Low-Cycle Dwell-Fatigue Life and Failure Mode of a Candidate Titanium Alloy Material TB19 for Full-Ocean-Depth Manned Cabin

WANG Fang,JIANG Zhe,CUI Wei-cheng

(Shanghai Engineering Research Center of Hadal Scicence and Technology,College of Marine Sciences,Shanghai Ocean University,Shanghai 201306,China)

0 Introduction

Deep-sea manned submersible plays an important role in ocean scientific research at deep sea space.Up to now,several 4 500-7 000 m level deep-sea submersibles have already been used to carry out routine scientific research(Li et al,2015)[1].And more and more researchers pay attention to full-ocean-depth manned submersibles with stronger ability for further exploring the deep sea trenches,which are now to be designed in several research centers(Cui et al,2015)[2].Designers of full-ocean-depth manned submersibles have continually strived to reduce the structural weight,by adopting some new ideas on design of manned cabin to increase the achievable range/speed/payload(e.g.Hawkes,2009[3];Taylor and Lawson,2009[4]),and proper material selection is a pivotal aspect hereinto(Cui et al,2015)[5].

Titanium alloy manned cabin is widely used for existing deep-sea manned submersibles.In the preliminary design stage of full-ocean-depth manned cabin conducted in Hadal Science and Technology Research Center of Shanghai Ocean University in China,the availability of high strength titanium alloys is considered as a top priority based on the successful application of Ti-6Al-4V ELI in other submersibles.However,due to the constraint of strength of Ti-6Al-4V ELI,if full-ocean-depth submersible is designed to have enough volume to take three occupants,manned cabin thickness will exceed current manufacture ability of most countries considering the thin-off amount and size of the sphere.Li et al(2015)[5]discussed the design of a full-ocean depth manned cabin with double intersecting spheres by finite element analysis and optimization theory.However,the connecting part of the two spheres may be difficult to construct for meeting all practical and theoretical requirements.Then the idea needs more investigation.Another choice is to use titanium alloy with higher strength level.The titanium alloy TB19 attracts designers’attention due to its extreme high strength of more than 1 200 MPa with other better performances such as relatively low density,corrosion resistance,high ductility,and ease of machining.It has been widely used in aerospace,marine,petrochemical and other engineering fields as an important structural material and functional material and its common properties have been introduced in literature(Liu et al,2010)[5].

In this paper,the low-cycle dwell-fatigue life and failure mode of TB19 are investigated and compared with that of Ti-6Al-4V ELI which has been practically proved to be satisfactory and experimentally investigated(Wang and Cui,2015)[6].Related experiments on TB19 with a uniform circular cross section diameter of 6mm are conducted in laboratory environment when maximum stress of load cycles approximates to yield strength.The failure modes and associated fractographic features have also been examined in detail.The results can be referred for material evaluation of full-ocean depth manned cabin.

1 Material and experimental procedure

TB19 is a high strength titanium alloy with nominal composition of Ti-3Al-5Mo-5V-4Cr-2Zr(%),which synthesizes the advantages of two phase titanium alloy and beta-annealed titanium alloy and widely used in aerospace,ships,petrochemical and other industries(Liu et al,2010)[5].In order to obtain basic data of mechanical properties for the alloy,the tensile specimens from the same forged plate were machined along two directions.And the tests were conducted in lab air,at room temperature,using a closed-loop servo hydraulic test frame.The mechanical properties of TB19 tested in the current research are given in Tab.1.

Tab.1 Mechanical properties of TB19

In order to further examine the microstructure and the local texture of the material,the fractured face of tensile specimen is characterized by using the Electron Back Scattered Diffraction(EBSD)technique using a Quanta650FEG scanning electron microscope.The lowmagnification and higher-magnification SEM images of TB19 tensile specimen fracture surface are respectively shown in Fig.1(a)and(b).The low magnification image shows relatively coarse grain size.The higher-magnification image shows fine intercrystalline dimples and transgranular dimples respectively in intergranular regions and transgranular regions.

