KHOA HOC & CONG NGHE
Mpt so phirang phap danh gia sir pha hoai moi cua lien ket han trong ket cau thep
Tom tat
Pha hoai mdi la mot trong nhiing nguyen nhan pha hoai phd bien doi vdi ket cau thep sii dung lien ket han. Nd thudng xay ra ddt ngot nSn khd dti bdo.
Mat khac, cdc quy dinh ve thiet ke mdi trong cac tai lieu va tifiu chuan Viet Nam difa ra cdn kha ngan gon, sg hitic va chiia giai thich co sdt ly thuyet ap dung.
Bdi viet de cap den mot so phiiong phap danh gia pha hoai mdi cua Uen ket han theo quy dinh cua tieu chuan Chau Au.
Abstract
Fatigue failure is one ofthe primary reasons for the failure of welded steel structures.
These failures often occur quite suddenly so it is difficuh to predict. On the other hand, the current fatigue design theory and codes in Vietnamese standards simplify this phenomenon and don't give the theoretical explanation. The article refers to some fatigue assessment methods of welded steel structures according to the Eurocode.
PCS. TS. Phgm Minh Hd
Cyc Gidm dinh Nhd nu&c ve chdl lit^ig cong trinh XD, Bo Xay d\nig
Email, [email protected] ThS. DSn Qudc Cirang Trung tdm Tin hpc mg dijng Email' danquocciiong@gmail com
PGS.TS. Pham Minh ThS. Dan Quoc C
1. Dat van de
Theo ASTM (American Society for Testing and Materialsl:
"Hipn tugng mdi Id qua trinh thay ddi k i t d u Idu ddi d vj tri cyc bb dien ra lien tyc xdy ra trong mbt vgt lidu chju cac dilu ki§nl4c dpng gdy ra sy thay ddi lap cua ung s u i t va bien dang d tgi m^
sd d i l m va nd cd the gdy ra cdc v i t nut Idn hogc phd hogi hain todn sau mdf sd vdng lap nhat djnh". Cdn chiing ta coi phS ho^
mdi Id dgng phd hogi dot ngpt, kem theo cdc v i t nut bgc tnrng, xdy ra khi vdt lipu thep dang Idm vipc trong gidi hgn ddn ha.
Hipn tugng mdi Id mdt trong nhOng nguydn nhdn chinh phd hogi k i t cau thep s u dyng lien k i t hdn. Tuy nhidn, cdc djnh ve thidt k l mdi trong cdc tai lieu va tieu chudn Vipt Nam ( _ ra cdn khd ngin gpn, so luge va chua gidi thfch co sd ly thuyft dp dyng. Hidn nay, cd nhilu phuang phdp dugc de cgp dldSijJ!
gia mbi tuy nhien hidu qua danh gia mbi cua cdc phuong ph^' Id khac nhau. Cdc phuang phdp danh gia mdi dugc de cdp mi cdch chi tilt de lam rd dugc hipu qua ddnh gid mdi.
2. Cac phirang phdp danh gid mdi 2,1. Cac yiu t6 anh hwimg
Cdc tai trong gay ra hidn tugng mbi Id cdc tai trpng c6,L trj thay ddi theo thdi gian d i n d i n cac irng s u i t trong clu w k i t c l u cung thay ddi theo. Cdc tdi trpng gdy mdi nhu tai Irong do phuang tidn giao thdng gdy ra, tai trpng do sy thay dli Sp Iyc, tai trong rung ddng, tai trgng do s y thay ddi nhidt d$, tSi trgng c l u tryc, tdi trgng do sdng n u d c . . . Trong sudt qud trinh sii dyng, cac lien k i t han thudng phai chju cdc tdi trpng cb bl^
dp khbng ddi hogc thay ddi theo thdi gian (Hinh 1). Tuy nhien, bidn dd ung s u i t (Ao) thay ddi do cdc tai trpng frdn gdy ra oS the dugc bieu dien nhu Id mpt hay nhilu bien dp ung suit khfc nhau.
