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ISSN 2229 – 5046
AN INVENTORY MODEL WITH WEIBULL DISTRIBUTION DETERIORATION
UNDER QUADRADIC DEMAND RATE USING PARTIAL BACKLOGGING
R. BABU KRISHNARAJ*
Associate Professor, Department of Mathematics
Hindusthan College of Arts & Science, Coimbatore – 641 028, Tamil Nadu, India.
V. NAGARANI
M. Phil (FT) Research Scholar, Department of Mathematics
Hindusthan College of Arts & Science, Coimbatore – 641 028, Tamil Nadu, India.
(Received On: 21-08-15; Revised & Accepted On: 16-09-15)
ABSTRACT
M
any Researchers have developed an inventory model to maximize the profit or to minimize the total cost for deteriorating items with respect to time. Deterioration in each product cannot be completely avoided and the rate of deterioration for each product will vary. We have studied an inventory model for two parameter Weibull distribution deterioration with quadratic demand rate, in which shortages are allowed and are partially backlogged and assumed that the backlogging rate is dependent on the length of the waiting time for the next replenishment. The results were described using numerical examples and sensitivity analysis.Keywords: Deteriorating products, partial backlogging, quadratic demand, shortage, weibull distribution and time
varying holding cost.
INTRODUCTION
The inventory control problem arises when it becomes necessary to create a stock of material resources or commodities with the purpose of meeting the demand within a given time span (finite or infinite). The challenge in any inventory control problem is to determine the quantity of products to be ordered and the moment for placing the order, which both affect the amount of the costs. Solution regarding the size of an order and the moment for its placement can be based upon the minimization of the corresponding overall costs function. The total costs of an inventory management system can be described as a function of its primary components as follows: Total cost of a inventory management system = Acquisition cost + Ordering cost + Holding cost + Deficiency losses. Recently R.Amutha and Dr.E.Chandrasekaran was developed the inventory model with weibull distribution deterioration and time-varying demand. In this paper, we have studied an inventory model for deteriorating with two parameter Weibull distribution using quadratic demand and time dependent holding cost. Shortages are allowed and are partially backlogged.
MATERIALS AND METHODS
• The inventory system deals with single item and the lead time is zero.
• Shortages are allowed and are partially backlogged. During stock out period, the backlogging rate is variable and is dependent on the length of the waiting time for next replenishment. So that the backlogging rate for negative inventory is
β
(
t
)
=
𝒆𝒆−ℷ(𝑻𝑻−𝒕𝒕) ,ℷ is backlogging parameter and (T-t) is waiting time (𝒕𝒕𝟏𝟏≤t≤T)•
θ
, be the deterioration rate. It follows the two parameter Weibull distributionθ
(
t
)
=
αβ
𝒕𝒕𝜷𝜷−𝟏𝟏 where0<α<1 and β>0
• Holding cost h(t) per item-unit is time dependent and is assumed to be
h
(
t
)
=
e
+
dt
when e>0,d>0• T is the length of the cycle, replenishment is instantaneous at an infinite rate and the planning horizon is finite
• The demand rate is
D
(
t
)
=
a
+
bt
+
ct
2• A,𝑪𝑪𝟏𝟏, 𝑪𝑪𝟐𝟐, 𝑪𝑪𝟑𝟑&𝑪𝑪𝟒𝟒 denote the set up cost, inventory carrying cost, deterioration cost per unit time,
shortage cost for backlogged items and the unit cost of lost sales respectively. All the cost parameters are positive constants.
Fig.1: Graphical Representation of Inventory System
MATHEMATICAL MODEL
During the period
(
0
,
µ
)
the inventory level is decreasing and at time 𝒕𝒕𝟏𝟏 the inventory reaches zero level, where the shortage starts and in the period (𝒕𝒕𝟏𝟏, T) some demands are backlogged.
