TECHNICAL FIELD The present invention relates to a method for estimating a life span reducing damage to an object which has come under load in operation, with at least a first parameter which corresponds to an operational position which generates said damage to the object, and at least a second parameter which corresponds to a condition for or around an object which does not by itself have the ability to generate the damage but which makes the damage caused by the operational position worse, being calculated by means of at least one operation, measurement, and said measurement being carried out repeatedly. The invention also relates to a method and a system for checking the use of the object.

The invention can, for example, be applied in a means for transport, such as a vehicle, a vessel or another means of transport, such as means for transport on rails. The word vehicle relates to various ground vehicles, such as vehicles with wheels or tracks. The invention is, in particular, suitable to be applied to a building machine such as a wheel loader, a digging machine or a frame or waist controlled vehicle also known as a dumper. The area of application of the invention is however not limited to these applications, it can also be applied in stationary devices.

The invention will in the following be described in a case where the object is a rotating organ in the gear box of a vehicle. The invention in this case relates to the calculation of surface fatigue of the teeth and the bearing, respectively. This should be seen as a preferred but in no way limiting application of the invention.

Said first parameter which corresponds to an operational position for the object relates to a parameter which on its own can generate damage to the

object. By the word"damage", fatigue of the object is intended, or in other words a reduction of the life span. An example of such an operational position is a torque transferred to the object during a number of revolutions. Other examples of such an operational position are various types of forces which come to bear on the object.

Said second parameter which corresponds to a condition at said operational position relates to a parameter which on its own would not cause any damage to the object. It is only in combination with the first parameter which corresponds to an operational position for the object that it can make the damage to the object worse. Examples of such conditions are the environment around the object, for example the amount of particles such as metal filaments in oil which has been supplied to the object.

PRIOR ART It is previously known to calculate the effect on the life span of a gear wheel by an applied torque during a number of revolutions. It is also known that specific parameters which correspond to operational conditions for the gear wheel with applied torque will affect its susceptibility for damage, i. e. it has an effect on the damage which the torque during said number of revolutions will cause to the gear wheel. An example of such an operational condition is the temperature of the oil which cools the gear wheel. There is, however, a problem in known models for calculation in taking into consideration such operational conditions with high precision.

SUMMARY OF THE INVENTION A primary purpose of the invention is to obtain a method for predicting the influence of a plurality of parameters on the life span of an object which, relative to prior art, gives higher precision when calculating the remaining life span of the object after use during a certain time. A secondary purpose of the invention is to obtain a method which generates values on load and/or

damage which in a storage-efficient manner can be stored in a memory with limited memory capacity.

These purposes are obtained by a total load which is defined by the total influence on the life span of the object by the operational position and the operational condition being calculated in such a way that variations in the damage-susceptibility of the object caused by the operational condition is adjusted for, and the total load is expressed as a product of a function for the operational position and a function for the operational condition. The total load is thus a measure of a simulative influence of both the operational position and the current operational condition (s), which, creates the conditions for high precision, since there in reality is an interaction between the operational position and the operational condition.

The other parameters, such as for example temperature and the amount of particles and water in a cooling medium for the object have a large influence on the damage which a specific load can cause on the object. A calculation of the remaining life span based on both the load which is defined by the first parameter and the condition which is defined by the second parameter creates conditions for a high precision in the result. In more detail, the invention takes into consideration the variations in the susceptibility to damage of the object which is caused by the other parameters.

In so doing, conditions are also created for storing a result of the calculation based on the first parameter which corresponds to a load and a plurality of other parameters which affect the susceptibility to damage of the object in a position (cell) in a memory unit. This leads to a reduced need for memory.

According to a preferred embodiment of the invention, the total load is calculated a plurality of times during the time when said measurements are carried out. This results in a further reduced need for memory, since a reduced number of measured values need to be stored. This also causes a

higher precision in estimating the damage. Preferably, the calculation of the total load is carried out continuously during the time that said measurements are carried out.

According to another preferred embodiment of the invention, the total load is calculated after each determination of the first parameter. Such a determination can in practice be a measurement of the first parameter. Only one value, the calculated value for the total load, is then saved for each increment in time. This creates the conditions for an even more efficient memory unit concerning memory space.