Thdng sd quan trgng nhat dnh hudng d i n kha ndng chju m^
cua lien k i t hdn Id gia tn bien dp irng s u i t Ao, sau mdi vdng Ifti mbt v i t nut se phat t n l n ldn han. He sd phan xu-ng cCia iJi^
suat R la ti sd cCia ling s u i t Idn nhdt ddi vdi ling suit nhb nhat bieu thj gia trj ung s u i t trung binh <5^ vd Id thdng sd thit' hai Suti hudng den kha nang chju mdt cua lien k i t hdn. Anh hudng ciS dng s u i t tmng binh a^ dugc bd qua trong trong thief ke cl#
mdi cua lien ket hdn do s y tdn tgi cua ung suit d u cao. Hi§n tugng mdi xay ra ddi vdi lien k i t hdn d gid trj img suit nhfi hon gidi han chay cOa vgt lidu.
Sd vdng Idp mdt cua mdf chi tilt (N) la tdng sd vdng Igp n™
trong giai dogn hinh thdnh vet nut (N,) vd giai doan phdt trig v i t ndt (Np). Cudng 66 mdi cOa lidn ket hdn phy thude tryc tiap vdo gid tri ung s u i t tgp tmng do anh hudng cua dang hlnh hpc cOa lien k i t hdn, vj b-f xult hipn v i t niit, khuylt tat hdn vd linfl
40
TAP CHi KHOA HOC K I E N T R O C - X A Y D I / N Gliirti h
inland It il^ilit, biend^^>'u
ggioibi ' iniiaiitiii
aHCftuj-Sloo
SUBltolf*
ymi*ii.
idssiiJi
Thdi gian
R > 0 I
Tai trgng moi c d bien dd khdng ddi | Tdi frgng moi c6 bi6n dd fhay ddj
Hinh 1. Cac loai tai trgng gay ra hien tu'dng mdi
^ ^
0- 1
' f l *
tf I 0? 4.
*
Ij
—7
' ' 1["
1 1
Chib4tdi,9cphanloaim6i / Gitjihan moi voi
W ;
"F
Si trpng CO bien do Miong doi
- |!
• ,, : . I ' l l
.4_. ' 1 i G|*| hen m6i
s4-^UIIk-xM-l-M4JI
5 o'' 1
^ ^ '
".-^^'^.^^^ ', '—H pT
^S;'^^! '
tt
ID"
II
la.'.:,- lUat d u Khi dgng hlnh hge cOa lien k i t han thay ddi bdi
°.r. •:ac thanh phan dugc len k i t bd xung se tam cho dp eung .^,,.. ai mbt sd vj tri cyc bp thay ddi, dan d i n s u tap trung img
^;°j ,iuat va tai nhu'ng diem cd gid trj ling s u i t tdp trung Idn se ' ' ' ' a v| tri xay ra pha hogi mdi. NhiJng khuylt tdt hdn ddng
'ai trb nhu nhung vet nirt mfii ban d i u va Idm cho giai tffl^f'Ioan hlnh thdnh vet nirt mdi dugc riit ngan di ddng ke.