The rate of change of inventory during, positive stock period (0,𝒕𝒕𝟏𝟏) is governed by the following differential equations,
=
dt
t
dI
(
)
-
(
a
+
bt
+
ct
2)
,0
≤
t
≤
µ
(1)αβ
+
dt
t
dI
(
)
)
(
)
(
21
ct
bt
a
t
I
t
β−=
−
+
+
,µ
≤
t
≤
t
1 (2)) ( 2
)
(
)
(
T te
ct
bt
a
dt
t
dI
=
−
+
+
−λ −,
t
1≤
t
≤
T
(3)With boundary conditions
I
(
0
)
=
S
,
I
(
t
1)
=
0
,Solving the equations, (1), (2) and (3) and neglecting higher powers of t, we get
2 3
( )
2
3
bt
ct
I t
= −
S
at
+
+
0
≤
t
≤
µ
(4))
(
)
2
(
)
(
)
1
(
)
(
3
)
(
2
)
(
)
(
1 12 2 13 3 1+1 +1 1 +2−
+2+
+
−
+
+
−
+
−
+
−
=
β β β ββ
α
β
α
t
t
b
t
t
a
t
t
c
t
t
b
t
t
a
t
I
(
)
3
3 3 1
+ +
−
+
+
β ββ
α
t
t
C
µ
≤
t
≤
t
1 (5)2 2 3 3 2 2 2 2 3 3 3 3
1 1 1 1 1 1 1 1
( )
(
)
(
)
(
)
(
)
(
)
(
)
(
)
(
)
2
3
2
2
3
3
b
c
a
b T
b
c T
I t
=
a t
− +
t
t
−
t
+
t
−
t
−
a T t
λ
− +
t
λ
t
−
t
−
λ
t
−
t
+
λ
t
−
t
−
λ
t
−
t
(
14 4)
4
c
t
t
λ
+
−
t
1≤
t
≤
T
(6)From the equations (4) and (5) after replacing
t
=
µ
, we get2 3
( )
2
3
b
c
I
µ
= −
S
a
µ
+
µ
+
µ
(7)2 2 3 3 1 1 2 2
1 1 1 1 1
( )
(
)
(
)
(
)
(
)
(
)
2
3
1
2
b
c
a
b
I
µ
a t
µ
t
µ
t
µ
α
t
βµ
βα
t
βµ
ββ
+ +β
+ +=
−
+
−
+
−
+
−
+
−
+
+
c
α
(
t
β 3µ
β 3)
β
+ +
+
−
Using Partial Backlogging / IJMA- 6(9), Sept.-2015.
Equating (7) and (8),
2 3
2 2 3 3 1 1
1 1 1 1
(
)
(
)
(
)
(
)
2
3
2
3
1
b
c
b
c
a
S
a
µ
µ
µ
a t
µ
t
µ
t
µ
α
t
βµ
ββ
+ +=
+
+
+
−
+
−
+
−
+
−
+
(
1 2 2)
(
1 3 3)
2
3
b
c
t
β βt
β βα
µ
α
µ
β
β
+ + + ++
−
+
−
+
+
)
(
3
)
(
2
)
(
1
3
2
3 3 1 2 2 1 1 1 1 3 1 2 1 1 + + + + + +−
+
+
−
+
+
−
+
+
+
+
=
β β β β βµ
ββ
α
µ
β
α
µ
β
α
t
c
t
b
t
a
ct
bt
at
S
(9)Using (9) in (4),
2 3
( )
2
3
bt
ct
I t
= −
S
at
+
+
2 2 3 3 1 1 2 2 3 3
1 1 1 1 1 1
( )
(
)
(
)
(
)
(
)
(
)
(
)
2
3
1
2
3
b
c
a
b
c
I t
a t
t
t
t
t
t
α
t
βµ
βα
t
βµ
βα
t
βµ
ββ
+ +β
+ +β
+ +=
− +
−
+
−
+
−
+
−
+
−
+
+
+
(10)
Total amount of lost sales
I
L, during the period(