According to another preferred embodiment of the invention, the calculated values are summed for the total load, and the result is stored in a first field in a memory unit. This creates the conditions for a continuous or on-line check of the influence of both the operational positions and the operational conditions. This feature is not limited to storing only the result, but at least the result of the summation is stored.

According to a further development of the previous embodiment of the invention, the second parameter is assumed to be constant during a plurality of measurements of the first parameter in a further calculation of the total load, and the total load is calculated with said second parameter at said constant value, the calculated values are summed, and the result is stored in a second field in said memory unit. In this embodiment, the second parameter is suitably set to a value which has been measured in real tests.

For example, this embodiment can be used as the basis for future dimensioning of the object/a device comprising the object. The embodiment is used in cases with small variations in the operational conditions.

According to another preferred embodiment of the invention, the number of load cycles are measured for the first parameter, and in a matrix in a memory unit which matrix comprises a plurality of different positions which each

corresponds to a specific operational position and at least one specific condition, the number of load cycles are added in the respective positions. Suitably, a matrix is used which has a number of predefined levels of said operational position on a first axis and a number of predetermined levels for said operational conditions on a second axis. The matrix can be expanded for further operational positions to an n-dimensional matrix. If a first parameter and two second parameters are measured, a three-dimensional matrix is used.

By means of this embodiment, the measured values can be stored in a memory unit, for example in a vehicle which comprises the object and can then at a later point in time be transmitted or transferred to a unit for calculating the total load. This embodiment brings about a saving in the calculation in the vehicle.

According to another preferred embodiment of the invention, at least one of the first and the second parameters are described as an exponential function.

This creates conditions for a simple and regarding memory capacity efficient way to express the total load with high precision. In addition, conditions are created for using the Palmgren-Miner partial damage theory.

According to another preferred embodiment of the invention, a total life span influencing damage in the object is calculated after a certain amount of time as the sum of each of the calculated total loads before this point in time. This life span influencing damage can be used in a number of various ways. For example, information regarding the remaining damage can be presented directly to the driver of the vehicle so that he is aware of the status of the object. This can lead to a milder operation of the vehicle/object. He can furthermore decide on continued operation of the vehicle, exchanging or repairing the object etc. The information is suitably presented on a display to the driver. According to one alternative, an electrical device such as a

computer is connected to the vehicle for presentation to service personnel, owners or others.

According to another example, this information is transmitted to a base station or a central terminal. This makes it possible to check the status of a plurality of vehicles and to plan the continued operation of these. When renting the vehicle, billing can be done according to damage caused.

According to another preferred embodiment of the invention, the first parameter is measured at a higher frequency than the second parameter. In other words, the first parameter is measured at shorter intervals in time than the second parameter. A measured value for the second parameter is in this case used for calculating the total load for several measured values of the first parameter which follow each other in time. In this way, the number of measurements is reduced. This is particularly preferred in cases when the second parameter at least in periods varies within a relatively small interval.

A further purpose of the invention is to obtain a system for communication between a base station and at least one remotely located stationary or mobily arranged machine, via transmitting and receiving organs for checking the operational status of the machine, with the machine comprising an object which is susceptible to damage, which system creates conditions for checking the operational status of the object and a position remote from the object. In particular, a system for predicting malfunctions or breakdowns of the object and taking measures before such malfunctions occur is intended.

This purpose is obtained by the system comprising a control unit, organs for measuring a plurality of operational parameters of the object, and organs for calculating damage done to the object based on the measured operational parameters.

Further preferred embodiments and advantages of the invention will become evident from the rest of the claims and the following description.

BRIEF DESCRIPTION OF THE FIGURES The invention will be described in more detail below with reference to the embodiment shown in the appended drawings.

In Fig. 1-4 four graphs are shown which each illustrate load strength functions.

In Fig. 5 the system according to the invention is shown.

In Fig. 6 a block diagram is shown of a specific part of the system according to a preferred embodiment.

DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS The word"damage"refers in the following to a weakening of an object or, in other words, a reduction of its life span. The term"operational position"refers to a load which by itself can inflict such damage to the object. In other words, a strength, also known as susceptibility to damage, of the object is reduced when it is exposed to a load. The strength is at each point in time dependent both on the internal structure of the object, such as its materials and of the operational conditions or environment which the object is in. Said operational condition does not thus on its own cause any damage to the object.

The object is exemplified in a first embodiment of the method by a component in the driving line of the vehicle. In particular, surface fatigue of the teeth on a gear wheel in a gear box is intended. The operational position (in the following called the first parameter) is the torque which the gear wheel transfers during a number of revolutions.

The strength of the object depends on the supporting ability of the flanks of the gear teeth. The strength is influenced by several operational conditions (in the following called second parameter) such as;

-temperature: reduces the viscosity of the oil and thus the supporting ability of the oil film, -the amount of particles in the oil : primarily particles in sizes which are equal in size to, or larger than, the size of the oil film. These are pressed together during operation between the cog surfaces, and reduce the strength against fatigue, -the amount of water in the oil.

Below, the method for calculation in the first embodiment and the theory behind it will be explained in detail with reference to the appended drawings.

In this embodiment the first and the second parameter are measured repeatedly.

The object which is exposed to said loads has, according to the description, above a so-called varying strength. First, there will below be explained a method which, as such, is known for dealing with so-called constant strength.

The term"constant strength"of the object means that the damage susceptibility of the object is not influenced by the environment or the conditions that it is in. There then will follow an explanation of the varying strength of the object according to a preferred embodiment.

Constant strength Many life span functions can be described as exponential functions. The Wöhler curve, also called the SN curve, is a well known example of fatigue in the case of load with constant amplitude, see (1) and Fig. 1 (with logarithmic scales).

Sm*N = C (1) where

S = signal level, for example tension, torque N = the number of cycles to fracture C = a constant for a certain curve The constant C can be perceived as a measurement of the object's tolerance for damage, that is the amount of damage it can absorb before a fracture occurs. This is, in other words, a measurement of strength.

Real loads are often applied with variable amplitude, so-called spectrum load. This can be described by means of the Palmgren-Minor partial damage hypotheses, see (2) and Fig. 1.

Partial damage value = E (ni/Ni) = p (2) where Ni = the number of cycles to fracture at the signal level SI ni = the number of load cycles in the spectrum at the same level Si p = partial damage value = 1 in the case of fracture, often the value 0,7 is used instead or another value.

The summation is done over the entire spectrum. The fatigue limit is ignored, since it is a phenomena which occurs in the case of load with constant amplitude. Formula (1) is introduced into formula (2): p = E (Sim*ni/C) = (1/C) * E (Sim*ni) (3) since the strength C is constant in this case.

The notion load (D) is introduced, where D = E (Sim*ni) (4)

From formulas (3) and (4) it is evident that p = D/C The ratio D/C thus indicates how large a portion of the damage tolerance which has been consumed by the load D.

As described above, the strength varies in many cases. The strength can, to be more exact, vary without decreasing successively. An example of this is surface fatigue in the case of cogs and bearings. Logging the damage in the form of load only, for example torque and number of revolutions without adjusting for the variation in strength will for this reason give a relatively poor precision.

It is therefore desirable to create a method which also takes into account the variation in strength.

Varying strength There is an interaction between load and strength. For example, a certain torque level will cause more damage at a high temperature (which brings about lower viscosity) of the cooling oil than in the case of low temperature. It is not feasible to record torque and temperature separately in order to then adjust for the temperature. For this reason, this is carried out according to the invention in each small time interval, i. e. continuously, or"on-line".

According to formula (1) above, the connection between load, signal level for example the torque M, life span and the number of revolutions N and fracture can be described by means of Mi'-Ni = Ci (5)

The decisive difference in the case of varying strength is that the tolerance for damage Ci is not constant, it depends on for example the temperature, the amount of particles, the amount of water etc. Another temperature would give another value of the strength Ci, see Fig. 2.

