System and method for fatigue forecasting and strain measurement using Integral Strain Gauge (ISG)

Abstract

The present invention relates to means, system and method for measurement of stress strain and fatigue forecasting by the means of Integral Strain Gauges (ISGs) capable of recording information from a surface of a tested object, mathematical processor for analysis of the information recorded on the surface of such gauges. Integral Strain Gauges produced from a custom made reaction sensitive materials.

Description

This application claims priority of provisional application No. 60/859,957 filed on Nov. 20, 2006.

1. FIELD OF THE INVENTION

Stress measurement and fatigue forecasting system of the present invention introduces a new revolutionary approach in a field of applied physics and chemistry, and more particularly in material science.

2. PRIOR ART

The measurement of stress-strain and fatigue conditions within a test object, such as a structural or mechanical element, has commonly been achieved using electrical (conventional) strain gauges. The major area of application of these gauges is a measurement of stress-strain values in static loading conditions. The principle of action of electrical gauges can be characterized as a differential type. This type of gauges reacts to a single cycle of loading and their reaction (resistance) correlates with a single parameter of loading, stress level S. Electrical strain gauges react only in a process of loading and after unloading the reaction disappears. It means those kind of gauges can not be used for cyclic loading and can not accumulate reaction and consequently the “history of loading” during some term of cyclic loading.

Electrical gauges sizes allow to measure average stress level according classical theoretical dependence:

S=F/A;

Where: S—stress value; F—force; A—area of cross section.

This approach does not consider microstructure (for example crystalline structure of metals) of real material therefore electrical gauges do not allow to measure stress value on the micro level (this stress value is responsible for material destruction), for example between crystalline grains of metals.

Electrical strain gauges do not react on the micro defects, micro irregularities of tested materials and roughness of a surface.

Electrical strain gauges have electrical wires, connectors and switches and for this reason can not be used in access challenged locations (such as on the satellites or gear boxes).

SUMMARY OF THE INVENTION

Integral Strain Gauges (ISGs) are made from a wide variety of a custom made materials by a wide variety of methods, such as electroplating, or other deposition methods, plastic rolling with following machining and chemical processing.

Chemical composition and mechanical properties of ISGs result in creation of a sensitive plane of such gauges capable of recording delicate changes of parts subjected to testing.

ISGs are generally attached to a surface of tested elements. Such elements are subjected to multiple types of testing loads depending on the purpose of the experiment. Reactions to such loads, among most commonly known ones to be classified as static strain loads and cyclic or so called repetitive loads are properly recorded by ISGs. Depending on the nature of trials, information recorded on ISGs can be retrieved through a number of loading cycles having a gauge detached for the convenience of the reaction analysis. Alternatively, depending on the final purpose of a test, ISGs can be attached on the testing element through out its entire lifetime having the reaction readings recorded in specified intervals. Reaction readings and processing can be attained by means of a mathematical processor programmed specifically to perform such calculations.

ISGs of the present invention have ability “to remember” the history of loading and accumulate the machine part fatigue events. This feature, similar to a concept of a “black box” on airplanes, allows analysis of structural damage or destruction of a tested part over a period of exploitation or predefined testing time.

Under the influence of cyclic loads or cyclic strains the internal reaction of the ISG changes and the outward effect appears on the surface of the gauge whereas the magnitude of the changes of the reaction pertaining to repetitive loads correlates with a number of cycles of the amplitude of a cyclic deformation.

ISGs of the present invention have a number of attributes pertaining their internal composition and structure including type of materials, multi layer formation and shapes. Additionally, ISGs can be classified as having isotropic or non-isotropic properties (identical or not identical structure in all directions). Regardless of the positioning of a non-isotropic version of an Integral Strain Gauge relative to the forces of strain applied upon the tested element, proper reaction is recorded by the gauge during the testing cycle. ISGs of an isotropic kind, typically record the reaction along lines of their mechanical composition.

Typical usage cycle of the present invention comprises classical calibration—testing model. First stage relates to calibration of ISGs and building of the γ−N (amplitude of cyclic deformation—number of cycles for a different grade of ISG reaction) dependency curve which is followed by second stage of testing. Calibration dependency may also be established using unique mathematical methods applicable to a specific situation.

