Scholarly article on topic 'U-shaped Bolts Fracture Failure Analysis'

U-shaped Bolts Fracture Failure Analysis Academic research paper on "Materials engineering"

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Procedia Engineering
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{"u-shaped bolt" / "fatigue fracture" / "micro cracks" / "surface decarburization"}

Abstract of research paper on Materials engineering, author of scientific article — Huanping Kong, Delin Liu, Tao Jiang

Abstract Left 1# u-shaped boltfractured when the automobile running 3766km in testing process, then 2# u-shaped bolt was repacked. Left 2# u-shaped bolt and right 3# u-shaped bolt were fractured whenthe automobile running 7778km. The failure mode and cause were analyzed by macro and micro observation, metallographic examination, hardness and tensile property testing, dimension examination.The results showed that u-shaped bolts were fatigue fracture. U-shaped bolts’ failure was related with densely distributed surface micro cracks, surface decarburization appearance, small dimension, and low strength of material.

Academic research paper on topic "U-shaped Bolts Fracture Failure Analysis"

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Procedía Engineering 99 (2015) 1476 - 1481

Procedía Engineering

www.elsevier.com/locate/procedia

"APISAT2014", 2014 Asia-Pacific International Symposium on Aerospace Technology,

APISAT2014

U-shaped Bolts Fracture Failure Analysis

Huanping Konga,b'c'* , Delin LIUa,bc, Tao JIANGa,bc

aAVIC Beijing Institute of Aeronautical Materials, Beijing 100095, China bAVIC Testing Innovation Cooperation, Beijing 100095, China cBeijing Key Laboratory of Aeronautical Materials Testing and Evaluation, Beijing 100095, China

Abstract

Left 1# u-shaped bolt fractured when the automobile running 3766km in testing process, then 2# u-shaped bolt was repacked. Left 2# u-shaped bolt and right 3# u-shaped bolt were fractured when the automobile running 7778km. The failure mode and cause were analyzed by macro and micro observation, metallographic examination, hardness and tensile property testing, dimension examination. The results showed that u-shaped bolts were fatigue fracture. U-shaped bolts' failure was related with densely distributed surface micro cracks, surface decarburization appearance, small dimension, and low strength of material. © 2015PublishedbyElsevierLtd. This is anopenaccess article under the CC BY-NC-ND license (http://creativecommons.org/licenses/by-nc-nd/4.0/).

Peer-review under responsibility of Chinese Society of Aeronautics and Astronautics (CSAA) Keywords: u-shaped bolt,;fatigue fracture; micro cracks; surface decarburization

1. Introduction

Due to perfect mechanical properties after quenching and tempering, 40Cr steel has become one of most widely used steel. 40Cr steel is mainly used for manufacturing mechanical parts that under moderate loads and moderate speeds, such as machine gears, shafts, worm, spline shafts, and other mechanical applications. Because of complicated working environment, 40Cr steel applications face to withstanding the combined effects of various loads, so that some applications are easy to failure if the production process not properly controlled [1,2]. U-shaped bolts installed in front of automobile leaf spring are used for connecting the engine and frame, which plays a

* Corresponding author. Tel.: +86-15210849599; . E-mail address: khp621@126.com

1877-7058 © 2015 Published by Elsevier Ltd. This is an open access article under the CC BY-NC-ND license (http://creativecommons.org/licenses/by-nc-nd/4.0/).

Peer-review under responsibility of Chinese Society of Aeronautics and Astronautics (CSAA) doi: 10.1016/j.proeng.2014.12.688

significant role in fixing and damping. Once faulty happen, it is dangerous for automobile safety and reliability, and enormous economic loss will happen.

The objects were 1 # ~ 3 # u-shaped bolts, which were made of 40Cr steel, specific heat treatment processes were: quenched at 850°C for 70min and oil cooling, tempered at 550 °C for 70min and air cooling. Failure property and cause were determined by macro and micro observation, metallographic examination, hardness and tensile property testing, dimension examination, which can provide reference to prevent re-occurrence of such failure.

2. The experimental approach

2.1. Macroscopic observation

U- shaped bolts and fractures macroscopic morphologies were shown in Fig. 1.

1# fracture originated from 1# bolt surface, showing line-source characteristic, and clearly extension lines could be seen. There were two flat fan-shaped areas on section, which was fatigue area covered by corrosion. 2# fracture originated from 2#bolt surface, showing line-source characteristic, and clearly extension lines could also be seen. There was one flat fan-shaped area on section, which was fatigue area presenting typical fatigue line morphology, and the remaining portion was shear lip area. 3# fracture originated from symmetrically distributed 3#bolt two sides, primary source and secondary source also showed line- source characteristics and typical fatigue line morphology. Extension lines and fatigue lines could be clearly seen. Primary and secondary fatigue zones accounted for 70% and 15% respectively, 15% intersection zone was final fracture zone.

Fig. 1. (a) 1# u-shaped bolt; (b) 2# u-shaped bolt; (c) 3# u-shaped bolt; (d) 1# fracture;(e) 2# fracture;(f) 3# fracture 2.2. Microscopic observation

Three fractures microscopic morphologies were shown in Fig.2.

1#~3# fracture source region showing line-source characteristics, no metallurgical defects has been seen under high magnification. Three cross sections presenting typical fatigue striation morphologies, secondary cracks can also be seen. Shear lip area and final fracture zone presenting typical dimple feature.

Fig. 2. (a) 1# fracture source region; (b) 1# fracture fatigue striation; (c) 2# fracture source region; (d) 2# fracture fatigue striation;(e) 3# fracture

source region; (f) 3# fracture fatigue striation

2.3. Metallographic examination

1#~3# metallographic samples were prepared near corresponding fracture. Prior etching, small pits and densely distributed micro cracks were found at specimen edge, which corresponded to bolts surface. Micro cracks' length less than 10^m, presenting bifurcation and tortuous appearance. 1# metallographic sample's small pits and micro cracks appearance were shown in Fig.3.