Fig. 3 corresponds to tests run at the same load level at different temperatures. It turns out that the fatigue function can be described by means of Tik*Ni = K1 (6) where T is the temperature, N is the amount of cycles to fracture, and k and K1 are constants.

The tests have been carried out at the same load level, which means that the strength C is in proportion to the life span N, which thus according to Fig. 4 results in Tik*Ci = K2 (7) Ci = K2/Tik is obtained from formula (7) and introduced into formula (3), and thus a partial damage value p is obtained: p = S (Sim*ni*Tik/K2) (8) A reference temperature To, preferably the one at which the test is carried out is choosen. The strength is then C = Co. Thus; K2=Tok*Co (9)

Formula (9) is introduced into (8) which results in p = (1/Co) *E (Si"'*ni* (Ti/To) k) (10) since Co is a constant. The expression Dkorr = E (Sim*ni* (Ti/To) k) (11) is thus a load value which is adjusted for variations in strength to a reference value.

Formula (10) can thus be expressed as p = Dkorr/Co The ratio Dkorr/Co thus gives a good indication of the damage portion which has been consumed.

Increasing to more varying strength parameters is trivial. If the particle ratio is p, the water amount v, and q and r are exponent and reference value respectively and the reference values are obtained at Co formula (12) is obtained: Dkorr = E (Sim*ni* (Ti/To) k* (pi/po) q* (vi/vo) r) (12) In the embodiment above, the strength function has been described as an exponent function. By so doing, simple expressions are obtained for load and strength. It is however within the scope of the appended claims to describe the strength function in another way.

Above, it has additionally been assumed that the influence of the load and the strength parameter on the life span can each be described by means of one or several exponential functions, which can generally be done with acceptable precision.

Preferably, the D value is summed successively. This means that the result can be placed in only one cell, irrespectively of how many parameters are taken into consideration. The memory need will thus be relatively small in spite of the large amount of information which is gathered. According to one application of the invention, a result of the calculation of the total load is stored in a position in a memory unit of the vehicle. The limited memory need is particularly advantageous in applications in a transport means such as a vehicle.

Another example of applications of the invention is in the case of cardan shafts whose strength decreases as the angle in the universal joint cross increases.

The invention is not limited to the above described examples of embodiments, a number of further variations and modifications are possible within the scope of the appended claims.

When calculating the total load, the first parameter is related to the ratio between a measured second parameter and a reference value for the second parameter. Said reference value for the second parameter is based on actual tests of the object. In other words, a normation is made of the operational conditions such as the temperature against values for these, which have been obtained in actual testing.

According to a further embodiment, a plurality of first parameters are measured at each measurement, and the total load is described as a function in which the life span influence of each of the first parameters is summed, It

is, for example, advantageous in the case of several torques or several forces.

A damage to the object which results in said total load is defined by the total load relative to the initial damage tolerance of the object. In other words, there is a knowledge about the life span of the object or its tolerance for loads at specific operational conditions. This knowledge can, for example, be obtained through fatigue testing.

A number of ways of utilizing the result of the calculation of the total load/influence of the life span are possible. Below, a number of examples of preferred such methods are indicated.

A user of the object can be billed for damage caused to the object. This can for example be used in the case of renting vehicles. Another area of application for the billing is sale. At the sale, the seller can sell a system which includes maintenance exchange etc. In other words, it is possible to bill for a degree of usage, such as calculated caused damage to the object. The billing can of course also take place based on further parameters.

According to another example, the remaining life span is predicted based on the calculated damage, and based on this prediction a decision is taken regarding the future operation of the object. The term"future operation"refers to, for example, measures such as maintenance, exchange, restoration etc.

Within the term"future operation"a change of task is also comprised.

Furthermore, the term"future operation"comprises a direct change of the operational condition by means of a so-called actuator, see below.

The task for a machine which comprises a number of objects which are susceptible to damage can thus be changed after operation during a certain amount of time, so that the built-in damage tolerance for all of these objects to a high degree is consumed at the end of the life span of the machine. In

addition, the task can be changed if a breakdown of the object is predicted to arise within a near future, so that the object is exposed to smaller load. The task for a machine which comprises two objects which are susceptible to damage, which objects can be exposed to different degrees of load in different applications, can thus be changed when a breakdown in a first of these objects is predicted to a near future, in such a way that the first object is relieved of load and the second object is given more load in the second application.