Fatigue forecasting of the present invention is performed once establishing correlations between ISG calibration curves and fatigue curves.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 Three layers of an ISG, top view;

FIG. 2 Three layers of an ISG, side view;

FIG. 3 Set of curves representing calibration dependency of a particular ISG type;

FIG. 4 Tested element (spring) with ISGs attached;

FIG. 5 Conical sample subjected to a twisting strain, variables used for establishment of a calibration dependency;

FIG. 6 ISG applied on a conical sample;

FIG. 7 Fatigue curve;

FIG. 8 Reaction of a maximum intensity R_NMax recorded on a surface of the ISG;

FIG. 9 Calibration specimen.

Let us now introduce a specific embodiment. It should be understood that various modifications and adaptations of such embodiment can be made without departing from the present invention.

DESCRIPTION OF A SPECIFIC EMBODIMENT

The first embodiment of the invention will now be explained by referring to FIG. 1, which depicts an internal structure of a particular version of an Integral Strain Gauge (ISG) for measuring and determining the stress-strain conditions and forecasting of a life time duration within a test object. ISG (10) is designed to enable a tester to measure the deformation when the Integral Strain Gauge (10) is applied to the test object (11) and thereby be used to approximate the fatigue, capacity and expected working life span of the test element (11).

During the testing stage, ISGs are attached to a surface of tested elements that subjected to a strain load and the reaction captured on such gauges is recorded appropriately.

Sensitivity of ISG's surfaces is dependant on the internal structure of its composition, creating a unique capacity of recording the reaction reflecting changes of the deformation of the tested materials based on a variety of factors such as reflection ability, size and density of grains, micro hardness, physical and chemical properties etc. Referring to FIG. 2, levels (12), (13), (14) existing on the ISG depicted for the purpose of description of the present embodiment.

Method of usage of ISGs can be further summarized in a number of consecutive stages.

Stage I. Calibration Test of a Particular Type of an ISG.

Calibration procedure is typically performed in a number of consecutive steps that begin with attachment of the ISG on a surface of a calibrating specimen. Such process can be performed by application of suitable adhesives or any other means of attachment. Upon completion, tested samples are loaded by a number of stress levels S₁, S₂, S₃ . . . S_n. While loading such specimen, tests are interrupted after a predetermined number of testing cycles N₁, N₂, N₃ . . . N_n, and the reaction pertaining each set is recorded accordingly. Based on the recorded information, calibration dependency curve is built in coordinates S-N (for different grade of ISG reaction R) using a mathematical equation or proximity of the data established during experimental process of a present stage.

Stage II. Creation of a Fatigue Curve.

Fatigue curve is typically built based on results of a fatigue testing of samples. Said samples are usually prepared from a material similar to that of a tested element. Technological process of creation of such sample it typically analogous to that of the tested element. Fatigue test can be performed on the real structural part as well.

Fatigue testing likewise testing performed for the establishment of the calibration dependency can be completed having various types of strain, such as a strain of twisting, stretching, bending or a composite of such depending on best proximity to the simulation of actual occurrence of the load.

Similar to the stage of calibration, reaction of the tested sample is obtained using ISG attached it the surface whereas number of cycles till the destruction of the tested sample is recorded. Such testing is conducted at various levels of strain for completion of the required sets of the experimental data. Accuracy of fatigue curve is typically dependant on the completeness of the data and larger array of levels of strain selected for the testing purposes. Additionally, such curve can be further described using mathematical equations.

Stage III. Life Time Prediction.

Following previously described stages, ISGs are attached on the surface of tested element subjected to a strain loading conditions. Such loading is interrupted after a predetermined number of cycles and the point of maximum reaction of strain is recorded. Using calibration dependency received previously curve corresponding to the intense reaction, corresponding to the maximum stress level of the present test is expressed. Such expression can be later resolved for a level of magnitude of acting strains on a micro-level which would allow calculation of the number of cycles prior to destruction once employing the fatigue curve of stage II.

STATEMENT OF OPERATION

Stage I. Calibration Test of a Particular Type of an ISG.

As an example of an application of the methodology of the present invention, let us describe calibration stage of an ISG of a conical specimen (15) as shown on FIG. 6 being subjected to a twisting strain applied by a testing machine following arrows (16).

