Method of manufacturing an elongate component

Abstract

A method of manufacturing an elongate element (10) using a punching operation assumes a polynomial relationship between punch depth (dpunch) and neutral axis, with the constants being a polynomial function of plastic deformation of the beam. Using finite element analysis, a relationship between the required plastic deformation, the second moment of area of the element and the neutral axis of the element can be derived.

Description

RELATED APPLICATION

This application claims priority to GB 1114438.3 filed 22 Aug. 2011, the entire contents of which is incorporated by reference.

BACKGROUND OF INVENTION

The present invention is concerned with a method of manufacturing elongate components. More particularly, the present invention is concerned with a method of manufacturing elongate aircraft components such as aircraft wing stringers by inducing plastic deformation by a differential contact three point bending operation.

Components such as aircraft stringers need to be shaped to the wing aerodynamic profile. In order to achieve this, stringers are plastically deformed by differential contact three point bending. By “differential contact three point bending” we mean a process by which the stringer is supported at two spaced positions on a first side. A punch is applied therebetween from a second, opposite side in order to induce a bending moment to cause local plastic deformation. Because the stringer deforms as the punch progresses, the contact area with the punch and the supports may change. This is not a classical three point bending load case (the forces are not point loads) and as such is defined as “differential contact”.

One problem with this method is that the stringer will spring back after the bending moment is released due to its elasticity. Known methods of forming are non-predictive. The operator will attempt to estimate the amount of punch movement required to provide a given plastic deformation. The punch is applied to the stringer by the estimated value and the plastic deformation is measured once the punch is retracted. If the level of deformation is too low, the operator will estimate a further punch distance and reapply the punch. Successive bending operations are applied at the same position until the required deformation is achieved. The punch information is then stored and applied to the next component, and so on until the required “first time” punch movement is refined to a satisfactory degree.

A typical 18 m stringer will be have up to 250 punching locations along its length. As such it is desirable to reduce the number of punching operations at each station.

Should the stringer be overdeformed (i.e. undergo too much deformation when punched), scrapping the part is not feasible as such parts are very expensive. As such, an inverse bending moment (again, estimated by the operator) is applied to the component to reverse the deformation. Such repeated and reverse application of plastic deformation to the stringer can cause problems such as work hardening and fatigue.

Because of the complex and changing geometry of the stringers used in the aerospace sector, prediction of the stringer plastic deformation by analytical methods is not appropriate.

An alternative is to use numerical simulation, such as finite element analysis (FEA) to predict the deformation of the stringer at each punch location. Not only would the analysis of a single stringer need to be repeated at each punch stage (in order to arrive at the required plastic deformation), but because of the changing cross section of the stringer along its length, these analyses would need to be carried out for each discrete punch position. This would be extremely time consuming and costly with respect to computing resource.

SUMMARY OF THE INVENTION

It is an aim of the present invention to provide an improved method of manufacture which mitigates the “trial and error” method of the prior art, whilst utilising the benefits of numerical simulation without undue burden.

According to the invention there is provided a method of manufacturing an elongate component comprising the steps of:

• • providing a punching apparatus configured to apply a differential contact three point bending load to the component by advancing a punch at a punch location between two supports by a punch distance,
  • calculating a punch travel (d_punch) based on the second moment of inertia of the component at the punch location (I_xx), the location of the neutral axis distance of the component at the punch location (Y) and the required plastic deformation (d_plastic) of the component at the punch location,
  • bending the elongate component at the punch location by moving the punch by the punch travel.

Preferably the punch travel is calculated assuming that the required plastic deformation (d_plastic) is related to the second moment of inertia of the component at the punch location (I_xx), and the neutral axis distance at the punch location (Y) by an nth order polynomial of the form:

$\frac{d_{\mathrm{plastic}}}{I_{\mathrm{xx}}}=\sum _{i=0}^n{B}_i{Y}^i$ where B, are functions of the punch travel (d_punch).

Preferably:

$B_i=\sum _{j=1}^m{{\mathrm{BB}}_{\mathrm{ij}}\left(d_{\mathrm{punch}}-d_{\mathrm{elastic}}\right)}^j$ where BB_ij are constants.

Preferably which n=m=2; i.e. the polynomials are quadratic.

Preferably the punch travel is calculated from the expression:

$d_{\mathrm{punch}}=\left[\frac{\left(\begin{array}{c}\pm \sqrt{\left[\begin{array}{c}{I_{\mathrm{xx}}\left({\mathrm{BB}}_{1b}{Y}^2+{\mathrm{BB}}_{2b}Y+{\mathrm{BB}}_{3b}\right)}^2+\\ 4I_{\mathrm{xx}}{d}_{\mathrm{plastic}}\left({\mathrm{BB}}_{1a}{Y}^2+{\mathrm{BB}}_{2a}Y+{\mathrm{BB}}_{3a}\right)\end{array}\right]}-\\ \left[{\mathrm{BB}}_{1b}{Y}^2+{\mathrm{BB}}_{2b}Y+{\mathrm{BB}}_{3b}\right]I_{\mathrm{xx}}\end{array}\right)}{2I_{\mathrm{xx}}\left({\mathrm{BB}}_{1a}{Y}^2+{\mathrm{BB}}_{2a}Y+{\mathrm{BB}}_{3a}\right)}\right]-d_{\mathrm{elastic}}$ where BB_1a, BB_1b etc are constants.