Fig.1 SEM image of TB19 fracture surface for tensile specimen(a)Low-magnification image;(b)Higher-magnification image

Fig.2 A schematic representation of dwell-fatigue load pattern in experiments

A schematic representation of the dwell-fatigue load pattern used in the experiments is shown in Fig.2.A load ratio of 0 is applied in all the experiments.There will be a load held period at peak stress in each cycle,which is denoted by dwell time Th.This is for simulating the service time of the manned cabin in the predetermined location of deep sea.However,considering the cost of the experiments in laboratory,the dwell time in actual operation of several hours could not be realized in experiments.The present experiments only used for explanation analysis of the dwell-fatigue phenomenon.Dwell-fatigue tests were carried out in laboratory environment on two cylindrical smooth specimens(Kt=1)with 6 mm in diameter,and 46 mm in gauge length for hot-rolled thick plate specimens machined from the same forged plate,using a polished surface.The tests were conducted using a testing machine specially designed for dwell fatigue with a wide range of load ratios and frequencies.Other requirements for loading are in agreement with the standard practice for strain-controlled fatigue testing(ASTM).Each load cycle includes loading-dwell-unloading process with axial force in tension and a zero load ratio.The applied maximum stresses are equal to 0.975σyand 0.959σyrespectively.The dwell period is 10 min and the load/unload period is 0.5 s.The dwell-fatigue tests were conducted up to failure in a stress-controlled mode.The strain accumulations and rupture life are recorded.In order to evaluate the properties of TB19,the test results are compared with those of Ti-6Al-4V ELI,which were reported by the authors(Wang and Cui,2015)[6].Optical and fractographic aspects of the fractured surface will be used to analyze the failure mode.

2 Test results and discussion

The test conditions are further described in Tab.2 with the corresponding specimen num-bers,maximum stress level,and dwell time.And the rupture life of each specimen is recorded.The results of TB19 can be compared with those of Ti-6Al-4VELI under similar stress levels and dwell time,tested using the same specimen size and under the same condition,which are together listed in Tab.2.To illustrate the failure process,the strain accumulation with cycles for specimen No.1-1 is shown in Fig.3(a),which exhibits three stages with an initial nearly instantaneous occurrence of elastic and perhaps plastic strain,followed by the gradual accumulation of strain,and tertiary rupture stage.The strain accumulation should comprise those from cyclic and dwell periods.Fig.3(b)depicts the strain from dwell time.It can be noted that the strain due to the dwell time will decrease with loading process to a negligible level,however,it is a relatively small amount on the whole comparing to the total strain accumulation.

Tab.2 Summary of test results on dwell fatigue life of TB19 and TC4 ELI(cycles to rupture)

Fig.3 Strain versus cycles behavior(a)Total strain versus cycles during dwell-fatigue process showing three stages;(b)Strain due to dwell time in each cycle

The comparison on strain versus cycle behavior between TB19 and Ti-6Al-4VELI can be seen in Fig.4.The comparison of results at two different peak stresses indicates that the rupture time of both candidate materials is lowered when the maximum loading stress is increased by just 2%.Both the strain accumulation rate during the whole deformation process of the TB19 specimen and total rupture strain are obviously lower than those of Ti-6Al-4VELI.The rupture time of TB19 is also more favorable,but it decreases more rapidly with increase of stress level.Fig.5 gives the comparison on rupture time of the total specimens for two materials.If linear tendency of stress-cycles curve for the two materials is assumed(which can be regarded as reasonable when the stress level is located in a relatively small region),TB19 will show a more advantageous property in the viewpoint of service life cycle comparing to Ti-6Al-4V ELI if they are chosen for deep-sea manned cabin materials.

Fig.4 Comparison on strain versus cycles behavior(Th=10 min)between TB19 and Ti-6Al-4V ELI

Fig.5 Comparison on normalized stress versus time to failure(Th=10 min)between TB19 and Ti-6Al-4V ELI

SEM analysis for TB19 on fracture surfaces is performed according to standard SEM operation procedure by putting the sample into SEM vacuum chamber to magnify observations at room temperature 23℃±2℃ and relative humidity 23℃±5℃.The same analysis has been conducted for Ti-6Al-4V ELI in the former research.The SEM observation is used to characterize the crystallography of the fracture surface of the dwell-fatigue specimen,which will enable the investigation of the relation between the load pattern and the fatigue crack nucleation and propagation characteristics in viewpoint of morphological and microstructural features.