Let us now describe the structure of a specimen depicted in further detail on FIG. 9. It was recognized that the true nature of tested materials, such as metals though commonly considered as a monolithic matter, rarely if ever behave as per commonly perceived patterns.

Tested specimen of FIG. 9 comprises a conical shaft (17), cappings (18) located on opposite edges of the shaft and provided for the purpose of fixation of the said shaft in spindles (19) of a testing machine. Internally, conical shaft consists of a double layer structure, namely inner rod (20) and the outer working shell (21). Outer working shell (21) is assigned for attachment of gauges undergoing stage of calibration.

Inner rod of the specimen is comprised of a durable metal. Outer working shell (21) is composed of an amorphous material such as plastic. Outer shell is attached firmly to the inner rod.

Thus, present specimen comprises a solid object composed of two types of materials.

Calibration stage of the test described herein facilitated by creation of calibration dependency γ−N (when R₁=const, R₂=const, R₃=const . . . R_n=const) for an ISG where composite calibration specimens are utilized through a predetermined number of testing cycles.

As shown on FIG. 6, ISG gauge (10) is fixed on the surface of the conical specimen (15) by means of a reaction sensitive adhesive. Conical structure (17) of such specimen creates a condition of coverage of the specimen at various radiuses on the surface of application.

Specimen is fixed in spindles (19) and subjected to cyclic strain (twisting) with constant amplitude of twisting (λ_a=const; where λ_a is the angle of twist). For a purpose of achieving precision dependency readings, amplitude of deformation of the sample is maintained constant through out the entire stage of a testing stage. Such procedure is interrupted after N₁, N₂, N₃ . . . N_i loading cycles or predefined periods of time for the purpose of reading of integral characteristics of the reaction from the surface of the ISG by the means of a scanner device. Referring to FIG. 5, we can further calculate the dependency of:

$\begin{array}{cc}{\gamma }_x=\frac{{\lambda }_i*3D^3*d_i^3}{2{\mathrm{dx}}^3L\left(D^2+d_iD+d_i^2\right)}.& \mathrm{Equation}1\end{array}$

Upon completion, received is a set of data where i—is a number of pair of values (N_i,ΣR_i) where

N_i—is the number of loading cycles after the i^th interrupt;
ΣR_i—is the integral characteristic of the reaction of the ISG and the extent of distribution of the reaction and micro-shear deformation on a surface of the ISG and the sample accordingly.

Additionally, in various sections of the specimen with diameters d₀, d₁ d₂, . . . d_j, points of maximum intensity of reaction R_0Max, R_1Max, R_2Max . . . R_jMax are identified and recorded for the purpose of subsequent calculation. Points of maximum reaction intensity are shown on of FIG. 8.

Based on Equation 1, described herein, and additional mathematical dependencies well known in the art, it is now possible to calculate the value of shear deformation γ and later the tangential stress τ for a particular material having known module of displacement G.

All of the data is received in an experimental (R,N) and computational (γ,τ) approach allows building of calibration dependency curve for a particular type of an ISG.

Let us now refer to FIG. 3, where:

In coordinates γ_Max(τ_Max)−N is described a number of curves each of which is built having R_Max=const.

Each of the curves in coordinates τ_Max−N represents characteristics of level of fatigue damage and thus can be described by a mathematical dependency:

$\begin{array}{cc}N=K\left[\frac{1}{{\tau }_{\mathrm{Max}}-D*C}-\frac{c_1}{{\tau }_{\mathrm{Max}}\left(c_1-DC\right)}\right].& \mathrm{Equation}2\end{array}$

Where:

N—number of cycles of the corresponding ISG reaction;
τ_Max—amplitude of tangential stress;
D—level of fatigue damage of the tested element material which corresponds to the ISG reaction R;
C,c₁,K—constants of the equation.

Thus set of curves of FIG. 3, in coordinates τ−N can be accurately described by a set of equations similar to that of Equation 2.

Stage II. Creation of a Fatigue Curve.

Let us now describe the second stage of the methodology of the present invention namely fatigue testing and creation of a fatigue curve. The fatigue test can be performed on samples with shape similar to the calibrating specimen (15) of FIG. 6, but manufactured of the same material and technology as a real structural part or on the real tested element. In the present description, twisting strain similar to that of a strain applied in stage I is acting upon specimen of FIG. 6.

