CN1869640A - Method for investigating fatigue crack expansion - Google Patents

Abstract

The invention discloses a fatigue crack extension testing method, making a sweeping test in the fatigue test course and obtaining the variation condition of vibration response of the test system varies with test piece crack extension on the forced vibration condition; using a numeric method to make sweeping data fitting and obtaining variation curve of resonant frequency with number of circulating times; after the test piece is broken, according to actually observed crack size and shape, prefabricating crack of a CAD model and making finite model analysis and obtaining a curve of resonant angular frequency of the system against test piece size, using the curve and able to obtain dynamic extension rule of test piece crack and then obtaining its curve of variation rate; and finally combining with calculation of stress strength factor range Delta K, and able to obtain a curve of crack extension rate.

Description

A kind of method of investigating fatigue crack expansion

Technical field

The present invention relates to a kind of crack growth test method that is applied to the component of machine torture test.

Background technology

The fatigure failure of mechanical component is its a kind of principal mode that inefficacy takes place.Conclude from various statistical data, its ratio is about about 80%～95%.Therefore problem of fatigue damage is major fields of Mechanical Reliability research.Fatigure failure is a very complicated problems, and the factor that influences fatigue strength is numerous, and most influence factor also can't provide strict Physical Mechanism at present and explain, also is difficult to provide more careful quantitative description from mathematics.

People from different viewing angles, have formed the engineering approximation describing method of some fatigure failures gradually in a large amount of production practices.Wherein, the rate curve of crack Propagation has been obtained good effect in the fatigue reliability estimation of a lot of actual engineering problems.Fatigue crack growth rate (da/dN-Δ K) is not only an important indicator of exosyndrome material fatigue crack-resistant toughness, also is to the traditional fatigue strength design and the important supplement of content of the test.Along with the progressively development of Theory of Fracture Mechanics and perfect, this parameter has obtained application more and more widely in Fatigue Design.

One of main task of fatigue crack propagation test is exactly to measure crack growth rate.The mensuration of these data then depends on the kinetic measurement of crack size a, and promptly crack size a loads the dynamic changing curve of times N with test.This measurement accurately whether test findings is had significant impact, be the key issue of fatigue crack propagation test.

The dynamic measurement method of existing crack size is a lot, and commonly used in the laboratory have portable measuring microscope direct-reading methods, potential method, flexibility method, eddy-current method, acoustic-emission and based on the mechanics method of experimental modal analysis etc.; And on-the-spot parts crackle is measured commonly used have eddy-current method, fluorescence method and magnaflux etc.For different subjects, the process of the test in the varying environment, various test methods in various degree application all arranged.But these methods all have a same limitation, promptly often can only be applied to material test or simple test specimen.For numerous actual parts, as engine crankshaft etc., because the complicacy of its structure and crack position and form is special, these methods all are difficult to implement.

Summary of the invention

The objective of the invention is for overcoming the limitation of original fatigue crack test method, a kind of method of novel investigating fatigue crack expansion is provided, makes that some have labyrinth and special process or crack position, form is special and parts that be not easy to observe can directly carry out the test of crack growth rate.

The method of investigating fatigue crack expansion of the present invention may further comprise the steps:

1) test specimen is installed on the load maintainer of forming by master, passive swing arm and eccentric wheel vibrator;

2) torture test is carried out in loading, and every is carried out frequency sweep test at regular intervals one time, and the resulting exciting angular frequency of frequency sweep-vibratory response acceleration information is carried out match by the prototype function that formula (1) is established by least square method:

$\mathrm{\α}=\frac{A+B{\mathrm{\ω}}^2}{1-C{\mathrm{\ω}}^2},\mathrm{\ω}\∈(0.3p,0.85p)---\left(1\right)$

In the formula: ω is the exciting angular frequency; α is the vibratory response acceleration; P is the resonant angular frequency of vibrational system; A, B and C are undetermined coefficient, obtain the dynamic change N-p curve of resonant angular frequency p with test cycle index N after the match;

3) finish in torture test, after test specimen ruptures, obtain the morphological feature and the size a of actual crack by actual observation fracture apperance fatigue striation;

4) it is prefabricated to utilize the actual observation data to carry out cad model, and the resonant angular frequency p of each crack size a drag is carried out finite element analysis, obtains the a-p relation curve;

5) utilize the corresponding relation of a-p curve and N-p curve, draw the a-N curve; According to the a-N curve, increase progressively quadratic polynomial or secant method calculates the da/dN-N curve with 7, the stress intensity factor range Δ K in conjunction with Finite Element Method calculates obtains the crack growth rate curve, i.e. da/dN-Δ K curve.

Beneficial effect of the present invention is:

Flow process by this test method, it is after test specimen ruptures, and under the situation that the particular location, form and the size characteristic that make crackle can be surveyed fully, the monitor message in the control test process utilizes the finite element simulation technology that the spreading rate of crackle is calculated accurately again.Therefore, the inventive method has been given full play to the technological merit of resonance loaded type fatigue tester and ingenious utilization the " posteriority " data, can tolerate farthest that the complicacy and the polytrope of position, crackle form and crack propagation mode appear in crackle in the torture test process.In fact under the enough meticulous situation of finite element model, its measuring accuracy is subjected to the influence of these factors hardly.The inventive method on its technology implementation route, embodied some with test method in the past different peculiar thinking.Make that some have labyrinth and special process or crack position, form is special and parts that be not easy to observe can directly carry out the test of crack growth rate.