Examples of these two objects can be a gear wheel in the driving line of the vehicle and a beam in the supporting structure (frame) of the vehicle. An example of a vehicle which can be used in applications with such varying loads is a waist or frame controlled vehicle, a so-called dumper. The dumper can be used in a mine, in which application primarily its driving line is loaded during operation, and can also be used at a construction site with uneven ground, in which application its supporting structure is exposed to load to a higher degree.

By means of said prediction, vehicles in a vehicle fleet can, with increased safety, be used in an optimal way with regard to the life span of the comprised components within various areas of use.

According to another example, the remaining life span is predicted based on the calculated damage, and the object is valued according to predicted remaining life span of the object. Alternatively, the evaluation can be carried out by a machine or a vehicle which contains the object.

According to another example, a signal is transmitted comprising information regarding said operational parameters, damage caused or remaining life span from a transmitter which is arranged in connection to the object to a receiver in a base station remotely located from the object for a decision regarding measures for the object. This brings about the possibility to check

the status of a plurality of objects/machines which comprise the object. It is particularly useful in the case where the object is arranged in a vehicle, so that the status of an entire vehicle fleet can be controlled from said base station. From the base station, signals can be transmitted to maintenance personnel, retailers, warehouses, repair shops, production units etc. for measures based on calculated remaining life span of the vehicle.

According to another example, the result of the calculation is used for design purposes. The calculated damage can be used for validation of a simulation module of the object in operation. Alternatively, the damage is used as a basis for dimensioning future objects which are intended to be exposed to similar operation.

In addition, the invention refers to a computer program product comprising data program segments for carrying out the steps according to the method described above when the program is executed in a computer. The term data program segment in other words refers to software parts. According to a further development of the invention, said data program segments are stored on a computer readable medium. The term computer readable medium refers to for example a disk, a CD-ROM disk or a hard drive.

In addition the invention refers to a system 1, see Fig. 5 and 6 for communication between a base station 2 and at least one remotely located stationary or mobily fixed machine 3, via transmitting and receiving organs 4, 5 for control of the operational status of the machine. The machine 3 is here arranged in a vehicle 12. The machine 3 is in more detail the driving organs of the vehicle 12. Said transmitting and receiving organs are arranged for the transfer of information via radio waves via antennas 5.

The machine 3 comprises an object 6 which is susceptible to damage. The object 6 is in Fig. 6 exemplified with a cog wheel in the transmission of the vehicle. The system 1 comprises organs 7 for measuring a number of

operational parameters of the object 6, a control unit (CPU) 14 operatively coupled to said measuring organs, and an A/D converter 15 operatively coupled between said measuring organs 7 and the control unit. The system additionally comprises an organ 8 operatively coupled to the control unit for calculating damage caused to the object based on the measured operational parameters and organs 9 (memory) for storing of a result of said damage calculation. The control unit 14 is arranged to receive a signal from the A/D converter 15, to communicate with said calculation organs 8 and memory 9, and to deliver a signal to the transmitting organ 4 for transmission to the antenna 5 of the base station 2.

Said measuring organs 7 consists of a plurality sensors for measuring the above-mentioned operational conditions and operational positions.

The calculation organs 8 of the system consist of a so-called predictor which is arranged to predict malfunctions or breakdowns of the object 6. Said calculation organs 8 are in more detail arranged to carry out the damage calculation according to the calculation process which has been described above. The evaluation of measured values including the prediction are thus carried out directly in the vehicle, and a result of this evaluation is transmitted to the base station 2. Alternatively, the calculation organ 8 is arranged in the base station. In this case, the measured values are stored directly in the memory organ 9 for later transmission or forwarding to the base station 14 for further processing. According to a further alternative, both the calculation organ 8 and the memory organ 9 are arranged in the base station.