•ai' 3o dd, qud trinh phd hogi mdi cua lidn k i t hdn ehi xet den
;'i.'. rong giai dogn phat friln vet nirt moi va y i u td eudng dd -:.'/dt li#u dugc bd qua do nhu'ng dnh hudng cua khuylt -,---' gt hdn
Hinh 2. Bieu do S-N stress), ung s u i t nhiet (hot spot stress) hay irng s u i t tai mep dudng hdn (effective notch stress). Bilu d l S-N (dugc gpi Id b i l u db Wohler) bilu thj mdi quan hd giOa bien dp irng s u i t (Ao) va sd vbng Igp gay phd hogi mdi (N) theo bieu thirc sau:
-(a=
iogiAa^) - mlog{A(T) (1)nf
2.2. S i l u rfd S-NH0,c« = ^ . ^ ^ ]
(2)09 De ddnh gid mbi cua lidn ket hdn cb the dimg phuang
^ilbhdp ddnh gtd theo bieu dd S-N vd ca hpc phd huy
^huang phdp s u dyng b i l u dd S-N Id phuang phap dugc 3U dyng phd bten d y a trdn ca s d xdc djnh gid trj bien i'^'p^ img s u i t tgi v| trt xdy ra phd hoai mdi vd so sdnh vdi
^ % e u do S-N d l xdc ^ n h sd vdng Idp mdi cOa k i t d u chju f'i^'iaugc (Hinh 2). Gid trj i>ng s u i t tgi vj tri xdy ra phdp hogi W'mdi dugc xdc dinh theo irng s u i t danh nghTa (nominal
0'
Trong dd"
N Id tdng sd vbng lap mdi cua k i t c l u ; A Q Id bien do dng suit;
No =2.10^ vdng lap mdt;
AOc Id gid trj phdn Idp mdi dugc xac djnh d N^;
m Id hdng sd vdt lieu.
S 6 18-2015
41
KHOA HOC & CONG NGHE
Ting irng sulit thirc tai m4p
ifng suat lie mSt Ou&ng ttiSng
A M
h Z
F I
-'•^^S):
M6p d y a n g han
a) Xac ^ n h uTng suat danh nghia siir d^ng phu'ang phap ngoai suy
Hinh 3. Xac djnh uTng suat danh nghTa
FAT.Aff,
b ] Xac d|nh i h i g suat danh nghla theo ly t h u y i t cd hoc
Cucmg d$ mdi a N=2xlO*
GacH hgn mdi, Agp Xic dmh or N^SxlO^ vdng 1:
Gidi hyi moi tich 11% Aot
S6 vong l|p mdi, N 2.3. Cac phwang phip danh gia mdi 2.3.1. Phuong phdp ipng s u i t danh nghTa Phuang phdp irng suit danh nghla (Nominal stress method) la phuang phdp phd biln vd dan gidn nhit cho vipc ddnh gid mdi cua lien k i t hdn. Phuang phdp ndy dya trdn ca sd ung suit danh nghTa Id gid tri irng suat tmng binh do ngogi Iyc tdc dyng. U'ng sudt danh nghTa khdng bao gdm gid frj ung s u i t tgp tmng vd cb the dugc tinh theo ly thuyit ca hpc ket cau hogc theo phuang phap ngogi suy (mg suit be mgt (Hinh 3). Phuang phdp img s u i t danh nghTa dugc quy dinh t^ong nhieu tteu chuan thilt k l nhu BS7608-1993, EN1993-1-9,... Khi sir dyng phuang phdp ndy, cdc chi tiet khao sdt phdi cd cdc yeu td v l hinh dgng, tai trpng tdc ddng tuang t y vdi chi tilt da dugc phdn logt trong cdc tteu chuin hay tdi lieu hudng dSn (Hinh 5). Cdc y i u to gdy tap tnjng irng s u i t dugc bb qua vi dd dugc bao g i m trong bieu dd S-N. Sd vbng lap mdl cua chi tilt se dugc xdc dinh theo bieu dd S-N (Hinh 4).
Hinh 4 . Bieu do S-N theo iimg suat danh nghTa [ 4 ]
2.3.2. Phuang phap irng s u i t nhidt
Phuang phap irng s u i t nhipt (Hot spot stress method) dugc sir dyng de ddnh gid mbi ciia lien k i t hdn t ^ nhu'ng trudng hgp lien kef cd dang hinh hge vd tdi ti98 tac dyng phirc tap ma phuang phap irng s u i t danh n ^ khdng danh gia dugc. Phuang phdp dya trdn co sd ^ suat nhiet tgi diem tdi hgn (mdp dudng hdn) id irng suffl tap tmng da xdt den dnh hudng cOa dgng hlnh h ^ iw d u do do viec phdn logi chi t i l t vd sd lugng bilu do S^<
diing d l ddnh gtd mdt gidm di dang k l (Bdng 1).
