EFFECT OF NOTCH DIMENSION AND LOCATION ON FATIGUE LIFE AND THERMAL BEHAVIOR OF LOW CARBON STEEL (ST37-2)

The effect of depth and location of the notch on the v-notched specimen on fatigue life of cantilever rotating beam of low carbon steel (ST37-2) is studied, by applying fully reversed cycle load of 150 N, and mean stress of Zero. The temperature variations during fatigue life were measured by infrared camera at three points in specimens. These points are the edge point, notch point and other specified points. Numerically, the finite element model of fatigue test was obtained using the ANSYS Workbench. The ANSYS model was based on the S/N curve measured experimentally. From the results, there is a notch position that changed the fracture position from the notch position to the edge position. Also, fatigue life can be increased by putting the notch in appropriate position on the specimen. The temperature variation at different points in the specimen gave a good prediction to the fracture position before the fracture occurs. Also, from the comparison between experimental and numerical results, the two curves were similar but the ANSYS model based on the experimental S/N curve gave a good prediction for fatigue life.


INTRODUCTION
Fatigue defined as "a form of failure caused by fluctuating or cyclic loads over a short or prolong period of time". On the other hand it was done at an ultimate stress less than the static yield strength of certain metal (Nestor, 2004). Fatigue behavior divided into two types, the first one is when high loads are applied that produced elastic and plastic strain, for every one cycle, likewise fatigue life in this region is relatively little less than 103 , and it is called lowcycle fatigue (LCF). The other type is called High -cycle fatigue (HCF), it is for lower stressless than (LCF) -, the deformations are totally elastic, and longer fatigue life more than 103cycles (Richard and Keith, 2015). Although Fatigue knowledge has been developed step by step utile this time. In the 1840s in railway industry, they noted that railroad axles failed at its shoulders.
That was the first major impact of failures due to repeated load. So the expression "fatigue" has been used in the 1840s and 1850s to explain failures occurring from repeated stress.
In 1850-1860 August WÖhler execute many fatigue testing experiments, and he presnts the concept of the S-N curve. After that a lot of researchers worked in this field to produce their theories for expanding knowledge on fatigue that we got to what it is now (Ralph et al., 2011).  Engineering, Vol. 8, No. 3, 2017 145 specimens of Low Carbone Steel and compared the experimental results with numerical results of ANSYS program. On the other hands there are a number of researchers work on heat generation during plastic deformation as Rabiei et al., (2000) studied the heat generation during the compression-compression fatigue failure on perforated, and Caroline et al., (2012) studied the heat generation due to plastic deformation to fracture.
In this work, the effects of notch dimensions (depth and location) on fatigue life are studied experimentally and theoretically using the finite element method (by ANSYS Workbench 15.0).
Experimentally, the S-N curve was plotted at the beginning, and then the effects of notch dimensions on fatigue life were studied. Also, the temperature increasing due to plastic deformation at the fracture point was measured using infrared camera (IR).

EXPERIMENTAL WORK
In this paper, the experimental work based on investigation of the effect of the notch depth and location on fatigue life, and monitoring the temperature change with the time during the tests.
This test divided into two parts: first one is done to draw S/N curve, and the second is done to investigate the effect of the notch depth and location on fatigue life as shown in Fig. 1. Low carbon steel alloy (ST37-2) is used in this work, and provided from The State Company for Mechanical Industry. The chemical composition of the specimens used in this work was shown in Table 1 and its mechanical properties was shown in Table 2.

Fatigue Testing Machine
Rotating bending machine type (WP 140 Apparatus of Gunt) was used in the test. This type with single cantilever beam specimen with constant amplitude and fully reversed load, as show in Fig. 2.

Fatigue test samples
The fatigue samples are machined in convenient dimension (ø = 12mm and L = 40 mm) and (ø = 8 mm and L = 106 mm) as shown in the Fig. 3. Two types of specimens are done; the first Depth 1.5mm Load 150 N and 60⁰ v-notches

Depth 1mm
Notch location 10-100mm Notch location 10-100mm With using IR camera type is smooth samples without notch used in the experiments to plot the S/N curve. While the second type is notched specimens used in the experiments to study the effects of notch location and notch depth on the fatigue life. This was done by doing v-notch with angle 60 o and two different depths 1mm and 1.5mm. Every notch depth localized in different locations on different samples. The v-notch are done at distance (10, 20, 30… 100) mm from the edge as show in Fig.   4.

MONITORING TEMPERATURE CHANGE
During the fatigue behavior test, infrared camera (IR) (Flir E50) is used to monitoring the temperature change during the fatigue life of the specimens, as shown in Fig. 5.

NUMERICAL INVESTIGATION
Finite element method is applied by using ANSYS Workbench 15.0 to do simulation with experimental work that based on experimental S/N curve, the models divided into more than (33,000) elements type triangle surface masher as shown in Fig. 6. Kufa Journal of Engineering, Vol. 8, No. 3, 2017 149 Fig. 7. Maximum Stress at Notch Area.

RESULTS
From experimental and numerical results it is found the following:  Fig. 8 shows the S/N curve of the tested material that is obtained from experimental fatigue test of smooth specimens with different applied loads, this curve used then in ANSYS program.  Table 3.
 Fig. 13 shows the pictures of thermal camera for temperature distribution along the specimen and temperature change during the fatigue test.
 Figs. 14, 15, 16 and 17 show the temperature change on significant points (edge, notch) during the fatigue life. The following points were found: 1. Maximum amount of heat generation occurred in fracture position because of the concentrating of stress and strain in that position.
2. Increasing in heat generation with increasing of strain rate, can be seen during the fracture occurring, where the temperature variation curve slope is increased.
 Fig. 18 shows the macrostructure of specimens at fracture position when the notch depth is (1, 1.5 mm) and for different notch positions. The cross sectional area of the specimen divided into two regions. The first one is smooth region which refers to the slowly crack propagation, while the second one is the rough region which refers to the sadden fracture. The sadden fracture area was inversely proportion with fatigue life.