After etching, three samples microstructures were observed. Bright decarburization layer existed at samples' surface, and the decarburization layer depths were about 100~ 150^m.Sample edge's microstructure mainly presenting ferrite morphology while center presenting fine and homogeneous sorbite microstructure. 1# sample decarburization layer morphology and microstructure appearance were shown in Fig. 4.

Fig. 3. (a) Small pit appearance; (b) Micro cracks appearance

Fig. 4. (a) decarburization layer appearance; (b) decarburization layer microstructure; (c) center microstructure

2.4. Hardness testing

Micro hardness testing was processed on 1#~3# samples, the average results were shown in tablel. From Table 1, it indicated that surface micro hardness were much lower than the center micro hardness, the difference reached 117HV. According to GB / T 1172-1999, the process of micro hardness conversion for rockwell hardness and strength were carried out. The results showed that, samples' surface rockwell hardness were much lower than the center rockwell hardness, the difference reached 12HRC. The phenomenon led to surface rockwell hardness not meet to technical requirement (32~39HRC). What's more, samples' surface strength converted were much lower than the center, the difference reached 254MPa.

Table 1. Hardness testing results.

Sample Location Average micro hardness result (HV) Rockwell hardness cnverted (HRC) Strength converted (MPa)

1# Surface 189.65 <20.0 <742

Center 306.16 32.0 996

2# Surface 217.07 <20.0 <742

Center 290.20 30.0 948

3# Surface 218.49 <20.0 <742

Center 299.68 31.5 984

2.5. Tensile testing

Tensile samples taken from bolts were prepared for performance test, the testing results were shown in Table 2. It indicated that tensile strength does not meet requirement(1040~1200MPa), yield strength met to requirement^ 785MPa), elongation met to requirement(^9%), percentage reduction in area met to requirement(^45%).

Table 2. Tensile performance results.

Sample Tensile strength (MPa) Yield strength (MPa) Alongation (%) Percentage reduction in area (%)

1# bolt 971 847 17.0 59.4

2#bolt 968 849 15.6 58.3

3#bolt 955 819 17.0 58.7

2.6. Dimension examination

Bolts dimensions were checked. The results showed that bolts dimensions (flat steel length, flat steel width, bolt root dimension) were small for the prescribed 95.2% ~ 96.2% , which does not meet technical specifications.

Table 3. Dimension examination results.

Sample Flat steel length(mm) Flat steel width(mm) Bolt root dimension(mm)

Measured value Requirement Measured value Requirement Measured value Requirement

1# bolt 26.70 17.40 22.86

2#bolt 26.60 28 17.28 18 22.82 24

3#bolt 26.72 17.42 22.84

3. Results and discussion

3.1. Failure mode

Three u-shaped bolts fracture originated from surface, showing line-source characteristics. Plat sections presenting typical fatigue striation morphologies, while final fracture zone presenting typical dimple feature. Therefore, u-shaped bolts' fracture properties were fatigue failure.

3.2. Failure process and cause

Fatigue fracture takes a large portion of failure cases. In generall, components fatigue fracture are relative with surface integrity, material fatigue resistance and structural integrity. The following parts were analyzed from these aspects.

Firstly, surface integrity. Surface integrity belongs to the physical category, including surface residual stress, surface roughness, surface microstructure and so on[3]. Several researchers study results show that surface integrity plays a crucial influence on the fatigue performance because of crack originating from components' surface or near surface [4-6]. This paper results showed that small pits and densely distributed micro cracks were found at bolts surface. By analysis that, micro cracks presented corrosive character, which generated at the surface etching procedure before painting. Small pits came from corrosive cracks mesh expansion. These densely distributed micro cracks appeared on the side surface of facture source, damaging surface integrity and inducing fatigue crack initiation. Densely distributed micro crack was the first reason for bolts' failure. What's more, this paper results showed that 100~150 u m decarburization layer were found at bolts surface. Many experimental studies have shown that the main factors affecting decarburization formation are: heat temperature, heat time, heat atmosphere [7, 8]. When furnace temperature too high or heat time too long or oxidation atmosphere at a high level, steel materials could decarburized. Because of surface carbon loss, bolts' decarburization layer strength were much lower than the center, the difference reached 254MPa. However, bolts' surface bear maximum load. Therefore, decarburization layer abnormal structure that further undermining surface integrity was the second reason for bolts' failure.

Secondly, material fatigue resistance. Bolt's material had low tensile strength, which did not meet technical requirement. High tempering temperature may be the cause of low strength. The low tensile strength was the third reason.

Thirdly, structural integrity. Component dimension is one part of structural integrity, which plays a significant role for regular service[9].This paper results showed that bolts dimensions (flat steel length, flat steel width, bolt root dimension) were small for the prescribed 95.2% ~ 96.2% , which does not meet technical specifications. Simply calculated, bolts' reduced size resulted in force area equivalent of 90% normal condition. In other words, bolts' average force increased up to 1.1 times of normal condition. Bolt's carrying capacity declined. Bolt's small dimension was the forth reason.

4. Conclusions

Three summary observations for u-shaped bolts failure can be made based on the results presented here.

• U-shaped bolts were fatigue fracture.

• U-shaped bolts' failure was related with densely distributed surface micro cracks, surface decarburization appearance, small dimension, and low strength of material.

• It is suggested that u-shaped bolts' surface integrity and manufacturing quality should be improved, and heat treatment process should be strictly controlled to enhance material's fatigue resistance.

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