Investigation of Microstructure and Stress Corrosion Cracking in Al-6061-T6 Alloy at Different Loads

Stress corrosion cracking (SCC) refers to the damage of mechanical components which are under the combined action of static load and corrosive environment. This phenomenon occurs in various applications including naval and aerospace industry where aluminum and steel alloys experience mechanical loadings in the presence of corrosive environments. In this research work, microstructural and environmental influence on corrosion behavior of Al-6061-T6 at different static loads was investigated. A new test fixture was developed for stress corrosion cracking. Dog-bone shaped tensile specimens of Al-6061-T6 were manufactured using CNC milling machine. Tests were conducted at constant loads of 200 N, 500 N and 800 N, in three different environments: dry ambient conditions, distilled water and 3.5% NaCl solution. Testing continued for different intervals of time i.e. 96 hours, 68 hours and 4.5 hours respectively. After each set of experiments, specimens were observed for cracks using metallurgical microscope. Detailed fractographic investigation of all the tested specimens was carried out using Scanning Electron Microscope (SEM). Excessive corrosion and material degradation was observed in specimens tested in distilled water and 3.5% NaCl environments. Microstructural analysis depicted pitting corrosion and crack deformation.  Some regions clearly showed that grain boundaries were attacked due to oxidation and chemical attack causing weakening of grain boundaries and resulted into intergranular corrosion. Precipitates and grain boundaries in Al-6061-T6 served as a reason of crack initiation due to hydrogen diffusion. Fractographic investigation provided the evidence of trans granular fracture as well as intergranular fracture which was observed as dimples and extensive ductile tearing.

Comparative Effectiveness of Expository Strategy and Problem Solving Approach of Teaching Mathematics at Secondary Level

A sound education in mathematics is important for any modern knowledge based economy. Mathematics is now important in many areas where it has not previously played much of role, for example, in biology, social sciences etc. If the proper mathematical foundations are not laid during the formative years of childhood and adolescence, it becomes increasingly difficult to address this weakness in later life. The attempts to confront these shortcomings during retraining in later life are generally wasteful and only partly successful. The present study was designed to investigate the comparative effectiveness of problem-solving approach on the academic achievement of secondary school students in mathematics. The major objectives of the study were: (1) To determine whether the problem solving approach is more effective than expository strategy of teaching on academic achievement of students in mathematics. (2) To determine whether the problem solving approach is more effective than expository strategy of teaching on attitude of students in mathematics. (3) To examine the effect of problem-solving approach on the academic achievement of low achievers. (4) To measure the effect of problem-solving approach on the academic achievement of high achievers. (5) To find out the effect of problem-solving approach on the retention of students in mathematics. To achieve the objectives of study, following null hypotheses were tested: (1) There is no significant difference between the pre-test mean achievement scores of experimental and control group. (2) There is no significant difference between the pre-test mean achievement scores of low achievers of experimental group and control group. (3) There is no significant difference between the Pre-test mean achievement scores of high achievers of experimental and control group. (4) There is no significant difference between pre-test mean attitude scores of experimental group and control group. (5) There is no significant difference between the post-test mean achievement scores of experimental group and control group. (6) There is no significant difference between post-test mean achievement scores of low achievers of experimental group and control group. (7) There is no significant difference between post-test mean achievement scores of high achievers of experimental group and control group. (8) There is no significant difference between post-test mean attitude scores of experimental group and control group. (9) There is no significant difference between mean achievement scores of experimental group and control group and control group on retention test. (10) There is no significant difference between mean achievement scores of high achievers of experimental group and control group on retention test. (11) There is no significant difference between mean achievement scores of low achievers of Experimental group and control group on retention test. Secondary school students studying mathematics constituted the population of study. The student of 10th class of Govt. Pakistan Girls Higher School, Rawalpindi (Pakistan) was selected as a sample of the study. The pre-test post-test equivalent- group design was used for the study. The sample size consisted of forty eight students who were divided into experimental group and control group, each consisting of 24 students by equating them on their previous knowledge in mathematics, as determined through a pre-test. A treatment of planned problem-solving approach was provided to the experimental group while the control group was taught by expository strategy for a period of six weeks.