Prior to beginning of the fatigue testing, an ISG is attached to the surface of tested element, fixed in spindles of a test rig and subjected to cyclic strain.

Tested element is subjected to the said strain according to the methodology of the present invention until an appearance of the reaction on the ISGs attached to the tested samples on multiple levels of strain τ₁, τ₂, τ₃ . . . τ_j. Through out the process, the fatigue testing is interrupted predetermined number of times corresponding to a number of cycles N₁, N₂, N₃ . . . N_j. Following every interrupt occurrence, reaction ΣR from the surface of the ISGs is read by the means of a scanning device. All of the data is properly recorded for the purpose of further analysis.

Though out the process, said testing procedures are continued until the finite destruction of samples for each level of strain τ. Numbers of cycles elapsed till sample destruction N_f is recorded accordingly for every level.

Following, each value of integral characteristic of ΣR identified and analyzed. Recorded are those points ISG having maximum intensity of reaction.

Using sets of data of present experiments in conjunction with values of calibration dependency (γ−N) received previously, maximum values of shear deformation (γ_Max) corresponding to maximum level of intensity of ISG reaction R_Max for all values of present strain (τ_j) and number of cycles (N_i) are calculated.

Shear deformation (γ_Max) is subsequently recalculated into tangential stress (τ_Max) and is recorded appropriately.

Such data allows us to build the fatigue curve in coordinates of γ_Max−N or τ_Max−N as shown on FIG. 7 having D=1.

Fatigue curve received in such method can be described in a similar mathematical dependency as the calibration dependency in coordinates (τ_Max−N) having D=1.

$\begin{array}{cc}{N}_f=K\left[\frac{1}{{\tau }_{\mathrm{Max}}-C}-\frac{C_1}{{\tau }_{\mathrm{Max}}\left(C_1-C\right)}\right].& \mathrm{Equation}3\end{array}$

Where:

N_f—is a number of cycles of loading preceding destruction of the sample;
τ_Max—real magnitude of micro-level strains;
K,C,C₁—constant parameters received throughout the stage of calibration dependency.

Stage III. Life Time Prediction.

Completing calibration dependency and fatigue testing stages final stage may begin. Approach to this stage is dependant on type of a strain applied against the tested object where 3 major conditions may occur:

• • 1. Level of strain is constant and the number of loading cycles can be predetermined;
  • 2. Level of strain is changing according to a set block of loads where the number of block phases is predetermined;
  • 3. Level of strain and the corresponding number of cycles is changing in random.

Let us now describe specifics of testing applicable to 3 major conditions of strain occurrences.

1. Under the first condition, through out exploitation of the element, the value of cyclic load such as, torque amplitude (T_a) is a constant value and the number of cycles can be defined precisely. Referring now to FIG. 4, ISG (24) is attached to the tested object (26) and subject it to a strain load till the reaction ΣR appears on the surface of the ISG. Number of cycles of loading preceding appearance of such reaction is as well as the point of maximum reaction intensity R_Max that corresponds with the maximum local strain τ_Max are recorded.

On the calibration dependency in coordinates (τ_Max−N) found are the curve that corresponds to the reaction R_Max and mathematical dependency for such a curve:

$\begin{array}{cc}{N}_f=K\left[\frac{1}{{\tau }_{\mathrm{Max}}-DC}-\frac{c_1}{{\tau }_{\mathrm{Max}}\left(c_1-DC\right)}\right].& \mathrm{Equation}4\end{array}$

Where:

τ_Max—amplitude of tangential stress;
K,C,c₁—constants of the present equation;
D—level of fatigue damage of the tested element material which corresponds to the ISG reaction R;
N_f—is a number of cycles of loading preceding destruction of the sample;

Substituting in to Equation 4 number of cycles of loading N, τ_Max can be calculated.

As a final point of the present stage, calculation of number of cycles of loading till the destruction of the element can be derived substituting the value of τ_Max in to the equation of the fatigue curve (Equation 3) and resolved with respect to parameter N_f.

2. Under the second condition, through out exploitation of the tested element, the value of cyclic load such as, strain level is changing according to a set block of loads where the number of block stages is predetermined. Such condition can be further described using following variables:
t_i—duration of the testing on the i^th stage of a block load;
g_i=τ_i/τ_Max—is a level of strain on the i^th stage of a block load.

