals (age: 23 ± 5 years) were recruited using convenience sampling and split into three race categories: short distance (nine subjects, 3–5 miles), middle distance (10 subjects, 10–13 miles), and long distance (three subjects, 26+ miles). Shear Wave Elastography (SWE) measurements were taken on both legs of each subject on the rectus femoris (RF), vastus lateralis (VL), vastus medialis (VM), soleus, lateral gastrocnemius (LG), medial gastrocnemius (MG), biceps femoris (BF) and semitendinosus (ST) muscles. For statistical analyses, a linear mixed model was used, with recovery time and running distance as fixed variables, while shear modulus was used as the dependent variable. Recovery time had a significant effect on the soleus (p = 0.05), while running distance had considerable effect on the biceps femoris (p = 0.02), vastus lateralis (p < 0.01) and semitendinosus muscles (p = 0.02). Sixty-seven percent of muscles exhibited a decreasing stiffness trend from before competition to immediately after competition. The preliminary results suggest that SWE could potentially be used to quantify changes of muscle mechanical properties as a way for measuring recovery procedures for runners

Analysis of the damage mechanisms in mixed-mode delamination of laminated composites using acoustic emission data clustering

© The Author(s) 2015. In this study, acoustic emission (AE) technique is used to investigate different time-to-failure mechanisms of delamination in glass/epoxy composite laminates. Woven and unidirectional layups were subjected to the double cantilever beam, end notch flexure, and mixed-mode bending tests and the generated AE signals were captured. Discrimination of the AE events, caused by different types of the damage mechanisms, was performed using wavelet packet transform (WPT) and fuzzy clustering method (FCM) associated with a principal component analysis (PCA). The FCM and WPT analyses identified three dominant damage mechanisms. Furthermore, different interface layups and different GII/GT modal ratio values (ratio of mode II strain energy release rate per total strain energy release rate) indicated different time-to-failure mechanisms incidence. Additionally, the damaged mechanisms were observed using scanning electron microscopic (SEM) analysis. The results showed that the dominant damage mechanisms in all the specimens are matrix cracking and fiber-matrix debonding. Besides, some fiber breakage appeared during the tests, and the percentage of this damage mechanism in the unidirectional specimens and mode I condition was higher than those in the woven specimens and mode II. SEM observations were also in good agreement with the obtained results. It was found that the presented methods can be utilized to improve the characterization and discrimination of damage mechanisms in the actual occurring modes of delamination in composite structures

Effect of vitamin C supplementation in the prevention of atrial fibrillation

Background: Recently, the role of inflammation and oxidative stress in the pathophysiology of atrial fibrillation (AF) after cardiac surgery has been emphasized. Vitamin C as an antioxidant important role in reducing the incidence of postoperative atrial fibrillation. This study aimed to investigate, administration of vitamin C, as a way to reduce the incidence of atrial fibrillation after coronary bypass surgery. Methods: In this double-blind clinical study, 290 patients in Rajaee Heart Center, from March 2013 to December 2014 who underwent coronary artery bypass surgery were randomly divided into intervention and control groups to receive vitamin C and placebo. The intervention group before the surgery in the operating room received 2 grams of vitamin C intravenously then one gram per day for four days prior to surgery. After the operation, the two groups were compared in terms of the following: Atrial and ventricular arrhythmias after surgery, ICU stay and hospital stay and duration of intubation. Results: 113 cases and 177 controls (191 men and 99 women) with a mean age of 55.40&plusmn;14.40 years in both groups (vitamin C and placebo) were enrolled. The incidence of postoperative atrial fibrillation was 55% in the placebo group to 35% in the vitamin C group decreased (P= 0.001). Duration of intubation in the intervention group 11.8 and the control group was 14.14 hours (P= 0.004). The amount of drainage was lower in vitamin C group (P= 0.003). Vitamin C had no effect on the rates of hospital and ICU stay (P= 0.075). There was no significant reduction in threatening arrhythmia (VT) and VF in this period (P= 0.159). Conclusion: Vitamin C supplements may reduce atrial fibrillation after coronary artery bypass surgery also can improve conditions such as reducing the duration of intubation. With regard to the safety, these supplements can be recommended for the prevention of atrial fibrillation before coronary artery bypass surgery

Effect of Heat Input Ratio on Residual Stress in Multipass Welding Using Finite Element Method and Ultrasonic Stress Measurement

The presence of residual stresses in components, particularly in those operating in high temperature, can lead to cracking and reduction of fatigue life. The present work is an attempt to study the effect of heat input ratio of multipass welding on the residual stresses. Firstly, the ultrasonic stress measurement technique is carried out in the weldment using specific welding parameters. The stress measurement conducted by ultrasonic method utilizes longitudinal critically refracted waves which are basically promoted parallel to the surface of material at certain depth. Moreover, the hole drilling method is utilized so as to validate the results of ultrasonic method. The finite element analysis is then carried out for the same welding condition so that the residual stress obtained by FEM could be compared and verified with those extracted from ultrasonic method. The different finite element analyses employing different heat input ratios are conducted in the 15Mo3 high strength low alloy steel while two different interpass time are considered. Finally, it has been concluded that heat input of the second pass has significant effect on longitudinal residual stress peak while heat input of the first pass is not much influential on residual stress magnitude if long interpass time is considered. On the other hand, when the second pass is deposited immediately after the first pass, the heat input ratio can be used as a criteria for evaluation of the maximum residual stress