Cfng s u i t nhidt thudng dugc xdc djnh tit mb hinh phan tCr huu han theo phuang phdp ngogi suy ung su^
l>ng s u i t tgi mep dudng hdn (Oto,) bao gdm: U'ng sw*
phdp tuyen (o^f-jj, (rng s u i t udn do dgng hinh hpc m tiet (Oben). Lrng s u i t phi h j y i n do hinh dgng cyc bd oia dudng hdn (Op^^). Nhu vay, irng s u i t nhipt (0|jss) diffc xdc djnh nhu sau:
42
TAP CH KHOA HOC K I E N T R U C - X A Y D U N Gl d n g 1. Phan I d ^ chi t i l t dira tren co- s d phiro-ng phdp umg s u i t danh nghTa va p h u v n g phdp ipng s u i t nhiet [4]
Phuong phap irng suit danh nghla U
7
L6p FAT C80 C71
r
^ .
irtdtii C63 C56
C71
C80
Chi tiet ket c i u tl^n L < 50mm
50 < L < 80mm 8 0 < L < 100mm i > 100mm i . > 100mm
« < 4 5 °
r > 150mm
- ^ ^
< % . " =
= ^
•C2^' r - v '
" ^ ^ ' ' ^ i —
Phuang phap irng suat nhiet Chi tiet ket cau han
i ^ .
Bang 2. Xac djnh irng s u i t nhipt cho md hinh phan tip dgc s u di^ng lud-i thd Md hinh phan tip dgc
Kich thude phln td Cdc diem ngogi suy Ngogi suy tuyin tinh
Ludi phan tir thd Loai a
t X t, max t X w 0.5tval,5t
^h«=1-5(T,s,-0.5ff,,, (4)
Loai b 10 X 10mm 5 va 15mm
<';„=1.5'',™,-0.5<T,„(5) Bdng 3. Xdc djnh d'ng s u i t nhidt cho md hinh phan tu* dgc sir dyng l u ^ i mjn
Mo hinh phan t u dgc Kich thude phan tu Cdc diem ngogi suy Ngogi suy tuyin tinh
Ngogi suy bde hai
Ludi phan tip mjn Loai a
S 0.4t X t; < 0.41 X w/2 0.4tvd1.0t (j,„=1.67(7o4,-0.67(T,o, (6)
£T^=2.52(To„-2.24(To9,+0.72(T,,, (7)
Logi b
< 4 X 4mm 4,8 va 12mm
lTy,55 = S C T ^ ^ ^ - 3 t 7 B „ „ + CT|2„„ (8) Ghi chii: w Id khodng cdch gtua hai mdp dudng hdn ciia tam bdn md dugc lidn kef
Hinh 5. Cdc thanh ph^n u'ng suat phan b o b i s n g tiet dien tafm d mep du'dng han
l>ng s u i t nhidt dugc phan thdnh hai loai (a) vd (b) xdc djnh theo phuang phap^phan t u hij'u han si> dung theo sy dnh hudng cua chieu ddy t i l t didn d i n sy phdn phuang phdp ngoai suy tuyin tinh va bac hai irng s u i t bd irng s u i t (Hinh 5). Gid trj ung s u i t nhidt cd t h i dugc b l mat sir dyng mb hinh phln tCr ddc vdi ludi mjn hoac xdc djnh theo bdng tra hd sd tdp trung d-nq s u i t cua mgt ludi fhb dugc xdc ^mh nhu mmh hga trong hinh 6 ,h]nh sd chi t i i t xdc djnh, theo phuang phdp phdn tif hiru hgn 7, bdng 2 vd bang 3
hose theo phijcng phip do bi^n d,ng. Cfng s u i t nhi^t 3 ^ ^ ^ ^ |.p ^ . . ^ . ^ ^^. , . j , ^.^^ |^., ^ , ^ ^ ^ ^ ^ ^ , ^ ^^^
S6 18 - 2015 4 3
KHOA HOC & C O N G N G H E
Cac ^em ngoai suy
M6 hinh ludi tho Hinh 6. Phu'dng phap ngoai suy tuyen tinh
• Lo?i a ' Loai b
U'ng suat mep ifng su^ phi tuy^n