The system further comprises an actuator 11 which is operatively coupled to the control unit 14 and the object 6. The actuator 11 is arranged to influence said operational condition and/or operational position based on values measured by said measuring organs 7. The actuator is, in this case, a cooling device for cooling the cooling oil which is supplied to the cog wheel 6. Alternatively, the actuator can consist of a filtering unit for filtering of

undesired particles in the oil. According to a further alternative, the actuator is a dehydrator for removing water from the oil. The actuator 11 is comprised in the appended patent claims with the term unit for taking steps and/or measures.

The system further comprises a unit 10 arranged for taking steps for the future operation of the machine/object based on said calculation. This unit 10 for taking steps for example consists of a station for spare parts, restoration or maintenance. The term"taking steps"for example refers to getting ready to remedy a predicted malfunction or an exchange of said object. According to one alternative, said unit for taking steps consists of a facility for producing new objects. The above used term"taking steps"of the unit 10 for taking steps should thus be seen widely. The unit 10 for taking steps comprises transmitting and receiving organs for communication with the base station 2.

According to the preferred example, the machine is arranged in a vehicle 12 but can alternatively be arranged in a vessel or in a transport means on rails. It is particularly advantageous to have the ability to monitor/control the status of said mobile means.

In the following, the system will be explained in the case that the object is arranged in a vehicle in the form of a building machine such as a frame or waist controlled vehicle 12. This should be seen as a preferred but in no way limiting application of the invention. The base station 2 can consist of a central unit in a facility in the form of a construction site. The management can from the central unit via wireless communication check the operational status of all the building machines 12 within the construction site. By carrying out said measurements and calculations at relatively short time intervals, the operational status of the vehicle fleet of the construction site can be checked essentially continuously. After such a check, decision can be taken regarding maintenance, repairs etc. According to an alternative example, the base

station is arranged to check vehicles positioned over a larger geographical area, such as a country or the entire world.

Said transmitting and receiving organs are arranged to transfer information relating to the status of the machine via a transmission signal in the form of radio waves or via satellite communication. Thus, wireless communication is at least used in part. Said transmitting and receiving organs can alternatively be arranged for optical connections or connection via hardware (in stationary objects) such as via land line, cable or wire.

The transmission of information takes place either periodically or at a request from the base station. A satellite positioning system, preferably the GPS system, is used for detecting the positions of the vehicles. In Fig. 5, this is illustrated with satellites 13.

In worldwide systems, the system suitably comprises a plurality of said units 10 for taking steps located at various positions in the world with the purpose of supplying a number of vehicles within a specific area with service.

According to an alternative use of the above-mentioned actuator, the control unit is arranged to directly activate it when the measured operational condition exceeds or falls below respectively a maximum or minimum value for the operational condition. In other words, the damage calculation is not used in this example of use.

Said machine can, within the frame of the appended claims, consist of a plurality of different devices arranged stationary and/or mobily such as combustion engines.

A second embodiment of the method according to the invention relates to the influence of tension load on an object. In particular, tension load on a part of a supporting structure is intended, such as a frame in a vehicle. The

invention can of course also be used in a supporting structure of a stationary device.

It is well known that the tension amplitude Sa, also known as the tension width AS (AS=2*Sa), is the parameter which prompts the crack growth of an object under load, i. e. controls the fatigue process, and that the mean tension Sm affects the influence of the tension amplitude. A high average tension accelerates the fatigue process. However, a constant average tension cannot on its own prompt crack growth. This second embodiment is thus analogous with the first embodiment. It is well known to measure tension on an object under load by means of elongation measurements, for example by means of strain gauges. By means of the strain gauge, only one parameter (strain) is obtained. According to the invention described below, a measurement signal from the strain gauge is divided into a first parameter (strain amplitudes) and a second parameter (average strain). The first parameter causes, as mentioned above, damage to the object. The second parameter in itself does not cause any damage but will affect the damage influence of the first parameter.

The expression that the first parameter and the second parameter are determined by at least one operation, that is measurement, should be interpreted to not only comprise a direct determination by means of measurement but to also comprise a measurement followed by another operation or processing of the measured parameter to calculate them.