Preferably d_elastic is calculated analytically from the cross section of the stringer at the punch location.

Constants BB_1a, BB_1b etc are preferably calculated statistically from a representative sample of numerical simulations, which may be finite element analyses.

SUMMARY OF THE DRAWINGS

A method in accordance with the present invention will now be described with reference to the accompanying figures in which:

FIG. 1 a is a view of a stringer undergoing a punching operation;

FIG. 1 b is a close-up view of the stringer of FIG. 1a, pre-punching;

FIG. 1 c is a close-up view similar to that of FIG. 1b during the punching operation;

FIG. 1 d is a close-up view similar to FIGS. 1b and 1c post punching;

FIG. 2 a is a cross section view of an I-stringer;

FIG. 2 b is a table of various stringer cross-section geometries;

FIG. 3 a is a table of results of various punching simulations;

FIG. 3 b is a graph of the results of FIG. 3a;

FIG. 4 a is a graph of the quadratic function of punch movement B1;

FIG. 4 b is a graph of the quadratic function of punch movement B2; and,

FIG. 4 c is a graph of the quadratic function of punch movement B3.

DETAILED DESCRIPTION OF INVENTION

Turning to FIG. 1a, an elongate aircraft wing stringer 10 is shown having an upper surface 12 and a lower surface 14. As shown in FIG. 1b, the example stringer is an I-beam having a top flange 24 and a bottom flange 26.

The stringer 10 is supported at its lower surface 14 on two space supports 16, 18 supported on ground 20. A punch 22 can be moved in a vertical direction in order to deform the stringer 10 between the two supports 16, 18. Such punching apparatuses are known and will not be described in detail here.

In order to deform the stringer to the profile of a desired aircraft wing, the punch 22 is pushed downwards with a force F by a punch deflection d_punch. d_punch is defined as the amount by which the punch 22 is advanced from a starting position in contact with the top surface 12 of the stringer 10 to the position of FIG. 1c.

As shown in FIG. 1c, as the punch 22 is advanced, the stringer 10 deforms both elastically and, eventually, plastically. The total deformation of the stringer at the point of contact with the punch is d_punch.

Turning to FIG. 1d, the punch 22 has been retracted and the stringer 10 will spring back by an elastic punch limit, d_elastic, leaving a final punched deformation of d_plastic. As mentioned above, the method of obtaining the required d_plastic is to progressively deform the beam, repeating the steps of FIGS. 1c and 1d, recording both d_punch and d_plastic until the desired deformation is met. Subsequent forming operations at that point along the beam are then used to refine this method until a suitable d_punch is found for the required d_plastic.

Stringer cross sections vary along their length in both size and shape. The prior art iterative process must therefore be carried out for each individual punch location, of which there are many.

Assuming that the second moment of inertia and the neutral axis of the stringer between the supports 16, 18 is constant, d_plastic can be calculated as follows:

$\begin{array}{cc}\frac{d_{\mathrm{plastic}}}{I_{\mathrm{xx}}}=B_1{Y}^2+B_{2}Y+B_3& \left(1\right)\end{array}$ where:

• I_xx=Second moment of inertia of the beam cross section,
• Y=Neutral axis of beam cross section,
• B1=Quadratic function of punch movement, d_punch (see below)
• B2=Quadratic function of punch movement, d_punch (see below)
• B3=Quadratic function of punch movement, d_punch (see below)B₁=BB_1a(d_punch−d_elastic)²+BB_1b(d_punch−d_plastic)  (2a)B₂=BB_2a(d_punch−d_elastic)²BB_2b(d_punch−d_elastic)  (2b)B₃=BB_3a(d_punch−d_elastic)²+BB_3b(d_punch−d_elastic)  (2c)where:BB_1a, BB_1b etc are material dependent constants to be determined.

This series of equations is indeterminate. Therefore a numerical solution is used. Values for the various constants (BB_1a, BB_1b etc) are derived from a number of selective numerical finite element analysis simulations. An example of a range of representative I-stringer geometries is shown in FIGS. 2a and 2b. The I-stringer shown in FIG. 2a has an upper flange having a thickness TF, a first portion to one side of the web having a length BFA and a second portion to the other side of the web having a length BFF; a web having a first thickness TWF at a first location and a second thickness TWA at a second location; and a lower flange having a thickness TA, a first portion to one side of the web having a length BAA and a second portion to the other side of the web having a length BAF. The I-stringer of FIG. 2a has a height H. The lower flange has a length a1 and a thickness a2 as shown in FIG. 2a. The upper flange has a length a3 and a thickness a4, as shown in FIG. 2a. The web has a height of a5 and a thickness a6 as shown in FIG. 2b.