D'ng suit nhigt Cac dilm ngoai suy
Lfng suat mep U'ng suit phi tuyen
Lfng suat nhi#t Cac diem ngoai suy
Loai b Hinh 7. Phu'dng phap ngoai suy bac hai
1-E405 l.E+06 2.E^06 S6 vbng Jap moi, [N]
Hinh 8. Bieu do S-N theo bien do u'ng suat nhiet [ 3 ]
4 4 TAP CHI KHOA HOC KIEN TRUC • XAY D U U G
I
Hinh 9. Vj tri xac djnh ban Itinh hihi Mgu tai mep va chan du'dng han [2]
V
=0 •••-. » l \; p,=p + si>' 1 PfT 'nun",!,
~ \ .
/ ; l" ;
i t Hlnh 10. Khai niein ve vCing tinh toan theo Neuber [6]
IS-KW l.E+06 2JE-KI6 sd vdng l | p mdi p>']
Hinh 1 1 . Bieu dd S-N [ 3 ]
>^!0
dya trdn gid tri bidn dd ung s u i t nhipt vd dugc so sanh vdi b i l u do S-tsl tuang ung (Hinh 8).
2.3.3. Phuang phdp irng s u i t d mdp dudng hdn C/'ng s u i t d mdp dudng hdn bao gdm tat cd cdc dnh hudng gia tdng i>ng s u i t tai vi tri mdp hope chan dudng hdn. Id tdng ciia ung s u i t nhipt (Oj,^^) vd ung s u i t phi tuyin do hinh d^ng cyc bp cua dudng hdn (Opeak)- ^ ^ ^ ddnh gtd mdi dya trdn co sd danh gtd irng suat d mep dudng han t^m gpi la phuang phdp ung s u i t d mep dudng han (Effective notch stress method). Phuang phdp
ndy dugc gidi thieu i l n dau tten bdt Radaj vd Sonsino [5] bdng each xdc iTinh gid frj irng s u i t tmng binh tai vung finh todn (micro-support) theo quy tdc Neuber vdi bdn kinh hi>u hidu pf =1mm ddi vdi t i m cd chilu day Idn han 5mm (Hinh 9). Bdn kfnh hO'U hidu dugc xdc djnh vdt trudng hgp gid dinh b i t Igi nhat Id cd p = 6, tuy nhien ddi vdl cac chi tilt lidn kef hdn gia frj bdn kinh cd s y thay ddi Idn nen trong tnj'dng hgp chilu ddt cua cua v i t n i i i frong viing tinh toan (p") md rgng den 0,4mm vdi hd sd rdng budc (s) Id 2,5 doi vdi k i t cau thdp thi gid tn bdn kinh hO-u hidu cd gid frj Id pf = 1 mm (Hinh 10)
SO 18-2015 4 5
KHOA HOC & CONG NGHE
Phucng phap
Phuo'ng phap Lpng s u i t danh nghTa
Phucng phap ung s u i t nhiet
Phuang phap ung s u i t d mep dudng hi
Uu diem
- De tinh loan
- Cb cac kit qua mlu thyc nghiem de so sanh
- Cb sdn cong thuc xac dinh - Co nhtlu chi tilt duac phan ioai trong cac tieu chuin - Phil hgp cho viec danh gia vet nirt moi xuat hien d mep va chan dudng han
- Co the sic dung mb hinh phan tu huu han de xac dmh ung suit nhiet
- Dp chinh xac b mijc chip nhan dugc
- Tinh toan dugc cho ddi vdi cac chi tilt cb hinh dang phirc tap
- Phil hgp vdi viec danh gia m cho tm'dng hgp vet nut moi a hien d chan va mep dudng lij chi vdi mpt bieu do S-N - Do chinh xac cao
N h u g c diem
- Phy thuoc vao chi tiet duoc phan loai
- Gidl han khi ap dung vdi chi tiet cb hinh dang va tai trong tac dung phirc tap
