CN1843829A - Automobile structure lightweighting design method based on structure intensity variation characteristic - Google Patents

Abstract

The invention discloses a method for reducing the weight of vehicle, based on the structural strength variable character. It comprises: 1, via strength calculation or test to get the strength change in constant load; 2, via the dynamic strength increase matrix of structure, attaining the critical load; 3, processing structure finite analysis on the structure, calculating the stress in critical load; 4, fixing the stress of weight reduced structure and designing relative structure. With said invention, the structure can attain longer service life and reduce weight. Said invention has high reliability and operational property. Since, there is not strength test on the sample; the user can calculate the strength increase in the sample load according to the lower-load strength character of structure, to calculate the critical load, to realize weight-reduced structure.

Description

Vehicle structure light-weight design method based on the structural strength variation characteristic

Technical field

The present invention relates to a kind of vehicle structure light-weight design method, the low load reinforcing of especially a kind of application structure Strength Changes and structure is realized the light-weight design method of vehicle structure.

Background technology

For a long time, the automobile design of China adopts the method for experience and analogy.And offshore companies such as America and Europe, Japan have experienced the way of progressive development, from initial extensive design (with experience and analogize to the basis) through strength reliability design (based on strength at repeated alternation achievement in research) up till now light-weight design (developing and high-tech technology) again based on high strength material to working load and material and structure.

In recent years, external big car company further develops to the lightweight direction in order to tackle fierce day by day global competition.They adopt high strength (as TRIP steel, aluminum alloy and synthetic material etc.) material to realize the car load lightweight, but increased cost greatly mostly.When vehicle structure, load character and the structure strength at repeated alternation Changing Pattern under various load lacks in-depth study, even adopt new material might not guarantee that also structural weight is the lightest.

The automobile of China is compared with external advanced level and is had big gap, the overweight car load dynamic property that causes of structure and fuel economy is poor, cost is too high then particularly outstanding.The automobile lightweight of a new round---widely apply the relatively backward market situation of new material and China's economy not too to coordinate.Though it is more reasonable that a lot of structure light-weight design all adopt the finite element analysis technology that structural stress is distributed, and lacks clear and definite feasible light-weight design interpretational criteria, just engineering stress is controlled under the permissible stress, makes structure be still infinite life.Therefore, this structure still has bigger loss of weight space.

Steel design part and material thereof have a key property, and under the effect repeatedly of certain cyclic loading, its static strength and fatigue strength values can change.In the automotive use process, framing member mainly bears the random load of alternation, and its intensity will change to a certain extent.Before fatigue crack initiation, intensity is increased for a large amount of low amplitude loads that are lower than (but approaching) ultimate load; A large amount of low amplitude loads (such as being lower than 40% ultimate load) far below ultimate load remain unchanged intensity.Because working load more at random, and most load value is smaller, and the intensity evaluation of framing member mainly is as the criterion with test load.For many years, framing members such as propons, back axle are finished static strength and fatigue life test by standard after, promptly can evaluate it and can satisfy the reliability of service requirement, ignored the evaluation of whether design level or material potentiality being given full play to etc. the aspect.

In order to instruct the light-weight design of structure with test results, be necessary after finishing static strength and fatigue life test, measure the residual intensity of structure, obtain the information of framing member Strength Changes state.If test back static strength rises, show that test load is still little load for this structure, the lightweight potentiality of structure are bigger.Rational structure is behind completion of test, and the static strength value of structure decays to the peak value of test load.When not having accurate theoretical foundation, for safety, can be controlled at a kind of safe slightly state by the bar structure design parameters, promptly behind the completion of test, its static strength value remains unchanged or decreases slightly.This " constant " is that static strength falls after rise again to original numerical value behind elevation process of experience.If the relative structure static strength of test load too small (intensity is superfluous greatly), the static strength value also will be kept " constant " in the process of the test.At this moment structure does not experience the low load reinforcing process, and the potentiality of material are not in full use.

Summary of the invention

The present invention seeks in order to lack clear and definite feasible light-weight design interpretational criteria in the automobile structure method of designing that changes present China, the automobile structure design still has the present situation in bigger loss of weight space, and a kind of vehicle structure light-weight design method based on the structural strength variation characteristic is provided.

Technical scheme of the present invention is: a kind of vehicle structure light-weight design method based on the structural strength variation characteristic is characterized in that concrete step is as follows:

1, determines the Strength Changes amount of structure under the specified load condition by strength test or calculating;

2, the dynamic strength model of growth formula (1) of integrated structure, obtain structure ultimate load (referring to the agricultural mechanical journal magazine, 1999,30 (4): " standing the research of vehicle part yield strength increasing law behind the reverse load by a narrow margin " literary composition on the 83-87)

$1g\frac{Q_b}{Q_0}=1g\frac{p_0}{p_a}1g\frac{n}{{10}^5}\left(p_a{<p}_0\right)---\left(1\right)$

The initial static strength of structure is Q in the formula ₀, structure is p in amplitude _aLoad under after CYCLIC LOADING n time, its static strength rises to Q _b, p ₀Ultimate load for structure; This equation has been set up the relation of the cycle number under structure static strength and fatigue load and the effect thereof, also reflects under the cyclic loading effect, and the static strength of structure is change (the stable rising).