0
,
T
)
is,∫
−
+
+
=
T − −t
t T
L
e
a
bt
ct
dt
I
1
)
)(
1
(
λ( ) 24
3
3
2
2
12
6
2
4 1 3 1 3 1 2 1 2 1 1 4 32
at
b
Tt
b
t
c
Tt
c
t
Tt
a
T
c
T
b
T
a
I
L=
λ
+
λ
+
λ
−
λ
+
λ
−
λ
+
λ
−
λ
+
λ
(11)Total amount of shortage units
I
s during the period(
0
,
T
)
is,∫
−
=
Tt
S
I
t
dt
I
1
)
(
2 3 2 2 3
2 3 4 3
2 2
1 1 1 1
1 1 1
2 3 4 2 3 4 3 4 5
4 5
1 1 1 1 1 1 1 1
2
2
2
6
3
12
3
2
3
12
3
2
30
2
3
4
3
4
5
S
b tT
ct T
b T t
b t T
aT
bT
cT
a T
at T
a T t
a t T
I
c T t
c Tt
at
bt
ct
a t
b t
c t
b T
c T
λ
λ
λ
λ
λ
λ
λ
λ
λ
λ
λ
λ
−
+
−
+
−
+
+
−
−
+
−
=
−
+
−
−
+
+
+
+
+
+
(12)Total amount of deterioration items
I
D, during the period(
0
,
T
)
is,∫
−=
1 1(
)
t
D
t
I
t
dt
I
µβ
αβ
1 2 3 2 1 2 2 2 3
1 1 1 1 1 1
2 3 1
1 2 3 2 1
1 1 1 1
2 1
(
1)
(
2
)(
3)
(2
1)
(2
2
)(2
3)
1
2
2(
2)
3
3(
3)
(
1)
(
1)(2
1)
(
2)
D
at
bt
ct
at
bt
ct
at
a
bt
b
ct
c
at
a
I
bt
β β β β β β
β β β β β β β β β
β β
α
α
α
β β
β β
β β
β β
β β
β β
µ
µ
µ
µ
µ
µ
α
µ
α µ
αβ
β
β
β
β
β
β
β β
β
β
α
µ
β β
+ + + + + + + + + + + ++
+
+
+
+
+
+
+
+
+
+
=
−
+
−
+
−
+
−
+
+
+
+
+
+
+
−
+
32 2 2 3
1
(2
2)(
2)
(
3)
(
3)(2
3)
ct
b
βα
βµ
βc
βα µ
α µ
β
β
β β
β
β
+ + +
+
−
+
+
+
+
+
+
(13)During period
(
0
,
T
)
total number of units holdingI
H is,dt
t
I
dt
e
dt
t
I
dt
e
I
tH
(
)
(
)
(
)
(
)
∫
−
+
+
+
−
+
+
−
+
−
+
−
+
=
µ β+ β+ β+µ
β+β
α
µ
β
α
0 2 2 1 1 1 1 3 3 1 2 2 11
(
)
2
)
(
1
)
(
3
)
(
2
)
(
){
(
e
dt
a
t
t
b
t
t
c
t
t
a
t
b
t
1
3 3 2 2 3 3 1 1
1 1 1 1 1
(
) }
(
){ (
)
(
)
(
)
(
)
3
2
3
1
t
c
b
c
a
t
β βdt
e
dt a t
t
t
t
t
t
t
βt
βµ
α
µ
α
β
+ +β
+ ++
−
+
+
− +
−
+
−
+
−
+
∫
+
(
1 2 2)
(
1 3 3) }
2
3
b
c
t
βt
βt
βt
βdt
α
α
β
+ +β
+ ++
−
+
−
+
+
2
2 3 3 4 4 5
1
{
2
2(
3)
3
2(
4)
4
2(
5)
2
H
aet
ae
ad
be
bd
ce
cd
I
β β β β β β
α µ
α µ
α µ
α µ
α µ
α µ
β
β
β
β
β
β
+ + + + + +
= −
−
−
−
−
−
+
+
+
+
+
+
+
3 3 4 4 5 2 3 3 4
1 1 1 1 1 1 1 1 1
6
3
8
4
10
2
2(
3)
