Fatigue Performance of 2017-T4 AL. Alloy Under sub-zero Temperature by Using Electromechanical Freezing System

The effects of sub-zero(-22C o ) cyclic rotating bending on the S-N behavior and cumulative damage are reported for 207-T4 aluminum alloy .Experimental characterization of fatigue behavior showed that the S-N curves behavior may described by the following Basquin's formulas:


INTRODUCTION
aterial1 fatigue1 is a failure1 mode that1 has been known, to researchers, and engineers since, the 19th century.Catastrophic accidents have happened, due to fatigue, failures of, structures, machinery, and transport vehicles [1].Fatigue, is commonly referred, to as a process, in which damage, is accumulated in a material, undergoing fluctuating, loading, eventually resulting, in failure, even if the maximum load, is well below, the elastic limit of, the material.Fatigue, refers to the process, which reduces, local strength, of engineering, materials [2].
Failure, occurring under, dynamic conditions, of stress application, are known, as fatigue, failure.Fatigue, failure is rather, unpredictable because it happens, without notice [3].
Fatigue, failure causes, (1) Fatigue, failure, is due, to repeated, loading: At least, half of all, mechanical failures, are due, to fatigue.Many books, and articles, have suggested between 50 and 90 percent, of all mechanical, failures are fatigue, failures (2) Most of these, are unexpected, failures.They include, simple items, such as door springs and electric, light bulbs, to complex,

Experimental Procedures Material Selection
Aluminum of the1 2017-T4 is used1 in the current work1 2017 -Like 2011 (which is free1 machining alloy1 compares favorably1 with free cutting1 brass.It is the most suitable1 alloy for machining1 on automatics, milling1 machines, lathes, planers, shapers1 and other machine1 tools, and is the most1 widely used alloy1 for all types' of screw machine1 parts.It can be1 machined at high1 speeds and 1comparatively heavy feeds).This is also a general-purpose alloy for automatic screw machine work.It is stronger1 than 2011, but harder1 to machine and1 does not have1 the fine chip 1associated with 2011.It is recommended1 for heavy-duty 1parts because of1 its high strength.Workability1 is fair, with ductility1 and formability 1considered better1 than 2014.Arc and resistance weld ability are satisfactory.Corrosion resistance is fair.It is used for1 rivets, fasteners, and aircraft1 components and it1 is primarily used1 in applications1 where electrical 1conductivity, formability, ductility [6].Chemical analysis of the metal used was tested at (State Company for Inspection and Engineering Rehabilitation1 (SIER) in Iraq).The results, which are compared1 to the American Society for Testing and Materials (ASTM B209) [7], Chemical analysis of the metal 1used was tested at (State Company for Inspection1 and Engineering Rehabilitation (SIER) in Iraq) are summarized in Table (1) .

Tensile test
Tensile tests (three specimens) were accomplished at the department of material Engineering, university of technology .using the test machine WDW-100 with capacity of 100KNwhich is shown the figure (1).The test specimen as in figure (2) is installed between the two large grips of the testing machine and then loaded in tension.Measuring devices record the deformations, and the automatic control and data-processing systems (at the left in the photo) tabulate and graph the results [8].Below the mechanical properties are listed in Table (2) and the results of the tensile tests are shown in Fig ( 3) .The alloys used in the present study were provided in form of rod of 2017A-T4 alloy.The above results are the average of three readings.

Fatigue test machine
A reverse bending fatigue machine type Schematic of schench rotating-bending fatigue machine, as shown in Figure ( 5) was used to carry out the fatigue testing.The tests were undertaken in stress control with a stress ratio R=-1 and the cycling rate is 1420 rpm (f=23.67Hz).In this rotating-bending fatigue machine used to calculate the number of cycles that it takes until the specimen fail by recording number of cycles from the cycle counter which is located in the left of the machine.The range of the loads that had been taken through the fatigue test were varied between (150MPa) and (350 MPa ) for constant stress level.Material used in this study is a rod (10mm diameter and 80 mm length) of 2017-T4 aluminum alloy (rod product) for fatigue test at room temperature (RT) and low temperature (-22C o ).

Operation freezer with Electrical Control Circuit:
The electric1 control circuit is1 turned by miniature1 circuit breaker 1which provides1 overload and short1 circuit protection.The controlled1variable, in this 1case, is the temperature1 which is measured1 by a thermocouple1 type K and converted1 to signal acceptable by the controller.The controller compares the temperature of the freezer measured1 by the sensor with1 the desired load 1temperature (the Set-Point) 1 and actuates 1the final control device1 which is a contactor.system consists1 of several equipment's 1like compressor, condenser, evaporator, expansion devices etc.A refrigerant1 compressor is a machine1 used to compress 1the refrigerant from the evaporator and to raise its pressure so that the corresponding temperature 1is higher than that 1of the cooling medium.The condenser1 is an important device 1used in the high 1pressure side of a refrigeration1 system.Its function is1 to remove heat of the hot vapour refrigerant discharged from the compressor.The evaporator is used 1in the low pressure 1side of a refrigeration 1system [9].A temperature control1 system is composed 1of essential elements1 which all affect1 its performance as 1shown in Figure (4):

Results and discussions A) S-N Curve Constant Amplitude Fatigue Results
The specimens were tested under constant amplitude fatigue stress control rotating bending at a stress ratio R=-1 at room temperatures (RT) and low temperature (-22C o ) to estimate the S-N curves.The results of this series are illustrated in Figure (7) for aluminum alloy 2017-T4 at room temperature and at sub-zero temperature (-22C o ) .The fatigue tests are presented in the curves shown in Figures (7) these curves give an indication about the variations in fatigue life.From these data, the fatigue life estimation equations were determined.The fatigue endurance limit can be calculated by using these equations.The percentage of enhancement in fatigue life was calculated for all groups.
S-N data for 2017-T4 AL. alloy at low condition of testing namely RT and subzero temperature (-22C o ) is illustrated in table (3).