Once a representative number of FEA simulations have been run, say 8 different values of d_punch from 4 to 11 mm for each of the cross sections listed in FIG. 2b, the results can be summarised as shown in FIG. 3a and plotted as shown in FIG. 3b, with values of c_plastic/I_xx vs Y. These plots, for each constant value of d_punch are thereby representative of equation (1) above.

From this analysis, a series of curves are retained such that the constants BB_1a, BB_1b etc can be plotted against (d_punch−d_elastic). (NB the elastic punch limit d_elastic is determined analytically). The least squares method used to calculate the values of BB_1a, BB_1b etc.

This allows the relationship between d_punch and each of B₁, B₂, B₃ to be plotted (see FIGS. 4a to 4c) and hence values of BB_1a, BB_1b etc to be determined by least squares regression.

Once all of the constants have been determined, the value of d_punch as a function of required plastic deformation d_plastic can be predicted using the following relation:

$d_{\mathrm{punch}}=\left[\frac{\left(\begin{array}{c}\pm \sqrt{\left[\begin{array}{c}{I_{\mathrm{xx}}\left({\mathrm{BB}}_{1b}{Y}^2+{\mathrm{BB}}_{2b}Y+{\mathrm{BB}}_{3b}\right)}^2+\\ 4I_{\mathrm{xx}}{d}_{\mathrm{plastic}}\left({\mathrm{BB}}_{1a}{Y}^2+{\mathrm{BB}}_{2a}Y+{\mathrm{BB}}_{3a}\right)\end{array}\right]}-\\ \left[{\mathrm{BB}}_{1b}{Y}^2+{\mathrm{BB}}_{2b}Y+{\mathrm{BB}}_{3b}\right]I_{\mathrm{xx}}\end{array}\right)}{2I_{\mathrm{xx}}\left({\mathrm{BB}}_{1a}{Y}^2+{\mathrm{BB}}_{2a}Y+{\mathrm{BB}}_{3a}\right)}\right]-d_{\mathrm{elastic}}$

In addition, the present invention provides a method of manufacturing using a prediction of the punch force required by the process.

Variations of the above embodiment fall within the scope of the present invention.

Claims

1. A method of manufacturing an elongate component comprising: providing a punching apparatus configured to apply a differential contact three point bending load to the elongate component by advancing a punch by a punch travel dpunch at a punch location between two discrete supports by a punch distance,calculating the punch travel dpunch based on a second moment of inertia Ixx of the elongate component at the punch location, the location of the neutral axis Y of the elongate component at the punch location, and a desired plastic deformation dplastic of the component at the punch location, wherein second moment of inertia Ixx, the neutral axis Y and the desired plastic deformation dplastic are related as follows:

2. The method of manufacturing an elongate component according to claim 1 in which n=2.

3. The method of manufacturing an elongate component according to claim 1 in which m=2 and n=2.

4. The method of manufacturing an elongate component according to claim 1 in which the punch travel is calculated from the expression:

5. The method of manufacturing an elongate component according to claim 1 in which the constants BB1a, BB1b, BB2a, BB2b, BB3a and BB3b are calculated statistically from a representative sample of numerical simulations.

6. The method of manufacturing an elongate component according to claim 5 in which the numerical simulations are finite element analyses.

7. A method of manufacturing an elongate component comprising: providing a punching apparatus configured to apply a differential contact three point bending load to the elongate component by advancing a punch at a punch location between two discrete supports by a punch distance;calculating analytically an elastic punch limit delastic;calculating a punch travel dpunch based on the second moment of inertia of the elongate component at the punch location Ixx, the location of the neutral axis of the elongate component at the punch location Y and the required plastic deformation dplastic of the component at the punch location, and bending the elongate component at the punch location by moving the punch by the punch travel dpunch,the punch travel dpunch is calculated assuming the required plastic deformation dplastic is related to the second moment of inertia of the elongate component at the punch location Ixx, and the neutral axis distance at the punch location Y by an nth order polynomial of the form:

8. A method of manufacturing an elongate component comprising: providing a punching apparatus configured to apply a differential contact three point bending load to the elongate component by advancing a punch at a punch location between two discrete supports by a punch distance,calculating a punch travel dpunch based on the second moment of inertia of the elongate component at the punch location Ixx, the location of the neutral axis of the elongate component at the punch location Y and the required plastic deformation dplastic of the component at the punch location, wherein the punch travel is calculated assuming the required plastic deformation dplastic is related to the second moment of inertia of the elongate component at the punch location Ixx, and the neutral axis distance at the punch location Y by an nth order polynomials of the form:

9. A method of manufacturing an elongate component comprising: providing a punching apparatus configured to apply a differential contact three point bending load to the elongate component by advancing a punch at a punch location between two discrete supports by a punch distance,calculating a punch travel dpunch is calculated from the expression: