JEC COMPOSITES MAGAZINE - Issue #88 - April 2014 - Special JEC Americas Spring Edition - 57

BVID

Designing for damage tolerance

innovation.
The core of this article will provide an overview of a model for predicting the residual
strength of aircraft structures. The Strip model is based on delamination growth caused
by surface ply (sublaminate) buckling/
blistering. This model has been developed
at the University of Bath over the past 6-7
years. It is very computationally efficient and
seeks to provide an alternative to the timeconsuming, design-specific and expensive
low-level test programmes currently used to
assess residual strength.

Dr Andrew T. Rhead
Prof. Richard Butler
Composites Research Unit
Department of Mechanical Engineering,
University of Bath

roducing future aircraft to meet
ACARE 2050 targets, irrespective
of predicted advances in engine
efficiency and aerodynamic performance,
will require airframe weight to be reduced by
approximately 15-20%. It is thus inevitable
that such aircraft will show an increasing
reliance on the favourable strength and
stiffness-to-weight ratio properties of carbon
fibre reinforced plastics (CFRP). In current
aircraft, vulnerability to impact damage,
and the 60% reduction in laminate strength
this can entail, is preventing the low mass
potential of CFRP from being exploited to
the extent seen in, for example, Formula 1
motor racing. Instead, safe but conservatively designed components are being produced at the cost of reduced fuel efficiency
and increased CO2 emissions. A lack of
understanding of the mechanisms through
which damage forms and propagates, and
the uncertainty this brings, has led to the
adoption of conservative design strategies
based on narrow empirical validation. The
significant cost of implementation of such
studies and the amount of data they generate
have created a design inertia that is stifling

Barely visible impact damage
During both the manufacture and operation of an aircraft, CFRP components are
vulnerable to impact. Sources include tool
drop during manufacture and maintenance,
runway debris, bird strike and hailstones.
The criticality of an impact depends on the
difficulty in detecting the impact site, the
distribution of damage (mainly delamination but also intra-ply cracks and fibre
failure) and the probability of an impacted
component being loaded beyond its residual
strength whilst damage remains undetected.
Owing to their easy detection, severe impacts such as bird strike - provided they are
immediately survivable - are not necessarily
the most dangerous. Barely visible impact
damage (BVID), which involves significant

internal damage yet leaves surface indentations that are undetectable on routine
inspections, is instead considered a worst
case as residual structural strength becomes
uncertain. Impact to CFRP laminates differs
from that of metallic structures in two key
ways: detectable surface deformation and
internal damage (see Figure 1). Impact to
CFRP components leaves far less surface
damage than it does on metals, making even
high-energy impacts difficult to detect on
routine inspections. This difficulty is intensified by the reasonably rapid and significant
relaxation of surface damage following
impact.
A number of factors contribute to the size
and structure of BVID including impact
energy, ply stacking sequence, resin and fibre
properties (particularly fracture toughness),
laminate thickness, bending stiffness and
boundary conditions provided by the surrounding structure.

Compression after impact
As with any aerospace structure, composite
components are subject to a number of
different failure modes including buckling,
open-hole failure and joint failure. Other
modes such as free-edge stress failure, kink
banding and surface ply buckling leading
to delamination growth are more uniquely
associated with composite structures.

No88 April 2014 /

SPecial JEC Americas 2014

Few in the aerospace community will doubt the severity of barely visible impact damage (BVID) and the constraints it places on the fuel
efficiency of the current generation of aircraft. However, the impact of
BVID becomes even more pronounced when considered in the light
of the stringent emission targets set out in ACARE 2050. This article
presents a model developed at the University of Bath for predicting
the residual strength of aircraft structures based on delamination
growth caused by surface ply buckling/blistering: the Strip model.

jec composites magazine 57

JEC COMPOSITES MAGAZINE - Issue #88 - April 2014 - Special JEC Americas Spring Edition

Table of Contents for the Digital Edition of JEC COMPOSITES MAGAZINE - Issue #88 - April 2014 - Special JEC Americas Spring Edition