(3) structural finite element analysis:

Set up the stress of structure and the relation of static strength by finite element analysis, set up the relation of stress and ultimate load indirectly with it; Cooresponding stress also obtains the ratio of structure static strength and strength at repeated alternation therefrom when calculating ultimate load, and when the structure light-weight design, it is constant with the ratio of static strength to keep structural fatigue intensity;

(4) carry out determining and the loss of weight design of loss of weight structural stress:

The structure loss of weight means that the stress that structure is born under identical load will increase, and according to endurance life of structure guaranteeing structural stress, also must satisfy structure simultaneously has enough rigidity.Therefore, light in structure quantize design process in will be through FEM (finite element) calculation repeatedly.

Method of the present invention combines stryctyrak test with theoretical analysis, have higher reliability and operability.Based on this method, structure intensity in use is not fully exerted.For new design structure, make a strength test owing to lack model machine, can estimate in the gain in strength under the test load structure according to the low load reinforcing property of structure, and then calculate the ultimate load of structure, realize the automobile light-weight design.

This method has been deepened the function of Finite Element Method in the structure light-weight design.Utilize this method, can make structure obtain finite life, thereby realize the lightweight on the fundamental significance.This method can be other ground transport vehicle, all kinds of mechanical processing machine and numerous fixed operation mechanical structures light-weight design Technical Reference is provided.

Description of drawings

Fig. 1 is the light in structure quantification method of designing diagram of circuit based on the Strength Changes feature;

Fig. 2 is the strain measurement point position view of bridge girder before certain.

The specific embodiment

The present invention is further illustrated below in conjunction with accompanying drawing and instantiation.

A kind of vehicle structure light-weight design method based on the structural strength variation characteristic: with the example that is designed to of 5 tons of autotruck propons, design cycle is seen Fig. 1, and method step is:

1, determine the Strength Changes amount of structure under the specified load condition by strength test:

According to the framing member static strength that specified load circulation back is measured, utilize formula (1) can obtain the ultimate load p of structure very easily ₀Ultimate load is similar to the fatigue limit of material in the physical sense, is 10 corresponding to making structure fatigue life ⁶Inferior load.A lot of uncertain factors and parameter be need consider owing to infer the fatigue limit of structure from material parameter, therefore, these uncertain factors and parameter taken all factors into consideration by the ultimate load that experimental result calculates.For the propons of 5 tons of bogies, its preceding bridge girder is a H-girder, and one group of propons is made a strength test (comprising the residual intensity strength detection), determines the as a result Strength Changes of propons under the test load condition, test results such as table 1:

The strength test of table 1. propons

The quiet full and down kg of propons	Initial static strength kg	Fatigue life cycle is inferior	Fatigue test load amplitude kg	Static strength kg behind the fatigue test
					3200	27600	10 6	4800	30920

As can be seen from Table 1, through 10 ⁶After the inferior CYCLIC LOADING test (recycle ratio r=0.1), the static strength value of this propons has increased by 12%.

$\frac{30920-27600}{27600}\&times;100\%=12\%$

2, the dynamic strength model of growth formula (1) of integrated structure, the ultimate load of acquisition structure:

With the data substitution formula (1) in the table 1, can obtain the ultimate load p of this propons ₀=5252kg.Be test cycle 1,000,000 times under the load of 5252kg promptly, will be damaged in amplitude.By China's standard, this propons is that mean life promptly reaches requirement 700,000 times under the load of 4800kg in amplitude.By the standard of external certain company, it is that test cycle is not damaged for 1,000,000 times and promptly reaches requirement under the load of 3200kg that the propons of this tonnage automobile only needs in amplitude.Even as seen by the standard evaluation of domestic harshness, this propons still has the leeway of big loss of weight.For satisfying further loss of weight under the condition of standard-required, at first must determine the stress level and the distribution of structure by finite element.

3. the finite element analysis of structure

Though, obtain the concrete numerical value of static strength and strength at repeated alternation (ultimate load) through overtesting and calculating, the size of further project organization, reference frame must be arranged.For this reason, set up the stress of structure and the relation of static strength, in the hope of setting up the relation of stress and ultimate load indirectly by finite element analysis.

Because finite element model provides the relation of load and stress more accurately, can be with the STRESS VARIATION of dangerous point as evaluation design-calculated benchmark.The stress calculation main or dangerous point that obtains preceding bridge girder through FEM (finite element) calculation (Fig. 2 and Fig. 3 are finite element model and measuring point distribution) the results are shown in Table 2.

The stress intensity of the main measuring point of bridge girder and FEM (finite element) calculation contrast before the table 2

Measuring point	1	2	3	4	5	6	7	8
									Result of a measurement (MPa)	-184.1	-217.4	-341.2	-262.4	312.1	320.2	304.1	228.7
Result of calculation (MPa)	-190.2	-221.9	-352.7	-282.6	322.3	329.7	313.6	247.9

Annotate: "-" expression is pressed, and "+" expression is drawn; The demarcation load of result of finite element is 10000kg.