3
2(
4)
adt
bet
bdt
cet
cdt
α
aet
βα
adt
βα
bet
βα
bdt
ββ
β
β
β
+ + + +
+
+
+
+
+
+
+
+
+
+
+
+
+
}
4 5 1 14
2(
5)
cet
βcdt
βα
α
β
β
+ +
+
+
+
+
(14)Therefore total unit cost per unit time is given by,
]
[
1
4 3 21
I
HC
I
DC
I
SC
I
LC
A
T
TC
=
+
+
+
+
2
)
3
(
2
3
)
4
(
2
4
)
5
(
2
[
1
1 1 5 1 1 4 1 1 4 1 1 3 1 1 3 1 1 2+
+
+
+
+
+
+
+
+
+
+
+
=
+ + + + + +β
α
β
α
β
α
β
α
β
α
β
α
β β β β β βaet
C
adt
C
bet
C
bdt
C
cet
C
cdt
C
A
T
TC
5 4 4 3 3 2 2 3
1 1 1 1 1 1 1 1 1 1 1 1 1 1
10
4
8
3
6
2
2
2(
3)
C cdt
C cet
C bdt
C bet
C adt
C aet
C ae
α µ
βC ad
α µ
ββ
β
+ +
+
+
+
+
+
+
−
−
+
+
3 4 4 5 1 2
1 1 1 1 2 1 2 1
3
2(
4)
4
2(
5)
(
1)
(
2)
C be
α µ
βC bd
α µ
βC ce
α µ
βC cd
α µ
βC
αβ
at
βC
αβ
bt
ββ
β
β
β
β β
β β
+ + + + + +
−
−
−
−
+
+
+
+
+
+
+
+
(
3
)
3 1 2
+
+
+β
β
αβ
βct
c
1
)
3
2
(
)
2
2
(
)
1
2
(
1 2 1 2 3 2 1 2 2 2 2 1 2 2 1 2 1 2 2+
+
−
+
+
+
+
+
+
+ + + +β
µ
αβ
β
µ
αβ
β
β
β
α
β
β
β
α
β
β
β
α
β β β β βa
c
at
c
ct
c
bt
c
at
c
2 2 3 3 2 1
2 1 2 2 1 2 2 1
2
2(
2)
3
3(
3)
(
1)
C
αβ
bt
µ
βC
αβ µ
b
βC
αβ µ
ct
βC
αβ µ
c
βC
α β
at
βµ
ββ
β
β
β
β β
+ + +
−
+
−
+
−
+
+
+
2 2 1 2 2 2 2 2 2 3
2 2 1 2 2 1
(
1)(2
1)
(
2)
(2
2)(
2)
(
3)
C
α β µ
a
βC
α β
b
βµ
t
βC
α β µ
b
βC
α β
ct
βµ
ββ
β
β β
β
β
β β
+ + + +
+
−
+
−
+
+
+
+
+
+
(
3
)(
2
3
)
3 2 2 2
+
+
+
+β
β
µ
β
α
βc
C
2
2 3aT
C
+
3
6
2
3 1 3 3 3 2 1 3 1 3T
ct
C
bT
C
T
bt
C
T
at
C
−
+
−
−
12
4 3cT
C
+
3
3 3 2 1 3 2 1 3T
a
C
T
t
a
C
T
t
a
C
λ
−
λ
−
λ
+
12
3
2
2
4 3 3 1 3 2 2 13
b
t
T
C
b
t
T
C
b
T
C
λ
−
λ
−
λ
+
3
2
30
2
2 1 3 5 3 4 1 3 2 3 1
3
c
t
T
C
c
t
T
C
c
T
C
at
C
+
−
−
+
λ
λ
λ
3
3 1 3bt
c
+
4
4 1 3ct
c
+
3
3 1 3a
t
C
λ
+
4
4 1 3
b
t
C
λ
+
5
2
2 4 5 13
c
t
C
a
T
C
λ
λ
+
+
12
6
4 4 34
b
T
C
c
T
C
λ
λ
+
+
−
C
4a
λ
t
1T
2 2
4 1 4 1
2
2
C a t
λ
C b t T
λ
+
−
3 3 4
4 1 4 1 4 1
]
3
3
4
C b t
λ
C c t T
λ
C c t
λ
Using Partial Backlogging / IJMA- 6(9), Sept.-2015.
Optimal value of t1can be obtained by solving the equation,
4 3 2
3 2 1
1 1 1 1 1 1
1 1 1 1 1 1
1
(
)
1
{
2
2
2
C cdt
C bdt
C adt
TC
C cet
C bet
C aet
t
T
β β β
β β β
α
+α
α
+α
α
α
++ + +
∂
=
+
+
+
+
+
∂
2 3 4
2 3
1 1 1 1 1 1
1 1 1 1 1 1
2
2
2
C adt
C bdt
C cdt
C aet
C bet
C cet
+
+
+
+
+
+
C
2at
1C
2bt
1 1β β
αβ
αβ
β
β
+
+
+
2 2 2 2 2 1 2 2 2
2 1 2 1 2 1 2 1 2 2 1
C
αβ
ct
βC
α β
at
βC
α β
bt
βC
α β
ct
βC
αβ µ
a
βC
αβ µ
bt
ββ
β
β
β
β
β
+ + +
+
+
+
+
−
−
C
ct
C
at
C
bt
C
ct
C
3aT
C
3bt
1T
2 1 2 2 1
1 2 2 1
2 2 2
1
2
−
−
−
−
−
−
+ +β β β
β β
β β
β
µ
β
α
β
µ
β
α
β
µ
β
α
β
µ
αβ
2 3 2 1
3
ct
T
C
a
T
C
+
λ
−
-C
a
t
T
C
b
t
T
C
b
t
T
C
c
t
T
C
3c
t
13T
22 1 3 2 1 3 2 1 3 1
3
2
2
2
λ
+
λ
−
λ
+
λ
−
λ
2 1 3 3 1 3 2 1 3 1
3
at
C
bt
C
ct
C
a
t
C
+
+
+
λ
+
+
3+
1 3
b
t
C
λ
C
c
λ
t
C
4a
λ
T
41
3
−
+
C
4a
λ
t
1
−
C
b
t
T
+
C
b
t
−
C
c
t
T
+
2 1 4 2 1 4 1
4
λ
λ
λ
}
0
3 1 4
c
t
=
C
λ
(16)The minimum total average cost per unit time is obtained for those values of
t
1for which
(
)
0
2 1 2
>
∂
∂
t
TC
(17)
By solving equation (16), the value of
t
1 can be obtained and then using this in equations (15) and (9), the optimal value of total cost (TC) and maximum inventory level (S) can be found out respectively.NUMERICAL EXAMPLE 1
Consider an inventory model with following parameters.
For, A=50, a=25, b=400, c=20, d=0.02, e=0.5, µ=2, α=0.5, β=0.01, ℷ=0.8,
C
1=
0
.
5
,
C
2=
0.8,
C
3=
0.02,
C4=0.5, T=3 in equation (15),
We get
t
1=
2
.
2961
and using this value in equation (16), we get TC=240.4963.SENSITIVITY ANALYSIS
Case (i): Using the above said parameters and only varying the deterioration parameter β, we get,
β t
1 TC
RESULT
Increasing the value of deterioration parameter β, the value of t1 and the Total Cost (TC) be decreased.
Case (ii):Using the above said parameters and only varying the deterioration parameter α, we get,
α T1 TC
0.4 2.3254 240.8955 0.5 2.2961 240.4963 0.6 2.2695 239.8117 0.7 2.2455 238.9070 0.8 2.2240 237.8915 0.9 2.2048 236.8135
RESULT
Increasing the value of deterioration parameter α, the value of t1 and the Total Cost (TC) be decreased.
CONCLUSION
In this paper, we have developed a model for deteriorating item with time dependent quadratic demand and partial backlogging. From the analytical solutions of the above model it is to be concluded that the total inventory cost be minimized when we increase the deterioration rate.
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Source of support: Nil, Conflict of interest: None Declared
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