The inventive method is with respect to traditional straight survey method, electrical measuring method and based on the crack growth test methods such as mechanics method of experimental modal analysis.It is not subjected to the restriction of sample dimensions, structure, technology characteristics and crack position, form, so long as utilize the torture test of resonance loading principle, can carry out crack growth test to any parts.

Description of drawings

Fig. 1 is the synoptic diagram of resonant mode torture test load maintainer;

Fig. 2 is that frequency sweep method crankshaft cracks spreading rate is measured flow process;

Fig. 3 is the frequency sweep illustrated example;

Fig. 4 is the prefabricated synoptic diagram of finite element analysis model crackle, a) the resonance mechanism three-dimensional perspective behind the precrack wherein, b) be a) in the enlarged drawing of semielliptic surface crack.

Embodiment

The method of investigating fatigue crack expansion of the present invention may further comprise the steps:

1) with test specimen, bent axle 1 for example shown in Figure 1 is installed on the resonant mode load maintainer of being made up of master arm 2, passive arm 3 and eccentric wheel vibrator 4.

2) load and to carry out torture test, and every at regular intervals, as 50,000 times or 100,000 times, carry out frequency sweep test one time.Frequency sweep test is in order to obtain the resonant frequency of vibrational system under the forced vibration condition, and its method is by applying the excitation of different frequency, and writes down under each frequency excitation the response acceleration value of system.Promptly obtain data sequence { ω _i, α _i| _I=1,2,3..., the sequence of actual measurement is drawn on the figure, can obtain frequency sweep figure, Fig. 3 has provided some examples of frequency sweep figure constantly.Along with the increase of test cycle index, the frequency sweep curve moves to left and shows the decline of its resonant frequency.

The resulting exciting angular frequency of frequency sweep-vibratory response acceleration information is carried out match by the prototype function that formula (1) is established by least square method:

$\mathrm{\α}=\frac{A+B{\mathrm{\ω}}^2}{1-C{\mathrm{\ω}}^2},\mathrm{\ω}\∈(0.3p,0.85p)---\left(1\right)$

In the formula: ω is the exciting angular frequency; α is the vibratory response acceleration; P is the resonant angular frequency of vibrational system; A, B and C are undetermined coefficient, use least square fitting and obtain after coefficient A, B and the C, and the calculating formula of resonant angular frequency p is:

$p=\sqrt{\frac{1}{C}}$

Obtain the dynamic change situation of resonant angular frequency p after the match, i.e. the N-p curve with test cycle index N;

3) finish in torture test, after test specimen ruptures, obtain the morphological feature and the size a of actual crack by actual observation fracture apperance fatigue striation.

4) it is prefabricated to utilize the morphological feature of crackle of actual observation and size a that cad model is carried out crackle.Concrete grammar is: on the CAD of former resonator system solid model, survey the crackle form factually in the actual crack position and size a draws an ellipse, with this ellipse is laminar entity of profile stretching, then round-corner transition being done at its edge handles so that its three-dimensional configuration with actual crack is similar to, at last, with the flawless cad model for being operated entity, with this thin slice it is done entity boolean reducing and finish the prefabricated of crackle, Fig. 4 is the partial enlarged drawing of prefabricated three-dimensional perspective of cad model crackle and crackle.Resonant angular frequency p to each crack size a drag carries out finite element analysis, obtains the a-p relation curve.

5) utilize the corresponding relation of a-p curve and N-p curve, draw the a-N curve; According to the a-N curve, increase progressively quadratic polynomial or secant method calculates change rate curve with 7, i.e. the da/dN-N curve.Stress intensity factor can be the 4th) calculate with Finite Element Method on the prefabricated model of CAD crackle finished of step, computing application 1/4 displacement method, the unit adopts 1/4 unusual yuan of 15 node.Stress intensity factor range Δ K in conjunction with calculating obtains the crack growth rate curve, i.e. da/dN-Δ K curve.

Claims (1)

1. the method for an investigating fatigue crack expansion may further comprise the steps:

1) test specimen is installed on the load maintainer of forming by master, passive swing arm and eccentric wheel vibrator;

2) torture test is carried out in loading, and every is carried out frequency sweep test at regular intervals one time, and the resulting exciting angular frequency of frequency sweep-vibratory response acceleration information is carried out match by the prototype function that formula (1) is established by least square method:

$\mathrm{\α}=\frac{A+{\mathrm{B\ω}}^2}{1-{\mathrm{C\ω}}^2},$ ω∈(0.3p，0.85p) (1)

In the formula: ω is the exciting angular frequency; α is the vibratory response acceleration; P is the resonant angular frequency of vibrational system; A, B and C are undetermined coefficient, obtain the dynamic change N-p curve of resonant angular frequency p with test cycle index N after the match;

3) finish in torture test, after test specimen ruptures, obtain the morphological feature and the size a of actual crack by actual observation fracture apperance fatigue striation;

4) it is prefabricated to utilize the actual observation data to carry out cad model, and the resonant angular frequency p of each crack size a drag is carried out finite element analysis, obtains the a-p relation curve;

5) utilize the corresponding relation of a-p curve and N-p curve, draw the a-N curve; According to the a-N curve, increase progressively quadratic polynomial or secant method calculates the da/dN-N curve with 7, the stress intensity factor range Δ K in conjunction with Finite Element Method calculates obtains the crack growth rate curve, i.e. da/dN-Δ K curve.

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