Table (3) S-N data for 2017-T4 AL. alloy at two condition
The fatigue behavior of 2017-T4 AL. alloy under two different conditions is presented in figure (7) and the fatigue parameters for the S-N curve equations can be shown in table (4).

Table (4) Fatigue parameters of 2017-T4 AL. alloy at two conditions of testing
From the fatigue data (experimental) of the specimens (2017-T4) as received and the specimens were cooled to (-22) C o , then at room temperature , the material (2017-T4 aluminum alloy) with applying stresses (350, 275, 200 and 150) MPa .So figure (8) represent comparison between the two cases to show the values of fatigue limit stress for 2017 aluminum alloy at two cases room temperature and low (-22C o ) .While below the 204 MPa stress level the specimens exhibited reduction in the Nf .It is suggested that above the 204 MPa stress level the fatigue resistance increased due to a strong bounding which is the responsible for improvement the fatigue behavior.But below 204 MPa the weakening of bounding exhibited reduction in the fatigue strength lives [10].

Figure (8) Values of stress fatigue limit for 2017 aluminum alloy
So that fatigue endurance limit reduced by 8.18% due to britteness and relaxation of residual stresses or stress concentration [11].The development and evaluation of fatigue damage in aluminum alloys at low temperature is still an open question [12].

B) Cumulative Fatigue Damage results:
The failure of structural components is difficult to assess, practically when the loading are not constant of light metals (aircraft structure ) [13].The prediction of fatigue life of samples subjected to variable loading is a complex subject .For the assessment of fatigue damage under block program loading the most widely used theory is the linear damage rule or ((Miners theory)).This rule has received much attention due to its simplicity [14].
Miners rule implies that damage accumulates linearly with applied cycle fraction, independent of life level.The damage sum is equal to unity according to Miner while the experimental results showed that the damage sum is less or greater than unity.This means that there is no load sequence effect that occurs during the fatigue loading history [15].It has been verified that light structure can exhibit highly nonlinear fatigue damage evolution.The Miners rule can underestimate or overestimate the fatigue life according to Miners rule, fatigue damage under block loading can be assessed by Where D is the damage equal to unity K is the number of blocks Ni is the applied stress cycles Nfi is the number of cycles to fatigue of block i For a sequence of two constant blocks ……….( Where L is low and H is high stress level.

The non-linear proposed model
Marco and starkey [16] suggest the following non-linear relation : Where α is a function of the applied load which can be determined experimentally.According to Perieira et al [17] and Alkawi et al [18] ,they defined fatigue damage for low -high and high-low constant block tests as X is defined the effect of loading sequence and surface treatment For the present work, X may be defined as For low-high loading For high-low loading Finally, equation ( 4) can be presented to take the form …… (7) for lowhigh

Applications of Miners rule and proposed model
The experimental results obtained from testing the specimens from low-high and high-low loading sequence can be illustrated in table ( 4) It is clear from that the fatigue life of variable amplitude in low-high load sequence is greater than the high-low load sequence life for both conditions RT and (-22C o ).Because the specimens is getting hardening due to change from low to high.So, the low to high load sequence is more damaging than high to low loading.These findings are well agreed with References [16] and [19].
Figure (7) shows the modified S-N curve data for (RT=23C o ) and (-22C o ) above the stress level 204 MPa and number of cycles 159475 the fatigue specimens exhibited increases in the number of cycles to failure (N f ).While below the 204 MPa stress level the specimens exhibited reduction in the Nf .It is suggested that above the 204 MPa stress level the fatigue resistance increased due to a strong bounding which is the responsible for improvement the fatigue behavior.But below 204 MPa the weakening of bounding exhibited reduction in the fatigue strength lives[10].So that fatigue endurance limit reduced by 8.18% due to britteness and relaxation of residual stresses or stress concentration[11].The development and evaluation of fatigue damage in aluminum alloys at low temperature is still an open question[12].B) Cumulative Fatigue Damage results:The failure of structural components is difficult to assess, practically when the loading are not constant of light metals (aircraft structure )[13].The prediction of fatigue life of samples subjected to variable loading is a complex subject .For the assessment of fatigue damage under block program loading the most widely used theory is the linear damage rule or ((Miners theory)).This rule has received much attention due to its simplicity[14].Miners rule implies that damage accumulates linearly with applied cycle fraction, independent of life level.The damage sum is equal to unity according to Miner while the experimental results showed that the damage sum is less or greater than unity.This means that there is no load sequence effect that occurs during the fatigue loading history[15].It has been verified that light structure can exhibit highly nonlinear fatigue damage evolution.The Miners rule can underestimate or overestimate the fatigue life according to Miners rule, fatigue damage under block loading can be assessed by ∑ …………(1) the basquin exponent determined from the S-N curve equation

LFigure ( 9 )
Figure (9) Comparison of fatigue life for two blocks loading, using experimental, miner and proposed model

Fatigue 2 -
The constant amplitude S-N curve showed change in the behavior of 2017-T4 Al. alloy.The change observed at stress level of 204 MPa and number of cycles 159475.Above this point the subzero condition raised the fatigue life while below it reduces the fatigue lives.3-The fatigue life under two step loading was predicated by Miner rule and proposed model.The two methods give safe prediction compared to experimental results.But the proposed model indicated safer than Miner rule.

Table ( 6
) gives the fatigue life predicted results according to Miner in comparison with the experimental results