Similar to the first condition, an ISG is attached on a surface of a tested element and reaction R_Max following N cycles of loading is recorded. For the purpose of this calculation, the number of cycles N corresponds to a whole number of block phases. Such reaction R_Max corresponding to the reaction R_Max2 of the calibration dependency having all of parameters defined (K,D_j,c₁C). Occurrence of the reaction R_Max shows that following N cycles of block load, damage value of D on the surface of the ISG corresponds to the reaction R_Max. Thus we can further describe dependencies for a block load condition as following:

$N=\frac{N_i}{t_i};$ $N=\sum _{i=1}^mN_i;$

Where:

N_i—is the duration of the testing on the i^th stage;
m—is a number of sages of a single block load.

Following dependencies can be described for a 3 stage loading block:

D=D₁+D₂+D₃

R_Max=R_Max1+R_Max2+R_Max3

Where:

D₁; D₂; D₃—is the damage on a surface of the ISG accumulated after every subsequent stage of a block load;
R_Max1; R_Max2; R_Max3—reaction on the ISG recorded after each subsequent stage of a block load.

Based on dependencies described above, value of the maximum equivalent strain (τ_Max,e) is derived based on the following equation:

$\begin{array}{cc}N=K\left[\frac{1}{{\tau }_{\mathrm{Max}·e}-DC}-\frac{c_1}{{\tau }_{\mathrm{Max}·e}\left(c_1-DC\right)}\right].& \mathrm{Equation}5\end{array}$

Where:

N—is a number of cycles of a block load;
K,D,C,c₁—constants of the present equation of the calibration dependency;
(τ_Max,e)—is a value of the maximum equivalent strain

Equation 5 is resolved for τ_Max,e which is substituted into the equation 3 for derivation of N_f, number of cycles of loading preceding destruction of the tested element.

3. Under the third condition, through out exploitation of the tested element, the value of cyclic load such as strain level is randomly changing. Number of cycles loading is not defined for this type of a condition.

Similar to the second condition, solution to the problem is based on the appearance of ISG reaction of a similar intensity that correlates to a corresponding level of a damage of the tested element.

Thus we can infer further relevance of strain value of (τ_Max,e) and equivalent number of lading cycles (N_E) based on the amount of a damage effect.

We can now suggest 2 distinct problem solutions.

Solution I. This method is based on usage of ISGs of a variable sensitivity to values of cyclic deformations. Calibration dependencies (τ−N) for said gauges established accordingly.

Testing of elements with ISGs (ISG₁, ISG₂) conducted until the appearance of a reaction of a similar intensity such as R_Max2 where the duration of the testing t₁ for ISG₁ and the duration t₂ for ISG₂ are recorded accordingly.

In such setup, equivalent cycle numbers for ISGs N_E1, N_E2 can be defined as:

N_E1=t₁*K_E; N_E2=t₂*K_E

Where:

K_E—is the coefficient of equivalency correlating to a cycle/duration ratio.

Based on such dependencies, following system of equations can be derived:

$\begin{array}{cc}{N}_{E1}=K_E*t_1=K\left[\frac{1}{{\tau }_{\mathrm{Max}e}-D_1C}-\frac{c_1}{{\tau }_{\mathrm{Max}e}\left(c_1-D_1C\right)}\right]N_{E2}=K_E*t_2=K\left[\frac{1}{{\tau }_{\mathrm{Max}e}-D_2C}-\frac{c_2}{{\tau }_{\mathrm{Max}e}\left(c_2-D_2C\right)}\right].& \mathrm{Equation}6\end{array}$

Solving this system of equations allows deriving values of (τ_Max,e), and K_E, and subsequently N_E1, N_E2.

Following, substituting of (τ_Max,e) into the equation of the fatigue curve (Equation 3) is resolved with respect to parameter N_f which can be measured in units of equivalent cycles N_fE or timing t_f.

Solution II. This method is based on usage of ISGs of a uniform sensitivity. In this method, for computation of equivalent strains (τ_Max,e) coefficient of equivalency K_E of calibration dependencies received for different criteria of reaction of a similar type of ISGs such as R_Max1, R_Max2.