An assumption which is independent of material is used to describe the influence of the average strain at fatigue, since one in general does not have knowledge regarding the material used. An equation proposed by Smith, Watson, Topper (SWT) satisfies this requirement. SWT is a special case (AS (R=-1), g=0,5) of the more general Walker's equation:

AS (R=0) = ASo = Smax (1~9) *AS9 (13) where g is a constant <1 and R=Smin/Smax. Smax = Sm+Sa, Smin= Sm-Sa, AS=Smax-Smin=2*Sa, ASo for R=0, AS1 for R=-1. A rearrangement of this equation gives: AS1/AS = Sa1/Sa = (1+Sm/Sa) (1-g) = (2/(1-R))(1-g) (14) In an evaluation with the Rainflow method of a measured input signal of the strain the strain width and the average strain are obtained. A damage value at varying load (R=-1) is: D1 = #(#S1im*ni) (15) The expression of AS1 from equation (14) is introduced into equation (15) whichgives: D1 = #((#Sim*ni)*(1+Smi/Sai)m*(1-g)) (16) Equation (16) is thus a measurement of the total load.

The Rainflow method is thus combined with a special case (the SWT equation) of Walker's equation in order to arrive at an expression for the total load. The invention is however not limited to Walker's equation, but a suitable function which adjusts for the influence of the average tension on the effect of the tension amplitude can be used.

Said calculation is carried out successively, and the adjusted damage value (the total load) is successively added to the same cell in a memory unit. Thus a method is obtained to adjust"on-line"for the influence of the average

strain. It is in an analogous manner possible to instead adjust to another R value (for example R=0 instead of as here R=-1).

The phenomena of the influence of the average strain or, to put it differently, of the R value can also be formulated in terms of fracture mechanics. The process is thus not limited to Walker's function. This constitutes a third embodiment of the invention. The maximal level where a crack in question does not grow is defined as a threshold value. The threshold value is very much dependent on the R value for most materials.

SKth=AKtho* (1-R) Ag (Klesnil and Lukas) or AKth= a+b* (1-R) AKth is the threshold value of the strain intensity index o for R=0. a and b are constants.

Another phenomena"Crack Closure"can be described as AKeff/AK = U = c+d*R (Elber) In both cases an adjustment of the life span is obtained depending on the R value which can easily be described as amplitude and average strain.

Furthermore, the evaluation of the signal is not limited to Rainflow. A combination of"Range Pair"and"Level crossing", or the latter alone can give an approximative information about the R value.

According to a further development of the second embodiment, the non- adjusted damage is added in another cell. The ratio between the adjusted

and non-adjusted damage will give the influence of the average strain on the damage in the time plane.

Naturally, it is possible to carry out the corresponding evaluation of a stored Rainflow matrix. This, however, needs more memory space. In addition, this does not give as good precision, since the amount of intervals is limited.

It is important that the parameter g can be chosen. In welding construction, it is assumed that residual strains are so great that the average strain can be neglected. In the case of spectrum load, which is in general the case, the high loads will however successively trigger the residual strains, which will lead to the average strain affecting the fatigue damage. By arriving at suitable values of the constant g it is possible to describe this phenomena in an adequate manner.

In the following, two examples will be given of applications of the second embodiment.

Example 1: Aeroplane In the air, the tension load on a wing will vary, that is the tension amplitude around an average tension which on the lower side of the wing is positive tensile stress. These varying average strains will increase and decrease respectively the damage effect of the strain amplitude. Contrarily, the average strain separately will give rise to none or very small damage influence.

Example 2: Load vehicles The average strain in supporting parts for example frames depends on whether the vehicle is loaded or not. This influences the damage effect of the variation in strain which is described with its amplitude or width. A constant average strain does not give rise to any damage influence in the case of fatigue. However, the shifts between, for example, two average strain levels

will in time contribute to damage. But the shifts will in the evaluation be treated as amplitudes.

The second embodiment of the method according to the invention is of course not limited to estimating damage/wear of an object caused by strain load. The measurement signal which is evaluated can be any signal, force, torque, pressure etc which can be related to strain.