Can see that from table 2 through FEM (finite element) calculation, the 6# measuring point that is positioned at the spring perch side-lower is the most dangerous.Because this finite element model provides the relation of load and stress more accurately, can be with this STRESS VARIATION as evaluation design-calculated benchmark.

Go on foot the concrete numerical value that has obtained structure static strength and strength at repeated alternation according to method the 1st, if further lightweight, by dynamic intensity equation, the ultimate load value of propons should suitably reduce, and simultaneously initial static strength also will decrease.But, replace load as intensive parameter with stress in order to increase comparability and generalization.

4. cooresponding stress when calculating ultimate load:

According to result of finite element, the cooresponding stress of initial static strength of this propons is:

S _b＝27600/10000×320.2＝883.75MPa

The load of determining according to FEM (finite element) calculation and the relation of stress, cooresponding stress was when ultimate load was 5252kg:

S ₀＝5252×320.2/10000＝168.17MPa

The ratio that can obtain this propons structural fatigue intensity and static strength thus is: the class chamber

k＝168.17/883.75＝0.19＝19％

During redesign propons scantling of structure, it is constant generally to keep this proportionate relationship.

5. carry out determining and the loss of weight design of loss of weight structural stress:

(1) the loss of weight structural stress determines

The structure loss of weight means that the stress that structure is born under identical load will increase.By the dynamic strength model of growth formula (1) of structure, relatively rational structure intensity should be able to make structure under loading, and the static strength value remains unchanged substantially.Like this, for present embodiment, the reasonable intensity of propons (by China's standard) should be that structure static strength value under (standard code) load of 4800kg remains unchanged substantially or has slightly and reduces.Therefore, the propons behind the loss of weight, its ultimate load should be p ₀₁=4800kg.

The loss of weight target that obtains this example thus is: the ultimate load 5252kg of propons original design is reduced to 4800kg.This passes through finite element analysis the Stress Control of structure is had essential distinction within permissible stress with at present popular.

According to the analytical model that finite element is set up, before the loss of weight, the test load width of cloth is that the cooresponding stress of 4800kg is:

S _ap＝4800×320.2/10000＝153.70Mpa

Behind the loss of weight, propons cooresponding stress when the test load width of cloth is 4800kg should be the cooresponding stress of former ultimate load, that is:

S _an＝168.17Mpa

When carrying out the finite element analysis second time (finite element model and measuring point such as Fig. 2 and Fig. 3), demarcating under the load (10000kg), the stress of same measuring point should be:

S _cn＝168.17×320.2/153.70＝350.34Mpa

By the structure that present stress is determined, under test condition finite life, promptly 10 ⁶Inferior.This is different from the infinite life design.

(2) loss of weight design

Propons is as safety member, and be to guarantee endurance life.For the cruising of vehicle, must guarantee that also propons has enough rigidity (comprising vertical stiffness, lateral rigidity and longitudinal rigidity).Therefore, when reducing scantling of structure, need to guarantee that the propons cross-sectional plane has enough height, center pillar has suitable width.According to new demarcation stress, finite element analysis model is made amendment.Mainly be to reduce scantling of structure, make stress appropriateness increase under the same load.

Through repeated calculation, with center pillar attenuate 4mm, upper and lower aerofoil respectively shrinks 4mm, and cross-sectional height reduces 2mm, at this moment, and the calculated stress at former measuring point place

S _cn＝347.75Mpa

The calculated stress of all the other measuring points all has raising in various degree.Consider engineering reality, such improvement is a basic feasible solution.If the conformability of the theoretical stress (350.34Mpa) of simple court dangers measuring point, many sizes are not easy to realize on the structure.By the preceding bridge construction that this size obtains, theoretical loss of weight is 4.5kg.For the originating firm of producing 120000 propons per year, but only this improves 540 tons of annual just saving in materials.

Claims (1)

1. vehicle structure light-weight design method based on the structural strength variation characteristic is characterized in that concrete step is as follows:

(1) by strength test or calculate to determine the Strength Changes amount of structure under the specified load condition;

(2) integrated structure dynamic strength model of growth formula (1), the ultimate load of acquisition structure:

$\lg \frac{Q_b}{Q_0}=\lg \frac{p_0}{p_a}\lg \frac{n}{{10}^5},(p_a<p_0)---\left(1\right)$

In the formula, the initial static strength of structure is Q ₀, structure is p in amplitude _aLoad under after CYCLIC LOADING n time, its static strength rises to Q _b, p ₀Ultimate load for structure;

(3) structural finite element analysis:

Set up the stress of structure and the relation of static strength by finite element analysis, set up the relation of stress and ultimate load indirectly with it; Cooresponding stress also obtains the ratio of structure static strength and strength at repeated alternation therefrom when calculating ultimate load, and when the structure light-weight design, it is constant with the ratio of static strength to keep structural fatigue intensity;

(4) carry out determining and the loss of weight design of loss of weight structural stress.

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