Following method of determining equivalent parameters τ_Max,e, K_E, N_E, N_E2, as well as resolution with respect to N_fE is similar to that described in Solution I.

Claims

1. Method of measurement stress-strain and life time prediction within a test object including following elements and procedural steps:

at least one Integral Strain Gauge (ISG) whereas creation of said gauge chosen to enhance sensitivity of the plain of the said gauge selected based on the mechanical properties of a tested element;

attachment of the said gauge on the surface of the tested element allowing transmission of stress reaction and further recording of said reaction on the surface of the ISG;

calibration tests of an ISG incorporate attachment of the ISG on the surface of a specimen, stress loading of the specimen and the reaction reading for the purpose of creation of calibration dependency in a form of mathematical equation;

creation of a fatigue curve incorporates attachment of an ISG on the surface of the tested element, stress loading of the sample till the point of destruction and the reaction reading for the purpose of creation of fatigue curve in a form of mathematical equation;

life time prediction incorporates attachment of an ISG on the surface of the tested element and reaction reading pertaining the maximum strain level recorded on the surface of the ISG and subsequent calculation of number of cycles prior to part's destruction.

2. ISG as set forth in claim 1, being attached on the surface of a loaded element bearing no external connections to its surface for the reaction reading or fixation purposes.

3. Calibration tests as set forth in claim 1, further comprising a step of recording of sets of relationships γ−N pertaining reaction on a surface of an ISG for each predetermined number of cycles under a strain force applied against the specimen.

4. Calibration tests as set forth in claim 3, having constant amplitude of deformation during the entire calibration stage.

5. Calibration specimen as set forth in claim 1, comprised of an inner elastic rod and an outer working shell of an amorphous material attached firmly to each other.

6. Calibration dependency as set forth in claim 1, described by means of a mathematical equation derived from sets of values of shear deformation and tangential stress computed based on the set of γ−N relationships for a different grade of ISG reaction.

7. Mathematical equation as set forth in claim 6 incorporates dependency between variables of: and is explicitly defined in the equation of N = K [ 1 τ - D * C - c 1 τ  ( c 1 - D   C ) ], where C, c1, K—are constants of the present equation.

N—number of cycles of the corresponding ISG reaction;

τ—amplitude of tangential stress;

D—level of fatigue damage of the sample material which corresponds to the ISG reaction R;

8. Creation of a fatigue curve as set forth in claim 1, includes stress loading of elements on multiple levels of strain conducted till the final destruction of elements on each level of said strains where the number of cycles prior to sample destruction and corresponding ISG reaction is recorded for each trial.

9. Fatigue curve as set forth in claim 8, is created based on the computational analysis of the calibration dependency, values of the shear deformation that corresponds to the maximum level of intensity of the reaction recorded on the surface of the ISG.

10. Fatigue curve as set forth in claim 8, is explicitly defined as a dependency having a number of variable parameters including number of cycles prior to parts destruction, maximum magnitude of micro-level strains, constants of the calibration dependency, constants pertaining mechanical properties of the tested element.

11. Life time prediction stage as set forth in claim 1 incorporates reading of maximum intensity reaction from the surface of the ISG attached to the surface of a tested sample, recording of the number of cycles preceding said reaction, engagement of a curve of the calibration dependency coordinates corresponding to the said reaction, and subsequent resolution for the finite number of cycles prior to destruction of the tested sample having the sample subjected to a constant level of strain and predetermined number of cycles.

12. Life time prediction stage as set forth in claim 1 incorporates reading of maximum intensity reaction from a surface of the ISG attached to a tested sample following block stages, computation of a number of cycles of loading corresponding to a present stage of a block loading, computation of values of a maximum strain and subsequent resolution for the finite number of cycles prior to destruction of a tested sample by substitution to an equation of a fatigue curve having the level of strain changing according to a set block of loads and a predetermined number of blocks phases.

13. Life time prediction stage as set forth in claim 1 incorporates reading of maximum intensity reaction from a surface of the ISGs of a variable sensitivity attached to a tested object, recording timings of appearance of reaction of a similar intensity, correlating such timings to a number of cycles of loading, solving the system of equations for a value of a maximum strain and subsequent resolution for the finite number of cycles prior to destruction of a tested element by substitution to an equation of a fatigue curve having the level of strain and the corresponding number of cycles is changing in random.