- Do chinh xac khbng cao
- Phu thubc vao phan mem xac dinh theo phuang phap phln tu huu han
- Khbng tinh toan dugc dbi vdi trubng hgp v i t nift mbi xult hien a chan du'dng han
- Chf ap dung dugc khi si> diin vbi phln mem phan ti> hiiu ha - Phu thoe vao mat dp lydi
Danh gia moi theo phuong phap irng suat a mep dubng han duoc d l cap theo hubng dan cua IIW su dyng bieu db S-N cb FAT225 (Hlnh 11) de danh gia mbi cho cac chi tiet cb hinh dang va tai trong tac dung bat ky. Tuy nhien, de xac dinh du'ac ung suat mep du'dng han c i n su dung cac phan m i m phan lu' hu'u han de mb phbng dudi dang mb hinh 3D hoac 2D. De dam bao do chinh xac cua gia tr| ung suit, mat do ludi tai vung tinh loan cung dugc chia vdl mat dp ludi mjn dam bao gia tri trong bang 4 Bang 4. L u d i phan tir xac djnh theo phuang phap phan tir huu han - Fricke
Loai phan tir Quadratic Linear
Gia trj kich thucc tucng doi
< 0.25/7,
<0 15p,
Gia tri kich thucc doi vdi ban kinh hij'u hieu
pC= 1mm
<0.25
<0 15
2.4. So sanh cac phuang phap danh gia moi Qua viec phan lich cac phuang phap danh gia si;i hoai moi cua lien k i t han trong ket cau thep va kit I vbi cac ket qua danh gia sy pha hoai mbi ciia lien kell chiu tai trong lap [1] de riit ra cac so sanh nhu bang 5
Cb nhilu phucng phap banh gia su pha hoai moii k i t c l u thep sic dung lien k i t han Mdi phuang phapi CO nhO'ng uu nhugc dibm nhit dmh. Phuang phap i s u i t a mep du'dng han cho phep ap dung cac phan m phan tu hO'U han nen cb do chinh xac va db tin cayca
Can thuc hien cac thi nghiem mbi tren cac mo h thuc te de danh gia k i t qua thuc nghiem va so sanh k i t qua thuc hien bang phuang phap sd.
Can tilp tuc nghien cuu bo xung cac quy dtnh ve^
danh gia mbt ciia k i t c l u thep sir dyng lien k i t han de dung danh gia trong thuc t l . / .
Phan bien: PGS.TS. Vu QuocA T * i IMu tham khdo
Ddn Quoc CUdng "Nghiii kit hdn chiu ldi Irong lap', Hd Noi. 2014
Uphd hoai mdi cua iiet vdn thac si. DH Kien IT
A. Hobbacher. 'Recommendations for fatigue design of welded joints and components'. International Institute of Welding December 2008
W. Fncke, 'Guideline for the fatigue assessment by notch amiysjs for welded structures". The International ^
Institute of Welding 2010. '^^
European Committee for Standardization, 'Eurocodel Design of Steel Structures - Pari 1-9: Fatigue'. Brusidi.
May 2005-
D. Radaj and CM Sonsino, "Fatigue assessment of yA joints by local approaches', Woodheadpublishing. IVH Neuber H, "Theory of Notch Stresses', Ann Arbor Edwards, 1946.
