Severing by tearing or breaking – Methods – With preliminary weakening
Reexamination Certificate
2000-06-22
2003-11-11
Shoap, Allan N. (Department: 3724)
Severing by tearing or breaking
Methods
With preliminary weakening
C225S094000, C225S101000, C029S888090
Reexamination Certificate
active
06644529
ABSTRACT:
BACKGROUND OF THE INVENTION
(I) Prior Art Background
Many methods have been used in fracturing connecting rods, that include:
(i) Passing an electron beam along a desired splitting plane as in U.S. Pat. No 3,751,080.
(ii) Providing holes in the fracturing plane through which the fracturing force is introduced as in U.S. Pat. No. 3,994,054
(iii) Using heat treatment or freezing to embrittle the fracture area as in U.S. Pat. No. 4,768,694
(iv) Applying a static or an impulsive force acting perpendicular to the fracture plane as in U.S. Pat. Nos. 4,860,419; 5,115,564; and 5,320,265.
(v) Actuating expanding mandrels using a wedge arrangement as in U.S. Pat. No 5,503,317.
However, most of the known methods for fracturing the connecting rods are based on the same principle: application of an “outward pressure” to the crank bore till the generated stresses are high enough to fracture the connecting rod. Some of these methods attempted to overcome the difficulty of fracturing such high strength material by reducing or weakening the cracking area, by using techniques, such as, the cryogenic cooling and the electron beam hardening, which have a deleterious effect on material performance.
Since connecting rods are made of high strength materials, the fracturing force is required to be of big magnitude. The use of big force has a negative effect on the quality of the fractured connecting rod, especially, with large size connecting rods in a high production environment. Despite the improvements, some disadvantages still exist such as: plastic deformation, lack of flexibility in adapting the same technique to different sizes of connecting rods, repeated breakage of force exertion elements of the machine, and poor quality of the fractured connecting rod. Moreover, some techniques are slow, costly, and technically very elaborate.
Before presenting the idea of the current invention, it is necessary to discuss the engineering principles on which the invention stands.
(II) Technical Background
(A) Fracture Mechanics:
Strength failures of load bearing elements can be either of the yielding-dominant (ductile) or fracture-dominant (brittle) types. In case of a cracked element, it may fail due to reaching the plastic collapse or fracture condition. Collapse and fracture are competing conditions, and the one satisfied first will prevail.
High-strength materials are more likely to fail in fracture mode before attaining the plastic collapse strength. Since connecting rods are made of high-strength materials, they generally fail under tensile forces due to reaching the fracture limit state.
Fracture may take place under one of two conditions, namely, plane stress or plane strain, depending on the thickness of the element. In general, connecting rods are thick enough to sustain plane strain fracture. In the presence of a V-notch or a crack, fracture occurs under essentially elastic conditions with a limited plasticity zone at the tip of the crack.
The stress intensity factor (K), is the characterizing parameter for crack extension. For each stress pattern, there is a corresponding value of the stress intensity factor. When the stress intensity factor reaches a certain value, crack propagates and collapse by fracture occurs. That critical value of the stress intensity factor under plane strain conditions, called the Plane Strain Fracture Toughness (K
Ic
), can be considered as a material property characterizing the crack resistance. Thus, the same value of K
Ic
should be obtained for a given material while testing specimens of different geometric shapes and sizes.
Lower temperature and faster strain rate decrease the plane strain fracture toughness for a specific material, while increasing the length of a pre-existing crack or decreasing the fracturing area will increase the stress intensity factor, if all other factors remain unchanged.
(B) Resonance of a Structural System:
The connecting rod, with all movement and rotation constraints imposed on it during the fracturing process, can be viewed as a structural system. Before explaining how to achieve and make use of a resonance condition in this fracturing technique, it is helpful to introduce the following definitions pertaining to an idealized structural system with finite number of degrees of freedom:
Degrees of freedom: the number of independent displacements required to define the displaced positions of all the masses relative to their original positions is called the number of degrees of freedom (DOFs).
Natural mode of vibration: a multi-degree-of freedom system (MDOF) would undergo simple harmonic motion, without a change of the deflected shape, if free vibration is initiated by appropriate distribution of displacements in various DOFs. In other words, for some characteristic deflected shapes, the system would vibrate in simple harmonic motion, and the initial shape would be maintained through out the motion. Each characteristic deflected shape (&PHgr;
n
) is called a natural mode of vibration of the MDOF system.
Natural vibration properties: the time (T
n
) required for a system to complete one cycle of the simple harmonic motion in one of its natural modes is called the natural period of that particular vibration mode. The corresponding natural cyclic frequency of vibration is f
n
, and the natural circular frequency of vibration is &ohgr;
n
, where:
T
n
=2&pgr;/&ohgr;
n
=1
/f
n
.
A vibrating system with N number of DOFs has N natural vibration frequencies &ohgr;
n
(n=1, 2, . . . , N), arranged in sequence from smallest to largest (&ohgr;
1
<&ohgr;
2
<. . . <&ohgr;
N
), with corresponding natural periods T
n
, and natural modes &PHgr;
n
.
The excitation frequency: the frequency of a harmonic force applied to a system is called the excitation frequency or the forcing frequency.
Damping: the process by which vibration steadily diminishes in amplitude is called damping.
SUMMARY OF THE INVENTION
The present invention employs a novel approach to fracture connecting rods. In this process, several factors are used to raise the stress intensity factor in the connecting rod up to the fracture point. Consequently, the use of single big force has been avoided with the application of several small magnitude forces. That eliminates many problems associated with the use of big forces. It also gives better control over the fracturing process, since the contribution of each factor is optimized to achieve the best results. For this process, a stress-riser should be provided in a prior process, using any of the known methods, in order to predetermine the fracture plane.
The present invention utilizes the following factors:
(a) Fatigue: if the stresses in a pre-notched connecting rod fluctuate due to the application of harmonic forces (or any time varying forces), the pre-existing crack (stress-riser) will extend incrementally depending on the range of fluctuation in the stress intensity factor. It is important to notice that the crack growth relates to the change of the stress intensity factor, not to its absolute value. Moreover, as the crack grows, the absolute value of stress intensity factor will increase.
(b) Resonance: during the fracturing process, the connecting rod will be in contact with many elements of the machine. These elements impose movement constraints, called geometrical boundary conditions, to the connecting rod. The connecting rod, with these boundary conditions, represents a distributed mass structural system, with an infinite number of degrees of freedom. However, it can be idealized and analyzed as a system with finite number of degrees of freedoms by using the finite element method.
If a MDOF structural system is subjected to an external force system, where the spatial distribution of the force components is independent of time, it takes a certain deformed shape. This shape does not necessarily resemble any of the natural vibration modes of the system. However, it has the same configuration as one of these natural modes, and with judicious selection of the external forces, the forced deformed shape can prese
Bhattacharjee Sudip
Guirgis Sameh Hakim Andrawes
Choi Stephen
Gierczak Eugene J. A.
Shoap Allan N.
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