The SEM images show similar fracture surface characteristics for all TB19 specimens.Taking the specimen failed under load condition of σmax=1 170 MPa and Th=10 min as an example,the low-magnification of specimen fracture surfaces(two parts denoted by A and B)is shown in Fig.6 comparing with those of Ti-6Al-4V ELI obtained under similar loading condition.In macroscopic observation,the fracture appearances of both materials are characterized by approximate brittle facets with very little necking.And three sub-surfaces can be separated from the fracture surface,which are origin area,extending area and fast fracture area,respectively representing the crack initiation,stable propagation and unstable propagation periods.However,the difference between the origin area and the extending area of TB19 is not as apparent as that of Ti-6Al-4V ELI.The crack is initiated from the specimen surface,which is not surprising considering the standard specimen treatment process.Under the high stress condition in the present experiments,the dwell-fatigue crack initiation sub-surface is relatively faceted with little ductility,while the evidence of cyclic growth with higher local plasticity characterized by wavy fatigue striation can be seen in stable crack growth period.The fracture surfaces of TB19 in the present study are different from those of other titanium alloys reported by some researchers when they study the difference between the dwell-fatigue and the nor-mal fatigue(e.g.Shen et al,2004[7];Sinha et al,2004[8]).It is not difficult to be understood as the fracture modes may be affected by many factors including load stress,load ratio,surface treatment process of the specimen and test condition.In the present work,the applied maximum stress in the dwell-fatigue load pattern is very high considering the stress concentration area in the manned cabin and relatively low cycle life design.

Fig.6 Low-magnification SEM images of the specimen fracture surfaces(1)Ti-6Al-4V ELI(σmax/σy=0.975 42,Th=10 min)and(2)TB19(σmax/σy==0.976 47,Th=10 min)

The high-magnification SEM images of fracture sub-surfaces of the investigated TB19 specimen are illustrated in Fig.7,which can be compared with those of Ti-6Al-4V ELI shown in Fig.8.The representing local areas in six sub-surfaces denoted by A1,A2,A3,B1,B2 and B3 for two material specimens are analyzed.Intergranular fracture together with dimples in crystal plane dominate in A1,A2,B1 and B2 regions of TB19 specimen,which combines brittle and ductile fracture characteristics,while A3 and B3 regions exhibit transgranular cleavage with dimples.Its high-magnification fracture surface image is different from that of Ti-6Al-4V ELI studied in previous research.The total surfaces of Ti-6Al-4V ELI specimen show mixed rupture with both ductile fracture characteristics with shear dimples in the observed regions of A1 and B1 sub-surfaces and brittle fracture characteristics with transgranular cleavage in the observed regions of A3 and B2 sub-surfaces.However,the ductile characteristic of the material is not significant under the test conditions with relatively small shear dimples.It should be noted that the slip bands and void can be observed in A2 and B2 of the stable crack growth area of Ti-6Al-4V ELI specimen,showing that it has transgranular creep fracture characteristics,which is quite different from that of normal fatigue.However,creep fracture characteristics of TB19 exist but are not very significant without obvious slip bands and voids observed.It can be used for explaining more satisfactory failure time tendency of TB19 than Ti-6Al-4V ELI.

Then the SEM analysis of the specimen explains the unique feature of room temperature dwell-fatigue of the candidate materials,and supports the idea that the dwell time effect should be considered when estimating the service life of the manned cabin.However,in the present study,the comparisons of normal fatigue and dwell fatigue as well as normal creep conditions have not been conducted for TB19,which will be included in future’s work aiming at obtaining the rupture lives under three conditions to see if there will be life debit considering cyclic dwell time,how is the magnitude and if the crack initiation mode under dwell-fatigue condition is similar to that under normal creep condition or normal fatigue condition.

Fig.7 High-magnification SEM image of the TB19 specimen fracture surface(σmax/σy=0.976 47,Th=10 min)

Fig.8 High-magnification SEM image of the Ti-6Al-4V ELI specimen fracture surface(σmax/σy=0.975 42,Th=10 min)

3 Summary and conclusions

The low-cycle dwell-fatigue life and failure mode of candidate material for full-oceandepth manned submersible,TB19,are investigated by experiments,which are compared with those of Ti-6Al-4V ELI.Experimental results show that the rupture time of TB19 specimen under dwell-fatigue condition is lowered when the maximum loading stress is increased by just 2%in high stress region.Both the strain accumulation rate during the whole deformation process of the TB19 specimen and total rupture strain are obviously lower than those of Ti-6Al-4VELI.The rupture time of TB19 is also more favorable,but it decreases more rapidly with increase of stress level.TB19 will show a more advantageous property in the viewpoint of service life cycle comparing to Ti-6Al-4V ELI if they are chosen for deep-sea manned cabin materials.

SEM analysis for TB19 on fracture surfaces is performed to characterize the crystallography of the fracture surface of the dwell-fatigue specimen and compared with those of Ti-6Al-4V ELI.In macroscopic observation,the fracture appearances are characterized by approximate brittle facets with very little necking and can be separated by three sub-surfaces.The high-magnification SEM images of fracture sub-surfaces show mixed rupture with both ductile fracture characteristics with shear dimples.It has creep fracture characteristics which are quite different from that of normal fatigue,but not so significant as Ti-6Al-4V ELI.

It appears that the candidate material,TB19,shows a better anti-creep property under the test condition but the cyclic damage mode should be differentiated from normal fatigue damage.However,most of current service life assessment methods for deep-sea manned cabin are based on normal low-cycle fatigue calculation.Further research should be conducted to obtain the life debit considering cyclic dwell time effect and design working stress.

The studies can provide material evaluation foundation for full-ocean-depth manned cabin.

Acknowledgments

This work is supported by the State Key Program of National Natural Science of China‘Structural Reliability Analysis on the Spherical Hull of Deepsea Manned Submersibles’(Grant No.51439004)and the General Program of National Natural Science Foundation of China‘Study on the Design and Life Calculation Method for the Maraging Steel Sphere of Fullocean-depth Manned Submersible’(Grant No.51679133).

[1]Li H,Wang F,Cui W C.Exploration on structural design of double intersecting spheres pressure hull of full ocean depth manned submersible[J].Journal of Ship Mechanics,2017,21(9):1160-1169.

[2]Cui W C,Wang F,Pan B B,Hu Y,Du Q H.Chapter 1 Issues to be solved in the design,manufacture and maintenance of a full ocean depth manned cabin,in advances in engineering research[M].Volume 11,Editors:Petrova V M,Nova Science Publishers,2015.

[3]Hawkes G.The old arguments of manned versus unmanned systems are about to become irrelevant:New technologies are game changers[J].Marine Technology Society Journal,2009,43(5):164-168.

[4]Taylor L,Lawson T.Project deepsearch:An innovative solution for accessing the oceans[J].Marine Technology Society Journal,2009,43(5):169-177.

[5]Liu X L,Yang R,Yan Y Y,Wang N Y,Song D J.Structural properties of TB19 heavy plate welded by electron beam welding[J].Chinese Journal of Nonferrous Metals,2010,20(s1):748-752.

[6]Wang F,Cui W.Experimental investigation on dwell-fatigue property of Ti-6Al-4V ELI used in deep-sea manned cabin[J].Materials Science&Engineering A,2015,642(905):136-141.

[7]Shen W,Soboyejo W O,Soboyejo A B O.An investigation on fatigue and dwell-fatigue crack growth in Ti-6Al-2Sn-4Zr-2Mo-0.1Si[J].Mechanics of Materials,2004,36(s1-2):117-140.

[8]Sinha V,Mills M J,Williams J C.Understanding the contributions of normal-fatigue and static loading to the dwell fatigue in a near-alpha Titanium Alloy[J].Metallurgical&Materials Transactions A,2004,35(35):3141-3148.